Part 3

Ruby Flash lamp Partially silvered end Laser output Power Cooling

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 in Figure 16 is 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 in Figure 17. The single large crystal is formed as it is pulled slowly from the “melt”, after which it is ground to size and polished.

LASING—A NEW WORD

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 word _laser_ and recall its meaning: _l_ight _a_mplification by _s_timulated _e_mission of _r_adiation. Our objective is to create a powerful, narrow, coherent beam of light. Let us see how to do this.

In Figure 18 we 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 in _a_) raises most of the atoms to the excited state, creating the required population inversion.

(a) Ruby crystal Pumping light Atom in ground state Excited atom Partial reflecting mirror Full reflecting mirror (b) Excited atom emits photon parallel to axis (c)

_Lasing_ begins 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....

SOME INTERESTING APPLICATIONS

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.

Cornea Lens Optic Nerve Beam angle Fovea centralis Iris Image 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 a section 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 20 shows 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 in Figure 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 use coherent light as an extension of ourselves, to probe into and to look at the world around us.

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.

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 in Figure 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 nuclear process, fission, which is used in atomic-electric power plants.[15]

Westinghouse Electric Corporation scientists, on the other hand, have used the concentrated energy of the laser, not to look at, but to _produce_ a 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.

Diamagnetic loop Laser beam Vacuum chamber Magnetic field Magnetic coils Electrostatic probe Plasma Lens Mirror To vacuum pump Camera

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:

—Earthquake prediction.

—Measurement of “tides” in the earth’s crust under the sea.

—Laser gyroscopes.

—Highly accurate velocity measurement (useful in certain assembly line and continuous manufacturing processes).

—Scanner for analyzing photographs of bubble chamber tracks and astronomical phenomena.

—Computer output and storage systems; perhaps even complete optical data processing systems.

—Lightning-fast printing devices.

—High-speed photography (Figure 25).

—Missile tracking and accurate alignment of antennas.

—Automatic flaw spotter for big radio antennas.

—Aircraft landing aid for poor weather conditions.

—Fast, painless dental drill.

—Cancer research.

A MULTITUDE OF LASERS

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 that the 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 in Figure 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 are _joules_ and _watts_. 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 of _power_; it is the rate at which (electrical) work is being done.

The joule is a unit of _energy_ and 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 (see Figure 27) and specially prepared glass, are providing competition. With proper auxiliary equipment, bursts of several _billion_ watts 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.

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 in Figure 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 in Figure 29.

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-power laser 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%.

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.

COMMUNICATIONS

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.