Pre-distortion adaptive optics: experimental results from bi-directional tracking links between DLR’s optical ground station and Alphasat’s TDP-1 terminal

Free-Space Optical Communication (FSOC) links between Earth-based Optical Ground Stations (OGSs) and satellites offer immense potential to securely and efficiently exchange vast amounts of information with worldwide coverage. However, atmospheric turbulence inhibits this potential by distorting laser beams, as they propagate through the atmosphere. Adaptive Optics (AO) systems are typically employed at the OGS to correct for these adverse effects and can increase the efficiency of laser light being coupled into an optical fibre for a downlink laser beam. Concurrently, the same AO system can be used to increase the coupling of laser light into an orbiting satellite by pre-distorting the uplink laser beam. In such a scenario, the downlink laser beam is used to measure the distortions that are applied by the atmosphere, and the conjugate of these distortions can then be applied to the uplink laser beam. The atmosphere then corrects the pre-distorted beam, resulting in a flat wavefront at the top of the atmosphere, as well as stable and efficient coupling of light into the satellite. This work showcases the successful experimental ground-to-satellite links in the spring of 2023 between DLR's recently commissioned OGS and TESAT's laser communications terminal (LCT-135)—i.e., part of the Technology Demonstration Payload No. 1 (TDP-1) on the geostationary satellite, Alphasat. Pre-distortion was successfully applied via an AO system testbed within the OGS, which resulted in extremely power efficient bi-directional tracking links with Alphasat. The findings of this work show that the application of pre-distortion AO not only improves the coupling of laser light at the satellite, but also reduces the scintillation experienced at the satellite, thus improving the robustness of the link.