Refracted-near-field analyses for refractive-index profiles of integrated LiNbO3 waveguides

The development of any new technology requires new characterization tools both for research and development laboratories and for production control. It is thus important to develop characterization methods for LiNbO3 waveguides. In this contribution we concentrate on a basic physical property of a waveguide, its refractive-index profile,1 for which we present a direct and nondestructive measurement technique. This technique generalizes the refracted-near-field (RNF) method, which is a well-known optical-fiber characterization tool.2 However, specific adaptations are necessary because of noncylindrical geometry, because of small dimensions, especially for the most interesting cases of single-mode guides, and because of high refractive indices, around 2.25, for which the index-matching liquids are corrosive and toxic and should thus be avoided. We have circumvented these difficulties by collecting the unguided light that emerges from one end of the sample when the beam is focused on the other end, as illustrated in Fig. 1. With this configuration we move somewhat away from the traditional RNF configuration; in particular, several sections of the incident beam will be detected.3