OPTICAL FREQUENCIES cover the wavelength decade from about 2 μm to 200 nm, that is from near infra-red to near ultra-violet. Though desirable for communications because of their huge bandwidth, they languished until a transmission medium of low attenuation was found. Aerial communications become increasingly difficult as the frequency is raised from microwave to infra-red because of increasing absorption by molecules and particles in the atmosphere. In addition a coherent1 source of light was essential as well as a means of modulating it. The coherent source was the laser, invented in 1958, and the low-loss medium was discovered in 1966. In that year, Kao and Hockham of STC Laboratories in England succeeded in sending light through an optical fibre, suggesting that transmission over several kilometres might be feasible with less attenuating fibres. By 1976 losses had been reduced to 0.5 dB/km in laboratory fibre specimens and the large-scale use of fibres for telephone trunk lines began. For some years all new trunk lines in the UK have been fibre-optic cables.
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