Efficient second-harmonic generation of ultrashort pulses with simultaneous quasi-phase matching and group velocity matching in periodic 90° domain structures of potassium niobate

Triple-line-shaped electrodes were devised to fabricate a periodically poled 90° domain structure of KNbO3 (PP90KN) of 20.25 μm period with uniform periodicity and reproducibility. Using the fundamental pulse of 141 fs duration at 1619 nm wavelength, an efficient broadband type-I quasi-phase-matched second-harmonic generation was realized in an 8.3 mm long PP90KN by satisfying both phase matching and group-velocity-matching in the telecommunication L-band at room temperature. Second-harmonic pulses showed no significant temporal and spectral distortion and had a pulse duration of about 144 fs, which agreed well with the theoretical value of 142 fs. The conversion efficiency of the second-harmonic generation was 28.7% at the fundamental pulse energy of 0.92 nJ, which is about 60% of the theoretical value.