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 (1,152 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. A more complete list can be found at Photonics Buyers' Guide - Lasers, HeNe and in the chapter: Laser and Parts Sources.
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.
Please refer to Helium-Neon Excitation and Lasing Process for the following description.
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 from which they can radiate at 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. (Only the three found most commonly in commercial HeNe lasers are shown in the diagram, above.) 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
886.5 nm " " 2s2->2p10 1.2 1.25 0.3
1,029.8 nm Near-IR Invisible 2s2->2p8 ???
1,062.3 nm " " " " 2s2->2p7 ???
1,079.8 nm " " " " 2s3->2p7 ???
1,084.4 nm " " " " 2s2->2p6 ???
1,140.9 nm " " " " 2s2->2p5 ???
1,152.3 nm " " " " 2s2->2p4 ??? 1.5
1,161.4 nm " " " " 2s3->2p5 ???
1,176.7 nm " " " " 2s2->2p2 ???
1,198.5 nm " " " " 2s3->2p2 ???
1,395.0 nm " " " " 2s2->2p? ??? 0.5
1,523.1 nm " " " " 2s2->2p1 ??? 1.0
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. (Strictly speaking, it is something called a "Voigt distribution" which is a conbination of Gaussian and Lorentzian - but that's for the advanced course. Gaussian is close enough for this discussion since the discrepency only shows up way out in the tails of the 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/(2*L) 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
The optical frequency of each line is than n * (c/2L). 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. There are very few commercial HeNe laser tubes that short. 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 with a slight loss in signal strength. Without a polarizer, the beat frequencies of a random polarized laser will tend to be at twice the mode spacing.
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: Stabilized Single Frequency 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 gain bandwidth of around 1.5 GHz (at 632.8 nm). (The width of the Doppler broadened gain curve depends on the lasing wavelength. At 3,391 nm, it is only about 310 MHz.) 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 beamsplitter 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 reasonably 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. But the
majority of tubes are well behaved in this regard.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes,
see An
Investigation of the Cavity Modes of the HeNe Laser.
(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.
There are many ways to actually "see" the modes of a laser including the
use of an instrument called a Scanning Fabry-Perot Interferometer (see the
section: Scanning Fabry-Perot
Interferometers). However, for a short tube with only 1 or 2 modes,
it's quite straightforward to interpret what's going on from the output
power and polarization alone. All that's need is a
photodiode and multimeter (or continuous reading laser power meter),
and polarizing filter. (A lens from a pair of polarized Sun glasses
or a photographic polarizing filter will do.) The power monitor can
be set up in the output beam and the polarizing filter in the waste beam
from the HR mirror. Alternatively, a non-polarizing beamsplitter can be used
to provide the two beams. Adding a polarizing beamsplitter oriented
so that it separates the two polarization orientations in one of the
beams can simplify the interpretation of the polarization changes.
Changing the orientation of the polarizer will affect the amplitude of
the intensity variations. For most HeNe lasers, the
longitudinal modes will generally remain at two fixed orthogonal
orientations, with adjacent modes usually being orthogonal to each other.
As the tube heats and the cavity length increases, the modes march along
under the gain curve with those at one end disappearing and new ones appearing
at the other end as described above. But for well behaved tubes, they
don't flip polarization. When the polarizer is oriented at 45 degrees
to the polarization axes of the tube, the reading will remain constant.
When aligned with the polarization axes of the tube, the reading will
fluctuate the most.
As a specific example, consider an HeNe laser tube with a mirror spacing
of 120 mm (about 4.75 inches, one of the shortest commercially available
laser tubes). This corresponds to a mode spacing of
about 1.25 GHz - rather close to the FWHM of 1.5 to 1.6 GHz for the neon
gain bandwidth. With this tube, at most 2 modes will be oscillating
at any given time. When the output power and polarization is monitored
while the tube is warming up, a very distinctive behavior will be observed.
