基于非局域调制不稳定性的水下偏振光自适应成像方法研究

Suppressing the scattering noise for the enhancement of weak optical image is the key of high-resolution imaging of turbid underwater targets, and is also the application bottleneck of the noise-filtering imaging technologies, such as the range gating. Based on the previous researches of nonlinear enhancement of weak signal, our project intends to investigate a self-adaptive imaging method by combining the underwater polarized light and non-local modulation instability effect. In detail, the non-local modulation instability of weak light, which is optimized by adjusting the polarization orientation of underwater polarized beam, will realize the spontaneous and order redistribution of scattered noise energy induced by weak signal, and enhance the signal-to-noise ratio of imaging. Firstly, based on the theory of coherent-polarization correlation, we will research the degradation mechanism of spatial coherence for underwater polarized beams, to propose the method of controlling the spatial coherence of scattering light by adjusting the polarization orientation. Then, we will investigate the condition of generating the non-local modulation instability in nonlinear medium, and reveal the mechanisms of spontaneous redistribution of noise energy and signal growth. On the basis of the results above, an experiment of self-adaptive imaging of polarized light in turbid water will be carried out, and the effective stadia will be increased by optimizing the parameters, such as polarization orientation and coherence length,. The project which focuses on the detection of turbid underwater targets, will innovatively construct a nonlinear self-adaptive method of improving the stadia, clarity and stability of imaging, and provide a theoretical basis and technical support for high-resolution imaging of underwater targets.

抑制散射噪声实现弱光图像增强，是浑浊水下目标高分辨成像的关键，也是阻碍距离选通等滤噪成像技术应用的瓶颈。本项目在前期开展含噪弱光信号非线性增强的工作基础上，拟研究一种水下偏振光和非局域调制不稳定性效应相结合的自适应成像方法，调节水下偏振成像光束的偏振取向优化散射光非局域调制不稳定性过程，实现弱光信号诱导的噪声能量自发有序再分布，从而增强成像信噪比。首先，基于相干偏振关联理论，研究水下偏振成像光束空间相干性退化机制，提出调节偏振取向调控散射光空间相干性分布的方法。然后，探索非线性介质中产生非局域调制不稳定性条件，揭示噪声能量自发再分布和信号增益机理。在研究基础上，开展水下偏振光自适应成像的实验验证，优化偏振取向和相干长度等参数实现成像视距有效提升。项目围绕浑浊水下目标探测需求，创新地构建一种实现成像视距、清晰度和稳定性提升的非线性自适应方法，将为水下目标高分辨成像提供理论依据和技术支持。

水下高分辨目标成像技术在雷达监测、搜索救援、测绘勘探等领域有着重要意义和应用需求，也是目前光信息处理领域的前沿热点。然而，蓝绿激光水下成像容易受到水分子、溶解质和悬浮颗粒等吸收和散射引起成像视距和清晰度急剧衰减，这也是线性滤波和距离选通等传统技术难以突破的瓶颈。因此，研究新型有效的高信噪比浑浊水下目标成像方法具有重要的学术意义和实用价值。项目系统地研究了一种基于非局域调制不稳定性的水下散射成像非线性重建方法。结合水下后向散射光的保偏特性和矢量光场偏振相干关联理论，提出了利用偏振调制实现图像退相干抑制的理论，推导了偏振光散射图像强度涨落和互相关函数，建立了基于非局域调制不稳定性效应的微弱信号增益模型，并实验完成了浑浊水质180度后向散射图像的有效恢复。以灰度平均梯度表征的图像清晰度平均提升2倍，图像互相关度提升约3.56倍，对应有效成像视距提升约2.5AL。结果表明，基于非线性光学效应能够有效实现成像视距、清晰度和稳定性的提升。该方法揭示了噪声能量的可利用性，避免了传统滤波方法造成的信号能量损失，并且对具有统计特性的噪声均具有适用性。围绕以上主要研究内容，项目组还研究了一种由纳米结构阵列组成的超透镜聚焦结构，可实现矢量光场紧密同轴聚焦，以及波长量级的散射光场相干长度调节，理论与实验结果吻合较好。此外，项目组还基于光折变聚合物取向增强效应引起的非线性模式耦合，在相空间实现了高频波数向傍轴区域的可控空间转移，结合相位反演在空域有效恢复了图像高频信息，有望应用于突破衍射限制的远场超分辨成像。

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