All About Helium-Neon (He-Ne) Lasers

The helium-neon tube is the staple of the laser experimenter. He-Ne tubes are in plentiful supply, including in the surplus market. They emit a bright, deep red glow that can be seen for miles around. Although the power output of He-Ne tubes is relatively small compared to other laser systems—such as CO2 argon, and ruby—the helium-neon la ser is perfectly suited for most any laser experiment. Its moderate power supply requirements coupled with its slim, coherent beam lend the He-Ne laser to inexpensive projects in holography, interferometry, surveying, lightwave communications, and much more.

In this section you’ll learn all about helium-neon lasers: what they are, how they work, and what you need to put a complete system together. In Section 6 you’ll learn how to place a bare He-Ne laser tube in an enclosure to make it easier to use, along with plans on building a He-Ne laser experimenter’s system. With the experimenter’s system, you’ll be able to perform numerous optical experiments with your helium-neon laser.

ANATOMY OF A HE-NE LASER TUBE

The helium-neon laser is a glass vessel filled with 10 parts helium with 1 part neon and is pressurized to about 1 mm/Hg (exact gas pressure and ratios vary from one laser manufacturer to another). Electrodes placed at the ends of the tube provide a means to electrify (ionize) the gas, thereby exciting the helium and neon atoms. Mirrors mounted at either end form an optical resonator. In most He-Ne tubes, one mirror is totally reflective and the other is partially reflective. The partially reflective mirror is the output of the tube.

The first helium-neon tubes were large and ungainly and required external cooling by water or forced air. The modern He-Ne tubes, such as the one in ill. 5-1, are about the size of a cucumber and are cooled by the surrounding air. The length and diameter of He-Ne tubes varies with their power output, as detailed later in this section.

Most helium-neon laser tubes are composed of few parts, all fused together during manufacture. Only the very old He-Ne tubes, or those used for special laboratory experiments, use external mirrors. The all-in-one design of the typical He-Ne tube means they cost less to manufacture and the mirrors are not as prone to misalignment.

Helium-neon lasers are actually composed of two tubes: an outer vacuum (or plasma) tube that contains the gas, and a shorter and smaller inner bore or capillary, where the lasing action takes place. The bore is attached to only one end of the tube. The loose end is the output and faces the partially reflective mirror. The bore is held concentric by a metal element called the spider. The inner diameter of the bore largely determines the diameter width of the beam, which is usually 0.6 mm to 1 mm.

ill. 5-1. A bare helium-neon laser tube. This one measures about 1¼ inches in diameter by 12 inches in length.

The ends, where the mirrors are mounted, typically serve as the anode (positive) and cathode (negative) terminals. On other lasers, the terminals are mounted on the same end of the tube. A strip of metal or wire extends the cathode (sometimes the anode) to the other end.

Metal rings with hex screws are often placed on the mirror mounts as a means to tweak the alignment of the mirrors. Unless you suspect the mirrors are out of alignment, you shouldn't attempt to adjust the rings. They have been adjusted at the factory for maximum beam output, and tweaking them unnecessarily can seriously degrade the performance of the laser.

The partially silvered mirror, where the laser beam comes out, can be on either the anode or cathode end. I found that on the many tubes I’ve tested, the beam extends out the cathode end. Many manufacturers prefer this arrangement, claiming it's safer and provides more flexibility. You can usually tell the output mirror by holding it against a light. You should see the blue tint of the anti-reflective coating. The totally reflective mirror generally isn't treated with an AR coating. A cutaway view of a laser tube with all the various components is shown in ill. 5-2.

ill. 5-2. A cross-sectional view of a typical He-Ne laser tube, with component parts indicated. The arrangement and style of your laser tube might be slightly different. Mirror wedge (On anode terminal)/Bi-plano fully-reflective mirror; Output beam

The facing mirrors of the He-Ne tube comprise what is commonly referred to as a Fab interferometer or resonator. With the mirrors aligned plane parallel to one another, as shown in ill. 5-3, the light bounces back and forth until the beam achieves sufficient power to pass through the partially reflected output mirror. In practice, the plane parallel resonator is seldom if ever used, because it's unstable and suffers from large losses due to diffraction.

ill. 5-3. Three ways to implement the optical cavity in a laser—plane parallel, confocal, and hemispherical. Most laser tubes use the latter method or a derivative of it.

