Free-space laser communications promise high-data rates for ground-to-space uplinks, but precise beam acquisition remains a critical challenge due to platform attitude errors and atmospheric turbulence. Conventional collimated beacon beams are often constrained by safety regulations, leading to the usage of defocused beams, which also promise efficient acquisition times. This Master thesis conducted at Airbus Netherlands B.V. investigates the near-field turbulence impact of defocused beams on far-field scintillation. The study comprises
theoretical modelling, wavefield propagation simulations, and experimental measurements to examine how varying the transmitter aperture radius affects scintillation relative to collimated beams under different turbulence conditions. The simulations and the theoretical model show that far-field scintillation grows as the beam becomes more defocused with increasing transmitter sizes for different turbulence conditions. Results indicate that while moderate defocusing doesn’t affect propagation performance, excessive defocusing in strong turbulence conditions amplifies irradiance fluctuations, potentially degrading acquisition efficiency from the computed amplitude statistics. Preliminary Experimental breadboard findings confirm the general trend but highlight discrepancies in absolute scintillation values due to laboratory downscaling constraints.

Maintaining a high speed, secure and robust connection to the world is of paramount importance
in modern society. This proved especially true during a recent volcanic eruption in the small island nation of Tonga, where the only optical fibre line to the country was severed due to the cataclysm, completely disconnecting the island from the rest of the world. The lack of communication made the disaster relief to the eruption more complex. The Emergency Communications Node (ECN) can be deployed within 48 hours to enable alternative means of communication that replace damaged communication lines...