CONTROLLED EMISSION

Making and Viewing a Hologram

What we are getting at, of course, is the fact that the coherence of the laser beam permits it to be concentrated into a tiny area. Thus whatever total energy is being sent out by the laser can be concentrated to the point where its effective energy is tremendous. The sun emits some 6500 watts per square centimeter. Laser beams have already reached 500millionwatts per square centimeter.

But the power of the laser does not derive solely from its ability to be focused. Even an unfocused beam is several times more powerful than the sun’s output (per square centimeter).

Figure 13The typical hologram, looks like a geometric design, but it contains more information than would an ordinary photograph. Theimages below, made from a hologram, show the detail, apparent solidity, and parallax effect of the reconstructed light waves. The parallax effect is the ability to see around the objects just as one could if they were really there. (Seefrontispiece.)

Model tank

Tank, from another angle

The crucial difference between the sun’s light or any ordinary kind of light and laser light lies in the extent to which the emission of energy can be controlled. In the production of ordinary light the atoms, as we know, emit spontaneously, or in an uncontrolled fashion. But if the atoms could be forced to take in the proper amount of energy, store it, and release it when we wanted them to, we would havestimulated, rather than spontaneous, emission.

This, however, is practically the same as the amplification principle we discussed earlier. In that case, a small radio signal is jacked up into a large one by stimulating an available power source to release its energy at the same wavelength and in step with the smaller signal.

The question is, how can we do this with light?

The laser and its parent, the maser, can be traced back half a century to its theoretical beginnings. The great physicist Albert Einstein is most widely known for his work in relativity. But he did early and important work on that other gigantic 20th century scientific achievement, the quantum theory.[10]In one of his papers, published first in Zurich, Switzerland, in 1916, Einstein showed that controlled emission of light energy could be obtained from an atom under certain conditions. When an atom or molecule has somehow had its energy level raised, the release of this stored energy could be stimulated by subjecting the atom or molecule to a small “shot” of electromagnetic radiation of the proper frequency.

Einstein wrote that when such a photon of energy caused an electron to drop from a higher to a lower orbit, the electron would emit another photon of the same frequency and in the same direction as the one that hit it.[11]In other words, the energy of the emitted photon would be added to that of the photon that stimulated the emission in the first place. Here,potentially, was light amplification. The three major factors, absorption of energy, spontaneous emission, and stimulated emission are diagrammed inFigure 14.

There the matter lay for more than 30 years.

In 1951 Charles H. Townes, then on the Columbia University faculty, was interested in ways of extending to still higher frequencies the range of microwaves available for use in communications and in other scientific applications. Townes and other scientists who were interested in the problem were to meet in Washington, D. C., on the 26th of April. The night before the meeting he slept in a small Washington hotel; but he awoke at 5:30—pondering, pondering the high frequency problem.

He dressed and took a walk, then sat on a park bench and savored the beauty of azaleas in bloom. But all the while his mind was running over the various aspects of the problem.

Figure 14An atom can release absorbed energy spontaneously or it can be stimulated to do so.

Suddenly the answer came to him.

Normally more of the molecules in any substance are in low-energy states than in high ones. He would change the natural balance and create a situation with an abnormally large number of high-energy molecules. Then he would stimulate them to emit their energy by nudging them with microwaves. Here was amplification.

He could even take some of the emitted radiation and feed it back into the device to stimulate additional molecules, thereby creating an oscillator. Thisfeedbackarrangement, he knew, could be carried out in a cavity, which would resonate (just like an organ pipe) at the proper frequency. The resonator would be a box whose dimensions were comparable with the wavelength of the radiation, that is, a few centimeters on a side.

On the back of an envelope he figured out some of the basic requirements. Three years, and many experiments, later the maser (microwaveamplification bystimulatedemission ofradiation) was a reality. The original maser was a small metal box into which excited ammonia molecules were added. When microwaves were beamed into the excited ammonia the box emitted a pure, strong beam of high frequency microwaves, far more temporally coherent than any that had ever been achieved before. The output of an ammonia maser is stable to one part in 100 billion, making it an extremely accurate atomic “clock”.[12]Moreover, the amplifying properties of masers have been found to be very useful for magnifying faint radio signals from space, and for satellite communications.

