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    Helium-Neon Lasers

    Sub-Table of Contents

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  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Characteristics, Applications, Safety

    Introduction to Helium-Neon Lasers

    A helium-neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this at first glance (though the design and manufacturing issues which must be dealt with to achieve the desired beam characteristics, power output, stability, and life span, are non-trivial). The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors - one totally reflective (called the High Reflector or HR), the other partially reflective (called the Output Coupler or OC) at the wavelength of the laser's output - complete the resonator assembly. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external (on precision high priced lab quality lasers). Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

    I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! :) HeNe's are simple in principle though complex to manufacture, the beam quality is excellent - better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! :) This really isn't possible with diode or solid state lasers.

    I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: "The Amateur Scientist - Helium-Neon Laser", Scientific American, September 1964, and reprinted in the collection: "Light and Its Uses" [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.

    Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.) See the chapters starting with: Amateur Laser Construction for more of the juicy details.

    However, for most of us, 'building' a HeNe laser is like 'building' a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.

    The most common internal mirror HeNe laser tubes are between 4.5" and 14" (125 mm to 350 mm) in overall length and 3/4" to 1-1/2" (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type - either bare or as part of complete laser heads - are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common.

    Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). Even more powerful ones have been built as research projects. The largest HeNe lasers in current production are rated between 35 to 50 mW.

    Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Common HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless switched on and off or modulated. (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to "cavity dump" a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren't generally useful for very much outside some esoteric research areas and in any case, you probably won't find any of these at a local flea market or swap meet! :)

    Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (611.9 nm), and even IR (1,1152 and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously but this may actually be a 'defect' resulting from a combination of high gain and insufficiently narrow band optics - these tubes tend to be unstable.

    Manufacturers include Melles-Griot, Spectra-Physics, Uniphase, and several others. (You may also find Aerotech and Siemens HeNe lasers though these companies have gotten out of the HeNe laser business.) HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability.

    HeNe lasers have been found in all kinds of equipment including:

    Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)

    Melles Griot catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don't think it is on their Web site.

    Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.

    Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser half a meter long, why bother with a HeNe laser at all? There are several reasons:

    However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say... If you need a stable, round, astigmatism-free, long lived, visible 5 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.

    Some Applications of a 1 mW Helium-Neon Laser

    There are many uses for even a 1 mW helium-neon laser. Most of these same sorts of things can also be done with a collimated diode laser (though some laser diodes may not have the needed coherence properties for applications like interferometry and hologram generation).

    Below are just a few possibilities.

    (Portions from: Chris Chagaris (

    For many more ideas, see the chapters: Laser Experiments and Projects and Laser Instruments and Applications and the many references and links in the chapter: Laser Information Resources.

    HeNe Laser Safety

    As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of HeNe lasers mostly dealt with in this document are rated Class II, IIIa, or the low end of IIIb (see the section: Laser Safety Classifications. For most of these, common sense (don't stare into the beam) and fairly basic precautions suffice since the reflected or scattered light will not cause instantaneous injury and is not a fire hazard.

    However, unlike those for laser diodes, HeNe power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the HeNe laser tube - especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.

    The high quality capacitors in a typical power supply will hold enough charge to wake you up - for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a HeNe tube attached or it did not start for some reason. There will likely be a lower voltage - perhaps 1 to 3 kV - on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn't likely to be enough to be lethal but it can still be quite a jolt. The HeNe tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won't really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!

    However, should you be dealing with a much larger HeNe laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW HeNe tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a homemade unit using grossly oversized parts)! It doesn't take much more under the wrong conditions to kill.

    After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge - and confirm with a voltmeter before touching anything. (Don't use carbon resistors as I have seen them behave funny around high voltages. And, don't use the old screwdriver trick - shorting the output of the power supply directly to ground - as this may damage it internally.)

    See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment for detailed information before contemplating the inside or HV terminals of a HeNe power supply!

    Now, for some first-hand experience:

    (From: Doug (

    Well, here's where I embarrass myself, but hopefully save a life...

    I've worked on medium and large frame lasers since about 1980 (Spectra-Physics 168's, 171's, Innova 90's, 100's and 200's - high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that's bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.

    I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.

    At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.

    I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can't even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that's called 'irony'.

    Comments on HeNe Laser Safety Issues

    (Portions from: Robert Savas (

    A 10 mw HeNe laser certainly presents an eye hazard.

    According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Theory of Operation, Modes, Coherence Length, On-Line Course and Tutorials

    Instant HeNe Laser Theory

    For much more than I can provide here (should you care), see the section: On-line Introductions to Lasers. These sites are well worth checking out as they include substantial material on HeNe lasers.

    The term laser stands for "Light Amplification by Stimulated Emission of Radiation". However, lasers as most of us know them, are actually sources of light - oscillators rather than amplifiers. (Although laser amplifiers do exist in applications as diverse as fiber optic communications repeaters and multi-gigawatt laser arrays for inertial fusion research.) Of course, all oscillators - electronic, mechanical, or optical - are constructed by adding the proper kind of positive feedback to an amplifier.

    All materials exhibit what is known as a bright line spectra when excited in some way. In the case of gases, this can be an electric current or (RF) radio frequency field. In the case of solids like ruby, a bright pulse of light from a xenon flash lamp can be used. The spectral lines are the result of spontaneous transitions of electrons in the material's atoms from higher to lower energy levels. A similar set of dark lines result in broad band light that is passed through the material due to the absorption of energy at specific wavelengths. Only a discrete set of energy levels and thus a discrete set of transitions are permitted based on quantum mechanical principles (well beyond the scope of this document, thankfully!). The entire science of spectroscopy is based on fact that every material has a unique spectral signature.

    The HeNe laser depends on energy level transitions in the neon gas. In the case of neon, there are dozens if not hundreds of possible wavelength lines of light in this spectrum. Some of the stronger ones are near the 632.8 nm line of the common red HeNe laser - but this is not the strongest:

    The strongest red line is 640.2 nm. There is one almost as strong at 633.4 nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak in an ordinary neon spectrum, due to the high energy levels in the neon atom used to produce this line. See: Bright Line Spectra of Helium and Neon. (The relative brightnesses of these don't appear to be accurate though at present.) More detailed spectra can be found at the: Laser Stars - Spectra of Gas Discharges Page. And there is a photo of an actual HeNe laser discharge spectra with very detailed annotation of most of the visible lines in: Skywise's Lasers and Optics Reference Section. The comment about the output wavelength not being one of the stronger lines is valid for most lasers as if it were, that energy level would be depleted by spontaneous emission, which isn't what is wanted!

    There are also many infra-red lines and some in the orange, yellow, and green regions of the spectrum as well.

    The helium does not participate in the lasing (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.

    It turns out that the upper level of the transition that produces the 632.8 nm line has an energy level that almost exactly matches the energy level of helium's lowest excited state. The vibrational coupling between these two states is highly efficient.

    You need the gas mixture to be mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines.

    A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.

    However, pure neon will lase superradiantly in a narrow tube (e.g., 40 cm long x 1 mm ID) in the orange (611.9 nm) and yellow (594.1 nm) with orange being the strongest. Superradiant means that no mirrors are used although the addition of a Fabry-Perot cavity does improve the lateral coherence and output power. This from a paper entitled: "Super-Radiant Yellow and Orange Laser Transitions in Pure Neon" by H. G. Heard and J. Peterson, Proceedings of the IEEE, Oct. 1964, vol. #52, page #1258. The authors used a pulsed high voltage power supply for excitation (they didn't attempt to operate the system in CW mode but speculate that it should be possible).

    (From: Steve Roberts (

    "Various IR lines will lase in pure neon, and even the 632.8 nm line will lase, but it takes a different pressure and a much longer tube. 632.8 nm also shows up with neon-argon, neon-oxygen, and other mixtures. Just about everything on the periodic table will lase, given the right excitation. See "The CRC Handbook of Lasers" or one of the many compendiums of lasing lines available in larger libraries. These are usually 4 volume sets of books the size of a big phone book just full of every published journal article on lasing action observed. It's a shame that out of these many thousands and thousands of lasing lines, only 7 different types of lasers are under mainstream use.

    There are many possible transitions in neon from the excited state to a lower energy state that can result in laser action. The most important (from our perspective) are listed below:

          (1)         (2)           (3)           (4)          (5)         (6)
         Output       HeNe       Perceived       Lasing      Typical     Maximum
       Wavelength  Laser Name    Beam Color    Transition   Gain (%/m)  Power (mW)
         543.5 nm    Green         Green        3s2->2p10   0.52   0.59    2 (5)  
         594.1 nm    Yellow    Orange-Yellow    3s2->2p8    0.5    0.67    7 (10)
         604.6 nm                  Orange       3s2->2p7    0.6    1.0     3
         611.9 nm    Orange      Red-Orange     3s2->2p6    1.7    2.0     7
         629.4 nm                Orange-Red     3s2->2p5    1.9    2.0
         632.8 nm     Red          "    "       3s2->2p4   10.0   10.0    75 (200)
         635.2 nm                  "    "       3s2->2p3    1.0    1.25
         640.1 nm                   Red         3s2->2p2    4.3    2.0     2
         730.5 nm             Border Infra-Red  3s2->2p1    1.2    1.25    0.3
       1,152.3 nm   Near-IR      Invisible      2s2->2p4    ???            1.5
       1,523.1 nm   Near-IR        "   "        2s2->2p1    ???            0.5
       3,391.3 nm    Mid-IR        "   "        3s2->3p4    ???  440.0    24


    1. Output Wavelength is approximate. In addition to slight variations due to actual lasing conditions (single mode, multimode, doppler broadening, etc.), some references don't even agree on some of these values to the 4 or 5 significant digits shown.

    2. HeNe Laser Name is what would be likely to be found in a catalog or spec sheet. All those that have an entry in this column are readily available commercially.

    3. Perceived Beam Color is how it would appear when spread out and projected onto a white screen. Of course, depending on the revision level of your eyeballs, this may vary someone from individual to individual. :)

    4. Lasing Transition uses the so-called "Paschen Notation" and indicates the electron shells of the neon atom energy states between which the stimulated emission takes place.

    5. Typical Gain (%/m) shows the percent increase in light intensity due to stimulated emission at this wavelength inside the laser tube's bore. This is the single pass gain and will be affected by tube construction, gas fill ratio and pressure, discharge current, and other factors. The first column is from various sources. The second column is from Hecht, "The Laser Guide Boook".

      Gain at 1,523 nm may be similar to that of 543.5 nm - about 0.5%/m. Gain at 3,391 nm is by far the highest of any - possibly more than 100%/m. I know of one particular HeNe laser operating at this wavelength that used an OC with a reflectivity of only 60% with a bore less than 0.4 m long.

    6. Maximum Power shows the highest output power lasers commercially available for each wavelength. The first number is rated power while the number in () is achieved output power for a particularly lively tube.

    See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.

    The most common and least expensive HeNe laser by far is the one called 'red' at 632.8 nm. However, all the others with named 'colors' are readily available with green probably being second in popularity due to its increased visibility near the peak of the of the human eye's response curve (555 nm). And, with some HeNe lasers with insufficiently narrow-band mirrors, you may see 640 nm red as a weak output along with the normal 632.8 nm red because of its relatively high gain. There are even tunable HeNe lasers capable of outputting any one of up to 5 or more wavelengths by turning a knob. While we normally don't think of a HeNe laser as producing an infra-red (and invisible) beam, the IR spectral lines are quite strong - in some cases more so than the visible lines - and HeNe lasers at all of these wavelengths (and others) are commercially available.

    The first gas laser developed in the early 1960s was an HeNe laser operated at 1,152.3 nm. In fact, the IR line at 3,391.3 is so strong that a HeNe laser operating in 'superradiant' mode - without mirrors - can be built for this wavelength and commercial 3,391.3 nm HeNe lasers may use an output mirror with a reflectivity of less than 50 percent. Contrast this to the most common 632.8 nm (red) HeNe laser which requires very high reflectivity mirrors (often over 99 percent) and extreme care to mimize losses or it won't function at all.

    When the HeNe gas mixture is excited, all possible transitions occur at a steady rate due to spontaneous emission. However, most of the photons are emitted with a random direction and phase, and only light at one of these wavelengths is usually desired in the laser beam. At this point, we have basically the glow of a neon sign with some helium mixed in!

    To turn spontaneous emission into the stimulated emission of a laser, a way of selectively amplifying one of these wavelengths is needed and providing feedback so that a sustained oscillation can be maintained. This may be accomplished by locating the discharge between a pair of mirrors forming what is known as a Fabry-Perot resonator or cavity. One mirror is totally reflective and the other is partially reflective to allow the beam to escape.

    The mirrors may be perfectly flat (planar) or one or both may be spherical with a typical radius (r = 2 * focal length) equal to the length of the cavity (L). The latter is a configuration called 'confocal'. Curved mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture and are not as efficient since less of the lasing medium volume is used (think of the shape of the beam inside the bore). The confocal arrangement represents a good compromise between a true spherical cavity (r = 1/2 * L) which is easiest to align but least efficient and one with plane parallel mirrors (f = infinity) which is most difficult to align but uses the maximum volume of the lasing medium. Based on my experience with commercial HeNe tubes, short ones (less than 8 inches in total length) seem to use planar mirrors while longer ones will tend to have at least one curved mirror. This makes sense since with a short bore, every fraction of a percent of gain is needed (implying the desire to use the maximum volume of the lasing medium) and aligning short resonators is going to be easier anyhow. See the section: Common Laser Resonator Configurations.

    These mirrors are normally made to have peak reflectivity at the desired laser wavelength. When a spontaneously emitted photon resulting from the transition corresponding to this peak happens to be emitted in a direction nearly parallel to the long axis of the tube, it stimulates additional transitions in excited atoms. These atoms then emit photons at the same wavelength and with the same direction and phase. The photons bounce back and forth in the resonant cavity stimulating additional photon emission. Each pass through the discharge results in amplification - gain - of the light. If the gain due to stimulated emission exceeds the losses due to imperfect mirrors and other factors, the intensity builds up and a coherent beam of laser light emerges via the partially reflecting mirror at one end. With the proper discharge power, the excitation and emission exactly balance and a maximum strength continuous stable output beam is produced.

    Spontaneously emitted photons that are not parallel to the axis of the tube will miss the mirrors entirely or will result in stimulated photons that are reflected only a couple of times before they are lost out the sides of the tube. Those that occur at the wrong wavelength will be reflected poorly if at all by the mirrors and any light at these wavelengths will die out as well.

    Summary of the HeNe Lasing Process

    The HeNe laser is a 4 level laser (see the table above for the specific energy level transitions for the common wavelengths):

    For most common IR wavelengths, level 4 is the 2s state and level 3 are various 2p states. However, the very strong 3.93 um line originates from the 3s state just like the visible wavelengths - and is the reason it competes with them in long HeNe tubes and must be suppressed to optimize visible output.

    The 's' states of neon have about 10 times the lifetime of the 'p' states and thus support the population inversion since a neon atom can hang around in the 2s state long enough for stimulated emission to take place. However, the limiting effect is the decay back to level 1, the ground state, since the 1s state also has a long lifetime. Thus, one wants a narrow bore to facilitate collisions with its walls. But this results in increased losses. Modern HeNe lasers operate at a compromise among several contradictory requirements which is one reason that their maximum output power is relatively low.

    Longitudinal Modes of Operation

    The physical dimensions of the Fabry-Perot resonator impose some additional constraints on the resulting beam characteristics.

    While it is commonly believed that the 632.8 nm (for example) transition is a sharp peak, it is actually a gaussian - bell shaped - curve. In order for the cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:

                       L * 2                 c * n 
                 W = ---------    or   F = --------- 
                         n                   L * 2

    The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal mode spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.

    Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one.

    For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,123, 948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabrey-Perot resonator and the reflectivity curve of the mirrors may look something like the following:

                    |                  632.8 nm
                   I|                     .
                    |                  |  |  |
                    |               |  |  |  |  | 
                    |            |  |  |  |  |  |  |  
               n=948,125  -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5

    Since the mode locations are determined by the physical spacing of the mirrors, as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity of the mirrors as a function of wavelength is quite broad (for all practical purposes, a constant), new lines will fill in from above and the overall shape of the function doesn't change.

    However, for very short HeNe tubes, the gain curve may be narrower than the spacing between modes. The effect is even more likely with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will actually turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion.

    Now for some actual numbers: The Doppler broadened gain curve for the neon in a HeNe laser has a half-width (the gain is at least half the peak value) on the order of 1,500 MHz. So, for a 500 mm long (high gain) tube with its mode spacing of about 300 MHz (similar to what is depicted above), 5 or 6 lines may be active simultaneously and oscillation will always be sustained (though there would be some variation in output power as various modes sweep by and compete for attention). However, for a little 10 cm tube, the mode spacing is about 1,500 MHz. If this laser were to be really unlucky (i.e., the distance between mirrors was exactly wrong) the cavity resonance might not fall in a portion of the gain curve with enough gain to even lase at all! Or, as the tube heats up and expands, the laser would go on and off. It is possible to widen the gain curve somewhat by using a mixture of neon isotopes (Ne20 and Ne22) rather than a single one since the location of their peak gain differ slightly. This would allow a smaller cavity to lase reliably and/or reduce amplitude variations from mode sweeping in all size HeNe lasers.

    A high speed photodiode and oscilloscope or spectrum analyzer can be used to view the frequencies associated with the longitudinal modes of a HeNe laser. The clearest demonstration would be using a short tube where exactly two longitudinal modes are active. This will result in a single difference frequency. A polarized tube is best as it forces both modes to have the same polarization (a photodiode will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this, though the polarization may drift with a randomly polarized laser.

    Passive stabilization (using a structure made of a combination of materials with a very low or net zero coefficient of thermal expansion or a temperature regulator) or active stabilization (using optical feedback and piezo or magnetic actuators to move the mirrors, or a heating element to control the length of the entire structure) can compensate for these effects. An internal etalon will also likely be part of such a system to select a single mode (frequency). However, the added expense is only justified for high performance lab quality lasers or industrial applications like interferometric based precision measurement systems - you won't find these enhancements on the common cheap HeNe tubes found in barcode scanners (which are long enough to not be affected in any case unless possibly if they are old and barely alive)! See the section: Frequency Stabilized Single Mode HeNe Lasers.

    Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which are the same in both resonators will produce enough gain to sustain laser output.

    The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale):

                    |                  632.8 nm
                   I|      .              .              .
                    |      |              |              |
                    |      |              |              |
                    |      |              |              |
               m=13,542   -1             +0             +1

    Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon's index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will only coincide with weak peaks in the main gain function shown above so that their combined amplitude (product) is insufficient to contribute to laser output.

    (From: Prof Harvey Rutt (

    The standard, small HeNe laser normally lases on only one transition, the well known red line at about 632.8 nm.

    The HeNe gain curve is inhomogeneously Doppler broadened with a line width of around 1.5 GHz. For a typical laser, say 30 cm long, the axial modes are separated by about 500 MHz. Typically, two or three axial modes are above threshold, in fact as the laser length drifts you typically get two modes (placed symmetrically about line centre) or three modes (one near centre, one either side) cyclically, and a slow periodic power drift results. Shorter lasers, less modes, more power variation unless stabilized. But it needs a huge HeNe laser to get ten modes, and since they are closer of course they still only spread over the 1.5 GHz line width.

    Most HeNe lasers which do not contain a Brewster window or internal Brewster plate are randomly polarized; adjacent modes tend to be of alternating orthogonal polarizations. (Note that this is not always the case and can be overridden with a transverse magnetic field, see below. See the section: . --- Sam.)

    Some frequency stabilized HeNe lasers are NOT single mode, but have two, and the stabilization acts to keep them symmetrical about line centre - i.e., both are half a mode spacing off line centre. A polariser will then split off one of them or a polarizing beam splitter will separate the two.

    (From: Sam.)

    The party line is that adjacent modes in a HeNe laser will be of orthogonal polarization. However, I've seen samples of small (e.g., 5 or 6 inch) random polarized tubes only supporting 2 active modes where this is not the case - they output a polarized beam that remains stable with warmup and in any case, applying a strong transverse magnetic field will override the natural polarization. So, it's not a strong effect. Only if everything inside the tube is precisely symmetric, will the modes alternate. Modes may also remain one polarization as they move through part of the gain curve and then abruptly - and repeatably - flip polarization.

    Longitudinal Mode Pulling

    It turns out that most lasers don't actually oscillate on exact multiples of the cavity resonance frequency, c/2L, as stated in introductory textbooks. (The exceptions would be where the gain curve is essentially flat but that's another story.) Longitudinal modes that aren't exactly centered on the gain curve will be at frequencies very slightly offset from these, pulled toward the center of the gain curve with those that are farthest away seeing the most shift. This is a well known effect called "mode pulling" with highly developed theory to back it up. (Mode pulling isn't unique to lasers. For example, a quartz crystal oscillator can be tuned over a small range using an external capacitor even though its resonance frequency differs from the output frequency.)

    When the laser beam hits a high speed photodetector like a photodiode, which is a non-linear (square law) device, in addition to the DC power term, there are the primary difference frequencies which are close to multiples of c/2L (but not exactly due to mode pulling), but also the differences of the difference frequencies - the second order intermodulation products - which will be at (relatively) low frequencies compared to c/2L. As the cavity length changes and the lasing modes drift across the gain curve, the mode pulling effect on each one varies slightly. But, small differences between large numbers can result in dramatic changes in these second order terms, rapidly rising and falling in frequency, and coming and going as modes drop off one end of the gain curve and appear at the other. The amplitude of the second order beat will be much lower than that of the primary beat but is still detectable with a spectrum analyzer, or in some cases with an audio amplifier.

    For a HeNe laser, the range of second order frequencies is typically in the 1 to 100 kHz range while for a solid state laser it will be in the MHz to 10s or 100s of MHz range. Note that there will generally not be any beat in the range from 0 Hz and some minimum frequency (e.g., 1 kHz or so in the case of the HeNe laser) as would be expected where the modes are almost symmetric on either side of the gain curve so there would be very low second order frequencies. Apparently, a self mode-locking effect occurs to force these to be exactly zero frequency over a small range of mode positions.

    For the effect to be present, the laser has to be able to oscillate on at least 3 longitudinal modes simultaneously. (With only 2 modes, there will be only a single difference frequency.) The doppler broadened gain curve of neon for the HeNe laser is about 1.5 GHz Full Width Half Maximum (FWHM). To get 3 modes requires the modes to be less than about 500 MHz apart implying a c/2L tube length of about 30 cm or more - typical of a 5 mW or more (rated) HeNe laser. It should be polarized to force all modes to be of the same polarization - orthogonal polarizations do not mix in a photodetector. For a randomly polarized laser which typically produces alternating polarizations for adjacent modes, a longer tube length would be required to guarantee enough same-polarized modes and/or a polarizer at 45 degrees to the beam polarizations could be added.

    This effect can be demonstrated using a medium length HeNe laser, high speed photodiode, and audio amplifier. Initially when the laser is turned on and is heating up and expanding the fastest, they may sound like clicks or pops or just non-random noise. As the expansion slows down, more distinct chirps and other interesting sounds will appear. The complexity of the symphony will also depend on the tube length and thus how many modes are oscillating.

    (From: Roithner Lasertechnik (

    You can "listen" to a single mode HeNe tube: Take an X-rated photodiode and an AC power amplifier - guide a small part of the HeNe laser beam to the photodiode (don't let it saturate!) - and listen to the "chirping oscillations" during warming up with a speaker. Hint: There are no birds inside the tube. ;-) Still - it sounds similar...! Looks like sin x/x...

    More on Resonator Length and Mode Hopping

    Here are some additional comments that address the common fear of the novice laser enthusiast that the resonator length has to be stabilized to the nm or else the laser will blink off.

    (Portions from: Steve Roberts (

    Flames expected, as I'm ignoring some of the physics and am trying to explain some of this based on what I observe, aligning and adjusting cavities on HeNe and argon ion lasers as part of repairing them. Anyone who only goes by the textbooks has missed out on the fun, obviously having never had to work on an external mirror resonator. It can be quite a education!

    Due to the complex number of possible paths down the typical gain medium, you will see lasing as long as the mirrors are reasonably aligned. The cavity spacing is not always that critical and will change anyway as the mirror mounts are adjusted (there will always be some unavoidable translation even if only the angle is supposed to be changed). No, lasers don't really flash on and off in interferometric nulls as you translate the mirrors - they instead change lasing modes. They will find another workable path. You will in some cases see this as a change in intensity but it is more properly observed on a optical spectrum analyzer as a change in mode beating. Eventually you can translate them far apart enough that lasing ceases, but this is a function of your optics not the resonator expansion.

    I have seen what you fear in some cases by adding a third mirror to a two mirror cavity with a low gain medium such as HeNe where the third mirror can be positioned in such a way to kill many possible modes. This usually occurs when I use a HeNe laser to align an argon laser's mirrors and the HeNe laser will flicker from back reflections. See the section: External Mirror Laser Cleaning and Alignment Techniques. But unless you have a extremely unstable resonator design, translation will just cause mode hopping, this becomes important on a frequency stabilized or mode locked laser if you have a precision lab application. Otherwise, most commercial lasers are not length stabilized in the least. There are equations and techniques for determining if you have a stable optical design - stable in this case meaning it will support lasing over a broad range of transverse and longitudinal modes. For examples see any text by A. E. Siegmund or Koechner. If your library doesn't have any similar texts, find a book on microwave waveguides. It might aid you in visualizing what is going on.

    Either an intracavity etalon or active stabilization systems are usually used on single frequency systems anyways, by either translating the mirror on piezos or by pulling on mirror supports with small electromagnets, or in the case of smaller units, heaters to change the cavity length on internal mirror tubes. An etalon is basically a precision flat glass plate in the lasing path between the mirrors, its length is changed by a oven and it acts as a mode filter.

    Length stabilization to the 50 or 100 nm you might have expected to be needed would be gross overkill anyhow, and would be impossible to achieve in practice by stablizing the resonator alone. Depending on the end use of the product, most lasers are simply built with a low expansion resonator of graphite composite or Invar, although in many products a simple aluminum block or L shape is used, a few rare cases use rods made of two different materials designed to compensate by one short high expansion rod moving the mirror mount in opposition to the main expansion. A small fraction of a millimeter is a more reasonable specification.

    (From: Prof Harvey Rutt (

    The basic idea, that the laser can only work at the frequencies where an integral number of half waves fit in the cavity, is perfectly correct. The separation between adjacent modes is just 1/(2*L) where L is the cavity length in cm. From this we get the separation in 'wavenumbers'. One wavenumber is 30 GHz, so in more usual units it is just 30 GHz/(2*L). Or, to make it easy, in a 50 cm long laser the modes are 300 MHz apart. That is not very far optically.

    The laser operates by some molecule, gas, ion in a crystal, etc. making a transition between two levels. But those levels are not perfectly 'sharp'; we say they are 'broadened'. The reason can be many things:

    In any case no transition is *perfectly* sharp, the fact that it has a finite lifetime gives it a certain width, but this is not often the real limit, something else is usually more important.

    These broadening mechanisms 'blur out' the line - we see optical gain over that *range* of frequencies, the gain bandwidth.

    An example is carbon dioxide. The 'natural width' is very small, of order Hz. The Doppler width at 300 °K is about 70 MHz. The collision broadened width increases about 7 MHz/Torr; so well below 10 Torr the width is Doppler limited, ~70 MHz; above 10 Torr pressure broadened (e.g. ~700 MHz at 100 Torr).

    If I take a typical HeNe laser it might 'blur' out over a GHz or so - **more** than that 300 MHz mode spacing - so there are *always* two or thee modes within the 'gain bandwidth' and it will always lase. For a glass laser there might be *thousands* of modes, because the glass gain is very wide indeed.

    But there *are* cases that go the other way. For carbon dioxide, at low pressure, the line is Doppler broadened and about 70 MHz wide, much **LESS** than that 300 MHz mode spacing. So short carbon dioxide lasers really do turn on and off as the cavity length changes, and you have to 'tune' the cavity length to get a mode inside the gain width. This mainly happens with short, gas lasers in the infrared.

    For a *high pressure* CO2 laser at 760 Torr (1 atm), the line width is several GHz, much more than the mode spacing, so the effect disappears.

    Transverse Modes of Operation

    Lasers can also operate in various transverse modes. Laser specifications will usually refer to the TEM00 mode. This means "Transverse Electromagnetic Mode 0,0" and results in a single beam. The long narrow bore of a typical HeNe laser forces this mode of oscillation. With a wide bore multiple sub-beams can emerge from the same cavity in two dimensions. The TEM mode numbers (TEMxy) denote the number (minus one) or configuration of the sub-beams.

    Here is a rough idea of what transverse modes might look like for a rectangular cavity:

                            O        OO        OOO      Each 'O' represents
         O        OO        O        OO        OOO       a single sub-beam.
       TEM00     TEM10    TEM01    TEM11      TEM21

    I have only shown the rectangular case because that's the only one I could draw in ASCII!

    Other (non-cartesian) patterns of modes will be produced depending on bore configuration, dimensions, and operating conditions. These may have TEMxy coordinates in cylindrical space (radial/angular), or a mixture of rectangular and cylindrical modes, or something else!

    To achieve high power from a HeNe laser, the tube may be designed with a wider but shorter bore which results in transverse multimode output. Since these tubes can be smaller for a given output power, they may also be somewhat less expensive than a similar power TEM00 type. As a source of bright light - for laser shows, for example - such a laser may be acceptable. However, the lower beam quality makes them unsuitable for holography or most serious optical experimentation or research. An example of a high power multimode HeNe laser head is the Melles Griot 05-LHR-831 which has a rated output power of 25 mW. Compared to their 05-LHR-827 which is a 25 mW TEM00 laser head, the multimode laser is about 2/3rds of the length and runs on about 3/5ths of the operating voltage at lower current.

