Continuous‐Wave Laser‐Driven Cascaded Harmonic Generation in a Periodically Poled Lithium Niobate Racetrack Resonator

Abstract Nonlinear parametric frequency conversion is essential in expanding the optical spectrum for various applications, from laser technology to quantum photonics. Cascaded harmonic generation, which sequentially utilizes generated harmonics to produce higher‐order frequencies, presents an effective method to achieve broader spectral coverage. However, efficient implementation remains a challenge due to optical nonlinearity and complex interactions in high‐intensity conditions. In this paper, an on‐chip configuration for nonlinear cascaded frequency harmonic generation is proposed, utilizing X‐cut periodically poled lithium niobate (PPLN). The approach employs a single poling period and a continuous‐wave (CW) telecom laser input, overcoming the limitations of previous methods reliant on multiple poling periods, multiple waveguides, or high‐peak‐power pulsed lasers. With a doubly resonant design, a second harmonic generation signal is observed in the near‐infrared region with a conversion efficiency of ≈2,200%W −1 . Furthermore, the comprehensive analysis on waveguide geometry simultaneously enables multiple phase‐matching conditions, supporting the generation of green and UV light in experiment through the cascaded nonlinear interactions. This work marks a significant advancement in integrated nonlinear photonics, showcasing the potentials for efficient, on‐chip cascaded frequency conversion with minimal complexity.