A general phase retrieval algorithm based on ptychographicaliterative engine for coherent diffractive imaging

Coherent diffractive imaging (CDI) is a lensless imaging technique and can achieve a resolution beyond the Rayleigh or Abbe limit. The ptychographical iterative engine (PIE) is a CDI phase retrieval algorithm that uses multiple diffraction patterns obtained through the scan of a localized illumination on the specimen, which has been demonstrated successfully at optical and X-ray wavelengths. In this paper, a general PIE algorithm (gPIE) is presented and demonstrated with an He-Ne laser light diffraction dataset. This algorithm not only permits the removal of the accurate model of the illumination function in PIE, but also provides improved convergence speed and retrieval quality.     

基于光栅分光法的相干衍射成像

提出一种可以通过单次曝光实现 PIE(ptychographic imaging engine)成像的方法, 该方法用正交光栅将入射细光束衍射为传播方向不同的子光束簇以对样品进行照明, 并用CCD同时记录各个子光束所形成的衍射光斑阵列. 该方法很好地克服了现有PIE方法的成像质量易受机械扫描误差影响和数据采集时间过长两个缺点, 具有很好的实用价值. 关键词： 相干衍射成像 相位恢复 显微成像 迭代算法     

非相干照明条件下的ptychographic iterative engine成像技术

在传统多波长相干衍射成像理论的基础上提出适用于 X-射线和电子束等非相干光源照明成像的改进多波长ptychographic iterative engine 方法, 同时将小孔形状和照明光谱信息用于叠代计算, 可以在非相干照明条件下精确重建出物体的强度透射像和相位透射像, 并对光源带宽对重建精度的影响进行了分析, 对于解决如何在非相干照明条件下对大尺寸物体进行精确相位成像的问题具有较好的科研和实用价值.     

中心挡板对扫描相干X射线衍射成像的影响

本文采用重叠关联迭代引擎算法, 系统地模拟研究了扫描相干衍射成像中中心挡板导致的低频信号丢失对重建图像质量的影响. 结果表明, 扫描相干衍射成像对中心挡板的承受能力远大于平面波单次相干衍射成像, 且选择小尺寸入射探针和较高重叠度(≥ 70%)可进一步降低中心丢失信号对扫描相干衍射成像的负面影响. 另外, 光斑扫描位置误差在重叠度较高时将超过中心挡板成为扫描相干衍射成像最主要的负面影响因素. 本文研究结果对扫描相干衍射成像实验中如何应用中心挡光板具有重要的指导意义, 将有助于进一步提高扫描相干衍射成像的分辨率. 关键词： 扫描相干衍射成像 位相恢复算法 低频丢失信号     

硬x射线同轴相衬成像的相位恢复

基于角谱的概念，分析了硬x射线同轴相衬成像过程，在传统相位恢复算法GS算法的基础上 ，提出了基于角谱传播的相位恢复算法——迭代角谱法（IASA），并利用数值法模拟研究了 相衬成像和相位恢复过程，从理论上验证了这一相位恢复算法. 关键词： 相位恢复 相位衬度 角谱 硬x射线     

基于电子束剪切干涉的PIE成像技术研究

提出了基于M?llenstedt电子双棱镜的电压扫描剪切干涉全场ptychographic iterative engine(PIE)显微成像技术.从低到高逐步改变电子双棱镜的电压,并同时记录所形成的剪切干涉条纹,待测样品透射电子束的强度和相位分布就可以用PIE算法得以快速重建,而且双棱镜的方向、位置和实际电场强度分布等诸多实验中不可避免地偏差都可以在迭代过程中自动得以更正.所提技术能够克服现阶段用电子束进行PIE成像的诸多技术困扰,从而有望推动PIE技术在电子显微成像领域的发展和应用.     

Phase imaging with rotating illumination

A modified extended-ptychographical-iterative-engine （ePIE） algorithm is proposed to overcome the dis- advantages of ePIE technique and reduce the influence of stage hysteresis or backlash error. The exit wave of a rotatable ＂screen＂ illuminated by plane wave is used as the illumination on the specimen, and the complex transmission functions of the rotatable object and specimen can be simultaneously reconstructed. Compared with the standard x - y scanning PIE algorithm, the proposed algorithm can completely avoid the influence of stage hysteresis （or backlash error）. The proposed algorithm also has higher convergence s!oeed and better accuracy than the standard PIE algorithm.     

上海光源硬X射线相干衍射成像实验方法初探

相干X射线衍射成像方法是一种先进的成像技术,分辨率可达纳米量级.国际上大多数的同步辐射装置和自由电子激光装置都建立了该成像方法,并有将其作为主要成像技术的趋势.上海光源作为目前国内唯一的一台第三代同步辐射光源,尚未建立基于硬X射线的相干衍射成像实验平台.随着一批以波荡器为光源的光束线站投入使用,使得该方法的建立成为了可能.本文基于上海光源BL19U2生物小角散射线站,通过有效的光路设计,搭建了相干衍射实验平台,在12 keV和13.5 keV能量点均获得了硬X射线相干光束,并基于小孔衍射测量了入射光束的空间相干长度.该平台支持常规和扫描相干衍射实验模式,对小孔衍射图样及波带片扫描衍射图样实现了正确的相位重建,证明了该平台初步具备开展硬X射线相干衍射成像实验的能力.硬X射线相干衍射成像实验平台为国内首次建立,将为国内该实验方法的发展和应用提供有效的软硬件支持.     

