Electro-Optic Property of Er³⁺-Doped LiNbO₃ Single Crystal for Integrated Optics

Er³⁺-doping effect on electro-optic property of LiNbO₃ crystal has been studied. A series of congruent LiNbO₃ crystals homogenously doped with different Er³⁺ concentrations up to 2 mol% in melt were grown. Unclamped electro-optic coefficients r₁₃ and r₃₃ of these crystals were measured by Mach-Zehnder interferometry. The results show that both r₁₃ and r₃₃ reveal little dependence on Er³⁺ doping level. r₁₃ hardly changes in the studied Er³⁺ doping level range 0-2 mol% within the experimental error of 3%. r₃₃ reveals a degradation tendency with a rise in the Er³⁺ doping level, but the degradation is no more than 5% in the Er³⁺ doping level range studied. The small effect is consistent with previous result that the performance of an electro-optic device based on Ti⁴⁺-diffused Er³⁺-doped LiNbO₃ waveguide is not influenced noticeably by Er³⁺-doping. A qualitative, comprehensible explanation is given for the small effect.