In 1870, John Tyndall demonstrated that light follows the curve of a stream of water pouring from a container, it was this simple principle that led to the study and development of applications for this phenomenon.
The advantages of using fiber optics
Because of the Low loss, high bandwidth properties of fiber cable they can be used over greater distances than copper cables, in data networks this can be as much as 2km without the use of repeaters. Their light weight and small size also make them ideal for applications where running copper cables would be impractical, and by using multiplexors one fiber could replace hundreds of copper cables.
There are many different types of fiber cable, but for the purposes of this explanation we will deal with one of the most common types, 62.5/125 micron loose tube. The numbers represent the diameters of the fiber core and cladding, these are measured in microns which are millionths of a metre. Loose tube fiber cable can be indoor or outdoor, or both, the outdoor cables usually have the tube filled with gel to act as a moisture barrier which stops the ingress of water. The number of cores in one cable can be anywhere from 4 to 144
Over the years a variety of core sizes have been produced but these days there are only three main sizes that are used in data communications, these are 50/125, 62.5/125 and 8.3/125. The 50/125 and 62.5/125 micron multi-mode cables are the most widely used in data networks, although recently the 62.5 has become the more popular choice. This is rather unfortunate, because the 50/125 has been found to be the better option for Gigabit Ethernet applications.
The 8.3/125 micron is a single mode cable which until now hasn't been widely used in data networking, this was due to the high cost of single mode hardware. Things are beginning to change because the length limits for Gigabit Ethernet over 62.5/125 fiber has been reduced to around 220m, and now, using 8.3/125 may be the only choice for some campus size networks. Hopefully, this shift to single mode may start to bring the costs down.
With copper cables larger size means less resistance and therefore more current, but with fiber the opposite is true. To explain this we first need to understand how the light propagates within the fiber core.
Light travels along a fiber cable by a process called 'Total Internal Reflection' (TIR), this is made possible by using two types of glass which have different refractive indexes. The inner core has a high refractive index and the outer cladding has a low index. This is the same principle as the reflection you see when you look into a pond.
This has the effect of scattering the signal because the rays from one pulse of light, arrive at the far end at different times, this is known as Intermodal Dispersion (sometimes referred to as Differential Mode Delay, DMD).
To ease the problem, graded index fibres were developed. Unlike the examples above which have a definite barrier between core and cladding, these have a high refractive index at the centre which gradually reduces to a low refractive index at the circumference.
This slows down the lower order modes allowing the rays to arrive at the far end closer together, thereby reducing intermodal dispersion and improving the shape of the signal. The easiest way to avoid Intermodal dispersion is to allow only one mode of propagation. So a smaller core size means higher bandwidth and greater distances.