Diffusion properties of Mg ²⁺ and Ti ⁴⁺ ions in optical-damage-resistant near-stoichiometric Ti:Mg:LiNbO ₃ waveguide

Secondary ion mass spectrometry (SIMS) was used to study the diffusion properties of Mg ²⁺ and Ti ⁴⁺ ions in a peculiar optical-damage-resistant near-stoichiometric (NS) Ti:Mg:LiNbO ₃ waveguide structure that a number of strip waveguides are embedded in a planar waveguide. The structure was fabricated on an initially congruent, undoped Z-cut substrate with a technological process in sequence of planar waveguide, strip waveguide, MgO diffusion and post Li-rich vapor transport equilibration (VTE) treatment. The results show that the Mg ²⁺-profiles in both strip and planar waveguide layers are desirably homogeneous with a concentration 1.7 ± 0.3 mol% above the optical damage concentration threshold. The Ti ⁴⁺-profile in the strip waveguide follows a sum of two error functions along the lateral direction with a width 13 ± 1.0 μm and a sum of two Gaussian functions in the depth direction with a 1/e depth of ∼11 μm all. The Ti ⁴⁺ ions in the planar waveguide obey a Gaussian profile. From the measured profiles, characteristic diffusion parameters such as mean Mg ²⁺ and Ti ⁴⁺ diffusivities and surface Ti ⁴⁺-concentration are evaluated. The diffusivity is discussed in comparison with the previously reported results on single Mg ²⁺, Ti ⁴⁺ diffusion and Mg ²⁺/Ti ⁴⁺ co-diffusion in pure (or bulk MgO-doped), congruent (or NS) LiNbO ₃ crystal. Finally, the keys to the succeeding in fabrication are highlighted.