Comparison of characterization of proton-exchanged waveguides in Y-cut MgO:LiNbO3 and Z-cut LiNbO3

Proton-exchanged waveguides of Y-cut MgO: LiNbO3 and Z-cut LiNbO3 are fabricated in benzoic acid. The effective refractive indices of guide modes are measured by prism coupling method at 632.8 nm. The refractive index profiles are obtained by the inverse WKB method and the surface refractive index increments are calculated. The results show that the waveguides have the linear-step index profiles and the slope coefficient of the profile for Y-cut MgO: LiNbO3 is much smaller than that of Z-cut LiNbO3 and indicate that the index profile of Y-cut MgO: LiNbO3 waveguides is more like step. The theoretical mode indices are obtained on the basis of the dispersion equations with the simple step and linear- step index profiles respectively. It is found that the linear- step profile is more accurate. The effective diffusion coefficients, diffusion constant and activation energy are achieved with the diffusion law finally.