Measurement Of Crystal Refractive Index Based On Michelson Interferometry

The refractive index of crystal is one of the most important optics parameter and it is also one of the most basic content to judge the physical characteristics of crystal. To the same crystal, if the wavelength different then we get the different refractive index. And the measurement of refractive index is vital to design device. There are many methods to measure the refractive index of the crystal, however, the accuracy and the range of the measurement is subject to different extent limitation because of the built-in conditions. The conventional methods are difficult to, sometimes unable to, meet the new requirement. The interference method for determination of refractive index has many advantages, such as easy-producing sample, extensive testing range, and the reutilization of sample, and has been a hot research issue in recent years.As one of the hottest topics in the field of measuring methods research in recent years, image measurement technology is a new measuring method with a lot of advantages of high speed, large dynamic range, abundant information and automatization. It has already been widely used in lots of industrial measurement areas. We combine Michelson interferometry and image measurement technology in order to realize entirely non-destructive, real-time, accurate measurement of refractive index for both isotropic and anisotropic optical materials.Aiming at the problem that the sample must be adjusted normally to the incident light in each measurement, which leads to not only complicated operation but also greater noise influence in small incident angle range, the formula for calculating RI from a non-zero incident angle has been derived. The measurement accuracy is restricted due to the accuracy of interference fringe counting. We break through traditional technique of fringe counting and develop a considerably simplified method based on phase analysis leading also to an increase of precision.Based on Michelson interferometer, a measurement system for refractive index has been developed. The system has better stability, good real-time ability, and is applicable to multi-wavelength measurement to fit dispersion curve. The refractive indices of CSBN crystals were measured accurately by our measurement system. The refractive indices are frequency dependent and increase dramatically when the wavelength decreases. As Ca²⁺ content increases, the refractive indices of CSBN crystals also increase. The Sellmeier's equations for refractive indices were obtained in terms of least-squares fit.