Laser

The special qualities of laser light make it ideal for a variety of applications. Some types of lasers, for example, are used to play music, read price codes, cut and weld metal, and transmit information. Lasers can also guide a missile to a target, repair damaged eyes, and produce spectacular displays of light. Still other lasers are used to align walls and ceilings in a building or to print documents. Some lasers even can detect the slightest movement of a continent.

Lasers vary greatly in size. One is almost as long as a football field. Another type is as small as a grain of salt.

A typical laser has three main parts. These parts are (1) an energy source, (2) a substance called an active medium, and (3) a structure enclosing the active medium known as an optical cavity. The energy source supplies an electric current, light, or other form of energy. The atoms of the active medium can absorb the energy, store it for a while, and release the energy as light. Some of this light triggers other atoms to release their energy. More light is added to the triggering light. Mirrors at the ends of the optical cavity reflect the light back into the active medium. The reflected light causes more atoms to give off light. The light grows stronger, and part of it emerges from the laser as a narrow beam. Some beams are visible. Others consist of invisible forms of radiation.

There are four main kinds of lasers. They are solid-state lasers, semiconductor lasers, gas lasers, and dye lasers.

In 1960, the American physicist Theodore H. Maiman built the first laser. At first, lasers had few uses, and scientists often thought of them as "a solution looking for a problem." Today, however, lasers rank among the most versatile and important tools in modern life.

Recording, storing, and transmitting information. The most common uses of lasers include the recording of music, motion pictures, computer data, and other material on special discs. Bursts of laser light record such material on the discs in patterns of tiny pits. The discs with recorded music and computer data are called compact discs (CD's).

A laser beam's tight focus allows much more information to be stored on a CD than on a phonograph record, making CD's good for holding data as well as music. Some CD's even can hold an entire encyclopedia. A disc used for storing data is usually called a CD-ROM (Compact Disc Read-Only Memory). Such discs store databases (large files of information held in computers) and are used widely by businesses, libraries, and government agencies.

Lasers can also read and play back the information recorded on discs. In a CD player, a laser beam reflects off the pattern of pits as the compact disc spins. Other devices in the player change the reflections into electrical signals and decode them as music. More lasers are used in CD players than in any other product.

Lasers are used to record movies on large platters called videodiscs. In addition, laser beams can produce three-dimensional images in a photographic process called holography. The images, recorded on a photographic plate, are known as holograms. They appear in advertising displays, artwork, and jewelry, and some are placed on credit cards to prevent counterfeiting.

One of the laser's greatest uses is in the field of fiber-optics communication. This technology changes electrical signals of telephone calls and television pictures into pulses (bursts) of laser light. Strands of glass called optical fibers conduct the light. An optical fiber is about as thin as a human hair. But one fiber can carry as much information as several thousand copper telephone wires. Laser light is ideal for this technology because it can be focused precisely and because all its energy can be introduced into the fiber. Fiber-optic transmission of laser light allows enormous amounts of telephone, TV, and other data to be communicated relatively cheaply.

Scanning involves the movement of a laser beam across a surface. Scanning beams are often used to read information. Many people have become familiar with laser scanners used at supermarket checkout counters. What looks like a line of light is actually a rapidly moving laser beam scanning a bar code. A bar code consists of a pattern of lines and spaces on packages that identifies the product. The scanner reads the pattern and sends the information to a computer in the store. The computer identifies the item's price and sends the information to the register.

Many other kinds of stores use bar code scanners. In addition, such scanners keep track of books in libraries, sort mail in post offices, and read account numbers on checks in banks. Laser printers use a scanning laser beam to produce copies of documents. Other scanners make printing plates for newspapers.

In entertainment, laser light shows are created with scanning laser beams. These beams can "draw" spectacular patterns of red, yellow, green, and blue light on buildings or other outdoor surfaces. The beams move so rapidly they produce what looks like a stationary picture. Laser scanners also produce colorful visual effects that create excitement at rock concerts.

Heating. A laser beam's highly focused energy can produce a great amount of heat. Industrial lasers, for example, produce beams of thousands of watts of power. They cut and weld metals, drill holes, and strengthen materials by heating them. Industrial lasers also cut ceramics, cloth, and plastics.

In medicine, the heating power of lasers is often used in eye surgery. Highly focused beams can close off broken blood vessels on the retina, a tissue in the back of the eyeball. Lasers also can reattach a loose retina. Laser beams pass through the cornea (front surface of the eye) but cause no pain or damage because the cornea is transparent and does not absorb light.

Doctors also use lasers to treat skin disorders, remove birthmarks, and shatter gallstones. Laser beams can replace the standard surgical knife, or scalpel, in some operations. The use of lasers permits extraordinary control and precision in cutting tissue and sealing off cuts. Thus, lasers reduce bleeding and damage to nearby healthy tissues.

In nuclear energy research, scientists use lasers to produce controlled, miniature hydrogen bomb explosions. They focus many powerful laser beams onto a pellet of frozen forms of hydrogen. The intense beams compress (pack down) the pellet and heat it to millions of degrees. These actions cause the pellet's atoms to fuse (unite) and release energy. This process, called nuclear fusion, may produce enough energy to solve the world's energy problems. Lasers have produced the tremendous heat needed to create fusion but have not yet produced usable amounts of energy.

