The argon laser is no more difficult to build than the helium-neon type, although in general the construction of lasers makes a considerably more severe demand on the craftsmanship of the experimenter than most of the projects that have been described in these columns do. Such demands are minimized, however, by an argon laser that has been designed recently for amateur construction by Sylvan Heumann of 410 Eucalyptus Avenue, Hillsborough, Calif. 94010. Heumann writes:

"The argon laser resembles the helium-neon laser in many ways. It consists essentially of a gas-discharge tube about two feet long, the ends of which are closed by a pair of flat windows of fused quartz that face a pair of small dielectric mirrors [left]. The tube glows dark blue when the gas is energized by a pulsed electric current of 15 to 20 amperes. Depending on the amount of the current, the energy of the ionized atoms is increased one or more levels above that of the ground state, which is the level of energy that characterizes electrically neutral atoms of argon gas. The electrical discharge is said to pump the atoms to an excited state. After a short time the atoms spontaneously fall to a lower energy state, one at a time, simultaneously emitting a quantum, or pulse, of light. The color of the emitted light varies according to the amount of energy that is liberated during the fall. Quanta of relatively low energy appear red and those of increasingly higher energy appear yellow, green, blue and so on.

"Occasionally a quantum of light that has been liberated spontaneously from an excited atom encounters another energized atom. The resulting interaction may cause the energized atom to fall to a lower energy level and simultaneously liberate a quantum of precisely the same color as the stimulating quantum. This is the phenomenon called stimulated emission. The initial photon causes the excited atom to fall somewhat earlier than it would if it were not disturbed. The two photons merge and proceed through space as a train of coherent, monochromatic light waves.

"The train may encounter a third excited atom and similarly cause it to contribute a photon to the growing packet of energy. Indeed, the train of waves may continue to accumulate energy by stimulated emission until it travels out of the gas. The argon laser is merely an apparatus designed to encourage the continued growth of the train by causing it to travel back and forth through the gas many times. This effect is achieved by the flat windows and the pair of dielectric mirrors associated with the laser tube. An occasional train of coherent light waves may travel along the axis of the tube and make its way through one of the windows and thence to the adjacent mirror. If the mirror is of high optical quality, it reflects most of the light directly back through the window and into the tube, where the light accumulates still more energy by the process of stimulated emission. The intensified light proceeds through the opposite window and the cycle of events is repeated.

"Not all the energy of the beam is reflected by the mirrors. Mirrors of perfect reflectivity cannot be made. Some of the energy is absorbed by the reflecting material and transformed into heat. Another portion, perhaps a few thousandths of 1 percent, makes its way through the reflecting material. This small portion constitutes the output of the laser.

"To create the current of 15 to 20 amperes that excites the gas a potential of some 2,000 volts must be applied to the electrodes of the tube. The resulting expenditure of power amounts to several kilowatts-enough to heat the tube beyond the melting point of the glass. In order to prevent destructive heating, power is applied to the laser in short, rather widely spaced pulses. In the design that I recommend the tube is so energized 120 times per second. The pulses persist only a few millionths of a second. Pulses of coherent light are emitted at the same rate and persist for less than 50 millionths of a second. The beam appears continuous to the eye, however, because the relatively sluggish chemical processes of vision cause each pulse to be seen for about a fiftieth of a second.

"The laser consists of a base assembly that supports a capillary tube 50 centimeters long with a bore of two millimeters. Each end of the capillary tube is sealed to a 15-millimeter tube that includes a quartz window and a neon-sign electrode. A ball-and-socket joint of glass in the 15-millimeter tube permits adjustment of the angle the windows make with the axis of the capillary. Air is pumped from the assembly and argon gas is admitted to it through a short length of seven-millimeter tubing sealed into the 15-millimeter tube at one end [Figure 2]. All parts are made of borosilicate glass except the quartz windows.

"Begin the construction by cutting a 51-centimeter length of capillary tubing. This tubing comes in standard lengths of four feet. With a corner of a flat file make a crosswise nick in the glass at the specified length, grasp the tubing on each side of the nick and pull it apart. Do not bend the glass. With a blowtorch that burns a mixture of oxygen and household gas, heat the cut ends just enough to round the sharp edges. When the glass cools, reheat one end until the bore closes. Blow into the opposite end to form a bulb about 18 millimeters in diameter. Let the bulb cool until it solidifies. Reheat the outer hemisphere of the bulb to softness, then blow forcefully to explode the softened glass. Strike off with the flat face of the file any tissue-thin fragments that cling to the expanded end of the capillary. Rotate the expanded end of the tube in the fire until the edge shrinks to a diameter of 15 millimeters. Similarly expand the other end of the capillary.

