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~ - ~ Right: Ultrapure glass fibers encased t ~protective ribbons are t ~ainstay of fiber .................. Fiber-Oplic ~ me Munson ............................... information revolution . I 'of' t2he 1960s created such an ticcom= ~.~7ton>~t - ~1 Avalanche of data that futurists saw it soon over whelming the telephone lines and radio waves that carry information around the globe. That vision and the invention of lasers spurred research into the tremendous potential of light, which can carry thousands of times more information than either electric signals or radio waves The work led in 1970 to a very transparent glass thread whose appearance quickly opened the door to fiber-optic communication, a technology able to channel the flood of information into the twenty first century. Today's optical fiber is thinner than a human hair. Yet it carries so much informa tion that four fibers two in each direction in a trans-Atlantic telephone cable are handling up to 40,000 calls at once. The cable, 38 called TAT-8 and laid in 1988, is the first transoceanic fiber-optic cable. TAT-7, the copper cable laid in 1983, carries less than a fourth as many calls and is twice as thick. The huge capacity of optical fibers translates into lower construction cost per telephone circuit. The first trans-Atlantic copper cable, for example, cost more than $1 million per circuit to install in 1956; 32 years later, TAT-8 cost less than $10,000 per circuit. l Waves of light carry more information because they have higher frequencies- measured in wave cycles per second, or hertz than either radio waves or the electric I waves on copper telephone wires. High- frequency waves can be switched on and off faster than low-frequency waves, so they can I E N G I N ~ 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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be divided into more pulses per second. The pulses and gaps between them represent the l's and O's of digital information. The infrared laser light used in optical telephone cables has a frequency of about 100 minion megahertz, 100 million times higher than a typical AM radio signal and 100 billion times higher than an electric telephone signal. Fiber-optic communication became practical with the development of fibers that carry light pulses a substantial distance F I B E R - O P T I C C O M M U N I C A T I O N The green light of a an argon laser flows through several miles of experimental optical fiber wound around a spool. Similar hair-thin optical fibers are the heart of cables that carry 40,000 or more simultaneous telephone calls on beams of laser light under oceans and across continents. 39
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Below: Fiber-optic cables carry many more communica- tion signals than do copper cables of similar size. Fiber cable laid in the trans-Atlantic link is less than half the size of copper cable used in an earlier link but carries more than four times as many telephone calls. before needing a repeater. Their cores are made of silica glass, one of the most transpar- ent solid substances known. Impurities in early glass fibers weakened pulses so much that light lost 99 percent of its energy after just 20 meters. But a new chemical process, pioneered in the 1970s, now produces cores of such high transparency that fibers in TAT- 8 need only 101 repeaters to boost signals expensive than lasers. Although their signals deteriorate over long distances, LEDs are efficient for short links between, for example, a compact-disc player and stereo amplifier or computers in a local network. Optical fibers can handle many times more information if they carry light pulses of several frequencies at the same time. The bandwidth of light (its range of frequencies) it..-'- .; - The C.S. (Cable Ship) Long Lines laid more than 3,600 miles of the TAT-8 fiber-optic cable, from the United States to a point off the coast of Europe. From there, British and French ships laid branch cables to their respective countries. In all, nearly 4,200 miles of cable were laid. 40 across the Atlantic. TAT-7, the copper cable, needs 662. The debut of a practical fiber coincided with the development of other technologies needed for fiber-optic communication. In 1970 a room-temperature, semiconductor laser was built that could transmit well- defined pulses through long-distance optical fiber. Photodetectors were developed that could handle the torrent of pulses pouring out the other end and convert them into electric signals. For short-distance systems, tiny light-emitting diodes (LEDs) were devised that use less power and are less is about 10 billion megahertz from infrared to ultraviolet and can be divided into millions of signal channels, each at a slightly different frequency. Radio waves cover a relatively tiny bandwidth, leaving little room for multiple channels. The bandwidth is even smaller for electric channels. Most optical fibers today carry light of only one frequency, although they are capable of transmitting several more. So far, I it has proved economical to use multifre- quency systems only in special cases of growing demand where limited numbers of fibers have already been installed. For I example, fiber pairs that were carrying 24,000 voice circuits between Chicago and Philaclelphia have been upgraded to transmit E N G ~ N E E R ~ 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 ~ FA R E
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two frequencies, so they now carry 48,000 circuits. Fiber-optic communication has other advantages over radio and electric channels. Light signals do not produce the external halo of electromagnetic waves that surround electric telephone lines and cause "cross talk" between them. Optical fibers are also relatively secure from unwanted eavesdrop- pers, who can easily intercept radio waves or tap electric telephone signals without being detected. Optical fibers have been quietly weaving into communication networks since the first commercial system opened in Chicago in 1 1977. Since then more than 1.5 million miles of optical fiber have been installed across the United States; copper cables are no longer laid for main lines between U.S. cities. An undersea fiber-optic cable that U.S. and Japanese cable ships laid across the Pacific was activated in 1989. Optical fibers have also been adapted for sensing movement in gyroscopes, linking industrial lasers to cutting and drilling tools, and threading light ~ into the human body for examinations and I laser surgery. I A future engineering challenge lies in ! building a computer that uses light instead of electricity. Theoretically, an optical computer I would be 1,000 times faster than the best modern supercomputers. Another challenge closer to home is in stringing fibers to individual users from the optical main lines that now exist. At present, copper wires connect homes to central telephone exchanges, where electric signals are convert- ed into light pulses that are channeled into the main lines. Telephone authorities in the United States, Great Britain, Japan, France, Canada, and West Germany have been running optical fibers to homes on an experimental basis since 1980. Once econom- ic and regulatory obstacles are overcome, the pace of replacing copper wires with optical fibers to individual homes in the United States should pick up. Economical optical switches and converters for such "end-to- end" systems still need to be developed. But early ~ the twenty-first century, many people could be enjoying hundreds of television shows, movies, newspapers, and books brought into their homes through a single, hair-thixl optical fiber. F ~ B E R - O P T I C C O M M U N ~ C A T ~ O ~ i A technician in Philadelphia checks a device that uses microprocessors to pinpoint a malfunction among signal regenerators along the heavily amused fiber-optics line between New York City and Washington, D.C. Left: Optical fibers can be threaded into the ureter, the tube leading from the bladder to the kidney, to bring in laser light that dislodges kidney stones. The photo here shows a laser chipping a kidney stone into fragments small enough to pass through I the ureter naturally. 41