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 the easiest type of laser to fabricate which produces 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. 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 may end up being only a few hours. 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 the approach to take! 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 sealed HeNe laser tubes are between 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 .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 .5 mW) and somewhat larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not quite as common.
Much larger HeNe tubes with internal or external mirrors (more than a *meter* in length!) and capable of generating up to 250 mW of optical power are also available and may turn up on the surplus market as well. 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 - you probably won't find one of these at a local flea market or swap meet!
Manufacturers include Aerotech, Melles-Griot, Siemens, Spectra-Physics, Uniphase, and others. (Note: Aerotech's HeNe laser division is now part of Melles Griot). Tubes, laser heads, and complete lasers from these companies are generally of very high quality and reliability.
HeNe lasers used to be found in all kinds of equipment including early laser printers, laserdisc players, small laser shows, optical surveying and tunnel boring systems, medical positioning systems, and supermarket checkout UPC and other barcode scanners.
Nowadays, most 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 670 nm deep red from a typical diode laser type.)
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, 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! Use a well insulated 1M resistor made from a string of four 250K, 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 Siemans. 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 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.
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.
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 laser (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.
It turns out that the upper level of the transition that produces the 632.8 nm line has an energy level that almost exactly matches the energy level of helium's lowest excited state. The vibrational coupling between these two states is highly efficient.
You need the gas mixture to be mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines.
A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.
There are many possible transitions from the excited state to a lower energy state that can result in laser action. The most important (from our perspective) are listed below:
Wavelength Color
-------------------------------------------
543.5 nm Green
593,9 nm Yellow
611.8 nm Orange
632.8 nm Red
1,152.3 nm Near Infra-Red
1,523.1 nm Near Infra-Red
3,391.3 nm Mid Infra-Red
See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge
for an easy way to see many of the visible ones.
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 more so than the visible lines. In fact, the first HeNe laser operated at 1,152.3 nm. HeNe lasers at all of these wavelengths are commercially available. However, those operating at 632.8 nm are by far the most common and least expensive.
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 of curvature equal to the length of the cavity. The latter is a configuration called 'confocal'. Spherical mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture.
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.
While it is commonly believed that the 632.8 (for example) transition is a sharp peak, it is actually a gaussian - bell shaped - curve. In order for the cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:
L * 2 c * n
W = --------- or F = ---------
n L * 2
where:
Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one.
For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,123, 948,124, 948,125, 948,126,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabrey-Perot resonator and the reflectivity curve of the mirrors may look something like the following:
| 632.8 nm
I| .
| | | |
| | | | | |
| | | | | | | |
_______|______.__|__|__|__|__|__|__|__|__|__._______
n=948,125 -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
Since the mode locations are determined by the physical spacing of the mirrors,
as the tube warms up and expands, these spectral line frequencies are going to
drift downward (toward longer wavelengths). However, since the reflectivity
of the mirrors as a function of wavelength is quite broad (for all practical
purposes, a constant), new lines will fill in from above and the overall shape
of the function doesn't change.
However, for very short HeNe tubes, the gain curve may be narrower than the spacing between modes. The effect is even more likely with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will actually turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion. 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) can compensate for these effects. However, the added expense is only justified for high precision lab quality lasers - you won't find these enhancements on the common cheap HeNe tubes found in bar-code scanners (which are long enough to not be affected in any case)!
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.
O OO OOO Each 'O' represents
O OO O OO OOO a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
Other (non-cartesian) patterns of modes may also be possible depending on
tube dimensions and operating conditions.
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 shorter 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.
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.
In particular, Unit 1-10 Helium-Neon Gas Laser--A Case Study, goes into considerable detail on the theory as well as some more practical information related to HeNe lasers.
This entire course contains great educational content for those who wish to gain a better understanding of the general principles of laser operation. But, it is designed at a level that probably won't put you to sleep with too much heavy math. :-)
Bellows Bellows
/\/\/\ Discharge tube with external electrodes /\/\/\
|| \________________________________________________/ ||
|| | | | | | | ||===> Laser
|| ___ __|_|________________|_|______________|_|__ ||===> Beam
|| / || | | | \ ||
\/\/\/ || | o | \/\/\/
Adjustable || +-----------o RF exciter o----------+ Adjustable
totally || partially
reflecting ||<-- to vacuum system reflecting
mirror mirror
Early HeNe lasers were also quite large and unwieldy in comparison to modern
devices. A laser such as the one depicted above was over 1 meter in length
but could only produce about 1 mW of optical beam power! The associated RF
exciter was as large as a microwave oven. With adjustable mirrors and a
tendency to lose helium via diffusion under the electrodes, they were a
finicky piece of laboratory apparatus with a lifetime measured in hundreds
of operating hours.
In comparison, a modern 1 mW sealed HeNe laser tube can be less than 150 mm (6 inches) in total length, may be powered by a solid state inverter the size of half a stick of butter, and will last more than 20,000 hours without any maintenance or a noticeable change in its performance characteristics.
