Introduction to Laser
Laser, a device that generates or amplifies waves of visible light or of such other forms of electromagnetic energy as ultraviolet and infrared radiation. The word laser is derived from the principle of the device itself: light amplification by stimulated emission of radiation. A related device, the maser, is used to generate or amplify microwaves.
A laser can produce a beam of coherent, highly directional, and highly intense light. The light is coherentthat is, the waves making up the beam have the same wavelength and are in phase with each other. Light from ordinary light sources does not have this property. The laser's light is directionalthat is, it does not spread out over a large area after it leaves its sourcebecause the light is coherent and is emitted from the laser in a single direction. (By contrast, light from an ordinary lamp bulb spreads out in all directions.) Because it does not spread out and because its waves are in phase, the light is also intense, diminishing little in brightness as it leaves the laser.
Most types of lasers can produce light of only one or a few specific wavelengths. Some types, however, can be adjusted to produce light at any of a wide range of wavelengths. Some lasers, called continuous wave lasers, produce a steady beam of light. Others, called pulsed lasers, produce light in pulses, each lasting only a fraction of a second.
Uses
Their unique properties make lasers valuable tools with a large variety of applications.
Laser beams are used for obtaining accurate alignment of parts in engineering projects such as the construction of buildings and the laying of pipelines. Precise measurement of long distances can be made by determining the time it takes for a pulse of light from a laser to travel to a distant object and return. Using this method, scientists have measured precisely the distance between the earth and the moon by directing laser beams from earth onto reflectors left on the moon by Apollo astronauts.
The intense light of powerful lasers can cut through or heat many materials more quickly and easily than can conventional tools. In industry, laser beams are used for a variety of manufacturing operations, from cutting textiles to drilling holes through hard metals. A laser can produce intense heating over a small area, making it ideal for certain types of welding.
Lasers are used for delicate operations on the eye, brain, and other organs where conventional operating methods are often difficult. In surgery to repair detached retinas, for example, the laser beam can be passed through the lens of the eye and, in effect, weld breaks in the retina; the lens and other transparent parts of the eye are unaffected by the beam. When lasers are used as surgical knives, the heat from the laser beam immediately seals off severed blood vessels in the tissue being cut, thus minimizing bleeding.
Tiny lasers that can be turned on and off millions of times a second are used to transmit telephone messages and other information as flashes of light through optical fibers. Lasers are also used in certain electronic devices to read specially encoded information. For example, some supermarket optical scanners use a laser to read the Universal Product Code on grocery items at the checkout counter, and compact disc players use a laser to read data recorded on a small plastic disc.
Laser-guided missiles have proven highly accurate; the target is illuminated by the laser beam and the missile's guidance system uses the beam's reflection to home in on the target. Experimental lasers that can disable missiles at a distance in the air have been successfully tested.
Lasers are used by chemists to analyze the composition of chemical substances and to study chemical reactions. In the field of nuclear energy, some scientists think very powerful lasers are the key to making a successful fusion reactor. The lasers would provide the tremendous heat needed to start the fusion process.
Another important application of lasers is in holography, a method of producing three-dimensional images.
Basic Principles
(Note—It is suggested that the article Radiation, introduction and subtitle Electromagnetic Radiation be read before reading the technical sections of this article.)
The laser makes use of some of the properties of the atom to generate or amplify light waves. Atoms absorb and emit energy in the form of photons (small bundles or particles of energy). An atom that has absorbed a photon is said to be excited (raised in energy). In returning to its normal, or ground, state, the atom gives up one or more photons. Usually, excited atoms return to the ground state in a spontaneous, random manner. However, an excited atom can be stimulated to give up a photon if the atom is hit by an outside photon having exactly the same energy as the photon the atom would have given up spontaneously. The outside photon and the photon given up move away from the atom and travel in the same direction and in coherent waves.
Any given atom can absorb and emit only photons with particular amounts of energy (particular wavelengths). Which wavelengths can be absorbed and emitted depends on which kind of atom is being used.
Usually most of the atoms in a substance are in the ground state. In a laser, most of the atoms are excited at one time, and are then made to emit their photons in an orderly way. When the photons are emitted, an intense, directional, and coherent beam is generated by the laser. The particular properties of the beam generated depend on the material used for the laser. A solid-state laser uses either a crystal or glass; other kinds of lasers use gases, liquids, or semiconductors.