    Sometimes, laser companies don't quite get it right either and a laser tube that is supposed to be TEM00 may actually be multi-transverse mode all the time or whenever it feels like it (e.g., after warmup). I have a 13.5 mW Aerotech tube that is supposed to be TEM00 but produces a beam that has an outer torus (doughnut shape) with a bright spot in the middle. This is probably due to one or both mirrors having a radius of curvature that is too short for the bore diameter and was probably a manufacturing reject. Everyone can have a bad day, even if it results in a bunch of dud lasers. :)

    Note that the mode structure implies nothing about the polarization of the beam. Single mode (TEM00) and multimode lasers can be either linearly polarized or randomly polarized depending on the design and for the multimode case, each sub-mode can have its own polarization characteristics. HeNe (and other) lasers will be linearly polarized if there is a Brewster window or Brewster plate inside the cavity. The majority of HeNe laser tubes produce a TEM00 beam which has random polarization. For internal mirror tubes, linear polarization may be an extra cost option. External mirror HeNe lasers also generally produce a TEM00 beam but are linearly polarized since the ends of the tube are terminated with Brewster windows.

    A photodiode and oscilloscope or spectrum analyzer can be used to view the frequencies associated with transverse modes. The transverse difference frequencies are very low compared to the longitudinal mode spacing so a really high speed photodiode isn't needed. A response of a few MHz should be sufficient. Typically less than 2 mm square silicon photodiode will have an adequate frequency response. But the modes do have to overlap on the detector so it may be necessary to spread the beam of a multimode HeNe laser using a lens. A polarized tube is best as it forces the modes to have the same polarization (a photodiode will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this, though the polarization may drift with a randomly polarized laser.

    Multi-Transverse Mode HeNe Lasers

    As noted, most HeNe lasers are designed to operate with a single transverse (spatial) mode or TEM00. However, to obtain the highest power for a given tube size or by a goof-up in design, a higher order mode structure may be produced. A non-TEM00 mode may be present if:

    All of these are really somewhat equivalent and simply mean that more than one mode fits inside the mode volume.

    Note that a speck of dirt or dust on the inside of a mirror or window (if present), or damage to an optical surface, can result in a multi-transverse mode beam even if the bore and mirror parameters are correct for TEM00 operation. Unfortunately, convincing a bit of dust to move out of the way isn't always easy on the inside of an internal mirror HeNe laser tube! Yes, though not common, it can happen. This is one reason not to store tubes vertically.

    Coherence Length of HeNe Lasers

    Common HeNe lasers have a coherence length of around 10 to 30 cm. By adding an etalon inside the cavity to suppress all but one longitudinal mode, coherences lengths of 100s of meters are possible. Naturally, such HeNe lasers are much more expensive and are more likely to be found in optics research labs - not mass produced applications.

    The following actually applies to all lasers using Fabry-Perot cavities operating with multiple longitudinal modes. It was in response to the question: "Why does the coherence length of a HeNe laser tend to be about the same as the tube length?"

    (From: Mattias Pierrou).

    In a HeNe laser you typically have only a few (but more than one) longitudinal modes. These cavity modes must fulfill the standing-wave criterion which states that must be an integer number of half wavelengths between the mirrors. In the frequency domain this means that the 'distance' between two modes is delta nu = c/(2L), where L is the length of the laser.

    The beat frequency between the modes gives rise to a periodic variation in the temporal coherence with period 2L/c, i.e. full coherence is obtained between two beams with a path-difference of an n*2L (n integer).

    If you have only one frequency, the coherence length is infinite (that is, if you neglect the spectral width of this mode which otherwise limit the coherence length). If you have two modes, the coherence varies harmonically (like a sinus curve).

    The more modes you have in the laser, the shorter is the regions (path-length differences) of good coherence, but the period is still the same.

    You can try this by setting up a Michelson interferometer and start with equal arm-lengths which of course gives good coherence. Then increase the length of one arm until the visibility of the fringes disappear. This should occur for a path-difference slightly less than 2L (remember that the path-difference is twice the arm-length difference!). If there are only two modes is the laser the zero visibility of fringes should occur at exactly 2L. Now continue to increase the path-difference until you reach 4L (arm-length difference of 2L). You should again see the fringes clearly due to the restored coherence between the beams.

    What is Mode Locking?

    The normal output of a HeNe or other CW laser is a more or less constant intensity beam. Although there may be long term variations in output power as well as short term optical noise and ripple from the power supply, these are small compated to the average intentsity. Mode locking is a technique which converts this CW beam to a periodic series of very short pulses with a length anywhere from picoseconds to a fraction of a nanosecond. The separation of the pulses is equal to the time required for light to make one round trip around the laser cavity and the pulse repetition rate (PRF) will then be: c/(2*l). For example, a laser resonator with a distance of 30 cm (1 foot) between mirrors, would have a mode locked PRF of about 500 MHz.

    Mode locking is implemented by mounting one of the mirrors of the laser cavity on a piezo-electric or magnetic driver controlled by a feedback loop which phase locks it with respect to the optically sensed output beam.

    Without mode locking, all the modes oscillate independently of one another with random phases. However, with the mode locked laser, all the cavity modes are forced to be in phase at one point within the cavity. The constructive interference at this point produces a short duration, high power pulse. Destructive interference produces a power of almost zero at all other points within the cavity. The mode locked pulse then bounces between the two laser mirrors, and a portion passes through the output coupler on each pass.

    As a practical matter, you probably won't run into a mode locked HeNe laser at a garage sale!

    HeNe Laser Output Power Variation with Warmup

    While not generally visible by eye alone except possibly for very short or tired (low gain) HeNe lasers, there is a quasi-periodic variation of output power with time. For the typical HeNe laser tube Shortly after turn-on, the frequency is quite rapid (a cycle every few seconds) and gradually slows down as the tube temperature reaches a steady state value (after a half hour or more).

    Thanks to Ryan Haanappel, here is a plot of the measured output power of a typical HeNe laser tube from power-on to 20 minutes: Typical HeNe Laser Output Power Versus Time During Warmup. More plots and photos can be found on Ryan's HeNe Lasers Experience Page.

    Examining the actual plot of output power versus time (or careful observation of a laser power meter reading) reveals that the curve is not simple but includes several types of behavior:

    Intensity Stability of HeNe Lasers

    There are at least three kinds of intensity variations present with HeNe (or other gas) lasers: long term as various longitudinal modes compete for attention, short term due power supply ripple or discharge instability, and beat frequencies between modes that are active.

    Common internal mirror HeNe laser tubes include a specification called "Mode Cycling Percent" or something similar. This relates to the amount of intensity variation resulting from changes in longitudinal modes due to thermal expansion. Typical values range from 20 percent for a small (e.g., 6 inch, 1 mW) tube to 2 percent or less for a long (e.g., 15 inch, 10 mW) tube. These take place over the course of a few seconds or minutes and are very obvious using any sort of laser power meter or optical sensor. Even the unaided eyeball may detect a 20 percent change. The more modes that can be active simulataneously, the closer those that are active can be to the same output power on the gain curve. Very short tubes or those with low gain (other wavelengths than 632.8 nm or due to age/use or poor design) may vary widely in output intensity or even cycle on and off due to mode cycling. (Note that since the polarization for each mode may be different, reflecting the beam of one of these HeNe lasers from a non-metallic reflective surface (which acts somewhat as a polarizaer) can result in a large variation in brightness as the dominant polarization changes orientation over time.) Trading off between tube size and mode cycling intensity variations is one reason that HeNe tubes with otherwise similar power output and beam characteristics come in various lengths.

    There are also stabilized HeNe lasers which use optical feedback to maintain the output intensity with a less than 1 percent variation. (They usually also have a frequency stabilized mode but can't do both at the same time.) An alternative to doing it in the laser is to have an external AO modulator or other type of variable attenuator in a feedback loop monitoring optical output power. See the next section for more info.

    Short term changes in intensity may result from power supply ripple and would thus be at the frequency related to the power line or inverter. These can be minimized with careful power supply design.

    Intensity variations at 100s of MHz or GHz rates result from beats between the various longitudinal modes that may be simultaneously active in the cavity. For most common applications, these can be ignored since they will be removed by typical sensor systems unless designed specifically to respond to these high beat frequencies.

    Also see the section: Amplitude Noise.

    Frequency Stabilized Single Mode HeNe Lasers

    The common HeNe laser, while highly monochromatic, does not produce just a single frequency (or equivalently, wavelength) of light. As noted in the section: Longitudinal Modes of Operation, several closely spaced frequencies will be active at the same time and their precise values and intensities will change over time. For many applications, this doesn't matter. However, for others, it makes such a laser useless.

    If you have, say, $5,000 to spend on a laser, you can buy something that actually produces a single frequency with specifications guaranteed stable for days and that don't change over a wide temperature range. While the operation of such a HeNe laser is basically the same as the one in a barcode scanner, several additional enhancements are needed to eliminate mode hopping and select a single output frequency. Some of these include:

    Optical feedback may then be used to maintain constant frequency or constant intensity by using the sensed output beam to drive the temperature regulator or piezo transducer. For example, the Melles Griot 05-STP series laser cavity permits a pair of orthogonal polarized longitudinal modes to be active and can provide very precise control by straddling these on the steep slopes of the gain curve (frequency stabilized mode) or positioning one on the flat portion of the gain curve (intensity stabilized mode). For some photos of the (quite simple) stabilized HeNe tube used in the Hewlett-Packard 5517 laser head, see the Laser Equipment Gallery (Version 1.86 or higher) under "Assorted Helium-Neon Lasers".

    It isn't really possible to convert an inexpensive HeNe tube that operates on several longitudinal modes into a frequency stabilized laser. Adding temperature control could reduce the tendency for mode hopping or polarization changes and the addition of powerful magnets can force a polarized beam and probably stabilize the discharge. But, selecting out a single longitudinal mode would be difficult without access to the inside of the tube. However, if the HeNe tube is short enough that the mode spacing exceeds about 1/2 the doppler broadened gain bandwidth for neon (about 1.5 GHz), it will oscillate on at most 2 longitudinal modes at any given time and these will each be linearly polarized and orthogonal to each-other. Then, stabilization is possible. See the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser for details.

    It may be possible with a combination of what can be done externally, as well as control of discharge current, to force a situation where gain is adequate for only 1 or 2 lines. Whether this could ever be a reliable long term approach for a HeNe tube that normally oscillates in many longitudinal modes is questionable but the experiments could be quite interesting. However, this may work for very short tubes which may only have 1 or 2 active modes to begin with - or with old and weak ones which now just barely lase in a single mode!

    What I don't think will have much success are optical approaches such as feeding light back in through the output mirror. Doing this would likely have the exact opposite of the desired effect but may work in special cases (its called injection locking).

    Coherent, Melles Griot, Spectra-Physics, and others will sell you a small stand-alone stabilized HeNe laser for $5,000 or so and Agilant (HP) and others have interferometers and other similar equipment which includes this type of laser. Those that I've seen use short HeNe tubes with feedback thermal control of the resonator length and all operate at the red HeNe wavelength (632.8xxxxxx nm to 8 or more significant figures). One typical system is described in the section: Coherent Model 200 Single Frequency HeNe Laser. However, a stabilized HeNe laser for green or other color visible HeNe wavelength or one of the IR wavelengths is also possible using the same principles.

    It used to be that the stabilized HeNe laser was the "Gold Standard". But nowadays, other technologies like diode pumped solid state lasers inherently have orders of magnitude narrower line width.

    On-line Introductions to HeNe Lasers

    There are a number of Web sites with laser information and tutorials.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    HeNe Laser Tubes, Heads, Structure, Power Requirements, Lifetime

    Early Versus Modern HeNe Lasers

    In the first HeNe lasers (see the diagram below), exciting the gas atoms to the higher energy level was accomplished by coupling a radio frequency (RF) source (i.e., a radio transmitter) to the tube via external electrodes. Modern HeNe lasers almost always operate on a DC discharge via internal electrodes.

          Bellows                                                Bellows
          /\/\/\      Discharge tube with external electrodes     /\/\/\
         ||     \________________________________________________/     ||
         ||             | |                | |              | |        ||===> Laser
         ||      ___  __|_|________________|_|______________|_|__      ||===> Beam
         ||     /   ||   |                  |                |   \     ||
          \/\/\/    ||   |                  o                |    \/\/\/
     Adjustable     ||   +-----------o RF exciter o----------+     Adjustable
      totally       ||                                              partially
     reflecting     ||<-- to vacuum system                         reflecting
       mirror                                                        mirror

    Early HeNe lasers were also quite large and unwieldy in comparison to modern devices. A laser such as the one depicted above was over 1 meter in length but could only produce about 1 mW of optical beam power! The associated RF exciter was as large as a microwave oven. With adjustable mirrors and a tendency to lose helium via diffusion under the electrodes, they were a finicky piece of laboratory apparatus with a lifetime measured in hundreds of operating hours.

    In comparison, a modern 1 mW internal mirror HeNe laser tube can be less than 150 mm (6 inches) in total length, may be powered by a solid state inverter the size of half a stick of butter, and will last more than 20,000 hours without any maintenance or a noticeable change in its performance characteristics.

    Structure of Internal Mirror HeNe Lasers

    The following applies to most of the inexpensive internal mirror low to medium power (0.5 to 5 mW) HeNe tubes available on the surplus market. Depending on the original application, the actual laser tube may be enclosed inside a laser head or arrive naked. :-)

    This fabulous ASCII rendition of a typical small HeNe laser tube should make everything perfectly clear. :-)

                   /                         _________________  \
            Anode |\  Helium+neon, 2-5 Torr   Cathode can ^   \  |
            .-.---' \.--------------------------------------.  '-'---.-.     Main
        <---| |::::  :======================================:   :::::| |===> beam
            '-'-+-. /'--------------------------------------'  .-.-+-'-'
     Totally    | |/  Glass capillary ^      _________________/  | |  Partially
     reflecting |  \____________________________________________/  |  reflecting
     mirror     |                                                  |  mirror
                |          Rb          +               -           |
                +---------/\/\---------o 1.2 to 3 kVDC o-----------+

    The main beam may emerge from either end of the tube depending on its design, not necessarily the cathode-end as shown. (For most applications it doesn't matter. However, when mounted in a laser head, it makes sense to put the anode and high voltage at the opposite end from the output aperture both for safety and to minimize the wiring length.) A much lower power beam will likely emerge from the opposite end if it isn't covered - the 'totally reflecting' mirror or 'High Reflector' (HR) doesn't quite have 100 percent reflectivity (though it is close - usually better than 99.9%). Where both mirrors are uncovered, you can tell which end the beam will come from without powering the tube by observing the surfaces of the mirrors - the output-end or 'Output Coupler' (OC) mirror will be Anti-Reflection (AR) coated like a camera or binocular lens. The central portion (at least) of its surface will have a dark coloration (probably blue or violet) and may even appear to vanish unless viewed at an oblique angle.

    For a diagram with a little more artistic merit, see: Typical HeNe Laser Tube Structure and Connections. And, for a diagram of a complete laser head: Typical HeNe Laser Head (Courtesy of Melles Griot). For some photos, see: Typical Small to Medium Size Melles Griot HeNe Laser Tubes. The ratings are guaranteed output power. These tubes may produce much more when new. Another type of construction that is relatively common is shown in the Hughes Style HeNe Laser Tube and a photo in Hughes 3227-HPC HeNe Laser Tube. These are probably disappearing though as Melles Griot bought the Hughes HeNe laser operation and is converting most to their own design but many still show up on the surplus market, including newer ones with the Melles Griot label. Another design that is similar is the NEC Style HeNe Laser Tube. Some specifications for various NEC HeNe lasers can be found at: NEC Lasers (but it's mostly in Japanese!). Most common higher quality HeNe tubes will be basically similar to one of these two designs though details may vary considerably. Most have an outer glass envelope but a few, notably some of those from PMS/REO, may be nearly all metal (probably Kovar but with an aluminum liner which is the actual cathode) with glasswork similar to that of Huches or NEC at the anode-end.

    Tubes up to at least 35 mW are similar in design but proportionally larger, require higher voltage and possibly slightly higher current. and of course, will be more expensive.

    HeNe tubes used in barcode scanners tend to use a simpler (possibly cheaper) design. Some typical examples are the Uniphase 098-1 HeNe Laser Tube and Siemens LGR-7641S HeNe Laser Tube. A typical small barcode scanner tube is shown in Uniphase HeNe Laser Tube with External Lens. That negative lens is used in the barcode application to expand the beam at a faster rate than with the bare tube. A second positive lens about 4 inches away is then used to recollimate the beam. (In many cases, the required curvature is built into the output mirror but not here. The lens was removed by soaking the end of the tube in acetone overnight.)

    CAUTION: While most modern HeNe tubes use the mirror mounts for the high voltage connections, there are exceptions and older tubes may have unusual arrangements where the anode is just a wire fused into the glass and/or the cathode has a terminal separate from the mirror mount at that end of the tube. Miswiring can result in tube damage even if the laser appears to work normally. See the section: Identifying Connections to Unmarked HeNe Tube or Laser Head if in doubt.

    Gas Fill and Getter

    In order for an HeNe laser to operate efficiently (as such things go) or at all, there must be a very precise and pure mixture of helium and neon gas in the tube. The total amount of gas in a typical 1 mW HeNe tube is much less than 1 cubic cm if it were measured at normal atmospheric pressure. It fills the tube only because the pressure is very low. However, with this small amount of gas, it doesn't take much contamination or leakage to ruin the tube.

    Mirrors in Sealed HeNe Tubes

    (See: Typical Small to Medium Size Melles Griot HeNe Laser Tubes for views of the types of mirrors and mirror mounts discussed below.)

    The mirrors used in lasers are a bit more sophisticated than your bathroom variety:

    Mirror Reflectances for Some Typical HeNe Lasers

    Here are some (approximate) typical OC reflectances for red (632.8 nm) HeNe lasers determined by measuring the actual transmission (R = 100 - T) of a red HeNe laser beam through the optic with a simple photodiode based laser power meter:

    The HRs in all cases showed greater than 99.9 percent reflectivity (T less than 0.001 - virtually undetectable on my fabulous meter).

    Due to the behavior of the photodiode at low light levels, the absolute precision of the readings is somewhat questionable. However, the relative reflectivities of these mirrors is probably reasonably accurate. Note, in particular, the high R of 99.4% for the very long external mirror laser compared to the low R of 97.7% (T of 2.3%) for a shorter internal mirror tube. I expect that in addition to the length of the bore, part of this difference is due to the absence of Brewster window losses in the internal mirror tube resulting in a higher gain so that more energy can be extracted via the OC on each pass.

    Mirrors for non-red HeNe lasers must be of even higher quality due to the lower gain on the other spectral lines. The OC will also have higher reflectivity for this reason. For green HeNe tubes (which have the lowest gain of all the visible HeNe wavelengths), the transmission is about 1/10th that of a similar length red tube. For example, the reflectivity of a typical green HeNe tube OC is 99.92 to 99.95 percent (.08 to .05 percent transmission) at 543.5 nm.

    Notes on making these measurements:

    More About HeNe Dielectric Mirrors

    In the mid 1980s, before Ion Beam Sputtered (IBS) coatings really made their commercial debut, some mirrors were still Epoxied (soft-sealed), particularly those with a lot of coating layers (like 20 or 30), mostly green, yellow, and IR HeNe lasers. These tubes need sharp cutoffs (to kill lasing on unwanted wavelengths) and/or ultra high reflectivity (due to their very low gain) in the coatings - which means a lot of layers. The packing density on Electron-Beam (E-Beam) coatings is not great, so water molecules get into all the layers. When you hard-seal the mirror by heating the frit, the water comes out and cracks the coating (called a 'crazed' mirror). Another problem with mega-stack E-Beam coatings is that the transmittance curve can shift as much as 10 nm (to longer wavelengths - the layers get thicker) during the oven cycle (again a water-thing). If you have to, say, highly reflect at 594.1 nm (for a yellow output tube) and highly transmit beyond 604.6 nm (to kill the orange and red), and your coating shifts 10 nm in the oven cycle, another batch of tubes ends up in the dumpster. :( No! Send the my way. :)

    Ion Beam Sputtered (IBS) coatings have a much higher packing density, so they withstand the (i.e., 450 °C) frit sealing temperatures and don't even shift 1 nm. Nowadays, everything is hard sealed, with the exception of the high-end (long precision) Brewster tubes. Hard-sealing a BK-7 window puts a lot of stress on it, and that just isn't acceptable on the high-Q tubes. So, those get fused silica windows optically contacted (lapped and polished surfaces that are vacuum tight.) (In fact, with this type of seal, if there is no adhesive present, the windows can be easily removed from your dead, leaky, or up-to-air tubes by heating the Brewster stem and window with a heat gun. The window can then be popped off with your thumbnail!)

    Random and Linear Polarized HeNe Tubes

    Most common HeNe laser tubes are randomly polarized since for many applications the polarization of the beam doesn't matter. As noted elsewhere, the term "random" here really doesn't mean that the polarization is necessarily jumping around to totally arbitrary orientations. It just means that nothing special is done to control it. Sometimes a randomly polarized tube will actually exhibit a fixed linear polarization but more likely it will have several longitudinal modes (how many will depend on tube length) with pairs that are orthogonal to each-other. Each of the modes will change their relative intensities periodically over time or may even switch polarizations as the tube heats and expands. (Adjacent longitudinal modes are usually orthogonally polarized.) For the special case of a short tube where only two modes fit under the gain curve (typically 5 or 6 inches in length) at the instants when they are equal, the output will appear to be non-polarized (constant intensity as an external polarizer is rotated in the beam) but as the modes shift under the gain curve, one or the other polarization will dominate.

    The main physical effect resulting in a particular polarization direction being favored in a random polarized HeNe tube is a slight preferred axis in the dielectric mirror coatings or slight misalignment of the mirrors. Where this is very small or the mirrors at opposite ends of the tube happen to be oriented so they effects cancel out, the resulting polarization may indeed not be restricted to a fixed pair of orthogonal orientations as the tube heats and parts expand. The polarization may be slowly rotating or flip between arbitrary orientations.

    Most linearly polarized HeNe laser tubes are similar to their randomly polarized cousins but include a Brewster plate or window inside the cavity which results in slightly higher gain for the desired polarization orientation Such tubes produce a highly polarized beam with a typical ratio of 500:1 or more between the selected and orthogonal polarization. External mirror HeNe lasers use Brewster windows and so are inherently linearly polarized. A magnetic field can also be used to force linear polarization and indeed, long before I observed this phenomenon, some commercial HeNe lasers offered a "polarization option" which was a set of magnets to be placed next to the bore. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.

    Linearly polarized HeNe lasers tended to be used in older laser printers (since the external modulator often required a polarized beam) and LaserDisc players (because the servo and data recovery optics required a polarized beam). Randomly polarized lasers were used in older barcode scanners since polarization doesn't matter there. Note the use of "older". Nowadays, this equipment all use diode lasers which are inherently polarized.

    More on Mode Cycling in Short HeNe Lasers

    As noted, a randomly polarized HeNe laser doesn't really produce arbitrary polarization but the individual longitudinal modes may switch polarizations as the tube warms up and expands. Where the distance between the mirrors is small - 5 or 6 inches as is the case with smaller HeNe laser tubes, only two adjacent modes will fit under the inhomogeneously Doppler broadened gain curve of neon. With only two active modes, effects of mode changes may be obvious even without anything more than Mark-I eyeballs and a polarizing filter but fancy equipment may be needed to fully characterize what's going on.

    (Portions from: Lynn Strickland (

    Our testing suggested that adjacent modes always have orthogonal polarization - (lets go with S and P designations). BUT, in some two-mode tubes, a given mode doesn't always REMAIN S or P as it changes in frequency (it flips polarization). In "flippers", certain frequencies only support one polarization. If this frequency range is around the center of the gain curve, most power will be of one polarization regardless of temperature (so it appears to be linearly polarized). (However, the extinction ratio varies over time, and is generally poor).

    Here's a test setup that shows what's going on if you have access to some nice instrumentation: Send the beam from a two mode, randomly polarized HeNe tube (Example: 05-LHR-006) into a scanning Fabry-Perot interferometer (this is mucho more expensive than your basic exorbitantly priced optical spectrum analyzer). (However, you can build a scanning Fabry-Perot interferometer if so inclined. See the sections starting with: Scanning Fabry-Perot Interferometers.) Put a polarizer in the beam path, aligned to maximize P polarization (or S polarization, doesn't matter). Normally, the P mode will remain P polarization at all frequencies under the gain curve. So as the frequency changes (due to cavity length changes with temperature), the P mode will trace out a nice pretty sort of Gaussian curve, the curve width being about 1.6 GHz FWHM. Bottom line, you can get P-polarized light at every frequency under the gain curve.

    In a 'flipper', your curve has missing sections. In other words, there are some frequencies where you cannot get P polarization. When the observed, P mode reaches one of these frequency ranges, it will flip and become S-polarized. When the flip occurs, the other, formerly S mode, turns into a P. If you're just looking at one polarization (as the experiment describes), the observed P mode disappears and pops up again at a frequency delta equal to the longitudinal mode spacing (where the S mode used to be). Some call it mode hop, but it really isn't, because both modes are still there. Both modes still have, and always had, orthogonal polarization - they just swapped. Some tubes flip at one point under the gain curve, some flip many times under the gain curve.

    This has to do with gain asymmetry. What brought it to our attention, is that when the polarizations flip, you get high frequency 'noise' if you have polarization sensitive components in your beam path. Solutions are to specify a laser that doesn't flip, go to a three mode (longer) laser, go to non-polarization sensitive optics all the way through the beam delivery/detection train, or put a bandwidth filter on your detector.

    A magnetic field will sometimes make a flipper stop, and sometimes make a non-flipper start - but not always. Sans magnetic field, over time (several thousand operating hours) our test population suggested that flippers always flip, non-flippers always behave.

    Polarization of Longitudinal Modes in HeNe Lasers

    It is well known that adjacent longitudinal modes in HeNe lasers (at least) tend to be orthogonally polarized. (See the previous section.) This is a weak coupling as a magnetic field, Brewster plate, or even some asymmetry in the cavity can affect it or kill it entirely. And some lasers will cause the polarization to suddenly flip as modes cycle through the gain curve.

    But what is the underlying cause?

    (From: A. E. Siegman (

    The reason that HeNe lasers can run - more accurately, like to run - in multiple axial modes is associated with inhomogeneous line broadening (See section 3.7, pp. 157-175 of my book) and "hole burning" effects (Section 12.2, pp. 462-465 and in more detail in Chapter 30) in the doppler-broadened laser transitions commonly found in gas lasers (though not so strongly in CO2) and not in solid-state lasers.

    The tendency for alternate modes to run in crossed polarizations is a bit more complex and has to do with the fact that most simple gas laser transitions actually have multiple upper and lower levels which are slightly split by small Zeeman splitting effects. Each transition is thus a superposition of several slightly shifted transitions between upper and lower Zeeman levels, with these individual transitions having different polarization selection rules (Section 3.3, pp. 135-142, including a very simple example in Fig. 3.7). All the modes basically share or compete for gain from all the transitions.

    The analytical description of laser action then becomes a bit complex - each axial mode is trying to extract the most gain from all the subtransitions, while doing its best to suppress all the other modes - but the bottom line is that each mode usually comes out best, or suffers the least competition with adjacent modes, if adjacent modes are orthogonally polarized.

    There were a lot of complex papers on these phenomena in the early days of gas lasers; the laser systems studied were commonly referred to as "Zeeman lasers". I have a note that says a paper by D. Lenstra in Phys. Reports, 1980, pp. 289-373 provides a lengthy and detailed report on Zeeman lasers. I didn't attempt to cover this in my book because it gets too complex and lengthy and a bit too esoteric for available space and reader interest. The early (and good) book by Sargent, Scully and Lamb has a chapter on the subject. You're probably aware that Hewlett Packard developed an in-house HeNe laser short enough that it oscillated in just two such orthogonally polarized modes, and used (probably still uses) the two frequencies as the base frequencies for their precision metrology interferometer system for machine tools, aligning airliner and ship frames, and stuff like that.

    (From: Sam.)

    Indeed, HP has several models of two-frequency HeNe lasers but the ones I'm familiar with actually use an external magnet to create Zeeman splitting. Rather than two longitudinal modes, a PZT or heater is used to adjust cavity length so that only a single mode is oscillating, which is split by the Zeeman effect. Then, the difference frequency (in the low MHz range) is used in the measurement system as a reference and possibly for stabilizing the (optical) frequency. See the section: Hewlett-Packard HeNe Lasers.

    The Spectra-Physics model 117A frequency stabilized HeNe laser is designed more like what you are describing - two modes, no magnets. A heater is used to adjust cavity length in a feedback loop using a pair of photodiodes to monitor the two orthogonal polarized modes. However, I would assume that based on its description, the desired operating conditions would be for it to run with a single mode (which it can with carefully controlled cavity length). See the section: Description of the SP-117A Laser. The Coherent and Melles Griot stabilized HeNe lasers are similar.

    Power Requirements for HeNe Lasers

    Power for a HeNe laser is provided by a special high voltage power supply (see the chapter: HeNe Laser Power Supplies and consists of two parts (these maximum values depend on tube size - a typical 1 to 10 mW tube is assumed):

    A few HeNe lasers - usually larger or research types - have used a radio frequency (RF) generator - essentially a radio transmitter to excite the discharge. This was the case with the original HeNe laser but is quite rare today given the design of internal mirror HeNe tubes and the relative simplicity of the required DC power supply.

