Mutual Coherence Enhanced Dual‐Comb Spectroscopy by Raman Gain Modulation

Abstract Dual‐comb spectroscopy (DCS) offers broad spectral coverage, high resolution, and rapid acquisition, making it an ideal spectroscopic technique. However, practical deployment remains limited by the inherent mutual incoherence of free‐running dual‐comb systems, which require complex phase‐locked architectures or non‐real‐time post‐processing algorithms to restore coherence. A mutually coherent DCS system is demonstrated, which operates without active feedback stabilization or post‐processing. By applying Raman gain modulation (RGM) to a 1645 nm single‐frequency laser, A mutually incoherent 1550 nm free‐running dual‐comb source is converted into a coherent 1645 nm system. Since both combs originate from the same single‐frequency seed laser, mutual coherence is passively enhanced. This approach suppresses radio‐frequency comb tooth jitter to the mHz level, enabling coherence times exceeding 20 s. DCS measurements of methane and carbon dioxide absorption cells achieve excellent agreement with the HITRAN database after averaging 20 000 interferograms over 9.5 s. Compared to conventional mutual coherence enhancement methods, the proposed RGM‐DCS system combines key advantages: low complexity, minimal cost, high stability, and outstanding technical scalability. This makes it a viable and practical solution for field‐deployable applications.