Self-heterodyne spectroscopy via a non-uniformly spaced frequency comb

Frequency comb spectroscopy has significantly advanced molecular spectroscopy across scientific research and diverse applications. Among its key performance metrics especially for time-resolved studies, sensitivity and measurement speed are paramount. However, a long-standing compromise between these parameters arises from the need for noise reduction. Here, we introduce a comb spectroscopy system that overcomes this limitation using a single frequency comb of non-uniformly spaced modes. The comb is generated using a simple optical setup, composed of a continuous-wave (CW) fiber laser and a single-sideband phase modulator (SSM). Our approach delivers optical-to-radio-frequency conversion comparable to dual-comb spectroscopy (DCS) but through a self-heterodyning architecture. By leveraging the intrinsic mutual coherence of the comb, this design achieves a noise-equivalent absorption (NEA) of 5.0×10^-6Hz^-1/2-an order-of-magnitude improvement over state-of-the-art DCS, coupled with long-term stability. The system resolves weak molecular overtone spectra on nanosecond timescales, in a single-shot measurement, with absorption noise as low as 2%, markedly lower than prior demonstrations. This integration of high sensitivity and speed resolves the core trade-off that has long constrained time-resolved spectroscopic analysis.