More than 1000 different lasers (an acronym for light amplification by stimulated emission of radiation) exist and can be classified according to the type of lasing material employed, which may be a solid, liquid, gas, or semiconductor. The characteristic wavelength and power output of each laser type determines its application. The first true lasers developed in the 1960s were solid-state lasers (e.g., ruby, or neodymium: yttrium–aluminum garnet ‘‘Nd:YAG’’ lasers). Ruby lasers were used as early as 1961 in retinal surgery and are today used mainly in surgery and scientific research, and increasingly for micromachining. Gas lasers (helium and helium–neon being the most common, but there are also carbon dioxide lasers), developed in 1964, were soon investigated for surgical uses. Gas lasers were first used in industry in 1969 and are still heavily used in high-power applications in manufacturing for drilling, cutting, and welding. Excimer lasers use the noble gas compounds for lasing. Dye lasers, which became available in 1969, use solutions of organic dyes, such as rhodamine 6G, which can be stimulated by ultraviolet light or fast electrons. Semiconductor lasers (sometimes called diode lasers) are generally small and low power. Emitting in either the infrared or visible range, semiconductor lasers produce light based on free electrons in the conduction band, which are stimulated by an electrical current to combine with others in the valance band of the material. Covering the wavelength range used in optical fiber communications (see Lasers in Optoelectronics), semiconductor lasers are today the most important and widespread type of laser.
Lasers are employed in virtually every sector of the modern world including industry, commerce,
transportation, medicine, education, science, and in many consumer devices such as CD players and laser printers. The intensity of lasers makes them ideal cutting tools since their highly focused beam cuts more accurately than machined instruments and leaves surrounding materials unaffected. Surgeons, for example, have employed carbon dioxide or argon lasers in soft tissue surgery since the early 1970s. These lasers produce infrared wavelengths of energy that are absorbed by water. Water in tissues is rapidly heated and vaporized, resulting in disintegration of the tissue. Visible wavelengths (argon ion laser) coagulate tissue. Far-ultraviolet wavelengths (higher photon energy, as produced by excimer lasers) break down molecular bonds in target tissue and ‘‘ablate’’ tissue without heating. Excimer lasers have been used in corneal surgery since 1984. Short pulses only affect the surface area of interest and not deeper tissues. The extremely small size of the beam, coupled with optical fibers, enables today’s surgeons to conduct surgery deep inside the human body often without a single cut on the exterior. Blue lasers, developed in 1994 by Shuji Nakamura of Nichia Chemical Industries of Japan, promise even more precision than the dominant red lasers currently used and will further revolutionize surgical cutting techniques.
Commerce throughout the world has been profoundly affected by laser applications, including the widely used barcode scanning systems used in supermarkets, warehouse inventories, libraries, universities, and schools. The red ‘‘scan line’’ is a laser spot rapidly moving across at 30 or 40 times per second. A photo diode measures the intensity of light reflected back from the object: since dark bars absorb light, the bar and space pattern in the barcode can be determined.
Lasers are widely employed for micromachining and automated cutting in industrial applications.
Laser welding is routinely used in large industrial applications such as automotive assembly lines, providing much cheaper, better, and dramatically quicker welds than those possible with traditional techniques. Laser alloying utilizes the precise and powerful application of lasers to melt metal coatings and a portion of the underlying substrate to create surfaces that have unique and highly desirable qualities. Alloying produces the desired effect at the precise location where it is needed, meaning that less expensive materials can be utilized for the remainder of the instrument. New alloys with unique properties have been developed using this technology over the past few years with resultant new applications. Laser diagnostic instruments have revolutionized studies across the entire range of the sciences and engineering. These include applications such as laser-induced fluorescence to measure tiny amounts of trace materials and laser Doppler anemometry, which enables fluid flow to be precisely monitored. Laser spectroscopy has revolutionized the study of very fast chemical reactions, the study of structural changes in complex molecules, and other areas of biology and chemistry. Laser photobiology and photochemistry are large and growing subdisciplines with their own conferences, journals, theories, and nomenclature. Lasers are widely used in telecommunications, especially in fiber-optic cables. These systems employ low-powered, computer-controlled, semiconductor lasers that transmit encoded information in rapid infrared pulses. Regular light cannot perform suitably because its waves are not in parallel and therefore become too weakened over long distances resulting in an unacceptable loss of essential information. Semiconductor lasers can also ‘‘read’’ the pits and lands on the surface of a compact disk or DVD (see Audio Recording, Compact Disk). Because the wavelength of blue light is shorter than red light, the blue semiconductor lasers developed in the 1990s will be able to form much smaller spots on the recording layer of the disc, increasing the density of optical data storage. The precise, unchanging nature of a lasergenerated light beam makes it ideal for a wide range of applications involving measurement. Surveyors, construction personnel, oceanographers, geologists, and astronomers use laser ranging between a source and a reflector some significant distance away to measure distances or to ensure proper alignment of objects. Transit time can be used to calculate distance to an extremely high level of accuracy. Geologists routinely employ lasers, for example, to measure regional deformation of the Earth’s crust to aid in understanding and predicting earthquakes. This same precision over long distances forms the core of modern laserguided weapons, ranging from hand-held sniper rifles to long-range missiles and other ‘‘smart’’ weapons. Lasers are used to ‘‘paint’’ a target that is then precisely honed in by the weapon ‘‘tuned’’ to that particular wavelength. Efforts to create laserbased missile defense systems are under development in the U.S, but as of the early twenty-first century, results suggest that success is many years away (see Missiles, Defensive).
Holography is a widely employed and enjoyed aspect of the modern application of lasers. In addition to being employed for artistic and esthetic purposes, holography is used in the manufacture of optical instruments, in analyzing materials without harming them, and in storing data in extremely compact form. Two absolutely identical light beams are essential to forming a holographic image, and lasers ideally perform this function.
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