Optimization of transverse electro-optic effect in LiNbO₃ and LiTaO₃ crystals

We analyze the transverse electro-optic (EO) effect in an arbitrary direction in LiNbO₃ (LN) and LiTaO₃ (LT) crystals based on the Fresnel index ellipsoid theory. A set of analytical formulas for calculating the birefringence in any direction in a crystal of the lowest symmetry is first deduced. The maximum magnitudes of the transverse EO effect appear along the directions that are turned out from the optic axis on the angles θ_m (φ = 30°, 150° or 270°) or 180°− θ_m (φ = 90°, 210° or 330°), the angles θ_m of congruent LN and LT crystals are 36° and 59.5°, respectively. For experimental verification, the (yzw)θ_m cut congruent LN and LT crystals were prepared. The half-wave voltage (HWV) of the (yzw)36° cut LN crystals is half that of the x-cut LN crystals and is only 30% that of the z-cut LN crystals, and the HWV of the (yzw)59.5° cut LT crystals is 7% higher than that of the x-cut LT crystals, the experimental results agree well with the theoretical analysis. The crystals with the optimized cell geometries are more practical because of the low HWV and lower sensitivity of birefringence to temperature.