Laser: The Light Fantastic

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The U. S. Navy is interested in lasers. Despite the understandable security classifications surrounding such interest, enough has been released in the open press to establish unequivocally that Navy research and development as well as operations people are either keeping a sharp eye on the rapidly progressing laser technology, or, they are sponsoring various research and development endeavors for the Navy.

For the Navy professional, the awareness of this level of service interest, combined with the continuing media reports of laser application potential, suggests that a discussion of laser "basics” will provide a useful introduction to a new and dynamic aspect of electro-optical science.

It all began in the late 1940s. At that time, the behavior of certain gases, such as ammonia (NH₄) was studied by various American physicists, notably Charles Townes and, as was learned later, by the Russians. These scientists concluded that, under the influence of "micro” radio waves, the dumbbell-shaped ammonia molecule could "vibrate” or resonate, and amplify microwaves, if certain physical conditions were met. This ammonia vapor was confined at low density in a tuned "cavity” and the microwave radio energy was "pumped” into the ammonia-filled resonator. Then, the ammonia molecule would oscillate in an exact, narrow, and fixed frequency band.

A reasonable acoustical analogy to this process is the action of an organ pipe under the influence of an air stream, with no intrinsic particular sound or tune, fed

across or into one end of the pipe. The air molecule in the pipe then oscillate, and set up a resonant column, which, depending on the size (and, to so®^c extent, shape) of the organ pipe, produce a pure tone The broad spectrum energy of the incoming air streaf has thus "reappeared” or been converted (in a manfl^£r of speaking) into audible narrow wavelength or p^utC sound energy. Townes called his device, first product in the early 1950s, by the acronym "maser” (Microwa'^c Amplification by the Stimulated Emission of Radi³' tion).

While the maser was an unqualified success as³ sensitive narrow-band detector for radio astronomy, also set Townes and other scientists to thinking abou¹ a maser which would work in the visible and infrared (IR) regions of the electromagnetic spectrum.

By 1957, Townes and his associates in the field bad done the basic theoretical work showing that intense narrow wave length or "single color” light could als’ be stimulated or produced in a crystal, such as a rub)'' if it were "pumped” externally by a flash lamp emitting broad spectrum white or blue light. The flash lamp light then would, like the maser’s microwaves, stimulate a "resonator” (now not an ammonia-filled cavity' but the crystal itself).

Sure enough, theory was soon followed by practitf when Theodore Maiman, at Hughes Aircraft Corporation in California, produced the first working mb light maser or "laser” (Light Amplification by Stimulated Emission of Radiation). Thus, in 1959, theory ha shown, and Maiman proved, that a ruby crystal rod--¹¹¹ his case only a couple of inches or so in length—^w*th one end partially silvered and the other fully silvered" would act as a "tuned cavity," which could drive tb^f ruby material to emit an intense, brief burst of monochromatic red light beam pulsing out from the sem¹ silvered end. The beam had also the unique proper'¹^ of being almost exactly "collimated”—i.e., practim . nonspreading, a process further improved by e^xtcrn lenses—and, coherent.  j

This latter property is the really unique feature

44 U. S. Naval Institute Proceedings, August 1973

the laser. Light from a household light bulb, a candle, the Sun, or any other source with which we are familiar, is incoherent—i.e., the light wave trains tumble upon each other in a random fashion. Let us draw two familiar analogies for a better grasp of this phenomenon. Tuning across a radio receiver’s band, one hears the hissing and roaring of the "noise” background produced by the myriad of natural or man-made electrical "noise-makers” such as lightning, sparks (as in cars), electric motors, etc. However, at regular intervals, the "pure” tones of a radio transmitter, such as music or speech are received. The radio emission of the transmitter is "continuous-wave” or coherent. In an acoustical comparison, we can say that a cupboard full of china, falling over, produces an "incoherent” noise, while our organ pipe or other musical instrument can produce a "pure” or coherent tone or sound.

