Proton‐Exchange Waveguides in Lithium Niobate: State of the Art and Applications

Proton exchange is one of the widely used methods for manufacturing optical waveguides in lithium niobate by partial replacement of Li + ions by protons H + , from an organic liquid source. Proton exchange allows the creation of stable low‐contrast buried waveguides with Δ n e ≈ 0.02 for navigation systems, telecommunications, sensors, nonlinear optics, and quantum technologies. New approaches to proton exchange under high vacuum open up the possibility for creating high‐contrast optical waveguides with Δ n e ≈ 0.1. Proton exchange can be used to modify the surface of lithium niobate and other ferroelectric crystals, to produce microfluidic chips, to study domain kinetics, as well as to form ridge optical waveguides in thin films, mixed, and doped single crystals. This review presents the physical principles and applications of proton exchange technology with postannealing, soft proton exchange, vapor‐phase proton exchange, high‐vacuum proton exchange, and reverse proton exchange. The prospects of proton exchange technology are also presented in conclusion.