The early lasers, however, were cumbersome and unreliable; the best of them failed after a few months of operation. Furthermore, it soon became obvious that the concept of projecting laser beams from point to point through the atmosphere, in analogy with microwave radio-relay systems, was unsatisfactory because the signal would be attenuated by fog, smog, rain and snow. Actually it is easier to transmit light pulses reliably from Arizona to the moon than it is to transmit them between downtown and uptown Manhattan.

There has been steady progress in making lasers compact, reliable and long-lasting and in circumventing the transmission of the light beams through the atmosphere. Moreover, although for some demanding applications lasers are still preferred, for others a simpler and cheaper device, the high-intensity light-emitting diode (LED), is adequate. The first promising alternative to transmitting light signals through the atmosphere consisted in sending optical signals through a light pipe: a carefully fabricated tube a centimeter or so in diameter provided with optical means (possibly local variations in the density of a gas in the pipe) for bending the rays wherever the pipe had to diverge from a straight line.

As an alternative to the light pipe, which presented many practical problems, communication engineers began studying the possibility of transmitting light through glass fibers. Bundles of glass or plastic fibers had been used for some time to carry light short distances, for example to light an instrument panel or to examine the interior of the stomach, but they were not nearly transparent enough for the purposes of lightwave communications. The materials commonly employed were less transparent than water. The glass fibers ultimately developed for communications are so transparent that if seawater were as clear as they are, one could easily see to the bottom of the deepest ocean.

Before considering the kinds of communication system one can build from lasers, light-emitting diodes and glass fibers let us examine how information from a source, such as a telephone, a television camera or a computer, is made suitable for transmission by light. In conventional "analogue" transmission systems the wave pattern of the original signal is used to modulate the amplitude of the energy entering the transmission line, in this case the amplitude of the light beam emerging from a light source and entering a glass fiber. At the far end of the fiber the light enters a photodetector that converts the varying intensity of the light into a corresponding electrical signal. The signal is amplified as needed to reproduce the incoming electrical waveform for presentation to the ear, the eye or an inanimate device such as a computer.

Even in the best fibers some of the light is lost by absorption and scattering, so that the strength of the light signal decreases geometrically as the signal travels from the source to the detector. For example, if the strength of the light signal falls to half its original value after the signal has traveled a kilometer, the strength will fall to a fourth of the original value at the end of the second kilometer, and so on. Thus for long-distance transmission the light source should be as powerful as possible and the detector as sensitive as possible, other things being equal.

At present this requirement is best met by high-intensity lasers and ultrasensitive photodetectors of the "avalanche" type, that is, detectors in which each incoming photon triggers an avalanche of electrons. It should be observed, however, that the maximum transmission range is much more dependent on losses in the fiber than on the power of the source or the sensitivity of the detector. For example, decreasing the loss by a factor of two will exactly double the range, whereas increasing the power of the source by a factor of two will normally increase the range by only about 10 percent (to be precise, by that length of fiber which will increase the loss by a factor of two).

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