Second-harmonic generation with broadened flattop bandwidth in aperiodic domain-inverted gratings

We report on a theoretical analysis for the broadened bandwidth of the fundamental waves in aperiodic gratings. The sequences and the length of the domains are optimized to realize the pre-designed wide bandwidth in the quasi-phase-matched (QPM) second-harmonic generation (SHG) by the simulated annealing (SA) method. About 3 nm prescribed flattop bandwidth as the full width at 95% maximum with 25% reduction of the nonlinear coefficient relative to that of a perfectly periodic QPM grating are obtained. Domains overgrown owing to the room-temperature electric poling technique show little influence on the broad bandwidth of the fundamental waves.