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~ - A carbon dioxide (CO2) laser cuts heavy-duty circular saw blades from '/4-inch steel sheet. The powerful CO2 laser, the workhorse of industry, has been applied to a wide assortment of tasks, ranging from tough metal work to delicate surgery. 34 I'.... lasers were invented nt~sts streamed of harness ;mg the unique properties of light to study the swift motion of molecules, atoms, and electrons. The military wanted light's awesome power to annihilate enemy tanks, planes, and missiles. Once the practical laser was built, however, the "glamorous blowtorch" began doing jobs no one had dreamed of. Today lasers play music, read price tags, carry phone calls, cut cloth, perform surgery, and test the quality of air. And although the military is still waiting for a light-ray weapon, lasers have become standard research tools for scientists and engineers in laboratories around the world. The word laser stands for light amplifica- tion by stimulated emission of radiation, and laser light is unlike any other. Light waves from a laser all have the same frequency, creating a beam with one characteristic color. The light is also coherent, its waves traveling in phase crest next to crest, trough next to trough. Coherency intensifies the waves' combined power, much as football fans intensify their combined sound by chanting in unison. In addition, the waves are almost perfectly parallel and so travel in nearly the same direction. This directionality keeps the waves concentrated in a narrow beam that widens only gradually over great distance. Incoherent light waves from the sun, light bulbs, and other nonlaser sources travel out of phase at different frequencies in a beam that quickly spreads and disappears from sight. Laser waves are no more powerful than waves of other light. But, because of their unique properties, they are easily focused to a point that can vaporize diamond and steel. Where continuous power of this magnitude is needed, the carbon dioxide laser has been the workhorse since it was introduced commercially in 1967. It drills holes in hard ceramics, cuts through composite materials, and heat-treats metals to harden them. A CO2 laser beam focuses to a fine point for the delicate work of cutting cloth or drilling holes in rubber baby-bottle nipples. Doctors use the CO2 laser as a surgical knife; the laser cauterizes blood vessels as it cuts, eliminat- ing much bleeding. Because light from a CO2 laser is infrared and thus invisible, a red, low-energy helium-neon laser is often used to aim it. Laser light is useful, too, in other areas of medicine. Its single-frequency nature lets a E N G I N E E R I N G A N D T H E A D VA N C E M E N T O F H U M A N W E L FA R E
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laser zap one kind of tissue while causing little harm to another. This is because some materials absorb more light energy at one particular frequency than at others. The colorless central portion of the eye, for example, absorbs little of the argon laser's green light, which is readily absorbed by blood-containing tissue at the back of the eye. Doctors, therefore, use the low-powered argon laser to spot weld detached retinas and seal the leaking blood vessels that often blind diabetics. Lasers also vaporize brain tumors, perform delicate inner-ear surgery, and remove warts and gynecological cysts. Laser light goes inside the body through fiber-optic endoscopes to burn fatty deposits out of clogged arteries, pulverize kidney stones, stanch bleeding stomach ulcers, and open blocked fallopian tubes. Coherent light is necessary for construct- ing the three-dimensional pictures called holograms. They are made by shining one part of a laser beam directly onto photo- graphic film while bouncing the other part off an object and then onto the film. Waves from the two beams interfere with each other in complex patterns that are recorded on the film. When the film is developed, these patterns act like a complex system of micro- scopic mirrors. They reflect back the object's image if the original laser light is shined on the film at the original angle. The patterns are so intricate that they reflect a slightly differ- ent image in slightly different directions. This lets you see the object from different angles and gives the image its three-dimensional quality. Holograms are difficult to counterfeit and so are used as tamper-proof seals on boxes of videocassettes and auto parts as well as on credit cards and passports. Double-exposure holograms are used widely for quality control in, for example, the aircraft tire industry. Disruptions in the delicate wave pattern on a double exposure of a tire reveal defective bulges only 6 millionths of an inch high. Since 1980 holograms have been used to direct the beam in many laser scanners that read price bar codes at checkout counters. Holograms on a whirling disc bend a red helium-neon laser beam in different directions, allowing it to scan for the bar code up to 1,800 times per second. The light pattern reflecting back to L A S E R 5 A researcher aims the beam from a YAG (yttrium- aluminum garnet) laser at a sample of gallium arsenide in an experiment to measure impurities in the semiconducting material. Rows of microscopic bumps on this video disc carry digital information that is read by laser and translated by . . ~ . computer Into visual Images. 35
