Chapter II: Part 2

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.

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 31 shows 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 than around 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 (see Figure 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, using _chemical 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 or storage. 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.

Sun
Parabolic Collector
Hyperbolic-cylindric secondary mirror
Semi-circular-cylindric tertiary mirror
Laser beam

A LASER IN YOUR FUTURE?

Atomic energy, only a scientific dream a few short years ago, is now providing needed power in many parts of the world. In the same way, the laser, also an atomic phenomenon, has made its way out of the laboratory and into the fields of medicine, commerce, and industry. If it hasn’t touched your life as yet, you need only be patient. It will.

Indeed the most exciting probability of all is that lasers undoubtedly will change our lives in ways we cannot even conceive of now.

SUGGESTED REFERENCES

Books

_ABC’s of Masers and Lasers_, Allan H. Lytel, Howard W. Sams and
Company, Inc., Publishers, Indianapolis, Indiana 46206, 1966,
96 pp., $2.25.
_The Laser: Light That Never Was Before_, Ben Patrusky, Dodd, Mead and
Company, New York 10016, 1966, 128 pp., $3.50.
_Masers and Lasers_, Manfred Brotherton, McGraw-Hill Book Company, New
York 10036, 1964, 224 pp., $8.50.
_Masers and Lasers_, H. Arthur Klein, J. B. Lippincott Company,
Philadelphia, Pennsylvania 19105, 1963, 184 pp., $3.95.
_The Story of the Laser_, John M. Carroll, E. P. Dutton and Company,
Inc., New York 10003, 1964, 181 pp., $3.95.
_Quantum Electronics: The Fundamentals of Transistors and Lasers_,
John R. Pierce, Doubleday and Company, Inc., New York 10017,
1966, 138 pp., $1.25.
_Lasers and Their Applications_, Kurt R. Stehling, The World
Publishing Company, Cleveland, Ohio 44102, 1966, 192 pp.,
$6.00.
_Understanding Lasers and Masers_, Stanley Leinwoll, Hayden Book
Companies, New York 10011, 1964, 96 pp., $1.95.
_Atomic Light: Lasers_, Richard B. Nehrich, Jr., Glenn I. Voran, and
Norman F. Dessel, Sterling Publishing Company, Inc., New York
10016, 1967, 136 pp., $3.95.

Articles—General and Historical

Advances in Optical Masers, A. L. Schawlow, _Scientific American_,
209: 34 (July 1963).
The Evolution of the Physicist’s Picture of Matter, P. A. M. Dirac,
_Scientific American_, 208: 45 (May 1963).
Filling in the Blanks in the Laser’s Spectrum, F. M. Johnson,
_Electronics_, 39: 82 (April 18, 1966).
The Amateur Scientist—How a persevering amateur can build a gas laser
in the home, C. L. Stong, _Scientific American_, 211: 227
(September 1964).
The Amateur Scientist—Homemade Laser, C. L. Stong, _Scientific
American_, 213: 108 (December 1965).
The Amateur Scientist—How to make holograms and experiment with them
or with ready-made holograms, C. L. Stong, _Scientific
American_, 216: 122 (February 1967).
The Maser, James P. Gordon, _Scientific American_, 199: 42 (December
1958).
The Quantum Theory: Early Years to 1923, Karl Darrow, _Scientific
American_, 186: 47 (March 1952).
Laser’s Bright Magic, T. Meloy, _National Geographic Magazine_, 130:
858 (December 1966).
Infrared and Optical Masers (original paper), A. L. Schawlow and C. H.
Townes, _Physical Review_, 112: 1940 (December 15, 1958).
Laser Market Enters Era of Practicality, W. Mathews, _Electronic
News_, 11: 1 (April 18, 1966).
Lasers, A. K. Levine, _American Scientist_, 51: 14 (March 1963).
Lasers, A. L. Schawlow, _Science_, 149: 13 (July 2, 1965).
Lasers and Coherent Light, A. L. Schawlow, _Physics Today_, 17: 28
(January 1964).
The Laser’s Dazzling Future, L. Lessing, _Fortune_, 67: 138 (June
1963).
Optical Masers, A. L. Schawlow, _Scientific American_, 204: 52 (June
1961).
Optical Pumping, A. L. Bloom, _Scientific American_, 202: 72 (October
1960).
Research on Maser-Laser Principle Wins Nobel Prize in Physics, J. P.
Gordon, _Science_, 146: 897 (November 13, 1964).
Resource Letter MOP-1 on Masers (Microwave through Optical) and on
Optical Pumping, H. W. Moos, _American Journal of Physics_,
32: 589 (August 1964), extensive bibliography. Available from
American Institute of Physics, 335 East 45th Street, New York
10017. Enclose stamped return envelope.
Advances in Holography, K. S. Pennington, _Scientific American_, 218:
40 (February 1968).
Applications of Laser Light, D. R. Herriott, _Scientific American_,
219: 140 (September 1968).
Holography for the Sophomore Laboratory, R. H. Webb, _American Journal
of Physics_, 36: 62 (January 1968).
Laser Light, A. L. Schawlow, _Scientific American_, 219: 120
(September 1968).
The Modulation of Laser Light, D. F. Nelson, _Scientific American_,
218: 17 (June 1968).