This fabulous ASCII rendition of a typical small HeNe laser tube should make everything perfectly clear. :-)
____________________________________________
/ _________________ \
Anode |\ Helium+neon, 2-5 Torr Cathode can ^ \ |
.-.---' \.--------------------------------------. '-'---.-. Main
<---| |:::: :======================================: :::::| |===> beam
'-'-+-. /'--------------------------------------' .-.-+-'-'
Totally | |/ Glass capillary ^ _________________/ | | Partially
reflecting | \____________________________________________/ | reflecting
mirror | | mirror
| Rb + - |
+---------/\/\---------o 1.2 to 3 KVDC o-----------+
(Note: the main beam may emerge from either end of the tube depending on its
design, not necessarily the cathode end as shown. A much lower power beam
will likely emerge from the opposite end if it isn't covered - the 'totally
reflecting' mirror isn't perfect.)
For a diagram with a little more artistic merit, see: Typical HeNe Laser Tube Structure and Connections. And, for a diagram of a complete laser head: Typical HeNe Laser Head (Courtesy of Melles Griot).
Tubes up to at least 35 mW are similar in design but proportionally larger, require higher voltage and current, and, of course, will be more expensive.
The discharge at this end produces little heat or damage due to sputtering.
The discharge at this end is distributed over the entire area of the can thereby spreading the heat and minimizing damage due to sputtering which results from positive ion bombardment. For this reason, although it may work (in fact, starting tends to be easier) a HeNe tube should not be run with reverse polarity for any length of time since damage to the anode would likely result.
The getter material is then available to chemically combine with residual oxygen and other unwanted gas molecules that may result from imperfect vacuum pumps and contamination on the tube's glass and metal structures. It will also mop up any intruder molecules that may diffuse or leak through the walls of the tube during its life. Helium and neon are noble gasses and ignore the getter. :-)
Should the getter spot turn milky white or red, it is exhausted and the tube is probably no longer functional.
If you had grown up during the vacuum tube age, the getter would be familiar to you since nearly all radio and TV tubes have getters (and CRTs still do).
You can tell which type you have by looking at a reflection off of the inner surfaces of the mirrors at each end (assuming the one at the non-output end is not painted or covered). A concave mirror will reduce the size of the reflection very slightly compared to a planar mirror.
The front (outer) surface of the mirror at the output end of the tube may be ground to a (slight) convex shape as well resulting in a positive lens which aids in beam collimation.
Since the reflection peaks at a single wavelength, this type of mirror actually appears quite transparent to other wavelengths of light. For example, for common HeNe laser tubes, the mirrors transmit blue light quite readily and appear blue when looking down the bore of an UNPOWERED (!!) tube.
However, long high power tubes (i.e., 20 mW and up) may require fixtures to maintain mirror alignment even when the mirrors are internal. Such tubes will not be stable by themselves because thermal expansion will result in enough change in alignment to significantly alter beam power - even to the extent of extinguishing the beam entirely at times! There may even be a 'This Side Up' indication (not related to the orientation for linearly polarized tubes) on the HeNe tube or laser head as gravity affects this as well (the alignment and thus power, not the gas, electrons, ions, or light!) and can significantly affect operation. I do not know if this sort of behavior is common or only likely with tubes that are marginal in some way.
In the case of a HeNe tube, the initial breakdown voltage is much greater than the sustaining voltage. The starting voltage may be provided by a separate circuit or be part of the main supply.
In order for the discharge to be stable, the total of the effective power supply resistance, ballast resistance, and tube (negative) resistance must be greater than 0 at the operating point. If this is not the case, the result will be a relaxation oscillator - a flashing or cycling laser!
Note: HeNe tube starting voltage is lower and operating voltage is higher when powered with reverse polarity. With some power supply designs, the tube may appear to work equally well or even better (since starting the discharge is easier) when hooked up incorrectly. However, this is damaging to the anode electrode of the tube (and may result in more stress on the power supply as well due to the higher operating voltage) and must be avoided (except possibly for a short duration during testing).
See the chapter: HeNe Laser Power Supplies for more information and complete circuit diagrams.
Most laser heads include the ballast resistor since it needs to be close to the HeNe tube anode anyhow (though you may still need additional resistance to match the tube to your power supply). The ballast resistor may be potted into the end cap with the HV cable, a wart attached to the HeNe tube, or a separate assembly. There may be an additional ballast resistor (e.g., 10K) in the cathode circuit as well.
The high voltage cable will likely use an 'Alden' connector which is designed to hold off the high voltages with a pair of keyed recessed heavily insulated pins. This is a universal standard for small-medium size HeNe laser power supplies (the longer fatter pin is negative).
Internal wiring may be via fat insulated cables or just bare metal (easily broken) strips. Take care if you need to disassemble one of these laser heads (the round ones in particular) as the space inside may be quite cramped.
