 What light is more than a million times brighter than the sun, within its narrow cone and frequency band? What light can hit the moon and the planets far beyond and can be seen by the human eye 600 billion miles away? What light promises to have thousands of times the capacity of our overwork radio channels? What light can be focused to a point less than one millionth of the size of a pinhead? This fantastic light, which can perform miracles, comes from an instrument called laser. The word laser is coined from the first letters of light amplification by stimulated emission of radiation. In a sense, it is an extension of maser or microwave amplification by stimulated emission of radiation, invented by Dr. C.H. Townes in 1953, when he was working under a joint services contract administered by the Signal Corps. What appeared to be nothing more than an obstruced scientific device then is now acclaimed a first-rate contribution to science and technology in this century. Its important implications in scientific, industrial and military applications lead us into asking many questions. What is laser? How does it work? Where does it fit in the modern army today? And where might it be in the army of tomorrow? First, laser is a device which generates coherent and monochromatic light. By coherent, we mean its light waves are in step or in phase, like perfectly drilled soldiers marching past the reviewing stand. By monochromatic, we mean that the light waves are nearly limited to one wavelength or frequency, just as all the soldiers are identical and marching in step at a constant pace. No other light in the world, whether from the sun or from lamps, is coherent and monochromatic. For ordinary light is generated by atoms excited in a random pattern at various wavelengths and at irregular intervals like an unruly crowd. Every atom has energy, which is derived in part from its orbiting electrons. To understand how laser works, one must know that every atom has its own well-defined set of energy levels. Ordinarily, it occupies the lowest level, but it can absorb a certain amount of energy of the right frequency and be excited to a higher level. When it falls back to the lower level, it gives up energy of a frequency in proportion to the energy difference between levels. The greater the difference in energy levels, the higher the frequency of radiation. Therefore, to set up a laser, the first step is to prepare an active medium that is an optically pure material containing atoms with a suitable set of energy levels. For example, a synthetic ruby crystal, the first successful laser material. It is made of aluminum oxide with a sprinkling of chromium atoms. As the ruby crystals are exposed to the so-called pumping action of white light, the chromium absorbs the green components of the light. This excites its atoms from the ground level to a higher level. They rapidly relax to a lower level called metastable level, and then somewhat more slowly they drop to the ground level and give off red light. This is why ruby crystals have their characteristic red fluorescence. Although such light spontaneously emitted is monochromatic, it is no more coherent than ordinary light. For the excited chromium atoms drop back to their ground level at random and give off red light at random. So the next step is to control the absorption and release of energy by the chromium atoms, making them give off a concentrated beam of light, all in step. A flash lamp is used to supply the energy to be absorbed by the chromium atoms. As energy is absorbed from the pumping light, more and more atoms reach the excited state, until there are more atoms in the high energy level than in the ground level. Now the stage is set for laser action, which is started by the spontaneous emission of a unit of light energy called a photon. Scientists have found that if a photon of the right frequency strikes the excited atom, it can stimulate the atom to emit another photon of the same frequency and in phase. So one photon comes in, two photons go out. The light was amplified by two through stimulated emission of radiation. This is laser. As long as these photons can strike more excited atoms along the direction of their travel, they will stimulate more emissions and swell their rank until the end of the road. Now we can keep them traveling and multiplying by putting them in an optical cavity that is between two parallel reflecting surfaces, perpendicular to the direction of travel, one of which is partly transparent by design. Only photons traveling along the axis of this system can be reflected back and forth inside the cavity. As the photons are reflected back and forth, a portion are transmitted through the partially transparent end. The remainder continue to build up until an intense coherent pulse passes through. This is the laser beam. The final laser beam is extremely directional, perpendicular to the end surface. The whole process usually takes less than a thousandth of a second. To sum it up, a laser is composed of an active medium within a cavity resonator and a pumping unit to supply the power. The output beam may be in pulses, as described before, or may be in a continuous