Ammonia gas was chosen for the first maser because molecules of ammonia have two individual energy states that are separated by a gap corresponding in frequency to 23,870 megacycles (23,870 million cycles) per second. Ammonia molecules also react to a nonuniform electric field in ways that depend on their energy level: low-level molecules can be attracted and high-level ones repelled by the same field. Thus it is possible to separate the low-energy molecules from the high, and to get the excited molecules into the cavity without too much trouble.

This procedure for getting the majority of atoms or molecules in a container into a higher energy state, is calledpopulation inversionand is basic to the operation of both masers and lasers.

It should be noted that two Russians, N. G. Basov and A. M. Prokhorov, were working along similar lines independently of Townes. In 1952 they presented a paper at an All-Union (U.S.S.R.) Conference, in which they discussed the possibility of constructing a “molecular generator”, that is, a maser. Their proposal, first published in 1954, was in many respects similar to Townes’s. In 1955, Basov and Prokhorov discussed, in a short note, a new way to obtain the active atomic systems for a maser, a method that turned out to be of great importance.

Thus on October 29, 1964, the Nobel Prize in Physics was awarded, not only to Townes, but to Basov and Prokhorov as well. The award was for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the “aser” principle.

Following the maser development, there was much speculation about the possibility of extending the principle to the optical region. Indeed the first lasers—lightamplification bystimulatedemission ofradiation—were called “optical masers”.

The difficulty, of course, was that optical wavelengths are so tiny—about ¹/₁₀,₀₀₀ that of microwaves. The maser principle depended upon a physical resonator, a box a few centimeters (or even millimeters) in length. But at millimeter wavelengths, such resonators are already so small that they are hard to make accurately. Making a box ¹/₁,₀₀₀ that size was out of the question. Another approach was necessary.

In 1958 A. L. Schawlow of Bell Telephone Laboratories and Dr. Townes outlined the theory and proposed a structure for an optical maser. They suggested that resonance could be obtained by making the waves travel back and forth along a relatively long, thin column of amplifying substance that had parallel reflectors at the ends.

After their theory of the optical maser had been published, the race to build the first actual device began in earnest. The winner, in 1960, was Dr. T. H. Maiman, then with Hughes Aircraft Company. (He is now president of Maiman Associates.) The active substance he used was a single crystal of ruby, with the ends ground flat and silvered.

Ruby is an aluminum oxide in which a small fraction of the aluminum atoms in the molecular structure, or lattice, have been replaced with chromium atoms. These atoms absorb green and blue light and hence impart a red color to the ruby. The chromium atoms can be boosted from their ground state into excited states when they absorb the green or blue light. This process, by which population inversion is achieved, has been given the name pumping.[13]

Pumping in a crystal laser is generally achieved by placing the ruby rod within a spiral flash lamp (Figure 15) that operates like those used in high-speed (stroboscopic) photography. When the lamp is flashed, a bright beam of red light emerges from the ruby, shining out through one end, which has been only partially silvered.

Figure 15A ruby laser system.

The duration of this flash of red light is quite brief, lasting only some 300 millionths of a second, but it is very intense. In the early lasers, such a flash reached a peak power of some 10,000 watts.

When Maiman’s device was successfully built and operating, a public relations expert was called in to help introduce this revolutionary device to the world. He took one look at the laser and decided that it was too small and insignificant looking and would not photograph well. Looking around the lab, he spotted a larger laser and decided that that one was better.

Dr. Maiman informed him in his best scientific manner that laser action had not been achieved with that one. But the world of promotion won out, and Dr. Maiman allowed the larger device to be photographed on the assumption—or was it hope?—that he would be able to get it to operate in the future. (He did.)