While a whole host of optically active—i.e., "lasing”—substances have been discovered since the genesis of the first ruby crystal device in 1959, and though various techniques for "pumping” have been established, laser types can still be broadly classified into three major divisions:

►    Solid-state or "glass” lasers. While the ruby is still the favorite solid-state glass-like material producing a red light, other "glasses” or crystals available include sapphires, garnets, yttrium-alumina-garnet (YAG), tita- nates, etc. These lasers are usually operable only in a pulsed mode, although continuous (CW) operation has been attempted.

►    The Semi-conductor. A rather elegant scheme, without bulky flash-lamps or similar paraphernalia; essentially, a crystalline "quasi-metal” element (usually quite small—say about */₂ to % cm. long and % cm. diameter) such as gallium arsenide, specially "doped” with zinc, when exposed to electro-motive force (EMF) across the ends, emits an intense, coherent light beam from one end (each end is polished flat) or, from a point within the crystal near one end. Emissions are usually in the red and infrared regions, and power outputs arc generally much lower than those achievable with other laser types. Continuous operation is possible although, because of heat developed across the crystal, it is usually immersed in a coolant, such as liquid nitrogen.

►    The Gas Laser. A rapidly-expanding class of laser technology; various gases, such as the chemically inert ones—neon, argon, xenon, krypton were used initially, but after the first applications, in I960, other gases were found capable of lasing; at present almost all gases or vapors have been tried and found responsive to coherent light generation.

Water vapor and carbon dioxide lasers are among the latest family of very high-powered devices which can be operated continuously; however, gas lasers still

cannot approach the peak (pulsed) power of solid-state or glass lasers.

The principle of operation is fairly simple: a gas containing no chemical impurities is contained in a glass or other tube with a fully silvered mirror at one end and a partially silvered one at the other. A radio frequency oscillator "pumper” is coupled by concentric coils or electrodes, to the tube or, small electrodes protruding into the tube, one each near the ends, are connected to a high-voltage source. The gas is "excited,” and, if the optical and physical conditions are right, the atoms resonate (a simplified analog) and coherent visible or infrared radiation is emitted fro® the partially silvered end glass plate, which must be exactly parallel to the other end. The original neon laser needed a small amount of helium mixed with 1^f before lasing would occur. Also, early gas lasers emitted mainly in the IR. By 1972, very' many variations and improvements have resulted in hundreds of new type* of pumping schemes, gas media, and configurations'

The unique properties of a laser, then, are: it p^r0' duces extremely intense beams of visible and infrared coherent radiation in a very narrow waveband regio®

After Maiman’s announcement of the laser, science and science fiction writers, noting its properties, quickl) speculated about its future applications as a "death ray' About that time, the Defense Department began ^t0 take notice of the laser’s possible uses, although nob at least seriously, as a possible science fiction death ra)' It was quickly realized that the relatively low poW^er available (at that time) and atmospheric absorption and scattering suffered by laser beams would preclude "death ray” applications even for very short rang* until immensely more powerful devices were built.

The Office of Naval Research (ONR), which had actually supported early maser and laser research, (with out demanding military payoffs) was interested in—an continued its support for—laser activities, mostly fr^0(l1 the standpoint of improving ONR’s general inventoO of basic and applied research data, and for finding n£ ways and means of communication, both above (®\ perhaps below) the seas. Thus, the laser’s property' ⁰ particular interest was its wide-band communication capability.

The amount of information that can be carried b) an electromagnetic "carrier” (radio or laser beams) proportional to its frequency."¹ The laser signal, f°^r

•Thus, a 5-cm wavelength carrier has a frequency of 6 X 10³mc., a laser with a wavelength of 5,000 angstroms (5 X 10 ^scm.)

(1A° X 10 *cm.) has an approximate frequency of 10⁹mc. Making assumption that the information you wish to transmit demands i bandwidth of about 1-mc., it is evident that the laser signal an at least 1,000,000 (10®) more channels than the 5-cm., radio-wave earn

Laser: The Light Fantastic 45

Sample, can "carry” at least 1,000,000 more channels ’tan the standard radio-wave carrier. Moreover, the Mastic collimation, or narrow beam characteristics, ^lchievable in the optical regions with lenses and mirrors fuarantees communications between our ships, aircraft ^0t spacecraft with little risk of interception by an ^emy. Of course, smoke, haze, or fog can weaken or block even the most intense beam and especially at the ^!fl wavelengths although it is in those spectral regions ’tat the most powerful lasers have been developed on ’be last few years.

