Fibre optics


Carrying sounds on light beams

Strands of the purest glass, some of them ten times finer than a human hair, are taking over from copper wire in cables used for transmitting telephone and television signals. The glass is so pure that you would be able to see through a block 12 miles (20km) thick as if it were a windowpane.

These fibre-optic cables can carry more information at higher speeds than copper cable, transmitting it as flashes of light. And they occupy only about one-tenth of the space of copper cable. Optical cables now in use can carry nearly 20,000 telephone calls at once. Sound, pictures and computer information can all be carried in the same cable, and the signals do not fade away as easily as they do in copper wire, so the cable needs fewer signal boosters.

When light is shone into one end of a fibre, it is internally reflected many times some 15,000 times per yard. Scarcely any light leaks out of the sides, because every fibre has an inner core that channels the light along it and an outer cladding that reflects it back into the core.

There are two main types of fibres. The finest, known as mono-mode fibres, transmit light as a single wave pattern, and the light signals can travel up to about 120 miles (190km), without being boosted.
In the thicker fibres, which are known as multimode fibres, up to 1000 wave patterns can be transmitted at different intervals, but some light is lost and so the signals need boosting about every 10 miles (16km).

How messages are transmitted In a fibre-optic telephone system, the electrical current produced by the telephone in response to the vibrations of the voice is first fed into an encoder. This measures the current strength about 8000 times a second and converts it into digitally coded electrical signals representing binary numbers - a series of ones and zeros.

The light transmission is by lasers. The type used in optical-fibre transmission is a semiconductor laser that produces invisible infrared light. This has a much higher frequency than the electric current in copper cable, so can carry much more information.

The electrical signals switch the laser rapidly on and off, producing digitally coded light pulses which pass into the optical fibre through a lens. At least 2400 million bits (binary digits) can be transmitted through a single fibre every second - equivalent to about 32,000 simultaneous telephone conversations. Because there are gaps in the signals of one call, numbers of calls are sent together, slotted between each other. This is known as multiplexing.

At the receiving end of the fibre cable, the light pulses are picked up by a photo-detector, which converts them back into electrical signals, and fed to a decoder. This changes them back to the pattern of electrical current transmitted from the telephone mouthpiece.

The transmitters and receivers used are so small they could both fit together inside a matchbox, and the laser generators are no bigger than grains of salt.

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