01C. Helium-Neon Lasers

Introduction

A helium neon (henceforth abbreviated HeNe) laser is basically a fancy neon sign with mirrors at both ends. Well, not quite, but really not much more than this. The gas fill is a mixture of helium and neon gas at low pressure. A pair of mirrors (one totally reflective, the other partially reflective at the wavelength of the laser's output) complete the resonant cavity. This is called a Fabry-Perot cavity (if you want to impress your friends). The mirrors may be internal (common on small and inexpensive tubes) or external. Electrodes sealed into the tube allow for the passage of high voltage DC current to excite the discharge.

I remember doing the glasswork for a 3 foot long HeNe laser which 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).

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 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, right? With luck, the laser produces a beam and only final adjustments to the mirrors are then required. 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. It just may not be your day! Nonetheless, if you really want to be able to say you built a laser from the ground up, this is the approach to take.

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 6" and 14" (150 mm to 350 mm) in overall length and 1" to 1-1/2" (25 mm to 37.5 mm) generating optical power from .5 mW to 5 mW.

Slightly smaller tubes (less than .5 mW), somewhat larger tubes (up to 20 mW), and much larger tubes with internal or external mirrors (a *meter* or more in length generating up to 250 mW of optical power), are also available and may turn up on the surplus market. Specialized configurations - a triple XYZ axis triangular cavity laser in a solid glass block for an optical ring laser gyro, for example - also exist but are much much less common - you probably won't find one of these at a local flea market!

Manufacturers include Aerotech, Melles-Griot, Siemens, Spectra-Physics, and many others.

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. (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 laser diode type.)

Nowadays, these applications are likely to use the much more compact lower (drive) power solid state laser diodes. Thus, a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as a HeNe laser half a meter long!

So why bother with a HeNe laser at all? There are several reasons:

* For many applications including holography and interferometry, the high quality stable beam of a HeNe laser is unmatched (at least at reasonable cost, perhaps at all) by laser diodes. In particular, the coherence length and monochromicity of even a cheap HeNe laser are excellent and the beam profile is circular (laser diodes usually have some amount of astigmatism) so that simple spherical optics can be used for beam manipulation.

* As noted in the chapter on laser diodes, it is all too easy to ruin them in the blink of an eye (actually, the time it takes light to travel a few feet). It would not take very long to get frustrated burning out $50 diodes. So, the HeNe laser tube may be a better way to get started. They are harder to damage through carelessness or design errors. Just don't get the polarity reversed or exceed the tube's rated current for too long - or drop them on the floor! And, take care around the high voltage.

* Laser diode modules at a wavelength of 635 nm may be somewhat more expensive than surplus HeNe tubes with power supplies. However, with the introduction of DVD players and DVDROM drives, this situation probably will not last long.

HeNe Laser Safety:

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

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

The high quality capacitors in a typical power supply will hold enough charge to wake you up - for quite a while even after the supply has been switched off and unplugged. Unless significantly oversized, this isn't usually a lethal amount of energy but 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 hurt you physically but your ego may be bruised if you then drop the tube and it shatters on the floor! Use an insulated 1 M, 2 W resistor to drain the charge before touching anything.

Comments on HeNe Laser Safety issues:

(Portions from: Robert Savas (jondrew@mail.ao.net)).

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.

Instant HeNe Laser theory:

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-orange 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.

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-Orange
1152.3 nm	Near Infra-Red
3391.3 nm	Mid Infra-Red

 

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 fact, the first HeNe laser operated at 1152.3 nm. HeNe lasers at all of these wavelengths are commercially available but 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.

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.

Modes of Operation:

The physical dimensions of the Fabry-Perot resonator impose some additional constraints on the resulting beam characteristics. While it is commonly believed that the 632.8 (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:

where:

* L is the distance between the mirrors (m)

* W denotes the possible wavelengths of oscillation (m)

* n is a large integer (order of 948,000 for W around 632.8 nm, L = .3 m)

* F denotes the possible frequencies of oscillation (Hz)

* c is the speed of light (approximately 300 million m/s).

           

The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(L * 2) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal modes 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. 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 :-).

For example:

This also means that as the tube warms up and expands, these spectral line frequencies are going to drift downward (toward longer wavelengths). However, since the reflectivity distribution of the mirrors remains constant, new lines will fill in from above so the overall shape of the output doesn't change.

Other (non-cartesian) patterns of modes may also be possible depending on tube dimensions and operating conditions.

Early vs. modern HeNe lasers

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

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 a stick of butter, and will last more than 20,000 hours without maintenance of any type or a noticeable change in its performance characteristics.

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

(Note: the main beam may emerge from either end of the tube depending design, not necessarily the cathode end as shown.)

* The anode (+) end is simply a small cylindrical metal electrode with a mirror fused or glued to its end.