One might think that it should be a periodic variation in output power with
a simple sinusoidal or similar characteristic. However, there will actually
be two peaks for each cycle: A large one corresponding to when there is a
single lasing mode at the center of the gain curve, and a smaller one when
there are two modes symmetric around the center of the gain curve. For
most tubes, the polarization of adjacent modes is orthogonal and will remain
fixed with the mode. So, as the modes cycle under the gain curve successive
large peaks will have opposite polarization. The small peaks will have
equal components of both polarizations. Even though two modes are
oscillating, the gain for each one is so much closer to the lasing
threshold that their combined power is still lower than for the single
mode at the peak of the gain curve. There may also be rather sudden
changes in output power as modes on the tails of the gain curve come and
go. However, for some tubes which are affectionately called "flippers",
the polarization of the modes will tend to suddenly change orientation
as they move through the gain curve. This should also be apparent when
viewing the beam through a polarizing filter.
For more on these types of experiments along with typical plots, see the
section: HeNe Laser Output Power Fluctuation
During Warmup.
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)
at 632.8 nm. 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 (but this would cut the power to the
photodiode by 50 percent or more).
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. ;-) But it sounds similar! Looks like sin(x)/x.
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.
(Note that it is easy in principle to convert the output of a TEM00 laser into
multimode by using a length of fiber-optic cable with lenses at each end to
focus the beam into it and collimate the beam coming out. If the core diameter
of the fiber is greater than that needed for the fiber itself to be single
mode, then the result will be that multiple modes will propagate inside
and the output will be multimode. To assure single mode propagation at
632.8 nm with the index of refraction of a typical glass fiber, a 4 um or
smaller core is needed. The actual core diameter of the fiber
will determine how many modes are actually generated. A core diameter of
10 um will result in a few modes while one of 125 um will produce
dozens of modes. Why this would be desired is another matter.)
However, all these modes will be exactly the same wavelength since they
originate from a single TEM00 beam.
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. I've
also seen an apparently factory-new Uniphase green HeNe laser that produces
a similar doughnut beam. Both of these are probably the result of one or
both mirrors having a radius of curvature that is
too short for the bore diameter. They may have been manufacturing goofups.
Everyone can have a bad day, even if it results in a bunch of dud lasers. :)
Good for us though. Everyone (well everyone who cares!) has seen a nice
TEM00 HeNe laser. How many have one that does three wavelengths with
different mode structures! :) (See the section:
The Weird Three-Color PMS HeNe Laser Head.)
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.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes,
see An
Investigation of the Cavity Modes of the HeNe Laser.
All of these are really somewhat equivalent and simply mean that more than
one mode fits inside the available active 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. I've heard of people successfully using a Tesla
(Oudin) coil to charge up the errant dust particle, causing it
to just out of the way via electrostatic repulsion. Your mileage may
vary. :)
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!
Note that while the frequency of the power variations in output power of a HeNe
laser goes to beyond the GHz range, the following deals with what can be
seen by human eyeballs with the aid of only a photodiode and multimeter
or chart recorder (or a PC with a data aquisition module).
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, and later in this section.
Examining the actual plot of output power versus time such as shown in
HeNe Laser Output Power Fluctuation During Warmup
(or careful observation of laser power meter readings) of a HeNe laser reveals
that the curve is not simple but may include 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.
I do not know exactly what the underlying cause is, but it has to
do with the lasing process itself.
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 that may have accumulated from
a period of non-use. (Or depending on how far gone they are,
the power may go down!)
Depending on the particular laser, the initial output power can
be very low even where the final output power exceeds rated power.
Goofups in design and manufacturering can result in various combinations of
these and other effects, though for the most part, HeNe laser companies
generally know what they are doing! :) But see the plots below for both
normal and abnormal behavior, and a link near the end of the section for
a case study of one dramatic example of and "oops". :)
For most of the plots, my "instrumentation" consisted of a pair of $2
photodiode feeding two of the analog inputs of a
DATAQ Chart
Recorder Starter Kit attached to my ancient 486DX-75 Kiwi laptop running
Win95. The photodiodes are reverse biased by 30 VDC from a +/-15 VDC power
supply with a variable load resistor to set the calibration. The
output is taken between the junction of the resistor and the photodiode, and
power supply common (0 VDC). One channel is shown below:
The values shown were selected for lasers with a maximum power output of
around 1 mW. For higher power lasers, R2+R3 can be decreased or an attenuation
filter can be placed in the beam. The later is preferred to avoid shifting
the 0 mW reference level, and is what I did for most of the plots.
