Laser, << LAY zuhr, >> is a device that produces a very powerful beam of light. Such a beam can travel over long distances or be focused to an extremely small diameter. Some tightly focused beams can drill 200 holes on a spot as tiny as the head of a pin. Some beams are powerful enough to pierce a diamond, the hardest natural substance known. A large laser system can trigger a small nuclear reaction. Laser beams have been sent to the moon and their reflections detected back on Earth.
The special qualities of laser light make it ideal for a variety of applications. It can be 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. Other lasers are used to align walls and ceilings in building construction or to print documents. Some laser devices can detect the slightest movement of a continent or precisely map a three-dimensional space.
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 or gain medium, and (3) an optical cavity, a structure enclosing the active medium. The energy source typically supplies energy in the form of electric current or light. The atoms or molecules of the active medium can absorb the energy, store it for a while, and then release it as light. Some of this light triggers other atoms to release their energy. 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. Beams can be produced with visible or invisible forms of radiation.
In 1960, the American physicist Theodore H. Maiman built the first laser. Today, lasers rank among the most versatile and important tools in modern life.
How lasers are used
Lasers can do a number of incredible things. Their unique qualities make them particularly useful in recording, storing, and transmitting many kinds of information. Lasers also are valuable in such processes as scanning, heating, measuring, and guiding. As a result of their wide use, lasers can be found in equipment used in homes, factories, offices, hospitals, and libraries.
Recording, storing, and transmitting information.
Bursts of laser light record music, computer data, and other material as patterns of tiny pits in the surface of special discs. Such discs include CD’s, DVD’s, and Blu-ray discs. Lasers also read and play back the information recorded on these discs. In a disc player or disc drive, a laser beam reflects off the pattern of pits as the disc spins. Other devices in the player or drive change the reflections into electrical signals, which are translated into music, computer data, motion pictures, and other information. More lasers are used in disc players and drives than in any other product.
Loading the player...Compact disc
Laser beams can produce three-dimensional images in a photographic process called holography. The images, recorded on a flat photographic plate, are known as holograms. They appear in advertising displays, artwork, and jewelry, and some are placed on credit cards to help prevent counterfeiting.
One of the laser’s most important uses is in the field of fiber-optic communication. This technology converts the electrical signals that represent telephone calls, television pictures, and computer data into pulses (bursts) of laser light. Strands of glass or plastic called optical fibers conduct the light. Such a fiber is about as thin as a human hair. But a single 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 almost all its power can be introduced into the fiber. Laser light can travel long distances within a fiber without diminishing much in intensity. However, if needed, a device called a fiber amplifier can be used to intensify the laser light within the fiber.
Scanning
involves the movement of a laser beam across a surface. Scanning beams are often used to read information. Laser scanners are used at the checkout counters in many stores. 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 cash register. Scanners are used to keep track of books in libraries, sort mail in post offices, and read account numbers on checks in banks. Laser printers use scanning laser beams to produce copies of documents.
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 or an animation. Laser scanners also produce colorful visual effects that create excitement at rock concerts.
Heating.
A laser beam’s highly concentrated 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. Laser beams of certain wavelengths can 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. Highly focused beams can pass through the cornea and cauterize (sear closed) broken blood vessels on the retina, a tissue in the back of the eyeball. Lasers also can reattach a loose retina. Lasers of a different wavelength can be used to reshape corneas, enabling some people to see clearly without glasses or contact lenses.
Doctors also use lasers to treat skin disorders, remove birthmarks and tattoos, 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 cauterizing blood vessels. Thus, lasers reduce bleeding and damage to nearby healthy tissues.
In nuclear energy research, scientists use lasers to produce controlled nuclear reactions. 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 hydrogen atoms in the pellet to fuse (unite) and release energy. This process, called nuclear fusion, may be able to 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 use lasers to measure distance. An object’s distance can be determined by measuring the time a pulse of laser light takes to reach the object and reflect back to the laser’s source.
In 1969 and 1971, astronauts placed mirrored devices called laser reflectors on the moon. Using a high-powered laser, scientists measured the distance between 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 Earth to the reflectors.
Laser beams directed over long distances 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 range finders to obtain information needed to make maps. Military personnel use them to determine the distance to a target.
Positioning.
Manufacturers use lasers to precisely position wafers of silicon for making computer chips. Such precision enables them to create chips with details mere tens of nanometers (billionths of a meter) wide.
Guiding.
A laser’s strong, straight beam makes it a valuable tool for guidance. For example, construction workers use laser beams to project perfectly straight lines 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.
How a laser works
Parts of a laser.
A typical laser has three main parts. These parts are an active or gain medium, an energy source, and an optical cavity. The active medium is a material that can be made to create laser light. Gases, liquids, solid materials, plasmas (electrically charged gaslike substances), and electrons 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 electric power, 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. Most often, one mirror has a fully reflecting surface and the other has a partly reflecting, partly transmitting surface. The laser beam exits the laser through the mirror with the partly transmitting surface.
Producing laser light.