The discharge at this end produces little heat or damage due to sputtering.

* The cathode (-) end is also a cylindrical metal electrode with a mirror fused or glued to its end but in addition, there is a large 'cylindrical can attached to the cathode and extending about half the length of the tube.

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.

* These mirrors are not silvered or aluminized (metal coated) but are a type called 'dichroic'. They are made by depositing many alternating layers of hard but transparent materials having different indexes of refraction. The thickness of each is precisely 1/2 the wavelength of the laser light (632.8 nm being the most common for a HeNe laser). This results in reflection by interference with very high (>99.9 %) efficiency - much greater than for even the best metal coated mirrors.

* One of the mirrors will be nearly totally reflecting and the other will only be partially reflecting at the laser wavelength. Since the reflection peaks at a single wavelength, these mirrors actually appear quite transparent to other wavelengths of light. For example, for common HeNe lasers tubes, the mirrors transmit blue light quite readily and appear blue when looking through an unpowered (!!) tube.

* The mirrors will likely not have any 'user' adjustments. However, the cylindrical end pieces are mounted by thinner sections of metal tubing so that some slight changes to alignment may be possible with appropriate fixtures. Don't be tempted: (1) grabbing the high voltage electrodes is not likely to be pleasant and (2) it is too easy to break the seal if you get carried away. There should be no reason for the alignment to have changed unless you whacked the tube - it was set at the factory.

* The main beam will emerge from the partially reflecting mirror but this may be at either end of the tube depending on model. For example, where the tube is enclosed in a metal barrel, the HV connections will be to the anode end and the beam will exit from the cathode end. With this arrangement, the positive output of the power supply and ballast resistor can be very close to the tube anode and the entire barrel can be connected to the negative output of the power supply and earth ground.

* Since the mirrors are not perfect, there will be a weaker beam visible from the other end if that mirror is not covered (blocked or painted over). One use of this is to permit monitoring of laser power for purposes of optical power regulation or other closed loop applications.

* The major discharge is forced to take place inside a thick glass capillary tube with an inner bore of roughly 1 mm. This concentrates the discharge forcing operation in the most common and desirable TEM00 mode.

* The cheap surplus HeNe tubes do not generally produce a fixed polarized beam. The polarization will either be random or slowly changing as the tube heats. Tubes with a specified polarization are also available but are generally more costly. Lasers with external mirrors and Brewster windows will be linearly polarized and really pricey (and more finicky).

* These tubes are nearly always operate in multimode (longitundial) with a TEM00 beam profile. See the section: "Instant HeNe laser theory" for more info.

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

- Operating voltage of 1,000 to 3,000 DC at 3 to 10 mA.

Like any discharge tube, the HeNe laser is a negative resistance device. As the current *increases* through the tube, the voltage across the tube *decreases*. The incremental magnitude of the negative resistance also increases with descreasing current.

- Starting voltage of 5 to 12 KV at almost no current. 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.

* With a constant voltage power supply, a series ballast resistor is essential to limit tube current to the proper value. A ballast resistor will still be required with a constant current or current limited supply to stabilize operation. The ballast resistor may be included as part of a laser head but will be external for a bare tube.

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!

* Every HeNe tube will have a nominal current rating. In addition to excessive heating and damage to the electrodes, current beyond this value does not increase laser beam intensity. In fact, optical output actually decreases (probably because too high a percentage of the helium/neon atoms/ions are in the excited state). You can easily and safely demonstrate this behavior if your power supply has a current adjustment or you run an unregulated supply using a Variac. While the brightness of the discharge inside the tube will increase with increasing current, the actual intensity of the laser beam will max out and then eventually decrease with increasing current. (This is also an easy way of determining optimal tube current if you have not data on the tube - adjust the ballast resistor or power supply for maximum optical output and set it so that the current is at the lower end of the range over which the beam intensity is approximately constant.)

* These may be 'bare' tubes or encased in a cylindrical or rectangular laser head - or something in between.

- Bare tubes require clip-on connections to the power supply or high voltage connector and an external ballast resistor.

Advantages: Less expensive, discharge is fully visible resulting in an interesting display.

Disadvantages: Fragile, exposed high voltage terminals, need to provide your own mounting, wiring, and ballast resistor.

- Laser heads will usually come with an internal ballast resistor (though you may still need additional resistance to match the tube to your power supply). The high voltage cable will likely use an 'Alden' connector. The Alden connector 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 HeNe laser power supplies (the longer fatter pin is negative).

Advantages: High voltage safely insulated, wiring is already done for you, relatively robust, easily mounted.

Disadvantages: More expensive, discharge not readily visible, repairs to wiring (unlikely to be needed) difficult.

