Compact self-referenced frequency comb for airborne lidar applications

Building on its experience with CHARM-F, the airborne demonstrator for the upcoming MERLIN mission, DLR’s Institute for Atmospheric Physics is developing a new generation of airborne differential absorption lidars for atmospheric trace gas measurements. Such measurements typically call for a very stringent long-term stability of the emitted wavelengths down to the 10-10 level, which can hardly be reached with conventional methods making use of broadened absorption lines in gas cells. Furthermore, such conventional methods can only cover narrow spectral regions in the vicinity of sufficiently strong absorption lines of the trace gas. Self-referenced optical frequency combs, used as spectral references, provide an elegant solution to these two limitations thanks to their ability to easily achieve order of magnitudes better than 10-10 long-term stability when locked to widespread and affordable RF references such as GPS-disciplined oscillators, and their ability to access any wavelength within a spectral region containing absorption lines of the trace gas of interest, provided that the comb spectrum can be enhanced in the corresponding region by optical amplification. Up to a few years ago, such devices were typically bulky and sensitive laboratory equipment but recent developments have led to the commercial availability of compact, rugged devices that can potentially be integrated into airborne lidar systems. The subject of this tender is a standalone device comprising a compact self-referenced frequency comb and the electronics and optical modules necessary to transfer the long-term stability of the comb to external continuous-wave lasers in two specific spectral regions containing absorption lines of carbon dioxide and methane.