Although one of the simplest in basic structure, given these requirements, the HeNe laser may not be the best home-built laser for the novice. However, later in this chapter, we present a number of alternatives to the HeNe laser fully built from the ground up using beach sand and copper ore. Therefore, it is still possible to experiment with partially home-built HeNe lasers (beyond just wiring together a HeNe tube and power supply) using various proportions of your own ingredients without doing everything from scratch. :)
Take precautions to avoid eye contact with the direct or reflected beam, whether or not it is visible. This includes the 4 pairs of beams reflecting off the Brewster windows which may be quite strong.
See the section: HeNe Laser Safety for more info. However, the home-built HeNe laser uses a different sort of power supply than CW commercial types (unless you are attempting something similar to one of these) so some of the specific details may not apply,)
For more information, see the chapter: Laser Safety and the more specific information in the section: HeNe Laser Safety. Sample safety labels which can be edited for this laser can be found in the section: Laser Safety Labels and Signs.
If anyone reading this has built (or even attempted) a HeNe laser from scratch, please send me mail via the Sci.Electronics.Repair FAQ Email Links Page!
(From: Mark Wilson (wilson_mark@htc.honeywell.com).)
"When I saw the Scientific American article on building your own HeNe laser, I decided that I wanted to build it. The Scientific American HeNe laser was extremely difficult to build and I could not have done it without a lot of help. I got Spectra-Physics to donate a set of laser mirrors to me, a glass shop in my home town to help me cut and grind the Brewster window angles on the tube. The tube was made from lead glass from a sign company, and I also used neon sign electrodes. The optical rail was a 3 foot long piece of 2"x6" extruded aluminum that I got from a glass company which used this material to make doors for commercial buildings.I followed the directions in the Scientific American article to the letter. I sealed the microscope slides to the glass tube using flexible colodian that I got from a pharmacy. I filled the tube at a sign company which had He, Ne, Ar and other gases on a glass manifold. I assembled the laser and made the power supply using a mercury rectifier tube and a neon sign transformer. I got the tube to lase for a brief time, but since it was not a hard-sealed tube it quickly died probably due to helium diffusion. The tube would light up but not lase for a while then that too stopped."
Refer to Typical Home-Built Helium-Neon Laser Assembly for a simplified diagram of the overall glasswork and power supply electronics.
Operation at wavelengths more than a few nm away from 632.8 nm (except possibly for 640.1 nm) would likely require changing the mirrors to have their reflectivity peak at that wavelength (unless they are very high reflectivity broadband coated), and using the appropriate angle for the Brewster windows (or implementing an adjustable Brewster angle as described in conjunction with the Home-Built Pulsed Argon and/or Krypton (Ar/Kr) Ion Laser Laser).
H o----------+ T1 T2 +---------o HV AC
)|| ||=||(
Variac )<--------------+ || ||(
0-115V )|| )|| ||(
5A )|| Neon Sign )|| ||( Well insulated
)|| Transformer )|| || +---+ HV wires to HeNe
+--+ 9kV,18mA )|| ||( | tube electrodes
| )|| ||( |
N o-------+-------------------+ || ||( |
||=||( |
| | +---|-----o HV AC
| | |
G o-----------------------------+---+-----+
_|_
////
However, there are two disadvantages to using a narrow bore:
I would recommend a one piece approach instead, with a thick-walled capillary fused directly to glass extensions (as used in the Vander Sluis et. al. paper) if your glass working skills and/or budget are up to it (or glued to metal extensions if using the no glass working approach).
No other aspect of the laser tube assembly itself is as important as the quality of the Brewster windows to the ultimate outcome of this project! While, certain types of distortion won't prevent lasing (some may even make it more exciting with complex mode structures), this is a complicating factor your first laser can do without.
The author of the SciAm article suggests the use of quartz instead of glass to minimize heat losses but that material has a high transmittance at the 3.391 um IR wavelength which interferes with operation at 632.8 nm (and other visible wavelengths). If you insist on quartz (perhaps to have the option of 3.391 um operation in the future), also obtain a pair of glass flats to add outside the tube to kill that line. If you do try lasing at 3.391 um, aluminum mirrors should be fine. This line will even lase with no mirrors (superradiantly) in a long enough tube.
CAUTION: Apparently, it is possible for an electron beam to be produced from the positive electrode during the high current bake-out step which can quickly melt a hole through the tube wall opposite the side-arm if left running for more than a few seconds. The visual effect will first be a spot of bright yellow sodium light from the point of impact. Use lower current and/or make that area of the tube (the outside of the turn) much thicker.
(From: George Werner (glwerner@sprynet.com).)
"I have read with interest your wealth of information on home-built HeNe laser because every problem you dealt with was one that we grappled with about forty years ago. I was a member of a group here in Oak Ridge that built the first HeNe laser in Tennessee about six months after Bell Labs announced the construction of theirs.Back in the 1960's we had a laser development group that decided that before we came out with a new kind of laser we should get laser experience by building one (HeNe) like Bell Labs. Ken Van der Sluis was the principal investigator. His prior experience with resonators was with the Fabry-Perot (FP) interferometer, so much of our construction was adapted from FP construction. The reflectors were 5 cm diameter in mountings that used 1/4 - 80 threads on the adjusting screws and Invar rods to maintain spacing, a case of overkill on every turn.
The finest window material we had were quartz FP flats 5 cm diameter x 1 cm thick, and these were cemented onto the Brewster-cut tube ends. Ken was measuring gain with a spectrometer, adjusting mirror angles, gas mix, gas pressure, and discharge current, trying to find the magic combination but with no luck. Then Ken realized that it was possible that the infrared transition at 3.391 um was depopulating the upper level of the transition we wanted to use. After adding a borosilicate crown glass (BSC) flat (which blocks 3.391 um) to one of the windows with masking tape, and some realignment of the mirrors, it wasn't long before he got the first spark of red light - the first HeNe laser light in Tennessee! Soon word got around and for the next day or two we had dozens of visitors to see this fascinating red sparkling light. (It was our opinion that Bell Labs did not know about the 3.39 um trap and that they were lucky to have tried BSC first.)
The lasers we made were made to get that sparkly red light and we were not concerned about the mode structure (except for our theoretical physicist who was fascinated by all the different multi-mode patterns we could get with our wide bore tube and he had names for them all). In fact, we soon abandoned using thick Brewster windows with optically flat surfaces, and sometimes used ordinary microscope slides.
This work led to our development of a demonstration laser which we took to universities and a few high schools mostly over the eastern United States, and also to South America and Hungary.
Everything can be found in the paper: "A Simplified Construction of a Helium-Neon Visible Laser", by K. L. Vander Sluis, G. K. Werner, P. M. Griffin, H. W. Morgan, O. B. Rudolph, and P. A. Staats in the American Journal of Physics, vol. 33(3), pp. 225-240, March 1965.
(Here is another paper from the same group at Oak Ridge National Lab that should probably be in another chapter but I put it here: "Conversion of a Simplified HeNe Gas Laser to Pulsed Operation with Ar, Kr, and Xe", H. W. Morgan, P. A. Staats, P. M. Griffin, G. K. Werner, and K. L. Vander Sluis, American Journal of Physics, vol. 37(9), pp. 938-939, September 1969. --- Sam.)
My most important contribution to the effort was that I was the inventor of the Laser Alignment Card, which you allude to later in this chapter. I could go on for several pages talking about lasers. We old-timers love to talk and reminisce!"
(George has since gone on for several pages talking about lasers and has contributed several sections relating to the home-built HeNe laser, amateur laser construction in general, and other laser topics.)
The HeNe laser presented in the paper is very similar to the SciAm design which isn't terribly surprising given that it was published in the same time frame. A photo in the paper shows the minimalist approach to laser design - the laser tube as well as the resonator mirrors supported by chemist's burette clamps on wobbly ring-stands with a Tesla-type leak tester for excitation! Well, maybe. Some of may partially home-built laser test rigs were a lot less stable. :) OK, this isn't what they recommend building (or what is described more fully in the paper and below) but it was included to drive home the point that you don't need a lot of sophistication to construct a working laser.
