1. Principle of light guidance

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 n ₁ sandwiched between two layers of a medium of index n ₂ less than n ₁ . The main properties are then transposed to the case of cylindrical optical fiber. We first describe the characteristics of the waveguide (§ 1.1 ...