This is perhaps the easiest laser to build and an excellent choice for a first laser in which valuable experience will be gained at working with lasers and high voltages. Although it is possible to build a tube 'from scratch' in the manner of Scientific American, Amateur Scientist, September 1964 this is expensive and required elaborate equipment (e.g. a turbomolecular or diffusion-pump based vacuum system including research-grade gases - See the PHTN1432 He-Ne reprocessing lab for details). In most cases the easiest approach is to purchase a commercially-built tube and power supply, available readily on the surplus markets. Once built, the laser serves well as an alignment tool for other laser projects.
Commercially-available HeNe lasers are often packaged in a cylindrical housing with a separate power supply as seen
here. The laser head itself contains the laser tube (with integral mirrors) as well as a ballast resistance and
a shutter which is required for class III lasers.
Aside from the common red (632.8nm) wavelength, HeNe lasers are also available at orange, yellow, and green wavelengths however these tubes are more expensive and in considerably shorter supply - they are a real find on the surplus market. To tell if a HeNe tube is red or a different colour simply look through the bore of the (unpowered) tube at a light (the mirrors are dielectric and so while they reflect well at one wavelength, they are quite transparent at other wavelengths). In the case of a red tube, the mirrors transmit blue light. If, when looking through the tube, you see violet or another colour, it is a green or yellow tube (if it's priced at $25, it's a steal!).
Most amateurs and professional alike will purchase a tube like that shown to the left.
The laser itself is a glass tube with integral mirrors - no user alignment is required.
For powering the laser tube you can purchase pre-built power supply modules (usually
a small block of epoxy encapsulating the entire supply with a high-voltage wire
coming out of it and several input wires). Such power supplies can be purchased on surplus
markets often for less cost than than the cost of the individual components.
Prices for a 1-5 mW red (632.8nm) tube are generally in the $25 range with a suitable supply
about the same price.
Aside from purchasing a pre-built, encapsulated power supply, a power supply may also be constructed which provides the required 1500 Volts at 2-5 mA to power the tube. Generally, this is relatively easy for one experienced at building electronic projects and an excellent first laser project. Note that an external ballast resistance is required with all tubes. This is usually in the range of 75K to 100K and is in series with the tube. This resistance is required since gas discharges usually exhibit a characteristic known as 'negative resistance'. This is NOT to imply the tube exhibits "negative ohms" but simply that it does not behave like an ohmic resistance in which voltage across the resistance rises as current does. In a gas discharge, an increase in current results in _decreased_ voltage ... effectively the electrical resistance of the tube drops as current increases. This will create instabilities in the discharge unless a positive resistance is placed in series with the discharge tube to overcome this - this is the purpose of the ballast resistance.
Put the ballast resistance as close as possible to the HeNe tube - in a packaged cylindrical tube this resistance is always housed in the laser head itself, not the power supply. Placing the resistor too far away from the anode of the tube results in stray capacitances which usually lead to instability in the discharge as well.
The completed laser must be packaged for both electrical and mechanical protection. I recommend packaging the tube in a housing made from 1.5" diameter copper plumbing pipe as I had done with this green HeNe tube which we now use in our photonics lab. This pipe is tough like nails and conducts well (obviously the outside of the housing is connected to ground). End caps may easily be fashioned from aluminum if you have access to a metal lathe. High voltage from the power supply is fed to the laser via 1.5m of RG-58/U coax cable in which the outer braid connects to the tube housing (this is ground) and the high-voltage lead from the power supply to the inner conductor. This assumes, of course, that the OC for the tube is the cathode and hence is mounted towards the front of the housing (this is true for most, but not all, tubes). For high voltage connectors an 'SHV' type connector, which resembles an extended BNC-type connector, may be used. These are available from large electronic supply houses. Ballast resistors are encapsulated inside the copper tube as well as they must be kept close to the anode of the tube - increased capcitance at the anode results in instabilities in the discharge. In the case of this green HeNe tube, the ballast resistor is 75K - made from two 150K, 5 Watt resistors in parallel. This makes a safe housing which may be mounted on an optical bench as well.
The power supply can be as simple as a high-voltage transformer (10kV) with a diode, caps, and a large series ballast resistor as follows:
This is about as simple a supply as it gets - although it is quite inefficient. The transformer is an old Allanson ignition transformer from an oil furnace. It is rated at 10kV with a center-tap at ground (the case). Rectifying the output using two ceramic TV-type diodes produces 5.0*sqr(2) = 7.07kV across the capacitors. Twelve caps were used in parallel total, each rated at 0.05uF for a total capacitance of 0.6uF. This is somewhat undersized and leads to a high AC ripple across the tube - optimally the capacity should be much higher.