Measuring. People also use lasers to measure distance. An object's distance can be determined by measuring the time a pulse of laser light takes to reach and reflect back from the object.

In 1969 and 1971, United States astronauts placed mirrored devices called laser reflectors on the moon. Using a high-powered laser, scientists measured the distance between the earth and the moon--more than 238,000 miles (383,000 kilometers)--to within 2 inches (5 centimeters). They made the measurement by shining laser light from a telescope on the earth to the reflectors on the moon.

Laser beams directed over long distances also can detect small movements of the ground. Such measurements help geologists involved in earthquake warning systems.

Laser devices used to measure shorter distances are called range finders. Surveyors use the devices to get information needed to make maps. Military personnel use them to calculate the distance to an enemy target.

Guiding. A laser's strong, straight beam makes it a valuable tool for guidance. For example, construction workers use laser beams as "weightless strings" to align the walls and ceilings of a building and to lay straight sewer and water pipes.

Instruments called laser gyroscopes use laser beams to detect changes in direction. These devices help ships, airplanes, and guided missiles stay on course. Another military use of lasers is in a guidance device called a target designator. A person using the device aims a laser beam at an enemy target. Missiles, artillery shells, and bombs equipped with laser beam detectors seek the reflected beam and adjust their flight to hit the spot where the beam is aimed.

An active medium is a material that can be made to create laser light. Gases, liquids, or solid materials can be used.

An energy source is any type of device that supplies energy to the active medium in a process called pumping. Lasers often use electricity, another laser, or a flash lamp as an energy pump. A flash lamp produces a bright flash of light, just as a camera flash does.

An optical cavity, also called a resonator, is a structure that encloses the active medium. A typical cavity has a mirror at each end. One mirror has a fully reflecting surface, and the other one has a partly reflecting surface. The laser beam exits the laser through the mirror with the partly reflecting surface.

The nature of atoms. Laser light results from changes in the amount of energy stored by the atoms in an active medium. The atoms of a substance normally exist in a state of lowest energy, called a ground state. Atoms also can exist in higher energy states, called excited states.

Atoms can change from a ground state to an excited state by absorbing various forms of energy. This process is called absorption. In many lasers, atoms absorb packets of light energy called photons. In most instances, the excited atom can hold the extra energy for only a fraction of a second before the atom releases its energy as another photon and falls back to its ground state. This process is called spontaneous emission.

Some atoms have excited states that can store energy for a relatively long time. These long-lived states can last as long as 1/1,000 of a second--much longer than the duration of most excited states. When a photon of just the right amount of energy shines on an atom in a long-lived excited state, it can stimulate the atom to emit (give off) an identical photon. This second photon has an equal amount of energy and moves in the same direction as the original photon. This process is called stimulated emission.

Producing laser light. Stimulated emission is the central process of a laser. One photon--the stimulating photon--produces another photon. It doubles the amount of light energy present, a process called amplification. The word laser comes from the first letters of the words that describe the key processes in the creation of laser light. These words are light amplification by stimulated emission of radiation.

Stimulated emission only occurs if there are atoms in the excited state. However, atoms in the ground state generally greatly outnumber those in excited states. For amplification to take place, more atoms of a substance must exist in excited states than in ground states. This condition is called a population inversion. In a laser, the energy source helps create a population inversion by pumping energy into the active medium. This energy places atoms in long-lived excited states and enables stimulated emission to occur. The mirrors in the optical cavity reflect the photons back and forth in the active medium.

Each interaction of a photon and an excited atom produces a chain reaction of stimulated emissions. This chain reaction causes the number of stimulated emissions to increase rapidly and produce a flood of light. Part of this intense light exits through the partly reflecting mirror as a strong beam.

Characteristics of laser light. Laser light differs from ordinary light in two major ways. (1) It has low divergence (spreading). (2) It is monochromatic (single-colored). Light with these two characteristics is known as coherent light.

Light from most sources diverges rapidly. Light from a flashlight, for example, fans out quickly and fades after a short distance. But laser light travels in an extremely narrow beam. It spreads little, even over long distances. For example, a typical laser beam expands to a diameter of only 1 meter after traveling 1,000 meters, or only 64 inches per mile.

Light consists of electromagnetic waves, and the color of light is determined by its wavelength (distance from one peak of a wave to the next). Ordinary light consists of waves of many wavelengths--and colors. When all these waves are seen together at the same time, their colors appear white--like those from a light bulb. But light produced by most lasers consists of waves with a very narrow range of wavelengths. Because this range is so narrow, laser light appears to consist of a single color. Some lasers can produce beams with several different colors, but each color band will be narrow. Some lasers produce an invisible beam. These beams consist of such forms of radiation as ultraviolet or infrared rays.

