Characterisation of the Turbulent Atmosphere for Free-Space Optical Communications

Abstract

Atmospheric optical degradation is a key inhibitor to the performance of a Free-Space Optical Communications (FSOC) link. The turbulent mixing of air creates a non-uniform refractive index within the atmosphere. Light propagating through this turbulent medium is subject to various propagation effects, including phase distortions, intensity fluctuations, beam wander, and speckling, all of which contribute to signal degradation at the receiver. The extent of this degradation is determined by the strength of the atmospheric turbulence encountered that the light interacts with. While the characterisation of such turbulent environments has been extensively studied within the field of astronomical instrumentation—typically in remote, high-altitude locations—these sites are not always suitable for communication links. Therefore, it is necessary to characterise turbulence in less optimal environments—this primary focus of this thesis.

This research investigates optical turbulence in diverse environments, including urban landscapes and varying zenith angles. The first part of the thesis examines the impact of changing zenith angles on the turbulence parameters and , crucial for ground-to-satellite communication links between transiting LEO satellites or the maintenance of a link between GEO-satellites and ground stations at high latitude. It was found that the scintillation index remains consistent with weak and strong fluctuation theories until zenith angles exceed approximately 80, where weak fluctuations increase asymptotically and strong fluctuations saturate, deviating significantly from observed values that reduce to near 0.3. Similarly, the Fried parameter aligns well with the theory up to a zenith angle of approximately 70. When approaching larger zenith angles, both Kasten and Young's theories and secant scaling overestimate the Fried parameter, tending to 0.9 cm, whereas true measurements tend towards 2.5 cm, indicating weaker turbulence than predicted.

The second part of the thesis presents the first measurements of atmospheric optical turbulence in an urban environment, specifically over London's financial district, where the Fried parameter, Rytov variance, and scintillation index were found to have mean values of 4.5 cm, 0.2, and 0.08, respectively. Vertical turbulence distributions were developed based on these measurements, which were then used to simulate realistic urban FSOC conditions. The study demonstrated that adding tip/tilt-only correction and full adaptive optics (AO) improved coupling by 8.86 dB and 11.6 dB, respectively, compared to the uncorrected case. For communication links established at zenith angles of 60 or higher, full AO was recommended due to the significant aberrations at these angles, with an improvement of 4.2 dB observed between tip/tilt-only and full AO systems.

Finally, a study was conducted at the Universitat Politècnica de Catalunya in Barcelona, collecting 40 hours of continuous optical turbulence data in an urban setting. The findings revealed significant daily fluctuations in turbulence, with mean values for Fried parameter, Rytov variance, isoplanatic angle, and coherence time of 6.2 cm, 0.36, 1.1 arcsec, and 3.6 ms, respectively. The study observed a decrease in the turbulence strength, reflected in each turbulence parameter, during the transition through sunset. The performance of a satellite-to-ground optical link is simulated investigating the impact of increasing beam waist in the uplink and increased receiver size on the downlink. Each of these scenarios investigate different adaptive optics configurations to compensate for the turbulence. It was found that in the strongest observed turbulence conditions, full AO offered at most a 7 dB improvement compared to no compensation on the downlink. Additionally, for uplink precompensation, full LGS-AO offers at most a 14 dB reduction in signal losses.