Effect of Thermal Treatment on the Properties of Proton-Exchange Waveguides in Lithium Niobate

Congruent X-cut lithium niobate samples were investigated on the proton exchange process and the properties of waveguides after thermal surface treatment have been carried out. It was shown, that after thermal surface treatment of lithium niobate, the proton exchange is less intense at identical parameters for all samples. This leads to smaller deformations of the lithium niobate crystal lattice at each step of the proton exchange process. The calculation of the density of etching pits, dislocations for planar proton exchange waveguides gives a decrease after thermal treatment. The prism coupling method shows a slight decrease in the refractive index at the surface of the lithium niobate crystal. These results expand the understanding of the relationship between the surface layer of a lithium niobate crystal and the creation of stable proton exchange waveguides. The proposed technique can be used to improve the thermal stability of integrated optical circuits based on different optical crystals.