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中国物理学会期刊

激光长程传输中的大气湍流非共轭智能校正方法

Non-Conjugate Intelligent Correction Method for Atmospheric Turbulence in Long-Range Laser Propagation

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  • 自适应光学系统广泛应用于天文观测、自由空间光通信等领域,相位共轭技术是其核心校正原理之一,通过变形镜加载探测像差的共轭相位实现相位畸变补偿。然而,在激光水平传输场景中,受大气湍流非共轭特性、湍流与衍射效应耦合影响,探测像差伴随相位奇点,导致传统共轭校正方法性能显著退化甚至失效。为此,本文提出基于机器学习的智能校正方法,旨在寻找优于理想相位共轭的最优补偿波面,提升激光大气水平传输湍流校正效果。首先,本文系统剖析随机并行梯度下降(SPGD)算法的校正机制,针对其局部最优、收敛缓慢等缺陷,提出融合自适应增益调整、动量机制与提前终止策略的改进方案。仿真结果表明,改进型SPGD算法在强湍流环境下可有效提升激光能量集中度。进一步地,探索了强化学习在相位优化中的应用,初步结果验证其具备挖掘更优校正相位的潜力。本研究证实了水平传输中存在优于理想校正相位的波面,为非共轭湍流校正提供了新算法思路与仿真支撑。

     

    Adaptive optics (AO) systems are indispensable for mitigating atmospheric turbulence-induced wavefront distortions in applications such as astronomical observation and free-space optical communication. The phase conjugation principle, which applies the conjugate of the measured aberration via a deformable mirror, serves as the foundational correction mechanism. However, in horizontal long-range laser propagation, the distributed and intense nature of atmospheric turbulence, coupled with diffraction effects, gives rise to strong non-conjugate characteristics. Under these conditions, the measured wavefront often contains phase singularities, causing conventional phase conjugation to degrade severely or even exacerbate beam distortion when the Fresnel number is small, thus fundamentally limiting its effectiveness. To transcend this bottleneck, this paper proposes a machine learning-driven intelligent correction framework designed to identify an optimal compensation wavefront that outperforms the ideal phase-conjugate solution. Firstly, we systematically deconstruct the standard stochastic parallel gradient descent (SPGD) algorithm, identifying its inherent limitations, comprising susceptibility to local optima, slow convergence, and sensitivity to initial conditions, in the context of non-convex, multi-peaked optimization landscapes induced by strong turbulence. To address these, we introduce a suite of enhancements: high-order Zernike polynomial representation (up to 65 orders) to expand the solution space, a Kolmogorov-spectrum-based exponential decay perturbation model to align exploration with physical priors, integration of the Adam optimizer for adaptive moment estimation and learning rate adjustment, an optimal parameter rollback mechanism for ensuring monotonic performance non-degradation, and an early stopping strategy with adaptive learning rate decay for computational efficiency. Our comprehensive simulations, based on a multi-phase screen wave-optics propagation model on the EasyLaser platform, systematically evaluate the improved SPGD algorithm across varying turbulence strengths and Fresnel numbers. The results demonstrate that the improved SPGD achieves a performance enhancement factor of over 20% compared to ideal phase conjugation at a Fresnel number of 1.2 in moderate-to-strong turbulence, conclusively verifying the existence of superior non-conjugate correction wavefronts. However, its performance improvement margin narrows significantly in weaker diffraction regimes, exposing the inherent local-search limitations of the gradient-based SPGD framework. To achieve more robust global optimization, we further pioneer the application of a deep reinforcement learning (RL) approach based on the Soft Actor-Critic (SAC) algorithm. By formulating the phase correction task as a Markov decision process and integrating convolutional neural network-based policy and value networks, the RL agent learns a direct mapping from high-dimensional wavefront and far-field intensity images to optimal Zernike correction coefficients. The maximum entropy framework of SAC enables systematic global exploration, effectively escaping local optima that trap gradient-based methods. Remarkably, under strong turbulence conditions where both ideal phase conjugation and the improved SPGD algorithm fail (e.g., demonstrating a performance factor of only 1.04 by SPGD), the RL-driven controller achieves a performance enhancement factor of 4.53, restoring a near-diffraction-limited far-field spot. This study establishes a transformative paradigm for turbulence correction by fundamentally moving beyond the phase conjugation principle, confirming the practical feasibility and profound potential of reinforcement learning for complex, high-dimensional, continuous wavefront control in adaptive optics.

     

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