GEOStar: demonstration of laser guide star adaptive optics for free space optical communications

Kurzfassung

Free Space Optical Communications (FSOC) links with satellites are limited by atmospheric turbulence in up and downlink. Adaptive Optics (AO) systems at the Optical Ground Station (OGS) can mitigate the adverse effects on the uplink by “predistorting” the transmitted laser beam such that its wavefront is corrected by the turbulence. The Point Ahead Angle (PAA) means that the downlink light is not a perfect wavefront reference for the AO system. GEOStar is a project created to demonstrate the feasibility of using a Laser Guide Star (LGS) in the direction of the uplink path to enable better predistortion of the transmitted beam. A novel setup uses a sub-pupil of the 1m diameter ESA-OGS to transmit the communications light to the satellite and a sub-pupil on the opposite side of the telescope aperture is used to launch the LGS. An LGS WFS observes the light from the LGS whilst a similar Near Infra-Red WFS observes downlink light from the satellite such that the measurements can be directly compared. A deformable mirror is used to predistort the uplink beam. The system is currently being integrated ready for shipment to Tenerife and measurements with the optical terminal TDP-1 on AlphaSat are scheduled for Q2 2024.