There are many kinds of optical fiber, but we choose silica fiber only, because this kind of fiber have the lowest loss when conducting light.

The fiber is consist of two layers of silica glass that have different refractive index. The inner layer is light core with a diameter of several μm to more than ten μm, while the outer layer has a diameter of 0.1 to 0.2 mm. Generally, the refractive index of inner layer surpass the outer layer for about 1%. According to the principle of reflection, when the incidence angle to the interface of inner and outer layer is bigger than the critical angle of total reflection, the light will be totally reflected back. So that, the light keep moving in the fiber until reaching the outlet.