"Next, select a cork that fits 15-millimeter tubing and bore an axial hole through it to fit a pencil-sized length of wooden dowel rod. Insert the end of the rod completely through the cork from the top. Insert the small end of the cork into the ball end of the ball member of the ball-and-socket joint. Align the dowel with the axis of the ball member so that when the dowel is rotated between the thumb and fingers, the glass turns without wobbling, as though it were in a lathe. Grasp the dowel in one hand and the capillary in the other. Bring the 15-millimeter tubing of the ball member into axial alignment with the capillary.

While rotating the glasses back and forth synchronously, move the ends of both pieces into the edge of the flame on opposite sides and heat the glass until about one millimeter of each edge softens. Remove the glasses from the fire and, while maintaining the back-and-forth rotation, press the aligned ends lightly together so that they fuse. Return the fused joint to the fire. Continue rotating the glass back and forth. Never let it stop. When it becomes soft, remove the work from the fire, stretch the joint about five millimeters and blow into the open end of the capillary until the glass expands into a rounded contour. You have now made a 'butt seal.' In the same way seal the remaining ball member to the opposite end of the capillary.

"By means of the same technique seal a 25-centimeter length of 10-millimeter tubing to a 25-centimeter length of 15-millimeter tubing. At a point three centimeters from the seal, cut the 15-millimeter tubing and seal to its end a 15-millimeter neon-sign electrode of the type that is coated internally with a mixture of barium carbonate and strontium carbonate. Similarly prepare a second electrode. Bend the 10-millimeter tubing of each electrode assembly to a right angle by softening a three-centimeter zone of the glass adjacent to the seal. Bend the softened glass by turning the ends upward. (Let gravity work with you.) When the bend is completed, promptly blow into the open end of the tubing to restore the original diameter of the curved portion. When the glass cools, cut the 10-millimeter tubing to the illustrated proportions.

"Seal the electrode subassemblies into the ball members of the capillary assembly. To make this seal blow a hole in the 15-millimeter tubing of the ball member by placing a stopper in the opening of the ball and heating a spot about 12 millimeters in diameter in the middle of the 15-millimeter tubing. Blow the softened spot into a hemisphere. Reheat about 70 percent of the hemisphere and blow forcefully to explode the upper part of the bulb. Butt-seal an electrode subassembly to the hole. Transfer the stopper to the opposite end and seal the second electrode subassembly. Finally, with the same procedure, seal a four-inch length of seven-millimeter tubing into the assembly at the position indicated by the drawing. (The basic techniques of making glass apparatus by hand are fully explained in Creative Glass Blowing, by James E. Hammesfahr and Clair L. Stong, W. H. Freeman Company, 1968.)

"The tube is completed by cementing the quartz windows to the socket members of the ball-and-socket joints. The windows transmit the laser beam with substantially no loss of light only if they are cemented to the glass tube at an angle of approximately 35 degrees 50 minutes with respect to the axis of the tube. My tubing was cut to this angle by a diamond saw of the type used by amateur mineralogists. The cuts can also be made with a blade of soft metal, such as brass, that is fed a slurry of No. 120 grit Carborundum and water. The blade, a strip of brass about .02 inch thick and 10 inches long, can be mounted for use in a hacksaw frame. The angle of the cut can be maintained with an improvised miter box. The cut end of the glass must be lapped smooth and flat by grinding the tubing against a sheet of plate glass with a slurry of No. 600 grit Carborundum.

"To cement the quartz windows in place, coat the mating surfaces of the balls and sockets lightly with high-vacuum stopcock grease, connect the laser tube to the vacuum pump through the seven-millimeter inlet and start the pump. Place the previously cleaned windows in contact with the cut ends of the socket members and assemble the sockets simultaneously to the balls. Suction will hold the joints and the windows in place. If this two-handed manipulation proves difficult, have an assistant position one of the windows while you position the other one. If the cut ends have been lapped flat, the tube assembly will be airtight. With a toothpick gently apply a bead of epoxy cement completely around the junction of the tube and window. The cement should not flow into the junction and it will not if the end of the tube has been lapped flat. Let the vacuum pump run until the cement solidifies. This operation completes the laser tube.

"The mechanical assembly can be improvised from almost any available materials. For the base I used a scrap of rectangular aluminum tubing four inches wide, two inches thick and 36 inches long of the type found in metal doors. The laser tube is attached to the base with a pair of adjustable fixtures that are convenient for aligning the tube coaxially with the mirrors. The fixtures can be made with ordinary hand tools. The mirrors are mounted in adjustable cells that include micrometer screws spaced radially at 120 degrees. The optical axis of the mirrors can be positioned as desired by manipulating the screws. Several of the accompanying illustrations give details of the supporting fixtures and mirror cells.

