Characterization of Annealed Proton Exchange Waveguides in Mixed Lithium Niobate-tantalate Solid Solution

The new lithium niobate-tantalate hybrid optical material platform can be used for special tasks in integrated photonics: waveguides, periodical poling, modulators and sensors. A comparison of structure and optical characteristics was performed between optical waveguides obtained by annealed proton exchange in lithium niobate (LN) and mixed lithium niobate-tantalate (LNT) solid solutions. The prism coupling method, IR-spectroscopy and X-ray diffraction were used to study the annealed proton exchange waveguides. The calculated increment of refractive index after proton exchange was 0.129 for LNT compared to 0.112 for LN. This unusual high increase of the refractive index of LNT is likely due to the presence of both compressive and tensile deformations of the crystal lattice of LNT as a result of proton exchange. The results of IR-spectroscopy indicate a more intense proton exchange in mixed crystals, which leads to greater structural disorder, in contrast to LN. The diffusion coefficients are higher for LNT unlike LN crystals and annealing relaxation of crystal lattice stresses is faster, subsequently. This is the first report on using proton exchange technology for optical waveguides fabrication in LNT solid solutions.