Fibre Optic Cabling

What is a multi mode fibre optic cable and what makes it different from a single mode fibre optic cable?

If we consider a fibre optic cable as a tube down which light can travel it will make our explanation a little easier. Consider also that when a light is turned on, light tends to travel in lots of different directions. Now if our fibre optic cable "tube" is large enough, then as light travels into it, some of the light will travel in a straight line, some light will travel at an angle; bounce of the inside of the tube and so on in a zig zag fashion; each direction of travel is a "mode". The effect of light taking different "modes" along the fibre and arriving at the far end at different times is known as modal dispersion.

Therefore any fibre cable whose core - "tube" - is sufficiently large enough to allow light to travel along the length of the fibre taking different paths, it is a multimode fibre cable.

The effect of modal dispersion is that the usable bandwidth of the cable is reduced. However some effort is made to counter the effects of modal dispersion by using "Graded Index fibres". These cables have a higher refractive index in the centre of the core, light travelling away from the centre of the core - where the refractive index is lower - speed up. The effect is that even though the modes that do not travel stright down the center of the core travel further, they do so travelling quicker than light through the centre of the core; resulting in more modes arriving at the same time at the end of the cable, reducing the effects of modal dispersion and therefore increasing the usable bandwidth of the cable.

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