Diffusion properties of Mg2+ and Ti4+ ions in optical-damage-resistant near-stoichiometric Ti:Mg:LiNbO3 waveguide

Secondary ion mass spectrometry (SIMS) was used to study the diffusion properties of Mg2+ and Ti4+ ions in a peculiar optical-damage-resistant near-stoichiometric (NS) Ti:Mg:LiNbO3 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 Mg2+-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 Ti4+-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 Ti4+ ions in the planar waveguide obey a Gaussian profile. From the measured profiles, characteristic diffusion parameters such as mean Mg2+ and Ti4+ diffusivities and surface Ti4+-concentration are evaluated. The diffusivity is discussed in comparison with the previously reported results on single Mg2+, Ti4+ diffusion and Mg2+/Ti4+ co-diffusion in pure (or bulk MgO-doped), congruent (or NS) LiNbO3 crystal. Finally, the keys to the succeeding in fabrication are highlighted.

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► Diffusion property of Mg2+ and Ti4+ in near-stoichiometric Ti:Mg:LiNbO3 waveguide.
► Mg2+ concentration is homogeneous and above optical damage threshold.
► Ti4+ follows sum of two error functions/Gauss function in lateral/depth direction.
► Surface/depth Ti4+ and Mg2+ diffusivities are 1.2/0.89 and 4.8 μm2/h, respectively.
► Adequate Ti and MgO film thicknesses and post Li-rich VTE are keys to fabrication.