Lasers

Type of physical science: Atomic physics

Field of study: Nonrelativistic quantum mechanics

A laser is a device which produces highly focused, intense, monochromatic light. It has revolutionized technology in nearly all fields.

Overview

The word "laser" is an acronym for the phrase "light amplification by the stimulated emission of radiation." Capable of producing powerful, concentrated beams of light, the laser seems to fulfill science-fiction writers' prophecies of a death ray, but its use in weaponry, while real, has proved far less spectacular than its myriad applications in science and industry.

Accordingly, the laser's development was one of the basic technological revolutions of the twentieth century and lasers have become essential tools in procedures as disparate as eye surgery and nuclear fusion.

The laser evolved from the theory of quantum mechanics, which was formulated in order to explain the behavior of subatomic particles and forces. According to the quantum mechanics model, electrons orbit an atom's nucleus at specific distances, called energy levels, and they can pass from one energy level to another. When an atom's electrons all orbit at the lowest possible energy level, the atom is in its ground state, its most stable condition. When the atom absorbs electromagnetic radiation (in the form of photons), its electrons jump to higher energy levels. Consequently, the atom is in an excited state, which can usually be maintained only for very short periods. Electrons tend to relapse (decay) to lower energy levels quickly, and when this occurs, the electron's decaying causes a photon to be emitted. It is this process that accounts for the "emission of radiation" portion of the acronym "laser."

Building upon his work in quantum theory in 1905, Albert Einstein elucidated the fundamental principle behind the laser--the photoelectric effect--in 1917. In fact, it was for this work, rather than for his better-known theories of relativity, that he received the 1921 Nobel Prize in Physics. Einstein theorized that, when a photon produced by an atom or molecule encounters a similar atom or molecule in an excited state, it can stimulate the second atom or molecule to emit an identical photon. Since the photon is the basic unit of light, this process accounts for the "light" and "stimulated" in "laser."

Stimulated emission of radiation rarely occurs in nature, although astronomers have observed it at microwave wavelengths in certain interstellar gas clouds. The technical means to employ the stimulated emission of photons in order to amplify light did not exist when Einstein first proposed the phenomenon, and for decades, the idea was regarded primarily as a curiosity.

Nevertheless, ten years after Einstein proposed it, English physicist Paul Adrien Maurice Dirac determined that the photons involved in stimulation are coherent; that is, both the original photon and the stimulated photon shared the same--or nearly the same--wavelength, frequency, and direction.

The race to build a laser began in the 1950's. In 1954, a research team led by Charles Hard Townes at Columbia University produced microwaves by stimulated emission of radiation (the maser). In 1957, Townes and Arthur L. Schawlow of Bell Telephone Laboratories published a paper in PHYSICAL REVIEW LETTERS identifying the conditions necessary to duplicate the success of the maser at visible light wavelengths. The paper is usually taken as the birth of the idea for a laser, but the history is not quite so simple. At about the same time, Gordon Gould, a graduate student at Columbia University, described the process in his unpublished notebooks, while in the Soviet Union, Nikolay Gennadiyevich Basov and Aleksandr Mikhailovich Prokhorov, also maser pioneers, were at work on laser theory. Because of their parallel work on masers and lasers, Townes shared the 1964 Nobel Prize in Physics with Basov and Prokhorov, and in 1987, after decades of litigation, Gould won three patent rights to laser technology.

A frenzy of research and development followed the Townes-Schawlow paper at American and Soviet laboratories in an effort to convert theory to practice. Hughes Research Laboratories' Theodore H. Maiman first created artifically stimulated light on May 16, 1960, producing red pulses from a ruby rod around which a flashlamp had been wound. By the end of 1960, other scientists had constructed working models of four other types of gas or chemical lasers. Since these early successes, researchers have stimulated a wide variety of materials to "lase."

Despite the many solids, liquids, and gases that provide the active media in different types of lasers, most lasers contain three basic components: a material whose atoms or molecules can be pumped to excited states, an energy source to effect stimulation, and a resonator to build up the light energy.

When all or most of the atoms of molecules in a lasing medium exist in an excited state, the condition is called a population inversion. The energy source, usually a flashlamp or another laser, "pumps" the atoms into the population inversion by infusing them with photons.