CAUTION: The case, if metal, of the laser head may be wired to the cathode of the HeNe tube and thus the negative of the Alden connector and power supply. This is not always the situation but check with an ohmmeter and keep this in mind when designing a power supply or modulation scheme. The case should always be earth grounded for safety if at all possible (or else properly insulated). DO NOT assume that a commercial power supply is designed this way - check it out and take appropriate precautions.
Note: Depending on design, the laser tube itself may be mounted inside the laser head in a variety of ways including RTV Silicone (permanent), or 3 or 4 set screws at two locations (front and rear) around the outside of the housing. The latter approach permits precise centering of the beam but don't overtighten the screws or you WILL be sorry! (Since RTV silicone has some compliance, very SLIGHT adjustment of alignment may still be possible even if mounted this way - don't force it, however.)
The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings (operating current, proper polarity, and not continuously restarting). Therefore, this is not a major consideration for most hobbyist applications. However, the shelf life of the tube depends on its construction. There are two types of (sealed) HeNe tubes:
Shelf life of hard sealed tubes is essentially infinite.
Shelf life of non-hard sealed tubes is limited by diffusion of the Helium atoms. Helium atoms are slippery little devils. They diffuse through almost anything. In the case of HeNe tubes, diffusion takes place mostly through the Epoxy adhesive used to mount the mirrors in non-hard sealed tubes (not common anymore) and through the glass itself but at a much much slower rate.
As the tube is used (many thousands of hours or from abuse), operating and starting voltages may be affected as well - generally increasing with the ultimate result being that a stable discharge cannot be initiated or maintained with the original power supply. See the section: How Can I Tell if My Tube is Good?.
IR (infra-red) HeNe laser tubes are manufactured as well (1,523.1 nm is most common probably because this wavelength is useful for testing of fiber optic data transmission systems). However, an invisible beam just doesn't seem as exciting!
Typical maximum output available from (relatively) small sealed HeNe tubes (400 to 500 mm length) for various colors: red - 10 mW, orange - 3 mW, yellow - 2 mW, green - 1.5 mW, IR - 1 mW. Higher power red HeNe tubes (up to 35 mW or more) and 'other-color' HeNe tubes (much lower - under 10 mW) are also available. However, these will be very large and very expensive.
The Melles Griot catalog states 473.612535 THz for the 632.8 line. For their stabilized system they claim 473.61254 THz. With c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J.Phys.E, Vol 18, 1985, p. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.
(From: D. A. Van Baak (dvanbaak@calvin.edu)).
Well, here it is exact:
Wavelength * 4
Divergence angle (half of total) in radians = --------------------
pi * Beam Diameter
So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence
angle will be about 1.6 mR. Note that although a wider bore should result
in less divergence, this also permits more not quite parallel rays to
participate in the lasing process. Also see the section: Improving the
Collimation of a HeNe Laser with a Beam Expander.
Lasers with external mirrors and Brewster windows (plates at the Brewster angle attached to the ends of the tube) will be linearly polarized and really expensive. They will also be more finicky as there may be some maintenance - the optics will need to be kept immaculate and the mirror alignment may need to be touched up occasionally. However, the fine adjustments will permit optimum performance to be maintained and changes in beam characteristics due to thermal effects should be reduced since the resonator optics are isolated from the plasma tube.
Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.
One possible cause for this artifact is that the output-end mirror (output coupler or OC) has some 'wedge' (the two surfaces are not quite parallel) built in to move any reflections - unavoidable even from Anti-Reflection (AR) coated optics - off to the side and out of harm's way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn't even know of their existence!
Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator - perfectly aligned with the tube axis - which could result in lasing instability. (However, the chance of this is probably minimal since anything making its way back inside from reflection from an AR coated surface AND through the 99%+ reflective OC would be extremely weak).
Thus, the ghost beam off to one side is likely a feature, not a problem!
If it isn't obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a 45 degree angle onto a white screen. There will be a pair of reflected beams - a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed.
It is conceivable that slight misalignment of the mirrors may result in similar symptoms but this is a less likely cause than the built-in wedge 'feature'. :-) However, if you won't sleep at night until you are sure, see the section: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes but ONLY apply the very slightest force in each direction to confirm that the beam is cleanest and most intense with the present alignment.
(From: Steve Roberts (osteven@akrobiz.com)).
The mirror is wedged to cut down on the number of ghost beams, however even with a wedged mirror there is almost always one ghost. Nothing is wrong with your coatings on the mirror, it is simply a alignment matter. The mirrors need to be "walked" into the right position relative to the bore. There are many many paths down the bore that will lase, but only a few have the TEM00 beam and the most brightness, this generally corresponds to the one with minimum ghosts.
See the section: Quick Course in Large Frame HeNe Laser Mirror Alignment for more information.
Magnets may be incorporated in HeNe lasers for several reasons including the suppression of IR spectral lines to improve efficiency (such as it is!) and to boost power at visible wavelengths, for the stabilization of the beam, and to control its polarization. There are no doubt other uses as well.
