An optical fibre is a piece of hair-thin glass material of a different refractive index. Such a fibre is cabable of transmiting data at the speed of light with no significant loss of intensity over long distances.
Fibre-optic links are based on the principle of ' total internal reflection '. When an electromagnetic wave, travelling in a medium with high refractive index, falls on the boundary of the surrounding medium of lower refractive index, a special phenomenon takes place. Up to a cerain angle of incidence, light passes through the boundary and enters the medium that has a lower refractive index. But if the angle of incidence is more than the critical angle, the light is reflected from the boundary and comes back to the first medium. This is called ' total internal reflectiont'. 
An optical transmission system based on the fibre-optics has three components, the transmission medium, the light source and the detector. The transmission medium is the ultra-thin glass fibre. The light source is either an LED (light emitting diode) or a laser diode, which emits light pulses when electric current is applied. the detector is a photo diode which generates electric pulses when light falls on it.
By attaching the LED or the laser diode to one end of an optical fibre and a photo diode at the other end, we can have an uni-directional data transmission system that accepts an electrical signal, converts and transmits it by light pulses and then reconverts the output to an electrical signal at the receiving end.
Advantages
It has very high rate of transmission of data.
It has better noise immunity.
It can transmit data over long distances.
Data is transmitted with high security.
The fibres are small in size.
Disadvantages
Limited physical arc of cable. Bend it too much and it will break!
Difficult to splice.
The cost of optical fibre is a trade off beween capacity and cost. A higher transmission capacity, it is cheaper than copper. At lower transmission capaciy, it is more expensive.
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