All-optical switching in a waveguide pair using unstable stationary solutions of second-harmonic generation

Coupled nonlinear waveguides are important components for all-optical signal processing. Their properties depend strongly on the existence and stability of states with no power exchange between the coupled waves (stationary solutions, nonlinear modes). In this contribution, all-optical switching based on unstable nonlinear modes of quasi-phase matched (QPM) second-harmonic generation (SHG) in a second-order nonlinear directional coupler is discussed. SHG in a waveguide pair is described by four normalized nonlinear coupling equations for the amplitudes of the single waveguide linear modes