The device shown inFigure 16is the true first laser. The all-important crystal rod is seen at the center. These crystals, incidentally, must be quite free of extraneous material; hence they are artificially “grown”, as shown inFigure 17. The single large crystal is formed as it is pulled slowly from the “melt”, after which it is ground to size and polished.

Figure 16Dr. Maiman’s first laser. Output was 10,000 watts.

Figure 17An exotic crystal of the garnet family is “grown” from a melt at a temperature of 3400°F.

Now we can begin to put together the various processes and equipment we have been discussing separately. Perhaps the best way to do this is to look again at the wordlaserand recall its meaning:lightamplification bystimulatedemission ofradiation. Our objective is to create a powerful, narrow, coherent beam of light. Let us see how to do this.

InFigure 18we imagine a laser crystal containing many atoms in the ground state (white dots) and a few in the excited state (black dots). Pumping light (wavy arrows ina) raises most of the atoms to the excited state, creating the required population inversion.

Figure 18Sequence of operations in a solid crystal laser. (a) Pumping light raises many atoms to excited state. (b) Lasing begins when a photon is spontaneously emitted along the axis of the crystal. This stimulates other atoms in its path to emit. (c) The resulting wave is reflected back and forth many times between the ends of the crystal and builds in intensity until finally it flashes out of the partially silvered end.

Lasingbegins when an excited atom spontaneously emits a photon parallel to the axis of the crystal (b). (Photons emitted in other directions merely pass out of the crystal.) The photon stimulates another atom in its path to contribute a second photon, in step, and in the same direction.

This process continues as the photons are reflected back and forth between the ends of the crystal. (We might think of lone soldiers falling into step with a column of marching men.) The beam builds up until, when amplification is great enough (c), it flashes out through the partially silvered mirror at the right—a narrow, parallel, concentrated, coherent beam of light, ready for....

Application of lasers can be divided into two broad categories: (1) commercial, industrial, military, and medical uses, and (2) scientific research. In the first case, lasers are used to do something that has been done in another way up to now (but not as well). Sometimes a laser solves a particular problem. For example, one of the first applications was in eye surgery, for “welding” a detached retina. The laser is particularly useful here because laser light can penetrate transparent objects such as the eye’s lens (Figure 19), eliminating the need to make a cut into the eye.

Figure 19Diagram of human eye showing laser beam focused on retina.

Surgeons have long wanted a better technique for treating extremely small areas of tissue. A laser beam, focused into a small spot, performs perfectly as a lilliputian surgical knife. An additional advantage is that the beam, being of such high intensity, can also sterilize or cauterize tissue as it cuts.

The narrowness of the laser beam has made it ideal for applications requiring accurate alignment. Perhaps the ultimate here is the 2-mile-long linear accelerator built by Stanford University for the United States Atomic Energy Commission. “Arrow-straight” would not have been nearly good enough to assure expected performance. A laser beam was the only technique that could accomplish the incredible task of keeping the ⅞ inch bore of the accelerator straight along its 2-mile length. A remote monitoring system, based on the same laser beam, tells operators when asection of the accelerator has shifted out of line (due for example to small earth movements) by more than about ¹/₃₂ inch—and identifies the section.[14]

Figure 20shows the 2-mile-long “klystron gallery” that generates the power for kicking the high-energy particles down the tube. The gallery parallels the accelerator housing and lies 25 feet beneath it (Figure 21). The large tube houses the optical alignment system and supports the smaller accelerator tube above. Target patterns dropped into the large tube at selected points produce an interference pattern at the far end of the tube similar to the one inFigure 13. Precise alignment of the tube is achieved by aiming the laser at the center dot of the pattern. Then the section that is out of line is physically moved until the dot appears in the proper place at the other end of the tube. It is the extreme coherence of the laser beam that makes this technique possible.

