Relation of refractive index to ti-concentration in near-stoichiometric Ti:LiNbO₃ waveguide

Multi-mode near-stoichiometric (NS) Ti:LiNbO₃ planar waveguide was fabricated by co-work of Li-rich vapor transport equilibration and in-diffusion of Ti-film on an initially congruent LiNbO₃ substrate. The Ti-concentration is profiled by secondary ion mass spectrometry. The refractive index profile is constructed from measured mode indices and correlated with the Ti-concentration profile. The results show that the index change and Ti-concentration follow an exponential relationship with a power index 0.75/0.49 for the ordinary/extraordinary ray. The relationship is different from that of either conventional congruent waveguide or homogeneously Ti-doped NS bulk material or NS waveguide fabricated by direct Ti-diffusion in an NS substrate.