I really liked the suggested supplier list (with 1965 prices!) which was thoughtfully included with my copy of the reprint of the paper. Optical windows for 25 cents; 1 liter flasks of He:Ne gas mixture for $6.50, and (small) neon sign transformers for $9.95. If only these companies were still n business today (using those same prices)! What I would give for a working time machine. :) One interesting thing is that while some items were quite inexpensive (if inflation wasn't taken into account), the dielectric mirrors were priced quite high - after all, this was new technology! And similarly, semiconductor rectifiers, which are dirt cheap today, were 10 or 20 times as expensive in 1965 dollars - much much more if inflation is included.
There is one piece of information that can be inferred from the paper that is lacking from all of the SciAm articles: The actual optical power output. Based on their use of a silicon photodiode detector, I expect that it peaked in the .5 to 1.25 mW range depending on gas fill ratio and pressure. This assumes a photodiode current sensitivity of .4 to .5 mA/mW. Note that this was with mirrors that were about half as efficient as modern ones (they had significant absorption losses) and they were both OCs so an equal mount of power exited both ends. Thus, it would appear that up to about 5 mW may have been possible by using a modern HR/OC pair. Maybe I could use that time machine to take them a set. :)
Here is a description of the HeNe laser presented in the paper summarized in my standard format. Some of the dimensions below were estimated as there is no dimensioned drawing in the paper. The authors suggest a variety of possible modifications as well. While the title of the paper implies a simplified approach, the authors did have access to a decent machine shop (including lathe and diamond cutoff wheel) and glass working shop (the glass fabrication looked perfect). However, like the SciAm lasers, this really isn't essential.
D1 R2 Rb
H o-------+ T1 T2 +-----+--|>|--+--/\/\--+--/\/\--+--o HV+
)|| ||=||( | 10KV |25K 20W | |
Variac )<--------------+ || ||( | _|_ C1 | |
0-115V )|| )|| ||( | R1 --- .25uF | v Spark gap
5A )|| Neon Sign )|| ||( | 170 | 2KV _|_ C3 ^ 4KV
)|| Transformer )|| ||( +--|-/\/\--+ --- .1uF |
+--+ 3kV,54mA )|| ||( | | 5W | C2 | 5KV \ R7
| )|| ||( | | _|_ .25uF | / 50K
N o----+-------------------+ || ||( | | --- 2KV | \ 10W
||=||( | | D2 | | |
| | +--+ +--|<|--+--------+--------+--o HV-
| | 10KV
G o--------------------------+---+
_|_
////
As noted, T2 is actually three, 3 KV, 18 mA, neon sign transformers wired in
parallel to obtain the 54 mA current rating. Rb, the ballast resistor,
consists of 4, 25 K, 20 W resistors in series with taps to adjust the tube
current.
Note that the voltage ratings of C1 to C3 are marginal at best. Should the spark gap fail, the output voltage could easily climb to nearly 10 KV.
A modern tube with a 34 cm discharge length would be rated about 5 mW when run from a normal HeNe laser power supply (DC, constant current).
For an HeNe tube, a wider bore doesn't necessarily permit higher power since the walls of the capillary apparently are important in depopulating higher energy levels. Gain actually decreases (inversely) with increasing bore diameter. I will assume that this is really equivalent to a 1 mm bore in a modern tube (and this may be optimistic).
Loss factor: .5.
Loss factor: .5.
Loss factor: .5.
Loss factor: .5.
Based on these considerations, I would be surprised if the original design produced more than .5 mW. But the good news is that it might be possible to approach 2 or 3 mW without too much effort using a narrower bore, large can style cathode, and modern HeNe laser power supply.
The very similar design described in the Verder Sluis et. al. paper (see the section: K. L. Vander Sluis et. al. HeNe Laser), had a maximum power on the order of 1 mW under optimal conditions based on their measurement of power using a silicon photodiode. So the wild guesses aren't all that far off. :) (However, it may have been capable of as much as 4 or 5 mW using a modern HR/OC pair.)
The SciAm article recommends using quartz windows because they have the lowest losses. We thought the same thing back in 1963 when we were trying to build a laser like the one that had been demonstrated by the Bell Labs people a few months earlier. Ken Vander Sluis had built our laser with two of our best quartz Fabry-Perot interferometer plates and was trying for days to make it run, with no luck.
Ken is a spectroscopist and understands how energy levels work, so he thought about it, and reasoned that the 3.391 um transition might be depopulating the upper level so that the visible transition wouldn't lase. For the 3.391 um wavelength, quartz transmits well, but borosilicate and other glasses do not. He got another pair of interferometer plates, made of borosilicate crown glass, and put them over the quartz plates, fastening them in place with masking tape. After realignment he found an increase in gain and before long he got that first sparkle of light, the first HeNe laser in Tennessee!
So the moral of the story is: Don't count on quartz windows giving you the best performance. "But", you may ask, "What about those lasers with no Brewster windows?" I have never measured their reflectance, but my guess is that the mirrors must have a multilayer surface that was designed with two criteria: high reflectance at 632.8 nm and low reflectance at 3.391 um.
Even with glass Brewsters, the 3.391 um effect rears its ugly head in long lasers, those with length of one meter or more. In these the path length in the plasma before encountering a window is long enough that the 3.391 um density has a chance to build up to undesirable levels before meeting a window. It's called "superradiance" and can be suppressed with magnets (preferential Zeeman splitting of the IR lines) and/or by grinding the inner surface of the tube to scatter low-angle reflected light. But, tubing with a roughened interior is not as strong as standard tubing.
(From: Sam.)
Yes, that is exactly how they are designed. With sufficiently high transmittance at 3.391 um and possibly the frosted bore as well, the need for magnets to split the energy levels via the Zeeman effect has also been reduced or eliminated. For example, even the Melles Griot 35 mW HeNe laser (their largest model) does not need magnets even though the tube is almost a meter long.
The alignment is performed with the laser powered but presumably not lasing since a 'spoiler glass' - a glass microscope slide - is placed in the optical path. While this probably is fairly reliable with minimal risk for the low gain HeNe laser described in the article, many other lasers - or even a longer HeNe laser - may have high enough gain that the losses introduced by the spoiler would NOT prevent lasing and a beam could appear without warning (once the mirrors are aligned well enough) as the adjustment screws are being tweaked! I would NOT recommend the procedure as described for any laser unless a more reliable method were used of preventing accidental lasing (like the use of a 50 percent neutral density filter) or you were absolutely sure of the maximum possible output power of your laser to be less than a couple of mW. Note that even with a spoiler, there is still a slight chance that a HeNe laser will lase if the glass is nearly perpendicular to the optical axis (due to constructive interference of the reflections from its surfaces). A neutral density filter would totally eliminate even this small possibility.
The description as presented is also somewhat ambiguous (but this is clarified below).
Note that if you don't have 20/20 (corrected) or better vision, this procedure may not be appropriate in any case since for the fine alignment, it's necessary to view the reflection from the mirror at the far end of the laser through its narrow bore - not easy with less than perfect eyes.
I would recommend using one of the other alignment techniques described in "Light and its Uses" or in the chapters of this document on HeNe and Ar/Kr lasers.
Having said all that, I am honored to have George Werner, the inventor of the this alignment technique while at Oak Ridge around 1963, address the safety issues and provide a clearer description of the procedure:
(From: George Werner (glwerner@sprynet.com).)
(The alignment card technique may have been independently invented elsewhere.)
First, the matter of safety. If you are looking through the alignment card at the time the laser starts to oscillate you will see a very bright light, but it won't be the last light you ever see. Have you ever looked at a camera flash bulb when it went off? Have you ever looked at the sun? These sources are too bright for normal viewing so nature gives us a defense for it - - we close our eyes immediately. As for the laser's brightness, the first burst of light, if adjustments are made slowly, is much less than the maximum output. Even this exposure can be avoided, as I will discuss later.
