Fiber types

There are two primary types of optical fibers in widespread use today, multimode and single-mode.
Simply put, multi-mode fibers are those with multiple pathways through which light travels. Within the core of these cables are several hundred layers of glass, each with a lower index of refraction as you move outward from the center. Since light travels faster in the glass with lower indexes of refraction, the light waves refracted to the outside of the fiber are speeded up to match those traveling in the center.
If that explanation is too complicated, don't worry. The most important thing to know about multi-mode fibers is that they are fine for distances of up to about a kilometer and slower bandwidths. That makes them suitable for data networks within buildings or between nearby buildings.
Single-mode fibers, on the other hand, are considered the more appropriate choice when running cable longer than a kilometer, when putting through a signal of more than 2 or 3 Gb/s, or when the system must be "future-proof."
The reason single-mode fibers have such a great capacity lies in their design. The core of a single-mode fiber cable is extremely small-usually between 8 and 9 microns (compared with 62.5 microns in the typical multi-mode fiber cable)-and only one light wave at a time can be transmitted down the core. Because of quantum mechanical effects, the light traveling in the very narrow core stays together in packets, rather than bouncing around the core of the fiber.
While this enables single-mode fibers to handle far more signal over far greater distances than their multi-mode cousins, single-mode fibers are considered to be a bit more difficult to terminate than multi-mode fibers. Plus, single-mode fibers require lasers as their light sources, which are more expensive than the LEDs that are typically used with multi-mode fibers.


Source:www.elec-toolbox.com/communications/classifications/fiber/fiber.htm