Laser light results from changes in the amount of energy stored by the atoms in an active medium. Atoms normally exist in a state of lowest energy, called the ground state. Atoms also can exist for a brief time 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. The atom then 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 1/1000 of a second. When a photon of just the right amount of energy strikes 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 exact same direction as the original photon. This process is called stimulated emission. Not only atoms, but also molecules, electrons, ions (electrically charged atoms), and excimers can serve as a medium to produce stimulated emission under proper conditions. Excimers are diatomic molecules—that is, molecules consisting of two atoms—that exist only in an excited state and are very short-lived.
Stimulated emission is the central process of a laser. One photon—the stimulating photon—produces another photon. This adds to the light energy already 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. But atoms in the ground state generally greatly outnumber those in excited states. For amplification to take place, more atoms of the medium must exist in excited states than in ground states. This condition, called a population inversion, is created by pumping. The energy pumped into the active medium 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 emission events 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 three major ways: (1) it has low divergence (spreading), and (2) it is monochromatic (single-colored), and (3) it is coherent, meaning the laser light waves travel in a highly organized manner.
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 a very narrow beam, 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 about 60 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, and so it 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 such forms of radiation as ultraviolet rays, infrared rays, or X 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 incoherent (ordinary) light, on the other hand, spread rapidly and travel in different directions. Incoherent light acts in much the same way as people walk along a sidewalk—with different strides and in many directions. A laser beam’s coherence allows it to travel long distances without losing its intensity.
Kinds of lasers
Lasers produce light either in a continuous beam or in pulses. The lasers that generate pulses, called pulsed lasers, supply all their energy in only a fraction of a second. As a result, they generally produce a greater peak power than lasers that produce a continuous beam, called continuous-wave lasers. Most continuous wave lasers range in power from less than 1/1000 of a watt to almost 30,000 watts. Some pulsed lasers can produce a short pulse of over 1 quadrillion (1,000 trillion) watts.
There are four main types of lasers. They are (1) solid-state lasers, (2) semiconductor lasers, (3) gas lasers, and (4) dye lasers.
Solid-state lasers
use a rod or disk of a solid material, often a crystal or glass, as the active medium. The most common crystal laser contains a small amount of the element neodymium (Nd) in an yttrium aluminum garnet (YAG) crystal. In some lasers, the neodymium is dissolved in glass. Nd:YAG and Nd:glass lasers are used widely in manufacturing to drill and weld metals. They are also found in range finders and target designators. Flash lamps are generally used to pump solid-state lasers. Sapphire crystals that contain titanium are also used as a solid-state medium. Titanium-sapphire lasers can produce variable wavelengths, a characteristic that is important for many applications. They can generate laser pulses that last only a few quadrillionths (thousandths of a trillionth) of a second.
The National Ignition Facility (NIF) operates the world’s largest and most powerful laser system. The facility, at Lawrence Livermore National Laboratory in Livermore, California, is the size of three football fields. It uses 192 separate lasers to direct light at a single point. In a tiny fraction of a second, the laser can generate almost 2 million joules of energy. Such concentrated energy generates heat and pressure similar to the fusion process of a star or a nuclear weapon. The facility was completed in 2009.
Semiconductor lasers,
also called diode lasers, use semiconductors as the active medium. Semiconductors are materials that conduct electric current but do not conduct it as well as copper, iron, or other true conductors. Semiconductors used in lasers include compounds of metals, such as aluminum, 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 lasers. They can be as tiny as a grain of salt. Semiconductor lasers are the most commonly used lasers because they are smaller and lighter and use less power than the other kinds. Their size makes them ideal for use in fiber-optic communications and in CD and DVD players.
Gas lasers
use a gas or mixture of gases in a tube as the active medium. The most common active media include carbon dioxide, argon, krypton, and a mixture of helium and neon. The atoms in gas lasers are excited by an electric current in the same way neon signs are made to light. Gas lasers are commonly used in communications, entertainment, eye surgery, holography, printing, and scanning. Gas lasers can produce infrared, visible, and ultraviolet light.
Carbon dioxide lasers are among the most efficient and powerful lasers. They convert 5 to 30 percent of the energy from their energy source into laser light. They can produce beams ranging from less than 1 watt to more than 1 million watts. Carbon dioxide lasers are often used to weld and cut metals. They also are used as laser scalpels and in range finders.
Dye lasers
use a chemical 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 dye molecules. 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.
Other kinds of lasers
include excimer lasers, X-ray lasers, and free-electron lasers. Many excimer lasers are used in eye surgery to sculpt corneas. X-ray lasers are used to probe and to create plasmas, and they can also be used to study the atomic structures of other materials. Free-electron lasers generate high-energy radiation of variable wavelengths. They have applications in the development and processing of metals, plastics, and other materials.
History
Lasers were not invented before the 1900’s chiefly because scientists did not know about stimulated emission. The process was first described in 1917 by the German-born physicist Albert Einstein. The next major advance in laser development came in 1954. That year, the American physicist Charles H. Townes created a population inversion in a device that amplified microwaves, an invisible form of radiation. The device was called a maser because it demonstrated microwave amplification by stimulated emission of radiation.
During the late 1950’s, researchers proposed designs for a device that would use stimulated emission to amplify visible 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 has opened a new era in home entertainment, communication, and computer technology.
