Fig. 1. Schematic of the MIR comb generation. The lens after the waveguide is to make the schematic easier to understand. In the actual system, we use an off-axis parabolic mirror to collimate the MIR light instead of the lens. LD, laser diode; EDF, Er-doped fiber; WDM, wavelength division multiplexer; Col, collimator; EDFA, Er-doped fiber amplifier; λ/2, half-wave plate; HNLF, highly nonlinear fiber; PPLN WG, periodically poled lithium niobate waveguide; and LPF, long-pass filter. Moreover, we also recorded the images of the PPLN WG using a phone camera (inset (a)) and a charge-coupled device (CCD) camera (inset (b)).

Fig. 2. (a) Autocorrelation trace and (b) measured spectrum of the compressor output.

Fig. 3. (a) Spectral profile of SCG. The spectral region is separately measured using a Yokogawa AQ6370 (900–1700 nm) and a Bristol 771B (1700–2400 nm). (b) Spectral profile of the MIR comb. (c) RIN of MIR light.

Fig. 4. (a) Schematic of the MIR dual-comb system. The recorded interferograms at different time scales of (b) 150 ms and (c) 0.02 ms. CW, continuous-wave laser; BPD, balanced photodetector; DAQ, data acquisition.

Fig. 5. Mode-resolved MIR dual-comb spectra at different wavelength scales of (a) 1.4 μm, (b) 0.002 μm and (c) 0.0002 μm.

Fig. 6. (a) Absorption lines of methane in the 3537–3547 nm region. (b) Zoom-in absorption line of methane at 3541.1–3541.3 nm. (c)–(e) Comparison results of the extracted gas absorption lines (blue dot) and the theoretical profiles from the HITRAN database (red line).