Optical feeder-link between ESA’s optical ground station and Alphasat

This paper describes the technical challenges and the experimental set-up for an optical feeder-link demonstration between ESA’s optical ground station in Tenerife, Spain, and the Alphasat satellite in geostationary orbit (25o East). In the absence of clouds, reliable optical feeder-uplinks require pre-distortion of the transmit beam to minimize the influence of the continuously changing atmospheric turbulence effect. Without beam pre-distortion, the quality of service (minimum impact of scintillations and outages) required for commercially viable feeder-links is hardly achievable. Power efficient feeder-links require the wave-front distortions of an optical downlink beam from a satellite to be inversely applied onto the optical feeder-uplink beam to the satellite with certain adaptations due to the difference in the uplink and downlink paths. The underlying assumption is that pre-distortion and atmospheric turbulence cancel each other out and the main goal of the planned experiment is to determine to which extent this is true. The closed-loop bandwidth of pre-distortions has to be higher than the atmospheric turbulence fluctuations and optical beam reception and transmission should use the same telescope aperture. The former requires a fast controller and the later poses stringent requirement on the stray-light performance of the set-up such that the powerful transmit beam does not blind the receiver. Avoidance of receiver blinding is a particularly difficult task when doing feeder-uplink tests with the coherent laser communication terminal on board the Alphasat satellite as will be explained. The paper will describe the design of the feeder-uplink system that will be installed in the OGS as well as initial measurements that have been performed to evaluate feasibility.