How does Optical Fibre work ??
Optical fibres are glass or plastics as thin as human hair, designed to guide light waves along their length. An optical fibre works on the principle of total internal reflection.
When light enters through one end of the fibre it undergoes successive total internal reflections from side wall and travels down the length of the fibre along a zigzag path. A small fraction of light may escape through side walls but a major fraction emerges out from the other end of the fibre.
Optical fibres are constructed either as a single fibre or a flexible bundle or a cable. A fibre bundle is a number of fibres in a single jacket. Each carries light independently.
An optical fibre has in general three coaxial regions.
- The innermost region is the light guiding region known as the core.
- It is surrounded by a coaxial middle region known as the cladding.
- The outermost region is called the coating. The refractive index of cladding is always lower than that of the core.
The purpose of cladding is to make the light to be confined to the core. Light launched into the core and striking the core to cladding interface at greater than critical angle will be reflected back into the core.
Since the angles of incidence and reflection are equal, the light will continue to rebound and propagate through the fibre. The coating protects the cladding and the core from abrasions and contamination and it also increases the mechanical strength of the fibre.
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Fibre optic Materials
Any material that is used for the synthesis of an optical fibre must satisfy some important qualities such as the following:
- The material must be transparent at a particular optical wavelength in order to guide optical pulses along the fibre.
- It should be possible to make thin, flexible and long fibres from the material.
- It should be physically possible to produce core and cladding having a slight difference in refractive indices by doping some impurities.
Depending upon the nature of applications optical fibres made of glass and plastics are used these days. For producing glass optical fibres, silica is preferred as the basic material whose refractive index is 1.458. The common optical fibres made of glass materials are:
- P2O5–SiO2 core and SiO2 cladding
- GeO2–SiO2 core and SiO2 cladding
- GeO2–B2O3–SiO2 core and B2O3–SiO2 cladding, and so on.