A confocal resonator uses two concave spherical mirrors of equal radius, each placed at the center of curvature of the other. The cavity uses a large portion of the gas volume and produces high power, but mirror adjustment is relatively critical. Yet another approach is the hemispherical resonator, which is primarily a plane mirror coupled with a spherical mirror. This type of resonator is very stable and easy to align, but its design wastes plasma volume, so power output of the laser is reduced.

The exact configuration of the resonator mirrors in a laser isn't a major consideration to the hobbyist experimenter. You’ll have little choice of the engineering of the tube when buying parts through surplus outlets. However, you might have a choice if you buy your tubes new or if you purchase a particular type of tube for a special application.

LASER VARIETIES

He-Ne lasers are available in three forms: bare, cylindrical head, or self-contained. Bare tubes are just that—the plasma tube isn't shielded by any type of housing and should be placed inside a tube or box for protection. Cylindrical head lasers (or just “laser heads”) are housed inside an aluminum tube. Leads for power come out one end of the laser. The opposite end might have a hole for the exiting beam or be equipped with a safety shutter. The shutter prevents accidental exposure to the beam. Both the bare tube and cylindrical head laser require an external high-voltage power supply (discussed below).

Lastly, self-contained (or lab) lasers contain both a laser tube and a high-voltage power supply. To use the laser, you simply plug it into a wall socket and turn on the switch.

Each form of laser has its own advantages and disadvantages:

* Bare tubes are ideal for making your own self-contained laser projects, such as laser pistols and rifle scopes, and fit in confined spaces. But because the tube and high- voltage terminals are exposed, they are more dangerous to work with. You must exercise considerable caution when working with bare tubes to avoid electrocution and injury from broken glass.

* Cylindrical head lasers are easy to use because the tube is protected and the high-voltage terminals are not exposed (but care must still be exercised to avoid shock from power applied to the leads). Laser heads are ideal for optical benches and holography (with the right type of mount). On the down side, the tubes tend to be large and are not easily mounted for use in hand-held devices.

* Self-contained lasers are designed to provide protection against tube breakage and high-voltage electrocution. They are often used in schools and labs where they can be easily set up for optical experiments. The built-in power supply operates from a wall socket, however, so the laser cannot be used where 117 volt ac current isn't available. Also, the bulk of the self-contained laser prohibits it from being used in hand-held devices.

THE POWER SUPPLY

Owning a plasma tube doesn’t mean you have a laser. The tube is only half the story; just as important is the power supply. You also need electricity for the power supply, either directly from a 117-volt ac wall socket or a 12-volt dc battery. Power supplies for lasers generate a great deal of volts but relatively few milliamps. The typical power supply generates from 1,200 to 3,000 volts at 3.5 to 7 mA. Generally, the larger the tube (and the higher the power output), the more juice it requires.

You have a number of choices for the power supply:

* Commercially-made laser supply, either ac or dc operated. These are the e to use and often come completely sealed as a precautionary measure. A common type of ac-operated commercially made He-Ne power supply is shown in ill. 5-4. Cost on the retail market is about $225; surplus is between $50 and $100. A high-voltage Alden connector (see end of section) is common on cylindrical head lasers. The female connector on the power supply matches with the male connector on the laser.

* Home-built power supply kit. Low-current power supplies can be built in your shop but require some specialized, hard-to-find parts. Building your own supply saves money and helps teach you about laser power requirements. Cost: $10 to $25 for parts; pre-packaged kits of parts and circuit board (available from some sources listed in Section A) cost $30 to $150.

* Salvaged high-voltage TV flyback transformer or module power supply. Many compact TVs use a self-contained flyback transformer that operates from 12 to 24 Vdc. The transformer, intended as the high-voltage source for the picture tube, generates up to 15 kV at a few milliamps. The modules are cheap ($10 to $20 on the surplus market) but their low current output makes them suitable only for small tubes.

ill. 5-4. The latest commercially made He-Ne power supplies are available in convenient, sealed packages, such as the one shown in the photograph. The power supply is about the size of a sandwich.

If you are just starting out with lasers, your first power supply should be a commercially made and tested unit, preferably new and not a take-out from existing equipment. Armed with a tube and ready-to-go supply, you’ll be able to start experimenting with the laser the moment you get it home. Then as you gain experience, you might want to build one or two supplies as extras or to power the various tubes you are bound to acquire. Section 12 presents several power supply circuits you can use to power tubes with outputs from 1 to about 5 mW.