The basic mechanism for the interaction of emitted light and magnetic fields is something called the 'Zeeman Effect' or 'Zeeman Splitting'. The following brief description is from the "CRC Handbook of Chemistry and Physics":
Magnet fields may affect the behavior of HeNe tubes in several ways:
As a result of the Zeeman Effect, if a gas radiates in a magnetic field, most of its spectral lines are split into 2 or sometimes more components. The magnitude of the separation depends on the strength of the magnetic field and as a result, if the field is also non-uniform, the spectral lines are broadened as well because light emitted at different locations will see an unequal magnetic field. These 'fuzzed out' lines cannot participate in stimulated emission as efficiently as nice narrow lines and therefore will not drain the upper energy states for use by the desired lines. The magnitude of the Zeeman splitting effect is also wavelength dependent and therefore can be used to control the gain of selected spectral lines (long ones are apparently affected more than short ones on a percentage basis).
Without the use of magnets, the very strong neon IR line at 3.39 um would compete with (and possibly dominate over) the desired visible line (at 632.8 nm) stealing power from the discharge that would otherwise contribute to simulated emission at 632.8 nm. However, the IR isn't wanted (and therefore will not be amplified since the mirrors are not particularly reflective at IR wavelengths anyhow). Since the 3.39 nm wavelength is more than 5 times longer than the 632.8 nm red line, it is affected to a much greater extent by the magnetic field and overall gain and power output at 632.8 nm may be increased dramatically (25 percent or more). The magnets may be required to obtain any output beam with some HeNe tubes.
I do not know how to determine if and when such magnets are needed for long high power HeNe tubes where they are not part of an existing laser head. My guess is that the original or intended positions, orientations, and strengths, of the magnets were determined experimentally by trial and error, not through the use of some unusually complex convoluted obscure theory.
The only thing I can suggest other than contacting the manufacturer is to very carefully experiment with placing magnets of various sizes and strengths at strategic locations (or a half dozen such locations) to determine if beam power at the desired wavelength is affected. Just take care to avoid smashing your flesh or the HeNe tube when playing with powerful magnets. Enclosing the HeNe tube in a protective rigid sleeve (e.g., PVC or aluminum) would reduce the risk of the latter disaster, at least. :-)
In this case, what is required is a uniform or mostly uniform field of the appropriate orientation rather than one that varies as for IR spectral line suppression though both of these could be probably be combined.
Also see the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
(In all of these diagrams, the orientation of the Brewster windows shown is totally arbitrary - for sealed HeNe tubes with internal mirrors, they would not be present at all!)
Polarity may alternate with North and South poles facing each other across the tube forming a 'wiggler' so named since such a they will tend to deflect the ionized discharge back and forth though there may be no visible effects in the confines of the capillary:
N S N S N S N S
|| //======================================================\\ ||
|| //======. .========================================. .======\\ ||
S ||| N S N S N S ||| N
'|' '|'
Alternatively, North and South poles may face each other:
N S N S N S N
|| //===============================================\\ ||
|| //======. .=================================. .======\\ ||
N ||| S N S N S ||| N
'|' '|'
N N N N N N N
|| //===============================================\\ ||
|| //======. .=================================. .======\\ ||
S ||| S S S S S ||| S
'|' '|'
+--+ +--+ +--+ +--+
N | | S N | | S N | | S N | | S
+--+ +--+ +--+ +--+
|| //==================================================\\ ||
|| //====. .========================================. .====\\ ||
||| +--+ +--+ +--+ +--+ |||
'|' N | | S N | | S N | | S N | | S '|'
+--+ +--+ +--+ +--+
Other axial configurations with opposing poles or radially oriented poles
may also be used or there may be a single long solenoid type of coil.
Output Tube Voltage Tube Supply Voltage Tube Size Power Operate/Start Current (75K ballast) Diam/Length ---------- --------------- ------------ ---------------- ------------- .3-.5 mW .8-1.0/6 KV 3.0-4.0 mA 1.0-1.2 KV 19/135 mm .5-1 mW .9-1.0/7 KV 3.2-4.5 mA 1.1-1.3 KV 25/150 mm 1-2 mW 1.0-1.4/8 KV 4.0-5.0 mA 1.2-1.8 KV 30/200 mm 2-3 mW 1.1-1.6/8 KV 4.0-6.5 mA 1.4-2.0 KV 30/260 mm 3-5 mW 1.7-1.9/10 KV 4.5-6.5 mA 2.1-2.4 KV 37/350 mmMelles Griot, Uniphase, Siemens, PMS, and Aerotech all show similar values.
Note how some of the power levels vary widely with respect to tube dimensions, voltage, and current. Generally, higher power implies a longer tube, higher operating/start voltages, and higher operating current - but there are some exceptions. In addition, you will find that physically similar tubes may actually have quite varied power output. These specifications are generally for minimum power (when new). Individual samples may be much higher. This is particularly evident in the Melles Griot listings, below.
