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

TL;DR

This paper presents a simplified self-heterodyne frequency comb spectroscopy system using a non-uniform comb generated by a CW laser and phase modulator, achieving high sensitivity and speed for molecular spectroscopy.

Contribution

It introduces a novel non-uniform comb spectroscopy system that overcomes traditional sensitivity-speed trade-offs with a simple setup and enhanced performance.

Findings

Achieves an NEA of 5.0×10^(-6) Hz^(-1/2), outperforming dual-comb spectroscopy.

Resolves weak molecular overtone spectra on nanosecond timescales in a single shot.

Maintains long-term stability and high signal-to-noise ratio.

Abstract

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 an extremely simple 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 simplified self-heterodyning architecture. By leveraging the intrinsic mutual coherence of the comb, this design achieves a noise-equivalent absorption coefficient (NEA) of 5.0*10^(-6) Hz^(-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, at a signal-to-noise ratio of 128. This integration of high sensitivity, resolution, and speed resolves the core trade-off that has long constrained time-resolved spectroscopic analysis.