Inside a Power Supply

Although the exact design of He-Ne power supplies varies, the principle is generally the same. He-Ne power supplies are familiar to anyone who has dabbled in high-voltage circuits, such as those required for amateur radio.

Here’s how a typical power supply works. An input voltage, say 12 Vdc, is applied to an oscillator circuit, such as a simple transistor, resistor, and capacitor. The values of the resistor and capacitor determine the time constant of the oscillator circuit. The oscillating input is fed to the windings of a step-up transformer. The voltage is stepped up to somewhere between 300 to 1,000 volts by the transformer and then passed through a series of high-voltage diodes and capacitors. These components act to rectify and multiply the voltage presented by the transformer. Depending on the number of capacitors and diodes used, the voltage multiplier doubles, triples, or quadruples the potential.

Power supplies that operate from 117 VAC usually don’t need the front-end oscillator circuit since the juice is already in the alternating current format required by the transformer (most transformers can’t pass dc current). The transformer steps up the voltage to anywhere between 300 and about 2,000 volts. If the output voltage is high enough, voltage multiplication by means of diodes and capacitors isn't required, but the ac component is removed by one or more diodes.

Some of the more advanced power supply circuits use a separate 6 to 10 kV trigger transformer to initially start or “ignite” the laser. The trigger transformer, similar to the kind used in photoflash equipment, fires only when the tube first turns on (but might continue firing if the tube doesn’t ignite). A circuit in the power supply senses when the laser starts to draw current (indicating that it has started), and shuts the trigger transformer off. A silicon-controlled rectifier (SCR) serves as the switch to turn the trigger transformer on and off.

High-voltage triggering isn't required for all tubes, especially those under 2 to 3 mW, but it's usually needed with higher output types. If your tube is hard to start— either refusing to ignite at all or just flickering—you may need to use a supply that has a high voltage trigger or at least one that supplies extra current. Most all commercially- made supplies have a built-in trigger transformer (or something equivalent) but home- brew supply circuits generally don't.

Using a high-powered laser with a power supply that can’t deliver the required current may actually damage the supply. TABLE 5-1 lists the average voltage and current requirements for a variety of tubes. The tubes are rated by their output only so the chart should be used only for estimating power requirements. Many other factors, such as polarization of the beam and operating mode, can affect the power output and change the voltage and current requirements. Obtain descriptive literature from the seller of the laser tube if you need more precise information.

Note that most He-Ne tubes can be safely operated over a range of currents. Manufacturer’s specifications usually list the recommended operating current for optimum performance. You can often safely increase or decrease the current slightly, for example, to produce a more powerful beam or to conserve battery power. Unless the specifications state otherwise, you shouldn't exceed 7-8 mA operating current.

A Warning about High Voltage Power Supplies

You’ve undoubtedly read this before in this guide, but the warning can’t be stressed enough: beware of high voltage power supplies. Never touch the output terminals of the supply when it's on or you may receive a bad shock. Turn the supply off and unplug it before working with the laser.

The capacitors in the voltage multiplier section of the supply can retain current even when the system is turned off. Before touching the tube or power supply, short the anode and cathode terminals together. If you can’t easily bring them together, keep a heavy-duty alligator-clip test cable handy. String it across the anode and cathode and short the leads.

The laser tube itself can also retain current after power has been removed. Always short out the anode and cathode terminals of the laser before handling the tube. Cylindrical head lasers with a male Alden connector can be discharged by touching the prongs of the connector against the metal body of the laser.

Table 5-1. Voltage/Current Levels for Typical He-Ne Tubes

POWER OUTPUT

The greatest difference among helium-neon laser tubes is power output. There are some He-Ne tubes designed to put out as little as 0.5 mW of power, while others generate 10 mW or more of light energy. The difference in power output isn't always visible to your eye because the spot made by a laser beam is brighter than your eye can register.

By far, the majority of helium-neon tubes are rated at 1-2 mW. This is adequate for most laser experiments and you rarely need more. In fact, lower power lasers are often easier to work with because the power supply requirements are not as stringent. You can get by with a smaller, lighter-weight power supply with a 1-2 mW tube. The higher power comes in handy, however, if you are engaged in holography (the more power the faster the exposure), outdoor surveying, and other applications where a bright beam is necessary.

