Laser is an 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 can be made extremely intense, highly directional, and very pure in frequency and thus colour. Laser devices now extend into the X-ray frequency range.
Lasers harness atoms to store and emit light in a coherent fashion. The electrons in the atoms of a laser medium are first pumped to an excited state by an energy source and 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, the underlying process for laser action, was first proposed by Albert Einstein in 1917. However, it was not until 1954 that American physicist Charles Townes created a population inversion in a device that amplified microwaves, a maser. The first laser was constructed in 1960 by Theodore Maiman in the United States.
Lasers have become valuable tools in industry, scientific research, communication, medicine, the military, and the arts.
Powerful laser beams can be focused on a small spot with enormous power density. Consequently, the focused beams can readily heat, melt, or vaporise 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 steel patterns, to synthesise 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.
Lasers are used for monitoring movements of the earth�s crust 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.
Laser light is highly directional and monochromatic, consequently 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.
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 fibres have been developed to transmit laser light for earthbound communication in telephone and computer systems.
Intense, narrow beams of laser light can cut and cauterise 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, vaporise lesions, and cauterise blood vessels. Laser techniques have also been developed for lab tests of small biological samples.
Laser guidance systems for missiles, aircraft, and satellites are being studied and constructed. The use of laser beams has been proposed against hostile ballistic missiles.