The objective of this activity is to mitigate the bottleneck in RF communication bandwidth applicable to telecommunication satellites. The activity shall develop and test the prototype of an optical feeder link system that enables extremely high data rate uplinks to future telecommunication satellites. The prototype shall consist of a data transmission system and an adaptive optical system for atmospheric distortion pre-compensation.

The use of optical (laser) feeder-links for next generation telecommunication satellites is receiving strong interest from commercial telecommunication providers because it will soon not be possible to support the projected growth in bandwidth using radio frequency bands (e.g. Ka-band). Moving to higher radio frequencies such as the Q/V or W-bands does not dramatically improve available bandwidth, while it increases atmospheric attenuation to a level comparable with the laser communication bands. Cloud coverage can be mitigated by site diversity, namely by using multiple (e.g. 10) feeder-link gateway stations in mutually uncorrelated locations (they must be at least 500 km apart). Studies on the optimum selection of European, Mediterranean and North African locations have already been performed. A second issue for optical feeder-links is atmospheric turbulence, namely the effect that makes stars twinkle at night also distorts laser beams. The light received by the GEO satellite from a ground station undergoes strong intensity fluctuations (fades and surges) that do not support high data rates. This effect can be compensated by adaptive optics (AO), where the beam distortions that ultimately cause intensity fluctuations are measured on ground (using a probe beam from space) and are inversely applied on the uplink beam. This pre-distortion on the uplink beam is then corrected by atmospheric turbulence rendering an undistorted beam reaching the satellite, provided the speed of the AO system is higher than the dynamics of the atmosphere (>300 Hz). Currently implemented AO systems are aimed at correcting the wave-front of starlight (for astronomy) and for optical communication ground receivers (e.g. in the EDRS transportable adaptive optical ground station developed by DLR). This activity will address the second issue described above by developing the prototype of an optical feeder link station and test it in an atmospherically relevant environment, namely a 14 km communication link between ESAs optical ground station (OGS) and a test station close to the top of mount Teide. Such a link characterises very well the relevant extent of effective atmospheric turbulence. The prototype will demonstrate the feasibility to perform pre-distortion on a feeder-link beam and its correction by atmospheric turbulence. The quality of the service will be measured in the OGS on the return beam by measuring the intensity fluctuations (with and without adaptive optics) and the uncorrected bit error-rate of a single communication channel. The system will be designed to ultimately achieve extremely high data rates by dense wavelength division multiplexing (DWDM); a technology used in terrestrial fibre communication networks.