Cylindrical head lasers, generally designed for use in telecopiers, laser facsimile machines, and supermarket bar code scanners, are generally engineered for high output. Most tubes in this class are rated at 5-8 mW, though a few—such as those made for laser facsimile devices—generate in excess of 10 mW. If you need lots of power check these out. Just be sure that the power supply delivers sufficient current and voltage to the tube.

Bear in mind that power output varies with tube age. A tube that originally produces two milliwatts when new may only generate 1.5 mW after several thousand hours of use. Careless handling also reduces the power output. Every shock or jolt may tweak the mirrors out of alignment, which reduces the power output. If the laser is abused, as it often is in industrial or commercial applications, the mirrors may become so out of whack that the tube no longer generates a beam.

The loss of output power is important to remember when buying used or surplus tubes. Even though the tube may have been rated at 2 mW, there is no guarantee that it’s still providing that much power. You can readily measure the power output of a laser if you use a calibrated power meter, such as the Metrologic 45-450.

PHYSICAL SIZE

Helium-neon laser tubes come in a variety of sizes, depending on power output. Most are about 1 to 1 1/4 inches in diameter by 5-10 inches long. Some very small tubes are designed for use in hand-held bar code readers and generate less than 0.5 mW. Such “pee-wee” tubes are available from a few surplus sources and are fun to play with, but they are extremely fragile. They can be permanently damaged by even a moderate jolt.

Few He-Ne lasers put out more than 10 to 15 milliwatts. These tend to be the largest are usually enclosed in a cylindrical head. The tube may measure about 1.5 inches in diameter and 13 inches long; the entire enclosure is 1.75 inches in diameter by 15 inches in length. A typical aluminum-housed laser head is shown in ill. 5-5.

Self-contained lab lasers can be most any size depending on the power output. Average size is approximately 24 by 4 by 3 inches (LWH).

ill. 5-5. A typical cylindrical laser head, removed from a Xerox laser printer. Laser heads such as these are common finds in the surplus market.

ill. 5-6. Only the single mode TEM-00 provides a solid beam throughout its diameter. Multi operation increases the overall power output of the laser but splits the beam into many segments. The blank areas in the beam are nulls.

ill. 5-7. A comparison of plane and circular wavefronts. The plane wavefront geometry of the typical laser decreases divergence and provides coherency across the entire diameter of the beam.

ill. 5.8. The Gaussian irradiance profile of an He-Ne laser (in TEM-00 mode). The peak of the irradiance profile is the center of the beam.

BEAM CHARACTERISTICS

The beam emitted from most helium-neon tubes doesn’t vary much between laser to laser. Except in special cases, the light has a wavelength of 632.8 nm, and can measure between 0.5 to 2 nm diameter. The diameter (or waist) of the laser beam is measured in a variety of ways and under different operating modes of the laser; hence the wide disparity in sizes. The diameter is less than the actual side-to-side measurement of the laser beam. Most manufacturers eliminate the outer 13.5 percent of the beam diameter, leaving the bright inside core.

As background, single-mode operation (sometimes called TEM of the laser produces a solid beam from side to side (looking from head-on). Multi-mode operation, which provides higher output power, causes the beam to separate into bands, as shown in ill. 5-6. Note that most He-Ne tubes work in TEM mode only and the tube must be specially built to operate in multi-mode.

In TEM mode, the round beam of a laser has a plane wavefront and a Gaussian transverse irradiance profile. This mode experiences the minimum possible diffraction loss, has minimum divergence, and can be focused to the smallest possible spot.

The plane wavefront, as illustrated in ill. 5-7, is a natural by-product of spatial coherence. All the waves in the beam are in lock-step, as if it were one big wave. The locking is constant across the diameter of the beam. This is in contrast to a circular wavefront, also shown in the figure, where the waves toward the outside radius of the beam are slightly behind the center waves. Note that you can easily convert the plane wavefront of laser light to circular wavefront simply by placing a lens in the path of the beam.

