In-flight enhancement of the optical frequency accuracy of an integral-path differential absorption lidar thanks to a rugged, airborne self-referenced frequency comb

Airborne and spaceborne integral-path differential absorption (IPDA) lidar has the potential to deliver column measurements of the major greenhouse gases influenced by human activity with the high accuracy that is required to significantly reduce the uncertainties in our estimations of surface fluxes of methane and carbon dioxide by inverse modelling. A prerequisite is the highly accurate knowledge of the emitted wavelengths, especially for carbon dioxide in the 1.6-µm region, where a long-term optical frequency knowledge accuracy of the online channel down to a few tens of kHz is required. Deutsches Zentrum für Luft- und Raumfahrt's airborne IPDA lidar for simultaneous measurements of carbon dioxide at 1.57 µm and methane at 1.64 µm, CHARM-F, uses a specifically developed frequency reference unit based on optimized wavelength modulation spectroscopy which can reach the required accuracy in a stabilized laboratory environment, but whose in-flight performance in the more challenging aircraft environment could not be independently validated. In the frame of the Carbon Dioxide and Methane Mission (CoMet) field campaigns in 2018 and 2022, CoMet 1.0 and CoMet 2.0 Arctic, respectively, a cooperation with Menlo Systems GmbH made it possible to bring a prototype of a new generation of portable and rugged self-referenced frequency combs (SRFCs) on board the German research aircraft HALO. This airborne frequency comb served as an independent frequency reference to characterize the performance of the carbon dioxide channel of CHARM-F's frequency reference system in flight. We report here on the frequency stability measurements carried out during the CoMet 2.0 Arctic campaign and demonstrate the potential of such portable SRFCs as next-generation frequency references for atmospheric lidars.