Real-time adaptive optics compensation for turbulence-impaired OAM-multiplexed optical communications

Orbital angular momentum (OAM) multiplexing provides additional spatial degrees of freedom for free-space optical communication (FSOC), but atmospheric turbulence induces wavefront distortion, desired-mode power loss, and inter-modal coupling, thereby degrading the received signal quality and bit error rate (BER). Wavefront-sensorless adaptive optics (AO) based on stochastic parallel gradient descent (SPGD) enables model-free compensation for turbulence-induced distortions. However, conventional SPGD often converges slowly and exhibits steady-state oscillations in high-dimensional control, prolonging insufficient wavefront correction and limiting the effectiveness of wavefront-distortion compensation. In this paper, we design a modified SPGD algorithm, called adaptive-decay and cosine-annealed Adam-SPGD (ACA-SPGD), to accelerate initial convergence and suppress steady-state fluctuations. ACA-SPGD dynamically adjusts the first-order momentum decay factor and employs a cosine-annealed step-size schedule to balance rapid optimization and stable convergence. To enable low-latency closed-loop operation and real-time AO compensation, we further implement a bandwidth-aware parallel pipelined architecture (BAPPA) with dual-channel streaming evaluation on an FPGA to execute ACA-SPGD and generate the 137-element DM control-voltage vector. Both the numerical and experimental results show that the proposed ACA-SPGD algorithm exhibits faster and more stable convergence performance than conventional SPGD and Adam-SPGD. The ACA-SPGD-based AO scheme effectively compensates for turbulence-induced wavefront distortions, thereby improving the communication performance of the OAM-FSOC system. For the present implementations, the runtime ratio (MATLAB/FPGA) is approximately 1283 ×, while the FPGA computational kernel maintains a computational error on the order of 10^-5 relative to the MATLAB reference implementation.