激光照明主动成像光场闪烁及散斑抑制分析研究

	激光照明主动成像光场闪烁及散斑抑制分析研究
	田玉珍
学位类型	博士
导师	郭劲
	2014-07
学位授予单位	中国科学院大学
学位专业	光学工程
摘要	激光照明主动成像，以激光作为发射光源，经大气湍流传输照明目标进行成像。由于照明光源的高相干性，使得其具有指向性好、发散角小以及作用距离远等特点；但同样是由于其相干性，照明光场必须经历大气湍流的微扰以及目标粗糙表面的调制，最终导致目标光场强度在空间域和时间域的随机起伏，光场均匀性被破坏，严重的影响主动成像系统性能，降低系统分辨率。为了提升系统性能，有效的抑制大气引入的湍流闪烁和粗糙目标引入的散斑现象，本论文对目标光场强度不均匀性的统计特性作了深入分析，并且提出了有效的抑制方法，在理论分析的基础上进行了仿真和实验研究，具体工作如下：（1）以Born和Rytov微扰近似理论为基础，将激光照明主动成像光场的研究分解为两个独立过程：第一，分析由发射光场经Kolmogorov谱弱湍流传输形成照明光场的湍流效应，给出了照明光场的强度均值、相干长度以及闪烁指数；第二，分析自由空间照明光场被粗糙表面目标调制后反射散斑场的统计特性，得到了回波散斑场对比度随表面粗糙度变化的关系，并且建立了对比度关于表面均方根高度和相关长度的函数表达式。（2）以照明光场和回波散斑场统计矩分析结果为基础，从引起光场强度不均匀性的本质出发，利用多光束破坏发射光场相干性及增加目标光场偏振多样性的方法，抑制照明光场湍流闪烁及回波光场散斑现象，建立了能量可调偏振态可调的多光束照明发射光场模型。以叠加场统计分布为基础，讨论了多光束和偏振多样性对照明光场湍流闪烁及回波光场散斑现象的影响，从统计学的角度解释了所提出模型的可行性。理论上证明多光束等能量发射，对光场不均匀性的抑制可以达到最佳效果，照明光场闪烁指数减小为单光束的1/N，回波光场的散斑对比度减小为单光束的1/√N；同样分析了照明光场的偏振多样性对湍流闪烁和散斑现象的影响。（3）在理论分析结果的基础上，建立了基于可变采样的split-step数值仿真光场传输模型。根据折射率起伏变量功率谱密度，以FFT相位屏为基础，形成了Kolmogorov谱大气湍流模型，利用该模型仿真计算了照明光场湍流效应；以表面随机高度函数为基础，形成了具有不同粗糙度的高斯型目标表面模型，利用该模型仿真计算了回波散斑场统计特性。在单光束照明光场和回波光场仿真的基础上，进行了多光束叠加光场的研究，并且与单光束计算结果进行分析对比，验证了多光束对湍流闪烁和散斑现象的抑制效果。（4）以理论分析结果和仿真计算结果作为依据，搭建了能量可调偏振态可调的多光束照明发射实验系统，以及大气湍流模拟实验系统，并且利用两实验系统进行了如下实验：一，单光束完全偏振照明湍流效应和散斑现象研究；二，多光束完全偏振照明湍流效应和散斑现象研究；三，多光束部分偏振照明湍流效应和散斑现象研究。实验结果与理论分析和仿真计算预期相吻合，证明等能量分光对光场不均匀性可以起到最佳抑制效果，并且利用多光束增加光场偏振多样性，将会在多光束的基础上进一步抑制回波光场散斑现象。
其他摘要	It is known as laser active imaging that targets illuminated by laser and imaged through atmospheric turbulence. Compared to ordinary sources of light, laser has far operation range, small divergence and high directionality, because of its coherence. And also due to this property, the uniformity of illumination filed, experiencing atmospheric disturbance and rough surface modulation, both of which lead to temporal and spatial intensity fluctuation, is serious undermined, finally, influence the imaging system property and reducing system resolution. Therefore, in order to improving the imaging system performance, this dissertation focuses on improving the uniformity of target filed and inhibiting the scintillation and speckle phenomenon based on the in-depth analysis of statistics properties of target field. The mainly work studied in the thesis is arranged as follows: (1) The study on target field of laser active imaging is resolved into two processes, based on Born and Rytov perturbation approximation: first, analysis the turbulence effect of illumination field after propagation through atmosphere, including the mean irradiance, coherence radius, beam wander and scintillation index; second, study the statistic properties of return speckle field, reflected by rough surface in free space propagation, deriving the influence of roughness on field uniformity, and establishing the relationship between speckle contrast and surface RMS height and correlation length. (2) Considering the nature of intensity fluctuation during the imaging process, this thesis adopts multi-beam emitting and polarization diversity of illumination field to weaken the coherence, based on the statistical property of illumination field and speckle field. According to this principle, the multi-beam emitting model, in which both energy of each emitting beam and the polarization state overlapped illumination field are adjustable, is founded, meanwhile the inhibition effects of this model, proved feasible, on turbulent scintillation and speckle phenomenon is discussed in terms of probability distribution density of the illumination field. The optimal inhibition effect, the scintillation index and speckle contrast of N beams emitting are N and √N times smaller than single beam, respectively, can be achieved by multi-beam emitting with equal energy of each one, and the influence of polarization diversity on irradiance fluctuation is also discussed. (3) Under theoretical analysis of illumination field, the simulation of field propagation is conducted based on split-step method, whose sampling interval is adjustable, and FFT phase screen, which is determined by turbulence properties, in Kolmogorov spectrum atmosphere; and the return speckle field, reflected by different roughness surface, is also simulated based on surface random height function. The comparison, between the simulation results of single beam illumination and those of multi-beam illumination, demonstrates that the multi-beam emitting does have restraining effects on scintillation and speckle, and that the optimal inhibition effect as expected by theoretical analysis is obtained. (4) According to the multi-beam emitting model, the experimental system is established, and the atmospheric turbulence simulation system with static phase plate is also established in lab. With these two system, some experiments are carried out, as follows: first, the scintillation and speckle phenomenon with linearly polarized single beam illumination; second, the scintillation and speckle phenomenon with linearly polarized multi-beam illumination; third, the scintillation and speckle phenomenon with partially polarized multi-beam illumination. All the experimental results, agreed with theoretical expectations and simulation results, demonstrate that the multi-beam emitting with equal energy of each one produces the optimal restraining effects on scintillation and speckle phenomenon, and increasing the polarization diversity, using multi-beam, of overlapped field in target field can further reduces the speckle contrast of return field, compared with linearly polarized multi-beam illumination, but useless for turbulence scintillation.
语种	中文
文献类型	学位论文
条目标识符	http://ir.ciomp.ac.cn/handle/181722/41472
专题	中科院长春光机所知识产出
推荐引用方式 GB/T 7714	田玉珍. 激光照明主动成像光场闪烁及散斑抑制分析研究[D]. 中国科学院大学,2014.

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