wave. Although the basic concepts are similar for all lasers, the rapid advances in this new technology have brought forth a wide range of significant types. For example, a helium neon gas mixture laser with radiofrequency exciter as the pumping power unit gives a red or an infrared beam in continuous operation. A family of single gas lasers using pure noble gases such as argon, krypton, and xenon adds many more output wavelengths in the infrared region, all of them continuous. The injection laser using semiconductor crystals such as gallium arsenide cubes, about one-third of a millimeter in size, is pumped directly by injecting electrons into it. This direct method of pumping produces a very efficient laser. The beam from this laser can be modulated and offers a convenient solution to the problems of using laser for communication. Although the first operating laser was created as recently as 1960, the U.S. Army has already initiated a number of projects along the most promising lines for Army applications. The item nearest to field operation is the laser rangefinder. By utilizing laser's unique combination of high intensity, narrow beam, and color purity, the Army has developed a lightweight rangefinder for forward observers with accuracy and range far surpassing existing equipment. In 1961, the U.S. Army Electronics Research and Development Laboratory developed a spinning reflector technique to squeeze a train of low power pulses into a single pulse of maximum peak power. The spinning reflector is time to be perfectly parallel with the fixed reflector at the critical instant. When the excited atoms reach a maximum number, a condition which allows the protons to build up into peak power by oscillation and then emit radiation in a single pulse. Both the U.S. Army Electronics Laboratories and Frankfurt Arsenal have developed a rangefinder based on this principle. The performance of these rangefinders meets all the objectives of range and accuracy in a package weighing less than 30 pounds. The eight-power telescope is used to line up the target with a six-degree field of view. The rangefinder is triggered and the range is read out in meters. In this block diagram you can see what happens in the fraction of a second from the time the trigger is pressed until the range reading appears. The firing trigger starts the pumping action which produces a laser beam of one milliradian with a peak power of several megawatts, lasting less than 20 billionths of a second. A photo diode samples the laser beam and sends a signal to open the gate of the range counter and timing begins. The part of the laser beam reflected back by the target enters the receiver's lens system. Goes through a filter tuned to 6943 angstrom wavelength with a narrow 25 angstrom band pass. Gets detected and amplified by a photomultiplier tube and electronic circuit and changes into a signal to close the gate of the range counter. Timing is now stopped and the range is displayed in meters. The whole operation from target sighting to range reading may take only seconds with elevation and azimuth readings obtained at the same time. 50 to 100 ranging operations can be made without recharging the power supply which is composed of nickel cadmium batteries performing satisfactorily from minus 40 to 125 degrees Fahrenheit. A refinement is the target discriminator which gives readings to the target even if there are other objects and smoke or dust along the line of sight. This new rangefinder can be adapted for tank installation. Connected with an electric ballistic computer, its accuracy and speed will greatly enhance the gunner's ability for a first round hit. Plans are now underway to mount a heavier model in an airborne platform. A 10 megawatt laser would be combined with the UBS-1 target locator system to obtain range and location of a target with a one-shot reading. This would be far superior to the conventional method of recognizing the same target twice for angle measurements from two positions. Coupled with computers, this new degree of accuracy and speed in ranging virtually eliminates one of the weakest links in an advanced fire control system. A potential application utilizing the Doppler effect is a laser radar for detecting a moving target as well as for measuring both its range and its speed. A laser's frequency is so high that even a very slight movement of the target produces a large Doppler shift. For example, using a typical infrared gas laser, a target approaching at a rate of only one inch per second can produce a Doppler shift of 60 kilocycles. Furthermore, the extremely tight beam of laser will make such radar virtually jam-proof. Laser light in the infrared region can be used for night illumination. It can then be converted to visible light for the viewer in order to see without being seen. This gives continuous viewing at long range for detection and recognition. Exploratory development is now in progress. Laser can be made into a super-search light. Its nearly parallel beam can illuminate a small area over long distance as dramatized by the historical event on the evening of May 9, 1962 when MIT engineers using a laser beam illuminated a two-mile-wide