During the manufacture of optical fibers, a glass fiber is produced that consists of two concentrically arranged types of glass with different refractive indices. The inner glass is the core glass, the fiber core, which is concentrically surrounded by and firmly bonded to the cladding glass.
The modes are guided in the core glass and reflected at the boundary layer between core glass and cladding glass. Core glass and cladding glass consist of high purity fused silica( SiO2). The core diameter depends on the optical waveguide and is 9 µm for monomode fibers and 50 µm and 62.5 µm for multimode fibers.