The capacitor across the input is intended to minimize noise
pickup. The resulting filter rolls off at around 0.1 Hz.
For reasonably well behaved HeNe lasers, even during the initial
warmup period, this bandwidth is more than adequate.
The sampling rate for all the plots is at least 10 Hz to allow for averaging
since the A/D seems to have an uncertainty of about 2 LSBs.
For monitoring power from the waste beam (which is much lower), a dedicated
beam sampler assembly was constructed which along with a photodiode preamp,
enabled power levels as low as a few uW to fully utilize the 20 V p-p range
of the A/D.
Although some of these plots aren't as nicely annotated as the
one above, zero power is near the bottom of the plot so relative power
variations can still be easily seen (who cares about absolute power
anyhow!) and the time/division is indicated. The plots are arranged by
increasing laser tube length.
For the following, "Total" means all the power in the beam; "Polarized"
means a polarizing filter has been inserted in the beam and aligned to
produce the largest difference between minimum and maximum output as the
modes cycle. (Only done for random polarized lasers.) The scale factor
for the "polarized" plot has been adjusted so that the peak amplitude
is approximately the same as for the "total" plot ease of viewing. However,
it should be understood, that the sum of the power in the two orthogonal
polarizations must add up to the total power. All are red (632.8 nm)
HeNe lasers unless otherwise noted.
Plot of Melles Griot 05-LHR-007 HeNe Laser Tube
During Warmup (Polarized). This is the same tube but with a
non-polarizing beamsplitter followed by orthogonal polarizing filters
inserted in the beam. The orientation of the polarizing filters is
adjusted for minimum transmission when its mode is not present since as can
be seen, the power actually goes to 0 mW for about half the period of each
polarized mode. Alternate similar height peaks on the total power plot
correspond to the same mode polarization. A careful examination will
confirm that they actually alternate very slightly in amplitude due to minor
variations in gain as a function of polarization. (I have adjusted
the scale factors to make the plot looks similar.) The reason why the peak
spacing on the two plots differs is that the tube was likely not
quite at the same temperature when each run was started.
Plot of Melles Griot 05-LHR-640 HeNe Laser Tube
During Warmup (Polarized). This is the same tube with the polarized
modes separately plotted. Similar comments apply for this tube as for
the 05-LHR-007, above.
However, the dramatic variation in mode amplitude
over the course of warmup is an artifact of the way that data is being
collected for this run and a peculiarity of the tube that doesn't
noticeably affect its useful output. Rather than using the output beam,
the P and S Modes are taken from the waste beam leaking through
the HR mirror at the back of the laser. The Total Power (Waste) is
then simply the sum (in an op-amp) of the modes. Compare this to the
Total Power (Output) curve, which was measured from the main beam.
The cause of the rear beam power variation is interference from multiple
internal reflections in the HR mirror glass - between the HR coated inner
surface and the uncoated outer surface. The result is a weak Fabry-Perot
etalon which varies the effective reflectance of the HR mirror. It doesn't
take much: A change from 99.975% to 99.95% would double the waste beam power
- from about 15 uW to 30 uW. The 15 uW lost from the main beam power
of about 1 mW is almost undetectable on the plot. The HR mirror glass
is apparently not wedged on these tubes so the surfaces are very parallel.
And indeed there was no ghost beam next to the waste beam as would be the
case if wedge was present. The cause was confirmed by putting a dab of 5
minute Epoxy on the outer surface of the mirror. The Epoxy is smooth and
clear enough to pass sufficient power for the photodiodes (though it is
reduced). But the Epoxy surface is lumpy enough to greatly reduce the power
variation. Why? The glass and Epoxy are fairly closely index matched
so that the dominant reflection is no longer from the planar glass surface
but from the lumpy surface of the Epoxy. There is minimal reflection
directly back along the optical axis and thus minimal etalon effect resulting
in a reduction of power variation from nearly 100 percent to under 10 percent.