My guess is that for tubes with identical specifications in terms of physical size, voltage, and current, the differences in power output are due to sample-to-sample variations. Thus, like computer chips, they are selected after manufacture based on actual performance and the higher power tubes are priced accordingly! This isn't surprising when considering the low efficiency at which these operate - extremely slight variations in mirror reflectivity and trace contaminants in the gas fill can have a dramatic impact on power output.
I have a batch of apparently identical 2 mW Aerotech tubes that vary in power output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the tubes indicating measured power levels at the time of manufacture).
And the answer to your burning question is: No, you cannot get a 3 mW tube to output 30 mW - even instantaneously - by driving it 10 times as hard!
I have measured the operating voltage and determined the optimum current (by maximizing beam intensity) for the following specific samples - all red (632.8 nm) tubes from various manufacturers). (The starting voltages were estimated):
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
---------- --------------- ------------ ---------------- -------------
.8 mW .9/5 KV 3.2 mA 1.1 KV 19/135 mm
1.0 mW 1.1/7 KV 3.5 mA 1.4 KV 25/150 mm
1.0 mW 1.1/7 KV 3.2 mA 1.4 KV 25/240 mm
2.0 mW 1.2/8 KV 4.0 mA 1.5 KV 30/185 mm
3.0 mW 1.6/8 KV 4.5 mA 1.9 KV 30/235 mm
5.0 mW 1.7/10 KV 6.0 mA 2.2 KV 37/350 mm
12.0 mW 2.5/10 KV 6.0 mA 2.9 KV 37/450 mm
The following are from Melles Griot. These are all red (632.8 nm) tubes
operating single mode - TEM00. Many are actually sold as part of complete
laser heads (all those where the model number is odd). You don't even want
to think about asking about the prices of these laser heads. Most are still
listed in the current catalog.
Red (632.8 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LHR- ----------------------------------------------------------------------------- .5 mW .46 mm 1.77 mR 1200 MHz 1.10/5 KV 3.2 mA 19/135 mm 002-246* .5 mW .49 mm 1.70 mR 1040 MHz 1.25/5 KV 4.5 mA 25/152 mm 700 .8 mW .46 mm 1.77 mR 1063 MHz 1.32/5 KV 4.0 mA 25/150 mm 211 1.0 mW .53 mm 1.50 mR 883 MHz 1.47/8 KV 4.5 mA 25/177 mm 900 1.0 mW .59 mm 1.35 mR 687 MHz 1.79/8 KV 6.5 mA 37/230 mm 111 2.0 mW .59 mm 1.35 mR 687 MHz 1.79/10 KV 6.5 mA 37/230 mm 121 2.0 mW .72 mm 1.10 mR 612 MHz 1.85/10 KV 6.5 mA 29/255 mm 080 2.0 mW .76 mm 1.06 mR 636 MHz 1.71/10 KV 5.0 mA 30/250 mm 073 2.5 mW .52 mm 1.53 mR 822 MHz 1.77/10 KV 4.5 mA 25/200 mm 691 4.0 mW .80 mm 1.00 mR 435 MHz 2.35/10 KV 6.5 mA 37/351 mm 140 5.0 mW .80 mm 1.00 mR 438 MHz 2.29/10 KV 6.5 mA 37/370 mm 151 7 mW 1.02 mm .79 mR 373 MHz 2.65/10 KV 6.5 mA 37/420 mm 171 10 mW .65 mm 1.24 mR 341 MHz 2.64/10 KV 6.5 mA 37/440 mm 991 17 mW .96 mm .83 mR 267 MHz 3.70/12 KV 7.0 mA 37/600 mm 925 25 mW 1.23 mm .66 mR 165 MHz 5.10/15 KV 8.0 mA 42/930 mm 827 35 mW 1.23 mm .66 mR 165 MHz 5.10/15 KV 8.0 mA 42/930 mm 927
Both random and linearly polarized models are available. The only other difference in specifications between these is the price - those that are linearly polarized are about 10 to 15 percent more expensive.
Maximum available power output is also lower - rarely over 2 mW (and even those tubes are quite large (see the tables below). However, since the eye is more sensitive to the green wavelength (543.5 nm) compared to the red (632.8 nm) by more than a factor of 4 (see the section: Relative Visibility of Light at Various Wavelengths), a lower power tube may be more than adequate for many applications. Yellow (594.1 nm) and orange (611.9 nm) HeNe lasers appear more visible by factors of about 3 and 2 respectively compared to red beams of similar power.