Laser light is highly organized, or coherent. The waves of a laser beam move in phase--that is, all the peaks move in step with one another. These waves travel in a narrow path and move in one direction. Thus, coherent light is like a line of marchers in a parade moving with the same strides in the same direction. The waves of ordinary light, on the other hand, spread rapidly and travel in different directions. Ordinary light is known as incoherent light. Incoherent light acts much like the way people usually travel along a street--with different strides and in many directions. A laser beam's coherence allows it to travel long distances without losing its intensity.

There are four main types of lasers. These types are (1) solid-state lasers, (2) semiconductor lasers, (3) gas lasers, and (4) dye lasers.

Solid-state lasers use a rod made of a solid material as the active medium. Substances made of crystals or glass are widely used. The most common crystal laser contains a small amount of the element neodymium (chemical symbol Nd) in an yttrium aluminum garnet (YAG) crystal. It is called an Nd:YAG laser. In some lasers, the neodymium is dissolved in glass. Flash lamps are generally used to pump the active media of solid-state lasers.

The world's largest and most powerful laser is an Nd:glass laser at Lawrence Livermore National Laboratory in Livermore, Calif. This laser, called Nova, is about as long as a football field. It produces laser light in pulses and is used for nuclear energy research. Its light is split into 10 beams, which are amplified to focus more than 100 trillion watts of power on a target for a billionth of a second.

Nd:YAG and Nd:glass lasers are used widely in industry to drill and weld metals. They are also found in range finders and target designators.

Semiconductor lasers, also called diode lasers, use semiconductors, which are materials that conduct electricity but do not conduct it as well as copper, iron, or other true conductors. Semiconductors used in lasers include compounds of metals such as gallium, indium, and arsenic. The semiconductor in a laser consists of two layers that differ in their electric properties. The junction between the layers serves as the active medium. When current flows across the junction, a population inversion is produced. Flat ends of the semiconductor materials serve as mirrors and reflect the photons. Stimulated emission occurs in the junction region.

Semiconductor lasers are the smallest type of laser. One kind is as tiny as a grain of salt. Another type is even smaller and can be seen only with a microscope. Semiconductor lasers are the most commonly used type of laser because they are smaller and lighter and use less power than the other kinds. Their size makes them ideal for use in CD and videodisc players and for fiber-optic communications.

Gas lasers use a gas or mixture of gases in a tube as the active medium. The most common active media in gas lasers include carbon dioxide, argon, krypton, and a mixture of helium and neon. The atoms in gas lasers are excited by an electrical current in the same way that neon signs are made to light. Gas lasers are commonly used in communications, eye surgery, entertainment, holography, printing, and scanning.

Many gas lasers produce infrared beams. The most important one is the carbon dioxide laser. It ranks among the most efficient and powerful lasers. Carbon dioxide lasers convert 5 to 30 percent of the energy from their energy source into laser light. Many other lasers convert only about 1 percent of the energy they get. Carbon dioxide lasers can produce beams ranging from less than 1 watt to more than 1 million watts. They are often used to weld and cut metals. They also are used as laser scalpels and in range finders.

Dye lasers use a dye as the active medium. Many kinds of dyes can be used. The dye is dissolved in a liquid, often alcohol. A second laser is generally used to pump the atoms of the dye. The most important property of dye lasers is that they are tunable--that is, a single laser can be adjusted to produce monochromatic beams over a range of wavelengths, or colors. Tunable lasers are valuable to researchers who investigate how materials absorb different colors of light.

During the late 1950's, researchers proposed designs for a device that would use stimulated emission to amplify light. Several people have received credit for developing the laser's basic design. They include Townes, American physicist Arthur L. Schawlow, the Russian physicists Alexander M. Prokhorov and Nikolai G. Basov, and the American inventor Gordon Gould.

Theodore H. Maiman of the United States constructed the first laser in 1960. His laser used a ruby rod as its active medium. Later that year, the American physicist Ali Javan constructed the first gas laser. In 1962, three separate teams of U.S. scientists operated the first semiconductor lasers. In 1966, the American physicist Peter Sorokin built the first dye laser.

Advances in laser technology and uses have soared since the early 1970's. Today, the enormous information-carrying capacity of optical fibers is opening a new era in home entertainment, communication, and computer technology. Even so, researchers remain convinced that the most exciting and revolutionary uses of lasers still lie ahead.

Contributor: Donald C. O'Shea, Ph.D., Prof. of Physics, Georgia Institute of Technology.

Related articles include:

Compact Disc; Fiber Optics; Holography.

What occurs in the process called stimulated emission?

Who built the first laser? When?

Why is the semiconductor laser the most commonly used type of laser?

How are lasers used in medicine?

Why is laser light known as coherent light?

What are the main parts of a typical laser?

What is a population inversion?

What is the origin of the word laser?

What is the world's largest and most powerful laser?

Asimov, Isaac. How Did We Find Out About Lasers? Walker, 1990. For younger readers.

Billings, Charlene W. Lasers: The New Technology of Light. Facts on File, 1992.

Bromberg, Joan L. The Laser in America, 1950-1970. MIT Pr., 1991.

Encyclopedia of Lasers and Optical Technology. Ed. by Robert A. Meyers. Academic Pr., 1991.

Hecht, Jeff. The Laser Guidebook. 2nd ed. McGraw, 1991.

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