Dual-comb spectrometer. The inset shows the output spectra of the frequency combs. Colorful lines represent optical beams, while black lines are electric cables. OFC: optical frequency comb; QWP: quarter waveplate; HWP: half waveplate; PBS: polarized beam splitter; BS: non-polarized beam splitter; BPF: optical band-pass filter; GC: gas cell; PD: photodetector; LPF: electronic low-pass filter.