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Attenuation is specified in db.km-1

Where

I out = outgoing intensity (intensity is measured in W.m-2)

I in = ingoing intensity (W.m-2)

Attenuation in a fibre is measured using an OTDR (Optical Time-Domain Reflectometer) which looks at the light reflected back long the fibre when a pulse of light is sent down the fibre. Another method is to send light from a continuous source of light and measure the power emerging at the other end of the fibre.

Optical Time Domain Reflectometer

Optical Time Domain Reflectometer

Causes of Attenuation

The light travelling along a fibre is attenuated, i.e. its intensity decreases as it moves along the fibre. This happens for 3 main reasons

Atomic absorption of light photons

Scattering of light by flaws and impurities

Reflection of light by splices and connectors

We will look at each of these factors in turn.

Atomic Absorption

The atoms of any material are capable of absorbing specific wavelengths of light because of their electron orbital structure. This absorption can be observed if you look into the edge of a pane of glass. The light which emerges has a green colour because so much red and blue light have been absorbed by the atoms of the glass. In the same way, as light passes along an optical fibre. more and more light is absorbed by the atoms as it continues on its path

Scattering by Flaws and Impurities

This type of scattering is called "Rayleigh Scattering". The amount of Rayleigh Scattering which takes place depends on the relative size of the scattering particle and the wavelength of the light. If the wavelength of the light is large compared to the size of the scattering particle then little light is scattered. If the wavelength of the light is small compared to the scattering particles then a lot of light is scattered. So long wavelengths are preferred in fibre optics because of the lower absorption. Thus 1500 nm is better than 1300 nm which is better than 850 nm

Reflection by Splices and Connectors

In a long fibre cable there may be many splices which join the individual lengths of fibre together. In a Local Area Network there will be many connectors because of the number of subscribers to the system. At each connector and/or splice some light will be reflected back along the fibre in the opposite direction. This will happen even for the most perfect splice or connector. Light reflected backwards does not leave the fibre but is no longer usefully available for the rest of the fibre, i.e. it is no longer part of the ongoing light.