    Operating Regions of a HeNe Laser Tube

    There are several distinct operating regions for a HeNe plasma discharge as a function of tube current each of which has its own properties. The following summary is partially extracted from the HeNe Laser Manual by Elden Peterson and is mostly just for curiosity sake as there is little reason to run a HeNe laser tube at anything other than close to the nominal current (which results in maximum power output and rated life) listed in the tube specifications except possibly to implement low level modulation for laser communications.

    Note that the visual effect of increasing current from dropout to cessation of output will just be a smooth increase and then decrease in coherent optical output power. To detect the single frequency or broadband noise will require a sensor and oscilloscope with a bandwidth of at least a few MHz.

    HeNe Tube Dimensions, Drive, and Power Output

    A large number of factors interact to determine the design of a modern HeNe laser. Beam/bore diameter, bore length, gas fill pressure, voltage, current, and mirror design, are all critical in determining how much output power will be produced - or whether a given tube will lase at all. Hundreds (at least) of technical papers and entire phone book size volumes filled with equations have no doubt been written on these topics and we can't hope to do anything serious in a few paragraphs, but at least, may be able to give you a feel for some of the relationships among power output, bore dimensions, gas pressure, and drive requirements in particular.

    You have probably wondered why the beam from a typical HeNe laser (without additional optics) is so narrow. Is it that making a tube with larger mirrors would be more costly?

    No, it's not cost. Even high quality and very expensive lab lasers still have narrow bores. The very first HeNe lasers did use something like a 1 cm bore but their efficiency was even more mediocre than modern ones. A wide bore tube would actually be cheaper to manufacture than one requiring a super straight narrow capillary. However, it wouldn't work too well.

    A combination of the current density needed in the bore, optimal gas pressure, gain/unit length in the bore, the bore wall itself aiding in the depopulation of lower energy states, and the desire for a TEM00 (single transverse mode) beam (there are multimode tubes that have slightly wider bores), all interact in the selection of bore diameter.

    In fact, there is a mathematical relationship between bore size, gas pressure, and tube current resulting in maximum power output and long life.

    The optimal pressure at which stimulated emission occurs in a HeNe laser is inversely proportional to bore diameter. According the one source (Scientific American, in their Amateur Scientist article on the home-built HeNe laser - see the chapter: Home-Built Helium-Neon (HeNe) Laser), the pressure in Torr is equal to 3.6 divided by the ID of the bore. I don't know whether this exact number applies to modern internal mirror tubes but it will likely be similar. Power output decreases on either side of the optimal pressure but a laser with a low loss resonator may still produce some output above twice and below half this value.

    Thus, as the bore diameter is increased, the optimal pressure drops. Aside from having fewer atoms to contribute to lasing resulting in a decrease in gain, below a pressure of about .5 to 1 Torr, the electrons can acquire sufficient energy (large mean-free-path?) to cause excessive sputtering at the electrodes. This will bury gas atoms under the sputtered metal (which may also coat the mirrors) leading to a runaway condition of further decreasing pressure, more sputtering, etc. Even with the large gas reservoir of your typical HeNe tube (which IS the main purpose of all that extra volume), there may still be some loss over time. A drop in gas pressure after many hours of operation is one mechanism that results in a reduction in output power and eventual failure of HeNe tubes.

    As a result, the maximum bore diameter you will see in a commercial HeNe laser will likely be about 2 mm ID (for those multimode tubes mentioned above where the objective is higher power in a short tube). Most are in the .5 to 1.2 mm range. This results in high enough pressure to minimize sputtering, maximize life, provide maximum power output, and optimal efficiency (to the extent that this can be discussed with respect to HeNe lasers! Well, ion lasers are even worse in the efficiency department so one shouldn't complain too much. Since total resonator gain is proportional to bore length and approximately inversely proportional to bore diameter (since the optimal pressure increases resulting in a higher density of lasing atoms), this favors tubes with long narrow bores. But these are difficult to construct and maintain in alignment. Wide bore tubes have lower gain but a higher total number of atoms participating with potentially higher power output at the optimal pressure and current density. Everything is a tradeoff!

    However, all this does provide a way of estimating the power output and drive requirements of a HeNe tube or at least comparing tubes based on dimensions. Assuming a tube with a particular bore length (L) is filled to the optimum pressure for its bore diameter (D), power output will be roughly proportional to D * L, discharge voltage will be roughly proportional to L (probably minus a constant to account for the cathode work function), and discharge current will be roughly proportional to D. (Note that D instead of the cross-sectional area is involved because the optimal pressure and thus density of available lasing atoms is inversely proportional to D.)

    So, do the numbers work? Well, sort of. Here are specifications for some selected Melles Griot red HeNe tubes rearranged for this comparison:

       Total    Bore      Bore    --- Ratio of ---  Discharge  Discharge   Output
       Lgth   Lgth (L)  Dia. (D)  L   D   (D * L)    Voltage    Current    Power
       135 mm   80 mm    .46 mm   1   1     1         900 V      3.3 mA     .5 mW
       177 mm  115 mm    .53 mm   1.4 1.15  1.6     1,130 V      4.5 mA    1.0 mW
       255 mm  190 mm    .72 mm   2.4 1.57  3.7     1,360 V      6.5 mA    2.0 mW
       370 mm  300 mm    .80 mm   3.8 1.7   6.4     1,800 V      6.5 mA    5.0 mW
       440 mm  365 mm    .65 mm   4.6 1.4   6.4     2,150 V      6.5 mA     10 mW
       930 mm  855 mm   1.23 mm  11.1 2.7  29.9     4,500 V      8.0 mA   25-35 mW
    (Bore length was estimated since the cathode-end of the capillary is not visible without X-raying the tube!)

    The general relationships seem to hold though large tubes seem to produce higher output power than predicted possibly constant losses represent a smaller overhead. As noted elsewhere there is also a wide variation even for tubes with similar physical dimensions. Oh well...

    There are more examples in the section:Typical HeNe Tube Specifications. You can do the calculations. And, some large IR HeNe lasers may use a somewhat wider bore. See the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications for a comparison of visible and IR HeNe tubes for the same model laser.

    Note that there are some multi-mode (non-TEM00) HeNe tubes with wider bores and a different mirror curvature that produce up to perhaps twice the power output for a given tube length. However, with multiple transverse modes, these are not suitable for many applications like interferometry and holography. They are also not very common compared to single-mode TEM00 HeNe tubes.

    Largest HeNe Laser?

    (From: Chris Leubner (

    The most powerful HeNe laser I have ever seen was 160 mW of real power and was the only time I've ever seen a HeNe laser burn anything before with raw beamage. It would slowly burn electrical tape placed in the beam and felt warm on your skin. It was made of two almost 6 foot long Spectra-Physics model 125 tubes hooked electrically to separate power supplies and optically in series in a custom made double-wide sized 125 head. Sadly, it doesn't work anymore and is currently resting piecefully in the NTC laser department's laser graveyard. :-(

    (From: Steve Roberts (

    I've seen a normal SP-125 break 160 mW on its own. Two tubes at only 160 mW sounds like it was misaligned, not that I'd like to try to align that one! :)

    The current record is for a Chinese researcher using 2 tubes with a flattened elliptical profile in a V fold resonator to get 330+ mW into a fiber. The beam shape and divergence from this are not what you would expect from a typical HeNe laser, even one that runs multi (transverse) mode. Remember that a HeNe laser's power is limited by collisions with the tube wall returning Ne atoms to the ground state, so using a flattened tube means more wall area, hence more power. Optimal gas pressure is a function of bore diameter as well. So you're limited to about a 1 meter tube in most cases by other optics reasons and sputtering. With collisions with the wall increased by a larger wall surface area, what the folks in China did is try tubes with different cross sections. To get enough length they folded the resonator using a 3 optic V-fold. You don't want to see the beam profile. It's nasty! It looks kind of like this: <{[=]}>. And the divergence is high as the optics need to fill that whole lasing volume.

    Please note, however, that going to a large rectangular or star shaped tube is not possible due to some quirks in the plasma at the pressure required for HeNe laser operation. Details are in a 1996 issue of Review of Scientific Instruments. A few years ago, Cornell University attempted to sell the rights to the unit in the United States, on behalf of the Chinese Inventor. U.S. patent and marketing were assigned to a group that sadly dropped the ball. At the time, the picture of the unit looked like one of those old foldaway sewing machines like my mom used to have, an ornamental blue box about the size of a PC Tower turned on its side with 4 wooden legs.

    Boosting the Power Output of a HeNe Laser?

    Unfortunately, given the existing laws of physics, there usually isn't much you can do to increase the output power of a HeNe laser above its specified ratings. Unlike an ion laser where higher tube current usually increases power output (at the expense of tube life), boosting current to a HeNe tube beyond the optimal amount actually *decreases* power output. Options like Q-switching don't exist for HeNe lasers.

    Bare HeNe Tubes and Laser Heads

    What you have may be a 'bare' tube or it may be encased in a cylindrical or rectangular laser head - or something in between:

    If you have a laser head that is missing the Alden connector, replacements should be available from the major laser surplus suppliers or salvage one from another (dead) head. I also have many available. Where the end-cap on a cylindrical laser head is also missing, there are no readily available commercial sources - fabricate one from a block of wood and paint it black or find some other creative solution. A suitable ballast resistance must also be installed between the positive power supply output and the HeNe tube anode.

    The cylindrical head serves another purpose besides structural support and protection. This is the distribution of heat and equalization of thermal gradients. Thus, removing a long HeNe tube in particular from its laser head may result in somewhat random or periodic cycling of power output due to convection and other non-uniform cooling effects.

    Often, particularly inside equipment like barcode scanners, you will see something in between: A HeNe tube wrapped in several layers of thick aluminum foil probably to help distribute and equalize the heating of the tube for the reason cited above. However, I haven't really noticed any obvious difference in stability when this wrap was removed. Spectra-Physics is very fond of this but others may have copied it to sell compatible tubes.

    HeNe Tube Seals and Lifetime

    Neon signs last a long times - years - how about HeNe laser tubes?

    The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings (operating current, proper polarity, and not continuously restarting). Under these conditions, end-of-life occurs when the oxide "pickling" layer of the cathode can gets depleted. Larger diameter (1.5 or 2 inch) tubes last the longest - up to 50,000 hours or more. Small diameter (0.75 or 1 inch) tubes have the shortest lifetime - 10,000 hours or so. Since even 10,000 hours is still very long - over 1 year of continuous operation - HeNe laser lifetime is not a major consideration for most hobbyist applications. Chances are that even a surplus laser will still have thousands of hours of life remaining.

    However, the shelf life of the tube depends on types of sealing method used in the attachment of the optics. There are two types of internal mirror HeNe tubes:

    A very few tubes apparently have frit at one end and a soft-seal at the other so check both ends. This probably applies only to some low gain "other color" HeNe lasers with a mirror that would be affected by even the relatively low temperature at which the frit melts.

    Note that other parts of most tubes (except for Brewster windows, if present) use glass-to-metal seals but since these must be manufactured at high temperature, they are not an option for delicate optics. The very best tubes with one or two Brewster windows do not use frit because even at the low temperature at which it is fired, there may still be some unavoidable stresses introduced - these tubes continued to be soft-sealed even after frit was common but now use optical contacted seals. With optical contacted seals, the two pieces are ground and polished optically flat and brought together under clean room conditions. The resulting seal is gas-tight. Just a bit of Epoxy is used for mechanical stability but it doesn't do the sealing.

    The HeNe gas doesn't 'wear out'. A HeNe tube, when properly connected has a substantial portion of its power dissipated by the bombardment of positive ions at the cathode (the big can electrode) which is made large to spread the effect and keep the temperature down and is "pickled" (coated) to reduce its work function. Hook a tube up backwards and you may damage it in short order and excessive current (operating current as well as initial starting current from some high compliance power supplies) can degrade performance after a while. Electrode material may sputter onto the adjacent mirrors (reducing optical output or preventing lasing entirely) or excessive heat dissipation may damage the electrodes or mirrors directly.

    As the tube is used (many thousands of hours or from abuse), operating and starting voltages may be affected as well - generally increasing with the ultimate result being that a stable discharge cannot be initiated or maintained with the original power supply. See the section: How Can I Tell if My Tube is Good?.

    (From: Lynn Strickland (

    Typical failure mechanism in a HeNe is cathode sputtering -- seldom gas leakage in the newer (like since 1983) tubes. Shelf life is stated to be about 10 years, but it's not uncommon at all to see HeNe lasers built in the early 1980's that still meet full spec.

    Interesting lifetime note - it used to be that you left a HeNe 'on' at all times to prolong life. Since hard-sealing, you should turn it off while not in use. If it's a 20,000 hour tube, and you only turn it on for a few hundred hours a year, it will last a heck of a long time. Not uncommon at all for the HeNe to outlive several power supplies. The larger diameter tubes tend to last longer, but it also depends on fill pressure and operating current (higher fill-pressure tubes last longer). The typical 5 mW red HeNe will commonly live to 40k to 50k operating hours.

    As for cathode sputtering, the tube has an aluminum cathode that is 'pickled' during the production process to add a layer of oxidation about 200 microns thick. The oxidation layer prevents aluminum from being bombarded away from the cathode during plasma discharge. As the tube ages, the oxide layer is depleted until aluminum is exposed. Sputtered aluminum can stick to the mirror, causing power decline, or to the inside of the glass envelope, causing the discharge to arc internally. This arcing, if allowed to continue for a period of time, will also cook the power supply. A tube with no oxidation layer on the cathode will die in about 200 hours of use. OR, once the oxidation layer is depleted, the tube will die in about 200 hours. This is why a HeNe life curve is usually pretty flat, then quickly degrading to nothing over about a 200 hour period.

    An Older HeNe Laser Tube

    The Spectra-Physics Model 084-1 HeNe Laser Tube was popular for applications like barcode scanners. It is rated at 2 to 3 mW when new. While the main glass tube and end-plates use glass-to-metal (hard) seals, the mirrors appear to be Epoxied in place (soft sealed). Thus, one would expect these tubes to leak over time. However, out of 31 that I have tested, 20 appear to be nearly as good as new showing only slight leakage which their getters have taken care of nicely and no detectable reduction in power output. (Of the others, 7 had weak or no output but most could be at least partially revived - see the section: Attempting to Revive Some Soft-Seal HeNe Tubes. The remainder were totally dead.)

    As is typical of Spectra-Physics internal mirror HeNe tubes, these have thick glass walls (at least compared to tubes from most other manufacturers). For the barcode scanner application (at least) there was an outer wrap (removable) of several layers of thick aluminum foil, apparently for thermal stabilization but it would also reduce electrical noise emissions and light spill from the discharge. (The foil wrap also seems to be common with more modern Spectra-Physics HeNe barcode scanner tubes when not installed in cylindrical laser heads.) A 100K ohm ballast resistor stack in heat shrink tubing was attached with a clip and RTV Silicone to the anode end-plate stud, and both ends were capped with rubber covers for protection (of the tube and user).

    The SP-084-1 is about 9-1/2" (241 mm) by 1" (25.4 mm) in diameter with a bore length of 5.5" (140 mm). Its output is a TEM00 beam about 0.8 mm in diameter exiting through a hole in the cover on the cathode-end of the tube. Power supply connections are made to a stud on the anode end-plate and the exhaust tube on the cathode end-plate. Their optimal operating point is around a tube current of 5 mA resulting in a total operating voltage (across tube + Rb) of about 1.9 to 2.0 kV using the 100K ballast.

    Note from the diagram that unlike modern tubes where the mirrors are on mounts that can be adjusted (by bending) after manufacturer, alignment of the SP-084-1 would appear to be totally fixed. Some possible ways of setting alignment might be:

    1. The mirrors were just glued in place expecting alignment to be adequate (but the end-plates do not appear to be specially machined).

    2. The mirrors were aligned at installation using external optics but before the tube had been pumped down and filled with helium and neon.

    3. The manufacturing process provided a means of adjusting the mirrors after filling but before the glue had fully set or by softening it with heat.

    4. There was some means of distorting the end-plates (but this doesn't seem likely given their thickness).

    From appearances, I would guess (2). Since the mirrors are slightly curved (non-planar), their position could be used to adjust alignment slightly - and some were attached very visibly off-center to compensate for end-plates fused to the glass tube at a slight angle.

    HeNe Lasers using External Mirrors

    While most of what you will likely come across are the common internal mirror HeNe tube, having the optics external to the tube is essential for some applications.

    A One-Brewster HeNe Laser Tube

    I was given a CLIMET 9048 HeNe laser head which contains what looks like a Melles Griot HeNe tube but with a frit-sealed Brewster window instead of an OC mirror at one end. In this case, it is the cathode-end which is nice since there is no high voltage to deal with near the Brewster window. But identical tubes also come with the Brewster window at the anode-end but why anyone would want this excapes me. :) (And, several other models of one-Brewster tubes are common - see the section: Melles Griot Brewster and Zero Degree Window HeNe Tubes.)

    The tube is a Melles Griot model 05-LHB-570. It has an internal HR mirror and Brewster window at the other end of the tube. The HR is similar to those on other Melles Griot tubes (including the use of a locking collar) though the somewhat more silvery appearance of its surface may indicate that it is coated for broadband reflectivity and/or perhaps for higher reflectivity than ordinary HRs. (The mirror reflectivity of the HR on at least some versions of the 05-LHB-570 is greater than 99.9% from 590 to 680 nm but I don't think this one, which is quite old, has these characteristics.) The total length is about 265 mm (10.5 inches) from the HR mirror to the Brewster window. There is also a power sensor inside the head for (I assume) monitoring what gets through the HR mirror (untested).

    CLIMET 9048 One-Brewster HeNe Laser Head shows the aluminum cylinder with its mounting flange at the Brewster window end, ballast resistor, and Alden connector. The other black wire attaches to the solar cell power sensor.

    These one-Brewster HeNe tubes are generally used in applications like particle counting which requires high photon flux to detect specks of dust or whatever. Access to the inside of the resonator is ideal since with appropriate highly reflective mirrors at both ends, several WATTs of "virtual" circulating power can be produced inside the cavity of this HeNe laser. Thus, for these applications, they have the benefits of a high power laser without the cost or safety issues. There are even HeNe tubes similar to this that will do up to 45 W using super high quality mirrors and Brewster window. And, of course, they are also super expensive. Of course, you can't siphon off all that power - only be extremely envious and frustrated that it is trapped in there - but also safe from any sneak attacks on an unsuspecting eyeball. :)

    With its wide bore, this tube has an optimal operating point (maximum power) of about 7.5 to 8 mA at about 1 kV (though the recommended current is actually 6.5 mA). This may just be a peculiarity of the sample I tested.

    I have constructed a simple mirror mount so that various mirrors could be easily installed and there is easy access to the inside of the cavity. See HeNe Laser Tube with Internal HR and Brewster Window with External OC for a diagram showing this laser assembly. Using various mirrors, both from deceased HeNe lasers as well as from laser printers and barcode scanners, output power reached more than 3 mW and the circulating power inside the resonator peaked at over 1 W (but not with the same mirrors). With optimum high quality mirrors, it should be capable of more power in both areas. Photos of this laser are shown in Sam's External Mirror Laser Using One-Brewster HeNe Laser Head.

    See the section: Sam's Instant External Mirror Laser Using a One-Brewster HeNe Tube for details on these experiments and the design of the mirror mount.

    I have attempted to get wavelengths other than boring 632.8 nm red out of this and similar 1-B tubes. However, all attempts have failed but one - installing a somewhat larger 05-LHB-670 in place of the dead tube of a PMS/REO tunable HeNe laser. (This 1-B tube did 7.5 mW with the same OC mirror as used above. The 1-B tube in the Climet head probably woudn't have enough gain.) The HR mirror on the tuning prism is broadband coated for 543.5 to 632.8 nm. In this case, I was able to convince just a few 604.6 nm orange photons to cooperate and lase. However, the only way to collect them was from the reflections off the Brewster window of the tube or prism, or from the HR mirror of the 1-B tube. The total orange power may have been 50 microwatts, if that.

    Designing a Helium-Neon Laser Tube

    (From: Lynn Strickland (

    H. Weichel and L.S. Pedrotti put out a good summary paper which includes the equations used in the design process of a gas laser. In particular, section V tells you how to calculate mode radius at any point, given mirror curvature, spacing and wavelength. If you know that, the aperture size (the capillary bore usually) and the magic number for the ratio between the two, you can design a TEM00 gas laser. Using a HeNe tube with a Brewster window, you could do some fun stuff with predicting aperture sizes and locations to force TEM00 operation.

    The paper was published by the Department of Physics, Air Force Institute of Technology, Wright-Patterson Airforce Base, OH. The title is "A Summary of Useful Laser Equations -- an LIA Report". Don't know where you'd find it, but the Laser Institute of America (LIA) might be a good start.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Wavelengths, Beam Characteristics

    HeNe Laser Wavelengths

    While what comes to mind when there is mention of a HeNe laser is a red beam, those with other wavelengths are manufactured.

    Typical maximum output available from (relatively) small HeNe tubes (400 to 500 mm length) for various colors: red - 10 mW, orange - 3 mW, yellow - 2 mW, green - 1.5 mW, IR - 1 mW. Higher power red HeNe tubes (up to 35 mW or more and over 1 meter long) and 'other-color' HeNe tubes (much lower - under 10 mW) are also available. However, these will be very large and very expensive.

    Exact Frequency/Wavelength of HeNe Lasers

    There is, of course, no single precise HeNe wavelength since any given cavity will only oscillate at the permitted longitudinal modes and the gain curve is something like 1.5 GHz wide. Thus, for a common HeNe laser, there is no single wavelength and those that are present drift over time (mostly due to thermal expansion of the cavity). A single mode frequency stabilized HeNe laser will have very nearly a constant single wavelength precise to 9 or more significant figures but it too will depend on the physical size of the laser's cavity - there is no one correct answer!

    For example, one typical stabilized HeNe laser from Hewlett-Packard, has a precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot (as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by the index of refraction of air, n=1.00027593).

    (Portions from: Jens Decker (

    The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their 05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow, with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.

    (From: D. A. Van Baak (

    Well, here it is exact:

    The metrologists' answer for a 632.8 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:

    under certain specified conditions, with uncertainty 2.5x10-11. See: "Metrologia", vol. 30., pp. 523-541, 1993-1994.

    HeNe Laser Beam Characteristics

    Compared to a diode laser, the beam from even an inexpensive mass produced HeNe tube is of very high optical quality:

    Ghost Beams From HeNe Laser Tubes

    If you project the output from some HeNe laser tubes (as well as other lasers) onto a white screen a meter or so away, you may see a main beam and a weak beam off to the side a few cm away from it. Maybe even another still weaker one after that.

    Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.

    One possible cause for this artifact is that the output-end mirror (Output Coupler or OC) has some 'wedge' (the two surfaces are not quite parallel) built in to move any reflections - unavoidable even from Anti-Reflection (AR) coated optics - off to the side and out of harm's way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn't even know of their existence!

    Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator - perfectly aligned with the tube axis - which could result in lasing instability. (However, the chance of this is probably minimal since anything making its way back inside from reflection from an AR coated surface AND through the 99%+ reflective OC would be extremely weak).

    Thus, the ghost beam off to one side is likely a feature, not a problem! The effects of wedge on both the output beam and a beam reflected from a mirror with wedge is illustrated in Effects of Wedge on Ghost Beams and Normal Reflections. Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".

    If it isn't obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a slight angle onto a white screen. There will be a pair of reflected beams - a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed (there may be higher order reflections as well but they will be VERY weak - see below). Where the mirror is curved, the patterns will be different but the wedge will still result in a line of spots at an angle dependent on the orientation of the tube.

    Wedge is often present on the other mirror (High Reflector or HR) as well (in fact, this appears to be more likely than the OC). Wedge at the HR-end won't affect the output beam at all but performing the reflectance test using a collimated laser (as above) at a near-normal angle of incidence may result in the following:

    With the exaggerated amount (angle) of wedge in Effects of Wedge on Ghost Beams and Normal Reflections, another effect becomes evident: The weaker spots are spaced further apart. It is left as an exercise for the student to determine what happens when a laser beam is reflected at an angle from such a mirror! Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".

    The appearance resembles that of a diffraction grating on such a beam (but for entirely different reasons). The behavior will be similar for an OC with wedge but because the HR mirror isn't AR coated, the higher order spots (from the HR) are much more intense.

    It is conceivable that slight misalignment of the mirrors may result in similar ghost beams but this is a less likely cause than the built-in wedge 'feature'. However, if you won't sleep at night until you are sure, try applying the very slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors as they are very fragile) in each while the tube is powered (WARNING: High Voltage - Use a well insulated stick!!!!).

    Depending on the type of laser you have, see the sections: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, Quick Course in Large Frame HeNe Laser Mirror Alignment, and External Mirror Laser Cleaning and Alignment Techniques, for more information.

    Another much simpler cause of an ugly beam from a HeNe (or other) laser is dirt on the outside of the output mirror or other external optics. Some HeNe laser heads have either a debris blocking glass plate glued at an angle to the end-cap or a neutral density filter to adjust output power. Even if AR coated, either of these may also introduce one or more ghost beams and if not perfectly clean, other scatter as well. I'm gotten supposedly bad HeNe lasers where the only problem was dirt on either the output mirror or external plate or filter.

    (From: Steve Roberts (

    The mirror is wedged to cut down on the number of ghost beams, however even with a wedged mirror there is almost always one ghost. Nothing is wrong with your coatings on the mirror, it is simply a alignment matter. The mirrors need to be "walked" into the right position relative to the bore. There are many many paths down the bore that will lase, but only a few have the TEM00 beam and the most brightness, this generally corresponds to the one with minimum ghosts.

    See the section: Quick Course in Large Frame HeNe Laser Mirror Alignment for more information.

    Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes

    As a practical matter, the only wavelength that is useful from an internal mirror HeNe laser is the one for which it was designed. However, it may be possible to at least obtain low power and unstable lasing at other wavelengths by extending the cavity using an external mirror. This probably works best obtaining a bit of red from a long "hot" yellow (594.1 nm) or orange (611 nm) tube since at least one mirror is likely coated broadband to include yellow through red. Due to the low gain of the non-red lines, going the other way - getting yellow from a red tube, for example - is not likely to succeed unless the tube is very long. But obtaining lasing at other red wavelengths - and even orange - may be possible with a moderate size red HeNe laser tube. Even a 1 mW tube may give you 1 or 2 other red lines. I doubt it will work at all with a green HeNe tube having mirrors that appear orange in transmission since both mirrors are probably too transparent at even the yellow wavelength (except possibly if two external mirrors are used). However, if a mirror is more red in transmission, there might be a chance. See the section: Instant HeNe Laser Theory for a table of HeNe lasing wavelengths and relative gains.

    Here's hot to get other wavelengths from your HeNe laser. Either a bare tube or complete laser head can be sued for these experiments.

    Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my "broken" sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm) from both ends (see the section: The Dual Color Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output, and all three colors were occasionally present at the same time - an impressive achievement for a HeNe laser. My setup is shown in 05-LYR-170 HeNe Laser Tube Mounted in Test Fixture for Multiline Experiments. The output from the tube's OC was directed at an AOL CD used as a reflective diffraction grating with the first-order beam projected on a white card several feet away. The lens from a pair of eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length) narrowed the spots to improve spectral resolution. This rig could easily resolve lines separated by less than 1 nm. The first external "red" mirrors I tried were from an SP-084 HeNe laser tube but due probably to their relatively short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get any red output. Mirrors designed for a longer laser worked better but there wasn't much difference between the behavior using an HR or OC (99 percent).

    Then to add to the excitement, with a bit of twiddling, I was able to obtain the other orange line (611.9 nm) as well, and at times, all 4 lines were lasing simultaneously! As expected, this additional line was only present when using an exteranl HR. Depending on the original makeup of the yellow and orange beam (for this tube, their absolute and relative intensities varied with time and were also a very sensitive function of mirror alignment), it was possible to get mostly red or to vary the intensities of the other colors, most easily suppressing yellow in favor of orange and red. The intensity of the red output was never more than 1 mW or so. Its transverse mode structure varied from TEM00 to a star pattern with nothing in the center. Strange. Due to both surfaces of the HeNe tube's HR mirror reflecting some of the intracavity beam resulting in a multiple cavity interference effect, there was a distinct lack of stability. To help compensate for this, a micrometer screw to precisely adjust cavity length without affecting mirror alignment would have been nice.

    I also tried this with the external mirror mounted beyond the tube's OC mirror but although there was a definite effect on yellow and orange lasing, it wasn't possible to obtain any red output. (For the 05-LYR-170, the OC already reflects red quite well and the HR doesn't.) Finally, I replaced the red external mirror with a green HR (from a tube of about the same length) mounted beyond the 05-LYR-170's OC (since its HR by appearance looked like it might be a good mirror for green). But, not surprisingly, while this could affect the lasing of the yellow and orange lines, I could detect no coherent green photons. However, I would expect that with a appropriately coated mirrors (or possibly two such mirrors, one beyond each end of the tube), obtaining lasing at the relatively high gain 640.1 nm red line would be easy - the usual "red" mirrors may deliberately kill this line to prevent it from lasing. Although I couldn't detect any evidence of lasing at the other red lines of 629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors as they have higher gain than the yellow and oranges. Another interesting one would be the "Border Infra-Red" line at 730.5 nm. Lasing at the IR lines might also be possible but they are so boring. :)

    Next, determined to do something with a more normal HeNe laser tube, I tried a Siemens tube but that refused to do anything interesting. Then, I tried a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam. Since the OC for this laser is probably around 99% reflective at most, peaking at 632.8 nm, I figured that it would be best to place the external mirror beyond the OC rather than the HR. And, with the same external HR as used above, it was possible to obtain 6 lasing lines, count'm 6: 629.4 nm, 632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm (all variations on red), ****AND**** 611.9 nm orange! However, since the output is being taken from the HR, none of the colors was more than a fraction of a mW.

