ABSTRACT
A geometrical-optics analysis shows that the position of a sharp cut-off in the radiation pattern determines the refractive index of the uncladded fibre, whereas the distance between certain successive minima gives the diameter. The following presents the analysis of back-scattering considering two internal reflections occurring within the fibre core. For low-loss optical fibres used as the transmission medium in optical communication systems it is possible to determine the physical properties of such cladded fibres with the help of back-scattering of light incident perpendicular to the fibre axis. The trajectory of the incident rays is followed through refractions and reflections where the relevant angles are defined. Light scattering in optical fibres is due to Rayleigh scattering and to inhomogeneities such as micro-bubbles, micro-crystals and micro-fractures, which in a first approximation may be considered as isotropic scatterings.
