Enhanced second harmonic generation performance in PPLN crystal using integrated fiber microlenses

This work presents the application of in-house fabricated fiber optic microlenses for ultrafast second harmonic generation (SHG) in a periodically poled lithium niobate (PPLN) LiNbO₃ nonlinear crystal. SHG is traditionally performed using free-space bulk optical components. In our work, we propose a compact, fiber-integrated solution that simplifies alignment and improves conversion efficiency. Microlens geometries were fabricated using a precisely controlled three-electrode arc discharge in a large-diameter splicing (LDS) system. We performed a comparative study between a bulk optics reference setup and a microlens-based setup, both of which used the same 1560 nm femtosecond laser source. At the output of each microlens setup, we measured the SHG optical power, optical spectrum, and pulse duration. The microlens configuration reduced the system's complexity. Additionally, it achieved higher conversion efficiency, with a maximum SHG power of 65.3 mW and an efficiency of 49.7%, surpassing the best performance of the bulk optics reference setup. These results demonstrate the usability of integrated fiber microlens systems in SHG nonlinear systems.