You will hear people talking about singlemode and multimode fibre. So what is the difference?

Light efficiently travels down a fibre only when the distance between each reflection point is a whole number of its wavelengths. Therefore only certain paths, or modes, are able to transmit light successfully.

In multimode fibre the core diameter is about half that of its cladding (typically 50 µm or 62.5 µm). This means there are several modes or paths available for the light to travel down the fibre. Each of these modes has a different path length. Therefore, as the light travels down the fibre, the higher order modes start to fall behind the lower order modes (the ones nearly travelling straight down the centre of the fibre). A sharp pulse at the beginning of the fibre becomes spread out and less distinct. This is called modal dispersion and limits how close pulses can be transmitted together (called the data rate) before they merge together and information gets lost.

In singlemode fibre the core is made so small (about 10 µm!) that only a single light path (a "single mode") can successfully propagate. This means that there is no modal dispersion, so much higher data rates can be achieved. However the small core means that it is harder (and more expensive) to reliably connect or join singlemode compared to multimode fibre.