Glass Fibers for Optical Communications

Abstract

Glass fiber optics have been used for many years for flexible light distribution bundles, faceplates for cathode-ray tubes, medical endoscopes, and similar purposes. Only recently (1) has serious research been directed toward the use of glass fiber as a long distance communication medium (2, 3). Such a communication system requires three main components: a transmitter, a receiver, and a transmission medium. The transmitter will most likely consist of a GaAIAs laser or light emitting diode (LED) driven with a digital signal. Such transmitters can operate at a wavelength of 800-900 nm at data rates up to hundreds of megabits per second. Another possible transmitter is based on an externally modulated Nd: YAG laser operating at 1060 nm. The receiver of an optical communication system will probably be a semiconductor photodetector that can be part of an integrated circuit chip used for processing the signal. Silicon avalanche diodes and silicon PIN diodes are sensitive at the GaAIAs wavelengths, and they have been built with bandwidths of at least a few hundred megacycles. Using lasers and avalanche photodiodes, losses of about 75 dB can be tolerated between repeaters operating at 1 megabit/sec, but only 45 dB loss is tolerable at 1000 megabits/sec. An excellent review of 'transmission and receiving devices has been published by Miller, Marcatili & Li (2). The transmission medium will be an optical fiber waveguide, consisting of a core of relatively high refractive index surrounded by a cladding of relatively low refractive index, The maximum tolerable loss per kilometer is governed by the distance over which transmission must take place. Losses as high as 100 dB/km may be acceptable for very short systems, as on airplanes or ships, whereas losses as low as 4 dB/km or less may be required for long distance systems where expensive repeaters must be spaced many kilometers apart. In addition to loss, another important characteristic of glass fiber waveguides