LASER

LASER

LASER, acronym for Light Amplification by Stimulated Emission of Radiation. Lasers are devices that amplify light and produce coherent light beams, ranging from infrared to ultraviolet. A light beam is coherent when its waves, or photons, propagate in step with one another. Laser light, therefore, can be made extremely intense, highly directional, and very pure in color (frequency). Laser devices now extend into the X-ray frequency range. Masers are similar devices for microwaves.

Principles of Operation

Lasers harness atoms to store and emit light in a coherent fashion. The electrons in the atoms of a laser medium are first pumped, or energized, to an excited state by an energy source. They are then “stimulated” by external photons to emit the stored energy in the form of photons, a process known as stimulated emission. The photons emitted have a frequency characteristic of the atoms and travel in step with the stimulating photons. These photons in turn impinge on other excited atoms to release more photons. Light amplification is achieved as the photons move back and forth between two parallel mirrors, triggering further stimulated emissions. The intense, directional, and monochromatic laser light finally leaves through one of the mirrors, which is only partially silvered.

Stimulated emission, which is the underlying process for laser action, was first proposed by Albert Einstein in 1917. The working principles of lasers were outlined by the American physicists Arthur Schawlow and Charles Hard Townes in their 1958 patent application. The patent was granted; however, it was later challenged by the American physicist and engineer Gordon Gould. In 1960 the American physicist Theodore Maiman observed the first laser action in solid ruby. A year later a helium-neon gas laser was constructed by the Iranian-born American physicist Ali Javan (1926–    ). Then in 1966 a liquid laser was constructed by the American physicist Peter Sorokin (1931–    ). The U.S. Patent Office court in 1977 affirmed one of Gould’s claims over the working principles of the laser.

Types of Lasers

Based on the laser medium used, lasers are generally classified as solid state, gas, semiconductor, or liquid.

Solid-state lasers

The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rod are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film. Solid-state lasers offer the highest power output. They are usually operated in a pulsed manner to generate a burst of light over a short time. Bursts as short as 12 x 1015 sec have been achieved, useful in studying physical phenomena of very brief duration. Pumping is achieved with light from xenon flash tubes, arc lamps, or metal-vapor lamps. The frequency range has been expanded from infrared (IR) to ultraviolet (UV) by multiplying the original laser frequency with crystallike potassium dihydrogen phosphate, and X-ray wavelengths have been achieved by aiming laser beams at an yttrium target.

Gas lasers

The laser medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor, and is usually contained in a cylindrical glass or quartz tube. Two mirrors are located outside the ends of the tube to form the laser cavity. Gas lasers are pumped by ultraviolet light, electron beams, electric current, or chemical reactions. The helium-neon laser is known for its high frequency stability, color purity, and minimal beam spread. Carbon dioxide lasers are very efficient, and consequently they are the most powerful continuous wave (CW) lasers.

Semiconductor lasers

The most compact of lasers, the semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties. The laser cavity is confined to the junction region by means of two reflective boundaries. Gallium arsenide is the most common semiconductor used. Semiconductor lasers are pumped by the direct application of electrical current across the junction, and they are operated in the CW mode with almost 100 percent efficiency. A method that permits even more efficient use of energy has been devised. It involves mounting tiny lasers vertically in such circuits, to a density of more than a million per square centimeter.

Liquid lasers

The most common liquid laser media are inorganic dyes contained in glass vessels. They are pumped by intense flash lamps in a pulse mode or by a gas laser in the CW mode. Tunable dye lasers are a type for which frequency can be adjusted with the help of a prism inside the laser cavity.

Free electron lasers

Lasers using electrons unattached to atoms and pumped to lasing capacity by an array of magnets were first developed in 1977 and are now becoming important research instruments. They are tunable, as are dye lasers, and, in theory, a small number could cover the entire spectrum from infrared to X rays. Free electron lasers should also become capable of producing very high-power radiation that is currently too expensive to produce.

Laser Applications

The use of lasers is restricted only by imagination. Lasers have become valuable tools in industry, scientific research, communication, medicine, the military, and the arts.

Industry

Powerful laser beams can be focused on a small spot with enormous power density. Consequently, the focused beams can readily heat, melt, or vaporize material in a precise manner. Lasers have been used, for example, to drill holes in diamonds, to shape machine tools, to trim microelectronics, to heat-treat semiconductor chips, to cut fashion patterns, to synthesize new material, and to attempt to induce controlled nuclear fusion. The powerful short laser pulse also makes possible high-speed photography with an exposure time of several trillionths of a second. Highly directional laser beams were also used for alignment in the construction of the Bay Area Rapid Transit system in San Francisco.

Lasers are used for monitoring crustal movements and for geodetic surveys. They are also the most effective detectors of certain types of air pollution. In addition, lasers have been used for precise determination of the earth-moon distance and in tests of relativity. Very fast laser-activated switches are being developed for use in particle accelerators, and techniques have been found for using laser beams to slow down atoms for extremely precise studies of their spectra.

Scientific research

Because laser light is highly directional and monochromatic, extremely small amounts of light scattering or small frequency shifts caused by matter can easily be detected. By measuring such changes, scientists have successfully studied molecular structures of matter. With lasers, the speed of light has been determined to an unprecedented accuracy, chemical reactions can be selectively induced, and the existence of trace substances in samples can be detected.

Communication

Laser light can travel a large distance in outer space with little reduction in signal strength. Because of its high frequency, laser light can carry, for example, 1000 times the television channels today carried by microwaves. Lasers are therefore ideal for space communications. Low-loss optical fibers have been developed to transmit laser light for earthbound communication in telephone and computer systems. Laser techniques have also been used for high density information recording. For instance, laser light simplifies the recording of a hologram, from which a three-dimensional image can be reconstructed with a laser beam. Lasers are also used to play audio compact disks and videodisks.

Medicine

Intense, narrow beams of laser light can cut and cauterize certain tissues in a small fraction of a second without damaging the surrounding healthy tissues. They have been used to “weld” the retina, bore holes in the skull, vaporize lesions, and cauterize blood vessels. Laser techniques have also been developed for lab tests of small biological samples and for use in place of dental drills.

Military

Laser guidance systems for missiles, aircraft, and satellites are being studied and constructed. Laser weapons for use against hostile ballistic missiles, as in the defense system urged by U.S. President Ronald Reagan in 1983, are also in development. The ability of tunable dye lasers to excite selectively an atom or molecule may open up more efficient ways to separate isotopes for construction of nuclear weapons.

Laser Safety

Because the eye focuses laser light as it does other light, the chief danger in working with lasers is eye damage. Therefore, laser light should not be viewed either directly or reflected. Lasers should be used only by trained personnel wearing protective goggles.