Structural and optical characterization for vapor-phase proton exchanged lithium niobate waveguides

We have studied the crystalline phase transition for vapor-phase proton exchanged (VPE) lithium niobate waveguide by using pure benzoic acid. The VPE layers in the z-cut lithium niobate crystal have been characterized by X-ray rocking curves, OH^- bond infrared absorption, and prism coupling measurements. The crystalline quality assessment by the X-ray rocking curve measurement shows desirable κ₂ phase formation for nonlinear optical waveguide applications. It has been observed that the crystalline phase makes a transition from the κ₂ to the β phase with the increase in the vapor-phase exchange time. The prism coupling measurement also showed a step-like index profile for our VPE samples. From the step index profile, we deduced an effective diffusion coefficient of 0.14 μm²h^-1, which is twice than that of the previously reported value. Shift in the OH^- bond absorption wavelength with crystalline phase change and the increase in absorption with the exchange time were also observed from our spectrophotometer measurement.