Spectroscopy of the Methane { u}3 Band with an Accurate Mid-Infrared Coherent Dual- Comb Spectrometer

We demonstrate a high-accuracy dual-comb spectrometer centered at 3.4 mu m. The amplitude and phase spectra of the P, Q, and partial R-branch of the methane { u}3 band are measured at 25 MHz to 100 MHz point spacing with ~kHz resolution and a signal-to-noise ratio of up to 3500. A fit of the absorbance and phase spectra yield the center frequency of 132 rovibrational lines. The systematic uncertainty is estimated to be 300 kHz, which is 10-3 of the Doppler width and a tenfold improvement over Fourier transform spectroscopy. These data are the first high- accuracy molecular spectra obtained with a direct comb spectrometer.