Communications by light beam is really an old story tat the Navy. Heliographs of sunlight signalling mir- ’ors supplemented signal flag communications from ships, (on sunny days only!) probably as far back as 'be Phoenicians. Then, came crude signalling lanterns, ^Iminating, by the late 19th century, in the familiar Aldis lamp. In all of these cases, communication was thieved by interrupting, with shutters, the light from ^a tamp, or by moving a reflected sunbeam in and out °f an observer’s field of view. This form of pulse communications is very limited in information-carrying ^ability because of mechanical shutter speed limitations; moreover, even if a pulse rate of, say 20-per-second were ^adiievable, the limited time response of the human eye tystem, about %₀ of a second, would restrict meaningful reception. After World War I, with the development of the photoelectric cell, or "electric eye,” at- ’empts were made in Europe and the United States to use this device, having almost unlimited pulse frequency response, as the receiver for intense searchlight beams, interrupted with electromechanical fast-vibrat- ^lng shutters, and later, with electro-optical shutters yielding pulse frequencies in the thousands of cycles fa second. However, these experiments came to ⁿ°ught, mainly because of excessive heat and light losses developed across the mechanical or electro-optical shutters.

The advent of the laser emission has changed all this, first, the "pure” or narrow wavelength distribution Permits operation upon the beam with little wasteful inversion into heat when electro-optical shutters or modulators are used. Secondly, the coherence of the laser beam permits "modulation” so that information may be carried.

Let us pause and briefly explain electro-optical "shut- ’ers” or modulators. A beam of light is an electromagnetic phenomenon consisting of clusters of radiant energy "photons.” These may be operated upon in pertain media, such as liquid nitro-benzene under the mfluence of electric fields to provide an electro-optical gate or shutter to interrupt the beam in controlled fences to convey information. The earliest and still most popular electro-optical gate or shutter is the "Kerr

Gate.” In its basic form it is an optically-flat glass cell, about % to 1 cm. across filled with an optically active medium, nitrobenzene. Two electrodes are immersed in this fluid and an electric field is established across them. When a light beam is polarized (such as with a Polaroid plate or sheet) and then passed through the "Kerr Cell,”f the plane of polarization is rotated in proportion to the amplitude and frequency of the electromotive force (EMF). A second polarizer, "crossed” (in terms of polarization angle) to the first, would normally block the light beam.

Interruption or modulation rates—i.e., "amplitude” modulation (AM) of the beam are possible up to frequencies of about 2 megacycles and higher with various solid electro-optically active crystals. This sort of amplitude modulation or impression of information on the "carrier” beam is used also in normal AM broadcasting. Laser beams, being coherent like radio waves, can also be "frequency” modulated, as in FM broadcasts. Only the continuous wave (CW) gas or solid-state lasers are readily capable of such modulation, by techniques too complicated to describe here. As in the FM radio broadcasting case, very broad undistorted transmission band- widths are possible.

It is possible, then, to consider the laser operating on the visible or the invisible infrared spectral regions as a powerful Navy communication tool. A laser re- ceiver/transmitter mounted on a controllable platform on a ship could be pointed at another ship, at a surfaced submersible—or one with only the laser transceiver above water—at an aircraft or spacecraft; or possibly, if the transceiver were mounted below the water line, at a submerged craft or diver.

For the last case, one might expect that sonar is the only practical and thoroughly established means of communication. This is almost true; however, as in the above-water case, the laser could provide a relatively inviolable communications link, unlike acoustic waves which can diverge or fan out in a most embarrassing fashion, in a water medium. Unfortunately for laser propagation, water is relatively opaque to most light wavelengths except in the blue-green regions. Therefore, an underwater laser, probably a gas (helium or argon or xenon) CW device would have to operate in those wavelengths where, because of the available lasing media’s physical limitations, it is difficult to obtain high transmission powers. Sediment or other turbidity-causing substances will sharply reduce laser penetration, just as will haze, smog, and clouds in the atmosphere. Hence, laser beam penetration, in relatively clear water, of about 1,000 feet would be quite an accom-

f "Polarized” light has its normally "omni" wave trains, constrained in one plane only.