    Lasing of the 650 nm line was hard to obtain - it only showed up for a few seconds off-and-on every few minutes and increasingly rarely after the tube warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which is a lot more expensive than my AOL CD. :) (The wavelength was referenced to the 632.8 line from the same laser resulting in a measurement error bound of +/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this could also be slightly shifted, the error may be higher.) Getting anything at 650 nm is really puzzling as there are no HeNe lasing lines between 640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it was fluctuating independantly of the others (later confirmed, see below). And all those other lines were quite accurately located corresponding to their handbook wavelengths in the diffracted pattern (and later confirmed with the OSA). So there is little reason to suspect that the funny one isn't as well. When present, it appeared as strong (or weak) as all the expected ones, (except of course, the original 632.8 nm line which was usually, but not always, the strongest). If 650 nm is not a HeNe lasing line - it's certainly not in the sequence of energy level transitions that produce all the other visible HeNe lines - one possible explanation is that there is some trace element present inside the tube and that is what's lasing, not neon. I figured this to be a distinct possibility since the particular tube I am using originally had gas contamination and I revived it by heating the getter. (See the section: Repairing the Northern Lights Tube.) Therefore, the 650 nm wavelength may not be present with another more normal tube. But as it turned out, contamination has nothing to do with it.

    I don't think the 730.1 nm line was present but given its low relative perceived brightness, it may not have been visible at all using my AOL Special CD diffraction grating but I couldn't find it with the OSA either. It took awhile to detect the evidence of the 635.2 nm line which only appeared sporatically (but it is the lowest gain of all the known ones above).

    A few days later, I tried the same experiment with a couple of my old Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so have almost certainly leaked over time (but still work fine). With my "hottest" SP084-1 (about 2.9 mW), I could almost duplicate the results of the 05-LHR-150 including the funny line around 650 nm but minus anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these, an SP084-1 HR worked almost as well for the external mirror as the longer RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1, a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW). With that, the only additional line was at 640.1 nm. Which particular lines appear in each case seem consistent with the length of the tubes (and thus the single pass gain) and the relative gain of the lasing lines.

    Some quick calculations predict that the real effect of the external HR mirrors is the obvious one - to increase the circulating power. A 1 percent OC (typical) followed by even a 90 percent external mirror would result in greater than a 99.9 percent effective mirror for a range of wavelengths/modes. An external 99.9 percent HR would result in an even better effective mirror. It looks like the reflectance peak is relatively broad with respect to wavelength (the transmission peak is rather narrow). Specific modes for each of the wavelengths will be enhanced or suppressed. This would also appear to be consistent with the apparent lack of need for the external mirror to result in a stable resonator. All it has to do is form a Fabry-Perot cavity.

    These have to be classified right up there in the really fascinating experiments department. Seeing any HeNe laser operating with multiple spectral lines is really neat.

    As always, depending on mirror reflectivity and other factors, your mileage may vary. But feel free to try variations on these themes. The results from using an HeNe HR beyond the OC of almost any red HeNe laser tube should be easily replicated (except perhaps for the funny 650 nm line). Almost any mirror will do something since even an aluminized mirror will be returning over 90 percent of the otherwise wasted photons to the cavity - enough to boost the gain of all but the weakest lines enough for lasing if everything lines up just right. Aside from getting zapped by the high voltage or dropping the tube on the floor, they are low risk, high reward experiments.

    (From: Bob.)

    For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed. For neutral helium, there is one at 667.8 nm. None of the other noble gases have wavelengths listed this short. As far as ionized species go, singly ionized argon has a line at 648.30 nm. Singly ionized krypton has a hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.

    For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm. There are no neutral lines of interest for atmospheric gases. The footnotes for the above line were listed as CW lasing in 0.02 torr of krypton. Whats the standard operating pressure of a HeNe laser? Not THAT far out of the ball park I would guess.

    (From: Sam.)

    The last one sounds promising and would make sense given the history of the particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe lasers operate in the 2 to 3 TORR range - about 100 times higher pressure, the partial pressure of any N2 contamination could very well be down around 0.02 Torr.

    However, I now know exactly where the 650 nm line is coming from and it has nothing whatsoever to do with contamination. The exciting writeup from someone who beat me to this by about 15 years follows in the next section preceeded by a condensed version, below.

    I've also found a commercial laser that appears to produce a very stable 650 nm line. See the section: The PMS/REO External Resonator Particle Counter HeNe Laser.

    (From: Stephen Swartz (

    Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a hallucination. The 650 nm line which is never discussed in most standard texts is not due to a "normal" transition of neon. It comes instead from a Raman transition. The 650 nm line is not often observed but when it is it will always be seen simultaneously with operation on a multitude of other lines. A large number of other "unusual" colors have been seen over the years. Higher power tubes with mirrors that are excessively broadband are your best bet for observing them. Often these lines flicker on and off over a few seconds to minutes time scale. A diffraction grating is a good way to look for them.

    (From: Someone at a major laser company.)

    The 650.0 nm Raman line is a known problem in that it competes for power with the 632.8 nm line intermittently, particularly in long tubes with high circulating power. Polarized tubes are much less susceptible to this effect and using a lower reflectance for the OC mirror helps since it reduces circulating power without affecting output very much (over a reasonable range).

    Bruce's Notes on Getting Other Lines from Red (633 nm) HeNe Laser Tubes

    This, to make a gross understatement, would appear to be the definitive word on coaxing other colors from surplus HeNe laser tubes. And I thought six lines (including the mysterious 650 nm line) was an achievement. :)

    (From: Bruce Tiemann (

    I have gotten many lines from many different HeNe lasers. In my experience almost every tube is capable of giving at least one other line than 633 nm. (Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633 nm.) I have never tried doing this with lasers that give other lines than 633 nm, but since that line has the highest gain, it should be no mean feat to at least get that line from lasers that are supposed to not give it. It is also not my experience that calculations to ensure resonator stability, etc., are necessary. Just try it! My best results, in terms of output power, were with a flat grating as the external feedback mirror, and my best results in terms of new lines was obtained with a flat dielectric mirror, formerly used as a facet in a polygonal scanning assembly. Flat mirrors are not stable at any separation for a diverging beam, and HeNe lasers are very rare that give converging beams for their output.

    The home stuff had the mirrors on blocks, with the steering accomplished by adjusting the HeNe tube by lifting one or the other end of the tube with sheets of paper, and the azimuth by moving the laser tube back and forth. The lab experiments were done with "real" mirror mounts, supplemented by a single PZT that tilted the feedback mirror a few microns.

    (I like PZTs a great deal, and would like to observe that you can get PZT elements from little piezo alarms, from which the useful element can be extracted with some hand-tools and the mind-set of a 9-year-old kid dissecting a bug. :) These are only about $1 each, as opposed to tens to hundreds of bucks for "real" PZTs that you buy from Thor, etc. One of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the micron scale, which is all that is needed for these experiments.)

    (From: Sam.)

    I have indeed done something similar using the piezo beeper from a dead digital watch to move a mirror in a HeNe laser based Michelson interferometer. With 0 to 25 V, it went through 4+ fringes which means over 2 full wavelengths at 633 nm. The configuration in these is called a "drum head" piezo element because the movement resembles that of a musical (depending on your point of view!) drum head with the most shift in the center. The piezo material itself doesn't change by very much in thickness but is constructed so it distorts to produce the shape change. With care, the piezo material can be cut to size or drilled to pass light through its center. Much more voltage could have been safely applied if needed.

    (From: Bruce.)

    Something I also did is cast the spots from a smaller (approximately 3/4 m) spectrometer directly onto the CCD element of a small camera with no lens. I also fabricated a beam block by taping little wires to the side of a block, that would protrude up just in the locations of the very bright lines, like 633, 650, and 612 nm, to block them, but letting light of other colors pass in the ample space between the wires. You could still see when the bright lines were on from light leaking around the wires, but it wouldn't wash out the image when they were.

    In this case, when the feedback mirror was tilted, speckle, which was cast everywhere, would kind of shift around all over the place, but the new lines looked like ghostly bullseyes, which would breathe in and out as the mirror was tilted, but remain in the same location unlike the speckle. This was an easy way to see the weakest lines like 624 nm, and it was also how I discovered 668 nm, the CCD being more sensitive than the eye in the deep red. (I searched for but did not find the normal laser line 730 nm even with this very sensitive method.)

    All in all this laser produced 17 different lines, many at one time, from a "single line" 633 swap-meet laser. :)


    The 650 nm discovery paper is:

    Later, in 1989, a Chinese group that doesn't read Applied Physics Letters published:

    The first one, at least, should be available from a university library.

    Other Spectral Lines in HeNe Laser Output

    While there is no such thing as a truly monochromatic source - laser or otherwise, the actual output beam of even an inexpensive HeNe laser is really quite good in this regard with a spectral line width of less than 1/500th of a nm. For a frequency stabilized HeNe laser, it can be 1,000 times narrower!

    But if you look at the output of a HeNe laser with a spectrometer, there will be dozens of wavelengths present other than one around 632.8 nm (or whatever is appropriate for your laser if not a red one). Close to the output aperture, there will be a very obvious diffuse glow (blue-ish for the red laser) visible surrounding the actual beam. So why isn't the HeNe laser monochromatic as expected?

    With one exception, this is just due to the bore light - the spill from the discharge which makes it through the Output Coupler (OC) mirror. As your detector is moved farther from the output aperture, the glow spreads much faster than the actual laser beam and its intensity contribution relative to the actual beam goes down quickly. It is not coherent light but what would be present in any low pressure gas discharge tube filled with helium and neon. However, the presence of these lines can be confusing when they show up on a spectral printout.

    The exception is that with a 'hot' (unusually high gain) tube or one with an OC that is not sufficiently narrow-band, one (though probably not more though not impossible) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though probably much more weakly than the primary line. For example, a red (632.8 nm) laser might also produce a small amount of output at 629.4 or 640.1 nm though this isn't that common. I have one 'defective' yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (604.6 nm), and another that produces in addition some of the other orange line (611.9 nm).

    (From: Prof Harvey Rutt (

    For gas lasers the plasma lines are typically 80 dB or more below the output (measured, of course, within the very small laser mode divergence). This is unlike most semiconductor lasers, which typically have broad 'shoulders' close in to the line, as well as 'lines' due to other modes and instabilities because the initial divergence of the diode is high, and spontaneous emission from the junction high, the broad background tends to be large.

    For gas lasers it is usually in the form of narrow lines at remote wavelengths, very easily removed with an interference filter and/or spatial filtering in the *rare* cases where it matters. There is presumably a weak broad background from processes involving free electrons (bound/free and free/free), but I've never seen it even mentioned, let alone observed it. More likely to be significant in the high current density argon laser than the very low current density HeNe.

    The only cases I have seen where the plasma lines caused problems were Raman measurements on scattering samples with photon counting detection, and weak fluorescence measurements which are similar.

    In most cases scattered light in the monochromator is much more of an issue (hence double monochromators for Raman) and will obscure plasma lines in many cases.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Magnets in High Power or Precision HeNe Laser Heads

    Effects of Magnetic Fields on HeNe Laser Operation

    If you open the case on a higher power (and longer) HeNe laser head or one that is designed with an emphasis on precision and stability, you may find a series of magnets or electromagnetic coils in various locations in close proximity to the HeNe tube. They may be distributed along its length or bunched at one end; with alternating or opposing N and S poles, or a coaxial arrangement; and of various sizes, styles, and strengths.

    Magnets may be incorporated in HeNe lasers for several reasons including the suppression of IR spectral lines to improve efficiency (such as it is!) and to boost power at visible wavelengths, for the stabilization of the beam, and to control its polarization. There are no doubt other uses as well.

    The basic mechanism for the interaction of emitted light and magnetic fields is something called the 'Zeeman Effect' or 'Zeeman Splitting'. The following brief description is from the "CRC Handbook of Chemistry and Physics":

    "The splitting of a spectrum line into several symmetrically disposed components, which occurs when the source of light is placed in a strong magnetic field. The components are polarized, the directions of polarization and the appearance of the effect depending on the direction from which the source is viewed relative to the lines of force."

    Magnet fields may affect the behavior of HeNe tubes in several ways:

    In principle, varying fields from electromagnets could be used for intensity, polarization, and frequency modulation. I do not know whether any are implemented in this manner.

    Typical Magnet Configurations

    Here are examples of some of the common arrangements of magnets that you may come across. In addition to those shown, magnets may be present along only one side of the tube (probably underneath and partially hidden) or in some other peculiar locations. I suspect that for many commercial HeNe lasers, the exact shape, strength, number, position, orientation, and distribution of the magnets was largely determined experimentally. In other words, some poor engineer was given a bare HeNe tube, a pile of assorted magnets, a roll of duct tape, and a lump of modeling clay, and asked to optimize some aspect(s) of the laser's output. :-)

    (In all of these diagrams, the orientation of the Brewster windows shown is totally arbitrary - for sealed HeNe tubes with internal mirrors, they would not be present at all!)

    For the magnet configuration used in a commercial laser, see the section: Description of the SP-124 Laser Head.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Internal Mirror HeNe Tubes up to 35 mW - Red and Other Colors

    Typical HeNe Tube Specifications

    Prior to the introduction of the CD player, the red HeNe laser was by far the most common source of inexpensive coherent light on the planet. The following are some typical physical specifications for a variety of red (632.8 nm) HeNe tubes (all are single transverse mode - TEM00):

       Output     Tube Voltage       Tube         Supply Voltage     Tube Size
       Power      Operate/Start     Current        (75K ballast)    Diam/Length
     ----------  ---------------  ------------   ----------------  -------------
      .3-.5 mW     .8-1.0/6  kV    3.0-4.0 mA       1.0-1.2 kV       19/135 mm
      .5-1 mW      .9-1.0/7  kV    3.2-4.5 mA       1.1-1.3 kV       25/150 mm
       1-2 mW     1.0-1.4/8  kV    4.0-5.0 mA       1.2-1.8 kV       30/200 mm
       2-3 mW     1.1-1.6/8  kV    4.0-6.5 mA       1.4-2.0 kV       30/260 mm
       3-5 mW     1.7-1.9/10 kV    4.5-6.5 mA       2.1-2.4 kV       37/350 mm


    At least one other basic specification may be critical to your application: Which end of the tube the beam exits! There is no real preference from a manufacturing point of view for red HeNe lasers. (For low gain "other-color" HeNe laser tubes, it turns out that anode output results is slightly higher gain and thus slightly higher houtput for the typical hemispherical cavity because it better utilizes the mode volume.) However, this little detail may matter a great deal if you are attempting to retrofit an existing barcode scanner or other piece of equipment where the tube clips into a holder or where wiring is short, tight, or must be in a fixed location. For example, virtually all cylindrical laser heads require that the beam exits from the cathode-end of the tube. It is possible that you will be able to find two versions of many models of HeNe tubes if you go directly to the manufacturer and dig deep enough. However, this sort of information may not be stated where you are buying surplus or from a private individual, so you may need to ask.

    The examples above (as well as all of the other specifications in this and the following sections) are catalog ratings, NOT what might appear on the CDRH safety sticker (which is typically much higher). See the section: About Laser Power Ratings for info on listed, measured, and CDRH power ratings.

    Note how some of the power levels vary widely with respect to tube dimensions, voltage, and current. Generally, higher power implies a longer tube, higher operating/start voltages, and higher operating current - but there are some exceptions. In addition, you will find that physically similar tubes may actually have quite varied power output. This is particularly evident in the Melles Griot listings, below.

    These specifications are generally for minimum power over the guaranteed life of the tube. New tubes and individual sample tubes after thousands of hours may be much higher - 1.5X is common and a "hot" sample may hit 2X. My guess is that for tubes with identical specifications in terms of physical size, voltage, and current, the differences in power output are due to sample-to-sample variations. Thus, like computer chips, they are selected after manufacture based on actual performance and the higher power tubes are priced accordingly! This isn't surprising when considering the low efficiency at which these operate - extremely slight variations in mirror reflectivity and trace contaminants in the gas fill can have a dramatic impact on power output.

    I have a batch of apparently identical 2 mW Aerotech tubes that vary in power output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the tubes indicating measured power levels at the time of manufacture).

    And, power output also changes with use (and mostly in the days of soft-sealed tubes, just with age sitting on the shelf):

    (From: Steve Roberts (

    "I have a neat curve from an old Aerotech catalog of HeNe laser power versus life. The tubes are overfilled at first, so power is low. They then peak at a power much higher than rated power, followed by a long period of constant power, and then they SLOWLY die. It's not uncommon for a new HeNe tube to be in excess of 15% greater than rated power."

    And the answer to your burning question is: No, you cannot get a 3 mW tube to output 30 mW - even instantaneously - by driving it 10 times as hard!

    I have measured the operating voltage and determined the optimum current (by maximizing beam intensity) for the following specific samples - all red (632.8 nm) tubes from various manufacturers. (The starting voltages were estimated):

       Output     Tube Voltage       Tube         Supply Voltage     Tube Size
       Power      Operate/Start     Current        (75K ballast)    Diam/Length
     ----------  ---------------  ------------   ----------------  -------------
        .8 mW        .9/5  kV        3.2 mA           1.1 kV         19/135 mm
       1.0 mW       1.1/7  kV        3.5 mA           1.4 kV         25/150 mm
       1.0 mW       1.1/7  kV        3.2 mA           1.4 kV         25/240 mm
       2.0 mW       1.2/8  kV        4.0 mA           1.5 kV         30/185 mm
       3.0 mW       1.6/8  kV        4.5 mA           1.9 kV         30/235 mm
       5.0 mW       1.7/10 kV        6.0 mA           2.2 kV         37/350 mm
      12.0 mW       2.5/10 kV        6.0 mA           2.9 kV         37/475 mm
    Melles Griot, Uniphase, Siemens, PMS, Aerotech, and other HeNe tubes all show similar values.

    The wide variation in physical dimensions also means that when looking at descriptions of HeNe lasers from surplus outfits or the like, the dimensions can only be used to determine an upper (and possibly lower) bound for the possible output power but not to determine the exact output power (even assuming the tube is in like-new condition). Advertisements often include the rating on the CDRH safety sticker (or say 'max' in fine print). This is an upper bound for the laser class (e.g., Class IIIa), not what the particular laser produces or is even capable of producing. It may be much lower. For example, that Class IIIa laser showing 5 mW on the sticker, may actually only be good for 1 mW under any conditions! The power output of a HeNe laser tube is essentially constant and cannot be changed significantly by using a different power supply or by any other means. See the section: Buyer Beware for Laser Purchases.

    Also see the section: Locating Laser Specifications.

    In addition to power output, power requirements, and physical dimensions, key performance specifications for HeNe lasers also include:

    With manufacturers like Aerotech, Melles Griot, and Siemens, a certain amount of information can be determined from the model number. For example, here is how to decipher most of those from Melles Griot (e.g., 05-LHP-121-278):

    The vast majority of Melles Griot lasers you are likely to come across will follow this numbering scheme though there are some exceptions, especially for custom assemblies. (Some surplus places drop the leading '05-' when reselling Melles Griot laser tubes or heads so an 05-LHP-120 would become simply an LHP-120.)

    For other manufacturers like Spectra-Physics, the model numbers are totally arbitrary! (See the section: Some Spectra-Physics HeNe Lasers.)

    HeNe Tubes of a Different Color

    Although a red beam is what everyone thinks of when a HeNe laser is discussed, HeNe tubes producing green, yellow, and orange beams are also manufactured. However, they are probably not found very often on the surplus market because they are not nearly as common as the red variety. Such tubes are also more expensive when new since for a given power level, they must be larger (and thus have higher voltage and current ratings) due to their lower efficiency (the spectral lines being amplified are much weaker than the one at 632.8 nm). Operating current for non-red HeNe tubes is also more critical than for the common red variety so setting these up with an adjustable power supply or adjusting the ballast resistance for maximum output is recommended.

    Maximum available power output is also lower - rarely over 2 mW (and even those tubes are quite large (see the tables below). However, since the eye is more sensitive to the green wavelength (543.5 nm) compared to the red (632.8 nm) by more than a factor of 4 (see the section: Relative Visibility of Light at Various Wavelengths), a lower power tube may be more than adequate for many applications. Yellow (594.1 nm) and orange (611.9 nm) HeNe lasers appear more visible by factors of about 3 and 2 respectively compared to red beams of similar power. To get an idea of the actual perceived color at each wavelength, see the section: Color Versus Wavelength.

    Infrared-emitting HeNe lasers exist as well. Yes, you can have a HeNe tube and it will light up inside (typical neon glow), but if there is no output beam (at least you cannot see one), you could have been sold an infrared HeNe tube. The IR may be visible with a video camera (assuming it doesn't have an IR blocking filter) or by using one of the IR detector circuits or an IR detector card as discussed with respect to IR laser diodes. IR HeNe tubes are unusual enough that it is very unlikely you will ever run into one. However, they may turn up on the surplus market especially if the seller doesn't test the tubes and thus realize that these behave differently - they are physically similar to red (or other color) HeNe tubes except for the reflectivity of the mirrors as a function of wavelength. (There may be some other differences needed to optimize each color like the He:Ne ratio, isotope purity, and gas fill pressure, but the design of the mirrors will be the most significant factor and the only one you can detect with a bare eyeball.) Even if the model number does not identify the tube as green, yellow, orange, red, or infra-red, this difference should be detectable by comparing the appearance of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of a normal (known to be red) HeNe tube. See the section: Determining HeNe Laser Color from the Appearance of the Mirrors. (Of course, your tube could also fail to lase due to misaligned or damaged mirrors or some other reason. See the section: How Can I Tell if My Tube is Good?.)

    As noted above, the desired wavelength is selected and the unwanted wavelengths are suppressed mostly by controlling the reflectivity functions of the mirrors. For example, the gains of the green and yellow lines (yellow may be stronger) are both much much lower than red and separated from each other by about 50 nm (543.5 nm versus 594.1 nm). To kill the yellow line in a green laser, the mirrors are designed to reflect green but pass yellow. I have tested the mirrors salvaged from a Melles Griot 05-LGP-170 green HeNe tube (not mine, from "Dr. Destroyer of Lasers"). The HR (High Reflector) mirror has very nearly 100% reflectivity for green but less than 25% for yellow. The OC (Output Coupler) also has a low enough reflectivity for yellow (about 98%) such that it alone would prevent yellow from lasing. The reflectivities for orange, red, and IR, are even lower so they are also suppressed despite their much higher gain, especially for the normal red (632.8 nm) and even stronger mid-IR (3,391 nm) line. Note that to manufacture a tube with optimum and stable output power, it isn't sufficient to just kill lasing for unwanted lines. The resonator must be designed to minimize their contribution to stimulated emission - thus the very low reflectivity of the HR for anything but the desired green wavelength. Otherwise, even though sustained oscillation wouldn't be possible, unwanted color photons would still be bouncing back and forth multiple times stealing power from the desired color. The output would also be erratic as the length of the tube changed during warmup (due to thermal expansion) and this affected the longitudinal mode structure of the competing lines relative to each other. Some larger HeNe lasers have magnets along the length of the tube to further suppress (mostly) the particularly strong mid-IR line at 3,391 nm. (See the section: Magnets in High Power or Precision HeNe Laser Heads.)

    And, the answer to that other burning question should now be obvious: No, you can't convert an ordinary red internal mirror HeNe tube to generate some other color light as it's all done with mirrors and they are an integral part of the tube. :) Therefore, your options are severely limited. As in: There are none. (However, going the other way, at least as a fun experiment, may be possible. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.) For a laser with external mirrors, a mirror swap may be possible (though the cavity length may be insufficient to resonate with the reduced gain of other-color spectral lines once all loses taken into consideration). But realistically, this option doesn't even exist where the mirrors are sealed into the tube.

    There are also a few HeNe lasers that can output more than one of the possible colors simultaneously (e.g., red+orange, orange+yellow) or selectively by turning knob (which adjusts the angle of a Littrow or other similar dispersion prism) inside the laser cavity using a Brewster window HeNe tube). But such lasers are not common and are definitely very expensive. So, you won't likely see one for sale at your local hamfest - if ever! One manufacturer of such lasers is Research Electro-Optics (REO). See the section: Research Electro-Optics's Tunable HeNe Lasers.

    However, occasionally a HeNe tube turns up that is 'defective' due to incorrect mirror reflectivities or excessive gain or magic :) and actually outputs an adjacent color in addition to what it was designed to produce. I have such a tube that generates about 3 mW of yellow (594.1 nm) and a fraction of a mW of orange (611.9 nm) but isn't very stable - power fluctuates greatly as it warms up. Another one even produces the other orange line at 611.9 nm, and it's fairly stable. But, finding magic 'defective' tubes such as these by accident is extremely unlikely though I've heard of the 640.1 nm (deep red) line showing up on some supposedly good normal red (632.8 nm) HeNe tubes.

    As a side note: It is strange to see the more or less normal red-orange glow in a green HeNe laser tube but have a green beam emerging. A diffraction grating or prism really shows all the lines that are in the glow discharge. Red through orange, yellow and green, even several blue lines (though they are from the helium and can't lase under any circumstances)!! The IR lines are present as well - you just cannot see them.

    See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.

    Actually, the color of the discharge may be subtly different for non-red HeNe tubes due to modified gas fill and pressure. For example, the discharge of green HeNe tubes may appear more pink compared to red tubes) which are more orange), mostly due to lower fill pressure. The fill mix and pressure on green HeNes is a tricky compromise among several objectives that conflict to some extent including lifetime, stability (3.39 um competition), and optical noise. This balancing act and the lower fill pressure are why green HeNes don't last as long as reds. Have I totally confused you, color-wise? :)

    The expected life of 'other color' HeNe tubes is generally much shorter than for normal red tubes. This is something that isn't widely advertised for obvious reasons. Whereas red HeNe tubes are overfilled initially (which reduces power output) and they actually improve with use to some extent as gas pressure goes down, this luxury isn't available with the low gain wavelengths - especially green - everything needs to be optimal for decent performance.

    The discharge in IR HeNe tubes may be more orange and brighter due to a higher fill pressure. Again, this is due to the need to optimize parameters for the specific wavelength.

    Determining HeNe Laser Color from the Appearance of the Mirrors

    Although most HeNe lasers are the common red (632.8 nm) variety (whose beam actually appears orange-red), you may come across unmarked HeNe tubes and just have to know what color output the produce without being near a HeNe laser power supply.

    Since the mirrors used in all HeNe lasers are dielectric - functioning as a result of interference - they have high reflectivity only around the laser wavelength and actually transmit light quite well as the wavelength moves away from this peak. By transmitted light, the appearance will tend to be a color which is the complement of the laser's output - e.g., cyan or blue-green for a red tube, pink or magenta for a green tube, blue or violet for a yellow tube. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered up!

    The actual appearance may also depend on the particular manufacturer and model as well as the length/power output of the laser (which affects the required reflectivity of the OC), as well as the revision number of your eyeballs. :) So, there could be considerable variation in actual perceived color. Except for the blue-green/magenta combination which pretty much guarantees a green output HeNe tube, more subtle differences in color may not indicate anything beyond manufacturing tolerances.

    The chart below in conjunction with Appearance of HeNe Laser Mirrors will help to ideentify your unmarked HeNe tube. (For accurate rendition of the graphic, your display should be set up for 24 bit color and your monitor should be adjusted for proper color balance.)

          HeNe Laser          High Reflector (HR)          Output Coupler (OC)
       Color  Wavelength   Reflection   Transmission    Reflection   Transmission
        Red    632.8 nm    Gold/Copper      Blue        Gold/Yellow   Blue/Green
       Orange  611.9 nm   Whitish-Gold      Blue       Metallic Green   Magenta
       Yellow  594.1 nm   Whitish-Gold      Blue       Metallic Green   Magenta
       Green   543.5 nm   Metallic Blue  Red/Orange    Metallic Green   Magenta
       Broadband (ROY)    Whitish-Gold      Blue
        IR     1,523 nm    Light Green  Light Magenta   Light Green  Light Magenta
        IR     3,391 nm       Gold (Metal) Coated         Neutral        Clear
    The entry labeled 'Broadband' relates to the HR mirror in some unusual multiple color (combinations of red and/or orange and/or yellow) internal mirror tubes as well as those with an internal HR and Brewster window for external OC optics. And, the yellow and orange tubes may actually use broad band HRs. The OCs would then be selected for the desired wavelength(s) and may also have a broad band coating.

    As noted, depending on laser tube length/output power, manufacturer, and model, the appearance of the mirrors can actually vary quite a bit but this should be a starting point at least. For example, I have a Melles Griot 05-LHR-170 HeNe laser tube that should be 594.1 nm (yellow) but actually outputs some 604.6 nm (orange) as well. It's mirror colors for the HR and OC are almost exactly opposite of those I have shown for the yellow and orange tubes! I don't know whether this was intentional or part of the problem And, while from this limited sample, it looks like the OCs for orange, yellow, and green HeNe lasers appear similar, I doubt that they really are in the area that counts - reflectivity/transmission at the relevant wavelengths.