* The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings. 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:

- Most better HeNe tubes (possibly all tubes manufactured in the last 10 years) are 'hard sealed' - the mirrors are fused to their respective electrodes by a low temperature glass 'frit' - sort of like solder for glass! These do not leak - at least not on any time scale that matters. Shelf life is essentially infinite.

- Older tubes have their mirrors just glued - Epoxied to the end electrodes. This adhesive leaks and such tubes have a shelf life of a few years - they fail by just sitting around doing nothing. This means that a bargain tube may not be such a bargain if it is beyond its expiration date (yes, just like dates on milk containers) as it may have a very limited life, be hard to start, weak or erratic, or may not work at all. You probably won't see any of these - at least not in a working condition.

* The efficiency of the typical HeNe laser is pretty pathetic. For example, a 2 mW HeNe tube powered by 1,400 V at 6 mA has an efficiency of less than .025 %. More than 99.975 percent of the power is wasted in the form of heat and incoherent light (from the discharge)! This doesn't even include the losses of the power supply and ballast resistor.

* The most common HeNe lasers by far produce light at a wavelength of 632.8 nm in the red-orange part of the visible spectrum - well into the region of the human eye's high sensitivity (but not as good as green). Thus, a 1 mW HeNe laser will appear brighter than a 4 mW laser diode operating at 670 nm.

* Green, yellow, and orange HeNe lasers are also available but are not nearly as 'efficient' as the common red-orange type. Thus, they are also much more costly for the same power since the spectral lines that need to be amplified are weaker at these wavelengths and therefore, the tubes must be larger.

Typical maximum output available from 'small' sealed tubes for various colors: green - 2.0 mW, yellow - 7.0 mW, orange - 7.0 mW, and red - 15 mW.

IR (infra-red) HeNe laser tubes are also available. However, an invisible beam just doesn't seem as exciting!

* The width of the beam as it emerges from the tube is typically between .5 mm and 1 mm - the inside bore diameter of the capillary discharge tube.

* The beam from a HeNe laser is already very well collimated even without external optics (unlike a laser diode which has a raw divergence measured in 10s of degrees). The divergence measured in milliradians (1 mR is less than 1/17th of a degree) is usually one of the tube specifications. A small HeNe tube may have a divergence of 1 to 2 mR. Divergence is affected mostly by beam (exit or waist) diameter (wider is better). Other factors like the ratio of length to bore diameter (narrower is better) may also affect this slightly. The equation for a plane wave source is:

So, for an ideal HeNe laser with a 1 mm bore at 632.8 nm, the divergence angle will be about .806 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 laser".

Typical HeNe tube characteristics

(Portions from: Steve Nosko (q10706@email.mot.com)).

The following are typical of small (bare) red-orange (632.8 nm) HeNe tubes:

Melles Griot, Uniphase, Siemens, PMS, and Aerotech all show similar values.

Note that for a given optical power level, there can be substantial variation in the tube size. Typically, longer tubes will require higher start and operating voltages. And no, you cannot get a 3 mW tube to output 30 mW - even instantaneously - by driving it 10 times as hard!

HeNe tubes of other colors exist but are probably rare on the surplus market. They are not that common to begin with and are more expensive when new since for a given power level, the tubes must be larger and thus have higher voltage and current ratings due to their lower efficiency (the spectral lines being amplified are much weaker than the one at 632.8 nm).

There are infrared HeNe tubes as well. Yes, you can have a HeNe and it will light up inside (typical neon orange 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 bu using one of the IR detector circuits or an IR detector card as discussed with respect to IR laser diodes.

As a side note... It is strange to see the 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.

How can I tell if my tube is good?

A variety of problems can prevent a HeNe tube from lasing properly or make it hard to start.

* Physical damage. Obviously if the tube arrived in pieces, this is a shipping, not a technical problem :-).

* Misaligned mirrors. Using the tube as a hammer might bend the mirror mount at one end or the other. There are ways of determining and adjusting this alignment but they require some optical components and special jigs. Without these, adjustments are hit or miss (mostly miss) and will more likely result in a broken seal than anything else :-(. Problems with mirror alignemnt are very unlikely to occur with these tubes unless you were working hard at it!

* Loss of gas fill. This may result in a total inability to start or sustain the discharge. There is usually a metal sealing nipple at one end. This might be damaged.

* Incorrect gas fill. There may be a glow but the laser output will likely be weak or non-existent. The normal color of the discharge is whitish red-orange - a somewhat unsaturated version of the red-orange glow of a (true) neon sign.

- Loss of helium (from diffusion through the glass or seals) will result in the glow becoming deeper red-orange and less white. There will be little or no emission at the wavelength of helium's spectral lines. It probably isn't worth the effort to refill but see the section: "Recharging HeNe tubes".

- A leak which has allowed some air to enter (but where it is not totally up to atmostpheric pressure) will result in a glow with a white or pink color. Depending on the actual pressure, the intensity will vary as well.