"It is possible to prepare the laser for operation with a fairly crude vacuum system. A mechanical pump capable of exhausting the tube to a pressure of .01 torr is adequate. (One torr is equal to the pressure exerted by a column of mercury one millimeter high.) The useful life of the tube tends to increase, however, with the effectiveness of the vacuum system. My laser operates for about 30 minutes on a charge of gas. I rarely disconnect it from the vacuum system. When laser action begins to fail, I replace the argon by opening a stopcock that lets the used gas flow into the pumps. Then I close the stopcock and open another one that admits fresh argon from the reservoir. The apparatus now operates for another 30 minutes.

"My vacuum system includes both a mechanical pump and a diffusion pump, a cold trap, a closed-end manometer, a vacuum gauge, a flask of argon and five stopcocks [below left]. (The diffusion pump, cold trap and gauges may be omitted.) The stopcock that connects the gas reservoir to the system should be of the high-vacuum type and need not have a bore larger than two millimeters. Remove the plug of this stopcock. With a file make a scratch about a third of the way around the plug beginning at the hole. Turn the plug clockwise and, by reducing the pressure on the file, let the depth of the scratch taper gradually to the surface. Make a similar scratch that extends in the same direction from the other end of the hole. Apply a thin film of high-vacuum stopcock grease to the plug and replace it. When the stopcock is operated, gas flows through the scratches slowly, enabling you to fill the tube with argon at a precisely controlled rate.

"Current for exciting the laser tube is drawn from a power supply at the rate of 120 pulses per second. The power supply consists of a variable transformer that feeds a neon-sign transformer. The high-voltage output of the neon-sign transformer is converted to direct current for charging a capacitor. Pulses of current are drawn from the capacitor by the tube when the argon gas is ionized by an Oudin coil, which generates a potential of 30,000 volts at a very high frequency. The amount of current depends on the adjustment of the variable transformer. Any neon-sign transformer can be used that is rated at an output potential of between 4,000 and 9,000 volts and a current of not more than 50 milliamperes. The variable transformer must be rated at a current of at least two amperes. The capacitor may be rated at one microfarad and at a breakdown voltage at least equal to that of the neon-sign transformer.

"A higher potential than is provided by the power supply is required to ionize argon gas; it can be developed from an Oudin coil of the type used for detecting leaks in the glass parts of vacuum systems. An example is the No. 15-34075V3 Vacuum Tester distributed by the Fisher Scientific Company, Springfield, N.J. 07081. Connect a wire from the high-voltage terminal of the coil to a piece of aluminum foil wrapped around the middle of the capillary tube. The high potential of the Oudin coil triggers pulses of pumping current from the capacitor. Place the capacitor within a few inches of the tube to minimize the length of the leads that connect it to the electrodes.

"When the laser has been assembled, connect the tube to the vacuum system and install the dielectric mirrors in their cells. One mirror should be spherical and have a radius of 120 centimeters. The other mirror should be flat. Ordinary silvered or aluminized mirrors will not work. With the mirrors installed, place a small incandescent lamp near the end of the tube so that the light that passes through the glass is reflected into the bore of the capillary by the inclined inner face of the quartz window. Look into the bore of the capillary through the mirror nearest the lamp and adjust the micrometer screws that control the distant mirror until the circles of light that are reflected from the walls of the tube and the bright spot from the distant mirror are concentric. Transfer the lamp to the distant end of the tube and adjust the mirror there.

"Close the stopcock of the gas reservoir and the stopcock leading to the air inlet. Open all stopcocks between the vacuum pump and the tube. Start the pump. With a low-temperature flame of the kind delivered by a propane gas torch heat all parts of the tube except the ball-and-socket joints and the quartz windows. The glass should be made hot enough to turn a piece of white tissue yellow in about 20 seconds. The heat drives adhering gases from the inner walls of the glass.

"The electrodes must now be brought to a dark red heat. I heat them one at a time by sliding over the glass envelope of the electrode a coil of wire that is connected to the output of a 75-watt amateur-radio transmitter. A variable capacitor is connected across the coil for tuning it to resonance with the frequency of the transmitter. The size of the variable capacitor and of the coil depends on the frequency at which the transmitter operates, but a coil of nine turns that is an inch wide and 1 1/2 inches long will work with most transmitters when it is tuned by a variable capacitor of about 150 picofarads.

"The electrode will begin to heat as the circuit approaches resonance. At full resonance the electrode may become hot enough to melt, so tune the circuit cautiously. Stop tuning at the point where the electrode becomes red hot. The heated electrode will liberate an astonishing amount of gas, enough to alter the characteristic sound of the vacuum pump. Maintain the metal at red heat for about three minutes, then similarly 'outgas' the second electrode. In addition to liberating gas from the metal, the heat burns away the nitrocellulose binder used for coating the interior of the electrode and converts some of the barium carbonate to metallic barium. Part of the metallic barium then combines with chemically active gases and vapor, such as oxygen, nitrogen, carbon dioxide and water vapor, that remain in the tube.

