  Infrared spectroscopy in the spectral fingerprint region from 6-12 um
accesses the largest molecular absorption cross-sections, permitting sensitive,
quantitative and species-specific measurements. Here, we show how dual-comb
spectroscopy--a form of high-speed Fourier-transform spectroscopy involving no
moving parts and capable of very high resolutions--can be extended to the 6-8um
wavelength band using femtosecond optical parametric oscillators (OPOs). By
acquiring dual-comb interferograms faster than the mutual decoherence time of
the OPO combs we implement line-shape-preserving averaging to obtain low-noise,
high-fidelity spectra of H2O and CH4 at approximately 0.3 cm-1 resolutions from
1285-1585 cm-1.
