A laser is any of a class of devices that produces an intense beam of light of a very pure single color. This light beam may be intense enough to vaporize the hardest and most heat-resistant materials. The word laser is an acronym derived from "light amplification by stimulated emission of radiation."

Atoms and molecules exist at low and high energy levels. Those at low levels can be excited to higher levels, usually by heat, and after reaching the higher levels they give off light when they return to a lower level. In ordinary light sources the many excited atoms or molecules emit light independently and in many different colors (wavelengths). If, however, during the brief instant that an atom is excited, light of a certain wavelength impinges on it, the atom can be stimulated to emit radiation that is in phase (in step) with the wave that stimulated it. The new emission thus augments or amplifies the passing wave; if the phenomenon can be multiplied sufficiently, the resulting beam, made up of wholly coherent light (i.e., light of a single frequency or color in which all the components are in step with each other), will be tremendously powerful.

Albert Einstein recognized the existence of stimulated emission in 1917, but not until the 1950s were ways found to use it in devices. The American physicists Charles H. Townes and A.L. Schawlow showed that it was possible to construct such a device using optical light. Two Soviet physicists proposed related ideas independently. The first laser, constructed in 1960 by Theodore H. Maiman of the United States, used a rod of ruby. Since then many types of lasers have been built.

The light produced by lasers is in general far more monochromatic, directional, powerful, and coherent than that from any other light sources. Nevertheless, the individual kinds of lasers differ greatly in these properties as well as in wavelength, size, and efficiency. There is no single laser suitable for all purposes, but some of the combinations of properties can do things that were difficult or impossible before lasers were developed.

A continuous visible beam from a laser using a gas, such as the helium-neon combination, provides a nearly ideal straight line for all kinds of alignment applications. The beam from such a laser typically diverges by less than one part in a thousand, approaching the theoretical limit. The beam's divergence can be reduced by passing it backward through a telescope, although fluctuations in the atmosphere then limit the sharpness of a beam over a long path. Lasers have come to be widely used for alignment in large construction--e.g., to guide machines for drilling tunnels and for laying pipelines.

A pulsed laser can be used in a light radar, sometimes called LIDAR, and the narrowness of its beam permits sharp definition of targets. As with radar, the distance to an object is measured by the time taken for the light to reach and return from it, since the speed of light is known. LIDAR echoes have been returned from the Moon, facilitated by a multiprism reflector that was placed there by the first astronauts to land there. Distances can be measured from an observatory on Earth to the lunar mirror with an accuracy of several centimetres. Simultaneous measurements of the mirror's distance and direction from two observatories on different parts of the Earth could give an accurate value for the distance between the two observatories. A series of such measurements can tell the rate at which continents are drifting relative to each other.

A vertically directed laser radar in an airplane can serve as a fast, high-resolution device for mapping fine details, such as the contours of steps in a stadium or the shape of the roof of a house. With a pulsed laser radar, returns can be obtained from dust particles and even from air molecules at higher altitudes. Thus air densities can be measured and air currents can sometimes be traced.

The high coherence of a laser's output is very helpful in measurement and other applications involving interference of light beams. If a light beam is divided into two parts that travel different paths, when the beams come together again they may be either in step so that they reinforce each other or out of step so that they cancel one another. Thus the brightness of the recombined wave changes from light to dark, producing interference fringes, when the difference in path lengths is changed by one-half of a wavelength. Such devices are called laser interferometers. Very small displacements can be detected, and larger distances can be measured with precision. With lasers, these measurements can be carried out over extremely long distances. Laser interferometers are used to monitor small displacements in the Earth's crust across geological faults. In manufacturing, such devices are employed to gauge fine wires, to monitor the products of automated machine tools, and to test optical components.

Lasers can be so monochromatic that a small shift in the light frequency can be detected. Light reflected from an object that is moving toward the laser is raised in frequency by an amount depending on the velocity of the object (Doppler effect). For a receding object, the frequency is lowered. In either case, if some of the original and the shifted light are recombined at a photodetector, a signal at the difference frequency (the difference in frequency between the original and the shifted light) is observed, and even small velocities can be measured.

The brightness and coherence of laser light make it especially suitable for visual effects and photography that simulate third dimensional depth--e.g., holography.

The light from many lasers is relatively powerful and can be focused by a conventional lens system to a small spot of great intensity. Thus even a moderately small pulsed laser can vaporize a small amount of any substance and drill narrow holes in the hardest materials. Ruby lasers, for example, are used to drill holes in diamonds for wire drawing dies and in sapphires for watch bearings. For biological research, a finely focused laser can vaporize parts of a single cell, thus permitting microsurgery of chromosomes.

Strong heating can be produced by a laser at a place where no mechanical contact is possible. Thus one of the earliest applications of lasers was for surgery on the retina of the eye.

Lasers are also used for small-scale cutting and welding. They can trim resistors to exact values by removing material and can alter connections within integrated arrays of microcircuit elements. A pulse of light from a laser can vaporize a sample of a substance for analysis by suitable instruments. By this method an extremely small sample can be analyzed without introducing contaminants.

The high brightness, pure color, and directionality of laser light make it ideally suited for experiments on light scattering. Even a small amount of light that is scattered with a change of wavelength or direction can be readily identified. In particular, a type of scattering known as the Raman effect produces characteristic wavelength shifts by which molecular species can be identified. With laser sources and sensitive spectrography, small samples of transparent liquids, gases, or solids can be analyzed. It is even possible to measure contaminants in the atmosphere at a considerable distance by the Raman scattering of light from a laser beam.

Laser beams can be used for communications. Because the light frequency is so high (around 5 1014 hertz for visible light), the intensity can be rapidly altered to encode very complex signals. In principle, one laser beam could carry as much information as all existing radio channels. Laser light can, however, be blocked by rain, fog, or snow so that, for reliable communications on Earth, the laser beam would need to be enclosed in a protective medium. Optical fibres made of glass and covered with a cladding material are employed for this purpose. Waveguides of this kind have been used increasingly in long-distance telephone systems since the early 1980s (see fibre optics).

Laser technology is integral to optical disc recording and storage systems. In such a system, digital data are recorded by burning a series of microscopic holes, commonly referred to as pits, with a laser beam into thin metallic film on the surface of a small-diameter disc. In this manner, information from magnetic tape is encoded on a master disc, which is replicated by a process known as stamping. In the read mode, laser light of low intensity is reflected off the disc surface and is "read" by light-sensitive diodes. The amount of light received by the diodes varies according to the presence or the absence of the pits, and this input is digitized by the diode circuits. The digital signals are subsequently converted to analog information on a video screen. Compact audio disc players work in much the same way except that the digital signals are transformed into sound impulses.

Lasers also are used in a major type of computer printer. Laser printers employ a laser beam and a system of optical devices to etch images on a photoconductor drum. The images are carried from the drum to paper by means of electrostatic photocopying.

Excerpt from the Encyclopedia Britannica without permission.