"To fill the tube with argon turn on the power supply and the Oudin coil, adjust the variable transformer for an output of approximately 70 volts and close the stopcock to the vacuum pump and the one to the trap. Open the stopcock to the argon flask just enough to allow a flow of gas. As argon enters the system the tube will begin to glow faintly and will increase in brightness as the gas pressure increases. The rate of gas flow differs with various stopcocks and must be determined experimentally.

"The tube may flash intermittently and the gas may change from light blue to a pinkish hue. The color change indicates the presence of an unwanted gas, such as oxygen. If the color change occurs, shut off the gas supply, let the tube operate for about a minute and then shut off the power. Open the stopcock to the vacuum pump and exhaust the system for about five minutes. Repeat the sequence of operations until the tube glows dark blue and retains this color for at least five minutes. Pump out the gas.

"The apparatus is now ready for laser action. With the variable transformer turned on and adjusted for an output of about 70 volts, admit argon gas to the tube as slowly as possible. The tube may glow dimly and flicker. Continue to add argon just to the point where the tube stops flickering. If the mirrors have been adjusted perfectly, a beam of greenish-blue coherent light will be emitted at the ends of the tube.

"If the beam does not appear, twist the micrometer screws (one at a time) back and forth about two or three angular degrees. This operation is known as 'fiddling' with the screws. Watch the ends of the tube carefully as you rock the screws. Stop when the beam appears. Then adjust the system for maximum beam intensity. Increase the output voltage of the variable transformer. Beyond a certain maximum voltage the intensity of the beam will decrease. Then, assuming that you have installed the quartz windows so that the faces are at right angles to the vertical plane, you will observe two spots of light on the ceiling. Rotate the windows so that the spots lie in the vertical plane that includes the bore of the capillary. Then tilt each window up or down to the angle at which the beam becomes most intense. Both of these adjustments are made possible by the ball-and-socket joints.

"Next, vary the gas pressure. Add gas slowly. Up to a certain pressure the intensity of the beam will increase. Thereafter it will decrease. Lower the pressure by pumping out gas. After 20 or 30 minutes of operation at maximum intensity the brightness of the beam may begin to decline. This indicates that atoms of argon are being buried under particles of eroded electrode materials, an effect known as 'sputtering.' The sputtering effect lowers the pressure of the gas. At this point I usually pump out the tube and refill it with fresh argon.

"You will observe that the color of the beam changes as the input voltage is increased. The change is caused by the successive appearance of coherent light at various wavelengths as current in the tube is increased. Laser action begins when the current reaches 1.45 amperes per pulse. The tube then emits coherent light at a wavelength of 4,880 angstroms, the wavelength for which the dielectric mirrors are coated. At 3.6 amperes laser action also begins at 5,145 angstroms. Thereafter light appears at the following wavelengths: 3.8 amperes, 7,465 angstroms; 4, 4,965; 5.2, 4,579; 6, 5,017; 6.9, 4,658; 10.5, 5,287, and 15 amperes, 4,727 angstroms. Each color can be separated into an individual beam by passing the output of the laser through a 60-degree glass prism. The individual beams can be used for the precise measurement of length. Collectively they will serve as a series of known wavelengths for calibrating apparatus and making numerous other experiments. Krypton gas can be used in this laser and will demonstrate still other spectral lines, but with lesser beam intensity.

"The dielectric mirrors, ball-and-socket joints, electrodes sealed in Pyrex, transformers, gas and other supplies can be bought from Henry Prescott, 116 Main Street, Northfield, Mass. 01360. A note of warning: The laser beam is hazardous. It can burn and destroy the retina of the eye. Two intense beams are emitted from the ends of the tube and two beams of lesser intensity are reflected into the room by the faces of the quartz windows. Mask all beams close to the tube except those with which you are experimenting. Avoid spurious reflections of the experimental beam by objects in the room. The power supply develops lethal voltages and the capacitor stores charge at high voltage. Handle them accordingly. When the apparatus is turned off, always short-circuit and thus discharge the capacitor by means of a wire supported in an insulating handle. As an added precaution it is well to connect a one-megohm, two-watt resistor permanently across the terminals of the capacitor to 'bleed off' automatically any charge that may accumulate spontaneously."

Bibliography

FUNDAMENTALS OF OPTICS. Francis A. Jellkins and Harvey E. White. McGraw-Hill Book Company, Inc., 1950.

INTRODUCTION TO LASER PHYSICS. Bela A. Lengyel. John Wiley and Sons, Inc., 1966.

CREATIVE GLASS BLOWING. James E. Hammesfahr and Clair L. Stong. W. H. Freeman and Company, 1968.

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