Solid-state LaserA characteristic type of solid-state laser is the ruby laser. It contains a rod-shaped crystal of synthetic ruby. This material, like natural ruby gemstones, is composed of aluminum oxide with trace amounts of chromium. The ends of the rod are cut parallel to each other and perpendicular to the sides. One end is heavily silveredthat is, coated with silver or other reflective materialso that it reflects almost all the light that strikes it. The other end is lightly silvered so that some of the light will be reflected and the rest will pass through.
A lamp that can produce an intense flash of white light surrounds the ruby crystal. When the lamp flashes, most of the chromium atoms in the crystal absorb photons. Within a fraction of a second, these excited atoms begin returning to the ground state, and, in the process, emit photons. When these photons strike chromium atoms that are still excited, they stimulate them to emit yet other photons. As this process continues, beams of coherent light are formed. The beam that is parallel to the sides of the ruby is reflected back and forth between the silvered ends until sufficient photons have joined it to make it powerful enough to escape through the lightly silvered end. (The beams that are not parallel to the sides escape from the ruby before they have a chance to build up much intensity.)
The ruby laser emits a red light. Other types of solid-state lasers can be used to generate light of other colors or to generate infrared radiation.
Gas LaserA gas laser contains one or more gases sealed in a glass tube. Two mirrors. one heavily silvered and the other lightly silvered, lie at either end of the tube. The most common type of gas laser uses a mixture of helium and neon. Other gases used include carbon dioxide and argon. In the helium-neon laser, energy is provided by an electrical discharge that excites the helium atoms. When the excited helium atoms collide with the neon atoms, the energy is transferred from the helium to the neon. The neon atoms emit photons that form a laser beam in essentially the same way as in the ruby laser. Helium-neon lasers can produce beams of red light, green light, or infrared radiation, depending on the mirrors used.
Liquid LaserThe typical liquid laser uses a fluorescent dye in a glass tube. As in the gas laser, two mirrors, one heavily silvered and the other lightly silvered, lie at either end of the tube. Energy to excite the molecules of the dye is provided either by a flash lamp or by an ultraviolet laser. Liquid lasers can be made to produce extremely brief pulses of light; some have produced pulses lasting less than a trillionth of a second. Dye lasers are used to produce beams of visible light of almost any color.
Semiconductor LaserA semiconductor laser is essentially a kind of electronic device called a junction diode. It is made of a semiconductor, typically gallium arsenide, that has been treated to form two types of materialsan n-type material, which has an excess of electrons, and a p-type material, which has a deficiency of electrons. The p-type material contains positively charged vacancies called holes.
When a voltage is applied across the diode, excess electrons of the n-type material combine with holes of the p-type material along the junction between the two types of materials. This process results in the release of energy in the form of photons. These photons, in turn, stimulate other electrons and holes to combine, and a coherent beam is formed along the plane of the junction. The back surface of the diode is generally coated with a highly reflective metal and the front surface is polished to make it partially reflective.
Most semiconductor lasers are used to produce beams of infrared radiation. A major advantage of semiconductor lasers is that they can be made very small. Some types of semiconductor lasers can be made to flash on and off millions of times per second.
HistoryIn the late 1950's, scientists began seeking ways to devise an optical maser--that is, a maser that would generate or amplify light (what is today called a laser). Preliminary studies were done by Charles H. Townes, inventor of the maser, with Arthur L. Schawlow, and by other scientists, including Gordon Gould, Nikolai G. Basov, and Aleksandr M. Prokhorov. The first successful laser was built in 1960 by Theodore H. Maiman of Hughes Research Laboratories. Maiman's laser contained a single large ruby crystal with two parallel surfaces silvered. The beam was emitted in a series of brief, intermittent pulses. Later in the same year, the first gas laser was operated at Bell Telephone Laboratories by Ali Javan and two collaborators. This laser used a mixture of helium and neon and emitted a continuous beam.
Books about Lasers
Beach, D.P., and others. Applications of Lasers and Laser Systems (Prentice Hall, 1993).
Hawkes, John, and Ian Latimer. Lasers: Theory and Practice (Prentice Hall, 1994).
Hecht, Jeff. Understanding Lasers: an Entry-L2evel Guide (Institute of Electrical and Electronics Engineers, 1994.)
For Younger Readers
Billings, C.W. Lasers: the New Technology of Light (Facts on File, 1992).