The Gaussian irradiance profile of a laser operating in TEM mode is shown in ill. 5-8. The center 86 percent of the beam is the brightest; the irradiance of the beam falls off as you approach the edges of the beam. You can visually see this effect when the beam is spread using a bi-concave lens. Shine the light at a green or black card and note how the intensity of the beam is greatest at the center and less at the edges. If the beam is so bright that it causes a halo even on a green or black card, wear dark green glasses or pass the beam through a deep green filter.

Even without external optics, the divergence (spreading) of the laser beam is typically not over 1.2 milliradians (mrads). That means that at 100 meters, the beam will spread 120 millimeters, or about 4.7 inches. Computing beam divergence is more thoroughly covered in Section 14.

ill. 5-9. The tiny piece of glass positioned near the plane parallel mirror in this tube causes polarized beam output.

He-Ne lasers exhibit random plane polarization. That is, the beam is plane polarized (as discussed in Section 8) in either one direction or 90 degrees in the other. The polarization randomly toggles back and forth, but the switch-over is extremely fast and generally not detectable. In this regard, all laser tubes are polarized, but there is no control over the plane of polarization.

Some helium-neon tubes are designed with internal polarizing optics that block one of the planes. These are called linearly polarized tubes. Polarization of the beam is accomplished not with filters but with a clear window placed at Brewster’s angle at the rear of the tube. The window is visible at the rear of the tube, as shown in ill. 5-9, and is a clear indicator that the tube is linearly polarized. In some lasers, the rear of the tube is terminated with the Brewster window but lacks a totally reflective mirror. You must then add an external mirror to operate the laser.

Linearly polarized tubes are more expensive than randomly polarized ones and are generally low in power—typically less than 2 mW (higher power polarized tubes do exist, but at a cost). However, polarized tubes are particularly handy in advanced holography, interfereometry, high-speed modulation, and other applications where you need a stable output and a linearly polarized beam.

Of course, you can always polarize a laser by placing a polarizing filter in the path of the beam, but this significantly reduces the intensity of the beam. In most applications you need as much beam intensity as possible.

Most linearly polarized tubes are rated by their polarization purity. The purity is expressed as a ratio, such as 300:1 or 500:1. That is, under normal operating conditions the tube is 300 or 500 times per likely to emit one plane of polarization over the other. Few applications, even precisely controlled laboratory experiments, require polarization purity greater than 500:1.

A laser beam might appear grainy or spotty when reflected off a white or lightly colored wall. This effect is called speckle and is caused by local interference. Even a smooth, painted surface has many hills and valleys in comparison to the wavelength size of the laser beam. These small imperfections in the surface bounce light in many directions. The uneven reflection causes constructive and destructive interference, where the light waves from the beam either reinforce or cancel one another.

Speckle is also produced in your eye as the beam strikes the surface of the retina. If you move your head, the speckle appears to move, too. Interestingly, beam speckle can be used to detect near- or short-sightedness. The speckle moves in the same direction as the head when viewed by a person with normal vision. But the speckle will appear to move in the opposite direction as the head when viewed by persons suffering from near- or far-sightedness. If you and any of your friends wear glasses, you can try this experiment for yourselves.

HE-NE COLORS

A He-Ne tube generates many different colors. You can see these colors by looking at a bare tube with a diffraction grating (an example is shown on our home page, taken with a criss-cross diffraction grating). The diffraction grating disperses the orange glow of the plasma tube into its component colors. Note that just about every primary and secondary color is present, with dark lines between each one. The dark areas rep resent in-between colors that are not generated within the tube.

Most all colors (plus infrared wavelengths you can’t see) are transmitted out of the tube before they can be amplified. That prevents them from turning into laser light. Here’s how it’s done: The cavity mirrors of the laser are coated with a highly reflective material that reflects the wavelength of interest (such as 632.8 nm) but transmits the others. As the beam inside the laser grows in strength, it overcomes the reflectance of the output mirror and exits the tube. You can verify the purity of the output beam by examining the red spot of the beam with the diffraction grating. You will see only one, well-defined color.

He-Ne’s emit a deep red beam at 632.8 nm because it's the strongest line. The other colors are weak or may not be sufficiently coherent or monochromatic. Yet there are some special helium-neon lasers that are made to operate at different wavelengths, namely 1.523 micrometers (infrared) and 543.5 nm (green). Green and infrared He-Ne lasers are exceptionally expensive (rare in the surplus market) and are designed for special applications.

