Chirped periodically poled lithium-niobate-on-insulator micro-waveguides for broadband second-harmonic generation.

Periodically poled lithium niobate (LN) waveguides based on quasi-phase matching schemes, benefiting from their high nonlinear coefficient (d) and strong optical confinement, are widely employed for implementing efficient second-harmonic generation (SHG). Here, we report broadband SHG in z-cut chirped periodically poled lithium-niobate-on-insulator (CPPLNOI) ridge micro-waveguides. Nearly 90-nm-wide SHG at the telecom band is achieved, along with an averaged normalized efficiency of 7.5%/(W·cm). We also demonstrate simultaneous generation of second as well as cascaded third and fourth harmonics under direct pumping of femtosecond pulses. This work would benefit applications for frequency conversion of a wideband coherent light source.