The alignment card I have used most recently has a hole about 2.5 mm in diameter. If you're worried about exposure you can make it smaller but that makes precise seeing more difficult. (In our report by Vander Sluis et al, we used .5 mm) The card stock is preferably heavier than normal filing cards, but they will do. The hole is made by drilling to ensure a round shape, but this leaves paper fibers protruding into the hole, which can be made to lay down by polishing the inner edge of the hole with wax or maybe glue. On the front side carefully rule a horizontal and a vertical black line across the hole. It is important that the intersection of these lines be on the hole, and sometimes I think that is easier to rule the lines first and make the hole afterward. A piece of a red gelatin filter, Wratten #29, is mounted on the back side of the hole. I have stuck it in place with a 1/2 inch circle of black masking tape with a 1/8 inch hole in it. Having an area of black around the hole as viewed from the back makes it easier to find and look through. In the deluxe model of card I use a fluorescent red surface on the front instead of white.
The card is used in this way: Position the card at one end of the laser, viewing through the near reflector, so that you can sight through the card hole and through the capillary to the far end. (To facilitate this it may help to place a strongly illuminated target beyond the end of the laser. The fine print on the back of a credit card is good for this purpose.) Holding the alignment card in this position, observe the crosslines as reflected from the back side of the reflector and adjust the reflector so that the pinhole image lies on the capillary axis. Now the adjustment at the near end is complete (we hope).
Next, take the card to the other end and repeat the operation. Note that at no time up to now has the laser been turned on so your eyes should be perfectly safe. NOW turn the laser on and it will shine with all its brilliance (it says here). :)
If, however, the laser doesn't lase, make another inspection of the adjustments. This is where the red filter is needed. If you try to sight down the capillary while the laser is turned on, all you see is a cloud of blue light if you don't have a red filter. One experimenter reported the red filter was ineffective. I suspect that he wasn't using a Wratten #29. The red cellophane from a box of Valentine candy won't work. It passes too much yellow. With a proper filter you can easily see to the far end of the tube through the luminous plasma. With the laser turned on, look through the card and plan your next adjustment of the mirror but don't make it. Then move your eye away from the pinhole and make the adjustment you planned. If it doesn't lase, you can look through the pinhole and repeat the motion to see what is happening at the far end of the tube. If your system is geometrically correct but still not lasing, you will see a brighter (but not brilliant) disc of light coming into position at the far end as the final adjustment is made. This is what is commonly called the "full moon" effect. If that bright spot is well centered when viewed from either end, then you can be assured that no further mirror tweaking is called for and you can turn your attention to gas pressure, current level and all those other problems.
I mentioned the use of a red fluorescent alignment card. What is the advantage of that? The only useful light reflected from a white card with red filter is the red component of the illuminating light. A fluorescent red card reflects the same red light but it also converts the blue, green and yellow light to red, giving us a brighter image.
I haven't found a laser where it was not possible to see the far end of the tube looking through the card. Maybe the sighting hole was too small (like 1 mm or less) or maybe there was fuzz in the hole, or maybe as I have said before the filter wasn't red enough.
In a very long narrow tube it is sometimes hard to determine straightness because internal reflections of a curved tube can give false images of the end opening. The curvature of the tube focuses the light in one plane, while at the same time in the other plane the strong focusing power of the bore radius is decreased at grazing angles, so that there is a curvature for which the reflected image has no astigmatism. Once I made a little light box to use as a target for this test. In front of a 15w light bulb I placed a wire screen mounted on a motor shaft to turn 6 rpm. I set it so that the target was moving left to right. Then if I looked at it through the capillary and saw an image moving right to left I knew that I was looking at a reflection.
When you have confirmed good alignment and the windows are clean, you may wonder if the gas mixture is right. In lasers that have been sealed a long time there is sometimes a noticeable loss of helium by diffusion through the glass and through the Epoxy of soft-seals. A good way to check the He:Ne ratio is to view the discharge spectroscopicly. I used to use a transmission grating (600 lines/mm) for this. In the yellow region you will find two lines close together. These are neon, 585.25 nm, and helium, 587.56 nm. If the mix is right these two lines should appear equal in brightness.
For a sealed tube, the helium lost by diffusion can be restored by putting the laser in an atmosphere of 100% helium (at 1 atm) for a day or two. 24 hours of inward diffusion this way is about equal to the outward diffusion of a year. (See the section: Rejuvenating HeNe Tubes.)
I do have a working green HeNe laser using a special one-Brewster HeNe tube with an HR optimized for green and a matching HR mirror. OK, so the output isn't anything to write home about - maybe a uW - but the circulating green photon flux is fairly impressive. However, this setup is just about perfect in every way with an optically contacted fused silica Brewster window and super high reflectivity mirrors for both HRs. See the section: A Green One-Brewster HeNe Laser for details. But it's amazing that such a short (26.5 cm) one-Brewster tube will lase green at all!
Also see the section: More on Other Color HeNe Lasers.
(From: Steve Roberts (osteven@akrobiz.com).)
From a letter in the IEEE Journal of Quantum Electronics by D.L. Perry, inventor of the green HeNe laser:
"Green #1 used a 65 cm long, 4 mm ID tube with a 7:1 fill ratio of He:Ne. Both optics had a 1% transmission at 594.1 (yellow) to kill that line. The current range was 16 to 40 mA (!!).The 611.8 nm (orange) line was used to align the laser (using red/orange optics). Then the green HR mirror was installed in the beam to align it and then the red/orange optics were removed."
He obtained a power of 50 uWatts with this setup.
(From: Sam.)
Hey, but that's still much greater than the output power of that green one-Brewster HeNe laser and also greater than the output power of my red two-Brewster HeNe laser described in the sections starting with: Sam's DIY External Mirror HeNe Laser - Some Assembly Required!.
Yes, it's possible because I have done it. The trick is to mill or saw a 1/2 inch aluminum plate to hold the window at 56 degrees to the axis of a hole you drill through it to receive the small bore laser tube. A side hole connects with this hole into which is glued a nipple for attachment to a vacuum system. Drill and tap a 6-32 hole somewhere for a screw to attach a wire because this piece is also an electrode. The one I made had a few extra square inches extending below so that I could put it in a beaker of water to dissipate the heat from the electrode. The allowable temperature of the aluminum is limited by the heat tolerance of the Epoxy you use to glue it together. Mine was a DC laser so only one end (the cathode) needed to be this large.
A photo of that part of the tube is shown in Cathode-End of Home-Built HeNe Laser Requiring No Glassworking. The aluminum block cathode and negative power supply lead can be seen with the Brewster window glued to its angled surface. The glowing bore of the tube extends toward the upper left corner. For scale, the platform is 3 inch aluminum channel and that's one of my early (white) alignment cards in the lower right.
The anode was similar but without the heat sink and vacuum attachment. Keep the kids away from it and/or make arrangements to insulate the high voltage electrode electrically but not thermally.
(From: Sam.)
Sputtering at the negative electrode would be my other concern. If it is near the Brewster window or internal OC mirror, a metal coating could form quite quickly, rendering the laser useless. I would recommend locating the cathode a few inches away - perhaps it could be a second aluminum block or an aluminum tube attached to the end of a glass side-arm glued into the block described above. The advantage of this geometry is that there is no direct line-of-sight path to the optics and thus sputtered material is much less likely to land there. Putting a few heat sink fins on this should provide adequate cooling if it becomes more than warm to the touch.
Even simpler: Use common pipe fittings at each end, one being a "T" for a side-arm mounted glass extension to which the aluminum cathode is attached.
(From: George Werner (glwerner@sprynet.com).)
Here is a laser design that I planned 35 years ago but never built. If you want to try it out you are welcome to it.
