大视场超分辨率同轴数字全息成像方法研究

With the fast development of charge-coupled device (CCD) and computer science, holograms can be recorded and reconstructed numerically without photographic plates. However, restricted by the finite size of CCD, the resolution of the reconstructed image in digital holography is usually lower than that in traditional optical holography and the reconstrcuted area is also quite limited. We propose a wide field-of-view super-resolution in-line digital holographic microscopy, in which a super resolution (SR) in-line hologram is recovered from a series of low resolution (LR) sub-pixel shifted holograms, and reconstructed by a hybrid phase retrieval algorithm using premium initial guess. Firstly, in order to record the LR holograms, an experimental setup based on lens-free in-line digital holographic microscope is established. The coherent noise in the reconstructed images is greatly suppressed via partially-coherent point illumination source. Secondly, these LR holograms is automatic registrated using scale invariant feature transform (SIFT) algorithm and recovered as a SR hologram. Thirdly, to reduce computional load, the SR hologram is divided into sub-field holograms. The complex amplitude of the object wavefront in every sub-field is retrieved using a hybrid phase retrieval algorithm, in which a premium initial guess in the object plane is calculated via angular spectrum back-propagation. A full-field phase distribuction in the object plane is recoverd by subtraction. This research would realize non-contract phase contrast imaging for micron structure samples at sub-micron horizontal resolution, nanometer level vertical resolution and centimeter level field-of-view. Giving the simplicity of its recording architeture, the proposed method can be used as a compact instrument for field microscopic observation, as well as three-dimensional imaging and measurement in other regions of the electromagnetic spectrum like THz, X-ray, and γ- ray.

针对探测器性能和计算能力对数字全息的制约，本项目基于亚像素微位移方法构建大视场超分辨率的同轴数字全息成像系统，利用低分辨率全息图序列重建出高分辨率全息图，通过相位复原和相位拼接实现大视场全息图的快速再现。首先，研究能够实现全息图亚像素微位移的光路结构，通过部分相干光扩展点光源的横向移动获取高质量全息图序列；在此基础上，通过特征匹配算法对序列自动配准，实现全息图的超分辨率空域重建；随后，通过基于初始值优化的相位快速复原算法，快速再现得到子视场物光波定量复振幅分布；最后，利用相减算法消除坐标差异，完成超大尺寸相位再现像拼接，实现亚微米级横向分辨率、纳米级相位测量精度、厘米级视场范围，快速再现的无损定量相衬成像。本项目能为微纳元件的研发、制造与评估提供一种大视场高分辨率检测方法，还能够作为便携式仪器实现现场原位显微观测，并应用在太赫兹波、X射线、γ射线等其它重要波段的三维成像和检测领域。

本项目对大视场高分辨率的同轴数字全息成像技术开展研究，研究进展如下：.首先，提出了多种同轴数字全息再现算法，大幅度提高计算效率。从约束条件、多物距、多波长、初始值优化四个方面对相位复原迭代算法的适用条件和计算效率进行了定量分析比较，提出了一种基于初始值优化的双物距迭代算法和一种多波长同轴全息再现算法。提出了一种基于数字全息的纯相位型空间光调制器相位调制特性测试方法和对应的同轴全息成像方法，通过在空间光调制器模拟不同距离下的相位传播因子，利用上述双物距算法快速迭代获得物面复振幅分布。.其次，提出了两种超分辨率数字全息成像方法。前者以原始全息图的幅值信息作为记录面的约束条件，从基于物光波低频分量所对应的全息条纹，外推得到高频分量对应的全息条纹，获得突破数值孔径的分辨率。后者是一种基于空间光调制器的动态光栅超分辨数字全息成像方法，可以动态控制衍射光栅的周期，自动调整成像系统的视场范围。.随后，提出了面向同轴数字全息的自聚焦算法和相位解包裹算法。对多种自聚焦算法进行定量分析比较，所提出算法可以准确获取同轴数字全息的再现距离。后者是一种多物距相位解包裹算法，通过不同记录面之间的往复迭代，有效获取物光波的连续相位变化。.最后，将同轴数字全息应用于太赫兹波段，并首次获得了生物样品的成像结果。传统的太赫兹成像光路受限于太赫兹波传输损耗和很强的衍射效应，而同轴数字全息由于物参光共路、结构紧凑等特点，尤其适用于太赫兹成像，可发展为一种可穿透非透明介质的、实时全视场、高分辨率动态获取定量振幅与相位信息的成像方法。与中物院激光聚变研究中心、第三军医大学和苏黎世大学等单位合作，开发了多套太赫兹同轴数字全息成像系统和一系列再现算法。分别提出基于探测器亚像素微位移和合成孔径的方法提高成像分辨率，在人体原发性肝癌组织切片的太赫兹相衬像中观察到纤维化组织，这一重大发现有助于肝癌和一系列肝脏疾病的早期诊断。