Articles—Special Subjects

Biological Effects of High Peak Power Radiation, S. Fine et al., _Life
Sciences_, 3: 209 (1964).
The Interaction of Light with Light, J. A. Giordmaine, _Scientific
American_, 210: 38 (April 1964).
Chemical Lasers, George C. Pimental, _Scientific American_, 214: 32
(April 1966).
Color Laser Stores Data, J. Eberhart, _Science News_, 90: 51 (July 23,
1966).
Communication by Laser, Stewart E. Miller, _Scientific American_, 214:
19 (January 1966).
Guidelines for Selecting Laser Materials, R. H. Hoskins, _Electronic
Design_, 13: _M_29 (July 19, 1965).
Holography: The Picture Looks Good, J. Blum, _Electronics_, 39: 139
(April 18, 1966).
How Dangerous Are Lasers?, L. H. Dulberger, _Electronics_, 35: 27
(January 26, 1962).
Injection Lasers, R. W. Keyes, _Industrial Research_, 6: 46 (October
1964).
Laser Potential in Deep-Space Link Grows, B. Miller, _Aviation Week
and Space Technology_, 84: 71 (January 31, 1966).
Laser Retinal Photocoagulator, N. S. Kapany et al., _Applied Optics_,
4: 517 (May 1965).
Laser Welding in Electronic Circuit Fabrication, J. P. Epperson,
_Electrical Design News_ (EDN), 10: 8 (October 1965).
The Light That Slices Inch into Millionths, (use of interferometry in
industry), _Steel_, 158: 38 (February 28, 1966).
The Optical Heterodyne—Key to Advanced Space Signaling, S. Jacobs,
_Electronics_, 36: 29 (July 12, 1963).
Photography by Laser, E. N. Leith and J. Upatnieks, _Scientific
American_, 212: 24 (June 1965).
Liquid Lasers, Alexander Lempicki and Harold Samelson, _Scientific
American_, 216: 81 (June 1967).
Plasma Experiments with a 570-kJ Theta-Pinch, F. C. Yahoda, et al.,
_Journal of Applied Physics_, 35: 2351 (August 1964).
A Sun-Pumped CW One-Watt Laser, C. G. Young, _Applied Optics_, 5: 993
(June 1966).
3-D Image Made at Home, _Science News_, 90: 185 (10 September 1966).
Scanning with Lasers, Robert A. Myers, _International Science and
Technology_, 65: 40 (May 1967).

Booklets

_Applications of Lasers to Information Handling_, The Perkin-Elmer
Corporation, Norwalk, Connecticut 06852, 1966, 32 pp., free.
Reprint of five talks given by company personnel.
_Laser Interferometer_, Airborne Instruments Laboratory, Division of
Cutler-Hammer, Inc., Deer Park, Long Island, New York 11729,
1965, 20 pp., free. Collection of article reprints.
_Laser: The New Light_, Bell Telephone Laboratories, Murray Hill, New
Jersey 07971, 19 pp., free. Full color, nontechnical brochure
presents some background, principles, and applications of the
laser.

FOOTNOTES

[1]Sometimes referred to as _hertz_ (abbreviated Hz), for the 19th
Century German physicist Heinrich Hertz; 1000 Hz = 1000 cps.

[2]Devised in France and officially adopted there in 1799, the metric
system uses the meter as the basic unit of length and has been
proposed for all measurements in this country.

[3]Named for the Swedish physicist Anders J. Angstrom.

