With the echoes amateurs can measure distances to reflecting targets miles away; the accuracy is a matter of a few feet. With targets that absorb radiation the effects of impacts range from the emission of fluorescent light to the initiation of chemical reactions, including photochemical reactions. Indeed, as a source of radiation for making photographs the laser is about 10,000 times faster than the high-speed strobe lamps ordinarily used by amateurs.

Many parts of the laser can be assembled with materials that accumulate in the scrap box of anyone whose hobby is electronics. A version of the apparatus that is particularly easy for amateurs to build has been developed and patented by James G. Small, a graduate student of physics at the Massachusetts Institute of Technology. Small explains the principles on which the apparatus is based and the details of its construction and operation

"It has been known for some time that a high-current electric discharge in nitrogen gas that is flowing at a relatively low pressure can generate a pulse of coherent radiation, which is to say a laser gain, at the wavelength of 3,371 angstroms. The wavelength lies in the ultraviolet region of the electromagnetic spectrum where lenses and windows of most kinds of glass are transparent. The laser action begins when a molecule of nitrogen at room temperature absorbs energy by colliding with an electron that moves in the discharge. The encounter leaves the molecule in an unstable state. Usually it spontaneously falls to a state of lower energy by emitting a photon of radiation at 3,371 angstroms.

"The emitted photon may encounter another excited molecule of nitrogen and merely by its proximity stimulate the

molecule to emit an identical photon. In this case the two particles of radiation join forces and proceed in the same direction, with their waves in lockstep. The resulting pulse of radiation contains twice the energy of each photon. This is laser action.

"The action will continue as long as the growing pulse encounters more excited molecules of nitrogen along its path than it does absorbing molecules. The process soon stops, however, because when a large number of molecules are suddenly excited, they will begin to randomly cascade to lower states of energy. Unfortunately in the case of nitrogen the molecules on the average linger at that lower level longer than at the upper one before moving on to still lower states. The number of molecules at the lower laser level builds up rapidly, eventually exceeding the number at the upper level and terminating the amplification In fact, the gas quickly becomes strongly absorbing to 3,371-angstrom emission. The laser turns itself off even though there are still excited molecules left Nitrogen lasers are therefore said to be self-terminating. The turnoff time is rather fast, usually less than 10 nanoseconds (billionths of a second), and it is responsible for the extremely short output pulse useful for radar and very-high-speed photography.

"The trick of inducing laser action in nitrogen lies in constructing a mechanism that will almost instantaneously send a huge current of electrons at a high voltage laterally through a column of the gas at a pressure of about 100 torr An appropriate switching mechanism, which can handle tens of thousands of amperes within nanoseconds, turns out to be quite simple both in principle and in construction. It was invented by Alan Dower Blumlein, a British electronics engineer.

"Essentially the device consists of two adjacent metal plates separated from a third plate of equal total area by a thin sheet of plastic insulation [Figure 2]. In effect the assembly behaves as an adjacent pair of interconnected capacitors. The space between the capacitors serves as the gap across which electric current can be discharged through nitrogen.

"The capacitors are interconnected electrically by a coil of copper wire. The capacitors can be charged by applying a potential difference between the interconnected plates and the mutual plate. Both capacitors charge to the same potential and the same polarity. No potential difference exists across the gap between them.

"The switching action devised by Blumlein develops when one of the capacitors abruptly discharges. The action is initiated by the breakdown of a spark gap that connects one adjacent plate to the mutual plate. Current moves through the spark gap when the charge accumulating on the capacitor assembly exceeds a predetermined voltage.

"As the charge rushes through the spark gap a steep difference of potential appears within the plate across a narrow boundary that separates the charged and discharged regions of the metal. The boundary has the form of a circular wave front that recedes from the spark gap at nearly the velocity of light. Shortly after the onset of conduction at the spark gap the voltage wave arrives at the center of the discharge gap between the plates. At this instant a potential difference appears across the center of the gap. Thereafter the advancing voltage spreads to the edges of the gap. In an apparatus 12 inches wide and 18 inches long a potential appears across the full length of the gap in less than .2 nanosecond, rising to its maximum value in about a nanosecond.