    I do not have any data for the 1,152 nm (IR) HeNe laser wavelength. If you have access to a 1,152 nm or any other non-red HeNe tube and would like to contribute or comment on their mirror colors (or anything else), please send me mail via the Sci.Electronics.Repair FAQ Email Links Page.

    More on Other Color HeNe Lasers

    Here are some comments on the difficulty of obtaining useful visible output from HeNe lasers at wavelengths other than our friendly red (632.8 nm):

    (From: Steve Roberts (

    You do need a isotope change in the gases for green, and a He:Ne ratio change for the other orange and yellow lines. In addition, the mirrors to go to another line will have a much lower output transmission. The only possible lines you'll get on a large frame HeNe laser will be the 611.9 nm orange and 594.1 nm yellow. The green requires external mirror tubes in excess of a meter and a half long and a Littrow prism to overcome the Brewster losses and suppress the IR.

    The original work on green was done by Rigden and Wright. The short tubes have lower losses because they have no Brewsters and thus can concentrate on tuning the coatings to 99.9999% reflectivity and maximum IR transmission. There is one tunable low power unit on the market that does 6 lines or so, but only 1 line at a time, and the $6,000 cost is kind of prohibitive for a few milliwatts of red and fractional milliwatt powers on the other lines. But, it will do green and has the coatings on the back side of the prism to kill the losses.

    Also look for papers by Erkins and Lee. They are the fellows who did the green and yellow for Melles Griot and they published one with the energy states as part of a poster session at some conference. Melles used to hand it out, that's how I had a copy, recently thrown away.

    Even large HeNe lasers such as the SP-125 (rated at 50 mW of red) will only do about 20 mW of yellow, with a 35 mW SP-127 you're probably only looking at 3 to 5 mW of yellow. And, for much less then the cost of the custom optics to do a conversion, you can get two or three 4 to 5 mW yellow heads from Melles Griot. I know for a fact that a SP-127 only does about 3 mW of 611.9 with a external prism and a remoted cavity mirror, when it does 32 mW of 632.8 nm.

    So in the end, unless you have a research use for a special line, it's cheaper to dig up a head already made for the line you seek, unless you have your own optics coating lab that can fabricate state-of-the-art mirrors.

    I have some experience in this, as I spent months looking for a source of the optics below $3,000.

    (From: Sam.)

    I do have a short (265 mm) one-Brewster HeNe tube (Melles Griot 05-LGB-580) with its internal HR optimized for green that operates happily with a matching external green HR mirror (resulting in a nice amount of circulating power) but probably not with anything having much lower reflectivity to get a useful output beam. In fact, I could not get reliable operation even with the HR from a dead green HeNe laser tube as the Brewster window would not remain clean enough for the time required to align the mirror. See the section: A Green One-Brewster HeNe Laser for more info.

    I would expect an SP-127 to do more than 3 to 5 mW of yellow, my guess would be 10 to 15 mW with optimized mirrors but no tuning prism. If I can dig up appropriate mirrors, I intend to try modifying an SP-127 to make it tunable and/or do yellow or green. :)

    (From: Lynn Strickland (

    You can find 640.1 nm in a lot of red HeNe lasers. I have a paper on it somewhere, and cavity design can influence it to a large extent. If you have a decent quality grating, it's pretty easy to pick up. 629 nm is the one you don't see too much.

    I'm no physicist, but the lower gain lines can lase simultaneously with the higher gain lines, no problem, as long as there is sufficient gain available in the plasma. It's really pretty easy to get a HeNe laser to output on all lines at the same time (if you have the right mirrors). The trick is optimizing the bore-to-mode ratio, gas pressure, and isotope mixture to get good TEM00 power. Usually the all-lines HeNe lasers are multi (transverse) mode. I don't know of anyone who makes them commercially though - at least not intentionally.

    Steve's Comments on Superradiance and the 3.39 um HeNe Laser

    Generally, when a gas laser is superradiant, there is a limit to its maximum power output (with exceptions for nitrogen and copper vapor laser, although nitrogen's upper limit is defined by the maximum cavity length into which you can generate a 300 ns or less excitation pulse.

    The 3.39 um HeNe laser's gain is still, like all other HeNe lines limited by a wall collision to return the excited atoms to the ground state. 3.39 um HeNe lasers have larger bores then normal HeNe lasers, and the bores are acid etched to fog them and create more surface area, but still the most power I've ever seen published was 40 mW - nothing to write home about. The massive SP-125, the largest commercial HeNe laser, could be ordered with a special tube and special optics for 3.39 um, and it still only did about 1/3rd the visible power. Superradiance and ultimate power are not tied together.

    The reason 3.39 um got all the writeups it did was that it started on the same upper state as all the other HeNe lines, was easily noticed when it sapped power from the visible line, and was, at the time, a exotic wavelength for which there were few other sources.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Melles Griot, Spectra-Physics, and Other HeNe Lasers

    For current production lasers, the manufacturers' Web sites often provide basic specifications. For older lasers, it's often difficult to obtain detailed specs so estimates based on physical size, and then testing may be the only option.

    Some Melles Griot HeNe Laser Tubes

    The following data came from a variety of sources including an old Melles Griot brochure, their 1999 catalog, and their HeNe Laser Specifications Page.

    All Melles Griot HeNe laser tubes are hard-sealed with essentially unlimited shelf life - 12 years is quoted but for all practical purposes, it is infinite. Most standard tubes have a planar HR mirror with a concave OC mirror with its curvature selected for maximum stability. This long radius hemispherical cavity configuration puts the beam waist at the HR with a slightly diverging beam from the OC. But a compensating curvature on the outer surface of the OC mirror results in a positive lens and the beam that exits the laser is quite well collimated.

    Melles Griot Red HeNe Laser Tubes

    The following HeNe tubes have been listed in various Melles Griot catalogs. These are all red (632.8 nm) tubes operating single longitudinal mode (TEM00) except for models 185 and 981 which are multimode (and thus produce much higher power for their size). Many are actually sold as part of complete laser heads (the model shown is actually for the laser head if it is an odd number, physical specs are for just the tube).

    The following data came from a variety of sources including an old Melles Griot brochure, the 1999 catalog, and the Melles Griot Web site. Go to "Product Info", "Lasers", "HeNe" or more directly to Melles Griot Lasers, "HeNe". Then click on any location.

    Red (632.8 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LHR-
       .5 mW  .47 mm  1.70 mR  1200 MHz  1.29/5  kV  3.3 mA   19/135 mm    002(1)
       .5 mW  .49 mm  1.70 mR  1040 MHz  1.25/5  kV  4.5 mA   29/152 mm    700
       .5 mW  .46 mm  1.77 mR  1063 MHz  1.32/5  kV  4.0 mA   25/150 mm    213
       .8 mW  .46 mm  1.77 mR  1063 MHz  1.32/5  kV  4.0 mA   25/150 mm    211
      1.0 mW  .53 mm  1.50 mR   883 MHz  1.47/8  kV  4.5 mA   29/178 mm    900
      1.0 mW  .59 mm  1.35 mR   687 MHz  1.79/8  kV  6.5 mA   37/226 mm    111
      2.0 mW  .59 mm  1.35 mR   687 MHz  1.79/10 kV  6.5 mA   37/228 mm    121
      2.0 mW  .72 mm  1.10 mR   612 MHz  1.85/10 kV  6.5 mA   29/255 mm    080
      2.0 mW  .76 mm  1.06 mR   636 MHz  1.71/10 kV  5.0 mA   30/250 mm    073
      2.5 mW  .52 mm  1.53 mR   822 MHz  1.77/10 kV  4.5 mA   25/198 mm    691
      4.0 mW  .80 mm  1.00 mR   435 MHz  2.35/10 kV  6.5 mA   37/353 mm    140
      5.0 mW  .80 mm  1.00 mR   438 MHz  2.29/10 kV  6.5 mA   37/353 mm    151
       7 mW  1.02 mm   .79 mR   373 MHz  2.65/10 kV  6.5 mA   37/410 mm    171
      10 mW   .65 mm  1.24 mR   341 MHz  2.64/10 kV  6.5 mA   37/440 mm    991
      12 mW  1.20 mm  3.40 mR    NA-MM   2.09/10 kV  6.5 mA   37/350 mm    185(2)
      16 mW  1.47 mm  1.40 mR    NA-MM   2.48/10 kV  7.0 mA   37/464 mm    981(2)
      17 mW   .96 mm   .83 mR   267 MHz  3.70/12 kV  7.0 mA   37/600 mm    925
      25 mW  1.23 mm   .66 mR   165 MHz  5.10/15 kV  8.0 mA   42/930 mm    827
      25 mW  1.42 mm  2.40 mR    NA-MM   3.20/10 kV  7.0 mA   42/590 mm    831
      35 mW  1.23 mm   .66 mR   165 MHz  5.10/15 kV  8.0 mA   42/930 mm    927


    1. Model 05-LHR-002-246 is a cute little tube that was found in a hand-held bar code scanner (Metrologic model MH290). There is also a model 05-LHR-001, even smaller being about 12 mm x 100 mm, which has a very short life and is only rated for intermittent operation, possibly due to its very small cathode area.

    2. Models 05-LHR-185 and 05-LHR-981 operate with multiple transverse modes (not TEM00) and therefore, they don't have a mode spacing specifications. They output greater power for their size compared to TEM00 tubes and may be used for applications like light shows but are unsuitable for holography and interferometry.

    The operating voltage across the tube itself can be found by subtracting the voltage drop across the ballast resistor (I*Rb), from the value listed in the table. Actual starting voltages are typically 3 to 5 times the tube operating voltage (though the specifications may be higher).

    Both random and linearly polarized models are available (change the "LHR" to "LHP" for most of those listed above). The only other difference in specifications for red HeNe lasers between these is the price (other-color polarized HeNe lasers tend to have lower output than their similar model randomly polarized counterparts). The price for a complete linearly polarized laser head was 10 to 15 percent higher in a catalog I have so you can imagine how much more the tube itself costs since that price differential is virtually all in the tube (at least in terms of manufacturing cost)!

    And speaking of prices, if you have to ask, you can't afford a new HeNe laser! But since you asked, prices (Summer 2002) from Melles Griot vary from around $300 for a 0.5 mW laser head to over $4,000 for one rated at 35 mW (power supply sold separately)! Fortunately, surplus prices tend to be much more reasonable - typically between 5 and 20 percent of these depending on actual age and condition as well as many other factors including your luck in finding a good deal. :)

    Melles Griot Other Color HeNe Laser Tubes

    The following "other-color" HeNe tubes have been listed in various Melles Griot catalogs. These are all single mode (TEM00), random polarized, unless otherwise noted. Linearly polarized versions are also available (e.g., yellow would be LYP instead of LYR for those models where this option is available). However, due to the lower gain of most of the "other color" lasing lines, linearly polarized tubes for these wavelengths have much lower output power than physically similar randomly polarized models. (For red tubes, the power outputs are usually identical.) Also note that the output power of these tubes in general is much much less than that of the red (632.8 nm) models with similar (in some cases identical) dimensions.

    Green (543.5 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LGR-
       .2 mW  .63 mm  1.26 mR   732 MHz  1.56/8  kV  4.5 mA   29/215 mm    025
       .5 mW  .80 mm  1.01 mR   438 MHz  2.39/10 kV  6.5 mA   37/351 mm    151
       .8 mW  .89 mm   .92 mR   373 MHz  2.62/10 kV  6.5 mA   37/410 mm    173
      1.0 mW  1.3 mm  1.00 mR    NA-MM   1.87/10 kV  6.5 mA   37/351 mm    161
      1.5 mW  .86 mm   .81 mR   328 MHz  2.75/10 kV  6.5 mA   37/475 mm    193
      2.0 mW  .86 mm   .81 mR   328 MHz  2.75/10 kV  6.5 mA   37/475 mm    393

    Yellow (594.1 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LYR-
      .35 mW  .63 mm  1.26 mR   732 MHz  1.62/8  kV  4.5 mA   29/215 mm    025
      .75 mW  .80 mm  1.01 mR   438 MHz  2.43/10 kV  6.5 mA   37/351 mm    151
      2.0 mW  .75 mm   .92 mR   373 MHz  2.59/10 kV  6.5 mA   37/410 mm    173
      2.0 mW  1.17 mm 1.00 mR    NA-MM   2.09/10 kV  6.5 mA   37/351 mm    161

    Orange (611.9 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LOR-
       .5 mW  .63 mm  1.26 mR   732 MHz  1.66/8  kV  4.5 mA   29/215 mm    025
      2.0 mW  .80 mm  1.01 mR   438 MHz  2.49/10 kV  6.5 mA   37/351 mm    151
      4.0 mW  1.17 mm 1.00 mR    NA-MM   2.07/10 kV  6.5 mA   37/351 mm    161

    Infra-Red (1,523 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LIR-
      1.0 mW 1.26 mm  1.59 mR   438 MHz  2.49/10 kV  6.5 mA   37/351 mm    151
      1.0 mW 1.33 mm  1.48 mR   373 MHz  2.97/10 kV  6.0 mA   37/410 mm    171

    Infra-Red (3,391 nm):

      Minimum  e/2               c/2L      Supply    Nominal
      Output   Beam    Diver-    Mode     Opr/Strt    Tube    Tube Size   Model
      Power    Diam    gence    Spacing   (Rb=75K)   Current  Diam/Lgth  05-LFR-
      1.0 mW  .83 mm  1.60 mR   438 MHz  2.50/10 kV  6.0 mA   37/351 mm    151

    Note: Some of the listed values for divergence in particular appear to be questionable. For example, for the same beam diameter, diffraction limited divergence should be proportional to wavelength. The discrepency for the 3,391 nm IR tube is particularly striking. Either the divergence or beam diameter are almost certainly incorrect. It probably doesn't matter much though because the 3,391 nm model is no longer manufactured.

    Melles Griot Brewster and Zero Degree Window HeNe Tubes

    Here are abbreviated specs for some Melles Griot one and two-Brewster window HeNe tubes and one and two 0 degree (perpendicular?) window HeNe tubes. The Brewster angle is probably optimal for 632.8 nm. Window tubes may be used where external adjustment of the polarization is required and for educational purposes where either a linearly or random polarized beam is desired in an external mirror laser. Power outputs below are for matched optics. Your mileage may vary using mirrors from your junk box!

    Brewster angle window HeNe tubes:

          Minimum   Supply    Supply   Nominal                      Number
          Output    Voltage   Voltage   Tube    Tube Size   Model     of
          Power    Tube Only  Rb=68K   Current  Diam/Lgth  05-LHB-  Windows 
          1.0 mW    1,430 V   1,870 V   6.5 mA  37/222 mm    270       1
          1.0 mW    1,460 V   1,900 V   6.5 mA  37/253 mm    290       2
          3.5 mW    1,080 V   1,520 V   6.5 mA  37/265 mm    370       1
          4.0 mW    1,030 V   1,470 V   6.5 mA  37/265 mm    570       1
          ??? mW    1,??? V   1,??? V   6.5 mA  37/265 mm    580       1
          6.0 mW    1,430 V   1,870 V   6.5 mA  37/351 mm    670       1

    The most common application for one-Brewster HeNe tubes was probably for particle counting since by using an external high quality HR mirror, the intracavity flux can be several watts which makes a speck of anything stand out! (Some large one-Brewster HeNe tubes can do as much as 100 W intracavity, not these!). Passing the air/gas/whatever flow through the cavity of a one-Brewster HeNe laser is similar to passing it through the output beam of a high power laser - at a fraction of the cost (and it's much safer as well since if anything macroscopic in size (like an eyeball or piece of paper) were to block the intracavity beam, lasing simply stops with no damage to vision and no risk of fire!

    The LHB models all have HR mirrors that are probably optimal for 632.8 nm (red) though newer versions, at least, may be quite broadband and better than 99.9 percent from 590 to 680 nm so operation at some of the non-632.8 nm wavelengths may be possible. However, older versions may not have such nice HRs.

    Other variations on these tubes are also produced (though they may be special order). I was given an 05-LGB-580 which has an HR optimized for 543.5 nm (green). With an external green HR, the behavior is very similar to the red version but with loads of circulating green photons instead of red ones. :) I was told that this tube may have been made for the sole purpose of confirming the quality of the mirrors to be used in normal internal mirror HeNe laser tubes. So, I doubt you could buy 1. Maybe 1,000, but not just 1! Applications for such a tube would be very limited due to the low gain as it stops lasing entirely in a few minutes after cleaning the optics just due to dust settling on the B-window.

    The 05-LHB-370, 05-LHB-570, 05-LHB-670, and 05-LHB-580 have wide bores and generally operate with multiple transverse modes to achieve maximum intracavity power in particle counting applications. The 05-LHB-270 and 05-LHB-290 have narrow bores like most conventional HeNe tubes. (The 05-LHB-270 appears physically similar to an 05-LHR-120 except for the Brewster window at one end.) The model 05-LHB-570 is the one-Brewster HeNe tube used in the CLIMET 9048 one-Brewster laser head described in the section: A One-Brewster HeNe Laser Tube. You can't tell from the model numbers but both Melles Griot and Hughes style designs may be used. For example, the 05-LHB-570 looks like a normal Melles Griot tube but with a Brewster angle window frit sealed to the metal end-cap instead of an OC mirror. The 05-LHB-580 looks like a Hughes style tube, but with an optically contacted Brewster window instead of an OC mirror (though some Hughes style polarized HeNe tubes are just one-Brewster tubes with an OC mirror attached to a glass tube that slips over the Brewster stem and is itself glued in place). Thus, the 05-LHB-580 is actually a much higher quality (and more expensive) tube than the 05-LHB-570 but you can't tell this from the catalog listing! Here are diagrams of each type:

    One possible explanation of why the Hughes style design is used for the high quality tubes with optically contacted Brewster windows is that since Hughes already produced HeNe tubes with a glass Brewster stem (as noted above), when Melles Griot took over the Hughes HeNe laser product line, making the modifications for the graded seal to accommodate the fused silica Brewster stem (needed to match the expansion coefficient of the fused silica window) was probably easier than starting with a metal end-cap.

    Zero degree AR coated window HeNe tubes:

          Minimum   Supply    Supply   Nominal                      Number
          Output    Voltage   Voltage   Tube    Tube Size   Model     of
          Power    Tube Only  Rb=68K   Current  Diam/Lgth  05-WHR-  Windows 
          4.0 mW    1,030 V   1,470 V   6.5 mA  37/269 mm    570       1
          6.0 mW    1,670 V   2,110 V   6.5 mA  37/351 mm    252       2
          8.0 mW    1,670 V   2,110 V   6.5 mA  37/351 mm    183       1
    Rather than mirrors, one or both ends of these HeNe tubes have optical flats with very high quality AR coatings to permit the use of external mirrors. One advantage of this arrangement is that external optics can be used to control polarization (the output beam of Brewster tubes is always linearly polarized and can't be changed).

    The 05-WHR-252 and 05-WHR-183 appear to be identical except for the number of windows - and the loss of 2 mW with the two window version!

    Some Siemens/Lasos HeNe Lasers

    Here are the HeNe lasers currently available from LASOS Laser-Fertigung GmbH. Many of these were originally manufactured by Siemens, since acquired by LASOS. I have listed them all with LGR prefixes though this really is reserved for bare tubes; LGK denotes a complete laser head or "laser module" as LASOS calls them. Red, green, yellow, and orange tubes all use the same prefix. (Some of these, mostly the larger ones, are actually only available as a complete laser head even though I've listed the LGR model number.)

    Legend for Type: SM=Single mode, TEM00; MM=Multimode, P=Linearly polarized.

    Red (632.8 nm):

         Power      Type     Model
         0.5 mW     SM      LGR-7656
         0.5 mW     SM P    LGR-7650
         0.6 mW     SM      LGR-7655
      0.75-1.0 mW   SM      LGR-7657
       0.8-1.4 mW   SM      LGR-7655-N
         1.0 mW     SM      LGR-7655-S
         1.0 mW     SM      LGR-7641-S
         2.0 mW     SM      LGR-7672
         2.0 mW     SM      LGR-7621-S
         2.0 mW     SM P    LGR-7634
       2.2-3.2 mW   SM P    LGR-7634
         5.0 mW     SM      LGR-7627
         5.0 mW     SM P    LGR-7628
         5.0 mW     MM      LGR-7621-MM
         5.2 mW     SM P    LGR-7628-1
       5.5-7.5 mW   SM P    LGR-7628-L
         7.0 mW     SM      LGR-7627-M
        10.0 mW     SM      LGR-7653-7
        10.0 mW     SM P    LGR-7654-7
        10.0 mW     MM      LGR-7627-MM
        15.0 mW     SM      LGR-7665
        15.0 mW     SM P    LGR-7665-P
        18.0 mW     SM      LGR-7665-18
        18.0 mW     SM P    LGR-7665-P18
        20.0 mW     SM      LGR-7665-20
        20.0 mW     MM      LGR-7658-7
        25.0 mW     SM P    LGR-7626-L
        25.0 mW     SM P    LGR-7626
        25.0 mW     SM P    LGR-7676-L
        28.0 mW     SM P    LGR-7676
        30.0 mW     SM P    LGR-7626-S
        30.0 mW     SM P    LGR-7676-S

    Green (543.5 nm):

         Power      Type     Model
         0.5  mW    SM      LGR-7770
         0.5  mW    SM P    LGR-7774
         0.5  mW    SM P    LGR-7786-P50
         0.75 mW    SM P    LGR-7786-P575
         1.0  mW    SM      LGR-7785-P100
         1.0  mW    SM      LGR-7770-S
         1.05 mW    SM P    LGR-7786-P
         1.5  mW    SM      LGR-7785-P150
         2.0  mW    SM      LGR-7785-P200
         2.5  mW    SM      LGR-7785-P250

    Yellow (594.1 nm):

         Power      Type     Model
         1.5  mW    SM      LGR-7511

    Orange (611.9 nm):

         Power      Type     Model
         2.0 mW     SM      LGR-7411

    More complete specifications are available at the LASOS Web site.

    Siemens LGK-7676 HeNe Laser Head

    This is among the largest HeNe lasers in current production. Its overall specifications are similar to that of the SP-107/127/907 though the details of its construction differ. Information can be found at Lasos's Helium-Neon Laser Modules Page. There are a number of versions of this laser with different output powers but they are all mechanically similar (probably a production sort in at least some cases). Photos of a packaged the LGK-7676 can be found in the Laser Equipment Gallery under "Siemens High Power Helium-Neon Laser".

    The LGK-7676 resonator consists of 4 full length 5/8 inch diameter rods joined by 10 thick plates. The tube is secured to these plates using sets of 4 screws with padded tips going in from all four sides at most locations. Some sets are adjustable for bore centering and optimizing straightness. The end-plates hold the mirror mounts. Coarse mirror adjustment is via some thinner rods attached to the ends of the main support rods, with pairs of nuts but no springs. This should permit the mirror mounts to be removed and replaced for cleaning of the optics without requiring coarse realignment. Fine mirror alignment uses Allen's head screws to press on rods which slightly warp the aluminum to which the actual mirror cell is attached. It works reasonably well with good sensitivity and repeatability except that the two adjustments at each end aren't quite independent. Note that with this scheme, walking of the mirrors requires turning the screws at the two ends in opposite directions. The fine adjustments are similar to those in the SP-907 but the coarse adjustments for that laser are three spring loaded nuts which means that removing the mirror mounts requires complete realignment.

    The entire resonator assembly is mounted on a thick piece of machined L-shaped aluminum fastened with screws at only two locations. However, under about half the thick plates (see above) on both sides of the "L" are adjustment screws to provide some sort of additional support.

    The LGK-7676 uses a coaxial tube with about half of its bore exposed (as opposed to the side-arm tube with totally exposed bore used in Spectra-Physics lasers). While this does result in a more compact package (overall dimensions under 3"x3"x39"), there is less space for IR suppression magnets. In fact, the LGK-7676 only has two sets of magnets (in proximity to less than 25 percent of the bore) for this purpose but could definitely use more. Adding moderate strength magnets (greater than refrigerator strength but much less than rare-earth disk drive strength) almost anywhere along the bore - even outside the large gas reservoir - resulted in a noticeable increase in output power - about 1 percent for a single magnet. I would guess that with enough magnets, a 10 to 20 percent boost would be possible.

    There are both anode and cathode ballast resistors of 81K and 27K, respectively. The power supply connector has 3 pins - anode, cathode, and Earth ground.

    The sample I tested is an LGK-7676S with a spec'd output power of at least 30 mW. Of course, since I like to spend as little as possible to acquire these things, mine is a high mileage tube which apparently served hard duty in some sort of high speed printer since there was toner all over it. These are turning up on eBay (and possibly from surplus outfits directly), probably being replaced when their output power drops below a certain value by tiny diode lasers. :)

    I was able to run it on my SP-255 exciter only by reducing the anode ballast resistor to 60K and removing the cathode ballast resistor entirely. Prior to this surgery, even with the input voltage to the SP-255 at 140 VAC (the upper limit of my Variac), it would only run for a minute or two (and only if it felt like it) and then cut out, not to restart for several minutes. With the modifications, it will now run all day at 120 VAC input, though restarting was sometimes still a bit of a problem until I added circuitry in an external pod to the SP-255 to boost its starting voltage. See the section: Enhancements to SP-255.) The SP-207 should be able to drive this laser without problems but I don't happen to have one of those.

    After bore straightening and mirror adjustments, I was able to squeeze more than 19 mW out of the laser at a somewhat reduced operating current (10 mA instead of the spec'd 11.5 +/- 0.5 mA). (I'm using the lower current only so I can look forward to increased power by a simple tweak in the future.) It would exceed 20 mW if when fully warmed up, the laser was shut off for 30 seconds and then restarted. But the output power would drop back to its previous level over the course of a minute or so once the "good" gas had time to migrate back out of the bore or something. :)

    Some Spectra-Physics HeNe Lasers

    Here is a chart of some older Spectra-Physics HeNe lasers. Most of these are from a 1988 catalog (along with 1988 prices). Not all information was available, thus the "???" in places. You can go to the Spectra-Physics Web site for current models but the hobbyist and experimenter is much more likely to acquire the classic ones below (unless very well endowed!). Typical output power when new may have been 50 percent or more greater than the value listed.
      Laser    Wave-  Mirrors  Output   Exciter  Original
      Model   length  Int/Ext   Power    Model    Price    Description/Comments
      107    632.8 nm    E      30 mW?    207    $  ?,???  Similar to 127 (1)
      115    632.8 nm    E       5 mW?    200    $  ?,???  RF excited, 24" resntr.
      116    632.8 nm    E       5 mW?    200?   $  ?,???    "     "  tuning prism
      120    632.8 nm    E       6 mW     256    $  1,980  Small lab laser (2)
        -01  1,152 nm    E       1 mW      "     $  2,800    "         "
        -02  3,391 nm    E     1.25 mW     "     $  2,800    "         "
      122    632.8 nm    E       5 mW?    253A   $  ?,???  Short version of 124 (3)
      123    632.8 nm    E      10 mW?     I     $  ?,???  Between 120 and 124
      124B   632.8 nm    E      15 mW     255    $  4,900  Popular lab laser (3)
        -01  1,152 nm    E       2 mW      "     $  5,500    "         "
        -02  3,391 nm    E       5 mW      "     $  5,500    "         "
      125A   632.8 nm    E      50 mW     261A   $ 16,000  Huge-head >125 lbs. (4)
        -01  1,152 nm    E      10 mW      "     $ 17,500   more than 6 feet long.
        -02  3,391 nm    E      10 mW      "     $ 17,500    "         "
      127    632.8 nm    E      35 mW      I     $ ??,???  39 inch resonator (1)
      130B   632.8 nm    E     1.5 mW      I     $  ?,???  Self contained (5)
      132    632.8 nm    I     1.00 mW     I     $    ???  Self contained (6)
      133    632.8 nm    I     2.00 mW    233    $    ???  Separate rect. head (5)
      134    632.8 nm    I       3 mW?     I     $    ???  Self contained
      135    632.8 nm    I     ??? mW     ???    $    ???  Separate rect. head (5)
      138    632.8 nm    I     ??? mW     ???    $    ???  Separate cyl. head
      142    632.8 nm    I       4 mW     248    $    ???  Separate rect. head (5)
      143    632.8 nm    I       5 mW?    ???    $    ???
      147    632.8 nm    I       8 mW     247?   $    ???  Separate cyl. head
      155    632.8 nm    I     0.5 mW      I     $    310  Educational laser (6)
      156    632.8 nm    I     ??? mW      I     $    ???    "        "
      157    632.8 nm    I       3 mW      I     $    525  Self contained
      159    632.8 nm    I       5 mW      I     $    630    "        "
      102R   632.8 nm    I       2 mW     212    $    610  Cyl. head, rand. pol.
      102P   632.8 nm    I     1.5 mW      "     $    ???  Cyl. head, lin. pol.
      105R   632.8 nm    I       5 mW     215    $    ???  Cyl. head, rand. pol.
      105P   632.8 nm    I       5 mW      "     $    ???  Cyl. head, lin. pol.
      117A   632.8 nm    I       1 mW     117A   $  3,500  Stabilized (7)
      118A   632.8 nm    I       1 mW     118A   $  ?,???    "    "    "
      119A   632.8 nm    I     0.2 mW     ???    $  ?,???    "    "

    1. The SP-107 (and the apparently similar SP-907) are versions of the SP-127 intended for OEM applications and configured in an open resonator (without covers). As far as I know, these are the only large-frame SP lasers still in production. They are probably physically very much like 'stretch' versions of the SP-124B. The original SP-207 exciter was similar to the SP-255 but its linear regulator appears to have a larger compliance range (with more pass transistors). The newer SP-207A/A-1/B power supplies are switchmode based potted bricks.