[4]The wavelength, indicated by the Greek letter λ (lambda) is related
to frequency (f) in the proportion λ (in meters) = 300,000,000/f.
(The number 300,000,000 is the velocity of light in meters per
second.)

[5]Microwaves are radio waves with frequencies above 1000 megacycles per
second.

[6]Ten to 30,000,000 kilocycles per second; this is low in the
electromagnetic spectrum, but not low in terms of the radio
spectrum, which has a low-frequency classification of its own.

[7]Primitive as early radios were by today’s standards, they brought a
new era to communication at the time. Unmodulated CW (continuous
wave) transmissions and crystal receivers were used to summon
rescuers in the _Titanic_ disaster of 1912, for example.

[8]Energy = h (Planck’s constant) × frequency. Planck’s constant is the
energy of 1 quantum of radiation, and equals 6.62556 × 10⁻²⁷
erg-sec.

[9]Each photon carries 1 _quantum_ of radiation energy, which is a unit
equal to the product of the radiation frequency and Planck’s
constant (see footnote page 15).

[10]Einstein was awarded the Nobel Prize in 1921 for his 1905
explanation of the photoelectric effect (in terms of quanta of
energy) and _not_ for his relativity theory.

[11]Einstein’s theoretical explanation applies in the case of
stimulation of a single atom. In practical stimulation,
directionality is enhanced by stimulating many atoms in phase.

[12]An atomic clock is a device that uses the extremely fast vibrations
of molecules or atomic nuclei to measure time. These vibrations
remain constant with time, consequently short intervals can be
measured with much higher precision than by mechanical or electrical
clocks.

[13]The 1966 Nobel Prize in Physics was awarded to Prof. Alfred Kastler
of the University of Paris for his research on optical pumping and
studies on the energy levels of atoms.

[14]See _Accelerators_, a companion booklet in this series, for a full
account of the Stanford “Atom Smasher”.

[15]For descriptions of fission and fusion processes, see _Controlled
Nuclear Fusion_, _Nuclear Reactors_, and _Nuclear Power Plants_,
other booklets in this series.

[16]A bit is a digit, or unit of information, in the binary
(base-of-two) system used in electronic data transmission systems.

[17]See _SNAP_, _Nuclear Space Reactors_ and _Power from Radioisotopes_,
other booklets in this series, for descriptions of nuclear sources
of power for space.

This booklet is one of the “Understanding the Atom” Series. Comments are invited on this booklet and others in the series; please send them to the Division of Technical Information, U. S. Atomic Energy Commission, Washington, D. C. 20545.

Published as part of the AEC’s educational assistance program, the series includes these titles:

_Accelerators_
_Animals in Atomic Research_
_Atomic Fuel_
_Atomic Power Safety_
_Atoms at the Science Fair_
_Atoms in Agriculture_
_Atoms, Nature, and Man_
_Books on Atomic Energy for Adults and Children_
_Careers in Atomic Energy_
_Computers_
_Controlled Nuclear Fusion_
_Cryogenics, The Uncommon Cold_
_Direct Conversion of Energy_
_Fallout From Nuclear Tests_
_Food Preservation by Irradiation_
_Genetic Effects of Radiation_
_Index to the UAS Series_
_Lasers_
_Microstructure of Matter_
_Neutron Activation Analysis_
_Nondestructive Testing_
_Nuclear Clocks_
_Nuclear Energy for Desalting_
_Nuclear Power and Merchant Shipping_
_Nuclear Power Plants_
_Nuclear Propulsion for Space_
_Nuclear Reactors_
_Nuclear Terms, A Brief Glossary_
_Our Atomic World_
_Plowshare_
_Plutonium_
_Power from Radioisotopes_
_Power Reactors in Small Packages_
_Radioactive Wastes_
_Radioisotopes and Life Processes_
_Radioisotopes in Industry_
_Radioisotopes in Medicine_
_Rare Earths_
_Research Reactors_
_SNAP, Nuclear Space Reactors_
_Sources of Nuclear Fuel_
_Space Radiation_
_Spectroscopy_
_Synthetic Transuranium Elements_
_The Atom and the Ocean_
_The Chemistry of the Noble Gases_
_The Elusive Neutrino_
_The First Reactor_
_The Natural Radiation Environment_
_Whole Body Counters_
_Your Body and Radiation_

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USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE 37830

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Transcriber’s Notes

—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by
_underscores_.