KTP crystal thickness distribution measurements based on laser feedback interferometry

KTiOPO(4) (KTP) crystal is a widely used nonlinear optical crystal, and it can meet high requirements for parallelism of crystal surfaces and the length in the light propagation direction in its application fields. In this paper, we present a method for measuring the thickness distribution of cuboid KTP crystals based on laser feedback interferometry (LFI). The accuracy of the measurements for the relative thickness distribution was 8.8 nm, and that of the absolute thickness can be improved by increasing the accuracy of the refractive index. This method is applicable to measurements of all light transmissive birefringent materials, and the results provide detailed instructions for crystal processing and polishing.