Theodore Harold Maiman succeeded in putting into operation the theoretical principles of the laser, creating a device with a host of immediate and obvious potential applications. Within months, private and public institutions were pouring money into laser research and development.
Lasers were once called optical masers
In order to appreciate the difference between laser light and ordinary light, one must examine how light of any kind is produced. In a light source, such as a fluorescent lamp, the atoms of gas the lamp contains must be excited to a state of higher energy than their normal, or ground, state. This is accomplished by flipping a switch, which sends a current of electricity through the lamp; the current jolts the atoms into the higher-energy state. This excited state is unstable, however, and the atoms will spontaneously return to their ground state by ridding themselves, in the form of light, of the excess energy they had acquired. The atoms return to the ground state “individualistically,” the light emitted by one atom unrelated to the light emitted by any other. The light emitted by a lamp full of atoms, therefore, is disorganized, emitted in all directions and randomly in time. This type of light, common to all ordinary sources, from fluorescent lamps to the sun, is ordinary, incoherent light.
Laser light is different. The excited atoms in a laser emit their excess energy not as individualists, but in a collective manner. The atoms remain in the excited state until there are a great many excited atoms. Then, they are stimulated to emit, not independently, but in an organized fashion, with all their light waves traveling in the same direction, crests and troughs perfectly aligned. This type of light is called coherent.
The concept of coherence did not arise in connection with lasers nor did stimulated emission, a subject studied by Albert Einstein in 1917. Maser and laser physicists introduced the idea of collecting a great many excited atoms, then stimulating them so as to amplify their radiation. Lasers could have been developed well before 1960, since optical spectroscopy was an established field a century ago. The laser’s precursor, the maser, however, could not have been produced before World War II, when microwave research began. Despite optical spectroscopy’s seniority, the maser preceded the laser because amplification was associated with electrical circuits, of which microwave devices had been components since the war. Thus, the laser’s tardy appearance can be attributed to conceptual, rather than technical, obstacles.
The idea of stimulating an assemblage of excited atoms to emit their excitation energy in masers was introduced independently in the Soviet Union by Nikolay Gennadiyevich Basov
In 1958, Townes, together with Arthur L. Schawlow, explored the requirements in a theoretical paper. They stated that, in principle, a laser could be constructed; they went on to describe its features. It would employ a technique called optical pumping to excite the atoms. (Optical pumping had been developed by Alfred Kastler
The material of which the laser is made is of prime importance; it must be one in which the atoms’ excited state lasts long enough for many excited atoms to accumulate. The next phases of laser action—stimulation and amplification—represents an engineering problem: how to construct the laser from the selected material.
It is a property of stimulated emission that stimulated light waves will be aligned exactly, crest to crest, trough to trough, and with respect to direction, with the radiation that does the stimulating. From the assemblage of excited atoms, one atom returns to its ground state, emitting light. That light hits one of the other excited atoms and stimulates it to fall to its ground state and emit light; stimulating and stimulated light are exactly in step. The light from these two atoms hits other excited atoms, stimulates them to emit in step, and the light from these stimulates still more atoms, until that first atom that returned to its ground state has produced a cascade.
If, while the light from the first atom is being amplified in this coherent fashion, another atom falls and emits in a different direction, its light will be amplified also, but will have a disturbing effect because of its lack of coherence with the light initiated by the first atom. The disturbing light must be eliminated. Engineering comes into play at this point. Maiman’s laser, a single crystal of synthetic pink ruby, was machined to form a cylindrical rod about 4 centimeters long and 0.5 centimeter across. The ends, polished flat and made parallel to within about a millionth of a centimeter, were coated with silver to make them mirrors.
If the first atom emitted light in a direction parallel to the length of the cylinder, all the light stimulated by that initial light would travel along the length of the cylinder. At the end of the cylinder, the mirror there would reflect the light back along the length to stimulate more light in the same direction. All the light would hit the mirror at the other end, be reflected back, stimulate more light, all in the same direction and, thus, steadily build up an increasing intensity of light.
If the disturbing atom emits light in a direction other than parallel to the length of the cylinder, either immediately or after a few reflections at the mirrors, the disturbing light would exit through the curved nonreflecting walls of the cylinder. The result is that no light, except that traveling along the length of the cylinder, builds up in intensity (is amplified). If the mirror at one end of the cylinder is constructed to let through a fraction of the light, the beam will emerge straight ahead with almost no spread.
To produce the initial excitation, which Schawlow and Townes believed could not be achieved with lamps available at that time, Maiman used a lamp similar to those used by photographers, which emitted an enormous burst of ordinary light in a fraction of a second. His lamp, helical in shape, was wrapped around his ruby rod, so that as much of the exciting light as possible would reach the rod. The lamp was connected to a large bank of condensers carrying a tremendous electrical charge, which produced sufficient energy to excite the atoms of the ruby to lase.
In the Soviet Union, F. A. Butayeva
When the laser was introduced, its impact was immediate: In the eighteen months following Maiman’s announcement that he had succeeded in producing a working laser, about four hundred companies and several government agencies embarked on similar work involving lasers. Activity centered on searching for new materials from which to construct lasers—solid, liquid, and gaseous—and on improving existing lasers. New materials were superior to the ruby because they did not require as much energy for excitation; others provided light of a different color from the characteristic red of the ruby. Substitutes were found for the metal end mirrors, which burned up after a few bursts of laser light.
Effort was devoted also to exploring applications of lasers; much of the effort was directed toward the use of lasers in communications. Because of the extremely high frequency of visible light, a single laser beam had more information-carrying capacity than all the radio, television, and other communications channels in existence. At the same time, there was equal activity in publicizing the near-miraculous potentialities of the device, in applications covering the spectrum from death rays to sight-saving operations. A popular film in the James Bond series, Goldfinger
In the first decade after Maiman’s laser, there was some disappointment. Lasers were used with much success in some areas of medicine, such as repairing detached retinas, and in scientific applications, particularly in connection with standards—the speed of light had been measured with great accuracy, as was the distance to the moon—but communications proved an area difficult for the laser to conquer. The difficulty was that light, including laser light, is scattered by clouds and fog. Later advances, however, would enable all of the laser’s promise to be fulfilled, in the fields of communications, medicine, and weaponry, among others.
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Bertolotti, Mario. The History of the Laser. Philadelphia: Institute of Physics, 2005. Thorough technical prehistory of lasers, devoted to the theoretical and practical physics preceding and underlying their invention. Includes two chapters on the actual invention of the laser and its applications. -
Lasers and Light: Readings from Scientific American. Introductions by Arthur L. Schawlow. San Francisco: W. H. Freeman, 1969. Compendium of articles written by distinguished scientists on subjects that provide a background for understanding lasers or that elaborate on topics, such as optical pumping. Makes an interesting, entertaining, informative package for those with an interest in lasers. -
Townes, Charles H. How the Laser Happened: Adventures of a Scientist. New York: Oxford University Press, 1999. Account of the invention of the laser by one of the Nobel laureates who contributed to its creation. Index.
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