    2. Several other exciters may be used with the SP-120 including the SP-247 and SP-249.

    3. In addition to the SP-124B, there were also SP-124 and SP-124A lasers. The SP-124 may have used the SP-253A exciter rather than the SP-255. However, the SP-122, a much smaller laser but similar in construction to the SP-124, appears to have used the SP-253A as well. The SP-253A exciter uses a switchmode power supply which sends 225, 300, or 375 VAC (pin selectable or by internal taps depending on specific model) to a potted voltage multiplier/start module in the laser head.

    4. There is also an SP-250 exciter for the SP-125 laser which is the RF power supply option. It appears to include an SP-200 RF exciter and other stuff in a larger box.

    5. The SP-135 is physically identical to the SP-133 so there must be a difference in power output or other beam characteristics. The power supplies for the SP-130B, SP-133, and SP-135 appear to be of similar construction though the one for the SP-130B is internal and the others are separate exciters. They all use a power transformer feeding a totally potted high voltage section but the SP-130B PSU also has a primary side pot to adjust tube current. With the potted HV section, they are for the most part, non-repairable and non-modifiable. It is likely that some other small SP lasers of this period probably had their power supplies constructed like this as well. The power supply for the SP-133, the SP-233, is capable of driving a 2 to 3 mW HeNe laser tube; that of the SP-130 can drive a 5 mW tube. Modern power supplies for their larger lasers are fully potted 'bricks' but the older exciters like the SP-255 and most others had wide open discrete circuitry.

    6. Some versions of the SP-155 and SP-132 are quite similar but I've seen at least one SP-132 with a much more spiffy enclosure (looks more like the SP-120 and larger lasers). The SP-132 is also slightly more powerful being rated 1.0 mW minimum while the SP-155 is rated 0.95 mW maximum (nominal power probably about 0.5 mW). The SP-156 looks very similar to the SP-155 but may be slightly higher power - perhaps the low cost version of the SP-132.

    7. The SP-117A is a frequency or amplitude stabilized laser consisting of a cylindrical head and mating power supply/controller. Its nominal frequency is 473.61254 THz. I don't know the SP-117A and SP-118A differ. See the section: Description of the SP-117A Laser.

    For more details on the popular large-frame Spectra-Physics HeNe lasers, see the next section.

    Spectra-Physics 120, 124, and 125 HeNe Laser Specifications

    Here are the nearly complete specifications for several high quality scientific/research type HeNe lasers which have resonators with external mirrors. The SP-125, in particular, is definitely a BIG laser - size-wise at laest - with a resonator length of over 1-3/4 METERS! These are the type of laser that might turn up in the long forgotten and dusty storage room of a university physics/optics or industrial research lab. Both of the larger models permit the mirror assemblies to be replaced (or at least as an option at the time of purchase) to select the output 'color' - normal HeNe red (632.8 nm), near IR (1152.3 nm), and medium IR (3391.3 nm). (It's possible that magnets may also need to be added/removed depending on the desired wavelength though this is not mentioned in the spec sheets. For an explanation, see the section: Magnets in High Power or Precision HeNe Laser Heads.)

    It should be possible to possible to obtain orange (611.9 nm), yellow (593.9 nm), and green (543.5 nm) output with similar modifications (at least for the longer lasers), though the gain of these lines is only a fraction of that for the red or IR lines (1152.3 nm and 3391.3 nm) so output power will be lower.

    Some photos of these lasers can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".

    Note: The specifications for the SP-124A and SP-125, below, were copied from an almost illegible scan of a fax of a copy of the original product brochure. Corrections are welcome! The specifications for the SP-120 were copied from an original user manual which, however, didn't list the tube operating voltage or current, so these values were sort of, well, guessed. :)

     Spectra-Physics Laser:      SP-120        SP-124B              SP-125A
       Wavelength (nm):          632.8   632.8 1152.3 3391.3   632.8 1152.3 3391.3
       Minimum Power (mW):        5.0     15     2.5   5.0      50     10     10
        Beam Diameter (mm):       .65     1.1   1.4    2.5      1.8   2.4    4.1
        Beam divergence (mR):     1.7     .75   1.0    1.5       .6    .8    1.4
        Transverse Mode:                              TEM00
        Degree of Polarization:                      1000:1
        Angle of Polarization:   Vertical (+/-5 Degrees except SP-120, +/-20 Deg.)
        Resonator Configuration:                  Long Radius
        Resonator Length (cm):     39         70.1                   177.0 
             Mode Spacing:      385 MHz     214 MHz                  85 MHz
        Plasma Excitation:    3.7 kV, 7 mA  5 kV, 15 mA      6 kV at 25 to 35 mA
                                                          (RF Opt: 15 W at 46 MHz)
        Starting Method:         ~8 kV         ~12 kV    Trigger pulse on isolated
                                (Direct from Exciter)      bar adjacent to tube.
        Beam Amplitude Noise:   <.3% RMS      <.3% RMS         <2% RMS (RF: <.5%)
        Beam Amplitude Ripple:  <.5% RMS      <.2% RMS        <.5% RMS (RF: <.6%)
        Long Term Power Drift:              <5% over 8 hours and 10 °C
        Warmup Time:           30 Minutes    30 Minutes             1 Hour
        Operating Temperature:                      10 to 40 °C
        Operating Altitude:            Sea Level to 3,000 m (10,000 ft.)
        Operating Humidity:                       Below Dew Point
        Power Supply:                    115/230 VAC, 50/60 Hz, +/-10%
        Exciter Model (DC):   SP-256 (1)      SP-255 (2)            SP-261A
        Input Power:            50 W           125 W                 456 W
        Laser Head Size:      3.26" (W) x    3.26" (W) x            ??? (W) x
                              3.66" (H) x    3.66" (H) x            ??? (H) x
                             18.48" (L)     32.00" (L)              ??? (L)
        Laser Head Weight:       7.5 lb         25 lb               100 lb
        Power Supply Size:    7.25" (W) x    7.25" (W) x            13" (W) x
                              3.72" (H) x    3.72" (H) x             6" (H) x
                              9.88" (D)      9.88" (D)              18" (D)
        Power Supply Weight:     7.5 lb        7.5 lb                30 lb


    1. Several other exciters may be used with the SP-120 including the SP-247 and SP-249.

    2. Some versions of the SP-124 (possibly those without the 'A' or 'B' suffix) may use an SP-253A exciter instead of the SP-255. These models require a mating boost/start module in laser head. See the section: Spectra-Physics Model 253A Exciter (SP-253A). The SP-122 laser, a short version of the SP-124 (actually shorter than the SP-120 but construction is like the SP-124), may have also used the SP-253A exciter.

    Actual power from these lasers may be much more than their ratings would indicate, especially when new: greater than 35 mW for the SP-124B and up to 200 mW (!!) for the SP-125A. (However, I don't know how likely such 'hot' samples, especially of the SP-125A, really were.)

    There is also a model 127 (OEM versions: SP-107 and SP-907) with the following partial specifications (632.8 nm). Beam diameter: 1.25 mm, divergence 0.66 mrad, length 38.75", height and width: about 4", power requirements: 5 kV, 11.5 mA, starting voltage: 12 kV + 6 kV pulse. This appears to be the only large-frame Spectra-Physics HeNe laser in current production. See the next section.

    Mirror sets for green (543.5 nm), yellow (594.1 nm), and orange (611.9 nm) were available for the longer lasers. (The SP-120 and SP-122 may be too short for the low gain green line.) There were also tunable versions of the SP-125 and possibly others. The SP-116 was a tunable version of the RF excited SP-115. These used a Littrow prism in place of the HR mirror.

    See the following sections for more information on these Spectra-Physics lasers.

    Spectra-Physics 107B HeNe Laser Specifications

    The SP-107 (and the apparently similar SP-907) is the same tube and resonator as is inside the SP-127 and is intended for OEM applications. As far as I know, the SP-107/127 is the only large-frame SP laser still in current production. Newer versions have a hard-seal tube with optically contacted Brewster windows and thus should not have the problems of the older SP lasers that go bad from non-use. Thus, if you're looking for a high power, high quality, HeNe laser that can be transported by one person and won't decay on the shelf, this is the one to get! I've seen like-new samples of these go for much less than $1,000 on eBay (about 1/5th the new price) and under $100 in at least partially working (and possibly restorable) condition.

    The SP-107/907 resonator is over 38 inches long and of the "Stabilite" design similar to that of the SP-122 and SP-124 but the mirror mounts differ. There is an internal L-shaped structure and outer thinner metal skin. There are two versions, differing the design of their mirror mounts:

    In addition to mirror alignment, there are a pair of bore centering brackets about 1/2 and 3/4 of the way relative to the cathode-end of the laser. These have an effect on both output power and beam shape. Carefully tweaking for maximum output power should done in conjunction with mirror alignment.

    The bare resonators have no beam centering adjustments and I don't see any on the packaged SP-127.

    The tube has a side-mounted cathode chamber like other SP lasers but it is quite oversize - about twice the typical diameter. The ballast resistors (2 at the anode-end, 1 at the cathode-end, all 27K ohms) are mounted externally in glass tubes sealed with rubber and heat-shrink tubing. IR suppression magnets are placed at every available location on two sides of the bore. Thin rubber boots seal the space between the Brewster windows and mirrors but these can be pushed back to permit cleaning of the windows and mirrors in-place (barely and not recommended unless the resonator has been previously disassembled as the optics stay quite clean). Some of these lasers include a metal cover and electrical heaters to decrease the warmup time required to achieve rated power and stability. CAUTION: The part of the rubber boots surrounding the tube are easily torn if the boots are removed since they tend to stick to the tube.

    Depending on specific model, the SP-107/127/907 has a minimum output power of 25 or 35 mW but may do much more when new. The following is from a Spectra-Physics datasheet. Only the specs for the red version are shown but any of the other HeNe lasing wavelengths (except possibly 3.391 nm which may require a wider bore tube and removal of the IR suppression magnets) should be possible by substituting appropriate optics. A yellow or green version would be nice. :)

     Spectra-Physics Laser:        SP-107B
        Wavelength (nm):           632.8
        Minimum Power (mW):        25 or 35
        Beam Diameter (mm):        1.25
        Beam divergence (mR):      0.66
        Transverse Mode:           TEM00
        Degree of Polarization:    500:1
        Angle of Polarization:     Horizontal (+/-5 Degrees)
        Resonator Configuration:   Long Radius
        Beam Waist Location:       Outer surface of output mirror
        Resonator Length (cm):     95
        Longitudinal Mode Spacing: 161 MHz
        Type:                      Hard-seal, cathode in side-arm
        Operating Voltage:         5 (+/- 0.4) kV, 11.5 (+/- 0.5) mA
        Starting Voltage:          ~15 kV
        Lifetime:                  Greater than 20,000 hours
        Beam Amplitude Noise:      <1% RMS
        Beam Amplitude Ripple:     <1% RMS
        Warmup Time:               20 Minutes (95% power)
        Operating Temperature:     10 to 50 °C
        Operating Humidity:        5-90% non-condensing
        Power Supply:              SP-207A (110/220 VAC +/- 10%)
                                   SP-207A-1 (100/200 VAC +/- 10%)
                                   SP-207B (90-130 VAC or 180-260 VAC)
        Laser Head Size:           3.7" (W) x 3.7" (H) x 38.75" (L)
        Laser Head Weight:         23 lb
        Power Supply Size:         2.4" (W) x 1.4" (H) x 10" (L)
        Power Supply Weight:       3 lb

    It is possible to run these lasers on the smaller linear SP-255 exciter but starting may be erratic or not work at all unless the AC line voltage is increased to 125 VAC for starting (it can then be backed off somewhat while operating). A bleeder resistor of 200M ohms or so rated for 15 kV can be installed to discharge the power supply capacitors after shutdown as starting of the longer SP-107/127/907 tube apparently requires the voltage to rise from close to 0 V to start reliably on the SP-255's whimpy starter. An alternative and better solution is to add a passive boost circuit to the starting multiplier of the SP-255. This can be in an external pod requiring no modifications to the exciter itself. (See the section: Enhancements to SP-255.) Make sure the laser head frame is securely connected to the power supply (and earth) ground. Since the operating voltage and current are well within the capabilities of the SP-255, the laser and power supply should both be happy once started (though the AC line voltage may still need to be slightly above 115 VAC to minimize drop out/restarts if there are line dips, expecially for a high mileage tube which may have increased operating voltage). Changing the jumpers to use one of the lower line voltage taps on the SP-255's power transformer would probably help in a marginal case (low line voltage, or a laser with a higher HeNe tube voltage or higher ballast resistance) where regulation can't be maintained with adequate current without using a Variac to boost line voltage.

    Description of the SP-115 Laser Head

    This is one of Spectra-Physics' first commercial HeNe lasers, probably from the mid-60s. It is RF (rather than DC) excited - basically a radio transmitter attached to a neon sign. :) Some photos of the laser and exciter can be found in the Laser Equipment Gallery (Version 1.88 or higher) under "Spectra-Physics Helium-Neon Lasers".

    The laser tube is about 20 inches long with separate bore and gas chambers side-by-side. The bore uses rather thin glass tubing and is a very large diameter for a HeNe laser - about 3 to 4 mm ID - consistent with early HeNe laser technology. The laser head is nicely mounted with lots of fine machined hardware. It has no IR suppression magnets. There are two RF connectors on the side for the Spectra-Physics model 200 RF-type power supply. One of the connectors is for the actual RF signal; the other is for starting. There is an impedance matching network located under the "tube deck". This consists of a series LC circuit (C is adjustable for peaking the tuning) between the RF input and case with the output taken from the junction of the L and C. The RF drives a dozen or so electrodes with alternating polarities in close proximity to the tube bore. The start connection goes to the input of a potted transformer which produces a several kV pulse when the "Start button" on the exciter is pressed. The starting pulse goes to a separate small electrode clamped near the center of the tube bore.

    The laser has external adjustable mirrors mounted on the very solid precision milled black anodized aluminum box support structure. Both mirrors have screw adjustments for coarse alignment not accessible from outside the case without removing the end-plates. The front mirror also has external fine adjustments in X and Y via two precision Lufkin micrometers and the rear mirror is mounted on a precision slide with an external micrometer adjustment for mirror separation (try to find that on any modern laser!). I don't know if the intent of this axial adjustment (over 1/2" of travel) was to fine tune the longitudinal or transverse modes or both. Since the resonator frame would experience little if any heating (and expansion), the micrometer could be used to center a longitudinal mode and maximize output for this low gain laser. In addition, the larger movement could possibly be used to select a particular transverse mode pattern, though actually achieving TEM00 operation in such a wide bore laser might not be possible.

    The power supply for the SP-115 is a high quality 15 to 25 watt 40.68 MHz RF source consisting of a crystal controlled oscillator and a power amplifier using a 4x150 tube. All active elements are tubes, of course, but out of character for the era, the oscillator and driver are built on a printed circuit board. Overall, the system looks like something straight out of the ARRL Handbook (which is probably where the design came from!).

    Not surprisingly, on the sample I have, the tube has leaked and only produces a weak purple glow when the RF is turned on. The getter has the "white cloud of death" syndrome and without an aluminum can cathode, there is no possibility of getter action anywhere else. (Not that a tube this far gone would have any chance of revival in any case. The tube would make an ideal candidate for refilling since the vacuum could be breeched by cutting the exhaust nipples at either end of the gas ballast without contaminating the Brewster windows.) The SP-200 does do a nice job of lighting 20 W fluorescent lamps and most likely screwing up radio reception in the neighborhood. :)

    There was also a Spectra-Physics model 116 laser which appears similar but has a tuning prism to enable wavelength selection. It goes without saying that a working sample of an SP-116 would be a real prize. :)

    Description of the SP-117A Laser

    The SP-117A is a frequency stabilized laser which produces a single longitudinal mode output with a nominal frequency of 473.61254 THz. It is still in production and has a sticker price of several thousand dollars. The SP-117A system consists of a cylindrical laser head and power supply/controller.

    The cylindrical SP-117A laser head has the normal strange SP high voltage cable/connector. Although I haven't yet seen it, I assume this mates with a conventional HeNe laser power supply inside the controller box. The laser head will run happily on the SP-248 exciter or any other HeNe laser power supply compatible with a 3 to 5 mW HeNe laser tubes. But of course, there will be no mode stabilization. So, it behaves more or less the same as other similar size HeNe lasers.

    In addition, there is a DB9 that is used for the control functions. It includes connections for a heater to adjust the cavity length of the HeNe laser tube, a pair of photodiode outputs for feedback mode stabilization, and an interlock.

    I have looked at the longitudinal modes of the SP-117A laser using my home-built scanning Fabry-Perot interferometer. The head was powered by an SP-248 exciter so there was no mode stabilization. As the tube heated and expanded, the modes would cycle under the gain curve (normal behavior for any HeNe laser!). It oscillates on at most two modes with a tendency for there to be two modes of reasonably equal amplitude. A single mode appeared for a relatively small portion of the cycle but I assume that is actually the desired result when using the feedback system. Having at most 2 modes is unusual for a HeNe laser of this power as 3 or 4 modes would be expected based on the tube length.

    This laser seems to be interesting in another respect: While the typical ordinary HeNe laser, the modes roughly follow the profile of the gain curve as they traverse it, with this tube, the mode on one side will tend to disappear and reappear on the other side of the gain curve relatively abruptly. I don't know whether this behavior is a peculiarity or a feature but it seems like it could be beneficial. ;-)

    (There is also an SP-118A but I'm not sure what the difference is.)

    Description of the SP-120 Laser Head

    The SP-120 is the same width and height as the SP-124, but just over half its length. Construction of the plasma tube is generally similar but it is shorter and there are fewer IR suppression magnets. However, the frame and mirror mounts differ. (The SP-122 is actually closer to a baby version of the SP-124 with virtually identical construction to that laser but is slightly shorter than the SP-120.)

    Photos of a SP-120 laser head and the SP-120 resonator and tube can be found in the Laser Equipment Gallery (version 1.85 or higher) under "Spectra-Physics Helium-Neon Lasers". The complete user manual for the SP-120 laser with SP-256 exciter can be found at, Manuals. On the one sample of the SP-256 exciter that I've seen, the current was set for 7.2 mA. However, I don't know if this is the default optimum setting for the SP-120 laser or whether it had been tweaked. (The specs list 7 mA at 3.7 kV.) There is also an SP-120S. It has been suggested that this simply means "with shutter" but I don't know for sure.

    The resonator uses three-screw adjustable mirror mounts for coarse alignment (tweaking these is a true pain!). Fine alignment is done via a pair of hex screw pan/tilt adjustments at each end which actually shifts the tube X-Y position without affecting the mirror. These are accessible via a pair of holes visible once the circular bezel/optics mount is unscrewed. It is possible to replace the tube in about 5 minutes without requiring major mirror re-alignment (no need to touch the coarse adjustments, only the tube centering).

    The resonator is constructed from 3 pieces of thick very nicely machined aluminum stock - an L-channel and 2 end-plates bolted together to form a very rigid structure. It is supported at only three points and essentially floats inside the outer case (the "Stabilite" name as discussed for the SP-124 laser, below) which isolates the resonator from external stress (or so it is claimed). So, the clunking you hear when changing the position of the laser head is normal.

    CAUTION: Unless the tube has been removed, there should be no need to clean the optics. Since there is no way to clean the Brewster windows with the tube in place and no way to clean the mirrors without removing them, it is a royal pain to be avoided. Remove, clean, install, test and tweak, repeat until output power comes back to what it was before attempting this stunt. :)

    Description of the SP-122 Laser Head

    The SP-122 wasn't nearly as common as the SP-120 or SP-124 based on the number of samples I've seen anywhere (exactly 1, which which I found on eBay), but is interesting in that the resonator is identical in every respect to an SP-124 except it is about 1/2 as long. The "Stabilite" frame, mirror mounts, and end-plate adjustments are the same, but there are fewer magnets. The tube is a bit shorter than the SP-120 tube and may have a slightly narrower bore but is otherwise identical. This was no doubt a more expensive laser to manufacture than the SP-120 so it probably never was very popular. The only obvious feature lacking in the simpler and probably slightly higher power SP-120 is the adjustable beam position. And, how much would anyone pay for that?

    The one I obtained also used the strange SP-253A exciter - a switchmode power supply which sends medium voltage AC to a voltage multiplier/boost module in the laser head. See the end of the next section for more on this. There is also an SP-123 which appears similar but with an internal power supply.

    Description of the SP-124 Laser Head

    I obtained an SP-124 laser in 'as-is' condition for nothing, so I won't complain. The HeNe tube is gasy so it makes a nice wall hanging at this point but has its uses in enhancing this section at least. :) A photo of a SP-124A laser head and SP-255 exciter can be found in the Laser Equipment Gallery (Version 1.43 or higher) under "Spectra-Physics Helium-Neon Lasers".

    The SP-124 laser head is a box about 76 mm (H) x 76 mm (W) x 813 mm (L) (3" x 3" x 32"), nicely massive for its size. There are threaded beam apertures at both ends though the HR is backed by a solid aluminum plate so I don't think much light would ever get through that even if there was leakage through the mirror!

    This is one of SP's "Stabilite" series lasers. This approach to frequency stabilization is based on a mounting system that employs optimally located pivots in an attempt to minimize the coupling of gravitational and vibrational torques and other distorting forces to the resonator cavity itself. In the SP-124, most of the mass of the laser head is in such an optimally mounted heavy solid frame with roughly an L cross section that runs nearly the full distance between the mirror mounts and attached to each of them at three points.

    Adjustments accessible externally at each end of the laser allow the beam alignment (X and Y) to be tweaked very accurately by moving the entire optics chassis relative to the head mounting studs (which accept 6-32 screws or rubber feet). The adjustment scheme is sort of interesting (to me, at least): A V-shaped block (bolted to the rosonator and case) sits between a pair of wedges (part of the mounting stud assembly) that can be moved up and down via a pair of screws (call them A and B) and retained in position by a stiff spring. Rotating both A and B equally in the same direction moves the beam in Y; rotating A and B equally in opposite directions moves it in X. The setting may then be locked.

    The external mirror HeNe tube is clamped in rubber mounts at its ends and also stabilized at the 1/3 and 2/3 (approximately) positions. Metal bellows join the tube mount brackets to the mirror mounts and, in conjunction with the rubber seals, prevent dust and dirt from getting on the inside surfaces of the mirrors and on the Brewster windows. The mirror mounts have hex head bolts for adjustments with set screws to prevent their settings from changing over time. An additional metal bellows joins the OC to the treaded output aperture.

    The HeNe tube itself is a bare capillary about 7 mm OD with a 1.1 mm ID (no, I didn't measure it - just trust the specs!). The cathode, getter assembly, and HeNe gas reservoir is in a side-arm at the output-end of the laser bent to run parallel to the bore. It is about 32 mm x 178 mm (1-1/4" x 7") with the 'can' electrode nearly filling the glass envelope. The anode is (naturally) at the other end of the bore along with the three 9.8K ohm (5 W at least) ballast resistors also in a parallel side-arm inside the gas envelope as apparently is the case with other Spectra-Physics lasers of this era. Interesting, they are just ordinary Ohmite power resistors. I guess this approach does reduce problems with high voltage insulation breakdown but it would be a shame if the laser went bad because a $.50 resistor failed and could not be easily replaced! The total value of about 30K ohms would seem to be rather low but might have been selected to match the needs of the SP-253A exciter (see below) or additional external ballast resistors may be required. The SP-124B version of this laser may use a more normal 81K ballast resistance.

    A series of relatively weak (e.g., refrigerator note holder strength) ceramic magnets 14 mm (W) x 22 mm (L) x 6 mm (H) (9/16" x 7/8" x 5/16") are mounted in close proximity under (15 magnets) and on one side (24 magnets) all along the length of the bore wherever they fit. (See the section: Magnets in High Power or Precision HeNe Laser Heads for an explanation of their purpose.) The approximate arrangement is shown below. I may have the poles backwards (which is of course irrelevant). A cheap pocket compass came in handy to determine the pole configuration!: The magnets were positioned with their broad faces about 2 mm from the bore.

        Magnets N_S_N_S_N_S_N_S_N_S   S_N_S_N_S_N_S_N   N_S_N_S_N_S_N_S_N
        on side |_|_|_|_|_|_|_|_|_|   |_|_|_|_|_|_|_|   |_|_|_|_|_|_|_|_|
       HR end ============================================================= OC end
      of bore ------------------------------------------------------------- of bore
        Magnets N_S_N S_N_S N_S N_S   S_N S_N S_N S_N     S_N S_N_S N_S_N
        below   |_|_| |_|_| |_| |_|   |_| |_| |_| |_|     |_| |_|_| |_|_|
               N_S_N                               +-----+-----+
        Where: |_|_| = 2 adjacent ceramic magnets: |N   S|S   N|
    I assume that the only reason there aren't 24 magnets below the tube is that the holes in the Stabilite frame got in the way.

    Apparently, there must have been a couple of power supply options for the SP-124. Most of these lasers appear to use the Spectra-Physics Model 255 Exciter (SP-255). This is a traditional HeNe power supply providing operating and start voltage through a high voltage BNC connector. However, the laser I have apparently is supposed to use an SP-253A Exciter, a model for which no one (including Spectra-Physics) seems to have any information or even acknowledge exists though I have since found out that the SP-122 laser, a model slightly shorter than the SP-120 but built more along the lines of an SP-124, may have also used the SP-253A (possibly a slightly different version or at least different jumper options). For more information on what I have found out so far about the exciter, see the section: Spectra-Physics Model 253A Exciter (SP-253A).

    Unfortunately, on the system I obtained, the boost/start module (which is what I assume was supposed to be inside the head to attach to the exciter) had been ripped out with the cable just chopped off and thus I can't even determine what was there originally. So, I removed the multiconductor cable and replaced it with a HV coax (terminated with a standard Alden connector) and wired it directly to the tube anode terminal and chassis ground (recall that the ballast resistors are inside the tube. Yes, I know, the 30K ballast resistance may be too low for use with the SP-255!)

    Using my SP-255 to power the head, I get a nice pink glow in the bore (more red than orange indicating a rise in pressure from slow leakage over the years) but as expected, no coherent light. The low ballast resistance is fine as far as maintaining a stable discharge (I don't know if this would still be the case if the gas pressure in the tube were correct). Maybe someday in the far distant future after that hot place freezes over AND those pigs start flying, I will get around to regassing the tube! :)

    Description of the SP-125 Laser Head

    This is definitely a LARGE-frame HeNe laser by weight alone: roughly 45 kg (100 pounds) for the head alone! And, it is around 2 meters (over 6 feet) long! Like its smaller cousin, the SP-124, there are models for normal red (632.8 nm) and two IR lines (1152.3 nm and 3391.3 nm). NEC made one almost as large (GLG5800, over 5-1/2 feet long) and Jodon has high power HeNe lasers but the SP-125 is still the largest production HeNe laser I've found.

    The SP-125A tube has a common cathode in the middle of the tube with two anodes, one at each end. The dual discharges are driven from its SP-261A Exciter which provides 6 kV at up to 35 mA. The SP-250 Exciter is also compatible with this laser.

    With a bit of rewiring of the laser head, one could feed the anodes separately reducing the individual current requirements so that a pair of power supplies similar to the SP-255 could be used. With this sort of scheme, it should also be possible to selectively power only one of the discharge paths for reduced beam output if desired. Yes, I know, why would you ever want *less* power? :)

    Two sets of ballast resistors in the laser head totaling 87K ohms (75K+12K) provide the operating voltage to each of the anodes of the dual discharge tube. They are located between the anodes and chassis ground (The SP-261A's output is negative with respect to ground. Thus, ground is the positive supply voltage). The HeNe tube's single cathode is attached directly to the negative output of the SP-261A.

    The starter operates in a manner similar to that of the method of triggering the xenon flashlamp in a typical electronic flash unit or solid state laser power supply - by pulsing an external electrode in close proximity to the HeNe tube bore. The whole tube is supported by metal rods which are insulated from the cavity structure by nylon disks. One of the rods is the trigger electrode. The starter runs off a voltage from the 75K/12K ohm taps of both ballast resistors ORed together so that it repeatedly generates a trigger pulse until BOTH discharges have been successfully initiated.

    The SP-261A also has a low power RF output (this isn't the same as the RF power supply option mentioned below) which drives a pair of plates in proximity to the HeNe tube. The RF is supposed to stabilize the laser power (presumably by some sort of discharge dithering process). However, the RF apparently also results in interference with local radio stations. :(

    An RF power supply option is/was also available. (Possibly some version of the SP-200 though the specs don't quite match for the one I have. See the section: Spectra-Physics Model 200 Exciter (SP-200).) This would replace theSP-261A and starter entirely by driving the tube directly with radio frequency energy - 15 W at 46 MHz. Note the greatly reduced power to the tube compared to the 150 to 210 W for the DC discharge! The drive is applied via coax from a BNC connector on the back of the laser to a resonant circuit about midway in the laser head. The two phases of the output of the resonant circuit connect to a pair of 0.1 inch diameter bars running the length of the tube about 0.6 inches from the centerline suspended from insulators.

    Unfortunately, many SP-125s that appear as surplus are not good for more than long boat anchors (or as a parts unit for salvage of the optics and frame). Unless the tube has been replaced relatively recently, being soft-seal, it has likely leaked to the point at which the getter can no longer clean up the contamination. Refilling is the only option and that cost would make what you paid for the laser look like pocket change. And, refilling a HeNe tube is generally not a realistic basement activity. So, if you come across an SP-125 at a low price, unless it is guaranteed to lase, buyer beware. An SP-125 sold "as-is" almost certainly means the seller couldn't get it to work (not that everything possible wasn't tried) since they likely know it is worth 10 times as much in operating condition!