There are infrared HeNe tubes as well. Yes, you can have a HeNe tube and it will light up inside (typical neon glow), but if there is no output beam (at least you cannot see one), you could have been sold an infrared HeNe tube. The IR may be visible with a video camera (assuming it doesn't have an IR blocking filter) or by using one of the IR detector circuits or an IR detector card as discussed with respect to IR laser diodes. IR HeNe tubes are unusual enough that it is very unlikely you will ever run into one. However, they do exist and may turn up on the surplus market especially if the seller doesn't test the tubes and thus realize that these behave differently - they are physically similar to red (or other color) HeNe tubes except for the center wavelength of the mirror reflectivity. (Of course, your tube could also fail to lase due to misaligned or damaged mirrors or some other reason. See the section: How Can I Tell if My Tube is Good?.)
Even if the model number does not identify the tube as green, yellow, orange, or infra-red, this difference should be detectable by comparing the appearance of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of a normal - red - HeNe tube. Since the mirrors are dichroic - functioning as a result of interference - they have high reflectivity only around the laser wavelength and actually transmit light quite well as the wavelength moves away from this peak. By transmitted light, the mirrors will tend to appear with a color which is the complement of the laser's output - e.g., cyan or blue-green for a red tube, pink or magenta for a green tube. The mirrors of an IR tube may appear neutral or even transparent. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered!
And, the answer to that other burning question should now be obvious: No, you cannot convert a red sealed HeNe tube to generate some other color light. As noted above, the mirror reflectance peak is different (and the cavity length may be insufficient to resonate with the reduced gain other-color spectral lines). For a laser with external mirrors, a mirror swap may be possible (though the Brewster angle windows may then not quite be at the proper angle). However, for sealed HeNe tubes, your options are severely limited - as in there are none :-(.
The following 'other-color' tubes are listed in the Melles Griot catalog. These are all single mode - TEM00, random polarized. Linearly polarized versions are also available but for some reason, unlike the red tubes (where the output power specifications are identical), have much lower output power. Also note that the output power of these tubes is much much less than that of the red (632.8 nm) models with similar (in some cases identical) dimensions.
Green (543.5 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LGR- ----------------------------------------------------------------------------- .2 mW .63 mm 1.26 mR 732 MHz 1.56/8 KV 4.5 mA 30/215 mm 025 .5 mW .80 mm 1.01 mR 438 MHz 2.39/10 KV 6.5 mA 37/360 mm 151 .8 mW .89 mm .92 mR 373 MHz 2.62/10 KV 6.5 mA 37/420 mm 173 1.5 mW .86 mm .81 mR 328 MHz 2.75/10 KV 6.5 mA 37/475 mm 193Yellow (594.1 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LYR- ----------------------------------------------------------------------------- .35 mW .63 mm 1.26 mR 732 MHz 1.62/8 KV 4.5 mA 30/215 mm 025 .75 mW .80 mm 1.01 mR 438 MHz 2.43/10 KV 6.5 mA 37/360 mm 151 2.0 mW .75 mm .92 mR 373 MHz 2.59/10 KV 6.5 mA 37/420 mm 173Orange (611.9 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LOR- ----------------------------------------------------------------------------- .5 mW .63 mm 1.26 mR 732 MHz 1.66/8 KV 4.5 mA 30/215 mm 025 2.0 mW .80 mm 1.01 mR 438 MHz 2.49/10 KV 6.5 mA 37/360 mm 151Infra-Red (1,523 nm):
Minimum e/2 c/2L Supply Nominal Output Beam Diver- Mode Opr/Strt Tube Tube Size Model Power Diam gence Spacing (Rb=75K) Current Diam/Lgth 05-LIR- ----------------------------------------------------------------------------- 1.0 mW .88 mm 2.20 mR 373 MHz 2.97/10 KV 6.0 mA 37/420 mm 171As a side note: It is strange to see the normal red-orange glow in a green HeNe laser tube but have a green beam emerging. A diffraction grating or prism really shows all the lines that are in the glow discharge. Red through orange, yellow and green, even several blue lines!! The IR lines are present as well - you just cannot see them.
See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.
The original work on green was done by Rigden and Wright. The short tubes have lower losses because they have no Brewsters and thus can concentrate on tuning the coatings to 99.9999% reflectivity and maximum IR transmission. There is one tunable low power unit on the market that does 6 lines or so, but only 1 line at a time, and the $6,000 cost is kind of prohibitive for a few milliwatts of red and fractional milliwatt powers on the other lines. But, it will do green and has the coatings on the back side of the prism to kill the losses.
Even Large HeNe lasers such as the SP 125 (100 to 200 mW of red) will only do about 20 mW of yellow, with a 30 mW 107 your probably only looking at 3 to 5 mW of yellow. And, for much less then the cost of the custom optics to do a conversion, you can get a two or three 4 to 5 mW yellow heads from Melles Griot. I know for a fact that a SP 107 only does about 3 mW of 612 with a external prism and a remoted cavity mirror, when it does 32 mW of 632.8 nm.
So in the end, unless you have a research use for a special line, it's cheaper to dig up a head already made for the line you seek, unless you have your own optics coating lab that can fabricate state-of-the-art mirrors.