"If the discharge gap is enclosed by a container of nitrogen gas at low pressure and the capacitors are charged to 20,000 volts, the resulting discharge will raise an enormous number of the nitrogen molecules to the excited energy state. Laser action follows during the next five to 10 nanoseconds. The coil of copper wire through which the capacitor assembly is charged responds sluggishly to changes in current. For changes that occur within nanoseconds the coil acts as an open circuit.

"A practical nitrogen laser includes the switching apparatus, a power supply and a source of nitrogen gas, preferably of the grade used by welders [see Figure 1]. The compressor from an old refrigerator can be connected backward to operate as a vacuum pump for reducing the pressure of the nitrogen An inexpensive water aspirator of the kind employed by chemists for vacuum filtering will work equally well.

"The capacitors of my laser were made from Type G-10 epoxy circuit board, which is clad with copper on both sides. This commercially available material serves widely in the electronics industry for interconnecting electronic apparatus. The unwanted metal is removed by etching to leave a network of conducting strips.

"My capacitors were formed on a single piece of circuit board that measures 30 x 45 x .04 centimeters (12 X 18 x .015 inches). Copper was etched from a two-centimeter (3/4-inch) margin around the edge on both the top and bottom sides of the board. An additional strip five centimeters (two inches) wide was removed straight across the top side to form the two adjacent plates of the capacitor, each of which measures 18 X 26 centimeters.

"Copper surfaces that are to be preserved can be masked with waterproof tape or with the enamel spray paint that is called 'resist' by the photoengravers who developed this etching technique. The unwanted metal is preferably etched with ferric chloride, although dilute nitric acid can be used. In the U.S. all materials required for building the laser, including the etching chemicals, are available from North Country Scientific (R.F.D. 1, Plymouth, N.H. 03264).

"The capacitors can also be fabricated from a sandwich of metal foils or plates bonded to a thin insulating sheet such as acetate or Mylar and even certain paints made with plastic resins. The insulating material should be as thin as possible but should not conduct or break down when the maximum potential difference is applied across the plates. Epoxy circuit board can be operated reliably at a potential of 1,000 volts per .001 inch of thickness.

"The housing for the discharge consists of a gastight box five centimeters square and 30 centimeters long (outside dimensions). It can be made by cementing together strips of clear plastic, such as Lucite, .63 centimeter (1/4 inch) thick. An effective cement is clear silicone seal, a rubberlike material that is available at most hardware stores.

"The ends of the box are cut at an angle of from 20 to 30 degrees to prevent the reflection of radiation from the surfaces of glass windows back into the plasma, where it would be further amplified. The end plates are made of plastic that is thicker than the plastic on the sides. The added thickness makes it possible to drill holes in the edges for hose connections. These holes join other holes that pass through the center of the plates and serve as outlets for the radiation.

"The output ports are closed by windows made of glass microscope slides. The windows can be sealed to the plastic with paraffin wax of the kind used in home canning. The wax can be melted and applied safely with a heat lamp. The windows could be cemented in place with silicone seal, but wax has the advantage of reversibility (an advantage that will be appreciated when the glass becomes coated with grease from the vacuum pump or collects other foreign substances).

"The discharge electrodes of copper foil are cemented between two strips of plastic, which form each side of the box, with the edges of the foil opposed and separated by about a centimeter [see above right]. The portion of the foil that extends outside the box has the form of a lazy S. The outer edge must be well soldered throughout its length to the foil of the epoxy board because the joint must conduct thousands of amperes.

"The spark gap must transmit maximum current in minimum time. It must be made of wide strap metal instead of wire to minimize its inductance. I made the clamps of the assembly with strap brass because of that metal's good conductivity, but aluminum strap could be substituted. An open air gap will work satisfactorily, but it is noisy and generates bright flashes of light that are rich in ultraviolet emission. I recommend that the gap be enclosed by an opaque tube of plastic or some other material that absorbs at least 90 percent of the emitted radiation.

