Nonlinear interaction between radiation and guided modes for the characterization of reverse-proton-exchanged waveguides

A new method that allows the determination of the effective refractive indices (n/sub eff/) of the guided modes of reverse-proton-exchange waveguides, even of the lower order ones, is presented and demonstrated. The method exploits the second harmonic generation process, where the fundamental (/spl omega/) field is given by prism-coupled ordinary radiation modes while the second harmonic (2/spl omega/) field by extraordinary guided modes. The power transfer is thus realized through the nonlinear d/sub 31/ coefficient of lithium niobate and involves a birefringence phase matching. When phase matching occurs, the effective index of the guided mode at 2/spl omega/ equals that of the ordinary radiation mode at /spl omega/, so that the former can be readily obtained from the well-known prism-coupling equation, provided that the coupling angle /spl alpha/ of the ordinary radiation mode is measured.