Having heard that laser light has bored through steel and is being used in microwelding, some have asked whether the laser will ever be used to weld bridge members and other structural girders. This is missing the whole point of the laser: It would be like washing your floor with a toothbrush (even one with extra stiff bristles)! There would be no advantage to using lasers for large-scale welding; present equipment for this operation is quite satisfactory and far less wasteful of input power. The sensible approach is to use lasers where existing processes leave something to be desired.

Until the advent of the laser, for example, there was no good way to weld wires 0.001 inch in diameter. Nor was there a good way to bore the tiny hole in a diamond that is used as a die for drawing such fine wire. It used to take 2 days to drill a single diamond. With laser light the operation takes 2 minutes—and there is no problem with rapid wear of a cutting tool.

So much for the first category of application. In the second category, namely use of the laser as a scientific tool, we enter a more theoretical domain. Here we usecoherent light as an extension of ourselves, to probe into and to look at the world around us.

Figure 20A laser beam was used (and continues to be used) for precise alignment of Stanford University’s 2-mile-long linear accelerator. This view shows the aboveground portion during construction.

Much experimental science is a matter of cooling, heating, grinding, squeezing, or otherwise abusing matter to see how it will react. With each new tool—ultrafast centrifuges, high- and low-pressure and extreme-temperature chambers, intense magnetic fields, atomic accelerators and so on—more has been learned about this still-puzzling world.

Since coherent light is something new, we can do things to matter that have not been done before, and see how it reacts. The laser is being used to investigate many problem areas in biology, chemistry, and physics. For example, sound waves of extremely high frequency can be generated in matter by subjecting it to laser light. These intense vibrations may have profound effects on materials.

Figure 21Subterranean view of Stanford accelerator housing. Alignment optics (laser systems) are housed in the large tube, which also acts as support for the smaller accelerator tube above it.

Figure 22Laser beam spot as observed at the end of the accelerator.

In the chemical field the sharp beam and monochromatic energy of the laser hold great promise in the exploration of molecular structure and the nature of chemical reactions. Chemical reactions usually are set off by heat, agitation, electricity, or other broadly applied means. None of these energizers allow the fine control that the laser beam does. Its extremely fine beam can be focused to a tiny spot, thus allowing chemical activity to be pinpointed. But there is a second advantage: The monochromaticity of coherent light also makes it possible to control the energy (in addition to the intensity) of the beam accurately by simply varying the wavelength. Thus it may be possible, for instance, to cause a reaction in one group of molecules and not in another.

One application in chemistry that holds great promise is the use of laser energy for causing specific chemical reactions such as those involved in the making of plastics. Bell Telephone Laboratory scientists have changed the styrene monomer (a “raw” plastic material) to its final state, polystyrene, in this way. The success of these and similar experiments elsewhere opens for exploration a vast area of molecular phenomena.

In another scientific application, the laser is being used more and more as a teaching tool. Coherence is a concept that formerly had to be demonstrated by diagrams, formulas, and inference from experiments. The laser makes it possible to see coherence “in action”, along with many of the physical effects that result from it. Such phenomena as diffraction, interference, the so-called Airy disc patterns, and spatial harmonics, always difficult to demonstrate to students in the abstract, can now be seen quite concretely.

Other interesting things can also be seen more plainly now. At the Los Alamos Scientific Laboratory, laser light is being used to “look” at plasmas; the result of one such look is shown inFigure 23. Plasmas are ionized gaseous mixtures. Their study lies at the heart of a constant search by atomic scientists for a self-sustained, controlled fusion reaction that can be used to provide useful thermonuclear power. This kind of reaction provides the almost unlimited energy in the sun and other stars. It is more efficient and releases less radioactivity than the other principal nuclearprocess, fission, which is used in atomic-electric power plants.[15]

Figure 23Shadowgraph of deuterium discharge taken in laser light. Turbulence of the plasma is clearly seen.