You can see the 543.5 nm green line by examining a regular red He-Ne tube with a diffraction grating. You will also notice a second, darker red line at 652 nanometers. These and other spectra are created as the helium and neon atoms drop from their raised, excited state to various transition levels. Ill. 5-10 shows a diagram of the energy levels within a typical He-Ne tube. As an aside, the invisible 1.523 micrometer line is the wavelength of the first helium-neon tubes that were developed by Javan, Bennett, and Herriott at Bell Telephone Labs.

ill. 5-10. This simplified view of the energy levels of the He-Ne laser shows how photons are created as the atoms decay from their level 3 metastable state to an intermediate level 4 terminal state. Many wavelengths of light are created in the helium-neon gas mixture, but selective filtering provides a beam with a nearly pure wavelength of 632.8 nm (some He-Ne tubes operate at 1523 nm and 543 nm).

THE ROLE OF THE BALLAST RESISTOR

Laser tubes operate at voltages between 1.2 and 3.0 kV (though some are higher or lower), but their current requirements are much more stringent. Most tubes in the 1- to 5-mW power output range are designed to consume between 3.5 and 5 milliamps of current. If consumption is any less, the tube won’t fire anymore and could be damaged.

A helium-neon laser tube uses a ballast resistor to limit its current and to provide stable electrical discharge. The value of the ballast resistor varies depending on the tube and power output, but it’s generally between 47k0 and 230k0. The ballast resistor is usually placed close to the anode of the tube to minimize anode capacitance and to provide more stable operation.

You can safely operate most lasers between the 3.5- to 7-mA band gap recommended for He-Ne tubes, but the lower the current, the better. The tube will last longer and the power supply will operate more efficiently. If you have a choice of ballast resistor values, choose the highest one you can that fires the tube and keeps it running. If the tube sputters or blinks on and off, it might be a sign that it’s not receiving enough current. Lower the value of ballast resistor and try again.

Commercially made power supplies designed for use with cylindrical head lasers are typically engineered without a ballast resistor. Rather, the resistor is housed in the end cap of the aluminum laser head—never forget this. If you use the power supply with a bare tube that lacks a ballast resistor, you can permanently ruin the laser. If in doubt, add the ballast resistor; you can always take it out later.

BUYING AND TESTING HE-NE TUBES

Not all helium-neon laser tubes are created equally. Apart from size and output power, tubes vary by their construction, reliability, and beam quality. After buying an He-Ne tube, you should always test it; return the tube if it doesn’t work or if its quality is inferior.

Should you need a laser for a specific application that requires precision or a great deal of reliability, you might be better off buying a new, certified tube. The tube will come with a warranty and certification of power output. Because you’re the only one handling the laser, you won’t be bothered with such headaches as misaligned or chipped mirrors—unless you misalign or chip the mirrors yourself!

The documentation—a sort of pedigree that comes with the best tubes—will specify actual output, often within a tenth of a mW. E.g., the tube may be rated by the manufacturer at 1 mW. That’s the nominal output figure, but the documentation for the particular tube you receive might state the output was tested at exactly 1.3 mW. If you purchase many new tubes, you’ll quickly discover that no two lasers are the same. Even though manufacturing tolerances are extremely tight, slight variations still exist. Much of this variation is due to mirror alignment.

Visual Inspection

The first step in establishing the quality of the tube is to inspect it visually. If the tube is used, be on the lookout for scratched, broken, or marred mirrors. Check both mirrors and use lens tissue and pure alcohol to clean them. Gently shake the tube and listen for loose components. Most tubes have a metal spider that holds the bore in place, and though it may rattle a bit, the spider shouldn't be excessively loose. If the tube is an old one, it might have externally mounted mirrors (with most He-Ne tubes, the mirrors are mounted within the glass envelope). Carefully check the mirrors and mounts for loose components and scratches.

Inspect the tube carefully under a bright light and look for hairline cracks. If the tube is encased in a housing that contains a shutter, be sure the shutter opens when the laser is turned on (open the shutter manually if it's not electrically controlled).

Checking Laser Operation

After inspection, connect the tube to a suitable power supply. Be sure to read the section below if you are unsure how to connect the tube to the supply. Place the tube behind a clear plastic shield or temporarily cover it with a piece of cardboard. Flick the power supply on and watch for the beam. Listen for a sputtering or cracking sound; immediately turn off the power supply if you hear any anomalies. Point the laser toward a wall. Tithe laser is working properly, the beam comes out one end only and the beam spot is solid and well-defined.