circle on the moon over a distance of 250,000 miles With an outstanding radar, night vision, and illumination, laser offers potential in the field of guidance and control. By training this unique light of a single frequency on a target, a missile guided by this particular frequency may home in on it silently and accurately. A great promise of laser is in communications. The bandwidth proportional to the frequency is potentially enormous. This means that laser opens for potential use, a huge new region of the electromagnetic spectrum with a communication capacity many times greater than that devoted to radio, TV, radar, and microwave combined. In the narrow band of visible light alone, there are frequencies enough to allow every person on Earth his own telephone circuit and still contain 80 million TV channels of the present bandwidth. And these are no idle dreams. In this experiment at MIT's Lincoln Laboratory, a gallium arsenide laser diode is being used to transmit a video signal by way of an infrared beam. This is the laboratory model transmitter. The diodes are contained in these small packages. The picture here is being transmitted over a very short distance. Notice that when the beam is interrupted, the video picture disappears. In similar experiments, a video signal has been transmitted over 30 nautical miles. This receiving element picks up the light on the 12-inch parabolic mirror, focuses it onto a photomultiplier tube, and displays it on this monitor. Today, antennas several hundred feet across are sometimes required for microwave reception. Using laser's pinpoint transmission for the same purpose would require an antenna only inches across. This would also have the advantage of making detection by the enemy practically impossible. But problems in this area are equally impressive. At the Harry Diamond Laboratories in Washington, D.C., the study of atmospheric interference of light has produced dramatic evidence on film. As a light beam travels through air turbulences, it is broken down into drifting blotches of bright and dark areas, marring its usefulness as a carrier. One suggestion is to send laser beams through pipelines. Shielding them from the whims of nature. The United States Army Electronics Laboratories is investigating possible solutions to other major problems, such as the modulation of extreme high frequencies and the insufficient power output of continuous wave lasers. The unprecedented power densities and extreme precision of laser make it a revolutionary welding cutting tool. Experiments have been conducted in micro welding and machining in this Army laboratory. At the Aberdeen Ballistic Research Laboratories, the study of the interaction of laser radiation with matter is carried on intensively. Here a very powerful laser concentrates half a million watts in one thousandth of a second on a small section of a steel razor blade. The result? Instant vaporization. Various materials are tested, including carbon and lead. By hanging the material on a pendulum, the shock produced by the powerful laser beam is graphically demonstrated. The medical world considers laser a new surgical tool and an experimental instrument of research. Experiments under an Army contract are now underway at the Medical College of Virginia. Here a solid state ruby laser is being used to produce current data on retinal burns, information which can be useful in the evaluation of ocular hazards from lasers. As a result of these and similar experiments, the laser may develop into an important surgical tool for many types of eye surgery. At the Air Force's Arnold Engineering Development Center in Tennessee, tests are being conducted by the United States Army for the National Cancer Institute and the United States Public Health Service. Here cancerous tumors in mice have been successfully destroyed by powerful infrared laser beams. This mouse has been anesthetized. This test film shot at seven thousand frames per second shows the impact of the laser beam. Here is one of the animals a few weeks after the laser operation. A concern of the whole scientific world, as well as the modern Army, which constantly improves itself by research and development, is laser's role as a research tool. Physicists can use it to investigate the structure of atoms and molecules. Chemists can use it as a source of energy for photosensitive chemical reactions. Biologists can use it to vaporize individual parts of bacteria or to probe into cells to learn the very process of life. It can be as sensitive as to detect changes in time and space in terms of a billionth of a second. Be as brutal as to vaporize any substance in a flash. It can probe as deeply as the inside of the atom. Without atmospheric interference, laser's coherent directional, pure and intense beam may well be the perfect carrier for signals and power in space. Light has always been a wonder to man since the beginning of human discovery which opened with the taming of fire. Followed by the harnessing of water, steam, electricity, and atomic power, we are now entering a new scene, man's new control of life. Where will it lead? On to dreams that are as yet undreamed. On to new miracles with light.