Successful first optical feeder link demonstration between a ground station and a GEO satellite applying adaptive optics pre-compensation ("Alpha-Up")

Targeting the optical feeder link application for geostationary Earth orbit (GEO), a bi-directional Adaptive Optics (AO) system was installed at the Coude bench of the ESA Optical Ground Station (Observatorio del Teide, Tenerife, Spain), and first uplink experiments were conducted with the TDP1 laser communication terminal (aboard Alphasat, 25 E). The AO system features a common downlink/uplink aperture, and uses the downlink beams wavefront, to pre-compensate the uplink beams wavefront. Key elements are a highly reflective 69-element deformable mirror in the common path, and a narrowband filter in front of the wavefront sensor, to sufficiently reject back-scattered uplink light. Wavefront pre-compensation can be confined to tip and tilt only, or extended to approx. 30 spatial modes. To evaluate the uplink performance of the system, one sensor of the TDP1 terminal recorded the temporal variation of the optical power received. Both, tip/tilt, and full pre-compensation of the transmitted wavefront could be demonstrated during several links. During links with strong boundary-layer turbulence, performance improvement of full pre-compensation over tip/tilt pre-compensation was demonstrated successfully: A factor-1.4 to 1.7 increase in terms of average power at the target, a much lower scintillation index (0.2 vs. 0.45), and a significant improvement in terms of fading statistics (+10 dB channel transmission at the 99% confidence level). During links with weak boundary-layer turbulence, good uplink performance was observed for both pre-compensation schemes. This can be expected, as there are no significant high-order wavefront aberrations that would need correction. Looking closer at the statistical distribution of the uplink irradiance, the variation between links with weak boundary-layer turbulence is attributed to the varying turbulence strength in the tropopause (anisoplanatism).