If one wanted to make a really long laser, one way to keep the 3.391 um radiation suppressed would be to stick a glass (not quartz, remember?) Brewster window in the optical path every 50 to 100 centimeters. Also, if we want to maintain the conditions of a conventional laser, we need to refocus the light rays periodically along the length. So the Brewster thing needs to be a lens rather than just a window. But a lens tilted to 57 degrees would have terrific astigmatism. OK, then have the window start with a reverse astigmatism such that when tilted to 57 degrees the astigmatism vanishes. Or, use a normal lens that is AR coated. If the AR coatings are really good then tilt should be unnecessary. (Note that the standard formulas for reflectivity won't work at an AR coated surface - I don't know if there is a Brewster angle for an AR coated surface.)
The change in focal length from tilt is a relatively simple matter for concave reflectors (cosine for one direction, 1/cosine for the other), but for lenses it is more complex. I did a rough check on focus of a tilted lens at 0, 30, and 60 degrees and it showed that the focal distance (can we still call it focal length?) is the original focal length times the square of the cosine of the tilt angle for focus in the plane of the tilt and times the square root of the cosine of the tilt angle in the orthogonal plane. Then I did some more exact ray measurements on the computer and instead of cos2. I found it was closer to cos2.55. (Close, but not exact). I contemplated computing the focus in the other plane but decided that was too much bother since I don't even know if anyone is going to read this. (Well, read?, yes; build?, hmmm. --- Sam.)
Therefore the lens/window, if it is to have a tilted focal length of 50 cm, should have a conventionally-measured focal length of 220 cm in one plane. and 67 cm the other way. The practical way to get such a lens is to deal with a manufacturer of spectacle lenses, so we have to translate to diopters and round to the nearest 1/8 diopter. Thus we end up with a refractive power of 0.50 diopters in one direction and 1.5 diopters in the other direction yielding tilted focal lengths of 45 cm and 50 cm (close enough). I guess the optometrist would call for "sph. +.50 , cyl +1.00". Make sure your lens maker understands that these curvatures are relative to a flat surface on both sides, or you may find you are given a lens that is strongly concave on one side as in standard spectacles. Perhaps each surface should be described as sph +.25 , cyl +.50. Check with the lens man.
After having one made and checking it out, order as many as you want for your laser. Each section has its own power supply and glass system (with a shared window) and they can be tested and added one section at a time. The completed design as I see it is a multiple confocal system with end mirrors of 100 cm radius and intermediate lens/windows with 50 cm focal lengths every 100 cm. The windows nearest the mirrors are flat. When testing an incomplete assembly the second concave mirror should be 100 cm away from the last lens/window.
In my imaginary design of this laser, each section has a glass ball joint near a window so that each section can be adjusted to be collinear with the rest, but I can also see it running as one huge glued-together contraption. After enough sections are added, it should lase without a terminating mirror, and this suggests that by that time it would have lost coherence. Who'd like to predict how much power it will produce? Who'd like to predict what limits the power?
The following sections describe various ways of ending up with a working HeNe laser that don't require quite as much in the way of support equipment and supplies as building one from the ground up. These include morphing a commercial HeNe tube into a home-built HeNe laser, using commercial one and two-Brewster HeNe laser tubes with your own resonator, and even a way of converting a cheap barcode scanner HeNe tube into a precision frequency stabilized laser. While perhaps not quite as rewarding as doing everything from scratch, the likelihood of success, particularly with the latter approaches, is much much greater.
Taking an existing HeNe tube and using it as the foundation for an external mirror laser would eliminate some of the hassle of constructing everything from scratch. Specifically, most glass work would be eliminated and by doing things in stages, the risks are somewhat reduced.
If you don't want to even think about vacuum systems and gas supplies, HeNe (and Ar ion) plasma tubes with Brewster windows for use with an external cavity ARE available from various sources. With one of these in-hand, and a matching conventional power supply (commercial or home-built), you can still experience the joy and frustration of constructing and aligning an external mirror laser head. I've even gotten lasing from a HeNe tube with a damaged OC mirror using an external mirror though I doubt there is another similar tube in the entire Universe so perhaps that isn't quite fair. :)
It then is *just* a matter of fabricating the laser platform and mirror mounts, and obtaining a pair of suitable mirrors. There would be NO excuse for failure!
However, the problem is that since such tubes are a lot less common - and mostly used as replacements in expensive high quality research lasers, their cost is considerable. Figure $600 to $1,000 or more depending on quality, size, and supplier. Check out the large well known HeNe laser manufacturers. Perhaps, if you can convince them it is for an educational project, they might let you have one that doesn't quite meet their specs for free or at cost.
Perhaps, after successfully constructing a laser head in this manner, you will have the confidence to proceed with a totally home-built design. The continuing saga of my (so far less than entirely successful) experience with this approach follows in the section: Sam's DIY External Mirror HeNe Laser - Some Assembly Required!.
The opposite situation is also a possibility: Build your own HeNe plasma tube but mount it in a used resonator. Depending on your resources, this might be an easier task (though I find that hard to imagine!). External cavity HeNe laser heads with dead tubes seem to turn up much more frequently than the other way around (for obvious reasons) and can often be obtained at attractive prices. In fact, the dead tube one of these contains might be a candidate for regassing!
You will need the vacuum setup and a source of the HeNe gas mixture, but the serious glass working can be postponed for another day.
The basic idea will be to start off with a laser resonator that once worked (a commercial HeNe tube) using a regular HeNe laser power supply. Inexpensive HeNe tubes and power supplies are readily available and therefore, much of the uncertainty can be easily eliminated so you can concentrate on the gas and vacuum issues.
I would recommend something in the 5 to 10 mW range - large enough to be interesting but not so long as to possibly require magnets or other special attention to operate reliably.
WARNING: The anode will be at a kV or more with respect to everything else! Cover, shield, or otherwise insulated it from accidental contact.
WARNING: Where this fill port is attached to the anode as is likely, not only must you take extreme care in working with anything connected to it, but there will have to be a long narrow gas flow path to prevent the high voltage from striking between the tube anode and the gas supply cylinder instead of the tube cathode. Even if your gas supply system is electrical isolated from ground, its large capacitance to free space would make powering the HeNe tube difficult.
Note: For this to work, both mirrors must be planar (which is difficult to align especially for a long narrow bore resonator) or must have a focal length significantly longer than the original tube length - otherwise, the added distance between them when mounted externally will mess up the cavity relationship that would have been present where one or both was concave.
In addition, it is almost certain that the reflectivity of the Output Coupler (OC) mirror - the one at the output-end of the laser that were part of the HeNe tube in the first place - will be too low for anything approaching optimal performance (and perhaps not even have enough gain to lase at all) once the losses through the Brewster windows are taken into account (especially those that aren't perfectly clean, high enough quality, and not exactly at the correct angle). Therefore, higher reflectivity optics for the OC with a curvature optimized for an external mirror HeNe tube similar in length to your creation should probably be used from the start to avoid a lot of frustration.
Note, however, that for clean, fused silica, very flat Brewster windows set at the proper angle, losses can be very low and even short one-Brewster HeNe tubes (e.g., 10 inches between HR and Brewster window) have enough gain to lase easily with quite low reflectance OCs (e.g., 97 or 98 percent - typical of the OCs in 25 to 30 inch internal mirror HeNe tubes. So, you may get away with using the original OCs if your Brewsters cooperate. :)
Another option is to forgo red entirely until you have something that lases at all and go for one of the IR lines: 1,162.3 nm, 1,523.1 nm, or 3,391.3 nm. I may be easier to get a short tube to opaerte - even one that won't work at all for red, especially at that last one - which will even lase superradiantly (without mirrors) in a moderate length tube. Of course, suitable optics will be needed as well as some means of detecting the IR. A silicon photodiode, CCD camera, or IR detector card can be used for the 1,162.3 nm wavelength; a phosphor plate or something else for the longer ones. Take special care if you do this as the IR is, of course, invisible, but can still cause eye damage. Personally, I'd go with the red - it's challenging but doable.
