Refractive Index in Ti:LiNbO3 Fabricated by Ti Diffusion and Post-Li-Rich VTE

Multimode near-stoichiometric (NS) Ti:LiNbO₃ planar waveguide was fabricated by Ti-diffusion followed by post-Li-rich vapor transport equilibration. The crystalline phase, composition and guided modes in the planar waveguide were characterized. The refractive index profile in the waveguide is constructed from the measured mode indices using the inverse Wentzel-Krames-Brillouin method and correlated with the Ti profile obtained from secondary ion mass spectroscopy analysis. The results show that the LiNbO₃ phase dominates in the waveguide layer and the waveguide has an NS composition. The index increase has a linear relation to Ti-concentration for both cases of ordinary and extraordinary rays, and the relationship is considerably different from those of some congruent or NS bulk materials or waveguides. A comparison of various congruent and NS bulk materials or waveguides allows to conclude that the relationship changes from congruent to NS composition, from one fabrication method to another and from bulk material to waveguide configuration.