Document created: 2 September 03
Air University Review, May-June 1975

The Laser

its function and its future

Only time--and research and development--will tell the laser's future. What is certain, however, is that the laser already has been adapted to warfare and that predictable advances in the military applications of it are expanding.

Communication links having undreamed of data rates, laser radars (ladars) for ultraprecise tracking, and laser guidance systems for unparalleled accuracy of munitions delivery--these are only a few of many near-term military possibilities. To see where we are going in laser research and development, let's examine what lasers can and cannot do, so that we can more realistically evaluate their potential.

The laser name is an acronym for "Light Amplification by Stimulated Emission of Radiation," but technically the laser is an oscillator, not an amplifier; however, the "accurate" acronym was never adopted, for obvious reasons.

Actually, the laser is a generator of light, a very special kind of light that does not occur in nature without man’s help.¹ It is emitted in only one frequency (e.g., "red" for a ruby laser--Figure 1), and all the light waves are coherent, that is, the wave crests and troughs occur at the same place. (Figure 2) The single frequency or wavelength is referred to as monochromatic (single-color) light.

The way in which this unnatural ugh occurs is based on discoveries in atomic physics made during the 1920s. It was found that, on a very small scale, matter could absorb or radiate energy only in certain allowed amounts. The energy in a light wave depends only on the frequency of the light wave; therefore, only, allowed frequencies (or wavelengths) can be absorbed or radiated by atoms. This is why the light coming from lasers is radiated at such a constant frequency--only red from the ruby laser, for example. The coherent property of the light also depends on the small-scale behavior of matter.

Coherent light means that all light waves are "in step with each other." This is an important property of laser light and explains why it can transmit energy over great distances. This coherent light is produced in the laser by the "stimulated emission" part of the laser process. One light ray passing through the excited lasing material is the stimulus, and the light rays emitted by other excited atoms are generated in-step (coherently) because of the stimulus.

Lasers consist of a working material (either a solid, a liquid, or a gas), which does the actual lasing. The material is put into an excited condition just prior to the onset of laser action by a process most often referred to as "pumping." Typical pumping methods include flashlamp light, electrical discharge, chemical reaction, etc. Pumping adds energy to the lasing material to put it into an excited condition, also referred to as a condition of "inverted population."

For those lasers that "lase" by having an electron fall from the high-energy (excited) state to a lower-energy (stable) and thereby emit laser light (Figure 3), population inversion means that more electrons reside in the excited state than in the stable state. The excited electrons were put there by "pumping" them up there. Lasing can be started by a random electron falling from the excited to the stable state by the normal emission process. Stimulated emission or lasing commences for the other excited electrons as the light wave from the normal emission passes by.

As more light waves are emitted, the lasing process (stimulated emission) is accelerated. Mirrors, put on each end of the laser material, can further accelerate the process as each light wave is sent through the lasing material more than once. One mirror is partially transparent so that the light can escape and become the laser output. (Figure 4) The spacing between the mirrors is rather critical to the coherence property. An exact number of wavelengths must fit between the mirrors to retain coherence in the output beam.

The laser schematic shown in Figure 4 is a solid state laser (like a ruby rod) pumped by a flashlamp. The lasing material could also be a liquid or even a gas. Further, the output could consist of pulses of light or be emitted continuously, depending on the laser type and design. Whatever the laser type or lasing material employed, all lasers operate basically the same way. The lasing material is put into an excited state, and, in its returning to a stable state by stimulated emission, coherent light is emitted.

Lasers are being used extensively in the fields of measurement, manufacturing, medicine, communications, computation, and warfare. In many instances lasers have improved established ways of doing things, while in others they have introduced entirely new and unique capabilities. The science of measurement (metrology), for example, has been mark improved by the introduction of laser techniques. Scientists and engineers have used lasers to measure the characteristics of shock waves (Schlieren photography and holography²), to measure the extent of air pollution (with transmissometes), and to measure the unique characteristics of gases and plasmas (by spectroscopy). Distances have been measured with fantastic accuracy in laser range finder radars, altimeters, seismographs, and even space-time experiments, to verify the consequences of Einstein's theory relativity. So accurate is the laser that it has become the new basis for the standard of length and has been used better determine the velocity of itself. Lasers are routinely used for alignment in tunnel mining; ring lasers can measure rotation on inertial platforms; and Doppler lasers can measure the city of moving objects. These new measurement capabilities have also been extended to the manufacturing field.

Lasers are used in the manufacture of several advanced technology components. Electronic microcircuits can built and inspected for quality by use of techniques. Many metal parts with complex geometry have been cut, drilled, tided by use of raw laser power. Precision holes can be drilled in hard alloys and diamonds. With automatic lasers, welds have been made much more reliable, and gyroscope rotors have been dynamically balanced. Chemical compounds have even been modified by laser radiation. Manufacturing is continually finding new uses for lasers.

