Extended Data Fig. 6: Cavity ringdown at comb-mode resolution.

a, Spectroscopy data collected at instrument resolution of 0.20 GHz for resolving the cavity transmitted comb lines spaced at 0.27 GHz frequency interval. Measurement is performed for calibrated 1.04(5) ppm CH₄ diluted in nitrogen at 100 Torr and 20 °C. Rovibrational absorption lines have a median width at about 0.61 GHz calculated on the basis of HITRAN data, which is considerably larger than the comb spacing. Spectroscopy data are an average from a total of 256 interferograms collected at 7.5 s each. Zero padding is used to oversample at one-tenth of the instrument resolution to accurately locate the comb line frequencies. Only the leading five Fourier harmonics are plotted for clarity. The right panel is the zoom-in to near 3,018.22 cm⁻¹. b, Ringdown absorption data determined from a. Stem lines in blue highlight the absorption coefficients measured by different comb lines. The data spanning 2 cm⁻¹ coverage uniformly sampled by 219 comb lines is globally fitted to the HITRAN database plotted in orange. Fitted concentration yields 0.97(5) ppm. We further fit each rovibrational absorption line to a Voigt profile (black). For this analysis, Gaussian linewidths are fixed at the calculated values based on HITRAN data at 20 °C. Fitted concentration is used as the true methane concentration. c, Molecular parameters for the Lorentzian half width at half maximum (HWHM) widths and line intensities are determined for individual rovibrational absorption lines. Experimental data in black are compared with HITRAN data in orange. Uncertainties in the HITRAN database for which linewidths reported at about 10% and line intensities at about 5% are indicated. Vertical orange dashed lines are a guide to the eye aligning extracted molecular parameters to their corresponding rovibrational absorption lines. d, Spectroscopy data collected at the modest instrument resolution of 0.80 GHz. In contrast to the measured data at comb-mode resolution in b, the two close-lying rovibrational absorption lines near 3,018.2 cm⁻¹ are less robustly resolved owing to the larger spectral resolution. Global fit to the HITRAN database yields 1.00(5) ppm fitted concentration for methane. To summarize, our data confirm: (1) both 0.80 GHz modest resolution or comb-mode resolution yield measured concentrations in good agreement with the calibrated value and (2) at comb-mode resolution, molecular parameters further extracted for absorption linewidths and line intensities are also in good agreement with the reported values from the HITRAN database.