Using Norland 65 UV cure optical cement to glue an angled plate to the
HR mirror reduced the ripples even more as shown in
Plot of Siemens LGR-7641 HeNe Laser Tube
With Variable Waste Beam Power During Warmup (Corrected).
More on this phenomenon can be found in the section:
Power Variations Due to Lack of Mirror
Substrate Wedge which explains the cause in more detail and additional
tests that were performed on this specific tube.
Plot of Melles Griot 05-LHR-151 HeNe Laser Head
During Warmup (Polarized). This is the same laser head but with the
two orthogonal polarizitions separated (as described for the shorter tubes,
above) and oriented for maximum variation ("ripple"). They are plotted
separately to reduce clutter. Since there are always modes of both
polarization present regardless of polarizer orientation, the output
power in doesn't go to zero as with the shorter laser but their ripple is
almost perfectly complementary. As expected, the size of the fluctuations
in each polarization - 5 to 10 percent - is more in line with the total power
behavior of a laser with only 2 or 3 modes. Even this amplitude seems
remarkable given the almost perfectly smooth behavior of the total (randomly
polarized) power. If the plots are examined very carefully, it will be noted
that their envelopes are not identical - there is a very subtle slow
variation over the course of the warmup period. This may be attributed to
a small rotation of the polarization axes as the tube expands. With
some samples of these lasers, it can be much more dramatic including
polarization flips whenever it feels like it. But such behavior
is still considered normal since for a random polarized laser, only the total
power really matters, not any peculiar gyrations the modes may go
through.
Plot of Melles Griot 05-LYR-173 HeNe Laser
Head During Warmup (Polarized). The same laser with a polarizing
filter in the beam. The fluctuations are larger as expected both
because of the fewer modes in the polarized beam, and the lower gain
of the 594.1 nm lasing line.
Since the laser head has optics to separate
the modes with orthogonal polarization, the raw beam already varies
by more than 2:1 in output power without any additional polarizer. Yes,
that is the actual spread - the vertical scale hasn't been stretched!
The actual HeNe laser tube inside is a specially selected Melles
Griot 05-LHR-120, which by itself would have a normal mode sweep with a
small ripple. From a cold start to lock takes about 20 minutes.
Plot of Coherent Model 200 Stabilized HeNe Laser
Head Near End of Warmup. This plot zooms in on the last
two cycles. Notice that there is a slight distortion on the rising part
of the second cycle in the plot. That is probably when the active feedback
is switched on. Before then, the heater is simply running at a constant
current to bring the tube up to operating temperature. It only takes
less than one full additional cycle to achieve lock. The amplitude is
then quite stable (uncertainty of less than 0.5 percent on the plot),
but the frequency stability which is d(power)*slope(frequency/power),
will be under 0.125 percent of the mode spacing of around 750 MHz, so
less than 1 MHz.
The peculiar shape of the mode
cycles is due to the fact that these are not linearly polarized modes
as with all the previous lasers. Rather, they are Zeeman-split modes
which include components differing in frequency by only a few MHz, rather
than the longitudianl mode spacing of many hundreds of MHz. (Around 800
MHz for this laser.) But the reason for the peculiar shape is not clear.
It may be due to a combination of Zeeman modes and normal
longitudinal modes but this is still under investigation.
The actual beat frequency is shown for the last few cycles and after locking
in both these plots. This is the actual measured frequency captured along
with the F1 and F2 modes, and total output power. (Showing the frequency
plot earlier would be a mess.) The beat only appears for a small percentage
of the mode cycles with some variation during the time it is present.
Much more on these Zeeman-split HeNe lasers can be found in the section
starting with: Hewlett-Packard HeNe Lasers
and Inexpensive Home-Built Frequency or
Intensity Stabilized HeNe Laser.