Medicine, too, has benefited enormously. Probably the best-known medical application of the laser is in eye surgery to repair detached retinas. Also used surgically as a bloodless knife, the laser light instantly cauterizes the cut. Small tumors in the eye may be cut out, wounds may be sutured, and small areas may be quickly disinfected. Dentists may soon employ a laser drill--painless, of course. Medical research into the very foundations of life is being pursued by selectively destroying minute cellular structures in cytoplasm to determine their individual functions. Thus, the laser has become an important new tool of geneticists.

In the field of communications, the laser offers significant advantages as a carrier of fantastic amounts of information. Because of the compactness of its beams, the laser information transmission beam can be made narrower than radio frequency systems, thereby concentrating the signal at the receiver terminal for more effective utilization. Due to the high frequency of light waves compared with radio waves, vastly greater data rates are possible. At the 1973 Air Force Association convention, a manufacturer showed that a laser beam could transmit seven TV channels simultaneously. High data rates make the possibility of using laser transmitters and receivers in data relay satellites very attractive.

The computer engineer, too, can apply optoelectronics and fiber optics in high-speed computer design. Light transmission is being investigated to determine just how fast computers can be made to operate. As in the measurement of shock waves by taking three-dimensional photographs called holograms, holograms can also be used as memory storage devices in computers. Although the stored information can only be read out and not modified (a new hologram must be constructed to change the memory content), the readout process is exceptionally fast. It is done by focusing a laser beam on the desired section of the hologram, and information is extracted in two (or three) dimensions (if phase information is used) rather than the one dimension available in modern-day computers. This parallel readout capability challenges computer engineers to find ways to use the vast capacity of holographic memories.

Military applications of lasers have only just begun but now are expanding. Perhaps the most publicized application has been laser-guided munitions. The idea of pointing a laser's narrow beam at a target (designation) and having a bomb home in on the target by sensing the reflected laser light (seeker) was applied in the Vietnam war with amazing success. Many families of laser-guided weapons were developed, including the initial Paveway laser-guided bomb system, the pod-mounted Pave Knife designator, -both the Pave Spike and the Long Knife pod-mounted follow-ons to the Pave Knife designator, Pave Storm fragmentation weapons, laser-guided artillery, and even laser-guided air-to-surface and surface-to-surface missiles. In "Operation Linebacker, air strikes were launched with surgical precision against key North Vietnamese military transportation and supply targets, many of which had not been previously attacked because of their proximity to dense population centers or civilian-oriented industries."³

Another system proved in Vietnam was used to acquire targets. Known as Pave Arrow, Pave Sword, and Pave Penny, laser seekers were pod-mounted on a variety of aircraft to acquire targets for either visual or automatic weapon delivery. On the ground, laser systems were used for a variety of other military applications like range finding, satellite tracking, "flashlights" for sniper-scopes, etc. Finally, in the area of communications systems, lasers are offering promise in line-of-sight communications and light radar (also called ladar).

Laser applications appearing on the horizon will be even more astounding. The search for new energy sources will depend quite heavily on lasers, as will medical research and new military systems. It may be possible in the near future to initiate the release of fusion energy by using high-energy laser beams. KMS Fusion, Inc., has a privately financed program to develop a system that does just that in a repetitive way, so that energy may be continuously extracted. If this effort bears fruit, and all indications are that it will, fusion reactors could be built to supply all the energy man would ever need, using a most plentiful resource, water, as the source of fuel. Although this approach appears to offer the greatest payoff mankind has ever had, if for some reason it does not work lasers may yet decrease energy costs by separating nuclear reactor isotopes more economically than present techniques do. Finally, lasers might someday transmit power across vast distances with little loss.

Through lasers, medical research can also be pushed into heretofore unknown realms. Recently it was discovered that laser radiation can alter the electrical conductivity of the blood, a discovery yielding--ultimately--only God knows what. But hopefully applications of this principle may lead finally to a cure for cancer. In any event, the laser has clearly given the medical researcher a new tool to make life better for all mankind.

Nuclear weapon development may involve the application of lasers.⁴ If developed, this technique would lead to really "clean" nuclear weapons, capable of being used without fear of radioactive contamination from fallout. Troops could be moved into attacked areas within a short time. These clean nuclear devices would also have peaceful applications, of course, such as digging canals or dredging harbors.

Much has been accomplished in a very few years, and the outlook is bright for numerous spin-offs to come. It is clear, in any case, that the laser is telling us again that the science fiction of the past sometimes does become the scientific fact of the future.

1.The description given here of how lasers work is an abbreviated version of that given in a previous Review article: "Lasers," C. Martin Stickley, Air University Quarterly Review, XIV, 3 (Summer 1963), 96-113.

2. Methods of recording the interference features of light waves. Shock waves bend light rays in such a way that boundary layers can be photographed. Holography records the phase of interfacing light waves, and a three-dimensional image can be reconstructed from this recording.

3. Keith E. Verble and Charles J. Malven, "Precision Laser Target Designation—A Breakthrough in Guided Weapons Employment," International Defense Review, no. 2, 1974, pp. 204-9.

4. Laser fusion research led to the possibility of developing a laser initiated thermonuclear weapon. Laboratory sources of nuclear radiation from laser fusion devices are also on the horizon.

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