A bifunctional system for measuring geometrical parameters and stress distribution in fiber optic preforms

A bifunctional system for nondestructive characterization of fiber optic preforms in transverse illumination is presented. It allows for measurements of geometrical parameters of the preform core as well as the distribution of stress-induced birefringence. The system measures the core thicknesses versus its angular orientation and by numerical fitting of the experimental data, the core orientation, its ellipticity and dimensions of ellipse axis are determined. After inserting the polarizer into the illuminating beam, the system is capable of measuring the preform birefringence induced by thermal stress. Maps of retardation distributions are obtained simultaneously at RGB channels of the CMOS camera with the use of the rotating polarizer method. This allows to determine the exact value of the stress-related retardation without interference order unambiguity and finally to calculate the stress-induced birefringence and axial stress distribution using inverse Abel transformation. Sample measurements for step index preform with nominally circular core are presented.