Diffusion properties of Tm³⁺ in congruent LiNbO₃ crystal

Diffusion properties of Tm³⁺ in congruent LiNbO₃ crystal have been investigated, together with other two related issues, i.e., Tm³⁺-doping contribution to refractive index of LiNbO₃ substrate and Li out-diffusion. Four X-cut and four Z-cut congruent LiNbO ₃ substrates locally coated with 15-31 nm-thick Tm-metal films were annealed in surrounding air under different temperatures of 1030°C- 1130°C for different durations of 20-70 h. After anneal, refractive index at Tm³⁺-doped and Tm³⁺-free parts of crystal surface was measured at the wavelengths of 1311 and 1553 nm and surface Li₂O contents were evaluated from measured refractive index. The results show that Tm³⁺ doping has a weak effect on substrate index and a small contribution to index increment in waveguide layer in comparison with Ti ⁴⁺- or Zn²⁺ doping. The Li₂O content at the Tm³⁺-doped surface equals that at the Tm³⁺-free surface. The Li out-diffusion depends mainly on the diffusion temperature. Below 1100°C, the Li out-diffusion is not measurable. At 1130°C, a 30-h diffusion procedure may cause 0.2-0.3 mol% slight loss of Li₂O content. Secondary ion mass spectrometry was used to study the Tm³⁺ diffusion properties. The results show that the diffused Tm³⁺ ions in all samples follow a complementary error function profile. From measured Tm³⁺ profiles, characteristic diffusion parameters such as diffusivity, diffusion constant, activation energy, solubility, solubility constant, and heat of solution were obtained and discussed in comparison with the case of Er³⁺ diffusion. In comparison with Er³⁺ diffusion, the Tm³⁺ diffusion shows similar anisotropy and temperature dependence of solubility. In the aspect of diffusivity, under lower temperature the Tm³⁺ has a lower diffusivity than the Er ³⁺, and their diffusivity difference reduces with the increased temperature and becomes null at 1130°C.