We present dispersive dual-comb spectroscopy of atmospheric CO2 across a 2-km open-air path. By sending a single comb through the open-air path, both molecular phase spectrum and conventional absorbance spectrum are obtained. The measured phase spectra match expected molecular lineshape models.
We discuss precision spectroscopy with a comb-based spectrometer at 3.4 μm. Our goal is to explore comb-based
spectroscopy as an alternative method for fast, highly resolved, accurate measurements of gas line shapes. The
spectrometer uses dual 1.5 μm frequency combs down converted to 3.4 μm via difference frequency generation (DFG)
with a stabilized 1 μm fiber laser. One 3.4 μm comb is transmitted through methane and heterodyned against the second,
offset comb to measure the gas absorption and dispersion. Doppler-broadened methane spectral lines are measured to
below 1 MHz uncertainty. We also discuss the higher sensitivity alternative of a comb-assisted swept-laser DFG
spectrometer.
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