Modeling and Optimization of Quasi-Phase Matching Via Domain-Disordering

Second-harmonic generation in a quasi-phase matched waveguide produced using a domain-disordered GaAs-AlAs superlattice is modeled including the effects of group velocity mismatch, nonlinear refraction, two-photon absorption, and linear loss. The model predicts our experimentally observed second-harmonic powers within an order of magnitude. Self-phase modulation and two-photon absorption led to reduced conversion efficiencies of up to 33% at input peak powers >50 W. Group velocity mismatch results in a reduction of 23% in conversion efficiency using estimated group velocities calculated from the measured effective refractive index. The modeling also shows that the conversion efficiency peaks at propagation lengths longer than the pulse walk off length and that duty cycle variations induced shifts in the tuning curves. Group velocity mismatch also increased the conversion bandwidth by ~ 30%. Incomplete modulation of chi(2) in disordered regions reduced the output conversion efficiency by up to two orders of magnitude. Grating-assisted phase matching led to a 7% efficiency drop for a Deltan of 0.045 at the second-harmonic and 0.01 at the fundamental. This model serves as a valuable tool to provide insight into the optimization of these devices.