Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation

We report on theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG) in LiTaO₃. QPM waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency. The waveguides were fabricated by adjusting the phase-matching condition either by changing the grating period of the SHG coefficient or by controlling the width of the waveguide. Consequently, the bandwidth of QPM-SHG was broadened to 0.35 nm with an SHG efficiency of 57 percent/W by modulation of the waveguide width and to 1.12 nm with an SHG efficiency of 29 percent/W by modulation of the grating period. The SHG efficiency and phase-matching characteristics in both types of waveguides showed good agreement with theoretical results