Frequency locking techniques for nonlinear frequency conversion: Principles, advances and challenges

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Frequency locking is a key enabling technique for resonator-enhanced nonlinear frequency conversion, where conversion performance is ultimately governed by intracavity power build-up and stable resonance. In practical systems, thermal, mechanical, and acoustic perturbations can detune the laser from the cavity resonance, leading to reduced intracavity power and degraded conversion efficiency, spectral quality, and long-term stability. Active frequency locking mitigates such detuning through real-time feedback, thereby sustaining resonant enhancement and stable nonlinear conversion. This Review summarizes the principles, recent advances, and outstanding challenges of frequency-locking techniques for cavity-assisted nonlinear frequency-conversion processes. We first examine three widely used error-signal generation schemes—Pound–Drever–Hall, Hänsch–Couillaud, and Tilt-Locking—and compare their physical mechanisms, implementation requirements, and performance trade-offs in sensitivity, bandwidth, and robustness. We then review the evolution of frequency-locking control systems from analog circuits to field-programmable-gate-array-based digital platforms, emphasizing their contributions to improved locking accuracy, broader bandwidth, and enhanced long-term robustness. Representative applications are further surveyed, including second-harmonic generation, sum-frequency generation, optical parametric oscillators, Raman lasers, and Brillouin lasers, together with commonly used cavity architectures such as ring, standing-wave, and monolithic resonators. Finally, we discuss key challenges related to thermo-optical-mechanical coupling, nonlinear feedback, noise transfer, and the trade-off among bandwidth, robustness, and system complexity, and outline future directions toward integrated control, higher-power operation, and extreme wavelength regimes.