Near-Field Turbulence Impact on Far-field Scintillation of Defocused Laser Beacons

Abstract

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.