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- ~ A fraction of a second after the flash of an ultraviolet laser beam, a "smoke" plume erupts from the corneal surface of the eye, shoots upward, and turns into a microscopic mushroom cloud during a surgery experiment. Laser beams of other frequencies pass harmlessly through the cornea to perform surgery inside the eye. 36 sensors from any successful scan will transmit the code. The directionality of laser light makes it very useful for aligning new buildings, tunnels, and pipes as well as leveling and grading land. And the ability to switch them rapidly on and off lets lasers produce the tiny pulses needed for timing measurements of long distances. Lasers can generate pulses of less than 0.1 billionth of a second far shorter than those made by mechanical or electrical switches. By timing pulses bounced off reflectors placed on the moon by U.S. astronauts and Soviet unmanned landers, I laser instruments measure the earth-to-moon distance with less than 1 centimeter of error. Scientists use even shorter laser pulses to observe the lightning movement of atoms, molecules, and chemical reactions. The pulses can, in effect, take "snapshots" quickly enough to prevent blurring. One laser system generates a pulse of just 6 quadrillionths of a second, fast enough to take step-by-step shots of a chemical reaction lasting only 100 quadrillionths of a second. Lasers are also good for measuring very slow movements. Geologists use them to I measure the almost imperceptible creep along the San Andreas Fault in California. A two-laser device on one side of the fault shoots red and blue beams at a reflector farther up the fault on the other side. One beam could be used to measure the distance. But by comparing two beams of different frequencies, geologists calculate how much the atmosphere has slowed the beams on a given day. With this information, they compensate for measurements taken under different atmospheric conditions. Shooting at a reflector 5 kilometers away, the instrument can detect a shift in the earth of only 2 millimeters. The ability to develop lasers with special talents opened the door to their use in communications in the 1970s. The break- through came with development of a semiconductor laser that operates at room temperature, is smaller than a grain of sand, and produces a light frequency that travels well through glass optical fiber. This laser made it practical to use fiber-optic cables for long-distance telephone lines that carry thousands of calls at once. The installation of fiber-optic telephone cables since then has expanded long-distance telephone service and reduced its cost. E N G I N E E R I N G A N D T H E A D VA N C E M The semiconductor laser quickly became the key to compact disc (CD) recordings, which store large amounts of information and can be played at home. CDs were invented in the Netherlands, and the first audio CDs were introduced in Japan in 1982. Information for a disc is translated into the l's and 0's of digital code and then stamped onto the upper side of the disc in a series of long pits. To read the information, a pin- point laser beam scans the bumps on the underside. Flat surfaces between the bumps reflect a strong return beam; bumps scatter the light and weaken the beam. Sensors detect differences between strong and weak beams, interpreting them as digital code. Music, video, and computer data can all be stored in digital form on compact discs. Other types of laser are especially useful for probing the environment. Government agencies and private organizations around the world use ground-based and airborne lasers to measure air pollution, monitor the weather, and study climate. Laser instru- ments have been used to study holes in the ozone layers over the North and South poles, particulates and gases over the Amazon rain forests, and dust drifting across the Atlantic from the Sahara. Laser radar, or lidar (light detection and ranging), detects airborne particles of dust, moisture, and chemicals by measuring the strength of laser light reflected back to the ; instrument. These fine particulates are invisible to normal radar. Weather scientists use lidar plus knowledge of the Doppler shift to study wind speeds. Light reflected off particles moving with the wind changes frequency, which gets higher if the particles are moving toward the observer and lower if they are moving away. A similar frequency shift causes a train whistle to sound higher lo while a train approaches and lower as it speeds away. Another system, called differential absorption lidar, uses laser beams at two I frequencies to detect the presence of a gas in the atmosphere and measure whatever is there. The first beam uses lidar to measure the light reflected back by particulates. The second beam, at a wavelength absorbed by a gas such as ozone, scans the same area. Some of its light is absorbed by the gas but some is reflected back by particulates. The amount of gas in the air and its location are revealed by comparing the return echoes of the two beams. 1 .. E N T O F H U M 1\ N W E L FA R E
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In the future, laser research will aim at reducing the size and cost of lasers while expanding their versatility. Technology is being developed for arrays of tiny semicon- ductor lasers which are limited in size that develop enough power for devices such as printers and facsimile machines. The search for a powerful laser that can be tuned to many frequencies has sparked interest in the free-electron laser. This laser generates light by sending electrons through a periodi- cally alternating magnetic field. Its periodici- ty or its strength can be altered to change the frequency of the light beam. At the same time, existing lasers are being applied to many new tasks. Lasers will find use in industries seeking greater speed and efficiency. They fit well into automation A laser beam shoots moonward from an observatory, left, in a test to learn whether a laser could hit a small target on the moon. Laser beams from this and another observatory appear as two tiny dots of light, above, sparkling on the dark side of the earth in a photo taken from the moon by a television camera on the Surveyor 7 lunar probe. This test led to the use of lasers for measuring the - earth to-moon distance. schemes because they can be operated by computer and used by robots. And, because light beams are quickly redirected with mirrors and fiber-optic cables, lasers can be assigned new jobs without costly retooling. The fastest growing field for lasers, though, is undoubtedly medicine. Lasers are being investigated for reshaping the cornea to correct eye problems. They are being tested for activating toxic anticancer drugs that accumulate in tumors but pass through the rest of the body. And lasers are being used experimentally for drilling new blood channels in weakened hearts and for other techniques aimed at reducing heart disease, the leading cause of death in the United States. LASERS I 37