Optical feeder link demonstrations between the ESA Optical Ground Station and Alphasat

Two adaptive optics systems, targeting the optical GEO feeder link application, have been tested at the ESA Optical Ground Station (OGS). Concerning the downlink, the so-called Cassegrain Adaptive Receive Optics (CARO) instrument attaches to the Cassegrain focus of the 1-meter telescope, and couples the light, received from the satellite, into a polarization-maintaining single-mode fibre with high efficiency. During a test campaign in April 2023, the instrument’s performance was validated, and 0.5 to 1.4 Microwatts were coupled into the fibre (average over each 17-minute link). Concerning the uplink, a bi-directional adaptive optics system (Alpha-Up) is installed at the Coudé bench of the ESA OGS. It features a common downlink/uplink aperture, and uses the downlink beam’s wavefront, to pre-compensate the uplink beam’s wavefront (using a high-power capable deformable mirror). In contrast to other on-going activities, no laser guide star is used. To evaluate the uplink performance of the system, one sensor of the laser communication terminal in space recorded the temporal variation of the optical power received. Both, tip/tilt, and full pre-compensation of the transmitted wavefront were demonstrated during several links in December 2022. During links with strong boundary-layer turbulence, full pre-compensation resulted in a significant improvement in terms of fading statistics (+10 dB channel transmission at the 99% confidence level).