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.
Below are just a few possibilities.
(Portions from: Chris Chagaris (pyro@grolen.com).)
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.
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 (dulmage@skypoint.com).)
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'.
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.
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 (osteven@akrobiz.com).)
"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
Notes:
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.
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.
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.
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
where:
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 (h.rutt@ecs.soton.ac.uk).)
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:
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.
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 (office@roithner-laser.com).)
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...
(Portions from: Steve Roberts (osteven@akrobiz.com).)
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 (h.rutt@ecs.soton.ac.uk).)
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.
Here is a rough idea of what transverse modes might look like for a
rectangular cavity:
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.
All of these are really somewhat equivalent and simply mean that more than
one mode fits inside the mode volume.
Where there is access to the inside of the cavity (as with a one-Brewster
tube), a laser that operates multimode can be forced to operate TEM00 with
a stop (aperture) between the external mirror and tube-end. However, there
will be a (possibly substantial) reduction is output power. Where both
mirrors are external, it may be possible to substitute longer RoC mirrors
to force TEM00 mode (again at the expense of some output power).
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.
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.
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!
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:
There is also usually an increase of power due to the heating
of the laser tube (independent of thermal expansion effects) as
well but this may be only a fraction of the effects of alignment.
In addition, especially with soft-seal tubes, there may be a
power increase as the cathode, acting as a weak getter, removes
contaminants from circulation.
Depending on the particular laser, the initial output power can
be very low even where the final output power exceeds rated power.
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.
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.
In particular:
See the section: On-Line Introduction to
Lasers for the current status and on-line links to these courses, and
additional CORD LEOT modules and other courses relevant to the
theory, construction, and power supplies for these and other types of lasers.
The following modules would be of particular interest for HeNe lasers (all
in PDF format):
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.
This fabulous ASCII rendition of a typical small HeNe laser tube should make
everything perfectly clear. :-)
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.
The discharge at this end produces little heat or damage due to sputtering.
This is a 'cold' cathode - there is no need to heat it (like the ones
in the electron guns of a CRT) for proper operation and no warmup period
is required before the tube can be started.
The discharge is distributed over the entire area of the can thereby
spreading the heat and minimizing damage due to sputtering which results
from positive ion bombardment. For this reason, although the laser may
appear to work (in fact, starting tends to be easier) a HeNe tube should
not be run with reverse polarity for any length of time (e.g., more than
a minute or so, preferably a lot less) since damage to the anode (now acting
as a cathode) and its mirror would likely result. See the section:
Damage to Mirror Coatings of Internal Mirror
Laser Tubes.
The can-shaped structure is also called a 'hollow cathode' for obvious
physical reasons - it is a tube electrode that is large in diameter and
hollow like a piece of pipe - and because the plasma discharge flows inside
of it. It operates in the abnormal glow current density gas discharge
region (should you care). The surface of the cathode can is also not
pure aluminum as it appears, but is processed with a very thin layer
of oxide which eventually gets depleted, and this is the main determination
of tube life. Hollow cathodes are usually used where a tube needs lots of
slow moving electrons to excite the gas. They are currently used mainly
in HeNe lasers but have been applied to other types of gas lasers having
modest current requirements.
Very old HeNe lasers (and some others, old and new, like argon ion) use a
heated filament which also acts as the cathode instead of the cold cathode
design. This structure can be much smaller than the cold cathode but the
added complexities of manufacture, the additional power supply, and the need
for a warmup period have delicated it only to those applications where there
is no other choice. See the section: Strange
High Power HeNe Laser for an example of this technology.
A very few, very tiny HeNe laser tubes, use a small ring-shaped cathode
made of either zirconium (expensive) or aluminum. These were likely
designed for special applications, presumably requiring very small size
or fast turn-on response (due to the reduced capacitance). The examples
of these HeNe tubes I've seen are about 5" long by 1/2" in diameter. Life
expectancy using the aluminum version (at least) is probably quite limited
due to sputtering (since the electrode is very close to the bore, which
promotes this due to the increased field gradient).
On some (mostly larger) HeNe tubes, the bore may be ground (but not polished)
on the outside, inside, or both:
Note that since the frosting process is done chemically (hydrofluoric
acid etch?), the bore will become marginally wider and care must be taken
that this doesn't result in multimode (non-TEM00) operation if it goes too
far!
There have been some experimental HeNe lasers built with an elliptical or
rectangular bore to get around the limits on power imposed by small bore
tubes. (Normally, gain is inversely proportional to bore size so just
using a large bore doesn't work.) Apparently, such lasers have generated
over 300 mW in a package the size of a PC tower but were never developed
commercially.
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.
The getter material is then available to chemically combine with residual
oxygen and other unwanted gas molecules that may result from imperfect
vacuum pumps and contamination on the tube's glass and metal structures
(e.g., from the surface as well as in fine cracks and other nooks and
crannies). It will also mop up any intruder molecules that may diffuse
or leak through the walls of the tube during its life. Helium and neon
are noble gases - they ignore the getter and the getter ignores them. :-)
Should the getter spot (if visible) turn to a milky white or red powdery
appearance, it is exhausted and the tube is probably no longer functional.
If you had grown up during the vacuum tube age, the getter would be familiar
to you since nearly all radio and TV tubes had very visible silvery getters
(and CRTs still do).
