Nondestructive determination of refractive-index profile and cross-sectional geometry of optical-fibre preform

A nondestructive method is presented to determine simultaneously the refractive-index profile and the cross-sectional geometry of a preform. In this method, a plane wave is made to impinge on the preform laterally and the power distribution of the scattered pattern is recorded. From the recorded power, the ray pathlength difference is calculated. After the preform has been rotated through 360° and the necessary data collected, a numerical inversion is performed to obtain the index profile and cross-sectional geometry.