However, near the very end of the warmup period (measured in terms of
mode cycles, not time) something very interesting
occurs: The tube seems to have reverted to being well behaved! This only
happens when the tube is approaching thermal equilibrium where each complete
mode cycle is taking over 90 seconds. There are perhaps 3 or 4 beyond
what is on the plot but the tube temperature is so close to its final
value that any disturbance like moving near the laser head will disrupt
the sequence. This behavior is consistent from run to run. The cause is
unknown, nor is it known whether the tube would continue to behave if
stabilization was attempted. But it might since the operating temperature
will be somewhat above the natural point of thermal equilibrium.
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
During First Part of Warmup is a closeup of the mode variations when
flipping. The shapes are nearly identical from the start of warmup until
the transition to normal behavior.
Also note that the frequency of the mode cycles for
a flipper is double that of a normal tube - each mode would normally be
what resulted from tracing the continuous curve and not taking the
discontinuities as is evident in
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
During First Part of Warmup (Combined). So following red-blue-red, etc.,
ignoring the green lines. And
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head
at Transition to Normal Behavior (Combined) is a closeup of the point
where flipping ceases. Note that the "envelope" of the mode plot is virtually
unchanged at this point but the green transitions have disappeared.
At the transition point, the period of a full
mode sweep cycle is about 80 seconds. There are then an additional 10
full cycles (only 4 or 5 are shown) requiring about an hour until
thermal equilibrium. There is more on flippers in the sectoin:
HeNe Mode Flipper Observations.
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 HeNe 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
(and in fact may use the identical model HeNe laser tube!),
several additional enhancements are needed to eliminate mode sweep and
select a single output frequency. Simply constructing the laser cavity of
low thermal expansion materials isn't enough when dealing with distances
on the order of a fraction of a wavelength of light! Active feedback is
needed. The most common implementation of these lasers starts
with a short tube that can only oscillate on at most 3 longitudinal modes.
It then adds optical feedback to keep them in a fixed location on the HeNe
gain curve by precisely adjusting the distance between the mirrors over
a range of about 1/2 the lasing wavelength. This is most often
done with a heating coil (inside or outside the tube), but a PieZo
Transducer (PZT, an expensive version of the beeper element in a digital watch)
may also be used. The PZT reduces the time for the system to stabilize
to a few seconds, compared to 10 or 20 minutes for the heater. But, for
a laser that will be left on continuously, this probably doesn't matter.
Some lasers use a means of cooling in addition to the heater like a
piezo fan, probably to allow the laser to run stably over a wider temperature
range. And a few including the Melles Griot 05-STP-909/910/911/912 (originally
based on teh Aerotech Syncrolase 100) use a miniature RF induction heater
surrounding the HR mirror mount to control only its length, not that of the
entire tube. With direct heating of such a small mass, the response is quite
fast. This also makes for a more compact package than a full tube heater.
Many schemes work well and it's amazing how dirt simple these really are,
considering their hefty price tags! It's easy to build perfectly usable
systems from a common surplus HeNe laser tube and a few common junk parts.
Note that an etalon inside the laser cavity could also
be used to select out a single longitudinal mode. For high power
lasers which would require long tubes supporting many modes, this
would be needed with both the overall mirror spacing and etalon being
feedback controlled. But for low power lasers (e.g. 1 to 3 mW),
the use of a short tube to limit the number of modes in conjuntion with
basic feedback control is a much less complex lower cost approach.
Stabilized lasers (or anything that needs to be regulated to some precision)
can be classified as "intrinsic" if what is used to regulate it is
a fundamental property of the device. Most commercial stabilized HeNe
lasers are of this type since they exploit the known and essentially
fixed frequency/wavelength and extent of the neon gain curve in the
E/M spectrum. Additional techniques may be used to further reduce
the uncertainty.
Most common commercial stabilized HeNe lasers usually fall into one of
two subclasses:
Some inexpensive (this is relative!) stabilized HeNe lasers only use a
single mode for frequency locking. When on the slope, this will be
reasonably stable after warmup once the output power has reached
equilibrium.
When the best intensity stability of the total output (without regard to
polarization) is desired, a non-polarizing beam sampler is used or the
signals from the two photodiode channels are summed and compared to the
reference.
Most commercial stabilized HeNe lasers for general laboratory applications
are of type (1) and operate with 2 orthogonal modes for frequency
stabilization, though some use 1 mode for intensity stabilization.