One possible inexpensive or free source for high quality Brewster windows is a defunct external mirror HeNe tube - but if you had one, you would probably be using that to build this apparatus entirely! Another possibility is a dead one-Brewster HeNe tube or a Hughes style polarized HeNe tube (which may actually be a one-Brewster HeNe tube with an external OC mirror glued to its Brewster stem). See the section: Determining Brewster angle.
Or, to keep the mirrors intact and mounted, use a file to score around the thin section to the point just before the metal is penetrated. Snap off the mount and immediate cap the end(s) of the tube to minimize the possibility of contamination. Don't remove the mirror glass from the metal mounts - they are more convenient to handle. Put each mirror/mount in s little plastic bag and set them aside in a tightly capped container until needed. (Even if, as recommended, you start with an OC mirror designed for use with an external mirror HeNe tube, after you get the thing lasing, you can go back and try the original OC to determine if it will work at all.)
If you really want to experiment, are doing this with an HeNe tube that was originally 30 cm or longer, and have a high frustration threshold, obtain a set of mirrors designed for an 'other color' HeNe tube and see if you can get something other than coherent red light from your contraption!
Note that even getting a short external mirror HeNe laser (e.g., bore length less than 30 cm or so) to operate on the red 632.8 nm wavelength may be difficult unless everything is perfect. And, there aren't that many commercial external mirror HeNe lasers in 'colors' other than red or IR - the gain is even lower than for red on the orange, yellow, and green lines so losses must be cut down to as near zero as possible! The very slight reflection from even high quality Brewster windows may be enough to prevent lasing unless the bore is long. Hint: Green has to lowest gain of the common 'other HeNe colors' so I would suggest avoiding that, at least until you have succeeded at yellow or orange! :)
You can use a pair of identical electrodes and the AC power supply described in "Light and its Uses". However, it would also be possible (with just a little more glass-work) to provide a large side-tube (which also provides a much greater gas reservoir) and aluminum (can) cathode as in commercial tubes with a regular HeNe (DC) laser power supply.
I have had a 30 inch long HeNe laser tube with a broken-off OC mirror (from overzealous attempts at alignment, don't ask!) sitting in a box in the attic for a couple of years now. It never really worked quite right anyhow with erratic power fluctuations and didn't come anywhere near its 20 mW rating even when all the planets were precisely aligned. :) The cause is unknown - possibly low gain due to a contaminated gas fill resulting in low gain. This tube seemed like it would be ideal for creating a nice long semi-home-built one-Brewster laser. And, I wouldn't feel at all guilty about making irreversible modifications - no chants or incantations required for the "gods of dead lasers" either! ;-) The HR mirror is known to be in good condition and properly aligned and there is a nice hole where the OC mirror used to be (the OC has since been reassigned to other projects and works fine so the tube's original problems weren't due to the OC). The exhaust tube is nice and long (about 3 inches) - apparently, this tube never actually was totally completed. It is also one of those peculiar HeNe tubes described in the section: Segmented HeNe Tubes. I expect this to be an advantage since the gas reservoir ends up being distributed throughout the bore and there should be less 'pumping' of gas from one end to the other by the current in the discharge. (Since I have a couple of other more normal tubes from the same source that also experience the lack of power and instability, I don't believe that is related to the segmented design.)
A diagram of the proposed assembly is shown in Sam's One Brewster Helium-Neon Laser Tube Conversion.
To accommodate my mediocre vacuum system, I intend to construct this as a flowing gas system. So, I will drill a hole for the gas fill port in the end-plate at the anode-end of the tube (the exhaust tube is, as usual, at the cathode-end) and attach a piece of metal capillary tubing to it with Epoxy.
I will attach the well cleaned Brewster stem salvaged from a Hughes style one Brewster HeNe tube to the OC mount stump. Although the diagram shows a sleeve and Epoxy seal, I may use a threaded pipe fitting so that the Brewster window can be easily removed or replaced if needed, or some other optic like an OC mirror could be substituted if desired. For initial testing, I have installed a piece of clear plastic tubing and a pair of small pipe clamps. This probably won't retain a decent vacuum but should work for flowing gas operation. And, the angle of the Brewster window can be adjusted if needed!
The external OC mirror will be mounted on an adjustable plate with everything attached to a rigid base.
My SP-255 exciter on a Variac should be satisfactory for powering this laser.
To be continued....
I acquired an external mirror HeNe tube for this exact purpose. Physically, the body of the tube looks like a Melles Griot internal mirror type (but no manufacturer label). Probably the closest current model would be the 05-LHR-120 HeNe tube used in the 05-LHR-121 laser head which is rated at 2 mW. Additional info can be found in the section: Typical HeNe Tube Specifications.
However, instead of the normal mirror mounts and internal mirrors, it has a pair of Brewster windows. Although such HeNe tubes are manufactured for use in research lasers, I suspect this was a one-of-a-kind for another reason: In Magic Marker on the side is printed: He3, Ne22, 2.8, which I assume refers to the isotope of helium and neon used in the gas fill and the gas fill pressure (2.8 Torr). Ordinary HeNe tubes may use normal He4 and Ne20 so my guess is that this was manufactured for someone's thesis project with a title like: "Determination of How Lasing Spectral Characteristics are Affected by Gas Isotope". The consensus is that isotope differences will have only minimal effect - and this is supported by my measurements. See the section: Performing the Single Pass Gain Test.
Actually, having said that before I know what I was talking about (assuming I do now!), the upper energy state of He3 is slightly closer to that of Ne20 so energy transfer is more efficient and thus gain will be modestly higher for a given tube length. This is particularly critical for "other-color" HeNe lasers where every bit of gain is critical. But apparently, virtually all modern HeNe lasers, regardless of color, are now filled with He3. See the additional comments below.
Tubes with Brewster windows are available from several companies including Melles Griot and Jodon. Sizes from 10 cm to over 100 cm (between the centers of the Brester windows) are available which will operate at the 632.8 nm (red) HeNe wavelength. My funny tube has a length of about 25 cm so it is well within this range. This tube appears to be similar to a Melles Griot 05-LHB-290 Brewster tube except for the strange gas fill. Thus, if manufactured properly (e.g., with the proper Brewster angle and properly aligned windows), it should work!
For the resonator frame, I used some aluminum scrap from an old chart recorder and 9 track tape drive (Like the perverbial cow, I use nearly everything!). Low expansion InVar or something else equally exotic and expensive would be better, but given my machine shop or lack thereof, I would much rather deal with aluminum!
The mirror mount assembly consists of three parts: a fine adjustment plate, a coarse adjustment plate, and a small slotted adapter to which the mirror optic itself is attached.
The tube itself (henceforth called the 'Tube Under Test' or TUT) is mounted by a couple of aluminum brackets and Plexiglas plates with the anode-end on ceramic insulators. The ballast resistor is also mounted on the frame with a Plexiglas cover to prevent accidental contact with the high voltage terminals. There is even a HV Warning sticker - what a concept! Power is provided via a 4 foot HV coax terminated in a male Alden connector.
Once this was all constructed, I checked that it would power up and then evaluated the TUT for gain. See the section: Performing the Single Pass Gain Test.
This one in particular must have been dropped since the capillary had broken completely off of its attachment at the anode-end of the tube and was rattling around inside. Given this state of affairs, I would expect the "gods of dead lasers" to understand the need for the sacrifice since I could think of absolutely no way it could ever be made to lase again (but I did provide the appropriate chants and so forth just to be sure!).
After evaluating several options on exactly how to remove the mirrors (retaining various amounts of the rest of the tube), I decided to cut them off at the narrow section of the mirror mount. This would minimize the possibility of damage to the optics while at the same time providing a convenient metal collar to attach to my mirror mount plate. To minimize contamination, rather than using a hacksaw or file, I scored a line with a sharp pair of wire cutters and then snapped them off. Then, I cleaned up the rough edges with a file after stuffing a the hole to prevent the entrance of metal particles.
