Aging effect in proton-exchanged optical waveguides based on lithium niobate crystals

The optical characteristics of planar waveguides produced by proton exchange in lithium niobate crystals in different acids (with and without additives) are unstable with time (“aging” effect). The aging effects in LiNbO$_3$ optical waveguides are investigated at room and elevated (50, 60, or 70$^\circ$C) temperatures. In all cases, variations in the refractive index of the waveguide layer with time, i.e., for two or three years at room temperature and for several weeks at elevated temperatures, are determined for waveguides prepared in different media. The dependences of the optical characteristics of waveguides on the nature of proton sources and the aging conditions are obtained experimentally.