Broad-band frequency references in the near-infrared: Accurate dual comb spectroscopy of methane and acetylene

The Doppler-limited spectra of methane between 176 THz and 184 THz (5870-6130 cm−1) and acetylene between 193 THz and 199 THz (6430-6630 cm−1) are acquired via comb-tooth resolved dual comb spectroscopy with frequency accuracy traceable to atomic standards. A least squares analysis of the measured absorbance and phase line shapes provides line center frequencies with absolute accuracy of 0.2 MHz, or less than one thousandth of the room temperature Doppler width. This accuracy is verified through comparison with previous saturated absorption spectroscopy of 37 strong isolated lines of acetylene. For the methane spectrum, the center frequencies of 46 well-isolated strong lines are determined with similar high accuracy, along with the center frequencies for 1107 non-isolated lines at lower accuracy. The measured methane line-center frequencies have an uncertainty comparable to the few available laser heterodyne measurements in this region but span a much larger optical bandwidth, marking the first broad-band measurements of the methane 2ν3 region directly referenced to atomic frequency standards. This study demonstrates the promise of dual comb spectroscopy to obtain high resolution broadband spectra that are comparable to state-of-the-art Fourier-transform spectrometer measurements but with much improved frequency accuracy.Work of the US government, not subject to US copyright.

► Dual comb spectroscopy allows one to measure the absorbance and phase of a sample. ► High frequency resolution, accuracy and spectral coverage is achieved. ► Absolute accuracy of approximately 0.2 MHz was demonstrated. ► Spectra of methane (176-184 THz) and acetylene (193-199 THz) were measured. ► First accurate broad-band measurements of the methane 2ν3 band.