Thereafter, the production of photons quickly multiplies: The excited atoms emit photons by stimulation from the pump, and those photons in turn encounter other excited atoms and stimulate them to produce photons--a process called a cascade. The millions upon millions of atoms in the active medium quickly radiate millions upon millions of photons in about the same direction. Additionally, after an excited atom has emitted its photon and decayed to a lower energy state, the pump will reexcite it so that it can again be stimulated to produce a photon, and so the cycle continues while the pump is active.

In order to intensify the process, the active medium is contained in a resonator, usually a tube with facing mirrors at either end. The light reflects back and forth between these mirrors, picking up increasing numbers of photons as it does so. At the same time, one of the mirrors, designed to reflect only a portion of the lasing photons and let the rest pass through, emits the laser beam.

Some types of lasers, however, do not employ this construction. Among them are salt-grain-size semiconductor diode lasers. Diode lasers achieve population inversion by passing electricity through a specially constructed semiconductor, typically made of gallium aluminum arsenide. Photons are emitted when electrons leap across the gap between two bands of specially treated semiconducting materials. Chemical lasers depend on the vibrationally excited molecules produced by chemical reactions, such as when hydrogen and fluorine atoms combine as hydrogen flouride. No external energy source is required, but chemical lasers do need resonators.

Free electron lasers create light when electrons in a particle accelerator pass through static magnetic fields, which slow them and cause them to emit photons.

Laser-produced light differs from regular light by having a single color and by being in phase and coherent. The monochromaticity results from the fact that the emitted photons have the same wavelength. Scientists measure wavelengths of light in fractions of a meter, and lasers have been built to emit from micrometer to nanometer wavelengths--from infrared to ultraviolet.

Even though humans cannot see infrared or ultraviolet light without special sensors, these wavelengths are included in the term "visible light" as it applies to the laser. Laser-produced light is in phase because the crests and troughs of each photon's wave pattern are congruent and travel at the same speed (the same frequency). By contrast, normal light, such as that produced by a light bulb or the sun, is polychromatic (a blend of different wavelengths) and incoherent. This model of laser beams is an ideal: No laser produces perfectly monochromatic, coherent light.

Rather, a beam encompasses a group of wavelengths and frequencies that are very nearly the same, and the amount of variation in a beam's wavelength is referred to as its line width. Even the most coherent laser beams--those with the narrowest line width--spread out with distance, dissipating their energy.

The content of electromagnetic radiation in a laser beam is its energy. It is proportional to the laser's wavelength and is measured in joules: The shorter is the wavelength, the higher is the energy. The power of a laser is the rate at which energy is produced and measured in watts, as with the power of a light bulb. The power potential varies enormously among types of lasers, from a few milliwatts in a diode laser to multiple gigawatts in a ruby laser. The effective power output of a laser, however, depends on whether it emits light inpulses or in continuous beams.

Only a few lasers can maintain high power levels in a continuous beam, such as the gas-dynamic carbon dioxide laser, which can produce 100 kilowatts. The power of pulsed lasers is measured as an average flow of energy per second in a given time period. The duration of a pulse may be as little as a femtosecond and as much as a millisecond; a laser may produce as many as a million pulses a second or as few as one a minute.

The measurement of an active medium's ability to amplify light is called its gain, which varies in accordance with the wavelength. No laser can emit the full potential of its medium's gain, since some energy is lost from even the most efficient resonators.

As research in laser design continues, so does laser versatility and power level. Some lasers can be tuned to lase at different wavelengths, while others can convert from pulses to continuous beams. In addition, the requirements of research and industry for greater power have led to increasingly intense beams by making more efficient resonators, using shorter wavelengths, merging beams, increasing the power of the energy pump, or magnifying the beam with lenses after it has left the laser.

Applications

Lasers participate in virtually all fields of modern technology and science. As a research instrument, manufacturing tool, means of communications, medical instrument, or component in electronic equipment, the laser has improved procedures in some cases and in others made them possible for the first time.

Most people encounter lasers, if nowhere else, in supermarkets. At the checkout counter, a red beam from a helium-neon or diode laser reflects from a rotatable mirror under the counter and hits the striped Universal Product Code on a package. The reflected light from this "bar" code is received by a detector and converted to electronic impulses that a computer reads as numerals--the one or zero of computer binary language--and then a price associated with the numerical code appears on the cash register. A similar process operates in compact disc use. The laser light reflects from tiny spots on the optical disc that correspond to binary code, and a detector converts the reflected light to electronic impulses so that a stereo system produces music or a computer retrieves data.

