On-chip nonlinear photonics: Advances and challenges in integrated PPLN optical parametric oscillators

Integrated periodically poled lithium niobate (PPLN) optical parametric oscillators (OPOs) are undergoing a fundamental shift from standalone functional components to programmable on-chip “system engines.” This review synthesizes recent breakthroughs on the thin-film lithium niobate (TFLN) platform, highlighting advancements in ultra-low thresholds, high conversion efficiencies, and spectral versatility, and the dichotomy of continuous-wave vs femtosecond pumping dynamics. Furthermore, we critically examine key microfabrication breakthroughs—particularly high-fidelity domain engineering across different crystal orientations, 3D nanodomain poling, and submicron poling for mirrorless OPOs—while exploring transformative applications in advanced on-chip light sources, integrated quantum photonics, and broadband frequency synthesis. This work serves as a brief guide for charting the future development of scalable, high-performance nonlinear integrated photonic systems.