    Also see the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications and Spectra-Physics Model 261A Exciter (SP-261A).

    (From: Marco Lauschmann (

    The SP125A is absolutely beautiful with much chrome and a metallic blue cover! It is nearly 2 meters long and looks like an older large-frame argon ion laser. A Spectra-Physica scientist noticed that this device will deliver twice the rated power with no problems. Others have claimed as much as 200 mW for the red (632.8 nm) model!

    Description of the SP-130 Laser

    This is probably one of the earliest self contained portable HeNe lasers ever sold. (The original "Model 130 Gas Laser Operation and Maintenance Manual" I have has diagrams date 1965 and 1966). It even has a genuine leather carrying handle. :) The overall construction is superb using precision machined anodized aluminum for the main structure as well as the covers - built along the lines of a Sherman Tank. The laser is cute - about 13.5"(L) x 4"(W) x 5"(H) and heavy for its size - about 11 pounds. The SP-130B uses a HeNe tube with Brewster windows and external mirrors all in a self contained box. Some photos can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".

    The tube inside the lasers in the photos is the typical small Spectra-Physics side-arm type (like those in the SP-155 and other similar lasers also shown on the Web page above) but with Brewster windows instead of mirrors. However, earlier versions may look a bit different with a side-arm for the anode as well and really early versions (SP-130, no B) actually used a heated filament for the cathode (though for some reason, the schematic of the SP130 with the heated filament is dated slightly later than the schematic of the SP130B with the cold cathode design).

    Based on the length of the tube, I would have expected its output power to be in the 2 to 5 mW range. However, from the specifications in the manual, it turns out to be only 0.75 mW when used with the hemispherical mirror configuration (planar and 30 cm radius of curvature), but capable of a TEM00 beam despite its wide bore (2.5 mm). With a confocal configuration using a pair of 30 cm mirrors, the beam is multimode (non-TEM00) and output power may be as much as 1.5 mW.

    When I obtained the first of these lasers (the one in the top two photos), the tube actually still lit up but there was no output beam. At first I thought it might even have a chance of working since the discharge color looked sort of reasonable, though somewhat less intense than I would have expected. Fiddling with the optics didn't yield any positive results. And then, when I wasn't looking, the discharge went out! As best I can tell, a crack must have opened somewhere in the tube and it is now at much higher pressure or up to air - bummer! I can find no visible damage or any evidence of this except that it won't start even on a much larger HeNe power supply and shows no signs of a glow from an RF source. So far, the getter hasn't changed color.

    I don't think this laser was ever really alive - the tube was probably gasy or helium deficient or something but I still can't explain what happened. The only place it could have leaked that I can't see is under the anode connection which is kind of potted but there shouldn't have been any heat there to cause such a problem.

    And to compound my disappointment, I dinged the OC removing the tube. Enough of it may be left to still work but the optics appear to be soft-coated as the AR coating came off totally by just barely touching it. However, that still hurts. Sometimes, you just have one of those days. :(

    The laser in the third photo was DOA with an up-to-air tube, seriously damaged mirrors (coatings mostly gone), and evidence of prior dissection attempts (cut wires, etc.). The tube in that one is probably one of the earliest non-heated filament types with a small cathode and separate side-arm for the anode.

    However, I have since obtained a third SP-130B which originally had a red/blue discharge. But while running for a few hours, the color gradually changed to a mostly correct white-ish red-orange. And, with an optics cleaning and alignment, this SP-130B actually lases. The output power is not up to spec - about 0.25 mW at maximum current (it's rated at 0.75 mW) - but that's still a bit amazing considering its age. See the section: Reviving a Spectra-Physics Model 130B Antique Laser for details.

    The internal power supply accounts for much of the weight and most of the height of the box and consists of:

    There is no actual starter - the open circuit voltage of the power supply is about 5,000 VDC but drops to around 1,500 VDC under load.

    For more info and schematics, see the section: Spectra-Physics Model 130 HeNe Laser Power Supply (SP-130).

    Now, the question becomes: Do I leave the dead ones intact as examples of antique lasers or replace their tube and optics with modern 3 mW barcode scanner tubes (about the largest that would fit height-wise, a 1 inch diameter tube) to have working lasers? I guess there's nothing special about 3 mW HeNe lasers so leaving them intact would be the best option. And, it would be a shame to only have 3 mW when the power supply is easily capable of driving at least a 5 mW tube. In order to do a test with an SP098-2 barcode scanner tube (actual output: 2.8 mW), I had to add 500K ohms of ballast resistance in addition to what is built into the power supply to get the current low enough so the adjustment would include the optimum current setting. (I can hear the antique connoisseurs breathing a collective sigh of relief!) Who knows, maybe someone will drop replacement tubes and mirrors in my lap someday! Hint, hint. :)

    Description of the SP-155 Laser

    The Spectra-Physics model 155 is a basic, bare bones, low power, low cost self contained HeNe laser probably used in countless classrooms to introduce uncountable bored students to geometric optics. :) Its rated power is 0.5 mW (Class II, 0.95 mW max) but when new probably approaches the safety limit (there is an adjustable glass plate neutral density filter inside to attenuate the beam if necessary) - a "hot" SP-155 tube might produce 1.25 mW or more.

    Photos of a typical SP-155 can be found in the Laser Equipment Gallery under "Spectra-Physics Helium-Neon Lasers".

    The HeNe laser tube is the classic Spectra-Physics side-arm design but with the anode electrode mounted about halfway along the length of the bore. The same tube with the anode mounted at the end would produce around 4 to 5 mW. In fact, the Spectra-Physics 157 (3 mW) and 159 (4 mW) lasers are virtually identical except for the tube's anode location and the use of a larger power supply. (The SP-156 may be similar but I haven't seen one to confirm.)

    The power supply for the SP-155 is a basic transformer/doubler/multiplier design with a single transistor current regulator. The power supply on later versions of the SP-157 and SP-159 lasers may be a potted brick instead of a discrete PCB but all of the SP-155 lasers appear to retain the older quaint power supply design. :)

    Note that other manufacturers sell (or have sold) lasers identical in appearance to the SP-155. For example, there is a Uniphase model 115ASL-1 and a Liconix L-388 (even though it is made by Uniphase). However, these use a hard-seal Uniphase barcode scanner HeNe tube (similar to a model 098 with a tiny collimating lens attached to its OC to reduce divergence) rather than the fancy Spectra-Physics side-arm tube we know and love. But their power supplies are similar or identical to that used in the SP-155. (There is also a Spectra-Physics model 155ASL which is physically identical to the Uniphase and Liconix lasers except for the name on the front. I assume it has the same construction though I haven't seen the insides of one up close and personal.)

    Also see the section: Spectra-Physics Model 155 HeNe Laser Power Supply (SP-155).

    Research Electro-Optics Tunable HeNe Lasers

    Research Electro-Optics, Inc. (REO) is probably the only manufacturer of commercially available HeNe lasers that can be tuned to 5 wavelengths by turning a knob.

    Here are some specifications for a couple of REO tunable lasers. Two models were listed in their 1992 catalog though only the LSTP-1010 shows up in a recent listing and on the REO Laser Products Page. As expected, both are linearly polarized (500:1) since they use a tuning prism external to the laser tube:

    Note the line at 604.6 nm (orange/yellow) which is almost never seen in other other-color HeNe lasers (at least isn't supposed to be there). :)

    And for only $4,050.00 plus shipping and handling, you can now buy your very own LSTP-1010 through Edmunds Industrial Optics. :)

    (From: Lynn Strickland (

    The five lines are 543.5, 594.1, 604.6, 611.9, and the common (red) 632.8 nm. You might see a flash at 629.4 nm and at 640.1 nm, but nothing to write home about. The 629 and 640 nm lines are so weak, and so close to 633 that they're sometimes hard to distinguish. There should be nothing at the IR lines (1,153, 1,523 or 3,391 nm).

    As originally designed, these lasers used a Brewster window tube with a Littrow prism as the wavelength selection mechanism. The tube's internal mirror was a broad band output coupler. Don't know if it's changed, but I doubt it.

    The fundamental design issue is that the optimum Bore-to-Mode Ratio (BMR) for green is much higher than for red. (BMR is the ratio of limiting aperture size to mode radius. To get TEM00 operation for green, the optimal number is about 4.2, for red it's about 3.5.) If you know the wavelength, mirror curvature, and spacing, you can calculate the mode radius at any point in the cavity. The capillary bore serves as the limiting aperture, so adjusting bore length and bore diameter sets the BMR, which in turn determines transverse mode purity.

    Thus, if you optimize the BMR for green power (which you have to do), the red is under-apertured, and has something like 50% off-axis modes. It's getting close to a doughnut-mode.

    REO builds some of the highest 'Q' Brewster tubes in the world (probably THE highest), exclusively for the company, Particle Measuring Systems (PMS). REO and PMS used to be one in the same, but the owner sold off the particle counter biz a few years back, for something like $75 million. They now have some sort of supply agreement. The REO tubes aren't the most robust or mechanically stable, but if you get them packaged right, probably some of the highest power you can get from a given tube length. This is mostly due to coatings (all Ion Beam Sputtered), and a super-polishing process they have for substrates. As they say, it's all done with mirrors. ;)

    A green Brewster tube IS a bitch! The original REO (PMS) tube was a 5 mW size - about 15" long. They did a soft-seal on the B-window; because it's fused silica. Don't know if they've gone to optical contacting/graded seal now - I'd hope so.

    I think REO added a 7 mW, maybe even a 10 mW size for power. I recall seeing some longer ones at a trade show. As for cavity power, I've seen an REO B-tube with 2 HRs do almost 45 Watts of intra-cavity circulating power. They're probably higher than that now. These puppies are like $1,700 each in volume and only sold to PMS - pretty hard to come by.

    (From: Sam.)

    There is a weak line at 635.2 nm which could also show up as its gain is higher than that of the 594.1 nm and 604.6 nm lines. 640.1 nm is actually quite strong - next in line after 632.8 nm. See the section: Instant HeNe Laser Theory for a listing.

    Here is a photo of the PMS One-Brewster HeNe Laser Tube and a closeup of the Littrow Prism Tuning Assembly from PMS Tunable HeNe Laser showing its proximity to the one-Brewster tube's Brewster window. There are adjustments for wavelength and transverse (alignment). The Littrow prism is the shiny thing at the far left. The Brewster window is next to it. There is normally a tight fitting metal cover to keep out dust which has been removed to take the photo. Except for the high quality internal OC mirror and window, the HeNe tube itself isn't that much different from the common variety, though the metal envelope - typical of PMS/REO tubes - may help stability. It does have a heater coil on the OC mirror mount, presumably to decrease warmup time. (These heaters are on some but not all PMS HeNe laser tubes.) The resistance is around 31 ohms and it runs on 9 VAC from a small transformer. The rest of this laser is unremarkable - a brick power supply and case. :)

    Proposal for High Efficiency Tunable HeNe Laser

    If this has been done (which is very likely), I'd like to see the reference or patent. If not, I'm claiming it as my invention. :)

    One problem that limits power in the REO tunable HeNe laser are losses through the Brewster window of the 1-B tube. The Brewster angle is only correct at a single wavelength so there will still be some Fresnel (reflection) at all the others. And, even super polished fused silica isn't perfect so there will still be some scatter. If these could be eliminated, the available power at all wavelengths would increase but this would be especially dramatic for the very weak 543.5 nm (green) and 594.1 (yellow). So, what I suggest is to place the tuning prism inside the tube envelope mounted on a two-axis bearing. Coupling through the glass can be via a pair of magnets to adjust tuning (pitch) and transverse mirror alignment (yaw). This is quite simple mechanically. Even simpler would be to attach the tuning prism assembly via a flexible metal bellows. In either case, 2 of 3 Brewster surfaces are eliminated from the intracavity beam path. The 3rd one is for the Littrow prism which unfortunately cannot be eliminated unless a high efficiency grating could substitute for the prism. Dust collecting on the optics is also, of course, no longer a problem. :)

    Coherent Model 200 Single Frequency HeNe Laser

    This is a HeNe laser that operates in a single longitudinal mode. It consists of a cylindrical laser head and separate controller/power supply.

    The HeNe laser tube is powered from a standard Laser Drive 6.5 mA, 2,100 V power supply brick via a HV BNC connector. There is no special control or regulation of this supply - it's turned on by the main power switch. But some thoughtful engineer included a high resistance bleeder to discharge the HV caps in the power supply brick after power is removed. :)

    The HeNe laser tube itself looks like a standard Melles Griot (not made by Coherent!) model, probably a 05-LHR-120 specially selected to produce only 2 longitudinal modes. It may also be filled with isotopically pure gases. The tube itself probably puts out more than 2 mW but the polarizing and beam sampling optics sucks up some of it. In addition, depending on the particular version, there is either a dielectric filter or polarizing filter in the end-cap. The dielectric filter cuts the output by about half but the this can be varied by 10 percent or so (though I'm not sure if this is intentional). The polarizing filter allows continuous adjustment of output power. (In both cases, the adjustment is done by loosening a set-screw and rotating the end-cap). According to the CDRH sticker, the output beam is supposed to be less than 1 mW. Given the wide swings in output power during warmup (see below), even with 50 percent attenuation, the peak output power may approach 1 mW.

    There is a thin film heater between the tube and laser head cylinder. A pair of photosensors monitor orthogonal polarized outputs from the tube. The controller monitors the lasing modes and maintain cavity length using the heater so that a pair of orthogonally polarized longitudinal modes straddle the gain curve. The beam sensor assembly can be rotated to align the photosensors with the 2 orthogonal lasing modes as this is arbitrary from tube to tube, but probably remains fixed for the life of the tube.

    The user controls consist of one (1) power switch. There are indicators for AC power and Status. After a warmup period of 20 minutes or so for the laser head to reach operating temperature, the Status indicator will change from Wait (red) to Ready (green). Doing anything that causes lock to be lost will result in a shorter delay of a couple minutes to re-establish it.

    The internal circuitry of the controller box is relatively simple and includes a 741 op-amp and LM311 voltage comparator along with a TO5 power transistor to drive the heater.

    Here is the pinout of the circular control connector as determined by my measurements. There may be errors.

      Pins  Wire Color  Function      Comments
      1,2    Blk/Wht    Heater Pwr    ~22 ohms
      3,4    Blk/Red    Temp Sense?   ~700 ohms at 25 °C; Increases with heat
      5,6    Blk/Blu    Photodiode 1  Anode is pin 5; Approximately 250 uA max
      7,8    Blk/Grn    Photodiode 2  Anode is pin 8; Approximately 50 uA max

    It would appear that the difference in sensitivities is the way it's supposed to be since this was similar on 3 heads. The controller and laser head are normally a matched pair so I assume there are adjustments inside the controller to equalize the responses.

    I picked up a controller and 3 laser heads in two separate eBay auctions for a grand total of $22.50 + shipping. The serial number on one of the heads matched that of the controller and while this head was initially hard to start, after running it for awhile on my HeNe laser test supply, it now starts normally.

    The controller originally had a dead HeNe laser power supply brick which is likely the reason it was taken out of service. I replaced that with an Aerotech LSS-5(6.5) which seems to be happy enough. Using a laser power meter, one of the two modes of the laser (the one present in the output beam) could be seen cycling up and down between about 0.60 and 1.40 mW with the orientation of the beam sensor assembly adjusted for maximum peak power. Each cycle took longer and longer as the tube warmed up to operating temperature, helped along by the heater. After about 15 minutes, it would appear to try to "catch" at certain power levels but couldn't quite remain there. (This behavior may have had nothing to do with the feedback control though.) Then suddenly, after about 20 minutes, the Ready light came on and a few seconds later, it locked rock stable at 0.95 mW. :) A second laser head behaved in a similar manner but with a slightly higher final output power of 1.02 mW. No adjustments were needed inside the controller despite the fact that the second head's serial number didn't match the controller's serial number. Possibly, even better stability or slightly higher stabilized output power could be achieved with some fine tuning. (The 1.02 mW head actually had higher peak power than the 0.95 mW head. The difference is probably in part due to the photodiode sensitivities.) With the fixed filter end-caps installed, the output power dropped to around 0.50 mW. I rather suspect that these are normal power levels for this system. A third head (which I haven't tested because its cables were cut) had the adjustable polarizer in its end-cap. With that installed on either working head, the output power could be continuously adjusted from near 0 mW to about 1 mW.

    Note that the Ready light comes on and then the laser locks in at the proper phase of the next mode cycle. So, basically the pea brain in the controller (no actual CPU of any kind!) decides that conditions are suitable and enables the feedback loop. Perhaps it's based on a combination of temperature and cycle duration or something. :) I've also seen the ready light come on even if the laser doesn't start and when one of the previously locked heads was plugged back in after a few minutes of cooling. In the latter case, the laser was indeed locked though it might not have been able to maintain it continuously since the tube was probably no longer really warm enough.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Viewing Spectral Lines in Discharge, Other Colors in Output

    Instant Spectroscope for Viewing Lines in HeNe Discharge

    It is easy to look at the major visible lines. All it takes is a diffraction grating or prism. I made my instant spectroscope from the diffraction grating out of some sort of special effects glasses - found in a box of cereal, no less! - and a monocular (actually 1/2 of a pair of binoculars). The shear number of individual spectral lines present in the discharge is quite amazing. You will see the major red, orange, yellow, and green lines as well as some far into the blue and violet portions of the spectrum and toward the IR as well. All of those shown in Bright Line Spectra of Helium and Neon will be present as well as many others not produced by the individual gas discharges. There are numerous IR lines as well but, of course, these will not be visible.

    Place a white card in the exit beam and note where the single red output line of the HeNe tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.

    As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube's output mirror.

    Dynamic Measurement of Discharge Spectra

    The following is trivial to do if you have a recording spectrometer and external mirror HeNe laser. For an internal mirror HeNe laser tube, it should be possible to rock one of the mirrors far enough to kill lasing without permanently changing alignment. If you don't have proper measuring instruments, don't worry, this is probably in the "Gee wiz, that's neat but of marginal practical use" department. :)

    (From: George Werner (

    Here is an effect I found many years ago and I don't know if anyone has pursued it further.

    We had a recording spectrometer in our lab which we used to examine the incoherent light coming from the laser discharge. This spectrum when lasing was slightly different from the spectrum when not lasing, which one can expect since energy levels are redistributed. As with most detectors, ours used a chopper in the spectrometer light beam and a lock-in amplifier.

    Instead of putting the chopper in the path of light going to the spectrometer, I put it in the path of the internal laser beam, so that instead of an open/closed signal going to the amplifier it was a lasing/not-lasing signal. What was recorded then was three kinds of spectrum lines: some deflected positive in the normal way, others deflected negative, and the third group were those that were unaffected by chopping, in which case when we passed over the line we only saw an increase in the noise level. Setting up such a test is easy. The hard part is interpreting the data in a meaningful way.

    Other Color Lines in Red HeNe Laser Output

    When viewing spectral lines in the actual beam of a red HeNe laser, you may notice some very faint ones far removed from the dominant 632.8 nm line we all know and love. (This, of course, also applies to other color HeNe lasers.)

    For HeNe lasers, the primary line (usually 632.8 nm) is extremely narrow and effectively a singularity given any instrumentation you are likely to have at your disposal. Any other lines you detect in the output are almost certainly from two possible sources but neither is actual laser emission:

    Since the brightness of the discharge and superradiance output should be about the same from either mirror, using the non-output end (high reflector) should prove easier (assuming it isn't painted over or otherwise covered) since the red beam exiting from this mirror will be much less intense and won't obscure the weak green beam.

    Note that argon and krypton ion lasers are often designed for multiline output where all colors are coherent and within an order of magnitude of being equal to each other in intensity or with a knob to select an individual wavelength. Anything like this is only rarely done with HeNe lasers because it is very difficult (and expensive) due to the low gain of the non-red lines. For more information, see the section: HeNe Tubes of a Different Color.

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Demonstration HeNe Lasers, Weatherproofing

    Putting Together a Demonstration HeNe Laser

    For a classroom introduction to lasers, it would be nice to have a safe setup that makes as much as possible visible to the students. Or, you may just want to have a working HeNe laser on display in your living room! Ideally, this is an external mirror laser where all parts of the resonator as well as the power supply can be readily seen. However, realistically, finding one of these is not always that easy or inexpensive, and maintenance and adjustment of such a laser can be a pain (though that in itself IS instructive).

    The next best thing is a small HeNe laser laid bare where its sealed (internal mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglas case with all parts labeled. This would allow the discharge in the HeNe tube to be clearly visible (and permit the use of the Instant Spectroscope for Viewing Lines in HeNe Discharge). The clear insulating case prevents the curious from coming in contact with the high voltage (and line voltage, if the power supply connects directly to the AC line), which could otherwise result in damage to both the person and fragile glass HeNe tube when a reflex action results in smashing the entire laser to smithereens!

    A HeNe laser is far superior to a cheap laser pointer for several reasons:

    Important: If this see-through laser is intended for use in a classroom, check with your regulatory authority to confirm that a setup which is not explicitly CDRH approved (but with proper laser class safety stickers) will be acceptable for insurance purposes.

    For safety with respect to eyeballs and vision, a low power laser - 1 mW or less - is desirable - and quite adequate for demonstration purposes.

    The HeNe laser assembly from a barcode scanner is ideal for this purpose. It is compact, low power, usually runs on low voltage DC (12 V typical), and is easily disassembled to remount in a demonstration case. The only problem is that many of these have fully potted "brick" type power supplies which are pretty boring to look at. However, some have the power supply board coated with a rubbery material which can be removed with a bit of effort (well, OK, a lot of effort!). For example, this HeNe Tube and Power Supply is from a hand-held barcode scanner. A similar unit was separated into its Melles Griot HeNe Tube and HeNe Laser Power Supply IC-I1 (which includes the ballast resistors). These could easily be mounted in a very compact case (as little as 3" x 6" x 1", though spreading things out may improve visibility and reduce make cooling easier) and run from a 12 VDC, 1 A wall adapter. Used barcode scanner lasers can often be found for $20 or less.

    An alternative is to purchase a 0.5 to 1 mW HeNe tube and power supply kit. This will be more expensive (figure $5 to $15 for the HeNe tube, $25 to $50 for the power supply) but will guarantee a circuit board with all parts visible.

    The HeNe tube, power supply, ballast resistors (if separate from the power supply), and any additional components can be mounted with standoffs and/or cable ties to the plastic base. The tube can be separated from the power supply if desired to allow room for labels and such. However, keep the ballast resistors as near to the tube as practical (say, within a couple of inches, moving them if originally part of the power supply board). The resistors may get quite warm during operation so mount them on standoffs away from the plastic. Use wire with insulation rated for a minimum of 10 kV. Holes or slots should be incorporated in the side panels for ventilation - the entire affair will dissipate 5 to 10 Watts or more depending on the size of the HeNe tube and power supply. (However, if you want to take this thing outdoors, see the section: Weatherproofing a HeNe Laser.

    When attaching the HeNe tube, avoid anything that might stress the mirror mounts. While these are quite sturdy and it is unlikely that any reasonable arrangement could result in permanent damage, even a relatively modest force may result in enough mirror misalignment to noticeably reduce output power. And, don't forget that the mirror mounts are also the high voltage connections and need to be well insulated from each other and any human contact! The best option is probably to fasten the tube in place using Nylon cable ties, cable clamps, or something similar around the glass portion without touching the mirror mounts at all (except for the power connections).

    Provide clearly marked red and black wires (or binding posts) for the low voltage DC or a line cord for AC (as appropriate for the power supply used), power switch, fuse, and power-on indicator. Label the major components and don't forget the essential CDRH safety sticker (Class II for less than 1 mW or Class IIIa for less than 5 mW).

    See: Sam's Demo HeNe Laser as an example (minus the Plexiglas safety cover), contructed from the guts of a surplus Gammex laser (probably part of a patient positioning system for a CT or MRI scanner). The discrete line operated power supply is simple with the HV transformer, rectifier/doubler, filter, multiplier, and ballast resistors easily identified. This would make an ideal teaching aid.

    See the suppliers listed in the chapter: Laser and Parts Sources.

    The Ultimate Demonstration HeNe Laser

    Rather than having a see-through laser that just outputs a laser beam (how boring!), consider something that would allow access to the internal cavity, swapping of optics, and modulation of beam power. OK, perhaps the truly ultimate demo laser would use a two-Brewster tube allowing for interchangeable optics at both ends, be tunable to all the HeNe spectral lines, and play DVD movies. :) We'll have to settle for something slightly less ambitious (at least until pigs fly). Such a unit could consist of the following components: Everything needed for such a setup is readily available or easily constructed at low cost but you'll have to read more to find out where or how as each of the components are dealt with in detail elsewhere in Sam's Laser FAQ (but I won't tell you exactly where - these are all the hints you get for this one!).

    A system like this could conceivably be turned into an interactive exhibit for your local science museum - assuming they care about anything beyond insects and the Internet these days. :)

    Weatherproofing a HeNe Laser

    If you want to use a HeNe laser outside or where it is damp or very humid, it will likely be necessary to mount the tube and power supply inside a sealed box. Otherwise, stability problems may arise from electrical leakage or the tube may not start at all. There will then be several additional issues to consider: For an example of something sort of kind of a little along these lines, see: Antony's HeNe Laser Page. :)

  • Back to Helium-Neon Lasers Sub-Table of Contents.

    Interesting, Strange, and Unidentified HeNe Lasers

    When Your Laser Doesn't Fit the Mold

    The vast majority of HeNe tubes and laser heads you will likely come across will be basically similar to those described in the section: Structure of Internal Mirror HeNe Lasers. However, when rummaging through old storerooms or offerings at hamfests or high-tech flea markets, you may come across some that are, to put it bluntly, somewhat strange or weird. I would expect that in most cases, these will be either really old, developed for a specific application, or higher performance lab quality models which are just not familiar to someone used to surplus specials. Consider these to be real finds if only for the novelty value! Refurbishing of the lab-grade lasers may be worth the effort and/or expense resulting in a truly exceptional (and possibly valuable) instrument. And, simply from an investment point of view, it is amazing what some old (and even totally useless dead) but strange lasers have fetched on places like Ebay Auction recently. See the section: Auctions. Here are some descriptions of what I and others have come across:

    Segmented HeNe Tubes

    I have several medium power HeNe tubes that do not have a single long bore (capillary) but rather it is split into about a half dozen sections with a 1 or 2 mm gap between them. Each of the short capillaries is fused into a glass separator without any holes. Two of these tubes look like the more common internal mirror HeNe tubes except for the multiple segments as shown below:

                   /       |            |            | _______  \
            Anode |\       |            |            |        \  | Cathode
            .-.---' \.-----'-----..-----'-----..-----'------.  '-'---.-.
        <---| |::::  :===========::===========::============:   :::::| |===> 
            '-'---. /'-----.-----''-----.-----''-----.------'  .-.---'-'
                  |/       |            |            | _______/  |

    Or, for a more aesthetic rendition, see: Helium-Neon Laser Tube with Segmented Bore.

    The third has Brewster angle windows at both ends with an external (fixed) HR mirror and an external screw-adjustable OC mirror. The cathode is also in a side-tube rather than the more typical coaxial can type but is otherwise similar.

    Only one of the 3 HeNe tubes of this type that I have works at all and it has a messed up gas fill probably due to age despite its being hard sealed. Its output is perhaps 1 or 2 mW (where it should be around 20 mW). However, to the extent that it works, there doesn't appear to be anything particularly interesting or different about its behavior. Of the other two tubes, one has a broken off mirror (don't ask) but before the mishap, did generate some decent power (perhaps 5 to 10 mW but still nowhere near its 20 mW rating) but erratically. I suspect this was due to a contaminated gas fill resulting in low gain rather than the segmented design since a couple of other similar length tubes of conventional construction behaved in a similar manner. The funky tube with the external mirrors was not hard-sealed at the Brewster windows and leaked over time.

    The only obvious effect this sort of structure should have on operation would be to provide gas reservoirs at multiple locations rather than only at the cathode-end of the bore as is the case with most 'normal' HeNe tube designs. I do not know whether this matters at all for a low current HeNe discharge. Therefore, the reason for the unusual design remains a total mystery. It may have been to stabilize the discharge, to suppress unwanted spectral lines, easier to maintain in alignment than a single long capillary, or something else entirely. Then again, perhaps, the person who made the tubes just had a spurt of excessive creativity. :)

    I have also acquired a complete laser head with a similar tube, rated 25 mW max with a sticker that says it did 22 mW at one time. It is unremarkable in most respects but does have a large number of IR suppression magnets arranged on 3 sides over most of the length of the tube. Currently, it does not lase because the gas is slightly contaminated but it is also misaligned. The discharge color is along the lines of "Minor - Low Outupt" in Color of HeNe Laser Tube Discharge and Gas Fill so there may be some hope.

    Strange High Power HeNe Laser

    This is a on-going project on finding information and restoring a strange HeNe laser acquired by: Chris Chagaris ( Research to determine the specifications and requirements involved postings to sci.optics, email correspondence, and a bit of luck - seeing a photograph of the mysterious laser in a book on holography.

    Here is the original description (slightly reformatted):

    (From: Chris Chagaris (

    I have recently acquired what I have been told is a 35 mW Helium Neon laser head. However, it is unlike anything I have ever seen before. (See the diagram, below.)