Well, since I pulled those mirrors off the little dead tube, I haven't heard of any global disasters so I guess the "gods of dead lasers" (GODLs) are satisfied with my chants. :) I did break a set of wire cutters trying to score a line (maybe that was my payment to the GODLs).
To mount the mirrors, I drilled a hole in each of my plates so they would retain their position by a press-fit. Then, with all the mirror mount screws tightened down, the plates with the mirrors were attached. To confirm that the mirrors were seated approximately correctly, I used my alignment HeNe laser to check for a return beam down the bore of the TUT. It didn't have to be exact (the coarse and fine adjustments will take care of that), but I wanted to be sure it wasn't really far off. A bead of Epoxy then assured that each mirror would stay in the proper position.
However, now I have 3 unknowns:
All the HRs tested at less than .1 percent transmission.
I know that I only have 2 percent to play with excluding losses through the Brewsters! So, these mirrors at least should be acceptable as long as the losses through the Brewsters are less than 1.3 percent or so. However, to have the best chance, I can just use an HR from another little tube (already have it so no need for sacrifices as someone else already went through that ritual) to see if I can get it lasing at all, then worry about the OC to get some power out at one end. Or, use Sam's special means of extracting power - a plate inside the cavity at almost the Brewster angle - 2 beams for the price of one! :)
(From: Daniel Ames)
Maybe my BEFIA (Beam Expansion for Interference Alignment Method) might come in handy with this one. (I guess that title sure could use some rewording, as the abbreviation sounds like a "beef processors union". :)
Important note: Be sure to offer the HeNe alignment chant, FIRST!
NOTE: This can be done with the same color HeNe, but the reflections will be substantially reduced in intensity. So, if using the same color R-Laser, (HeNe) use a bright fluorescent sticker or card for the viewing screen and dim the lights.
This procedure should only take a minute or two of your time, or forever. Your mileage my vary. :)
On your alignment jig, have you thought of any way other than the manual (slip and slide) method for lateral adjustment?
Geometrically speaking, it is much easier to move the TUT for aligning, than the reference laser. It works out to be a much less critical movement. The distance of movement of say 1/100th of a degree times the distance between the two lasers, is much greater than 1/100th of a degree times the length of just the TUT. It makes dialing in the alignment much easier, especially if the distance between the two lasers is more than the absolute minimum.
My (unorthodox) method:
It's reversed from the norm. I put the TUT on the alignment jig, and the reference laser was just positioned and secured at the approximate center of the jig's vertical and horizontal travel.
Although with my (unorthodox) method, the TUT still needs to somehow secured to the Jig.
Either way, what about putting a piece of metal, maybe aluminum, under the TUT for a smoother lateral positioning? Just a thought, maybe you already thought of this. :)
But I'm sure that you have a plan. :)
(From: Sam)
Right... I am quite convinced that alignment of the A-Laser relative to the TUT's bore really isn't a problem at this point.
I would have expected the reflectivity of the OC from that old laser to be similar to what was required for my funny tube because although its tube was (past tense, no longer intact) much longer, the bore was about twice as wide resulting in lower gain/inch. That OC is also curved which should ease alignment requirements. Unfortunately, I couldn't try the HR because it had been damaged. See the section: A Really Old HeNe Laser for a description of that laser.
However, there have as yet been no confirmed sightings of any flashes regardless of which optics were used, the phase of the moon, or wishful thinking. :(
At this point I am therefore left with 2 of the 3 unknowns: Absolute single pass gain of the funny tube and the curvature, quality, and cleanliness of the mirrors. Or....
I just noticed that there is some possibility that the funny gas fill with the non-standard isotopes of helium and neon might have been used to make this HeNe tube producing a green beam at 543.5 nm. See the section: More on Other Color HeNe Lasers. However, for all my tests, I have used red probe beams and mirrors designed to reflect red at 632.8 nm. Perhaps my problem all along is that I should have gone green!
After a pleasant interlude of getting a HeNe tube with a single Brewster window to work (see the section: A One-Brewster HeNe Laser Tube), I returned to this effort. I suspected that part of the problem was that I hadn't paid enough attention to the cleanliness of the Brewster windows. With the one-Brewster tube, even a single spec of dust or fine coating of who-knows-what could drastically reduce the output power. With two Brewsters, such effects would be much worse.
So, I went back to optics from the large-frame Spectra-Physics laser (and are what are shown in the photo, above). I hoped these would have the best chance of lasing short of a pair of long focal length HRs which I currently don't have. (The OC from the old lab laser might be even better if it has higher reflectivity - I may try that in the future.)
I discovered that by watching the scatter from the Brewster window closest to the alignment laser (A-Laser), it was possible to tweak the mirrors so that the spot caused by the beam from A-Laser and the return from the HR mirror at the other end of the tube could be superimposed. If this was done with the OC's reflection smack in the middle of the A-Laser's output aperture, there would be an increase in intensity and fluctuations in intensity due to mode cycling of the A-Laser and light bouncing back and forth between the A-Laser's OC and the OC of my resonator. At this point, alignment was really very close.
While gently rocking the mirrors I got what were unmistakable flashes for the first time. More cleaning and blowing off of dust and I was finally able to get a few photons of coherent 632.8 nm light coming from the funny tube.
Actually, a grand total of about 19 uW. (That's 19 whole microwatts - not milliwatts or megawatts!) It's a nice TEM00 beam - just not very bright! :)
Part of the problem may be that the inside of the Brewster on the cathode-end of the funny tube seems to have a lot of scatter - about as much as I get from the Brewster window of the one-Brewster tube with perhaps 100 times as much circulating light flux between the Brewster and the OC. How do you clean the inner surface of a Brewster window on a sealed tube? :(
Another and perhaps more significant characteristic is that when first turned on, the output power may be more than 2-1/2 times greater (more than 50 uW!!) and then decays to the lower value over the course of a minute or two. If it is turned off for a minute or two, the behavior will repeat. This could indicate a gas fill problem as I've seen similar behavior with an old Spectra-Physics 084-1 soft-seal HeNe tube. The mechanism would be that discharge current is causing the gases to be redistributed to the detriment of lasing gain or the optical power that can be extracted from the population inversion (sounds impressive at least!). The color of the discharge isn't obviously incorrect but could be a bit more pink than normal, though the spectrum appears normal. However, I may attempt to reactivate the getter in any case but this will have to wait until I get my induction heater working - there is no way to do this easily with my solar heater or by loading the entire laser into the microwave! However, I have tried the RF exciter test for gas fill problems and the results would seem to indicate that there is no detectable contamination.
I do believe at present that my OC reflectivity is marginal and I should be able to get a bit more power out of this tube by locating a mirror with 99.6% or greater reflectivity. As noted, I have tried a couple of HRs (which would certainly satisfy the reflectivity criteria) without even a single pair of coherent photons being ejected from the laser but since they originated from small internal mirror HeNe tubes, their focal lengths may have been too short.
Anyhow, this is success! I don't know how much more I can squeeze out of it regardless of optics but at least the entire effort resulted in a working laser - even if you do need to have someone point out the location of the beam!
I have left the two-Brewster laser as well as the A-Laser (just in case) set up against the back wall of my laser lab bench and turn it on from time-to-time just to be sure I wasn't imagining things. It continues to work at about the same power (or lack thereof) level, generally without requiring any mirror tweaking to peak it, only brushing off the Brewster windows.
(From Steve Roberts (osteven@akrobiz.com).)
You've got a research tube. And, being as short as it it, probably one designed for a single longitudinal mode. The foggyness on a hene is bad news... Do you get chaotic fluctuations as the mirror is moved slightly or if you put your finger on the tube? If so you have dirt in the path.
I suspect your tube was designed for spectroscopy games, or perhaps to be locked to a iodine or methane cell as a standards laser for metrology. Or maybe somebody was redoing the isotope work to see if anybody missed something.
