Nickel-indiffused x, y-cut LiNbO₃ optical waveguides

A study of the fabrication process and optical characterization of Ni indiffusion into x and y cut LiNbO₃ substrate. The formation of waveguides in air ambient is clean and fast with the diffusion time well away from the T_c and out-diffusion regime of LiNbO₃. Optical characterization of the planar waveguides uses prism measurement technique and IWKB method, to establish the diffusion depth, d and the changed in refractive index, Δn. The derived normalized dispersion relation satisfies the Gaussian refractive index profile dispersion for n_e (TE) mode along the optical axis and n_o (TM) mode along both the optical and non-optical axis. For the samples studied, the profile attains that of the Gaussian in the initial waveguide formation and retains the profile for long annealing time and high temperature. The acquired value of Δn_e<0.04 and Δn₀<0.02 along the optical axis are comparable to that of Ti:LiNbO₃ and the anomaly, Δn_e/Δn_o<1, is valid for the non-optical axis only for high temperature annealing and thick Ni samples. The above measurements lead to a derivation for a simple model for the index profile of the channel waveguide, given the film thickness, diffusion time and temperature.