连续太赫兹波双物距叠层定量相衬成像

针对连续太赫兹波叠层成像重建算法收敛较为迟滞的问题,提出一种连续太赫兹波双物距叠层成像方法及相关重建算法,使用不同记录距离形成的差异化衍射图幅值作为重建算法记录平面的约束条件,增加了记录平面数据多样性和衍射信息冗余度.仿真结果表明,本方法可以加快算法收敛速率,有效减少迭代次数,提高连续太赫兹波定量相衬成像计算效率.随后构建了基于2.52 THz光泵连续太赫兹激光器的双物距叠层成像实验装置,应用双物距记录方法及改进算法重建获得了聚丙烯基字母图案样品的幅值和相位分布,结果表明改进方法可以减少算法迭代次数,提升计算效率,同时改善相位像重建结果保真度.     

Resolution enhancement of random adsorbed single-molecule localization based on surface plasmon resonance illumination

Single-molecule localization (SML) is a powerful tool to overcome the diffraction limit in optical imaging, because the fluorescence emitted by single molecules can be observed with nanometer accuracy when the optical background and associated noise are made sufficiently small. Random adsorbed SML has been successfully demonstrated for superresolution imaging on metal surfaces. To optimize the random adsorbed SML, we developed a new illumination method based on surface plasmon resonance (SPR). The enhancement of the fluorescence signal and the reduction of background noise were achieved simultaneously. A high localization resolution of 15?nm was demonstrated with this new SPR illumination system.     

Optical diffraction tomography microscopy with transport of intensity equation using a light-emitting diode array

Optical diffraction tomography (ODT) is an effective label-free technique for quantitatively refractive index imaging, which enables long-term monitoring of the internal three-dimensional (3D) structures and molecular composition of biological cells with minimal perturbation. However, existing optical tomographic methods generally rely on interferometric configuration for phase measurement and sophisticated mechanical systems for sample rotation or beam scanning. Thereby, the measurement is suspect to phase error coming from the coherent speckle, environmental vibrations, and mechanical error during data acquisition process. To overcome these limitations, we present a new ODT technique based on non-interferometric phase retrieval and programmable illumination emitting from a light-emitting diode (LED) array. The experimental system is built based on a traditional bright field microscope, with the light source replaced by a programmable LED array, which provides angle-variable quasi-monochromatic illumination with an angular coverage of ±37 degrees in both x and y directions (corresponding to an illumination numerical aperture of ∼0.6). Transport of intensity equation (TIE) is utilized to recover the phase at different illumination angles, and the refractive index distribution is reconstructed based on the ODT framework under first Rytov approximation. The missing-cone problem in ODT is addressed by using the iterative non-negative constraint algorithm, and the misalignment of the LED array is further numerically corrected to improve the accuracy of refractive index quantification. Experiments on polystyrene beads and thick biological specimens show that the proposed approach allows accurate refractive index reconstruction while greatly reduced the system complexity and environmental sensitivity compared to conventional interferometric ODT approaches.     

Phase retrieval using an aperture-array filter under partially coherent illumination

Recently we have proposed a deterministic phase retrieval method using an aperture-array filter to reconstruct a complex-valued object from a single diffraction intensity pattern. We describe here the effect of quasi-monochromatic partially coherent illumination on the object reconstruction by the phase retrieval method, and then present the method of eliminating the effect of the partially coherent illumination from the diffraction intensity pattern via a simple Fourier deconvolution operation provided that the complex degree of spatial coherence of the illuminating beam is known. The usefulness of this method is shown in computer-simulated examples of the object reconstructions under Gaussian Schell-model partially coherent illumination.     

Phase-diverse coherent diffractive imaging: high sensitivity with low dose

This Letter demonstrates that coherent diffractive imaging (CDI), in combination with phase-diversity methods, provides reliable and artefact free high-resolution images. Here, using x rays, experimental results show a threefold improvement in the available image contrast. Furthermore, in conditions requiring low imaging dose, it is demonstrated that phase-diverse CDI provides a factor of 2 improvement in comparison to previous CDI techniques.     

Atom-scale ptychographic electron diffractive imaging of boron nitride cones

Ptychographic coherent diffractive imaging (CDI) has been extensively applied using both x rays and electrons. The extension to atomic resolution has been elusive. This Letter demonstrates ptychographic electron diffractive imaging at atomic resolution, permitting identification of structure in a boron nitride helical cone at a resolution of order 1 ?, beyond that of comparative Z-contrast images. A scanning transmission electron microscope is used to create a diverging illumination in a defocused Fresnel CDI geometry, providing a robust strategy leading to a unique solution.     

