High-precision determination of the cut angle of an electro-optic crystal by conoscopic interference

To precisely determine the cut angle of an electro-optic crystal, an improved conoscopic interference measurement method, developed to address the large influence of crystal alignment and melatope positioning errors on measurement results, is presented in this paper. Ray-tracing formulas are derived to calculate the cut angle and evaluate the measurement errors. A Twyman-Green interferometric partial system is employed to make auxiliary adjustments to the orientation of the crystal sample to ensure that the front surface of the crystal is completely perpendicular to the optical axis during the test. In this way, the alignment error can be controlled within 0.050 mrad. An interferogram processing method, by which the centers of gravity of each independent conoscopic speckle are connected, is put forward to position the crystal melatope. The average result is 3.734 mrad when the cut angle of a 6-mm-thick KDP electro-optic crystal is measured 12 times, with the measurement repeatability being approximately 5 times less than that of a sophisticated x-ray diffractometer. The experiment reveals that by using the method, the melatope positioning error can be reduced to less than 0.60 μm. The improved method avoids the shortcomings of traditional conoscopic measurement methods, with the measurement error being reduced from 1.750 to 0.100 mrad, and provides technical support for future applications of high-precision and low-cost crystal orientometers.