Refractive index profile of GRIN optical fibre considering the area under the interference fringe shift: II. The mismatching case

The multiple-beam interference: Fizeau fringes technique is used to obtain the refractive index profile of multimode graded index (GRIN) optical fibre s. Expressions have been derived to determine the profile shape parameter a lpha and the cladding/core maximum refractive index difference Delta n when using a mismatching immersion liquid. The derived expressions consider the area enclosed under the interference fringe shift. The method is a general ization of the method presented previously by Mabrouk and El-Bawab (1997 Pu re Appl. Opt. 6 247). The method has the advantage that it gives the exact solution for alpha and Delta n, moreover, it is a simple relation between t he measured quantities to determine the refractive index profile. The opto- thermal coefficients of the cladding and at different points along the core are determined for two graded index optical fibres.