Westinghouse Electric Corporation scientists, on the other hand, have used the concentrated energy of the laser, not to look at, but toproducea plasma (Figure 24). They blasted an aluminum target the size of a pinhead with a laser beam, thereby vaporizing it and creating a plasma. The calculated temperature in the electrically charged gas was 3,000,000° centigrade. This is pretty hot, but still not hot enough for a thermonuclear reaction.

Figure 24Plasma heating by laser light.

The temperature of a plasma necessary to sustain a thermonuclear reaction is so high (above 10,000,000°C) that any material is vaporized instantly on coming into contact with it. The only means developed so far to contain the plasma is an intense magnetic field, or “magnetic bottle”; containment has been accomplished for only a few thousandths of a second at most. The objective of the Westinghouse research, which was supported by the Atomic Energy Commission, was to study in detail the interaction of the plasma with a magnetic field.

We do not have room to describe more applications in detail, but it may be interesting to list a few other uses of lasers—some commercial and some still experimental:

Figure 25Twenty-two caliber bullet and its shock wave are photographed from the image produced by a doubly exposed laser hologram. The original hologram was exposed twice by a ruby laser within half a thousandth of a second as the bullet sped past at 2½ times the speed of sound.

It is almost self-evident that no single device, even one as incredible as the laser, could accomplish all the feats mentioned in the preceding paragraphs. After all, some of these applications require high power but not extremely high monochromaticity, while in others the reverse may be true. Yet, by its very nature, any laser produces a beam with one, or at the most a few, wavelengths, and many different materials would be needed to provide the many different wavelengths required for all the tasks listed.

Also, the first laser was a pulsed device. Light energy was pumped in and a bullet of energy emerged from it. Then the whole process had to be repeated. Pulsed operation is fine for spot-welding and for applications such as radar-type rangefinding, where pulses of energy are normally used anyway. With lasers smaller objects can be detected than when using the usual microwaves. But a pulsed process is not useful for communications. In other words, pulsing is good for certain applications but not for others.

And of course solid crystals are difficult to manufacture. Hence, it was natural for laser pioneers to look hopefully at gases. Gas lasers would be easier to make—simply fill a glass tube with the proper gas and seal it.

But other advantages would accrue. For one thing the relatively sparse population of emitting atoms in a gas provides an almost ideally homogeneous medium. That is, the emitting atoms (corresponding to chromium in the ruby crystal) are not “contaminated” by the lattice or host atoms. Since only active atoms need be used, the frequency coherence of a gas laser would probably be even better than that of the crystal laser, they reasoned.

It was less than a year after the development of the ruby laser that Ali Javan of Bell Telephone Laboratories proposed a gas laser employing a mixture of helium and neon gases. This was an ingeniously contrived partnership whereby one gas did the energizing and the other did the amplifying. Gas lasers now utilize many different gases for different wavelength outputs and powers and provide the “purest” light of all. An additional advantage is thatthe optical pumping light could be dispensed with. An input of radio waves of the proper frequency did the job very nicely.

But most significant of all, Javan’s gas laser provided the first continuous output. This is commonly referred to as CW (continuous wave) operation. The distinction between pulsed and CW operation is like the difference between baking one loaf of bread at a time and putting the ingredients in one end of a baking machine and having a continuous loaf emerge at the other.

When a non-expert thinks of a laser, he is apt to think of power—blinding flashes of energy—as illustrated inFigure 26. As we know, this is only a small part of the capability of the laser. Nevertheless, since lasers are often specified in terms of power output it may be well to discuss this aspect.

The two units generally used arejoulesandwatts. You are familiar with a watt and have an idea of its magnitude: think, for example, of a 15-watt or a 150-watt bulb. A watt is a unit ofpower; it is the rate at which (electrical) work is being done.

Figure 26High power is demonstrated as a laser beam blasts through metal chain.

The joule is a unit ofenergyand can be thought of as the total capacity to do work. One joule is equivalent to 1 watt-second, or 1 watt applied for 1 second. But it can also mean a 10-watt burst of laser light lasting 0.1 second, or a billion watts lasting a billionth of a second.

