Fibre cables can be identified by the type of paths

Fibre cables can be identified by the type of paths that the light rays, or modes, travel within the fibre core. There are two basic types of fibres: multimode and high-speed dispersing machine single-mode. Multimode fibre cores may be either step index or graded index. Step index multimode fibre derives its name from the sharp step like difference in the refractive index of the core and cladding. In the more common graded index multimode fibre the light rays are also guided down the fibre in multiple pathways. But unlike step index fibre, a graded index core contains many layers of glass, each with a lower index of refraction as you go outward from the axis.
The result is that a graded index fibre equalizes the propagation times of the various modes so that data can be sent over a much longer distance and at higher rates before horizontal sand mills light pulses start to overlap and become less distinguishable at the receiver end. This virtually eliminates any distortion due to the light pulses overlapping. The core of the single mode fibre is extremely small, approximately five to ten microns.