Fibre Optic Cabling

So, what is a single mode fibre optic cable and what makes it different from a multimode 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 light travelling and bouncing off the "tube" walls will travel farther than the light that went straight. Thus for different modes of light setting off at the same time at one end of a fibre, they would arrive at the far end at different times, this is clearly not desireable as it will affect our signal. If we consider a pulse of light entering the fibre as a square wave, the effect of multiple modes will cause the pulse to be rounded at the far end. Also, pulses too close together - higher data rate - may result in pulses running into one another and cause data loss. Therefore, one important consideration is that light traveling in multi modes in a fibre will limit our useable bandwidth.

Enter Singlemode fibre. In effect what single mode fibre does - in simple terms - is reduce the size of our tube such that it is only possible for light to travel along the tube in one single mode. Since all the light energy will now follow the same path to the end of the fibre, our pulse of light cannot spread out and so Singlemode fibre has a very much higher bandwidth.

For a fibre to act as a single mode fibre the core ("tube") diameter must be less than 10 times the wavelength of the light being used, therefore singlemode fibres operating at 1310nm must have a core diameter less than 13 microns.

Singlemode fibre tends to be used mostly for high bandwidth telecomms applications, operating at data rates of 2.5Gb/s or more over distances in excess of 80km.

The problem with Singlemode fibre is that even though it costs no more, if not less, than Multimode fibre; it is more difficult to launch much light into the fibre, this usualy means the need for high power lasers and sensitive detectors which are much more expensive than the equipment needed to work over Multimode cables.

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