Fabrication, characterization and index profile modeling of high-damage resistance Zn-diffused waveguides in congruent and MgO:lithium niobate

A study of the fabrication and optical properties of planar waveguides fabricated in MgO:LiNbO₃ and LiNbO₃ substrates by diffusion of a ZnO film is presented. Transmission electron microscopy was used to show that using ZnO instead of metallic zinc as a source, and maintaining the ZnO film thickness below a prescribed value, greatly reduces second phase precipitation and produces usable waveguides. Dopant and refractive index profiles were characterized by electron microprobe analysis and interference microscopy, respectively. The dependence of the Zn diffusion coefficient on temperature and the dependence of the refractive-index change on Zn concentration are inferred from these measurements. A simple model is also reported which predicts the index profile of the waveguide given the film thickness, diffusion time and temperature. The validity of the model is demonstrated by comparison between calculated profiles and profiles measured by prism coupling and IWKB analysis