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_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 (ω) field is given by prism-coupled ordinary radiation modes while the second harmonic (2ω) field by extraordinary guided modes. The power transfer is thus realized through the nonlinear d₃₁ coefficient of lithium niobate and involves a birefringence phase matching. When phase matching occurs, the effective index of the guided mode at 2ω equals that of the ordinary radiation mode at ω, so that the former can be readily obtained from the well-known prism-coupling equation, provided that the coupling angle α of the ordinary radiation mode is measured.