Computer systems also use lasers to read and produce print. In optical character readers, lasers can scan printed material, associate the letters or numerals' shapes with binary codes, and store this information in the computer memory. Laser printers do essentially the opposite, converting computer binary language to print through a process that is similar to that of photocopy machines.

Telecommunications companies, more than any other single industry, have capitalized on lasers, making them a fundamental part of transferring information. Phone companies use optical fibers to carry laser-borne calls long distance. The optical fiber is a thin, hollow tube whose interior is highly reflective; the laser light, carrying millions of calls, follows the optical fiber for as much as 150 kilometers for conversion into sound or retransmission over longer distances. Lasers can also send signals as far as 20 kilometers through the atmosphere, which makes narrow-beam communications possible from point to point on Earth (a method much valued by the military) and from ground to satellite.

Manufacturers use lasers in a variety of ways for materials processing. Carbon dioxide and neodymium-yttrium aluminum garnet lasers have the ability to slice, pierce, weld, solder, and heat-treat metals. Therefore, these types of lasers have improved construction large and small--from assembling cars to punching the holes in the rubber nipples of baby bottles--because of their high power and precise focus. The computer industry also relies on lasers for cutting, etching, and annealing semiconductors. Lasers can also be used to examine the surface of materials for signs of stress and microscopic fracturing through interferometry, a boon to quality assurance and safety.

Scientists use lasers for precision measurements. The ability of laser light to travel long distances without losing energy makes it well suited to measuring such distances. For example, lasers have determined the distance from Earth to the Moon to within a few centimeters, a technique sometimes called lidar on the analogy of radar. Furthermore, the composition of clouds in the atmosphere can be analyzed because the particulate matter in clouds absorbs, scatters, or reflects laser light in characteristic, analyzable ways.

Chemists can likewise assess the structure and nature of solids, liquids, and gases with lasers. Excited matter emits a characteristic light signature, or spectrum. In fluorescence spectroscopy, lasers excite an undetermined substance and detectors record its spectrum, which can then be identified. Spectroscopy and laser analysis of the atmosphere are particularly useful in measuring the extent and nature of pollution. The energy structures of molecules and atoms--even protons and electrons--can be elucidated through such processes, and chemists can induce chemical reactions that are otherwise extremely difficult or impossible to obtain by exposing molecules to specific wavelengths laser light, a field called laser chemistry or photochemistry.

Lidar and similar long-distance capabilities have applications for the military in the detection and identification of aircraft, range-finding, and missile guidance. The much-discussed use of lasers to destroy missiles--the Strategic Defense Initiative, or "Star Wars," research--may make them weapons in their own right, although the enormous power required can now come only from bulky, awkward devices. In a more manageable offensive tactic, moderate power lasers are trained on enemy vehicles to blind their guidance systems or sensors, rendering them ineffective.

Medicine has succeeded in turning the laser into a flexible, precision weapon in its battle with disease. Delivered through fiber optics, laser light's controllable power and fine focus make it suitable for a variety of surgical techniques, including the removal of tumors, the coagulation of blood vessels, eye surgery, and the eradication of plaques that clog arteries.

Because a doctor does not directly touch tissue when using lasers, the procedure is exceptionally antiseptic. Under some circumstances, patients may recover faster because the laser cauterizes severed blood vessels, allowing incisions to heal quickly and bleed little.

Photochemistry also may make lasers valuable in diagnosis. Fluorescence spectroscopy, in particular, holds promise as a means to distinguish healthy tissue from cancerous tissue accurately. Pulsed lasers also can be used in measuring the thickness of skin components and analyzing fine lacerations in delicate tissue, such as that in the eye.

Perhaps the most ambitious use of lasers, however lies in the attempt to build a fusion reactor--a reactor producing energy by forcing atoms together, unlike current atomic reactors, which split atoms. The process of fusion is understood and is the basis of the hydrogen bomb, but harnessing the vast energy output from fusion for peaceful application has proved extremely difficult. One of the methods under study to achieve controlled fusion--inertial confinement--depends on high-power pulsed lasers to compress and heat small pellets of deuterium and tritium (isotopes of hydrogen) into dense plasma for short periods.

This survey is only a bare summary, as lasers are necessary in many other areas. For example, lasers are used to create three-dimensional photographs of holography and the "laser shows" on stages and at planetariums. In addition, the number of uses for lasers increases steadily.

