Mid-infrared dual-comb spectroscopy is of great interest owing to the strong spectroscopic features of trace gases, biological molecules, and solid matter with higher resolution, accuracy, and acquisition speed. However, the prerequisite of achieving high coherence of optical sources with the use of bulk sophisticated control systems prevents their widespread use in field applications. Here we generate a highly mutually coherent dual mid-infrared comb spectrometer based on the optical–optical modulation of a continuous-wave (CW) interband or quantum cascade laser. Mutual coherence was passively achieved without post-data processes or active carrier envelope phase-locking processes. The center wavelength of the generated mid-infrared frequency combs can be flexibly tuned by adjusting the wavelength of the CW seeds. The parallel detection of multiple molecular species, including C2H2,CH4,H2CO,H2S, COS, and H2O, was achieved. This technique provides a stable and robust dual-comb spectrometer that will find nonlaboratory applications including open-path atmospheric gas sensing, industrial process monitoring, and combustion.

The determination of airflow parameters is essential to the research of critical information on environment monitoring, chemical kinetics, and aerodynamic and propulsion applications. During the past few decades, tunable diode laser absorption spectroscopy has become a common and efficient tool for the flow velocity measurement based on the Doppler shift of the absorption line. Dual-comb absorption spectroscopy (DCAS), as a state-of-the-art Fourier-transform broadband spectroscopic technique, not only can detect multiple trace molecules in parallel but also can extract Doppler shifts to derive the flow velocity through the analysis of dozens of molecular absorption lines simultaneously with high precision. Here, we report a proof-of-principle demonstration of the velocity measurements of acetylene at various flow velocities by means of a high-resolution and broadband DCAS. Mode-resolved Doppler-shifted rotational-vibrational lines in the P branch of acetylene molecules are obtained. A model for multiline Doppler frequency determination is investigated and experimentally verified. The flow velocity measurements with a measuring uncertainty down to the submeter per second over the range from 8.7 m/s to 44.8 m/s at an effective time resolution of 1 s and a measuring uncertainty of 1.97 m/s at 0.1 s are demonstrated. With broadband mid-infrared frequency combs covering atmospheric transmission windows, the open-path measurement for monitoring diffusion of the weak pollutant source would be realized.