Optical fibre systems

Abstract

The present state of research and development into optical fibre systems for applications in telecommunications is reviewed and some of the principal problems remaining are discussed. Attenuations close to the intrinsic limits of the materials available have been reached in laboratory fibres and losses in optical cables installed under normal working conditions are below 5 dB/km. Bandwidths available range from 20 MHz km, in step-index multimode fibres with light emitting diode sources, to 10 GHz km with single-mode fibres and semiconductor lasers. If a truly monochromatic laser source operating in the region of minimum material dispersion becomes available then individual fibre capacities up to, or beyond, 100 GHz km are feasible. The major problems in cabling have already been largely overcome but further improvements in fibre strength, homogeneity and reproducibility are awaited. The difficulties are technological rather than fundamental and will succumb to good innovative engineering within the next few years. The same may be said of the requirements for such mundane, but vitally important, components as splices, connectors, couplers and even the lowly jack plug. Excellent and encouraging progress is being made with all of these items. Of the major hurdles remaining, that of a suitable optical source is by far the most difficult. The lifetime and reliability of existing semiconductor lasers are improving only slowly and need to be increased by at least an order of magnitude. It would also be an advantage if their line width, coherence and beam quality could be made to approximate more closely those of an ideal laser. Fortunately light emitting diodes can also be used if adequate lasers do not become available, but at the expense of system bandwidth and repeater spacing.
Technological forecasting is fraught with hazards for the unwary but it is reasonable to expect systems to be operating in the telephone network in the 1980s at capacities from 140 Mbit/s to 500 Mbit/s at repeater spacings of at least 5 km and perhaps as high as 20 km. Serious study of the application of optical fibres to underwater cables will also have begun. If simple fibre cables can be made cheaply enough for use in installations to individual subscribers a wide range of new developments become possible, but these problems are more relevant to the 1990s.

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