fibre optique
optical fiber
[ accès général ]


cotton, Bob; oliver, Richard


"Optical fibres are fine strands of high-transparency glass which act as 'waveguides' for the transmission of single-frequency light beams. Fibre-optic communication systems require a light source (emitter), an optical fibre, a light detector, and connectors and couplers to link these components together. The message (ie the picture or audio signal) is encoded digitally and converted to pulses of laser light energy by the emitter. The optical fibre provides a 'guide' which the light energy follows until it is 'received' by an optical detector. This detector converts the pulse back into electronic pulses and restores the signal to match the original information. At first, optical signals could only be sent a short distance without amplification, but advances in engineering gradually increased the range from 30 kilometres (18 miles) in 1981 to 400 kilometres (240 miles) in the early 1990s. A new generation of fibre optics being developed at the University of Southampton (UK) will enable signals to be sent hundreds of miles.
Fibre-optic technology stems from research on waveguides by G C Southworth and others in the 1920s. Their research resulted in the development of the coaxial cable (1928) followed by hollow-tube wave guides for electromagnetic radiation. In 1960, the first LASER (Light Amplification by Stimulated Emission of Radiation) was produced. In the late 1960s, researchers at ITT's Standard Telephone Laboratories in the UK established a framework for the design of optical fibres that would allow the practical transmission of information using laser light beams as data carriers. By 1980, the technologies necessary to make this a reality (in the form of solid-state laser emitters and light-emitting diodes) were in place.
At 120 microns, fibre-optic cables are about the thickness of a human hair, with a glass core of about 7 microns in diameter. Operating at very high frequencies of infrared, they can support an almost unlimited bandwidth and can be used to carry large volumes of low-band multiplexed signals (such as telephone calls) or wide-band signals for television and video conferencing. According to communications expert John Alvey (former chief engineer at British Telecom), fibre-optic cables can carry a thousand times more signals than can be transmitted over the entire radio bandwidth. 'In this way', states Alvey, '350 million phone calls could be carried over a single fibre optic at any on time... or over 50,000 HDTV signals.' (D H Mellor (ed.), 'Communications and Technology' Ways of Communicating, 1990)."

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