I have some experience in this, as I spent months looking for a source of the optics below $3,000.
I don't know whether it is also possible to obtain orange (611.8 nm), yellow (593.9 nm), or green (543.5 nm) output with similar modifications - the resonator gain may be too low. The 1152.3 nm and 3391.3 nm IR lines are very strong and high gain.
Note: The specifications below were copied from an almost illegible scan of a fax of a copy of the original product brochure. Corrections are welcome!
Spectra-Physics Laser: Model SP-124B Model SP-125A
-----------------------------------------------------------------------------
OUTPUT
Wavelength (nm): 632.8 1152.3 3391.3 632.8 1152.3 3391.3
Minimum Power (mW): 15 2.5 5.0 50 10 10
BEAM CHARACTERISTICS
Beam Diameter (mm): 1.1 1.4 2.5 1.8 2.4 4.1
Beam divergence (mR): .75 1.0 1.5 .6 .8 1.4
RESONATOR CHARACTERISTICS
Transverse Mode: TEM00
Degree of Polarization: 1E-3
Angle of Polarization: Vertical (+/- 5 Degrees)
Resonator Configuration: Long Radius
Resonator Length: 70.1 cm 177.0 cm
Axial Mode Spacing: 214 MHz 85 MHz
Plasma Excitation: 5 KV at 15 mA 6 KV at 25 to 35 mA
(RF Opt: 15 W at 46 MHz)
AMPLITUDE STABILITY
Beam Amplitude Noise: <.3% RMS <2% RMS (RF: <.5%)
Beam Amplitude Ripple: <.2% RMS <.5% RMS (RF: <.6%)
Long Term Power Drift: <5% over 8 hours and 10 Degrees C
Warmup Time: 30 Minutes 1 Hour
ENVIRONMENTAL CAPABILITY
Operating Temperature: 10 to 40 Degrees C
Operating Altitude: Sea Level to 3,000 m (10,000 ft.)
Operating Humidity: Below Dew Point
POWER REQUIREMENTS
Power Supply: 115/230 VAC, 50/60 Hz, +/-10%
Input Power: 125 W 156 W
PHYSICAL CHARACTERISTICS
Laser Head Weight: 11.4 KG (25 lb) 45.4 KG (100 lb)
Power Supply Weight: 3.5 KG (7.5 lb) 13.6 KG (30 lb)
Place a white card in the exit beam and note where the single red output line of the HeNe tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.
As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube's output mirror.
For HeNe lasers, the primary line (usually 632.8 nm) is extremely narrow and effectively a singularity given any instrumentation you are likely to have at your disposal. Any other lines you detect in the output are almost certainly from two possible sources but neither is actual laser emission:
Close to the output mirror, you may see some of this light seeping through especially at wavelengths in the green, blue, and violet, for which the dichroic mirrors are nearly perfectly transparent. However, such light will be quite divergent and diffuse and won't be visible at all more than a couple of inches from the mirror.
The result will be a weak green beam that can sometimes be observed with a spectroscope in a very dark room room. It isn't really quite as coherent or monochromatic as the beam from a true green HeNe laser and probably has wide divergence but nonetheless may be present. It may be easier to see this by using your spectroscope to view the bright spot from the laser on a white card rather than by deflecting the beam and trying to locate the green dot off to one side.
Note: I have not been able to detect this effect on the short HeNe tubes I have checked.
Note that argon and krypton ion lasers are often designed for multiline output where all colors are coherent and within an order of magnitude of being equal to each other in intensity. This is not to my knowledge ever done with HeNe lasers, at least not the sealed tube variety.
The next best thing is a small HeNe laser laid bare with the sealed (internal mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglass case with all parts labeled. This would allow the discharge in the HeNe tube to be clearly visible (and permit the use of the Instant Spectroscope for Viewing Lines in HeNe Discharge). The clear insulating case prevents the curious from coming in contact with the high voltage (and line voltage, if the power supply connects directly to the AC line), which could otherwise result in damage to both the person and fragile glass HeNe tube when a reflex action results in smashing the entire laser to smithereens!
A HeNe laser is far superior to a cheap laser pointer for several reasons:
Important: If this see-through laser is intended for use in a classroom, check with your regulatory authority to confirm that a setup which is not explicitly CDRH approved (but with proper laser class safety stickers) will be acceptable for insurance purposes.
For safety with respect to eyeballs and vision, a low power laser - 1 mW or less - is desirable - and quite adequate for demonstration purposes.
The HeNe laser assembly from a barcode scanner is ideal for this purpose. It is compact, low power, usually runs on low voltage DC (12 V typical), and is easily disassembled to remount in a demonstration case. The only problem is that many of these have fully potted "brick" type power supplies which are pretty boring to look at. However, some have the power supply board coated with a rubbery material which can be removed with a bit of effort (well, OK, a lot of effort!). For example, this HeNe Tube and Power Supply is from a hand-held barcode scanner. A similar unit was separated into its Melles Griot HeNe Tube and HeNe Laser Power Supply IC-I1 (which includes the ballast resistors). These could easily be mounted in a very compact case (as little as 3" x 6" x 1", though spreading things out may improve visibility and reduce make cooling easier) and run from a 12 VDC, 1 A wall adapter. Used barcode scanner lasers can often be found for $20 or less.
