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 microm) to visible in a LiNbO3 poled waveguide coupler.