Although one of the mirrors in an He-Ne tube should be completely reflective, they aren’t always so. Occasionally, the totally reflective mirror allows a small amount of light to pass through and you see a weak beam coming out the back end (this is especially true if the mirror isn't precisely aligned). Usually, this poses no serious problems unless the coating on the mirror is excessively weak or damaged or if the mirrors are seriously out of alignment.

All lasers exhibit satellite beams—small, low-powered spots that appear off to the side of the main spot. In most cases, the main beam and satellites are centered within one another, so you see just one spot. But slight variations and adjustment of the mirrors can cause the satellites to wander off axis. This can be unsightly, so if it matters enough to you, you might want to choose a tube that has one solid beam.

The satellites are caused by internal reflection from the output (partially reflective) mirror. The main and satellite beams are out of phase to one another because their path lengths are different. Some applications, such as advanced holography or interferometry, require you to separate the satellite beams from the main beam. You can do so with a spatial filter or metal flap.

When projected against a white or lightly colored wall, the beam from the laser might “bloom” with a visible, lightly colored ghost or halo. The ghost, which generally has a blue cast to it, sometimes makes it difficult to see the shape of the beam itself. Wearing a pair of green safety goggles helps to reduce the ghost, allowing you to see just the beam spot. You can also tack a piece of flat black or dark green paper on the wall and project the beam at it to reduce the effects of the ghost.

With just the beam itself cast on the wall or paper, inspect it for any irregularities. It should be perfectly round. If there are smears, try cleaning the output mirror and try again. Note that the beam will show nulls (dark spots) if the tube isn't operating in TEM mode, as described above. These cannot be eliminated by cleaning.

Should the tube start but no beam comes out, check to be sure that nothing is blocking the exit mirror (or, is the laser an infrared type?). Clean it and try again. If the beam still isn’t visible, the mirrors might be out of alignment. You cannot re-align mirrors that are fused to the glass tube unless the tube has alignment rings or wedges—and then the process depends largely on trial and error (and usually results in error).

Each ring is equipped with three or four hex screws. You readjust the mirrors by tightening and loosening one or all of the screws. Use a hex wrench that's completely insulated with high-voltage tape and take care not to touch any exposed metal parts. With the laser on, watch the output mirror and try loosening one of the screws. Does a beam appear? Keep trying, noting how you adjust each screw.

You might be able to make the beam appear by pressing down slightly on the wrench. That stresses the ends of the tube and can bring the mirrors into partial alignment. The idea is to work slowly and note the positions of the mirrors at each adjustment interval. Alignment becomes very difficult if both mirrors are out of whack.

If the tube doesn’t ignite at all, check the power supply and connections. Try a known good tube if you have one. The tube still doesn’t light? The problem could be caused by:

* Bad tube. The tube is “gassed out,” has a hairline crack, or is just plain broken.

* Power supply too weak. The tube may require more current or voltage than the levels provided by the power supply.

* Insulating coating or broken connection on terminal. New and stored tubes may have an insulating coating on the terminals. Be sure to clean the terminals thoroughly. A broken lead can be mended by soldering on a new wire.

Sputtering Tubes and Other Problems

Some “problems” with laser tubes are really caused by the power supply. In fact, if your laser doesn’t work, suspect the power supply first. Even commercially made power supplies can burn out, especially if they are used with a tube they aren’t designed for.

One common problem is when the tube sputters when you turn it on. This fault is most often caused by a tube that isn’t receiving enough current. Inspect the connections from the power supply to the tube—make sure that there is no arcing. An arcing connection can cause the tube to blink.

If the tube isn't designed for the power supply you are using, the supply (or tube) might have a ballast resistor that excessively restricts current. Locate the ballast resistor—it’s usually on the power supply or mounted directly on the anode terminal of the tube. Try a slightly lower value, but be sure the resistor is rated for at least 3 watts (5 watts is even better).

Note that the tube may be unstable if the ballast resistor is too high or too low. The trick is to find the value that works with the tube. Specifications that accompany new tubes may indicate the recommended ballast resistor value; in most cases, you must experiment until you find the one that yields the best results.