Your best bet on the isotope thing is to contact Spectra Gases and ask them what isotopes they sell in the hot HeNe mix.
The following excerpts is from: "Laser Fundamentals" by William Silfast, ISBN: 0-521-55617-1:
"and a single isotope of neon (Ne20) is used to keep the gain bandwidth to a minimum and thereby increase the gain.""Using a natural mixture of neon will reduce the gain by approximately 10%. Additional modes will then only develop from the Ne22 isotope if the much smaller gain in the frequency range of that isotope exceeds the losses within the laser cavity."
"The shift between Ne20 and Ne22 is approximately 1 GHz, whereas the bandwidth due to doppler broadening is on the order of 1.5 GHz".
From what I can tell, Ne22 has a difference in gain of -9.8% in the mix (best guess on the sign, as the graph in the text is ambiguous. Naturally occurring neon is: 90.8% Ne20, (10 neutrons), .26% Ne21 (11 Neutrons), and 8.9% Ne22 (12 neutrons).
Naturally occurring helium is 99.9998% He4 and .00013% He3, so somebody wanted a real shift in the hyperfine spectrum of a HeNe laser, I would suggest asking why on the USENET newsgroup scl.physics.research.
I don't know about the chance of the other color lines lasing on a short tube like that, but I'd get two pieces of Newport BD-1 coated mirror and find out. It's 99.99% reflectivity across the visible spectrum and well into the IR. I doubt you'll see green in less then a 1 meter tube with brewsters but yellow is a strong candidate.
(From: Sam.)
I see that Spectra Gases does list He3 and Ne22 on their Visible and Infrared Laser Gases Page but you have to call for more info.
As suggested, I posted to the USENET newsgroup sci.physics.research (as well as alt.lasers). Here is the one reply so far:
(From: Excimer (species8672@email.com).)
My good friend Chris Leubner - laser expert extraordinaire - was very interested in your laser:
"I think you have found a very unique HeNe laser tube. Helium 3 comes from tritium. He3 also has a higher energy state than normal He4. So the laser is quite efficient at operating at an otherwise weak line. It is most likely designed to operate at a wavelength of 1.523 um. This wavelength is used for infrared spectroscopy and fiber analysis. It most likely came from some sort of spectrometer or fiber optic analyzer. Definitely hold on to this laser! It is a very rare find!"
PS: Try and see if it would work with other types of mirrors... You never know...
(From: Sam.)
OK, so now I need a set of mirrors good for 1.523 um.... :)
But now, perhaps the final word:
(From: Lynn Strickland (stricks760@earthlink.net).)
Most HeNe lasers are filled with He3 and an equal mixture of Ne20 and Ne22. This broadens the gain curve and provides a little more power. Some want just Ne20 or just Ne22, usually for frequency references.
The center of the gain curve for Ne20 and Ne22 are (can't remember for sure) about 500 MHz apart. If you want a precise frequency reference, you wouldn't want the mixed neon isotopes because the center frequency could vary anywhere in that 500 MHz range.
As for the He, no one really uses He4 in HeNe lasers any more - only He3.
This should be the experimenters' dream laser combining low cost, ease of use, safety, simplicity, flexibility, and a visible beam while still providing convenient access to the inside of the resonator. With only very modest metal working skills and a hacksaw, file, drill, and tap, a one-Brewster HeNe laser tube and compatible power supply can be turned into a an external mirror (well, one mirror at least - which is really all you probably need in most cases) laser for experimentation with the high photon flux inside the resonator; effects of mirror reflectivity, curvature, and location; or just the thrill of seeing several hundred mW to several WATTs of HeNe laser light bouncing off specs of dust - along with the frustration of knowing that you can't really get at it! :)
The safety aspect in particular of this design makes it an ideal laser for experiments requiring access to the cavity. There are no high voltages near the Brewster window and mirror mount assembly, the tube is fully enclosed in a robust aluminum cylinder, and the output beam power will generally be well below the Class IIIb threshold. Even though there is Class IIIb power inside the cavity, it is in a sense 'virtual' - if anything interrupts that beam, including an unsuspecting eyeball, it simply disappears as lasing stops.
And, unlike most commercial external mirror HeNe lasers which locate the mirrors as close to the ends of the tube as possible, you can mount the mirror for your one-Brewster HeNe tube at almost any distance to provide either easy access to the circulating photons or to just show off with a several hundred or more mW beam visible in the air. For example, with the 60 cm radius of the HR typically found in these one-Brewster HeNe tubes, a planar mirror will work as far away as about 30 cm (~1 foot) from the Brewster window; another 60 cm mirror could in principle be mounted up to 90 cm (~3 feet!) away though adjusting its alignment would be quite a treat. :) In the design described below, we'll be a bit less ambitious, but see the section: Mirror/Optics Test Jig Using One-Brewster HeNe Laser Tube.
I have a limited quantity of CLIMET 9048 laser heads (with or without power supplies), as well as Melles Griot 1-B laser tubes, available for sale. See the section: Sam's Stuff for Sale or Trade and Items Wanted.
The parts list and mirror mount drawing is provided below:
(Not listed is any hardware required to mount the laser head and mirror mount assembly to a baseplate or enclosure.)
The left photo in Sam's External Mirror Laser Using One Brewster HeNe Laser Head shows the complete system with mirror mount and an SP-084-1 OC mirror installed in the Simple Mounting Cell for Salvaged HeNe Laser Tube Mirrors. A Melles Griot 05-LPM-379 power supply brick set for 6.5 mA provides the excitation. The middle photo shows the adjustable mirror mount and support standoffs. This assembly can be easily swapped to another similar one-Brewster HeNe head requiring at most a touch-up of the mirror alignment. For the right photo, an SP-084-1 HR mirror has been installed in place of the OC to maximize the internal circulating power. The scatter of the 500+ mW circulating photons from the random dust particles (in a relatively dust-free office environment) is quite visible.
Note that this HeNe tube operates reliably from a small HeNe laser power supply like the Melles Griot 05-LPM-379 because it has a wide bore and thus a low operating voltage (1,470 V from the power supply at 6.5 mA assuming a 68K ballast resistance). However, the 05-LPM-379 would appear to be a bit marginal for starting (8 kV instead of the 10 kV listed for the tube) and one recommendeded Melles Griot power supply is actually the 05-LPM-939 which has a somewhat higher maximum starting (and operating) voltage. While these tubes will work on either supply, starting is very quick with an 05-LPM-939 even for a tube that might (on a bad day) take a minute or more to start using an 05-LPM-379.
Mirror Mount Plates for One-Brewster HeNe Laser has the mechanical details for compatibility with the CLIMET 9084. The only critical dimensions are the locations of the 4 corner holes and center hole. Everything else can be modified for use with your particular mirror(s). If you don't have some aluminum scrap, even Plexiglas or other rigid plastic can be used in a pinch. The hardware should be readily available from any electronics distributor or your junk box. :). The fixed aluminum plate and 4th standoff can be eliminated with a slight reduction in stability as shown in Anode-End One-Brewster HeNe Laser Tube Mounted in Test Fixture. (This also happens to be one of the less common tubes with the Brewster window connected to the high voltage.)
If your junkbox is bare and you don't want to 'invest' in standoffs, 6-32 threaded rods or long screws and some extra nuts and washers could be substituted instead with slightly lower rigidity and ease of set up, but the standoffs are really much better. In any case, don't be tempted to use too thin a material for the mirror mount plate (not less than 1/8" for aluminum) as the adjusting screws may warp it enough to really confuse things. :( One-Brewster HeNe Laser Head with Very Simple Mirror Mount shows such a setup with a piece of a barcode scanner spinner mirror for the OC (though it actually is more of an HR in terms of reflectivity). This arrangement isn't fancy or elegant but is quite stable and relatively easy to align.