If you can sustain a discharge but it is the wrong color (weak or white/pink color), you may have one of those really old Epoxy sealed tubes that leak and air has leaked in. Again, probably not worth repairing.

* Damaged electrodes or mirrors due to running with the power supply polarity reversed or greatly excessive current for a prolonged period of time. I don't know exactly what the physical effects might be but I would suspect metal sputtering from the negative electrode coating the mirror at that end of the tube. Buy another tube.

Improving the collimation of a HeNe laser

The following applies to any laser which outputs a substantially parallel beam but is written specifically for HeNe lasers. Collimation of laser diodes require a slightly different approach.

Although the divergence of a HeNe laser is already pretty good without any additional optics, the rather narrow beam as it exits from the tube (typically 1 mm) results in a beam with a typical divergence between 1 to 2.5 mR (order of 1 mm per meter or worse) if no other optics are used.

As noted in the section: "HeNe laser operation", beam divergence is inversely proportional to the beam diameter. Thus, it can be reduced even further by passing the beam through a pair of positive lenses - one to focus the beam to a point and the second to collimate the expanded beam.

A small telescope can be used to collimate a laser beam and will be easier to deal with than individual lenses. (This is how laser beams are bounced off the moon but the telescopes aren't so small.) Using a telescope is by far the easiest approach in terms of mounting - you only need to worry about position and alignment of two components - the laser tube and telescope.

If you want to use discrete optics:

* The focusing lens should have a short focal length (F1) such as a microscope or telescope eyepiece (e.g., F1 of 10 mm) or low power microscope objective (e.g., 10X). Note: the objective lens from a dead CD player has an ideal focal length - about 4 mm - but is aspheric and would probably not be that great but give it a try!

This will focus the laser beam to a (diffraction limited) point F1 in front of the lens from which it will then diverge.

* The collimating lens should be a large diameter medium focal length (e.g., 15 mm D2, 100 mm F2) lens placed F2 from the focus of the first lens.

For optimal results, the ratio of collimating lens diameter to focal length (D2/F2) should greater than or equal the ratio of HeNe beam diameter to focusing lens focal length (D1/F1). This will ensure that all the light is captured by the collimating lens.

The beam will be wider initially but will retain its diameter over much longer distances. For the example, above, the exit beam diameter will be about 10 mm resulting in nearly a 10 fold reduction in divergence.

Adjust the lens spacing to obtain best collimation. A resulting divergence of less than 1 mm per 10 meters or more should be possible with decent quality lenses - not old Coke bottle bottoms or plastic eyeglasses that have been used for skate boards :-).

HeNe laser tube life

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

Lifespans of HeNe laser are long - 20,000 or more operating hours are typical. 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.

The gas doesn't 'wear out'. However, excessive or reverse current can degrade performance after a while. Electrode material may sputter onto the adjacent mirrors or excessive heat dissipation may damage the electrodes themselves.

A HeNe tube, when properly connected has much of its heat dissipated by the bombardment of positive ions at the cathode (the big can electrode) which is made large to spread the effect and keep the temperature down. Hook a tube up backwards and you may damage it in short order!

Recharging HeNe tubes

"I have two large tube green HeNes and and a 1 micron IR HeNe that are dead from obvious low helium pressure (spectrum from grating shows only weak He lines) has anyone had any success with putting tubes in a pressure chamber filled with Helium so it diffuses the other way?"

HeNe tubes which do not laser well or at all due to loss of helium can sometimes be rejuvenated by soaking them in helium at normal atmospheric pressure for a few days or weeks.

The point to realize is that it is the partial pressure of each gas that matters. Neon is a relatively large atom and does not diffuse through the tube at any rate that matters. However, helium is able to diffuse even when the pressure difference is small.

Even for a HeNe tube at 2 Torr, the partial pressure of helium is much greater than its partial pressure in the normal atmosphere. So, helium leaks out even though the total pressure outside is several hundred times greater. Conversely, soaking a HeNe tube in helium at 1 atmosphere will allow helium to diffuse into the tube at several hundred times the rate at which it had been leaking out. Thus, only a few days of this treatment may be needed if the problem is low helium pressure.

This hardly seems worthwhile for a $25 1 mW HeNe tube but it is something to keep in mind for other more substantial types.

(From: Mark W. Lund (lundm@xray.byu.edu)).

I have rejuvenated HeNes with low Helium pressure. Since the partial pressure of 1 atmosphere helium is much higher than inside the tube you don't really need to use high pressure, or even increased temperature. I just put them in a garbage bag and blasted some helium into it from time to time. The length of time necessary in my case was a few days, but depending on the glass type, thickness, and sealing method this may vary. It would be good to test the power every couple of days so you don't overshoot too much.

One warning, helium has a lower dielectric strength than air, so don't try to operate the laser in helium, it may arc over.