Fig. 1. Adjustment of frep and fceo. Cavity filtering on comb when (a) frep<FSR and when (b) frep>FSR. CC, current filtering center; NC, next filtering center. Cavity transmission when (c) frep is not appropriate, (d) frep is appropriate and (e) both frep and fceo are appropriate.

Fig. 2. Experimental setup. (a) Measurement part; (b) acquisition part. ECDL, external cavity diode laser; CW1, CW2, continuous-wave lasers; PDH, Pound–Drever-Hall detection; EOM, electro-optical modulator; CPL, coupler; PBS, polarization beam splitter; L, lens; PZT, piezo actuator; DCS, dual-comb spectroscopy; Q, quarter-wave plate; H, half-wave plate; blue lines, optical fibers; red lines, spatial beams.

Fig. 3. Optimization of frep and fceo and coupled spectrum. Cavity transmission in the experiment when (a) frep is not appropriate, (b) frep is appropriate, and (c) frep and fceo are appropriate. The nonuniform intervals between adjacent two peaks appearing in (b) and (c) were due to the nonlinearity of the piezo movement. (d) Cavity finesse and corresponding frequency detuning of FSR. (e) The spectra of the original comb and the transmitted light in cases of (a), (c), and fine-tuning of fceo.

Fig. 4. Spectral measurement of ACE-DCS system. (a) The spectrum of N2O sample around 6450  cm−1; blue lines, the measured spectrum after Fourier transforming the IGM, exhibiting each individual comb mode; red lines, the fitted spectrum using the highest points of each comb mode, revealing clear absorption features. (b), (c), and (d) Mode-resolved spectrum in different scales, spanning 14  cm−1, 0.23  cm−1, and 0.005  cm−1, respectively. (e) Transmittance and phase around 6370  cm−1; (f) transmittance of the P(16) line of the (5000→0000) band and the Voigt fitted result and the line profile provided by HITRAN 2016.

Fig. 5. Diagram for the postcorrection algorithm. Clk., clock; Sig., signal.