Simulation study of phase retrieval for hard X-ray in-line phase contrast imaging

X-ray phase contrast imaging (XPCI) is a novel method that exploits the phase shift for the incident X-ray to form an image. For light elements such as carbon, hydrogen and oxygen, the phase-shift term can be up to 1000 times greater than the absorption term in the hard X-ray energy region. So XPCI has attracted much attention in recent years. Various methods for XPCI have been proposed and demonstrated on synchrotron devices and other X-ray sources[1―13], particularly the in-line metho…     

Structured-illumination-based lensless diffractive imaging and its application to optical image encryption

In this paper, we propose a new method in the structured-illumination-based lensless diffractive imaging using variable grating pitches. When a phase grating pitch is sequentially changed, a series of diffraction patterns can be recorded by a charge-coupled device (CCD) camera. Subsequently, a phase retrieval algorithm with a rapid convergence rate is developed to recover a high-quality object from the recorded diffraction patterns. The proposed method is further applied to optical image encryption, and simulation results are presented to demonstrate validity of the proposed method.     

Single-pulse coherent diffraction imaging using soft x-ray laser

We report a coherent diffraction imaging (CDI) using a single 8 ps soft x-ray laser pulse at a wavelength of 13.9 nm. The soft x-ray pulse was generated by a laboratory-scale intense pumping laser providing coherent x-ray pulses up to the level of 10(11) photons/pulse. A spatial resolution below 194 nm was achieved with a single pulse, and it was shown that a resolution below 55 nm is feasible with improved detector capability. The single-pulse CDI might provide a way to investigate dynamics of nanoscale molecules or particles.     

Linear algorithms for phase retrieval in the Fresnel region. 2. Partially coherent illumination

Analytical expressions for the spatial distribution of the spectral density and intensity of projection images are derived for a broad class of object transmission functions in the case of partially coherent Schell-model-type incident illumination. The expressions are linear with respect to the phase distribution in the transmitted wave. Associated methods for phase retrieval are discussed with the emphasis on a technique that allows simultaneous “automatic” phase retrieval and deconvolution of projection images of homogeneous objects by means of mutual cancellation at a specified defocus distance of the Fresnel diffraction effects and the image blurring due to the point-spread function of the imaging system.     

Analysis of a multi-wavelength multi-camera phase-shifting profilometric system for real-time operation

Real-time accomplishment of a phase-shifting profilometry through simultaneous projection and recording of fringe patterns requires a reliable phase retrieval procedure. In the present work we consider a four-wavelength multi-camera system with four sinusoidal phase gratings for pattern projection that implements a four-step algorithm. Successful operation of the system depends on overcoming two challenges which stem out from the inherent limitations of the phase-shifting algorithm, namely the demand for a sinusoidal fringe profile and the necessity to ensure equal background and contrast of fringes in the recorded fringe patterns. As a first task, we analyze the systematic errors due to the combined influence of the higher harmonics and multi-wavelength illumination in the Fresnel diffraction zone considering the case when the modulation parameters of the four gratings are different. As a second task we simulate the system performance to evaluate the degrading effect of the speckle noise and the spatially varying fringe modulation at non-uniform illumination on the overall accuracy of the profilometric measurement. We consider the case of non-correlated speckle realizations in the recorded fringe patterns due to four-wavelength illumination. Finally, we apply a phase retrieval procedure which includes normalization, background removal and denoising of the recorded fringe patterns to both simulated and measured data obtained for a dome surface.     

平场复用多焦点结构光照明超分辨显微成像

多焦点结构光照明显微技术(multifocal structured illumination microscopy, MSIM)能在50μm的成像深度内和1Hz的成像速度下实现两倍于衍射极限分辨率的提升,相比传统的宽场结构光照明显微技术,具有较大的成像深度和层析能力,更适合应用于厚样品的长时程三维超分辨成像.然而, MSIM存在成像速度慢、图像处理过程复杂等问题.本文提出了一种基于平场复用多焦点结构光照明的快速超分辨显微成像方法和系统(flat-field multiplexed MSIM, FM-MSIM),通过在照明光路中插入光束整形器件,将高斯光束转变为均为分布的平顶光束,提高激发点阵的强度均匀性和扩大视场;通过将每个衍射受限的激发点沿y方向延长,形成新的多路复用多焦点阵照明图案,提高能量利用率,减少扫描步数,进而提高成像速度和信噪比;结合基于多重测量矢量模型的稀疏贝叶斯学习图像重构算法,简化图像重构步骤,在保证空间分辨率的同时实现至少4倍于传统MSIM的成像速度.在此基础上,利用搭建的FM-MSIM系统进行了BSC细胞微管样片和小鼠肾切片标准样片的超分辨成像实验,实验结果证明...