Ti diffusion in Ti : LiNbO3 planar and channel optical waveguides

Abstract

Optical mode measurements have been used to determine the anisotropic diffusion coefficient and surface index changes for planar Ti : LiNbO₃ waveguides at 0.633 μm. Measured values for the diffusion coefficient at 1000 °C are D_y=9.4×10⁻¹³ and D_z=1.4×10⁻¹² cm²/sec. The ratio of the extraordinary to the ordinary surface index change (Δn_e/Δn₀) was found to vary between 1.3 and 1.8, depending on diffusion temperature and Ti film thickness. Ti and Li concentration profiles were measured by secondary‐ion mass spectrometry. Additional high concentration peaks, 0.2–0.3 μm wide for both ions, were found superimposed on otherwise well‐behaved diffusion profiles. This observation is interpreted to result from a tendency towards Li‐Ti‐O compound (Li₂TiO₃ or Li₂Ti₃O₇) formation at the diffusion temperature in a dilute mixture with LiNbO₃. Such compound formation has the effect of impeding the Ti diffusion into the LiNbO₃ substrate. Electron microprobe measurements were used to measure lateral diffusion from channel waveguides. Under nominally identical conditions, lateral diffusion coefficients varying by as much as a factor of 3 have been observed. The minimum lateral diffusion coefficients observed at 1000 °C were D_y=9.7×10⁻¹³ and D_z=1.4×10⁻¹² cm²/sec.