18 June 2009

Fiber-optics

The invention: The application of glass fibers to electronic communications and other fields to carry large volumes of information quickly, smoothly, and cheaply over great distances. The people behind the invention: Samuel F. B. Morse (1791-1872), the American artist and inventor who developed the electromagnetic telegraph system Alexander Graham Bell (1847-1922), the Scottish American inventor and educator who invented the telephone and the photophone Theodore H. Maiman (1927- ), the American physicist and engineer who invented the solid-state laser Charles K. Kao (1933- ), a Chinese-born electrical engineer Zhores I. Alferov (1930- ), a Russian physicist and mathematician The Singing Sun In 1844, Samuel F. B. Morse, inventor of the telegraph, sent his famous message, “What hath God wrought?” by electrical impulses traveling at the speed of light over a 66-kilometer telegraph wire strung between Washington, D.C., and Baltimore. Ever since that day, scientists have worked to find faster, less expensive, and more efficient ways to convey information over great distances. At first, the telegraph was used to report stock-market prices and the results of political elections. The telegraph was quite important in the American Civil War (1861-1865). The first transcontinental telegraph message was sent by Stephen J. Field, chief justice of the California Supreme Court, to U.S. president Abraham Lincoln on October 24, 1861. The message declared that California would remain loyal to the Union. By 1866, telegraph lines had reached all across the North American continent and a telegraph cable had been laid beneath the Atlantic Ocean to link the OldWorld with the New World.Another American inventor made the leap from the telegraph to the telephone. Alexander Graham Bell, a teacher of the deaf, was interested in the physical way speech works. In 1875, he started experimenting with ways to transmit sound vibrations electrically. He realized that an electrical current could be adjusted to resemble the vibrations of speech. Bell patented his invention on March 7, 1876. On July 9, 1877, he founded the Bell Telephone Company. In 1880, Bell invented a device called the “photophone.” He used it to demonstrate that speech could be transmitted on a beam of light. Light is a form of electromagnetic energy. It travels in a vibrating wave. When the amplitude (height) of the wave is adjusted, a light beam can be made to carry messages. Bell’s invention included a thin mirrored disk that converted sound waves directly into a beam of light. At the receiving end, a selenium resistor connected to a headphone converted the light back into sound. “I have heard a ray of sun laugh and cough and sing,” Bell wrote of his invention. Although Bell proved that he could transmit speech over distances of several hundred meters with the photophone, the device was awkward and unreliable, and it never became popular as the telephone did. Not until one hundred years later did researchers find important practical uses for Bell’s idea of talking on a beam of light. Two other major discoveries needed to be made first: developdevelopment of the laser and of high-purity glass. Theodore H. Maiman, an American physicist and electrical engineer at Hughes Research Laboratories in Malibu, California, built the first laser. The laser produces an intense, narrowly focused beam of light that can be adjusted to carry huge amounts of information. The word itself is an acronym for light amplification by the stimulated emission of radiation. It soon became clear, though, that even bright laser light can be broken up and absorbed by smog, fog, rain, and snow. So in 1966, Charles K. Kao, an electrical engineer at the Standard Telecommunications Laboratories in England, suggested that glass fibers could be used to transmit message-carrying beams of laser light without disruption from weather. Fiber Optics Are Tested Optical glass fiber is made from common materials, mostly silica, soda, and lime. The inside of a delicate silica glass tube is coated with a hundred or more layers of extremely thin glass. The tube is then heated to 2,000 degrees Celsius and collapsed into a thin glass rod, or preform. The preform is then pulled into thin strands of fiber. The fibers are coated with plastic to protect them from being nicked or scratched, and then they are covered in flexible cable.The earliest glass fibers contained many impurities and defects, so they did not carry light well. Signal repeaters were needed every few meters to energize (amplify) the fading pulses of light. In 1970, however, researchers at the Corning Glass Works in New York developed a fiber pure enough to carry light at least one kilometer without amplification. The telephone industry quickly became involved in the new fiber-optics technology. Researchers believed that a bundle of optical fibers as thin as a pencil could carry several hundred telephone calls at the same time. Optical fibers were first tested by telephone companies in big cities, where the great volume of calls often overloaded standard underground phone lines. On May 11, 1977, American Telephone & Telegraph Company (AT&T), along with Illinois Bell Telephone, Western Electric, and Bell Telephone Laboratories, began the first commercial test of fiberoptics telecommunications in downtown Chicago. The system consisted of a 2.4-kilometer cable laid beneath city streets. The cable, only 1.3 centimeters in diameter, linked an office building in the downtown business district with two telephone exchange centers. Voice and video signals were coded into pulses of laser light and transmitted through the hair-thin glass fibers. The tests showed that a single pair of fibers could carry nearly six hundred telephone conversations at once very reliably and at a reasonable cost. Six years later, in October, 1983, Bell Laboratories succeeded in transmitting the equivalent of six thousand telephone signals through an optical fiber cable that was 161 kilometers long. Since that time, countries all over the world, fromEngland to Indonesia, have developed optical communications systems.Consequences Fiber optics has had a great impact on telecommunications. Asingle fiber can now carry thousands of conversations with no electrical interference. These fibers are less expensive, weigh less, and take up much less space than copper wire. As a result, people can carry on conversations over long distances without static and at a low cost. One of the first uses of fiber optics and perhaps its best-known application is the fiberscope, a medical instrument that permits internal examination of the human body without surgery or X-ray techniques. The fiberscope, or endoscope, consists of two fiber bundles. One of the fiber bundles transmits bright light into the patient, while the other conveys a color image back to the eye of the physician. The fiberscope has been used to look for ulcers, cancer, and polyps in the stomach, intestine, and esophagus of humans. Medical instruments, such as forceps, can be attached to the fiberscope, allowing the physician to perform a range of medical procedures, such as clearing a blocked windpipe or cutting precancerous polyps from the colon.