Refractive index change in Ti-diffused near-stoichiometric LiTaO₃ waveguide and its relation to Ti-concentration

Ti-diffused near-stoichiometric(NS) LiTaO₃ multimode planar waveguide was fabricated by diffusion of Ti-film coated onto a congruent LiTaO₃ substrate and post Li-rich vapor transport equilibration process. The crystalline phase in the Ti-diffused layer was analyzed by powder x-ray diffraction and the Li₂O-content was evaluated from the measured birefringence. The Ti-concentration profile was analyzed by secondary ion mass spectrometry. The modes guided in the planar waveguide were characterized by prism-coupling technique. The refractive index profile was constructed from the measured mode indices, and correlated with the Ti-concentration profile. The results show that the waveguide retains the LiTaO₃ phase and is in an NS composition environment. Both the refractive index and Ti-concentration follow Gaussian profile. The extraordinary and ordinary index change profiles are almost same, implying that the electric field profile of the mode guided in the NS Ti:LiTaO₃ waveguide would be polarization-insensitive. The ordinary and extraordinary index changes and the Ti-concentration follow a similar exponential relationship with a power index ∼1.3. Comparison shows that the Ti:LiTaO₃ waveguide shows considerable differences from the Ti:LiNbO₃ in Ti diffusivity, index change profile and its relation to Ti-concentration, and polarization dependence of guided mode.