Multistep cascading in periodically poled directional couplers

We suggest a novel scheme for multistep cascaded parametric processes based on a periodically poled nonlinear directional coupler and derive the conditions for simultaneous phase matching of the second-harmonic generation and sum-frequency mixing processes. We predict an efficient wavelength conversion by the cascaded third-harmonic generation from the third optical communication window (1.53–1.56 m) to visible in a LiNbO₃ poled waveguide coupler.The authors acknowledge a support of the Bulgarian Ministry of Science and Education (grant F1201/ 2002) and the Australian Research Council