Passing through the ballast resistance the tube ignites on the 7kV (open-circuit) potential and begins conducting. When the tube has started, the voltage across it drops to about about 1200V which is a problem now since the ballast resistance has 5.8kV across it and a current of 5mA through it. First off, the resistance must have a value of 5800/.005 = 1.16 Mega-ohms. As well it must dissipate 5800*.005 = 29 Watts! A single resistor of this size is not commercially available so the resistance is composed of many smaller 5W resistors in series-parallel arrangement. 250K and 470K power resistors were used to make-up this resistance. Considering the resistor dissipates almost five times the power that the laser tube does this is (electrically speaking) a horrendously wasteful power supply - but it does work. Some HeNe power supplies from the 1960's worked in this same manner.
All modern supplies use a lower-voltage transformer, usually 300 to 600 V, and a voltage doubler or quadrupler circuit to generate the required 1500V to run the tube. To ionize the gas and ignite the tube a much higher voltage pulse is required. It is supplied by a series of small diodes and capacitors which serve to multiply the transformer's voltage to the required level. This start circuit 'kicks' the tube to ignite it and then ceases to run when the tube has lit. Because the main supply has an output of only 1500V, the ballast resistor has only a few hundred volts across it. A more practical resistance of about 60 to 80K are required (300/.005) and the power dissipated in the ballast resistance is a meager 1.5 Watts (300 * .005). It is easy to see why the previous quick-and-dirty design is hardly optimal (See the link on the sidebar for a description of various HeNe power supplies). The encapsulated 'block' power supplies work in a similar manner although these usually employ switching power supply technology operating at high frequencies (> 40KHz) allowing physically small transformers and capacitors to be used.
When a HeNe tube is operating properly a red (or other colour) beam appears. As well, a blue (for a red HeNe) or violet (for a green HeNe) diffuse output appears through the mirrors on the ends of the tube. This is not laser output but rather spontaneous emission from the discharge itself. The mirrors on the tube ends are dielelectric and reflect extremely well at their designed wavelength however they are somewhat transparent to other wavelengths. In the case of a red HeNe tube, the mirrors reflect red light (at 633nm) very well however they pass blue light and so blue light from the discharge itself is emitted from the tube ends. When a HeNe tube 'dies', this blue emission is all that will be seen - a laser beam will not appear.
On Reprocessing Old HeNe Tubes
HeNe lasers may be repumped but do not expect a repumped tube to have a long lifetime. The setup and use of high vacuum systems is a science in itself however a basic system can be used to repump a tube for experimental purposes.
I have personally used a single-stage vacuum pump with two lecture bottles of HP (high purity) grade Helium and Neon gases to reprocess an old HeNe tube. These bottles are sold outright and are available from most scientific supply houses or by special order from most welding gas suppliers. Expect a lecture bottle to cost around $100. The vacuum system (built many years ago) consisted of a single-stage pump, a Bourdon gauge, several isolation valves, and two needle valves to regulate the flow from the bottles (which are under considerable pressure).
In order to reprocess a tube the tube is first opened and a vacuum fitting attached. If the tube is a newer Melles Griot or Uniphase type this connection may be made to the gas port on the tube which protrudes 5-6mm on the end cap (slip a piece of 1/4" OD copper tube on top of the opened port and use silver solder to make the connection). For an older tube, such as the Metrologic tube below (a soft sealed tube), a new copper filling tube must be epoxied to an opening in the tube. Using a file I opened the old glass filling stem and epoxied 1/4" copper tube onto this. I now question whether epoxy was the best choice as I am unsure of the vapour pressure of the epoxy used. There are high vacuum epoxies such as 'Torr Seal' which are made for such use but these are considerably more expensive than the hardware-store variety. I have used regular (24 hour cure) epoxy and it worked although outgassing of this material could have contributed to it's short lifetime when sealed.
The tube must first be purged of impurities - easier said than done considering that the single-stage pump employed could likely only attain an ultimate vacuum level of 0.10 torr. To clean the tube of impurities I backfilled to 100 torr with pure helium, pumped the tube down to 1-2 torr, and ran the tube (from the regular HeNe power supply) for a few minutes to heat it which drives adhering gases off the elements of the tube. The tube is then opened to the vacuum pump and the contaminated helium pumped out. This cycle is repeated many times (five or six) until the pink discharge remains pink after a minute of discharge - shades of blue or violet in the discharge indicate the presence of nitrogen or oxygen. Oxygen is liberated from tube elements such as the aluminum cathode as it heats and its presence indicates that further helium flushing cycles are required however nitrogen may indicate the presence of an outright leak. Liberal application of melted vacuum wax to suspect connections usually solves this problem - melt the wax in a tin can and apply it using a small brush. If a better vacuum pump, such as a dual-stage pump capable of an ultimate vacuum of 1*10-3 torr is employed, one might determine that leaks are plugged to a sufficient level when the entire tube is evacuated to the point where all discharge stops (i.e. the system is capable of pressures of under 2*10-3 torr at which point discharge is not possible) and stays dark when sealed. If such a tube were sealed (using the valve on the tube stem) and the discharge started again, one might well assume a leak (or a large quantity of outgassing) ... without an RGA (mass spectrometer) one might use a spectroscope to view the discharge to determine the source of the new gas (either oxygen or nitrogen). In my case (using a single-stage pump), the pump was incapable of evacuating the tube to the point of extinguishing discharge so vacum leak testing had to be done "the hard way".
