Contribution of diffused Er ³⁺ ions to refractive index of LiNbO ₃ crystal and relation of Li out-diffusion to Er ³⁺-diffusion condition

Contribution of diffused Er ³⁺ ions to refractive index of LiNbO ₃ crystal and relation of Li out-diffusion to Er ³⁺-diffusion condition have been studied. Three X-cut and three Z-cut congruent LiNbO ₃ plates locally coated with 10, 15, or 20 nm thick Er metal films were annealed at 1100°C or 1130°C for 60 or 100 h under different atmospheres of surrounding air, wet O ₂ or wet Ar. After the anneal, the ordinary and extraordinary refractive indices at Er ³⁺-doped and Er-free parts of crystal surface were characterized at the 633, 1311, and 1553 nm wavelengths by measurement of critical angle. The difference of surface index values at the Er ³⁺-doped and Er-free parts enables to identify the Er ³⁺-doping effect on substrate index. From the known birefringence, the surface Li ₂O contents at the Er ³⁺-doped and Er-free parts were evaluated. A comparison with the Li ₂O content data before the anneal treatment allows to obtain the anneal-induced Li ₂O loss and hence identify the anneal condition effect on the Li out-diffusion. The results show that the contribution of Er ³⁺-doping to the ordinary index is within the measurement error of 1 × 10 ^-3 and that to the extraordinary index is within ± 1.7 × 10 ^-3, which is just slightly out of the measurement error. The diffusion temperature is while the diffusion environment is not the key factor responsible for the Li ₂O out-diffusion.