Second-harmonic generation and cascaded nonlinearity in titanium-indiffused lithium niobate channel waveguides

We summarize and discuss the results of our second-harmonic generation experiments in titanium-indiffused lithium niobate optical channel waveguides. The wave-vector mismatch in the nonlinear wave interaction was varied with temperature tuning around the second-harmonic resonances. Fundamental depletion and second-harmonic tuning curves show a strong power dependence, which is an indication of an intensity-dependent wave-vector modification of the interacting modes. This nonlinear refractive effect (which is called cascaded nonlinearity) is characterized with interferometric measurements of the resulting nonlinear phase shift of the fundamental. Large nonlinear phase shifts $(>2\pi )$ appear in regions of low fundamental depletion $(<10\%)$ because of a nonuniform wave-vector mismatch along the waveguide. At resonance a maximum fundamental depletion of more than 90% was observed. All the measured results are explained well theoretically with a coupled-mode model that has proved to be a reliable design tool for fabricating waveguide devices for applications of the cascaded nonlinearity.