Novel bifunctional systems for measuring the refractive index profile and residual stress birefringence in optical fibers and preforms

Abstract

Two measuring systems are presented enabling determination of the refractive index profile and its anisotropy as well as principal stress components in optical fibers and preforms, respectively. The system for optical fibers is a scanning-type, differentiating interferometer used to measure directly the wavefront derivative, from which, after the inverse Abel transformation, the index profile is obtained. The required high sensitivity of measurement is achieved by applying the sinusoidal modulation of the input beam ellipticity and the homodyne detection of the first harmonic component of the output intensity. After removing the Wollaston prism, the system can be used to measure the retardation function that is related to the fiber residual birefringence. The dynamic spatial-filtering technique, used until now to measure the ray deflection function, has been modified for testing the preforms. An optionally applied linear modulator of ellipticity of the input beam was added, to enable the measurement of the retardation function also. The system can be easily switched from the measurement of the ray deflection function to the measurement of the retardation function by moving only a single element.