Method for determining the parameters of gradient proton-exchanged waveguides in single crystals and solid solutions

Subject of study. The determination of the parameters of gradient waveguides required for the fabrication of integrated optical circuits with low optical losses is presented. Aim of study . This work aims to develop a new approach for determining all key parameters of gradient proton-exchanged waveguides in single crystals and solid solutions. Method . Confocal Raman microscopy, the prism coupling method, and a numerical solution of the classical transport equation are used to reconstruct gradient waveguide profiles. Main results. Gradient profiles of proton-exchanged waveguides formed in lithium niobate crystals and lithium niobate–tantalate solid solutions of X - and Z -cuts are reconstructed by measuring the OH-group line in the Raman spectrum. The reconstructed profiles are in good agreement with those obtained by the prism coupling method and diffusion theory, enabling accurate determination of the waveguide layer depth. The refractive index increment is determined from a calibration curve calculated in this work that establishes the relationship between the Raman scattering intensity in the OH-group region and the refractive index increment. This approach allows the estimation of all the main parameters of proton-exchanged waveguides at any point, which is not achievable using conventional methods. Practical significance. The results can be applied for the rapid nondestructive characterization of gradient proton-exchanged waveguides in integrated photonics devices.