An alternative is to purchase a .5 to 1 mW HeNe tube and power supply kit. This will be more expensive (figure $5 to $15 for the HeNe tube, $25 to $50 for the power supply) but will guarantee a circuit board with all parts visible.
The HeNe tube, power supply, ballast resistors (if separate from the power supply), and any additional components can be mounted with standoffs and/or cable ties to the plastic base. The tube can be separated from the power supply if desired to allow room for labels and such. However, keep the ballast resistors as near to the tube as practical (say, within a couple of inches, moving them if originally part of the power supply board). The resistors may get quite warm during operation so mount them on standoffs away from the plastic. Use wire with insulation rated for a minimum of 10 KV. Holes or slots should be incorporated in the side panels for ventilation - the entire affair will dissipate 5 to 10 Watts or more depending on the size of the HeNe tube and power supply. (However, if you want to take this thing outdoors, see the section: Weatherproofing a HeNe Laser.
Provide clearly marked red and black wires (or binding posts) for the low voltage DC or a line cord for AC (as appropriate for the power supply used), power switch, fuse, and power-on indicator. Label the major components and don't forget the essential CDRH safety sticker (Class II for less than 1 mW or Class IIIa for less than 5 mW).
See the suppliers listed in the chapter: Laser and Parts Sources.
____________________________________________
/ | | | _______ \
Anode |\ | | | \ | Cathode
.-.---' \.-----'-----..-----'-----..-----'------. '-'---.-.
<---| |:::: :===========::===========::============: :::::| |===>
'-'---. /'-----.-----''-----.-----''-----.------' .-.---'-'
|/ | | | _______/ |
\_______|____________|____________|__________/
The third has Brewster angle windows and external screw-adjustable mirrors.
It also has its cathode in a side-tube rather than the more typical coaxial
can type. It is otherwise similar.
Only one of the 3 HeNe tubes of this type that I have works at all and it has a messed up gas fill probably due to age despite its being hard sealed. Its output is perhaps 1 or 2 mW (where it should be around 20 mW). However, to the extent that it works, there doesn't appear to be anything particularly interesting or different about its behavior. (Of the other two, one has a broken mirror and the funky tube with the Brewster windows was non-hard sealed and leaked.) The reason(s) for the unusual design are therefore not known at this time. It may have been to stabilize the discharge, suppress unwanted spectral lines, or something else.
The HeNe laser head was manufactured by Jodan Engineering Associates of Ann Arbor, MI. Output power is 15 to 25 mW, 2 mm diameter beam, probably multimode (not TEM00).
It requires approximately 2,000 V at 18 to 22 mA (for two discharge feeds, see the diagram below). Starting uses a large trigger transformer to produce an ionizing pulse via external electrodes (rather than a voltage in series with the anode). This is similar the way a xenon flash lamp is triggered.
The construction is certainly non-standard (at least compared to modern sealed HeNe lasers) and is shown below:
Capillary tube/external starting electrodes
Starting pulse o-------+----------------------+
_|_ _|_
|| //==================================================\\ ||
|| //=====. .==================. .=================. .=====\\ ||
||| | | |||
Mirror '|' 25K | | 25K '|' Mirror
Anode 1 +---/\/\---o +HV | | +HV o---/\/\---+ Anode 2
.---------------' '--------------.
---|-+ +-|---
| ) Main Spare ( |
---|-+ +-|---
'--------------------------------'
Gas reservoir with heated cathodes and getters
This tube uses a heated cathode (like an old vacuum tube) requiring a filament
supply of 6.3 V at 2.05 A. Only one of the two shown are needed, the other is
a spare. Modern HeNe lasers use cold cathodes (the 'can' electrode). However,
some other gas lasers (e.g., argon ion) still use heated cathodes.
A series of magnets (not shown) along axis of discharge: Suppress the 3.39 um line which competes with the 632.8 nm line and can rob up to 25% of its power
Jodon Laser Head shows the construction in more detail including the magnet locations and orientations.
The current status of this project is that the laser needs to be regassed. Chris is equipped to do this and has acquired the needed HeNe gas mixture. Stay tuned.
One other thing that is most interesting is that the original list price from the HP catalog for the laser head alone is about $9,000!
(From: Angel Vilaseca (100604.1242@compuserve.com)).
Here is a quick description of the unit:
The label on the unit says:
HENE GAS LASER, Hewlett-Packard, P.N. 05517-60501 Date of mfg. 4-12-93, Date of instl. 4-19-93, Ser. no. 591-3 Made in USA, Licensed by Patiex Corporation, under patent no. 4,704,583.