46 U. S. Naval Institute Proceedings, August 1973

plishment, and no amount of power increase or wavelength tinkering would extend this range significantly. This limitation will probably relegate the laser to an ancillary role for most undersea naval requirements.

An interesting use for lasers as ocean bottom "profilers” or even airplane-submarine communication links, has been hinted at in various unclassified papers and articles. A laser transceiver mounted on a helicopter flying about 1,000 feet above the sea’s surface, would be pointed downwards, penetrate the water (if clear) and bounce off the bottom, returning to the "chopper” (or plane) to record an accurate contour of the bottom topography no matter what the aircraft’s speed might be. Assuming clear water, scanning through depths of 100-300 feet might well be achievable. By independently receiving the sea surface reflection (yes, this is possible with modern electronics even though the difference in arrival time of the two signals would be a fraction of a microsecond) a wave height and distribution profile could also be gotten. Such experiments would be of special interest to the Navy’s Oceanographic Office which has been a prime agent for developing interest and activities in physical oceanography. Navy and Coast Guard and Environmental Protection Agency oceanographers are also interested in ocean surface bio-luminescence which may indicate the presence of various organic substances such as plankton or fish oils, and, petroleum, phenol, and other pollutants. Such luminescence could be stimulated, even in the daytime, by laser illumination in the yellow or greenish regions and the luminescent (i.e., fluorescent) patches could be identified with low light level TV scanners, or photographic cameras.

Turning now to possible military/naval applications of lasers, the reader is undoubtedly aware that the popular press has, during the past decade and on regular occasions, made much of the "death ray” implications of the laser. Such speculation has been heightened during the past couple of years by public announcements by various companies, of very high-power laser development, with emissions in the far IR regions, and with C0₂ and water vapor being used as the emitting—i.e., lasing—medium. Similar claims have been made for various new types of glass or ruby, or solid- state emitters, and for the so-called laser amplifiers, wherein a cascade of laser emissions is produced by one laser "pumping” another until the final output of a series may be millions of times greater than that of the original laser generator.

Although tremendous power densities are available, permitting such spectacular demonstrations, at close range (say 50 meters) of melting steel and "burning” holes through metal and cement walls, atmospheric

absorption very quickly attenuates the laser energy- especially in the IR regions—to a harmless level. There seems to be no way in which this fundamental limitation can be circumvented, even if immensely powerful lasers were developed, at very great expense, for the visible optical regions, where atmospheric absorption is much reduced. In the unlikely and unfortunate case that warfare in the vacuum of space were to occur, then the science fiction scene of argosies of spacecraft battling and melting each other, at distances of thousands of miles, with laser rays, might well become a gruesome reality a few decades from now. In the meantime, an ocean-bound Navy mariner need have little concern about destructive laser attacks from land, ships, airplanes or spacecraft.

If a shipboard laser beam is aimed at a fairly distant target, even at the horizon, enough energy is usually reflected back, without any damage having been done to the target, to be detectable by a sensitive optical detector, using special photo-electric or "photomultiplier” tubes. Thus, since the light’s transit time is finite, the interval between emission and reception is easily measured (as with the airborne profilometer) yielding a distance measuring device. These "radar” lasers have, in fact, been built in the United States for various non-military uses in geophysics or surveying. One commercial device, called a "lidar,” attains distance measurement accuracies to within a centimeter over, say, a 20-km. range. Such lidars can also be mounted on naval aircraft to supplement other range/altimetry devices. While laser range-measuring techniques will not replace other traditional electromagnetic shortwave or micro- wave gear, they can usefully supplement such equipment where compactness, very high resolution, security, and fairly low power demands are needed. Once again, however, fog and heavy precipitation can seriously hamper effective operations.