These include the Coherent 200, Spectra-Physics 117
and 117A (and the identical Melles Griot 05-STP-901), various models from
Teletrac, Zygo, and others. The interferometry lasers used in metrology
manufactured by Agilent (formerly Hewlett Packard) and others are of type (2).
For example, in the Melles Griot 05-STP series of
frequency and intensity stabilized HeNe lasers, the 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 flatter
portion of the gain curve (intensity stabilized mode). Those from other
companies are generally similar.
For some photos of the (quite simple) Zeeman split 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". And for more information
on these lasers, see the sections starting with:
Hewlett-Packard HeNe Lasers.
It isn't really possible to convert an inexpensive HeNe tube that operates on
many longitudinal modes into a single frequency 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 usually orthogonal to each-other. Then, stabilization
is possible using very simple hardware. In fact, even if the mode spacing is
a bit smaller - down to 500 or 600 MHz - then only 2 modes will be present
most of the time but 3 may pop up if one is close to the center of the gain
curve. This, too, is an acceptable situation since the tube can be stabilized
with the modes straddling the gain curve and then only 2 modes will oscillate.
For intensity stabilization, 4 modes may even be permitted.
Note that while the modes of a random polarized and
linearly polarized tube are similar (except for polarization), a random
polarized tube is desirable to be able to use a tube that supports 2 modes
to with the benefits they provide, but be able to eliminate the second mode
in the output. Also 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 modes even for a longer tube. Whether this could ever be a
reliable long term approach for a HeNe tube that normally oscillates in
many longitudinal modes is questionable.
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 (it's
called injection locking and is used with great success for other
applications).
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 (and are even more expensive!). Other manufacturers includ Zygo,
Teletrac, Nikon, Micro-g Solutions, SIOS, NEOARK, and Nikon. The lab lasers
that I've seen all 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). OMore 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.
Observing Longitudinal Modes of a HeNe Laser
Monitoring the output power of any HeNe laser
while it's warming up will show a variation in output power due to
longitudinal mode cycling. There is even a specification called the
"Mode Sweep Percentage" which indicates how large the variation is
in relation to the output power. For short tubes, the power fluctuations
can approach 20 percent; for long tubes, they may be less than 2 percent.
Longitudinal Mode Pulling
It turns out that most lasers don't actually oscillate on exact multiples of
the cavity resonance frequency, c/2L, as stated in introductory textbooks.
(The exceptions would be where the gain curve is essentially flat but that's
another story.) Longitudinal modes that aren't exactly centered on the gain
curve will be at frequencies very slightly offset from these, pulled toward
the center of the gain curve with those that are farthest away seeing the
most shift. This is a well known effect called "mode
pulling" with highly developed theory to back it up. (Mode pulling
isn't unique to lasers. For example, a quartz crystal oscillator can be
tuned over a small range using an external capacitor even though its resonance
frequency differs from the output frequency.)
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 Fluctuation During
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).
Plots of HeNe Laser Power Output and Polarized Modes During Warmup
Here are some plots I made of power output versus time for several typical
HeNe laser tubes and heads from nearly the shortest available to
mid-size. (Beyond this, the appearance will be very similar, but possibly
with even a smaller fluctuation in power due to mode cycling.)
Most are from Melles Griot but the behavior of lasers from other
manufacturers will be very similar. The majority are healthy samples
but a few show some rather dramatic peculiarities.
There are also plots of a Coherent model 200 and Hewlett Packard model 5517A
frequency stabilized HeNe lasers from power-on to locking.
R1 PD1
+15 VDC o----/\/\----|<|----+
100 |
/
\<----------+----+---o A/D Input (+/-10 V range)
/ R2 | |
\ 25K | /
R3 | C1 _|_ \ 200K ohms (Zin of A/D module)
-15 VDC o-----/\/\----------+ 1uF --- /
68K | \
| |
0 VDC o-------------------------------+----+----o A/D Ground
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.
Stabilized Single Frequency HeNe Lasers
The common HeNe laser, while highly monochromatic, may 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 generally 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.
