Effect of pre-annealing of lithium niobate on the structure and optical characteristics of proton-exchanged waveguides

The influence of pre-annealing of lithium niobate in the range of 400–800 °C was studied at each step of formation of proton-exchange waveguides. Pre-annealing affects the structure and optical properties of lithium niobate. There is a relationship between the initial structure of lithium niobate and the optical characteristics of proton-exchange waveguides. The linear dependence between the deformations of the annealing proton exchange waveguides and the refractive index is established. Pre-annealing leads to an improvement in the structure of the subsurface layer of lithium niobate, as well as a slight decrease in the refractive index due to an increase in the homogeneity and a lower concentration of protons inside the crystal. The crystal structure becomes most uniform at a temperature of 500 °C. The results can be useful for different optical crystals used for applications.