Description
A particular type of glass fibre is the so-called fibre optic, which is used as a light guide. If a large number of very thin fibres are put together (the diameter is about 0.13 mm), so as to form a cable, each of them is a guide for light rays which are inserted at one end and are carried, by reflection, along the whole length of the fibre.
To avoid loss of light through the side walls, each fibre consists of two concentric layers: the inner glass one, the core, has a high refraction index (silica glass speckled with germanium and rare earths), surrounded by a sheath of silica glass, which is less refractive (cladding).
The two types of glass are chosen so that in their interface there is total reflection of light. By doing so, the light ray, which is propagated in the central part of the fibre, is reflected on the sheath towards the inside, then goes back towards the wall and so on. The optical fibres are made of ultra-pure glass to reduce any dimming of the light transmission to a minimum. Currently fibres of over 100 km length are used, without having to retransmit the light.
Fibre optics are used in various fields both in electronics and optics (for example in the survey of high energy particles and in medical optics) in addition to lighting and furnishings.
In those used in telecommunications, the electronic signal is converted into a light signal, transported along the optical fibres, and, at the end, reconverted into an electronic signal.
Compared to copper cables, optical fibres have the advantage that they can transport an enormously superior amount of information with the same diameter and are not affected by electrical and magnetic interference.