Coherent light scattering measurements on single and cladded optical glass fibres

This paper considers the scattering, at normal incidence, of a laser beam by single and cladded optical glass fibres with a view to estimating their radii and refractive indices. For parallel and perpendicular polarizations of the incident light, the scattered intensity was calculated using the wave theory. The scattering pattern from a single fibre was also calculated by geometrical optics. The light scattered by a cladded fibre in air was photographed for fibres of three different overall diameters. A similar experiment was conducted for the fibre immersed in a liquid having a refractive index near to that of the cladding. Matching was achieved by thermal means. In both cases considered, the experimental results were found to be in agreement with theoretical results. For a cladded fibre in air, information about the core could not be obtained but a measurement of the angular fringe spacings and positions enabled the cladding diameter and index to be calculated. For a cladded fibre immersed in an index matching liquid, it was found that a measurement of the angular positions of minimum intensity enabled the core diameter to be calculated.