The  Optical Fibre Cable.

The job of the cable is to convey the modulated light from transmitter to receiver.


There are three broad types of cable used  in communications technology. The diagram below (provided by the US Navy) shows the typical characteristics of each:


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The two step index types have a core and cladding in which the refractive index changes abruptly causing reflection to occur at the cladding. In multimode the diameter of the core is relatively large, in single mode very small. Graded index has a subtle change in refractive index from core to cladding. In combination with timed pulsing, graded index reduces dispersion dramatically as it cuts down the differing velocities and pathways. Graded index is therefore the preferred choice in telecommunications trunk systems. LASER light is the chosen source as it has a narrow spectral emission thereby further reducing the number of wavelengths carried.

Purer very high quality (and expensive) systems will use single mode. There is no dispersion and we have only to worry about Rayleigh scattering. The problem should not be too great at 1550nm, and there should be no Mie scattering at all in cables built to this specification. Both types of step index tend to be used only in short runs (500 m or so).

Our model transmitter serves only one core, but in a real transmitter we may be serving hundreds. Below is an industrial cable designed for trunk work:


Barely visible in the right hand lower corner is the core and cladding. each grey section could contain a number of these, each green may contain a number of greys although only one is shown here. In the centre are steel strengtheners, the two white sections are moisture proofers and the knurled section the armour. If each of the red sections is multiplexed, and has its own light source, dozens  of channels  of telecommunications are possible. if many of these are trunked together, or more are packed in one cable,  it becomes thousands, and so on.