Refractive index profile in titanium-diffused congruent lithium tantalate optical waveguide

Titanium-diffused lithium tantalate (Ti:LT) multimode planar waveguide was produced by in-diffusion of titanium thin-film deposited onto a Z-cut congruent LT substrate. Titanium concentration (C_Ti) in the titanium-diffused layer was profiled by secondary-ion-mass-spectrometry. Modes guided in the waveguide were characterized by prism coupling technique. Refractive index in the waveguide is profiled on the basis of the measured mode indices, and is related to the C_Ti profile. The results show that both the refractive index and the C_Ti profiles obey to a Gaussian function. The profiles of titanium-induced extraordinary and ordinary index changes are almost identical, implying that the index change in the Ti:LT waveguide has a small anisotropy and the confinement to the mode field is polarization-insensitive. The ordinary and extraordinary index changes reveal a similar exponential dependence on the C_Ti with a power index of ∼0.8.