"The laser must be charged to a potential of from 10,000 to 20,000 volts. A power supply that delivers .001 ampere of direct current will charge the capacitors to the ionizing potential of the spark gap several times per second. An alternating-current supply, such as a current-limited, neon-sign transformer rated at an output of 20 milliamperes, would cause the gap to conduct at twice the line frequency (120 discharges per second). Operation at this rate, however, would necessitate some means for cooling the gas if the maximum peak power is to be obtained. Warm nitrogen at low pressure does not lase well. The gas should be allowed to cool to approximately room temperature after each pulse.

"The output is at a maximum when the discharge occurs in relatively pure nitrogen. The gas need not flow continuously. The discharge housing can be flushed with gas, pumped down to 100 torr and sealed off. The pulse-repetition rate of the sealed unit must be less than one pulse per second.

"Pulses at this rate can be generated with a battery-operated power supply [see above left]. This device consists of a low-voltage transistor oscillator, a step-up transformer and a voltage-multiplying rectifier. The transformer is made from an automobile ignition coil. Carefully cut the top from the housing of the coil. Remove the windings, along with the iron core. The innermost winding in most coils will consist of approximately 25,000 turns of fine wire. Carefully unwrap from the fine wire the outer, heavier-gauge primary winding and in its place wind two 20-turn coils of 20-gauge enameled wire, which should be closely spaced.

"Connect one of the coils in series with the collector of the 2N3055 transistor or an equivalent transistor such as the 2N3236, 2N5039, 2N6271 or HST9203. This transistor needs no heat sink, as it is operated conservatively. The 2N1613 is a low-powered transistor. It is connected in the Darlington configuration to speed the switching time of the larger transistor. Other transistors that will work nicely for this purpose include the 2N696, 2N2222 and 2N3642.

"The circuit is grossly over-designed. Experimenters outside the U.S. will find that almost any silicon NPN transistor with a beta of 30 or more and a power dissipation of more than half a watt can be substituted for the 2N1613. For the larger transistor try any NPN of the silicon type with a collector current rating of five amperes or more that is housed in a TO-3 case. Surplus dealers in the U.S. have been selling large transistors of this kind at four for $1. When transistors are substituted, the values of resistors R1 and R2 may have to be altered experimentally within a factor of three to ensure reliable oscillation. The frequency of oscillation should be a few kilohertz.

"The power supply will operate from a six-volt battery at from 1.5 to three amperes and will deliver 20,000 volts at current sufficient to charge the laser to ionizing potential in from three to eight seconds. The electrical efficiency is quite low (about 1 percent), primarily because of the inefficient but simple transformer. Do not omit the two-microfarad capacitor that connects from the chassis to the junction between the emitter of the first transistor and the base of the second transistor. Without this capacitor the unit may tend to oscillate at a frequency too high for efficient transformer operation, depending on the loading of the high-voltage output.

"The reader may wonder why no mention has been made of laser mirrors. The optical gain of the rapid discharge is so large that emission becomes superradiant, which means that the unit will lase without an optical cavity. Radiation that is spontaneously emitted from molecules at one end of the laser can be amplified so strongly by the time it reaches the other end that the laser approaches saturation, meaning that it reaches the limit of its amplifying ability. Output is emitted from both ends of the column of excited gas, but a mirror at one end will more than double the power at the other end.

"The pulses can be detected easily by holding a piece of bleached cloth in front of either window. The cloth will fluoresce brightly. Almost any bleached cloth will function as a fluorescent screen. In general anything that glows in 'black light' works well: shirts washed in detergents containing fabric brighteners, 'psychedelic' posters, some kinds of paper such as white business cards, some types of clear mineral grease, the radiant dial of a watch or a clock and, of course, any of the dyes for dye lasers (fluorescein, relatively concentrated solutions of rhodamine BG and so on).

