Abstract
Sensor-less adaptive optics (SLAO) based on stochastic parallel gradient descent (SPGD) is effective for the compensation of atmospheric turbulence in coherent free-space optical communication (CFSOC) systems. However, SPGD converges slowly and easily falls into local extremes. Therefore, we propose a novel NadamSPGD algorithm for efficient wavefront correction that combines Nesterov-accelerated adaptive moment estimation (Nadam) and SPGD. Specifically, Nesterov’s accelerated gradient momentum (NAG) and adaptive gain coefficients are integrated to conventional SPGD to accelerate its convergence speed and avoid converging to extremum points. Theoretical analysis, numerical simulations and experimental results demonstrate that NadamSPGD can increase the convergence speed by ~50% and significantly improve the robustness of parameters, and thus more efficiently suppress the negative effects of atmospheric turbulence on mixing efficiency (ME) and bit error rate (BER). Our algorithm also presents better dynamic performance under strong turbulence and high Greenwood frequency conditions, and it is more suitable for real-time SLAO systems. This study proves that the NadamSPGD algorithm is suitable for SLAO in the CFSOC system and is a viable substitute for SPGD to improve the quality of optical communications.
Highlights
The results demonstrate that NadamSPGD can significantly increase the convergence speed and robustness in Sensor-less adaptive optics (SLAO), and the negative influence of atmospheric turbulence on mixing efficiency (ME) and bit error rate (BER) can be efficiently suppressed, which is of great significance to the coherent free-space optical communication (CFSOC) system
The theoretical theoretical analysis analysis and and numerical numerical simulations simulations demonstrate demonstrate that that the the negative negative influence influence of of varying varying degrees of atmospheric turbulence on system can be supdegrees of atmospheric turbulence on ME and BER of the CFSOC system can be suppressed pressed by NadamSPGD
NadamSPGD exhibits improved dynamic capabilities as the Greenwood frequency increases, and it is more suitable for real-time SLAO systems
Summary
The free space optical communication (FSOC) system has developed rapidly in modern communications due to the advantages of security, communication speed and licensefree operation [1,2]. Summarizing, the above studies obtained promising results, most of them were aimed at specific optical problems, lack applications in the CFSOC system, and cannot be applied to achieve efficient ME and decrease the BER directly. The feasibility of these algorithms in the CFSOC system requires further verification and implementation. NadamSPGD combines Nesterov’s accelerated gradient (NAG) momentum and the adaptive gain coefficients with SPGD to improve the correction speed and robustness of SLAO without noticeably increasing the complexity of the algorithm.
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