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相干衍射成像是一种对光源相干性要求非常高的无透镜计算成像技术. 相干性包括时间相干性和空间相干性两方面, 在很多实际场合具有理想相干性的光源很难获得. 导致空间部分相干的因素主要有光源发光区域内各部分不同步的发光机制、检测器像素有限大小引起的非零宽度点扩散函数以及样品的不稳定性; 而时间部分相干则是由光源的能量扩散引起, 表现为扩展的发射光谱带宽. 空间和时间相干性退化的直接结果是衍射图样的清晰度和可见度的降低, 导致相干衍射成像中基于全相干模型构建的重构算法无法实现准确的物体波前重构. 解决方法包括改良实验装置来直接提高光束的相干性, 和通过优化相位恢复算法来补偿相干性退化的影响. 基于Wolf的相干模表示方法和Nugent等建立的部分相干衍射成像理论框架, 针对不同相干衍射成像技术中低相干性问题的各种解决办法相继被提出, 总体历经了从需要提前测出照明光的空间相干特征 (相干长度或者复相干因子) 或者时间相干特征(光谱分布) 到不需要任何已知光源相干特征的发展过程. 本文重点介绍相干衍射成像的部分相干性理论、以及部分空间和部分时间相干光衍射成像中重构算法的研究进展.Coherent diffraction imaging (CDI), a type of lensless imaging method, relies on the use of light source with high-degree coherence to compute highly resolved complex-valued objects. The coherence of light source consists of temporal coherence and spatial coherence. In practice, it is difficult to obtain a fully coherent source. Spatial decoherence can be generated in the following three scenarios: no synchronization mechanism for the whole radiation source, a finite (non-zero) point spread function of the detector, and the sample variation within exposure time. Partial temporal coherence means that the beam is not quasi-monochromatic, behaving as the energy spread of the illumination. The consequence of reduced degree of temporal and/or spatial coherence in CDI is the decrease of visibility in the measured diffraction intensity. A fundamental assumption of CDI is the full temporal and spatial coherence, and even a relatively small deviation from full coherence can prevent the phase retrieval algorithm from converging accurately. It is necessary to break the barrier of limited coherence by improving the experimental setups directly or optimizing the phase retrieval algorithms to mitigate decoherence. Based on the Wolf’s model of coherence-mode of light and the framework of CDI using partially coherent light proposed by Nugent et al., various methods have been proposed to solve the problems induced by low coherence. Those methods generally experience a similar development process, that is, from the requirement for measuring the spatial (coherent length or complex coherent factor) or temporal (spectrum distribution) coherence properties to without the need for such priori knowledge. Here in this work, the principles of partial coherent CDI, and the major progress of CDI with partial spatial- and temporal-coherent light are reviewed.
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方法 是否需要光谱信息 可适用的波的类型 最大带宽 实验结果 中心波长/nm 空间分辨率 Multiwavelength CDI[100] Yes Discrete Several harmonics (35 ± 1) (165 ± 5) nm Two-step CDI[101] Yes Discrete Several harmonics 810 1 μm PolyCDI[64] Yes Continuous 11% 0.888 (150 ± 5) nm Numerical monochromatization[105] Yes Continuous >10% 710 2.5 μm Two-pulse imaging[113] No Continuous No limitation 47 6.7 μm -
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