In general, the crystal (ruby) lasers are the most powerful, although other recently introduced materials, such as liquids (seeFigure 27) and specially prepared glass, are providing competition. With proper auxiliary equipment, bursts of severalbillionwatts have been achieved; but the burst lasts only about 100 millionths of a second. For certain uses, that’s just what is wanted: a highly concentrated burst of energy that does its work without giving the material being “shot” a chance to heat up and spread the energy, perhaps damaging adjacent areas.

Figure 27Active substance for a modern liquid laser is made in an uncomplicated 10-minute procedure. Bluish powder of the rare earth, neodymium, is dissolved in a solution of selenium oxychloride and sealed in a glass tube.

Since the joule gives a measure of the total energy in a laser burst it is not applicable to CW output. Power in this area began low—in the milliwatt (one thousandth of a watt) region—but has been creeping up steadily. A recent gas laser utilizing carbon dioxide has already reached 550 watts of continuous infrared radiation. This is the giant 44-footer shown inFigure 28. An advantage of gas (and liquid) lasers is that they can be made just about as large as one wishes. By way of comparison, the smallest gas laser in use is shown inFigure 29.

Figure 28A giant 44-foot gas laser produces 550 watts of continuous power and is expected to reach 1000 watts. Glowing of the tube comes from gas discharge, not from laser light, which is in the infrared region and cannot be seen.

One of the least satisfactory aspects of the laser has been its notoriously low efficiency. For a while the best that could be accomplished was about 1%. That is, a hundred watts of light had to be put in to get 1 watt of coherent light out. In gas lasers the efficiency was even lower, ranging from 0.01% to 0.1%.

In gas lasers this was no great problem since high power was not the objective. But with the high-power solid lasers, pumping power could be a major undertaking. A high-powerlaser pump built by Westinghouse Research Laboratories handles 70,000 joules. In more familiar terms, the peak power input while the pump is on is about 100,000,000 watts. For a brief instant this is roughly equal to all the electrical power needs of a city of 100,000 people.

Two relatively new developments have changed the efficiency levels. One, the carbon dioxide gas laser, is quite efficient, with the figure having passed 15%. The second is the injection, or semiconductor laser, in which efficiencies of more than 40% have been obtained. Unless unforeseen difficulties arise this figure is expected to continue to rise to a theoretical maximum of close to 100%.

Figure 29A miniature gas laser produces continuous output in visible red region.

The semiconductor laser is to solid and gas lasers what the transistor was to the vacuum tube; all the functions of the laser have been packed into a tiny semiconductor crystal. In this case, electrons and “holes” (vacancies in the crystal structure that act like positive charges) accomplish the job done by excited atoms in the other types. That is, when they are stimulated they fall from upper energy states to lower ones, and emit coherent radiation in the process. Aside from this the principle of operation is the same.

The device itself, however, is vastly different. For one thing it is about the size of this letter “o” (Figure 30). For another, it is self-contained; since it can convert electric current directly into laser light—the first time this has been possible—an external pumping source is not required. This makes it possible to modulate the beam by simply modulating the current. (A different approach has been to modulate a magnetic field around the device. This, it turns out, can also be done with some newer solid crystal lasers.)

An additional advantage offered by the semiconductor laser is simplicity. There are no gases or liquids to deal with, no glassware to break, and no mirrors to align. Although it will not deliver high power, it can already deliver enough CW power for certain communications purposes. Its simplicity, efficiency, and light weight make it ideal for use in space.

Figure 30A tiny injection laser works in infrared region. The beam is visible because photo was taken with infrared film. The laser itself is a tiny crystal of gallium arsenide inside the metal mount being held between the fingers.

Future deep space missions are expected to require extremely high data transmission rates (on the order of a million bits[16]per second) to relay the huge quantities of scientific and engineering information gathered by the spacecraft. Higher data rates are necessary to increase both the total capacity and the speed of transmission. By comparison, the Mariner-4 spacecraft that sent back TV pictures of Mars had a data rate of only eight bits per second—a hundred thousand times too small for future missions. The use of lasers would mean that results could be transmitted to earth in seconds instead of the 8 hours it took for the photos to be sent from Mariner-4.

