The absolute frequency reference unit for the spaceborne methane-sensing lidar mission MERLIN

Lidar systems have become an important technology in a variety of industrial and scientific applications. For terrestrial applications, such systems have been developed over the last decade and are commercially available. In space, this technology does not have a comparable maturity level and is still far from being a standard technology. In this paper, we present the operating principle, the design, and performance of the spaceborne absolute frequency reference used in the instrument of the French-German methane remote-sensing lidar mission, MERLIN. The MERLIN instrument operates on a methane absorption feature at 1645.55 nm. To achieve the accuracy goal for the column-integrated methane concentration of 3 ppb, absolute frequency accuracy levels in the low MHz range are required. The absolute frequency reference unit provides the stabilized seed lasers for the high-power laser sources at the required wavelengths and measures the absolute frequency of the outgoing laser pulses by means of a wavemeter. The focus of this paper is on the implementation of the absolute frequency referencing and the obtained frequency stability and accuracy.