(C). Applications in telecommunications:
Great interest in communicating at optical frequencies was created in the 1960s with the advent of lasers, which made available highly coherent optical sources. In the field of communications laser offer two unusual advantages. These are bandwidth and information transmission rate. Since the optical frequencies are of the order of 5x1014Hz, lasers have a theoretical information capacity exceeding that of microwave systems by five orders of magnitude, which is approximately equal to 10 million TV channels. The use of optical fibres as reliable and versatile media to transmit laser light over long distances. Owing to successful reduction in optical fibre losses (0.16 dB/km at 1.55nm wavelength) more than 10 million km of fibre has been installed world-wide in various optical networks. Today, bit rates for long-haul fibre links typically range between 400 and 600 Mbits/s, and in some cases up to 4Gbit/s, which is far beyond the reach of traditional copper cable communication systems.
Important components in an optical fibre communication system include transmitter, fibre link, receivers . In addition we need repeaters, connectors, couplers, multiplexers, and demultiplexers. An optical fibre communication system is illustrated schematically in Fig.18. In each channel, a light source which is dimensionally compatible with the fibre core is used as the transmitter. Semiconductor light emitting diodes (LEDs) or laser diodes are used as the transmitter source. The transmitter circuitry converts electrical input signals into optical signals . Optical outputs from different channels are multiplexed by a multiplexer and then coupled into the optical cable. At the end of the optical fibre link, the transmitted optical signals are first demultiplexed and then detected by the photo detectors in the receivers. in each channel. Semiconductor PIN and avalanche photo diodes are the two principal photo detectors used in the optical fibre communications. The outputs of the photo detectors are amplified and reshaped before they are delivered to the user (e.g., a telephone or a computer terminal).
