Critical angle method applied in determining the refractive indices and axis direction of crystal

Abstract

Measurement of birefringence is crucial for the investigation of polarization optical components, including frequency-doubling crystal, electro-optic crystal, liquid crystal modulator. The commonly used measurement methods are based on determining the phase difference after transmission, and scarcely any method based on reflection is utilized. We propose an improved method of measuring the refractive index curve mode of anisotropy crystals during rotation, based on which we are able to determine the refractive indices and optical axis directions of the crystal according to a retrieval method. The measurement method involves precisely rotating and scanning the index ellipsoid and extracts two refractive indices from images captured at every rotation angle. The retrieval method processes two sets of data in a group and gives the fitted parameters, including the refractive indices and the optical axis direction in space. Simulation results are given, and two types of crystal, quartz and selenite, exemplifying uniaxial crystal and biaxial crystal, are experimented. The experiment results show that the fitted parameters are in good agreement with the theory. Compared with existing methods, our method has two main advantages. The sample does not need to be cut into a specific shape or prior information of the sample obtained, meanwhile the complete optical axis direction in space, including the intersection angle to the surface plane and the azimuth angle in the plan, can be determined, which benefits the evaluation and improvement of the material.