An optical fiber is a flexible, transparent glass/fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Transmission is the transfer of a signal as the light pulse along a glass (silica) fiber.
Optical fibers carry light signals down them in what are called modes. Different ways of traveling: a mode is simply the path that a light beam follows down the fiber. One mode is to go straight down the middle of the fiber. Another is to bounce down the fiber at a shallow angle. Other modes involve bouncing down the fiber at other angles, more or less steep.
Single Mode
The simplest type of optical fiber is called single-mode. It has a very thin core about 5-10 microns (millionths of a meter) in diameter. In a single-mode fiber, all signals travel straight down the middle without bouncing off the edges (yellow line in diagram). Single-mode fibers generally carries the Cable TV, Internet, and telephone signals, wrapped together into a huge bundle. Cables like this can send information over 100 km (60 miles).
Multi Mode
Another type of fiber-optic cable is called multi-mode. Each optical fiber in a multi-mode cable is about 10 times bigger than one in a single-mode cable. This means light beams travels through the core at following a variety of different paths (yellow, orange, blue, and cyan lines)—in other words, in multiple different modes. Multi-mode cables can send information only over relatively short distances and are used (among other things) to link computer networks together.