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Abstract

Free-space optical (laser-based) communication (FSOC) links have become an attractive solution to the increasing demand for efficient, secure, and high-speed global data transfer. Of particular interest is these systems’ ability to send data from optical ground stations to orbiting satellites via bidirectional FSOC links. However, atmospheric turbulence-induced phase and intensity fluctuations can severely degrade the signal quality for such FSOC links. This thesis addresses this critical challenge by developing and experimentally validating a pre-distortion adaptive optics (AO) system that both corrects turbulence-induced distortion on the downlink laser beam and applies pre-distortion onto the uplink laser beam (to have it arrive at the satellite with minimal turbulence-induced distortion). A comprehensive literature review is given on laser communications, atmospheric turbulence, and adaptive optics, followed by results from Monte Carlo simulations of a pre-distortion AO system at the German Aerospace Center’s (DLR’s) optical ground station near Munich, Germany. The simulation results show that the pre-distortion AO system can mitigate the total uplink losses by 10 dB, decrease the link variability by an order of magnitude, and improve the laser power thresholds required to maintain 99.9% link uptime by 20–40 dB. An experimental pre-distortion AO system was constructed to test such FSOC links with a geostationary satellite (Alphasat) some 38,000 km away. The experiments were the first demonstration of a pre-distortion AO system in bidirectional coherent (homodyne) FSOC tracking links, as the optical phase-locked loop was closed at Alphasat, and their results showed a consistent 3–5 dB reduction in total uplink losses, a decrease in link variability by a factor of two to four, and improvements of 10–20 dB in the link uptime thresholds. Moreover, the optical power needed for such links was only 260 mW. This is, to the author’s best knowledge, a world record for the lowest optical power used to maintain an FSOC tracking link with a geostationary satellite. The insights gained from this work are now guiding tests of a laser-guide-star-assisted pre-distortion AO system installed within the European Space Agency’s optical ground station.