IGCSE
Physics Notes: Transmitting Signals Via Optical Fibre Cables
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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