In assessing the present and predictable state of laser technology, it is apparent that significant applications will be found within the repertoire of Navy Oceanographic Research projects.:): Some other uses may also accrue within the Navy’s Research Laboratories or maintenance facilities, with lasers being used as physics laboratory research tools for the former, and, possibly as a micro-welding devices for minute, precise welding applications for the latter.

For Fleet ship and aircraft operations, radar and communications above water—and, visibly, in a few selected situations, underwater, there will probably be major applications for the near future, where very private or secure links must be maintained, and where

| Navy Research scientists pioneered in the use of a maser as a sensitive detector for Radio Astronomy Antennae at the NRL.

Laser: The Light Fantastic 47

large amounts of data must be transmitted.

Beyond these general areas of laser applications in which the Navy might be interested, it is possible to discuss more specifically some Navy developments which have surfaced in unclassified meetings, hearings, reports, or other media.

As an indication of overall Navy activities, it appears that about 250 to 300 laser-related projects are either completed, current, or planned. As has been stated, communications and ocean surface or bottom pro- filometry are obviously suited for laser applications, and various classified projects cover these areas.

Since the Navy is, of course, very much interested and involved in guided missile operations, it is reasonable to conclude that, just as for the Air Force, electro- optical guidance, display, countermeasures and interception systems are part of the Navy research and development repertoire.

The major challenge faced by the Navy is probably that of Arctic-Ship missile interdiction and defense. Surface ships surely are lucrative targets for surface- to-surface and air-to-surface missiles (of which the Soviet Union may have an arsenal of over 11,000), and presently-contemplated anti-missile weapons and electronic countermeasures (ECM) need all the sophistication accruing from modern technology—and then some—if they are to be even reasonably effective against airborne hypersonic or ballistic missiles.

As stated earlier, the notion of a laser beam zapping an icbm out of an approach trajectory may seem far fetched—and it probably is. However, a slower-moving, trackable cruise missile may well be more vulnerable to laser interception, if, for example, its delicate aerodynamic balance were upset by super-heated air pockets,” or if its control surfaces were fused or otherwise damaged.

In 1970, published data estimated that about $75 million was spent that year by the three military services 2nd the DoD on laser-thermal-weapon research and development. It is only reasonable to conclude that the Navy is spending a sizable portion of this funding on ship-based thermal-weapon air defense systems, with the carbon-dioxide, high-energy laser as a possible prime contender.

Other Navy missile laser applications, and in an entirely different field, have been hinted at from time to time. For example, the Navy has already demonstrated the ability of the Bulldog air-to-ground missile—a Bullpup missile modified with a laser seeker head—to hit moving ground targets designated or illuminated” with a laser, by a forward air controller. Operational evaluation of this weapon, which has achieved remarkable accuracy, will continue to assess 'ts usefulness as a close support item for the Marines.

Undoubtedly, similar laser applications may be found for the Phoenix, Sidewinder, Sparrow, and Agile missiles.

Not only self-propelled guided missiles, but also bombs dropped from aircraft, can be directed with remarkable accuracy by laser guidance. Such "smart” bombs began to be used routinely in Vietnam in the spring of 1972. Most of them are modified World War II bombs weighing 500, 2,000, or 3,000 pounds. Special modification kits have been developed, resulting in an optical laser-seeker or sensor in the nose, and movable guide vanes in the rear.

In an operation, aircraft work in pairs—the first called the "designator,” shines a laser beam on the target. The second drops the seeker-equipped bomb. The bomb’s seeker senses the unique (i.e., coherent) laser energy reflected from the target. The target’s location data relative to the bomb are fed into a minicomputer within the bomb. The computer sends guidance instructions to the tail-mounted steering vanes. The designator or "Judas” plane must keep the target illuminated, even when evading hostile reaction; the crewman operating the laser beam must show considerable dexterity during the various evasive maneuvers his pilot may be taking.

A "smart” bomb may cost at least $5,000—ten times as much as a conventional one—but the damage it can do may be hundreds of times greater.

Several limitations to this system include cloud, dust or smoke (perhaps artificial) cover over the target, and the need for the bomb-dropping plane to be reasonably

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accurate in its aim at the outset, since the bomb, not having any propulsion, can deviate only so far from its "normal” ballistic drop course, to a target.