The getter electrode can be seen in photos of a Typical
Small to Medium Size Melles Griot HeNe Laser Tubes. However, no getter
spots are visible. I have found many tubes where there is a getter electrode
present but the getter spot is undetectable. Some modern getters use a
zirconium based material which is colorless as opposed to old style getters
which were barium based with a very visible spot. (Really long life HeNe
tubes like those from Hewlett-Packard actually use a zirconium cathode. They
are rated for a 100,000 hour life!) It's also possible that the getter
was included as insurance and never activated. I suppose that modern
vacuum systems and processing methods are so good and hard-seal tubes
don't really leak, so there is not as much need for a getter as there
used to be.
Note that a high mileage HeNe (or other gas discharge) tube may exhibit
metallic deposits (usually) near electrodes which look similar to the getter
spot. However, these are due to sputtering and won't change appearance if
there is a leak! The tube is usually near death at this point in any case.
The mirrors used in lasers are a bit more sophisticated than your bathroom
variety:
However, note that for a sufficiently long HeNe tube (one with high enough
gain), it would be possible to use a pair of freshly coated or protected
aluminum mirrors though performance would be pretty terrible. And, getting
a useful beam out of such a laser would be difficult because aluminized
mirrors tend to not be even partially transparent! I've gotten
a 10" long HeNe tube with an internal HR and Brewster window at the other
end to lase using the aluminized mirror from a barcode scanner - just
barely. But the first HeNe laser would not have been possible without
dielectric mirrors despite its length since the wide bore resulted in
very low gain.
I have also come across HeNe laser output mirrors with a slight *negative*
RoC - they are convex rather than concave with respect to inside the cavity.
At first I thought these were a mistake, coating the wrong sides of the
mirror glass or something like that. But the slightly convex curvature
does indeed result in a stable resonator configuration and actually has
a slightly lower divergence than a similar concave mirror when tested in
my one-Brewster external mirror HeNe laser (though I can't
tell if this might also have been more due to the curvature of the outer
surface). I have since found a sample of a HeNe laser tube (probably from
a barcode scanner) that had such a mirror, though it's certainly not a
common configuration.
You may be able to tell which type you have by looking at a reflection off
of the inner surfaces of the mirrors at each end (assuming the one at the
non-output end is not painted or covered). Assuming the outer surfaces
are flat, a concave mirror will reduce the size of the reflection very
slightly compared to a planar mirror. If wedge is present, the reflections
from the front and back (interior) surface of the mirror will shift apart as
you move further away (though this may be tough to see on the Anti
Reflection (AR) coated output mirror since the reflection from the AR
coated surface will be very weak). See the section:
Ghost Beams From HeNe Laser Tubes.
To further complicate matters, the front (outer) surface of the mirror at
the output-end of the tube may be ground to a (slight) convex or concave
shape as well resulting in either a positive lens which aids in beam
collimation or a negative lens with increases the divergence.
Since the reflection peaks at a single wavelength, this type of mirror
actually appears quite transparent to other wavelengths of light. For
example, for common HeNe laser tubes, the mirrors transmit blue light quite
readily and appear blue when looking down the bore of an UNPOWERED (!!) tube.
Blue light from the electrical discharge will also pass out of the
mirrors as a diffuse glow when running. No, you don't have a blue HeNe
laser!
Also see the section: Mirror Reflectances
for Some Typical HeNe Lasers.
There should be no reason for the
alignment to have changed unless you whacked the tube - it was set at the
factory. However, if you suspect an alignment problem, it is easy to check.
Then, you can decide if attempting an adjustment is worth the risks. See
the section: Checking and Correcting Mirror
alignment of Internal Mirror Laser Tubes.
However, long high power tubes (i.e., 20 mW and up) may require fixtures to
maintain mirror alignment even when the mirrors are internal. Such tubes
will not be stable by themselves because thermal expansion will result in
enough change in alignment to significantly alter beam power - even to the
extent of extinguishing the beam entirely at times! There may even be a
'This Side Up' indication (not related to the orientation for linearly
polarized tubes) on the HeNe tube or laser head as gravity affects this as
well (the alignment and thus power, not the gas, electrons, ions, or light!)
and can significantly affect operation. I do not know if this sort of
behavior is common or only likely with tubes that are marginal in some way.
There is a slight benefit to having the output coupler mirror at
the anode-end of the tube due to the typical long-radius hemispherical
cavity configuration. With the bore running almost to the mirror mount,
more of the mode volume is inside the bore and thus the gain will be
slightly higher. But the difference is only really significant for "other
color" HeNe laser tubes which have very low gain and these are more likely
to use anode-end output configuration.
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:
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!)
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.
(Portions from: Lynn Strickland (stricks760@earthlink.net).)
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.
But what is the underlying cause?
(From: A. E. Siegman (siegman@stanford.edu).)
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 gLongitudinal 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.)
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.
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.
O OO OOO Each 'O' represents
O OO O OO OOO a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
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:
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.
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.
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).
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.
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.
On-line Introductions to HeNe Lasers
There are a number of Web sites with laser information and tutorials.
EXP01 Emission and Absorption
EXP03 Fabry-Perot Resonator
EXP06 HeNe Laser
EXP20 Laser Safety
EXP27 Bar Code Reader
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
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. :-)
____________________________________________
/ _________________ \
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-----------+
Beam quality - There is a statistically significant reduction in
diffraction rings (stray light) around the main beam with a frosted bore
ID, though some designs are more susceptible to this than others. However,
sometimes requirements for a particular spot size or output power limit
options and the frosting will help.
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.)
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:
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. :)
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.
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.
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.
