Multimode Fibre, in contrast to Single Mode Fibre, is a form of optical fibre through which light is sent in a number of different "modes". In simplest terms, we can think of modes as paths that the light can take through the core of the fibre. In other words, with Multimode Fibre the light takes multiple paths, rather than only one, as is the case for Single Mode Fibre.
There are two main types of Multimode Fibre: step index and graded index. Step index Multimode Fibre relies on the phenomenon of Total Internal Reflection, in which light passing through a material (i.e. the glass in an optical fibre's core) hits the inside edge of that material and is reflected back inside of it, rather than being refracted out. When, as in step index Multimode Fibre, this process repeats, light can continue to be reflected for a very long time, and in this way light (and so signals) can pass along the length of the fibre.
There are, however, complications to this idea, because an effect known as modal dispersion occurs. Modal dispersion means that in any given fibre, because some light is taking a longer path through the fibre than other light, the different signals encoded in that light become increasingly spread out (or dispersed) over time and distance, overlapping one another and becoming ever more difficult to interpret. For this reason, Multimode Fibre in general (but particularly step index Multimode Fibre) tends to provide very poor signals when used over long distances.
Graded index Multimode Fibre is a form of Multimode Fibre designed to suffer less seriously from modal dispersion. In this form, the core of the fibre is graded rather than stepped, meaning that the refractive index of the fibre does not suddenly change as it passes from core to cladding, but smoothly transitions from a high refractive index at the centre of the core to a low refractive index at the edge. This has the effect that the light is made to curve, to the extent that it never actually reaches the edge of the fibre core. Instead, it is bent back in towards the centre, and so to the other side of the fibre core, where the same occurs again.
This has an interesting effect, because light travels more rapidly at the edge, where the refractive index is lower, and so as it moves through the fibre core, passing up and down the refractive index gradient, will always tend towards an average path. This drastically reduces the effects of modal dispersion, meaning that graded index Multimode Fibre can operate over significantly longer distances than its step index equivalent.
But even this trick is not perfect, and there is still a fair amount of attenuation, from modal dispersion and other factors, in graded index Multimode Fibre (at least when compared to Single Mode Fibre). For this reason, UTEL do not use Multimode Fibre, but it should still be noted that graded index Multimode Fibre does offer significant improvement on the distance capabilities of step index Multimode Fibre, which, coupled with the reduced operational cost of the Multimode technique (when compared to the high laser costs of Single Mode Fibre), can make graded index Multimode Fibre a useful tool for some data transfer applications, for instance CCTV (Closed Circuit Television), LAN (Local Area Networks) and University Campus Networks.