There are many promising areas of application for the new light-guide technology. For example. television signals could easily be carried over a single fiber, thereby opening up new possibilities for both entertainment and business purposes. Buildings could be "wired" with almost invisible fibers to provide internal communication services. The parts of computers could be interconnected with fibers. It is in telephony. however. that one can expect some of the first important applications.

Today much of the copper cabling that interconnects metropolitan telephone- switching centers goes through underground ducts where space is at a premium. Adding new duct space is both costly and inconvenient. Lightwave communication systems with their high capacity and small size could make better use of the existing underground ducts and help to postpone the need for new ones. Moreover. since adjacent switching centers in many cities are less than seven kilometers apart, light-wave systems might not require any amplifiers in manholes to boost signals along a typical route.

Before the completion of the Chicago installation Bell Laboratories and the Western Electric Company tested a prototype light-wave system under simulated field conditions last year in Atlanta. Two light-guide cables 640 meters long, each containing 144 fibers, were pulled through standard underground ducts and subjected to tests simulating a typical urban telecommunication environment. The installation work did not break any of the fibers, and the pulling operation, which required the negotiation of sharp bends, did not degrade the performance of the light guides. As in the present Chicago system, each pair of fibers carried the equivalent of 672 two-way voice channels. The light sources were gallium aluminum arsenide lasers operating at a rate of 44.7 million bits per second. At the receiving end the light pulses were converted into electrical signals by avalanche photodetectors. As part of the Atlanta experiment the ends of some individual fibers were joined to create a continuous communication path about 70 kilometers long. With the help of 11 regenerators, or amplifiers, virtually error-free transmission was achieved in one direction over the full path for a sustained period. The Chicago installation closely follows the Atlanta experimental system except that LED's are being used in addition to lasers as light sources.

Apart from some references to the anticipated red uction of fiber losses in the future, everything I have described here is based on current technology. It would be contrary to all previous experience to believe we shall not witness further dramatic developments. For example, a number of industrial and university investigators are conducting experiments with integrated optics, which include techniques for processing light signals within thin films, the optical equivalent of integrated microelectronic circuits. Such optical circuits may someday eliminate the need for converting light pulses to and from electrical signals in amplifiers along transmission paths. In addition both theoretical and experimental work is proceeding on the possibility of switching light pulses directly, obviating the need for first converting the light signals into their electrical equivalent. The hope is to develop optical switches to replace the present electromechanical and electronic devices, thereby making it possible to connect telephone calls in greater numbers and at higher speeds than ever before.

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