Ion Exchange for Optical Waveguides in LiNbO3 and LiTaO3

Abstract

This report describes the use of silver-lithium and thallium-lithium ion exchange to produce waveguides in lithium niobate and lithium tantalate. Fabrication of optical waveguides is a necessary prerequisite for making integrated optics devices. At present the most usual technique for making such guides is the diffusion, at high temperatures, of titanium into LiNbO3. While this method has proved successful for numerous applications, it has certain shortcomings. First, for some applications, wavelength filters, for example, an index increase (Δn) higher than that available with titanium diffusion is desirable Second, LiTaO3, rather than LiNbO3, may be he preferred substrate because of its much lower susceptibility to optical damage. Since LiTaO3 has a Curie temperature considerably lower than any practical diffusion temperature and therefore requires repoling after diffusion in order to recover its electro-optic properties, diffusion of titanium, or of other metals, is not an appropriate technique for this material. Although it has been shown that an electric field can be used to drive copper ions into LiTaO3 at temperatures low enough that repoling is unnecessary,1 the presence of copper produces a colored crystal with strong absorption in both the visible and near IR.2