Free‐Running Single‐Cavity Dual Combs With Hz‐Level Relative Linewidth

ABSTRACT Single‐cavity dual‐comb lasers provide a compact and efficient source for dual‐comb spectroscopy in gas sensing applications; however, achieving sufficient free‐running mutual coherence for comb‐line‐resolved, high‐resolution measurements remains challenging. Here, we present a symmetry‐engineered bidirectional single‐cavity dual‐comb laser based on an all‐polarization‐maintaining fiber architecture. The system exhibits exceptional free‐running mutual coherence, achieving Hz‐level relative linewidths without active feedback or phase correction. The time‐averaged absolute jitter of the dual‐comb repetition‐rate difference reaches 4.7 × 10 − 7 min − 1 , representing an improvement of nearly two orders of magnitude over previously reported free‐running systems. As a spectroscopic demonstration, we resolve ≈49,000 comb lines over a 5.4 THz optical bandwidth and measure the absorption spectrum of carbon monoxide ( 1 2 CO), faithfully retrieving molecular line shapes with millisecond acquisition times. This architecture provides a compact and robust free‐running platform for broadband molecular spectroscopy and millisecond‐scale, line‐shape‐resolved gas sensing.