"If the ultraviolet pulses are focused by a cylindrical lens to a line on the surface of the dye rather than to a point, the dye will often lase superradiantly in visible light along the direction of the line. No optical mirrors will be needed. Indeed, the nitrogen laser makes an ideal 'pump' for the dye laser. When it is employed as a pump, it opens up most of the visible spectrum to new experimental investigation [see "The Amateur Scientist", SCIENTIFIC AMERICAN, February, 1970].

"Rumor has it that some types of Day-Glo plastic will lase when pumped with a sufficiently intense ultraviolet pulse. I have seen motion pictures of this effect but have not done the experiment myself. The smooth surfaces of the plastic appear to function as the cavity of the optical laser.

"The ultraviolet laser can readily be scaled to higher powers. A discharge path one meter long can develop an output pulse of almost a million watts, although there is a trick to it. Because the laser turns itself off so quickly, radiation does not have time to travel the full length of the column before the gain automatically drops to zero.

"This problem can be solved with a traveling-wave discharge. Move the spark gap to a corner of the capacitor. The voltage wave will then arrive first at the end of the discharge channel nearest the spark gap and will race down the channel in step with the growing pulse of emission!

"As I have mentioned, all these lasers work best when the discharge channel is filled with flowing nitrogen at low pressure. Helium can be added with almost no effect other than raising the total pressure. With a sufficiently high percentage of helium the laser will work at atmospheric pressure, thereby eliminating the vacuum pump.

"During the development of this apparatus I have enjoyed the cooperation of Norman Kunit and other members of Ali Javan's laser group at M.I.T. I want to thank them for their spirited encouragement. I am particularly grateful to our chemist, Ray Mariella, Jr., for his suggestion of the aspirator vacuum pump.

"Finally, the experimenter must keep in mind that this is a high-powered apparatus and therefore a hazardous one. The ultraviolet emission from the laser and from the unshielded spark gap can harm the eyes. Avoid looking directly into the beam of this laser or any other one, just as you would avoid looking directly at the sun or at the arc of an electric-welding rig.- Do not touch the capacitors until they have been completely discharged. Indeed, before touching any part of the apparatus make it a habit to short-circuit the spark gap with a yoke of wire supported at one end of an insulating rod about a foot long. (It is a good idea to cover the exposed high-voltage surfaces with sheets of Lucite.) Also keep in mind the fact that coherent energy, like sunlight, can be hazardous both in the direct beam and on a bounce as a specular reflection from a mirror or a smooth metal surface. Never project direct or reflected pulses into places where there may be people."

The accompanying map [above] charts the acidity of all rain that fell in the U.S. during the two-week interval from March 15 to March 31, 1973. It shows that heavy pollution acidifies substantially all rain that falls east of the Mississippi. More important, it dramatizes the kind and quality of data that can be gathered simultaneously throughout a vast area by enlisting the enthusiastic cooperation of grammar school students (16,000 of them in this case).

The study was conceived and organized by Aaron E. Klein, who is editor of Current Science, a weekly publication that reaches a million grammar school children. A safe, simple and inexpensive technique whereby youngsters can measure the acidity of freshly fallen rain within an accuracy of pH .2 was suggested by Walter Scott Houston, a staff member of Current Science. The study was directed locally by teachers. The results were forwarded to Frederic Godshall of the National Oceanic and Atmospheric Administration for processing by computer. The study appears to have set some kind of record for number of amateurs simultaneously involved in a scientific project. Certainly it demonstrates the worth of a national resource that ought not to be left to languish for want of appropriate projects.

Bibliography

A SIMPLE PULSED NITROGEN 3371 A LASER WITH A MODIFIED BEUMLEIN EXCITATION METHOD. J. G. Small and R. Ashari in The Review of Scientific Instruments, Vol. 43, No. 8, pages 1205-1206; August, 1972.

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