All-optical switching due to cascaded second-harmonic generation in directional couplers with laterally varying phase mismatches

The local normal mode approach is employed to analyze cascaded second-harmonic generation (SHG) in directional couplers with different phase mismatches in both waveguides. The couplers are assumed to be fabricated in ferroelectric crystals, which are periodically domain inverted with different periods in both waveguide regions. These components are proposed to imitate the switching in nonlinearly asymmetric third-order directional couplers by using the cascaded second-order effect, which can mimic third-order interactions with effective third-order nonlinearities proportional to the reciprocal phase mismatch. The results for the second-order directional couplers are compared to those for the respective ideal third-order ones.