Light Conversation

"Mr. Watson, come here! I want you!"

That command--shouted in 1876 by Alexander Graham Bell and heard in another room by his assistant, Thomas Watson, over Bell's first working telephone, was repeated in Boston last week. The occasion: an American Telephone and Telegraph banquet commemorating the 100th anniversary of the telephone. To demonstrate a century of progress, the teen-age descendants of Bell and Watson who re-enacted the historic moment then placed a call that was transmitted between two modern telephones not by electrical current or radio waves but by a beam of light passing through a hair-thin glass fiber. Proclaimed

AT&T Chairman John deButts: "I anticipate that by the early 1980s cables of glass fibers will be carrying thousands of simultaneous messages between major switching centers in our big cities."

Morse Code. The idea of using light to convey information far predates the new fiber-optics technology demonstrated so dramatically by AT&T. Primitive man sent signals by building fires or waving torches; ships still use shuttered signal lamps to flash messages to each other. Proof that light could be sent along a curved "pipe"--like electricity flowing through a wire--was provided by British Physicist John Tyndall in 1870. He showed that light shining down on a tank of water could be carried by a stream pouring from a hole in the side of the tank to illuminate the spot on which the stream fell.

It took many decades of research and three basic developments to make communication by optical wires a reality. One development was the invention in 1960 of the laser, a device capable of generating an intense narrow beam of light that, for all practical purposes, did not diverge. Miniaturized lasers make it possible to couple powerful light beams accurately with hair-fine glass fibers. Another was the perfection, by Corning Glass Works, of a fiber of glass so pure that it could transmit light long distances. The third accomplishment was the devising, by engineers at Bell Labs and elsewhere, of methods of integrating fiber optics into modern telephone systems.

In a conventional telephone hookup, sound waves entering a microphone are converted into electrical pulses, which travel along a copper wire to another phone, where they are converted back to sound waves. In a typical optical arrangement (see diagram), sound waves entering a telephone microphone are converted into electrical signals. These signals pass through an encoder, which converts them into electrical pulses that switch a laser on and off, interrupting a light beam being sent into the end of a fiber. The light thus travels in a series of pulses, not unlike Morse code, that race along the glass "wire." At the end of their journey, these light pulses are picked up by a photodetector, which converts them back to electrical pulses. These, in turn, are fed into a decoder for translation into an electrical signal that vibrates a diaphragm in the receiver, reproducing the voice.

Fibers have enormous advantages over wires. Because they do not "leak" light as copper wires "leak" electricity, fibers should eliminate the cross talk and static that can occur when one telephone wire spills some of its signal into a neighboring line. Measuring as little as one-thousandth of an inch in diameter, the fibers are also far less bulky than wires --an important consideration in cities, where underground cable conduits are already overcrowded. Eventually, the fibers may also prove cheaper. Supplies of copper are limited; silicon, the chief ingredient of glass fiber, is one of the most plentiful materials on earth.

Field Test. Bell Labs is currently field-testing an experimental fiber-optics communications system in Atlanta. But much work must still be done before glass replaces copper in regular systems. Engineers are still trying to find efficient ways of joining the threadlike fibers together. Researchers are working to increase the lifetime of the lasers used to generate the fine beams upon which optical communication depends; the lasers now in use have a projected lifetime of 100,000 hours; researchers would like to increase this to 1 million hours. Scientists are also developing integrated optical circuits, the optical equivalent of the chips that operate digital watches and pocket calculators. Bell engineers are particularly interested in using the circuits to boost or amplify light beams, or to switch them from one fiber to another.

Police in Bournemouth, England, are now using an optical system developed by International Telephone and Telegraph to link their radio room with a computer data bank that enables them to keep track of their patrol cars. Fiber-optics circuits are being tested as control systems in U.S. military aircraft and ships; a Japanese power company is using fiber-optics circuits, which are not affected by nearby high-tension lines, to control some of its equipment.

Amnon Yariv, professor of electrical engineering at California Institute of Technology, predicts that optical circuits will permit and, indeed, encourage an increase "by a factor of thousands" in the amount of information flowing in and out of the average home as people use their phone lines more and more to gain access to everything from their checking accounts to computers and consumer services. A single glass fiber can now be made to carry up to 672 one-way conversations simultaneously. This means that eight fibers, a bundle no thicker than a pencil lead, could do the job now being done by a 3-in. telephone cable.

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