Here is the parts list for the simplified setup:
Or go a bit less basic as shown in Enhanced Simple Mirror Mount for One-Brewster HeNe Laser Head, built by Dave (Ws407c@aol.com) for one of these (purchased from me). He added springs and wing nuts which allow for easy adjustment (possibly too easy though as bumping one will mess it up!). Actually, the photo makes the mirror mount look much spiffier than it does in person. :)
Almost any planar or high Radius of Curvature (RoC=r, more than about 12 inches) high reflectivity (R, more than about 94 to 96 percent at 632.8 nm) good quality first surface mirror will result in lasing action if mounted next to the Brewster window. However, the range of positions beyond this for the resonator to be stable will depend on the actual RoC as noted above. Here are the rules:
Where:
In practice, lasing may not continue quite to the limits but should come close.
While OC mirrors from 5 or 6 inch barcode scanner HeNe tubes have adequate reflectivity, their RoC may be so short (typically 26 cm for the OC) that no lasing is possible until the mirror is more than 60 cm from the internal HR (more than a foot from the Brewster window). And, some longer HeNe tubes like the Siemens LGR-7641S use the same 26 cm radii for the OC mirror so tube length alone is no guarantee of a suitable OC curvature.
Some examples of the approximate range of positions (*) where an external mirror (e.g., OC) of a particular RoC should work with the internal HR having an RoC of 60 cm:
Distance to HR: 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 cm
| | | | | | | | | | | | | | |
26 cm OC: ( =\ *********
45 cm OC: ( =\******* *****************
60 cm OC: ( =\*************************************
80 cm OC: ( =\************* ***********************
Planar OC: ( =\*************
Dielectric mirrors are much better than aluminized mirrors but the latter may work in a pinch (though not that well, and some just don't have the required reflectivity even though they may look identical). I've gotten several mirrors from HeNe laser based barcode scanners and an old HeNe laser based laser printer to work with these HeNe tubes. A high quality dielectric mirror with very high reflectivity (e.g., greater than 99.9 percent such as a HeNe laser HR) and low losses should result in a great deal of circulating power inside the resonator - possibly up to a WATT or more and a very visible beam in there unless you are in a clean-room, but only a weak output beam. The OC from a typical medium length HeNe tube will result in a more modest 300 or 400 mW inside the resonator but a useful output beam of 1.5 to 5 mW. A mirror from that laser printer produced 750 mW inside the cavity with a 0.9 mW output. And those barcode scanner spinner mirror chips result in very high circulating power with only a few hundred mW of output. As a matter of fact, it is likely that these non-laser dielectric mirrors are actually probably better quality than the laser-quality mirrors of the 1970s.
Even with just a bare tube or laser head without the external mirror mount, it is quite easy to test a mirror by holding it about 2 to 3 inches away from the Brewster window positioned so that the reflection of the light of the discharge from the bore is centered around the Brewster mount. Then, rocking the mirror about this position should yield flashes quite quickly if the mirror has adequate reflectivity and is of high enough quality. Thus the lasing ability of a newly acquired one-Brewster tube or head can be easily determined without constructing the mirror mount as long as a suitable mirror is available. Or, evaluating a newly acquired mirror using a known good one-Brewster tube.
These HeNe tubes usually can produce a beam which is TEM00 or multimode depending on the mirror and a stop inserted inside the cavity. (This should happen on a red HeNe laser when the ratio of the aperture diameter to mode radius is about 3.5:1.) The higher order mode structure is quite interesting (not just a rectangular array). Higher quality mirrors will result in a more well defined mode structure. There is enough gain that additional Brewster angle optics (even a cheap microscope slide) can be introduced inside the resonator to act as an etalon, and possibly optics that are just AR coated as well.
Note that there should generally be no need to touch the alignment of the internal mirror to get these to lase unless someone before you had mucked with it. I don't particularly recommend attempting this alignment though since unless the output beam is obviously non-circular (oval or cut off) even with the external mirror aligned for maximum output power, any benefit will be minimal. However, where there is a locking collar present, some careful tweaking (basically walking this mirror and your external mirror) is relatively low risk and may result in some additional output power by centering the intracavity beam in the bore. Only attempt this while the tube is lasing (unless you enjoy going through the entire alignment procedure using an external alignment laser) and take care with the high voltage! Where there is no locking collar, a standard Melles Griot locking collar from a dead HeNe laser tube can be installed.
First, I fired the unit up on a Melles Griot 05-LPM-379 power supply brick to confirm that the tube was intact and had the correct discharge color. It did, though I figured this power supply might be a bit marginal. From the bore diameter of at least 1.4 mm, it would appear to be a tube which would tend to produce a beam with multiple transverse modes and would require a higher current than typical for narrow bore TEM00 HeNe tubes for maximum power output. During the subsequent tests, I used an adjustable HeNe laser power supply (the one described in the section: Aerotech Model PS2B HeNe Laser Power Supply (AT-PS2B) with a Variac (and its internal regulator disabled). A tube current of about 7.5 mA resulted in maximum power output. Note, however, that Melles Griot actually recommends 6.5 mA for the tube current and it turns out that the 05-LPM-379 power supply brick will provide this with no problem.) I don't know how life expectancy will be affected by runnnig at the higher current and the ballast resistor supplied with the CLIMET 9048 laser head may overheat after awhile.
The OC-end of the laser head has a flange with conveniently located holes to attach the external optics. I used 4, 2-1/4" x 1/4 threaded spacers to mount a pair of 2"x2"x1/8" plates, the second of which is adjustable via using a hex wrench via cap-head screws and split washers used as springs. My mirror mount. :) See the HeNe Laser Tube with Internal HR and Single Brewster Window and External OC.
Based on the geometry (assuming that the HR mirror has a radius of curvature of 60 cm as I had been told and later verified), a stable resonator should result for an external mirror at a 30 cm distance from the HR as long as its radius is between +30 cm and planar (concave) or -30 cm and planar (convex). This means that except for some short radius barcode scanner HeNe tube mirrors, almost anything else with enough reflectivity at 632.8 nm should work. At this point, I didn't really know the value of the required reflectivity to achieve threshold.
I had several possible mirrors to try both from deceased internal mirror HeNe tubes as well as from a couple of external mirror HeNe lasers. Initially, for rough alignment, I used another HeNe laser (the A-Laser) firing down the bore of the 9048 without the OC in place. The returned a strong nicely focused reflection which (indicating a curved/concave OC) and centered in the A-Laser's output aperture. Then, without disturbing anything, the candidate OC-mirror was installed and the mirror mount adjusted to center its reflection in the A-Laser's output aperture.
I first tried the OC from a dismembered tiny barcode scanner HeNe tube - a Melles Griot 05-LHR-002-246. No amount of fiddling resulted in any output beam. Nor did the use of its companion HR. (Using an HR mirror in place of the normal OC to test a laser results in the lowest lasing threshold since it maximizes round trip gain. Thus, it should be easiest to get going where losses are unknown. For a high power laser, this can be risky since the oscillations in the resonator could build up to a sufficient level to actually damage the optics. However, for a low power (at least) HeNe laser, such effects are unlikely.) I assume that these mirrors were unsuitable either because the reflectance was too low (for the OC) and/or they were curved with a radius of curvature that was too small (almost certainly the latter). (Later I did achieve lasing with that same HR. I don't really know what caused it to fail the first time.)
Next, I tried the HR mirror from an unidentified (but probably Hughes) internal mirror HeNe tube, using the same alignment technique. And, almost as soon as I touched the adjustment screws to center the its reflection, a beam appeared! I almost missed it shining back into the A-Laser but then noticed the really bright scatter off of the Brewster window. With the a bit of tweaking and HR mirror adjusted for maximum output, the beam was weak (maybe 10 uW, just the minimal transmission through the HR that is normally considered waste!) but this was success! While not exactly strong, it was stable. Of course between the OC and the Brewster window, there was probably several hundred mW bouncing back and forth as evidenced by the dancing illuminated specs of dust. :)
As expected, the laser produced a beam with multiple transverse