Simultaneous measurement of thickness and group refractive index in birefringent crystals

In order to measure the group refractive index and thickness of birefringent crystals simultaneously, a method based on spectral-domain interferometry is proposed. The optical path of each reflection layer of birefringent crystal is converted into spectral interference fringe of corresponding frequency by the principle of spectral-domain interferometry, and the accurate optical path calculation is realized by fast Fourier transform and Hanning-windowed energy centrobaric method. A two-step measurement method was designed to measure the crystal thickness and group refractive index of ordinary ( o ) and extraordinary ( e ) light synchronously by comparing the changes of the optical path before and after the insertion of the sample. The comparison between experimental and theoretical results of LiNbO 3 crystals shows that the accuracy of the thickness measured by this system is better than 1 μm and the relative error of the group refractive index is better than 0.15%. The measurement method is simple to implement and has high precision, which is of practical significance for realizing the rapid and high-precision measurement of optical parameters of birefringent crystals.