Increase in the signal-to-noise ratio of the beat note between a frequency comb and a continuous-wave laser

For most comb-based applications, the signal-to-noise ratio (SNR) of the beat note between the comb and the continuous-wave laser is a critical parameter that affects the stability of the system and the accuracy of the measurement. We analyze the SNR model of the beat note and discuss some key aspects of implementing an optimized gated-detection scheme. High-speed balanced detection is introduced to enable detection of more interfering photons, while suppressing optical-intensity-noise interference. The gated detection is achieved by our combining a self-made low-noise, delay-adjustable pulse-generator circuit, which can suppress phase-random white noise. Compared with traditional direct-detection schemes, we demonstrate a 20-dB improvement in performance and achieve a beat-note SNR of more than 60 dB within a 300-kHz resolution bandwidth. Further experiments on frequency-comb locking show that this scheme has high reliability and robustness.