                    Capillary tube/external starting electrodes
       Starting pulse  o-------+----------------------+
                              _|_                    _|_
        ||     //==================================================\\     ||
        ||   //=====. .==================. .=================. .=====\\   ||
                    |||                  | |                 |||     
      Mirror        '|'  25K             | |           25K   '|'        Mirror
             Anode 1 +---/\/\---o +HV    | |   +HV o---/\/\---+ Anode 2
                         .---------------' '--------------.
                      ---|-+                            +-|---
                         |  ) Main               Spare (  |
                      ---|-+                            +-|---
                  Gas reservoir with heated cathodes and getters
    Jodon Laser Head shows the construction in more detail.

    Here is one reply Chris received by email from someone else named Marco. As you will see, this turns out to be a dead end.

    (From: Marco.)

    "Hi Chris,

    This seems to be a really old one, or from other location than west Europe, Japan, and the USA. The 'SM' could be an abbreviation for Siemens, they had manufactured lasers from 1966 to 1993; until last year Zeiss/Jena has taken over the production; and since 1997 Lasos has overtaken the production by a kind of management buy-out. You can send them the number, it will be possible that they know it. Contact Dr. Ledig. I will also look around if I can help you further.

    HeNe lasers with a heated filament are no longer built. To see if it still runs you can attach a 3.3 V supply to the filament and see if it glows red, not more, to much heat will destroy it. You could use transformers from tube amplifiers for the filament and an old HeNe laser power supply for the anode.

    This laser will need around 5,000 V and 10 mA I think. If you could only get a smaller power supply, you may not see any laser beam, but you can see if it will trigger."

    (From: Sam.)

    Here are my 'guesses' about this device. (I have also had email discussions with Chris.)

    I agree with much of what Marco had said.

    Unfortunately, Chris has determined that regassing will be required and he is equipped to do this but there will be some delay in the results.....

    (From Chris (a few months later).)

    Well, tonight while looking through the "Holography Handbook" I spied what looked suspiciously like that elusive laser I have. It said it was made by Jodon Engineering Associates of Ann Arbor, MI. I immediately called them and was fortunate to have the engineer (Bruce) who has built their tubes for the last 18 years answer the phone. I told him of my plight and read off the numbers that were on the plasma tube. Sure enough, it was one of their early lasers. They have been manufacturing HeNe's since 1963. He provided me with many of the details that I had been searching for.

    I explained that I planned on trying to re-gas this antique and he offered to help with what ever information I needed. It is truly refreshing to find someone in the industry that is willing to help the amateur without an eye on just making a profit.

    I finally located a small supply of HeNe gas, just yesterday. While visiting North Country Scientific to purchase a pair of neon sign electrodes (in Pyrex), I mentioned my need for a small amount of laser gas for my laser refurbishing project. (This was formally Henry Prescott's small company that supplied all the hard to find components for the Scientific American laser projects.) Lo and behold, there on a shelf, covered with dust, were a few of the original (1964?) 1.5 liter glass flasks filled with the 7:1 He/Ne gas mix. He let them go at a very decent price!

    (Hopefully, those tiny weeny slippery He atoms have not leaked out! --- sam)

    Now, about the magnets:

    The magnets are of rectangular shape, one inch long, 3/4 inch in width and 3/8 inch thick. There are a total of 26 magnets placed flat against the top (14) and flat against the bottom (12) of the plasma tube as viewed from the side. All but the ones on the very ends of the plasma tube are attached exactly opposite from one another, top and bottom. (See Jodon Laser Head for placement and field orientation).

    They are placed with the long side (1") parallel to the plasma tube with the north and south poles along this axis.

    They appear to be of ceramic construction and not very powerful. Sorry, I don't have any means of measuring the actual field strength.

    The current status of this project is that the laser needs to be regassed. Chris is equipped to do this and has acquired the needed HeNe gas mixture.

    To be continued....

    Photos of a similar but much larger Jodon HeNe laser (3.39 um IR in this case) can be found in the Laser Equipment Gallery (Version 1.41 or higher) under "Jodon Helium-Neon Lasers".

    Hewlett-Packard HeNe Lasers

    Here are descriptions of a few stabilized Hewlett-Packard HeNe lasers, used for applications like very precise interferometric measurement. (More information on inteferometers based on two frequency lasers can be found in the section: Interferometers Using Two Frequency Lasers.) These lasers are now sold under the Agilent name but since they were developed by HP, that's the designation I'll continue to use!

    The first is the HP-5501B laser head from the HP-5501A Laser Interferometry Measurement System. Position/distance resolution down to better than 10 nm (that's nanometer as in 0.000000001 meter!) were possible with this equipment. Of course, only the laser remains) but the specifications say something about the frequency stability of the laser head.

    One other thing that is most interesting is that the original list price from the HP catalog for the laser head alone is about $9,000 (now over $12,000)!

    (From: Angel Vilaseca (

    Here is a quick description of the unit:

    I have other HeNe lasers but this one really seems to be a class (or several!) above all others...

    The label on the unit says:

      HENE GAS LASER, Hewlett-Packard, P.N. 05517-60501
      Date of mfg. 4-12-93, Date of instl. 4-19-93, Ser. no. 591-3
      Made in USA, Licensed by Patiex Corporation, under patent no. 4,704,583.

    (From: Sam.)

    The Patent is rather interesting but I'm not sure it relates directly to this laser.

    There are photos of some version of the HP-5501A laser head in the Laser Equipment Gallery (Version 1.87 or higher) under "Assorted Helium-Neon Lasers". The tube used there matches my strange tube but not the description above which appears to be more like the tube in the HP-5517A (though not identical). I think this likely older 5501 tube looks much cooler than the newer 5517 versions. :)

    The HP-5501 laser head lases in two modes that are polarized orthogonal to each other. These are split and sent down different paths. The two beams rather than creating an interference pattern are beat together to and sent to a detector that outputs a difference signal. If the difference between the two beam paths changes by one wavelength of the laser (about 632.8 nm but accurate to many significant digits!), the phase of the difference signal will change by 360 degrees. The laser outputs a reference signal from beating the signals together internally. This is compared to the detector signal and an electronics package counts off the phase shifts and uses it to determine the distance traveled. The laser is supposed to have an accuracy on the order of 10 parts per billion over the life of the instrument.

    (From: Wong Sy Ming (

    I picked up a HP-5517A laser head for S$50 (that's about US$30) and I have to say it's an extremely fine piece of equipment, about the same as the HP-5501B. The datasheet (which may be found by searching for "5517" on the Agilent Web Site) claims a "vacuum wavelength stability" of 0.002 ppm(!!!) over 1 hour and 0.02 ppm over it's entire 50,000 hour lifetime. Quite incredible, isn't it? It also says it has a wavelength of 632.991372 nm and a wavelength accuracy of 0.1 ppm. (that's for the "consumer grade" model, the "military calibrated" one is 0.02 ppm).

    I got a rather more complete version than the one above. It came within its original casing, an inverter and a whole lot of electronics (don't know what they were for so I just took them out).

    The tube is really non-standard, it has only one thick white HV wire coming out of the back and two smaller wires (red and purple, just like the HP 5501B) and the tube connects to the HV power supply through only the ONE HV cable (for the anode). I discovered later (by poking around with a separate little inverter power supply) that the not-so-obvious cathode connection is via the red wire.

    The two smaller wires are connected to a "connector board" (that's what it says on the PCB) which has a big multiway connector on it, but I just ignored it and connected a 12 VDC power supply to a 470 uF or so capacitor on the board, and the tube lights up! It states a maximum power of 1 mW but the beam looks much brighter than that (probably due to the magnets along the tube which were drawing all my tools to them).

    The power supply is a Laser Drive, Inc. model 111-ADJ-1, which appears to be adjustable (due to the model number and the presence of a third wire which goes to a small preset on the PCB) but I didn't fiddle with that. It only takes 0.5 A at 12 VDC which is quite incredible. CAUTION: Do NOT just connect a 12 VDC power supply to the two red and black wires from the power supply or you will get quite a nasty shock. I don't know why.

    I wasn't able to trace where the two smaller wires from the tube went. The tube also has additional optics to expand the beam size to 6mm.

    (From: Sam.)

    The two unmarked wires and that stuff you removed were needed to actually obtain the incredible stability that HP (now Agilent) claims. I think you got a shock playing with the power supply because the HV return is via the black input wire since there is no second HV connection to the supply.

    These are called "Continuous Wave Two Frequency Lasers" or more specifically: "Helium-Neon Lasers with Automatically Tuned Zeeman-Split Two-Frequency Output". They have an extremely precise wavelength of: 632.991384 nm and 0.002 ppm short term wavelength stability.

    A diagram of the general approach is shown in Interferometer Using Two Frequency HeNe Laesr.

    A permanent magnet does the Zeeman splitting resulting in a pair of circularly polarized outputs at two very slightly different frequencies, F1 and F2 (difference of between 1.5 and 4 MHz depending on model and specific sample). The distance between the mirrors in the HP-5517 is feedback controlled by a heating coil wrapped around the bore to force the laser tube to maintain the position of the lasing line within the doppler broadened gain curve. I assume that a wave plate somewhere in the optical path converts the circular polarized output to orthogonal polarized components which are used externally. F1 is reflected from the thing being measured or tested (e.g., disk drive servo writer) and F2 is reflected from a fixed reference. The difference frequencies (F1-F2) and (F1-F2)+dF1 are then analyzed to determine precise position, velocity, or whatever. This approach has lower noise, greater stability, and is therefore more accurate than the common single frequency interferometer. By using cavity length control to lock the difference frequency to a known reference, the actual optical wavelength/frequency can be set very accurately. Using the MHz range beat signals makes straightforward signal processing and is more immune to noise than the baseband optical signals.

    There are some photos of the HP-5517 laser head as well as the HP-5501 HeNe laser (with descriptions) in the Laser Equipment Gallery under: "Assorted Helium-Neon Lasers".

    (From: Michael (

    The HP-5501A and HP-5517A are dual frequency interferometers where the two frequencies come from the Zeeman splitting of the energy levels. The laser produces two frequencies polarized normal to each other which are beat together internally to create a reference signal (which is just the difference between the two laser frequencies and is about 1.5 to 2 Mhz for the HP-5501A and HP-5517A and a bit higher on the newer ones). The beam is then sent to a beam splitter outside the laser which sends the vertical polarization one way and horizontal the other. One of these frequencies will be the reference beam, the other will be reflected from the object whose distance is being measured. The configuration looks a lot like a Michelson interferometer but the beam splitter is different. When the two beams are recombined and beat together, the resulting beat signal can be compared against the reference signal from the rear panel of the laser head. If you crunch through the math you will see that if the object being measured moves through a distance of one wavelength, the phase of the beat signal will move through one complete cycle (2*pi).

    Notes on the HP-5500 and HP-5501 Two Frequency HeNe Laser

    The HP-5501 appears to be the successor to the functionally similar HP-5500. (I have samples of the HP-5500C and HP-5501A.) They both use the same HeNe laser tube. But the HP-5501 is in a much smaller lighter case with simplified optics and different electronics. See Interior of the HP-5501A Laser Head - Right Side and Interior of the HP-5501A Laser Head - Left Side. The HeNe laser tube dominates the interior space in both views. The high voltage piezo driver power supply brick is visible under the magnets at the center of the tube. The HeNe laser power supply brick is underneath the output end of the tube. The piezo driver electronics circuit board at the far right end of the right side view. The optical sensor circuit board is at the far left of the left side view.

    Both the HeNe laser power supply and piezo power supply bricks run off the -15 VDC power supply. An interlock switch (easily defeated) prevents operation with the cover removed. In the HP-5500C, the HeNe laser power supply has an additional input that may be a current adjust signal, and the piezo driver power supply provides 0 to 1.5 kVDC when fed a control voltage of 0 to 15 VDC relative to the negative input. The However, in the HP-5501A, the potted power supply bricks have no inputs other than power. Rather, current and voltage control are accomplished by externally regulating the input current.

    The output of the laser tube is passed through a 1/4 wave waveplate to convert the circular polarization to linear polarization, and then through a 1/2 wave waveplate to rotate the linear polarization by an arbitrary, but fixed angle to line the two linearly polarized components up with subsequent optics. The beam is then expanded to about 7 mm and passed through an angled partially reflecting plate located just beyond the collimating lens on the laser tube assembly. This deflects about 30 percent of the beam to a polarizing beamsplitter which sends each component to its own photosensor to provide the frequency control feedback. A control loop uses these signals to adjust the PZT, and thus tube length, so that the two signals are of equal amplitude. The difference of the two signals is the frequency/phase reference.

    The HP-5501A laser head requires +/-15 VDC and +5 VDC for power. This is all that is needed to power up the laser but the interlock switch must be closed prior to applying power. It will lock even if not connected to the interferometer. At least I assume it's locked - after a few seconds, the "Retune" LED goes off, similar to if the "Retune" button is pressed. I haven't yet looked at the output with a photodiode or scanning Fabry-Perot interferometer. I have since acquired an operation and service manual for the HP 5501A which confirms the information above.

    Notes on the HP-5517 Two Frequency HeNe Laser

    Interior of the HP-5517A Laser Head shows the major laser/optics components of the Hewlett-Packard 5517A Dual Frequency Interferometer. The actual HeNe laser tube is inside the gray cylindrical housing which also has a powerful cylindrical magnet for Zeeman splitting the HeNe laser lines to create the difference reference frequency in the interferometer application. It's very well made but not particularly unusual except for an internal heater coil used to control cavity length very precisely. See the previous sections for more information on these two-frequency lasers. The output optics consists of a beam expander/collimator (the black and silver object just to the left of the power supply danger label) and an additional optical assembly to the left of this. Its front and rear halves contain what appear to be AR coated optical quality mica pelicles oriented at slight, but different angles. The front and rear sections can be rotated independently and they were sealed with blue paint once the perfect orientations were found.

    After thinking about this, I came to the conclusion that the two mica (or whatever) pieces might act together as an adjustable waveplate where their orientations with respect to each other control the relative delay of their e and o axes, and their overall orientation determines the orientation of any linear polarization components in the output beam. (The Zeeman split beam directly out of the HeNe laser tube should be circularly polarized.) If this were set up for a 1/4 wavelength retardation, these could be used convert the circular polarization of the Zeeman split beam back into linear polarizations that could be separated out with a polarizing beamsplitter at the detectors. In fact, having acquired an operation and service manual for the HP 5501A (which functions in a similar manner), this is exactly how it works.

    If two modes were oscillating simultaneously as might be the case with the reasonably short HeNe laser tube in the HP-5517, then it should be possible to recover their two polarizations and use this in the temperature control feedback loop to stabilize the difference frequency - which ultimately determines the accuracy of the interferometer measurements.

    One interesting characteristic of the HP-5517 HeNe laser tube I have is that the difference frequency only appears for perhaps 10 percent of the time as it heats up - possibly only when the dominant longitudinal mode is near the center of the gain curve. I don't know if this is the normal behavior for these lasers but suspect that is is based on experiments with common barcode scanner HeNe laser tubes inside strong magnetic fields. With those as well, the beat frequency would come and go as the tube heated and expanded with this effect becoming more pronounced when the magnetic field encompassed the entire tube as it does with the HP-5517.

    Experimenting with and without the presence of the mystery optics showed some effect. With them removed, there was absolutely no indication of a polarization preference in the output beam at any time. When the optics were installed and aligned to the original blue paint, the symmetry of the beat waveform, if nothing else, was polarization dependent. In addition, just after the output beat appear as well as just before it disappeared, the polarizer would suppress the beat entirely when oriented so that its axis was parallel or at 90 degrees to the axis defined by the blue paint. These perhaps weren't quite as dramatic as the effects I was hoping for but confirmed some of the speculation at least.

    A Really Old HeNe Laser

    This one isn't really that strange but it must be quite old. The American Optical Corporation model 3100 was a red (632.8 nm, the usual wavelength) HeNe laser that used an external mirror tube with a heated filament for the cathode. The cover bears a sticker from El Don Engineering, 2876 Butternut, Ann Arbor, Michigan 48104, Phone: 1-313-973-0330. The laser was serviced and repaired on 9/28/80 and its output was 2.3 mW, TEM00. I wonder if they still exist. :)

    The 3100 appears to be similar (or identical) to the Gaertner (model unknown) HeNe laser shown in the Laser Equipment Gallery (Version 1.72 or higher) under "Assorted Helium-Neon Lasers".

    The bore is about 2.5 mm in diameter which is extremely wide for a red HeNe laser and thus it was probably multi-mode (not TEM00). The Brewster windows are Epoxy sealed so needless to say, it no longer worked (aside from the slight problem that when I received the tube, it was in pieces. :( (All I have are the HeNe tube bits, its mounting clamps, mirrors, and mirror mounts, so some of the description below is inferred from limited information.)

    HeNe Tube:


    Power Supply:

    The Dual Color Yellow/Orange HeNe Laser Tube

    Multiline operation is common in ion lasers where up to a dozen or more wavelengths may be produced simultaneously depending on the optics and tube current. However, most HeNe lasers operate at a single wavelength.

    Through screwups in manufacturing (incorrect mirror formula, extra "hot" emission, etc.), an occasional HeNe laser may produce weak lasing at one or more wavelengths other than the one for which it was designed. For red tubes, the most likely spurious wavelength is a deeper red at 640 nm since it is also a fairly high gain line. For a low gain yellow laser, orange is most likely since it is relatively close wavelength and any goofup with the will may allow it to lase.

    I have a tube made by Melles Griot, model number 05-LYR-170, which is about 420 mm long and 37 mm in diameter and can be seen as the middle tube in Three HeNe Tubes of a Different Color Side-by-Side. Its only unusual physical characteristics are that the bore has a frosted exterior appearance (what you see in the photo is not the reflection of a fluorescent lamp but the actual bore). Apparently, larger Melles Griot HeNe tubes are now made with this type of bore - it is centerless ground for precise fit in the bore support. I don't know if the inside is also frosted; that is supposed to reduce ring artifacts. And, of course, the mirrors have a different coating for the non-red wavelengths.

    According to the the Melles Griot catalog, this is a HeNe laser tube operating at 594.1 nm with a rated output of 2 mW. However, my sample definitely operates at both the yellow (594.1 nm) and orange (604.6 nm) wavelengths (confirmed with a diffraction grating) - to some extent when it feels like it. The output at the OC-end of the tube is weighted more towards yellow and has a power output of up to 3 mW or more (you'll see why I say 'up to' in a minute). The output at the HR-end of the tube has mostly orange and does a maximum of about 1 mW. Gently pressing on the mirrors affects the power output as expected but also varies the relative intensities of yellow and orange in non-obvious ways. They also vary on their own. The mirror alignment is very critical and the point of optimum alignment isn't constant. In short, very little about this tube is well behaved. :)

    Why there should be this much leakage through the HR is puzzling. The mirror is definitely not designed for outputting a secondary beam or something like that as there is no AR coating on its outer surface. Thus, that 1 mW is totally wasted. Perhaps, this was an unsuccessful attempt to kill any orange output from the OC. The OC's appearance is similar to that of a broadband coated HeNe HR - light gold in reflection, blue/green in transmission. The HR appears similar to one for a green HeNe laser - light metallic green in reflection, deep magenta in transmission. (However, it's hard to see the transmission color in the intact tube. The OC may be more toward deep blue and the HR may be more toward purple.)

    As would be expected where two lines are competing for attention in a low gain laser like this, the output is not very stable. As the tube warms up and expands - or just for no apparent reason - the power output and ratio of yellow to orange will gradually change by a factor of up to 10:1. Very gently pressing on either mirror (using an insulated stick for the anode one!) will generally restore maximum power but the amount and direction of required pressure is for all intents and purposes, a random quantity. If the mirror adjuster/locking collar is tweaked for maximum output at any given time, 5 minutes later, the output may be at a minimum or anywhere in between.

    I surmise - as yet unconfirmed - that at any given moment, the yellow and orange output beams will tend to have orthogonal polarizations. But, as the distance between the mirrors changes, mode cycling will result in the somewhat random and unpredictable shifting of relative and total output power as the next higher mode for one color competes with the opposite polarized mode of the other. Is that hand waving or what? :)

    A few strong magnets placed along-side the tube reduce this variation somewhat. I'm hoping that adding some thermal control (e.g., installing the tube in an aluminum cylinder or enclosed case) may help as well. I was even contemplating the construction of a servo system that would dither the cathode-end mirror mount to determine the offset direction that increases output and adjusts the average offset to maximize the output. This might have to be tuned for yellow or orange - an exclusive OR, I don't know if maximizing total optical power will also maximize each color individually.

    Using an external red HR or OC (99 percent) mirror placed behind the tube's HR mirror, I was able to obtain red at 632.8 nm as well as a weak output at the other orange line (611.9 nm), and at times, all four colors were lasing simultaneously. :) See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.

    (From: Steve Roberts (

    Ah, the Melles Griot defects... These show up from time to time and are highly prized in the light show community for digitizing stations and personal home lumia displays.

    The yellow/orange combo is a not a goof. I've seen a 7 mW version of that that was absolutely beautiful, but rejected because it was too hot. It's probably slight differences in the length of the tube or bore size. They cut them for a given mode spacing, but fill them all at once with the same gas mixture. A few companies do make dual line tubes, but you can imagine the initial cost is murder.

    I used to have a short tube that switched from red (632.8 nm) to orange (611.9 nm) that appeared brighter then the red when it felt like it.

    I sometimes wonder if there are a few more HeNe transitions we don't know about. I know they exist in ion lasers. I have seen a 575 nm yellow line in krypton that's not on the manufacture's data and a red in Kr that is between 633 and 647 nm. I had that red in my own laser. 575 nm is preferred for show lasers because it doesn't share transitions with 647 nm like 568 nm does.

    When I was interviewing at AVI in Florida they used 4 color 4 scan pair projectors for digitizing - 6 mW of yellow, 5 mW of green, and 8 mW of red, all from HeNe lasers. The blue came up from an ILT ion laser in the basement to each of the four stations via optical fiber. The guy who owned AVI said if you call Melles Griot and ask nicely they will grade some tubes for you for a slight extra cost. Methinks they make all the special colors up and tune them in power somehow, so they can make a price differential, those lines should be consistent by now.

    Every two years of so it seems Melles Griot cleans out their scrap pile, and somebody always seems to get there hands on them, grades them and sells em.

    (From: Daniel Ames (

    The yellow and orange HeNe energy transitions are very similar and possibly competing with each other, especially if the optics are questionable. I have learned that Melles Griot and other HeNe laser manufacturers sometimes suffer from costly mistake on a batch of tubes due to the optics being incorrectly matched to the tube and/or the optics themselves not being correct for the desired output wavelength. One such batch was supposed to be the common red (632.8 nm) but the optics actually caused the gain of the orange to be high enough that the output contained both red and orange (611.9 nm). Then I believe they are rejected and tossed out, only to be saved by professional dumpster divers to show up on eBay or elsewhere. Actually, these misfits such as the yellow/orange tube can be quite fascinating. It would be interesting to shine a 632.8 nm red HeNe laser right through the bore of that tube while powered and see what color the output is. I have been told that if you shine a red HeNe through a green HeNe that it will cause the green wavelength to cease. I have not had this opportunity to try this, so I do not know for sure what really happens, maybe the red just overpowered the green beam. This could be verified with 60 degree prism or diffraction grating on the beam exiting the opposite end of the green tube. Happy beaming. :)

    (From: Sam.)

    I have tried the experiment of shining a red HeNe laser straight down the bore of a green HeNe laser (my green One-Brewster tube setup). I could detect no significant effect using a low power (1 or 2 mW) laser. This isn't surprising given that the intracavity power of the green laser was probably in the hundreds of mW range so the loss from the red beam would be small in a relative sense. However, wavelength competition effects are quite real as evidenced from experiments with the two color 05-LYR-170 tube.

    The Weird Three-Color PMS HeNe Laser Head

    I recently picked up a surplus PMS (now Research Electro-Optics) LHYR-0100M HeNe laser head (with power supply) on eBay for a whopping $30 including shipping. This model supposedly produces a pure yellow (594.1 nm) multimode beam with a minimum power output of 1 mW. See REO LHYR-0100M. But mine is happily outputting the yellow (594.1 nm) and two orange (604.6 and 611.9 nm) lines (determined by splitting the beam with a diffraction grating, something I routinely do with all newly acquired HeNe lasers!).

    Its actual total power output after warmup is over 2.50 mW. The 594.1 nm (most intense, TEM01* doughnut) and 604.6 nm (TEM10* or TEM10 depending on its mood) are relatively stable but the 611.9 nm (least intense, TEM01) visibly fluctuates. Nonetheless, overall power stability and mode cycling behavior are similar to that of a typical medium power red (632.8 nm) HeNe laser, which contrasts dramatically with the very unstable yellow/orange Melles Griot laser described above. REO does have a couple of dual wavelength HeNe laser heads listed on their Laser Products Page but nothing like this. They are 1,152/3,391 nm and 1,523/632.8 nm.

    There is also an additional mystery 2 pin connector on this laser head. The resistance between pins is about 20 ohms and I assume it to be a heater on the OC mirror, though driving it with about 10 V had no detectable effect whatsoever.

    However, I wonder if there is also some screwup in the REO model descriptions as the size of this laser head actually matches that of the REO LHYR-0200M, being almost 17" in length rather than the 13" listed for the LHYR-0100M. I kind of doubt that shorter length can be accounted for by dramatic improvements in HeNe laser technology since my sample was manufactured (1988), though I suppose that's a possibility. But the electrical specifications of the two lasers are supposed to be identical, which doesn't make sense and I don't believe in coincidences. :) And the output power of my sample peaks at 6.5 mA which isn't consistent with the specs for either the LHYR-0100M or LHYR-0200M which are both 5.25 mA.

    The PMS/REO External Resonator Particle Counter HeNe Laser

    This is a particle counter assembly labeled: ULPC-3001-CPC, 18861-1-16 with the actual HeNe laser tube labeled: LB/5T/1M/E(HS), PMS-4877P-3594. The unit is shown in PMS/REO ULPC-3001 Particle Counter HeNe Laser Assembly. When I found it on eBay, the listing was for a One-Brewster tube. However, this one is really strange. For one thing, it is not a Brewster tube but rather a somewhat normal internal mirror HeNe laser tube. Well, at least normal by PMS/REO standards - mostly metal with Hughes-style glasswork at the anode-end. Except it is a very multimode tube having an output that is rather high (greater than 7.5 mW) for its length (11 inches between mirrors) and power requirements 1,900 V/5.25 mA. That would be only modestly interesting. But there is an additional mirror beyond the OC (inside in the area between the two red dots next to the red sticker at the left) which forms an external resonant cavity with the (internal) OC mirror. The external HR mirror is actually coated on the end of a transparent crystal about 1 cm in length, mounted by a pair of electrodes attached to opposite sides which most likely is piezoelectrically active and probably twists when a voltage is applied to it. A photodiode is mounted beyond the crystal (far left in photo). Based on the mounting arrangement, it would appear to be used for slightly wobbling the external mirror. The signal from the photodiode shows resonance effects at several relatively low frequencies (two dominant ones are around 175 and 350 kHz). The waste beam from the HeNe laser HR mirror can actually be seen to flicker and become much lower in power at the resonance points. The crystal and photodiode may be used to dither the output so that the effects of the inherent laser noise are eliminated. I doubt its supposed to be a very high frequency because the wires to the electrodes are not shielded. It might also be used in a feedback loop at low frequencies.

    Any hapless particles that may pass through the beam in the cavity between the OC of the HeNe laser tube and the external mirror will result in scatter detectable from the side. A large reflector and aspheric lens collects the side-scatter and focuses it on another photodiode (under yellow CAUTION sticker). There is a preamplifier in the box.

    It gets better. Viewing the waste beam out the unused HR-end of the tube (far right) with a diffraction grating reveals that the tube is lasing on the normal red line, but also on both of the HeNe orange lines (604.6 nm and 611.9 nm), two other red lines (629.4 nm and 635.2 nm), *and* on the very rare Raman shifted red line at 650 nm. And there may be others but it's difficult to resolve them since the beam is multimode and the spectra cannot be focused to small spots. This is similar but even better than what I've observed in my experiments using external mirrors with normal internal mirror HeNe laser tubes although this one seems particularly stable with little obvious variation in the intensities of the lines, at least over a period of a few minutes. Obtaining the 650 mm line is particularly unusual, especially since it is so stable. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes. These non-632.8 nm lines are probably not an objective of the design but are just an interesting artifact.

    I have estimated the reflectivities for the three mirrors which are in this laser. These values are based on measurements of the output power of the HeNe laser tube without the external mirror (about 8 mW after warmup) and the assumption that the internal OC is about 99 percent:

                                                Power with external HR?
        Mirror Description     Reflectivity         No         Yes
        HeNe laser tube HR     99.99%             0.9 mW     1.00 mW
        HeNe laser tube OC     99% (assumed)      8.00 mW   80.00 mW
        External HR            99.9%                --       0.09 mW

    The "Power" refers to the optical power passed by the specified mirror depending on whether the external HR mirror is present and aligned. In the case of the HeNe laser tube OC with the external HR, this is the circulating power in the external cavity which is what's available for the particle scatter. Note that the circulating power inside the HeNe laser tube is around 10 WATTS but isn't accessible.

    And here are some comments on particle counter technology:

    (From: Phil Hobbs (

    There exist particle counters using external resonant cavities, and also intracavity Nd:YAG setups. Intracavity measurements *look* as though they give you amazing sensitivity, but they usually don't. Not only is the circulating power amazingly sensitive to temperature gradients and tiny amounts of schlieren from air currents, but the signal you get is wildly nonlinear and highly position-dependent. Intracavity measurements are a great way to lose sleep and hair. Passive cavities are usually much better, and nonresonant multipass cells are better still.

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