Fig. 1. (a) Experimental setup of the coherent dual-comb system. The MIR comb delivered a 3-nJ mid-infrared pulse train at a repetition rate of 108.4 MHz. Then the mid-infrared pulses were focused into the APPLN waveguide, generating multiple OFCs output covering from the mid-infrared to ultraviolet region. Ge, AR-coated germanium wedges. λ/2, half-wave plate; L1, AR-coated molded aspheric lens; L2, aluminum-coated off-axis parabolic mirror; BS, beam splitter; PD, photodetector. (b) Cross-section geometry of the lithium niobate waveguide. (c) Overview of the waveguide, pumped by the mid-infrared pump pulses. (d) Generated harmonics in the visible and near-infrared regions, after being dispersed with a prism and observed on white paper and infrared detector cards.

Fig. 2. (a) Output spectra of the generated harmonics from H1 to H9. (b) Simulated evolution process in the frequency domain. (c) Simulated evolution in the time domain, revealing the harmonic generation location and the corresponding poling periods. Each harmonic is normalized, and the group velocity of the pump pulse is set to zero to center the temporal profile.

Fig. 3. Dual comb spectra from H1 to H8. (a) The first column shows the dual-comb spectra retrieved from the measured interferogram, which is coherently averaged 150,000 times. (b) The second column represents the Fourier transformation result of continuous data stream of time length 1 s with 1000 times of coherent averaging. (c), (d) The third and fourth columns are the zoomed-in plots of (b).

Fig. 4. Dual-comb spectroscopy of methane absorption. The gas pressure of methane is 50 mbar. The effective light path length is 8 cm, 1 m, 8 m, and 8 m for H1–H4, respectively. (a), (b), (g), (h) Dual comb absorption spectra at 3550, 1750, 1185, and 900 nm region. The spectra were coherently averaged 75,000 times. (c), (d), (i), (j) Extracted absorption lines (gray) from the dual-comb spectra and the theoretical absorption profiles (red) from the HITRAN database. (e), (f), (k), (l) Magnified view of part of the methane absorption lines. Gray lines stand for experimental extracted lines, red lines for theoretical profiles from HITRAN, and purple lines for residual between theoretical result and experimental observation.

Fig. 5. Wavelength tuning characteristics of the dual-comb system, with the MIR pumping wavelength tuning from 3.3 to 4.3 μm. (a) Tunable spectra for H1 and H2, measured with a fast Fourier transform analyzer (Bristol 771B). (b) Tunable spectra for H3 and H4, measured with an optical spectrum analyzer (Yokogawa AQ6370). (c) Tunable spectra for H5–H9, measured with a fiber spectrometer (Ocean Optics HR4000).

Fig. 6. Fourier transformation results of 10-s-long continuous data stream. The linewidth of individual comb tooth is 0.1 Hz in RF domain for every harmonic from H1 to H8. It equals the Fourier limit of 10-s-long recording time, demonstrating the coherence of our dual-comb sources. OF, optical frequency domain; RF, radio frequency domain.