Enhancing broadband second harmonic generation in a thin film lithium niobate racetrack resonator with tunable coupling

Second harmonic generation in thin film periodically poled lithium niobate (PPLN) is constrained by an efficiency–bandwidth trade-off and fabrication-sensitive scaling. We demonstrate a racetrack resonator incorporating a short PPLN section and a tunable Mach–Zehnder interferometer coupler that enables in situ control of the coupling condition, compensating fabrication tolerances by enabling post-fabrication optimization near critical coupling. The telecom pump is resonantly enhanced, while the near-infrared second harmonic is generated in a single pass, eliminating dual-resonance requirements. This architecture enables an approximately 35-fold efficiency enhancement over an equivalent non-resonant structure across a ∼7 nm pump-wavelength tuning range. Our approach provides a practical and fabrication-tolerant platform for resonantly enhanced integrated frequency doubling.