Optical fibres

The structure of optical fibre transmission lines takes the very simple [Suematsu & Iga 1982] form of a cylindrical glass core of refractive index n1 surrounded by a cladding glass of refractive index n2 where n2 < n1. Normally most of the propagating energy is contained in the core but there is always a radially-decaying evanescent field in the cladding, which may extend over several wavelengths in the case of single-mode fibres. Both core and cladding materials must therefore have very low absorption and scattering losses