The ballast resistor is often located in the end cap of laser tubes that are encased in an aluminum housing. Remove the end cap, if possible, by twisting it off. You might need to loosen one or more set screws. The ballast resistor is typically sealed inside the end cap with silicone rubber. Peel the rubber away and exchange the resistor with a lower value, If the new resistor works, repot it in the end cap with fresh silicone rubber sealant.

Long lead lengths can also cause sputtering. The longer the lead, the greater the loss of current. Most laser power supplies cannot power a laser with leads longer than about 12 to 18 inches. Small “pee-wee” power supplies, particularly those that run on 12 volts dc, should have no longer than 2- to 4-inch leads. Bear in mind that the length of the anode lead is of most importance; the cathode lead can be any reasonable length.

Some laser power supplies have a current adjust pot; turn it to see if it makes a difference. Exercise caution, however, and avoid yanking the current so high that you cause damage to the tube. You can test the current consumption of any tube by inserting a meter between the laser and power supply. Section 6, “Build a He-Ne Laser Experimenter’s System,” provides the details. Few helium-neon lasers can be safely operated above 7 mA of current.

Hard-to-start tubes flick on but quickly go out. If the power supply incorporates a trigger transformer, the tube might “click” on and off once every 2 to 3 seconds (correlating to the time delay between each high-voltage trigger pulse). Tubes that haven’t been used in a while can be hard to start, so once you get it going, keep it on for a day or two. In most cases, the tube will start normally.

Hard starting can also be caused by age and de-gassing, two factors you can’t fix. A tube that simply won’t start might suffer from inadequate current, regardless of ballast resistor, power lead length, and other variables. Be sure to use a power supply that delivers sufficient current. You will have better luck starting the tube if the power supply incorporates a high-voltage trigger transformer.

POWERING THE TUBE

Most surplus He-Ne tubes are sold without instructions of any type. It’s up to you to know how to use it. For the uninitiated, this can seem frightening, but the detective work is actually simple. Follow these steps and you can get just about any tube working in a manner of minutes.

All He-Ne tubes have an anode (positive terminal) and cathode (negative terminal). In some cases, the terminals are marked with an “A” and “C” (sometimes “K”) or simply “+“ and “—“. A few others have a red dot that indicates the anode. Following the polarity specified, connect the power supply so that the positive lead connects to the anode and the negative (or ground) lead connects to the cathode.

By far, most He-Ne tubes have no markings at all, and you are left wondering which one is which. Connecting the tube backwards to the power supply can damage the sup ply, the tube, or both. Also note that some power supplies must have a load on the output terminals or damage to the high-voltage components could result.

You can readily identify the cathode of most tubes by looking for the ring-shaped “getter” that rests near the inside wall of the tube. The getter is usually made of a zirconium-aluminum alloy and serves to remove trace gas residues after the tube is sealed during manufacture. In most all cases, the getter is connected to the cathode or at least to the spider, which is connected to the cathode. The area around the getter might have a silver discoloration, but this isn't an indication that the tube is bad nor that it has been used excessively.

Another tip-off is the filling line originally used to pump the gasses into the tube. The filling line usually (but not always) denotes the cathode end. In fact, the line often doubles as the cathode connection.

Lasers enclosed in aluminum housings typically have leads coming out of an end cap. These leads terminate in a male high-voltage Alden connector, designed for use with a corresponding female connector attached to commercial laser power supplies. The thin prong of the connector denotes the anode.

If there is no connector, or another connector type is used, you can often identify the polarity of the leads by their color. As usual, red denotes positive (anode) and black (or sometimes white) denotes negative (cathode). Lasers that use a shielded coaxial cable use the inside conductor for the anode and the outside shield for the cathode.

USING THE TUBE

If you have not used your He-Ne laser yet, test it by connecting the power supply and applying juice. If the supply is one that’s commercially made, there might be a 1- to 3-second delay before the laser turns on. This time delay allows the capacitors to charge and is also a CDRH requirement. If nothing happens after about 5 seconds, disconnect the power and check all the connections.

Once you get the laser running, keep it on for an hour or so. The tube will become warm, but if the supply is properly adjusted (that is, it’s not delivering too much current), the laser won't be damaged. The “burn in” period provides a safety net; if the tube and supply are going to fail, it will likely be within the first hour or so of use. This allows you plenty of time to return the tube and /or power supply and obtain a replacement.