Principle of light guidance
Guided propagation of light

An optical fiber is a thin glass cylinder in which light is trapped by total internal reflection. To understand how light propagates in a fiber, we can consider a geometrically simpler system, the symmetrical planar waveguide, consisting of a planar layer of a medium of refractive index n₁ sandwiched between two layers of a medium of index n₂ less than n₁ . The main properties are then transposed to the case of cylindrical optical fiber. We first describe the characteristics of the waveguide (§ 1.1 ), then calculate the modes of such a guide (§