Heterodyne beats between a continuous-wave laser and a frequency comb beyond the shot-noise limit of a single comb mode

We demonstrate a simple and robust technique for improving the signal-to-noise ratio (SNR) of heterodyne beats between a low-power frequency comb and a continuous-wave (cw) laser. By exploiting the pulsed nature of the comb, a SNR better than the shot-noise limit for the beating of a single mode can be obtained. This technique relies on electrical gating of the beat signal to remove as much shot noise as possible from the measurement. This technique can also be understood as efficiently using the beating of the cw laser with many comb modes simultaneously. We demonstrate a simple experimental setup which achieves a 50-fold improvement in the shot-noise limited SNR of a conventional heterodyne setup and is in long-term agreement with a conventional heterodyne setup, at least to better than 1 part in 10(17). The resulting analog signal is completely compatible with existing heterodyne setups and can be used as an upgrade to an existing setup which would benefit from the SNR improvement. DOI: 10.1103/PhysRevA.87.023802