Реферат: Волоконная оптика
However, the new invention did not prove to be very practical. Sunlight was only available during the daytime. And even then, bad weather such as fog, rain, or snow blocked the beam of light.
In spite of these problems, throughout his life, Alexander Graham Bell thought the photophone was his most promising idea. He felt certain that someday people would use beams of light to talk to each other.
For nearly one hundred years, scientists like Bell dreamed of using light to communicate. They knew that light and electricity traveled as vibrations or waves. And they knew that many more light waves could he transmitted in one second than electrical waves. For this reason, light could carry more information than electricity flowing in copper wires.
Not until the 1960s and 1970s did two inventions make the dream possible. During this time, scientists invented lasers. Lasers are powerful sources of a special kind of light. Other researchers developed optical fibers.
An optical fiber is a flexible thread of very clear glass—thinner than a cat's whisker and up to six miles long. Laser light can pass through the length of an optical fiber and still stay bright. Because optical fibers can serve as pipelines for light, they also are called lightguides.
In the mid-1970s, these inventions were teamed together. Now pulses of light flash through optical fibers carrying information and messages over great distances. This important new technology is called fiber optics.
Glass fibers are replacing copper wires for many reasons. The fibers are not as expensive for telephone companies to buy and install. They weigh a lot less than copper wires — making them easier for workers to handle. A single four-and-one-half-pound spool of optical fiber can carry the same number of messages as two hundred reels of copper wire that weigh over sixteen thousand pounds!
Optical fibers also take much less space than copper wires. This is very important in crowded cities where bulging, overloaded telephone cables have little room for additional lines. Optical fibers can help unsnarl this telephone traffic jam.
The fibers are better, too, because light is nut affected by nearby electrical generators, motors, power lines, or lightning storms. These often are the cause of noisy static on telephones or information errors in computer systems connected by copper wires.
As electrical signals pass through copper wires, they become weakened. Devices called repeaters are used to strengthen the electrical signals about every mile along each line. In a fiber optic system, repeaters are needed only every six miles or so to boost the light signals. And experiments have shown that this distance can be stretched many more miles. This means that installation costs for a fiber optic system are less now and can be cut further in the future.
However, the most important reason for using glass fibers is that they can carry much more information than copper wires. A single pair of threadlike glass fibers can transmit thousands of telephone calls at once. A cable as thick as your arm and containing 256 pairs of copper wires would be needed to handle the same number of conversations.
Pairs of fibers (or wires) are used for two-way communication. One fiber carries your voice to the listener at the other end of the line. The other member of the pair transmits the other person's reply to you.
Optical fibers are less expensive, easier to install, and more dependable than copper wires. With tight from a laser, they can transmit thousands of times more information than electricity in copper wires. The new technology of fiber optics is a better and faster way to communicate.
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Where Are Optical Fibers Used?
All over the world, the copper wires of telephone trunk lines are being replaced by modern glass optical fibers.
One of the first attempts to use an optical fiber system in the United States was in 1977 in Chicago. There, two offices of the Bell Telephone Company and a third building for customers were connected successfully by twenty-four light-carrying glass fibers. The fibers were threaded through telephone cables already under the city streets. The total length of the fibers was about 1.5 miles.
In 1978, Vista-United Telecommunications at Walt Disney World near Orlando, Florida was first to use fiber optics commercially in the United States. Telephones throughout the 28,000-acre park are linked by fiber optic trunk lines. Video transmissions by glass fibers are made to many individual hotel rooms on the property from one location. Lighting and alarm systems also use optical fibers.
In EPCOT Center (Experimental Prototype Community of Tomorrow), there are information booths equipped with television-tike, two-way video screens and speakers. The screens and speakers are connected by optical fibers to a central office. A visitor can activate the screen by touching it and select the information needed. Or the guest can talk to an operator who appears on the screen if requested.
American Telephone and Telegraph has in service a fiber optic trunk line that connects Boston, New York City, Washington, D.C., and Richmond, Virginia. The trunk line is part of a project 780 miles long. The light cable used is only about the thickness of a garden hose. Nevertheless, it can carry eighty thousand calls at once.
By July 1988, American Telephone and Telegraph will have laid a fiber optic cable beneath the ocean between North America and Europe. The cable is called TAT-8 because it is AT&T's eighth /transatlantic telephone cable. TAT-1, a copper cable, was completed in 1956 and could carry fifty-one calls at a rime. TAT-7, the last copper cable, was laid in 1983. It can handle about eight thousand calls at once. ТЛТ-8 will transmit forty thousand calls at one time. Even with TAT-8, a second fiber optic transatlantic cable, TAT-9, probably will be needed by 1991. Another undersea cable, between California and Hawaii, is planned.
The Japanese telephone company, Nippon Telephone and Telegraph, has placed glass fiber cables from one end of the country to the other. By 1990, similar lines will join Japan to Hong Kong, Australia, and New Zealand.
A fiber optic system in Munich and other cities of West Germany is called Bigfon. It transmits a video picture along with voice. In addition, Bigfon sends and receives copies of documents and other important papers.
Over fifteen hundred customers in Biarritz, France, use videophones and television channels made possible by fiber optics.
In the remote countryside of Manitoba, Canada, two towns are part of an experiment. Elie and St. Eustache have become "glass-wired" communities. Optical fibers connect keyboards and television sets in homes in these towns to distant computers. People who live there use the keyboards to get television shows, radio broadcasts, weather forecasts, news, farm and stock market reports. In addition, over three hundred items for sale at a large, well-known department store, Hudson Bay Company, can be viewed on television. To make a purchase, a customer types an item code, number of items wanted, size, color, and credit card number on the keyboard. Hudson Bay Company receives the order and ships the goods directly to the customer.
Near Tokyo, in Japan, there is an optical fiber communications network known as HI-OVIS (Highly Interactive Optical Visual Information System). With this two-way system people can take an active part in educational classes such as piano lessons. They also can learn about schedules for airlines, trains, and concerts, and get up-to-the-minute news and weather reports.
New installations for communications at Kennedy Space Center in Florida use fiber optics. These include the Space Shuttle control center and operations building for Launch Complex 39. In addition, the Space Center's fiber optic system is used to check out experiments, such as those on board Skylab, before launch. Eventually, all of the facilities for the Shuttle at Kennedy Space Center will use fiber optic systems.
There arc many other uses for fiber optics. A medical instrument known as an endoscope is made from bundles of optical fibers packed inside a long, slim, bendable tube. A doctor slips this medical "spyglass" into a patient's throat, stomach, lungs, or intestines to look for anything abnormal. One bundle of fibers carries light to the tip of the probe. Another bundle of fibers transmits pictures back to an eyepiece. This allows a doctor to see inside the human body without surgery. And sometimes it locates early stages of serious diseases, such as cancers, that X-rays may miss. Miniature tools within a separate channel in the endoscope tube can remove samples of tissue for a closer look.
Veterinarians examine horses, cats, clogs, and other animals with similar fiber optic scopes. Pets sometimes choke on foreign objects. With the probe of the scope, the animal doctor can locate the object, snare it, and quickly remove it.
People peer into dangerous or hard-to-see places with industrial fiber optic scopes too. Workers can look inside and check radioactive reactors in nuclear power plants, the jet engines of airplanes, turbines, boilers, pipelines, gear boxes, and many other types of machinery.
Image conduits are large pipelines for light. They are formed from thousands of optical fibers that have been bundled and fused together into one unit. They can directly transmit images or pictures from one place to another. If the conduit is tapered on one end, it can be used to make an image larger or smaller. And if the fibers in the conduit are twisted, the picture can be turned upside down.