After the tube has been sufficiently cleaned (and leaks in the system, even small ones, patched) the tube is ready for filling with the helium-neon gas mixture. I began by overfilling the tube with 25 torr of neon and 250 torr of helium (10:1 ratio for He:Ne) since these pressures could be accurately read on the gauge (pressures below 10 torr were impossible to read accurately on the mechanical gauge). I then opened the valve to the vacuum pump until the pressure in the tube was reduced enough (lasing starts below 4 torr and optimal pressure is about 1.8 torr) and the laser produces an output beam. The tube may then be sealed via the valve on the tube. Using the rather crude vacuum system described I had been able to reprocess that Metrologic tube however it runs sealed for only a few minutes before lasing ceases ... likely due to impurities collecting in the tube (everything from oxides from the electrodes to outgassing from the epoxy. Water vapour is particularly toxic to a HeNe laser). Do not expect a tube reprocessed in this manner to last for a long time, but it might be useful for experimentation.
If a better vacuum system is employed, one which includes a diffusion or better-yet a turbomolecular pump to allow high vacuum (better than 10-6 torr) to be obtained, the tube will last much longer. The pump-down sequence is similar to that described however after each 'cleaning' cycle the tube is pumped-down as far as possible to remove as much contaminated residual gas as possible. I have such a system available for a thin-film and vacuum technology course I teach Seen Here. The system includes a mechanical roughing pump, turbomolecular pump, and a host of valving including a gas manifold. Gas pressures are measured using dual Baratron capacitance manometers (extremey accurate in a mixed-gas environment) and for analysis of the remaining gas in the tube a residual gas analyzer (RGA) is attached to the system. By all means this is a 'professional' system valued at over $30,000 and so is quite unobtainable for the average amateur laser experimenter.
I have attached a Melles-Griot laser tube to the system by soldering a copper stem to the tube as in the photo below. The tube does indeed lase when repumped with power levels of over 2mW (for a tube rated originally at 5mW) achieved. One would not expect to obtain maximum power since a mixture of natural neon (containing 9.6% of 22Ne and the rest 20Ne isotopes) was employed - high performance commercial tubes are filled with isotopically-pure neon gas. Monitored with the RGA, the tube was flushed until ultimate purity was achieved: multiple flushes with research grade helium were performed until the appearance of water vapour and hydrogen contamination in the tube were minimal. The tube was then filled to overpressure (10 torr) with a mixture of helium and neon and pressure reduced to the optimum value somewhere between 2 and 4 torr. Pressure is read on a Baratron gauge which reads gas pressures independent of the actual gas involved (many vacuum gauges like thermocouple gauges must be compensated for the specific gas involved).
With proper flushing (which takes several hours) performance is good as is lifetime. I have had one tube which, when isolated from the vacuum system (using the valve evident in the photo), operated for over an hour with a power decrease of only 5% evident! The next day the tube ran fine, again with power approaching that of the previous day. And yes, the valve held vacuum all that time (it was a Nupro valve designed for high-vacuum work) - cheaper valves would likely have leaked a little overnight causing the laser to fail by morning.
Photos of a HeNe tube modified for reprocessing. The stainless steel filling-stem was opened using a rotary tool and a 1/4" copper stem was soldered to the tube using silver solder (normally used for jewelery). The stem then attaches to an isolation valve via a compression fitting. From there, it is connected to a vacuum system via flexible vacuum line.
Types Of TubesHeNe tubes have evolved over the past 20 years. In the early 80's the soft-sealed tube, in which mirrors were attached to the tube by epoxy, was popular. These tubes did not have particularly long lifetimes - usually only a few years of shelf life. Light helium atoms would eventually diffuse through the relatively porous seals changing the gas mix until the tube no longer lases. These tubes thrived on use ... the best was to prolong their life was to run them often. It was possible, occasionally, to re-infuse helium into the tube by placing the tube into a chamber filled with helium at high pressure (about 30 psi). After a week or so enough helium would often infuse into the tube to allow it to operate again. Once the tube is too far gone, though, this will not work and opening the tube for reprocessing is the only option.
The tube in the above section is a Metrologic tube, popular in the late 70's/early 80's. Another,
smaller Metrologic tube is pictured below:
Other Gases In a HeNe Tube ?
This can indeed be done. See the Neon Lasers Page on this site for details in which pure neon is used to yield an output at 614.3nm. A pure-neon laser uses different lasing transitions than the helium-neon laser and hence different wavelengths result. Electrical pumping is also performed using a radically different approach that that described here for the HeNe laser.