Femtosecond laser direct writing quasi-phase matched type-II waveguide in lithium niobate

In this paper, we demonstrate a quasi-phase matched type-II waveguide fabricated by femtosecond laser direct writing in Z-cut MgO doped lithium niobate. Compared with the previous work that employs type-III configuration to realize the all-optical fabricated quasi-phase matched waveguide, our type-II waveguide has a simpler dual-line structure that can support TM-mode quasi-phase matched second harmonic generation which employs d33 naturally, thus the fabrication process can be significantly simplified. In experiment, an enhanced second-harmonic generation with a normalized conversion efficiency of 8.76 %W−1cm−2 is observed, corresponding to a nonlinear modulation depth of 0.89. The proposed design and fabrication method expand the application of femtosecond laser direct writing type-II waveguide in the field of monolithic integrated nonlinear devices.