One of the problems to be solved in using lasers for deep space communication, oddly enough, is that of pointing accuracy. Since the beam of laser energy is narrow, it would be possible for the radiation to miss the earth altogether and be lost entirely unless the laser were pointed at the receiver with extreme precision. Aiming a gun at a target 50 yards away is one thing; aiming a laser from an unmanned spacecraft 100 million miles away is quite another. It is believed, however, that present techniques can cope with the problem.

Another peculiarity of laser communication is that it will probably be accomplished faster and more readily in space than here on earth. Powerful though laser light may be, it is light and is therefore impeded to some extent by our atmosphere even under good conditions. Data transmissions of 20 and 30 miles have already been accomplished in good weather with lasers.

But if you have ever tried to force a searchlight beam or shine automobile headlights through heavy fog, rain, or snow, you will appreciate the magnitude of the problem under these conditions. The use of infrared frequencies helps to some extent, since infrared is somewhat more penetrating, but the poor-weather problem is a serious one.

A possible solution is the use of “light pipes”, similar to the wave guides already in use for certain microwave applications over short distances. But as often happens, new developments create new needs; how, for example, can we get the laser beam to stay centered in the pipe and follow curves? A series of closely spaced lenses, about 1000 per mile, probably would accomplish this, but too much light would be lost by scattering from the many lens surfaces.

Scientists are experimenting with a new kind of “lens”, one that uses variations in the density of gases to focus and guide the beam automatically. Since there are no surfaces in the path of the light beam, and since the gas is transparent and free of turbulence, the laser beam is not appreciably weakened or scattered as it travels through the pipe.

Figure 31Laser light beam being guided through a “light pipe” by a gas “lens”. Heating coil (lower left) or mixture of gases (lower right) are two possible ways of maintaining proper density gradient in the gas.

Figure 31shows how the gas focusing principle might be used to guide a beam through a curving pipe. The shading represents the density of the gas. Several means have been developed to keep the gas denser in the center thanaround the outside. When the pipe curves, the light beam starts moving off the axis of the pipe. The gas then acts like a prism, deflecting the light beam in the direction of the curvature of the “prism”.

In communication between distant space and earth, a light pipe might be a little cumbersome; hence it may prove necessary to set up an intermediate orbiting relay station that will, particularly in cases of poor weather, intercept the incoming laser beam and convert it to radio frequencies that can penetrate our atmosphere with greater reliability.

Powering space-borne lasers will, of course, be a problem. Indeed one of the major unsolved problems in production of spacecraft and long-term satellites is the provision of an adequate supply of power. Fuel cells and solar cells have helped but do not give the whole answer.[17]

One other approach has already been developed: a sun-pumped laser. Sunlight focused onto the side of the laser (seeFigure 32) provides the pumping power, enabling the device to put out 1 watt of continuous infrared radiation, enough for special space applications. Descendents of this device could produce visible light if this is deemed desirable.

Another approach, usingchemical lasers, is even more intriguing and may have greater consequences. Chemical lasers will derive their energy from their internal chemistry rather than from the outside. A mixture of two chemicals may be all that is needed to initiate laser action aboard a spacecraft or satellite. (Chemical lasers also offer the promise of even greater concentrations of power than have been achieved heretofore, which may make them useful in plasma research.)

With all these possibilities, it may still be that spacecraft will need more power than is available on board. The narrow beam of the laser offers one more fascinating possibility, especially in the case of satellites relatively near earth. The light of a laser might actually be used to beam energy to a receiver, either for immediate use orstorage. It would then become possible to “refuel” satellites at will, giving them much greater capabilities.

If available laser power is great enough, laser beams might even be used to push satellites back into their proper orbits when they begin to wander off course, as they almost invariably do after a while.

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