Spectral Mode Enhancement in Coherent-harmonic Dual-comb Spectroscopy Enables Exceeding 300-fold Averaging Time Reduction

Abstract: Dual-comb spectroscopy (DCS) is a novel Fourier-transform spectroscopy not relying on mechanical scanning and capable of simultaneously achieving high speed, high spectral resolution, and broad optical bandwidth. Nevertheless, it suffers from low signal-to-noise ratio (SNR) per single acquisition due to the dynamic range limitation of photodetectors imposed by the high-peak-power mode-locked pulses, making coherent averaging an essential means to improve SNR, at the price of compromising the exceptional time resolution and placing more stringent demands on mutual coherence and stability. In this study, a novel approach to enhance SNR by exploiting the spectral mode enhancement mechanism in coherent-harmonic pulses is demonstrated. As a proof-of-concept, two frequency combs with mode spacing of $\sim$12.5 MHz, operated at a 20th harmonic repetition rate of $\sim$250 MHz, are employed, demonstrating a $>$300-fold reduction in averaging time for comparable SNR in conventional DCS. This reduction is expected to be further enhanced through integration with ultra-high repetition rate combs like microresonator combs. This new approach promises both a recovery of the inherent high-speed capability and a mitigation of the coherence-time requirements, thereby making it possible to significantly facilitate subsequent DCS investigations and field deployments.