Context

The laser brought home one of the grandest and most esoteric theories of twentieth century science: quantum mechanics. While by no means proving that theory, the laser nevertheless has made it familiar, usable, specific, and tactile. Unlike the other famous product of the theory, atomic power, lasers can be owned and operated--even built--by those who have little knowledge of the physical laws governing them. Furthermore, the laser dramatically underscores the genius of Einstein because, like his other discoveries, the photoelectric effect has created opportunities for technology and research that classical physicists did not dream of and engineers are still laboring to exploit.

The laser also gives reason to soften the indictment against the destructive nature of modern physics. When researchers in the early 1960's rushed to build lasers with an ever-increasing array of active media, component configurations, and power sources, they foresaw few immediate applications, leading one observer to quip that the laser was a solution looking for a problem. When the laser was finally harnessed for practical use, however, most developments were not for weaponry, as was the case with atomic energy, but for peaceful purposes. Far from kept secret, the various methods of constructing a laser have been published as soon as they have been established.

The importance of lasers in the future is difficult to assess adequately. Fusion power based on laser-produced inertial confinement promises more abundant, cheaper, and cleaner energy production. The National Aeronautics and Space Administration (NASA) is investigating laser systems for the propulsion of manned vehicles to the planets. Optical computing designs may lead to far more powerful supercomputers. In fact, the practical applications of lasers are so varied and under such intensive development that no overview can capture their potential ability to transform technology as radically in the twenty-first century as electricity transformed twentieth century technology.

Principal terms

COHERENCE: the degree to which light waves are in phase and have the same wavelength

ENERGY LEVEL: the distinct states of constant energy at which electrons can exist in an atom's shell

FREQUENCY: the number of light waves that pass a given point per second, inversely proportional to wavelength

PHOTON: the light quantum; a packet of light in the form of an energy wave

RADIATION: the emission of energy in the form of waves or particles

STIMULATED EMISSION: the process by which a photon knocks loose another photon from an excited atom

WAVELENGTH: the distance that a wave of light travels in a single cycle of oscillation

Bibliography

Bromberg, Joan Lisa. "The Birth of the Laser." PHYSICS TODAY 41 (October, 1988): 26-33. An excellent, detailed history of the laser's development, concentrating on the race in the United States to construct the first working model. The article assumes a basic knowledge of laser physics.

Burkig, Valerie. PHOTONICS: THE NEW SCIENCE OF LIGHT. Hillside, N.J.: Enslow, 1986. Burkig has written an overview for a general readership. While less than half the book deals with lasers directly, the rest is useful background on the properties and uses of light. The text is accompanied by black-and-white illustrations and photographs.

Deutsch, Thomas F. "Medical Applications of Lasers." PHYSICS TODAY 41 (October, 1988): 56-63. For readers who are familiar with both medical technology and lasers, this article surveys the current applications of lasers in surgery, therapy, and diagnosis well. Briefly discusses future developments.

Hecht, Jeff. UNDERSTANDING LASERS. Indianapolis, Ind.: Howard W. Sams, 1988. An excellent introductory book for those who want to understand the laser in depth. In addition to clear narrative and well-explained equations, Hecht offers many simple illustrations and quizzes at the end of every chapter for readers to test their comprehension. The reader needs only a rudimentary knowledge of physics and chemistry.

Laurence, Clifford L. THE LASER BOOK: A NEW TECHNOLOGY OF LIGHT. New York: Prentice-Hall, 1986. Although at times repetitious, this book provides painstakingly clear descriptions of the principles and uses of lasers. Diagrams illustrate the processes of lasing, and photographs reveal the nature and variety of applications. Includes a list of speciality periodicals, organizations, and conferences.

Mauldin, John H. LIGHT, LASERS, AND OPTICS. Blue Ridge Summit, Pa.: TAB Books, 1988. With deft explanations and useful illustrations, Mauldin lays out the background knowledge necessary to understand lasers and optics in some depth. A knowledge of mathematics is necessary to appreciate the presentation fully.

Taylor, J. R., and P. M. W. French. HOW LASERS ARE MADE. New York: Facts on File, 1987. Brief and well illustrated with color diagrams and photographs, this pamphlet offers a concise, nonmathematical explanation of lasing principles and an excellent summary of the practical applications. It does not, however, provide instructions on how to make one.

Electrons and Atoms