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Could such smart bombs be a threat to surface ships? This would depend, probably, on the technical sophistication of the intended smart bomb recipients. The crew of a primitive enemy’s coastal vessel, suspecting that they were an intended laser bomb target, would probably break all existing records for abandon ship. They would have no defense. A more sophisticated enemy, however, could well apply a variety of countermeasures once he sensed with ever-alert wide spectrum energy detectors that he was being illuminated. Of course, he would also detect the designator aircraft, the bomber, etc. Such fearsomely complicated laser sensors or "sniffers” could then, having established the designator’s laser frequency, emit an intense and spurious counter-signal saturating the incoming bomb’s detector. He might even be able to use his counter laser illuminator to destroy the bomb in mid-air.

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48 U. S. Naval Institute Proceedings, August 1973

A more far-out prospect is the use of spacecraft, either low altitude (200 n.m.) orbiters or synchronous stationary satellites (at 22,000 n.m.). These, by recognizing an enemy fleet, would act as designators, carrying batteries of laser illuminators for the precise terminal guidance of long-range hypersonic missiles. Far-fetched perhaps, but so was the "smart” bomb five years ago. A fleet commander may yet find a cloud deck his best ally.

Other unclassified releases have hinted at Navy testing of an 8-inch naval shell fitted with a laser-seeker for terminal guidance at targets 13,000 yards distant. Again, better-than-average impact accuracies were implied, although how the shell’s ballistic trajectory is modified in mid-course has not been revealed.

Those unique, most fearsome of all weapons in the Navy’s stock, the Polaris and Poseidon missiles, may also be fitted, some day, with inertial laser guidance systems. While space limitations forbid an explanation of a "laser inertial guidance,” it can be said that a laser beam, travelling within an optical (glass or reflecting metal) ring, or square or other closed path, can sense instantly, minute changes in its position or attitude (and therefore the missile’s attitude). An "error signal” can then send corrective data to the interpretation and

control system. Thus constrained, the laser beam acts like a mechanical gyroscope, but has enormously great® stability and sensitivity and much faster response.

Naturally, the Navy must be alert to—and, hopefully, aware of—a putative adversary’s laser activities so that appropriate ECMs can be developed. Neither we, nor an enemy can do much, if anything, against self- contained non-radiating systems, such as the inertial guidance systems, or a passive detector. However, it is known that the U. S. Navy is studying countermeasures against laser rangefinders (in which it is definitely interested) and warheads with laser acquisition devices. The TRW Systems Corporation has been developing high-powered, chemically-pumped, hydrogen- fluoride lasers radiating in the infrared 2.3-to-3 micron region, wherein most IR detectors have their peak response. Such a laser could "saturate” an enemy’s laser radiation sensors, just as spurious electromagnetic radiation can be used to confound enemy radar.

In summary, it can be said that the Navy probably spends around $30-40 million a year on laser research and development, much of it devoted to development of very-high-powered items (in the millions of watts per pulse) with gas or water vapor lasers surely in the technical lead. The U. S. Air Force, being interested in range-finding and "death-ray” weaponry, apparently exchanges technical data with the Navy in pertinent areas.

Since there will surely be a laser in the Navy’s technology inventory, it behooves both the involved technical individual and every professional to understand something of the physics, the capabilities and the limitations of this amazing manifestation of physical optics and electronics.

Educated in Toronto, Canada, and having served in the Royal Canadian Armoured Corps from 1941 to 1946, Mr. Stchling became a U S. citizen in 1950. He was employed as a physicist by the American Optical Company (1948 to 1950) and by the Bell Aircraft Corporation as rocket group leader (1950 to 1955) before joining the Naval Research Laboratory Senior Vanguard staff in 1955. In 1959, he transferred to NASA, serving there until 1965, when he joined Electro-Optical Systems Corporation/Xcrox. Since returning to government service in 1966, he has been science and technology advisor to several government groups. Author of four books, he has published numerous articles and papers on technology, space, and marine science