High-performance adaptive optics control in strong turbulence scenarios for free space optical communication

Kurzfassung

Current satellite communication is limited by the highly saturated radio frequency spectrum. Free Space Optical Communication (FSOC) enables higher-throughput data links to satellites. However, its performance is limited by atmospheric turbulence, which distorts the laser beam and induces scintillation at the receiver, reducing data rates. Adaptive optics (AO), widely used in astronomy to mitigate turbulence effects, employs Deformable Mirrors (DMs) for real-time correction using measurements from Wavefront Sensors (WFSs) and a control algorithm. AO has been proposed for FSOC to enhance link robustness, but AO loops can become unstable under rapidly varying turbulence typical of daytime operations. Optimal control techniques based on stochastic models can anticipate turbulence behaviour and improve FSOC performance. This research aims to design, simulate, and implement high-performance AO control strategies, starting from Linear Quadratic Gaussian (LQG) regulators successfully tested in astronomy. Models and control schemes were adapted for FSOC, with emphasis on low-Earth orbit (LEO) links. Diffraction effects were included for accurate simulation. A zonal-based approach was chosen, naturally encoding frozen flow as wind translation. Theoretical covariances from the von Kármán structure function were used to generate auto-regressive turbulence models. A predictive control library was developed in Python and integrated into DLR’s ELSiE simulator. The phase grid was oversampled by a factor of two using weighted averaging across subaperture edges. A horizontal testbed link was used in preliminary simulations comparing tuned LQG regulators with a tuned integrator. Under identical conditions, coupling efficiency dropped by 56.4% for the integrator and 69.8% for the LQG controller when scintillation was included. These results provide the first quantitative evidence of the critical role of scintillation in FSOC simulations. Atmospheric turbulence profiles for a worst-case daytime LEO link over DLR’s Oberpfaffenhofen Optical Ground Station (OGS-OP) were used to simulate LQG regulators at three elevations. Oversampling phase points by a factor of two improved performance by 34.5% compared to a simplified Fried geometry grid. The robustness of LQG regulators to elevation model mismatch was also evaluated. Results show high resilience to model inaccuracies, consistently outperforming the integrator. The best LQG regulator significantly reduced the bit error rate (BER), enabling higher throughput with less post-processing, and improved the secret key rate (SKR) by up to 70% under low-elevation, strong-turbulence, and scintillation conditions. Scintillation effects were further assessed for LEO links by comparing geometric and angular spectrum propagation at 2 and 5 kHz. Physical propagation introduced more fade events, degrading overall performance, yet the LQG regulator at 2 kHz outperformed the integrator at 5 kHz, effectively mitigating intensity fluctuations. Finally, an initial attempt to mitigate scintillation using a degraded-measurement replacement strategy was conducted. Although inconclusive, the results indicate that dynamic configurations warrant further investigation. Overall, this work demonstrates that FSOC-AO systems using predictive LQG control can substantially enhance link stability and throughput, supporting Europe’s secure quantum communication infrastructure and enabling higher data rates for next-generation missions such as HydRON and EAGLE-1.