Unlocking a lower shot noise limit in dual-comb interferometry

Optimizing the signal-to-noise ratio (SNR) is critical to achieve high sensitivities across broad spectral ranges in dual-comb interferometry. Sensitivity can be improved through time-averaging, but only at the cost of reduced temporal resolution. We show that it is instead possible to use high-bandwidth detection combined with frequency-domain averaging of multiple copies of the dual-comb beat note. By controlling the signal and noise stationarity properties, one can even reduce the fundamental shot noise contribution compared to the normal, single copy, dual-comb operation where integration time is matched to, or larger than the repetition period. In principle, the use of N-a aliased frequency-domain copies will improve SNR by up to vN(a), or equivalently, reduce acquisition time by a factor of N-a. We demonstrate dual-comb interferometry using N-a = 5 aliases, achieving the predicted fivefold reduction in shot noise power density at low frequencies. Over the full spectrum, unaveraged relative intensity noise limits the SNR, but we measure a 1.65x fold improvement in detection of CO2, corresponding to a 2.7x reduction in acquisition time for a given precision.