its simplest form a step-index fibre consists of a cylindrical core surrounded by a cladding layer whose index is slightly lower than that of the core. Both core and cladding use silica as the basic material, the difference in the refractive indexes is realized by doping the core or the cladding or both. Dopants such as GeO2 and P2O5 increase the refractive index of silica and are suitable for the core. On the other hand, dopants such as B2O3 and fluorine decrease the refractive index of silica and are suitable for the cladding. The major design issues for the optical fibres are related to the refractive-index profile, to the amount of dopants, and the core and cladding dimensions. Figure 1 shows typical index profiles that have been used for different types of fibres