High-precision measurement system for the Z-cut angle of uniaxial crystals based on conoscopic interference

Pockels cells require the surface normal of the potassium dihydrogen phosphate (KDP) crystal to be aligned parallel to the crystal's optical axis. Minor cutting errors, notably incorrect Z-cut angles, can impair the electro-optic modulation capability. We propose a high-precision measurement system for the Z-cut angle of uniaxial crystals based on conoscopic interference. This system uses a motion compensation trajectory, derived from reflected light coordinates, to correct tilt and rotation errors. This approach obviates the need for real-time posture monitoring during tests and maintains crystal alignment to within 0.02 mrad. The ray tracing model correlates the Z-cut angle with melatope trajectory radii in the interference patterns, achieving a spatial resolution error below 0.3 µrad/pix, mainly due to lens reinstallation errors. Experimental results indicate that the Z-cut angles achieves a Peak-to-Valley (PV) of less than 0.03 mrad and a Root Mean Square (RMS) of less than 0.01 mrad. Compared to the best parameters reported by other researchers, this system offers 2.25 times better repeatability and 1.65 times finer spatial resolution.