Simultaneous measurement of refractive-index and thickness for optical materials by laser feedback interferometry

The refractive index measurement by ordinary interferometers cannot avoid the air disturbances in the optical path. A novel approach is presented in this paper based on the Nd:YAG microchip laser feedback interferometry (MLFI) with 1064 nm wavelength. For eliminating the air flow and electric-heating influence the heterodyne modulation and quasi-common path in the MLFI are used. The simultaneous measurement with high accuracy of the refractive index and thickness is realized. The measurement results for three kinds of materials are presented including N-SF57 glass with high index up to 1.81057. The measurement uncertainty of refractive index is better than 0.00002 and of thickness is better than 0.0006 mm.