Transmitting Signals via Optical Fibres

Fibre optic cables are used for many telephone and TV transmissions. Fibre optic cables are long, thin, strands of glass or clear plastic and carry signals in the form of electromagnetic wave – light or infrared. The electromagnetic waves are transmitted as pulses, with the signal being switched on and off many times a second to produce a string of zeros and ones. In the transmitter, electrical signals are changed into light signals by a light emitting diode (LED) or a diode laser. At the receiving end, the light signals are picked up by a photodiode and changed back into electrical signals.

Optical fibre cable is thinner, lighter and cheaper than copper cables and can carry more signals. Signals in copper cables get weaker very quickly – and the loss in signal depends on the frequency of the signal - so many amplifiers are needed if the cable is long. The signals also accumulate noise unavoidably and these are also amplified, reducing the quality of the signal. Signals in optical fibres get weaker much more slowly and are less prone to interference.

The amplifiers can be much further apart. Optical fibres also have the advantage that they cannot be 'tapped', so that conversations are inherently more private.

Most long distance telephone calls are now carried by fibre optic cables, as is most computer data and internet traffic.

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