Microscopic imaging and holography with hard X-rays using Fresnel zone-plates

The imaging properties of a Fresnel zone-plate (FZP) were used for magnified imaging of microobjects using hard X-rays. The experiments were done using 14 keV synchrotron radiation. The coherence properties of the radiation produced by an undulator allowed the recording of real images and holograms from an object in one single exposure. These images result from the positive and the negative first order diffracted beams respectively. The X-ray microscope worked at an X-ray magnification factor of 12 and could resolve structures of 0.3 μm in size. By going to slightly defocused conditions we obtained magnified images of the holographical nearfield diffraction pattern (in-line holograms) of the object.     

Elimination of zero-order diffraction in digital off-axis holography

A simple experimental method of eliminating zero-order diffraction in the reconstructed image of off-axis digital holography is presented. Holographic diffraction grating acting as a beam splitter, an off-axis holography system is formed. The holograms of object with different recording parameters are obtained by adjusting the reflecting mirror in the recording optical system to vary the incidence orientation of the object beam in CCD to introduce a phase shift. The zero-order image can be eliminated by numerically processing the holograms of object with different recording parameters. The theoretical analyses have been done in detail and the experimental results are also given. The zero-order image eliminated, the area of reconstructed image increases remarkably, the image quality can then be significantly improved and the better resolution obtained. The experimental results show that the method presented in this paper is feasible, simple in optical system and easy in operation and data processing.     

Parallel-quadrature phase-shifting digital holographic microscopy using polarization beam splitter

We present a digital holography microscopy technique based on a parallel-quadrature phase-shifting method. Two π/2 phase-shifted holograms are recorded simultaneously using polarization phase-shifting principle, slightly off-axis recording geometry, and two identical CCD sensors. The parallel phase-shifting is realized by combining circularly polarized object beam with a 45° degree polarized reference beam through a polarizing beam splitter. DC term is eliminated by subtracting the two holograms from each other and the object information is reconstructed after selecting the frequency spectrum of the real image. Both amplitude and phase object reconstruction results are presented. Simultaneous recording eliminates phase errors caused by mechanical vibrations and air turbulences. The slightly off-axis recording geometry with phase-shifting allows a much larger dimension of the spatial filter for reconstruction of the object information. This leads to better reconstruction capability than traditional off-axis holography.     

一种消除数字离轴全息零级像的实验方法

提出一种消除数字离轴全息零级像的实验方法.这种方法是将全息衍射光栅作为分光元件,形成数字离轴全息记录系统,通过调节记录光路系统中平面镜,改变物光波到达CCD的入射方向而引入相移,从而得到不同记录参数的全息图.再通过对不同记录参数全息图的数字处理,即可达到零级像消除的目的,并对这种方法进行了理论分析和实验验证.给出了利用这种方法获得的实验结果.实验结果证明了该方案的可行性,并且具有光路系统简单、操作和处理容易等特点.     

High-resolution quantitative phase microscopic imaging in deep UV with phase retrieval

High-resolution three-dimensional (3D) microscopic imaging requires the use of short wavelengths. Quantitative 3D imaging techniques, such as digital holographic microscopy, require interference between the object beam and a known reference background for the extraction of phase information. At shorter wavelengths, due to short coherence lengths, it may be difficult to implement a two-beam off-axis setup. Thus, a single-beam technique, which provides complete phase information, may be better suited for short wavelengths. This Letter describes the development of a quantitative microscopy technique at 193 nm using multiple intensity samplings and phase retrieval.     

Angular multiplexing in pulsed digital holography for aperture synthesis

A new aperture-synthesis approach in a pulsed digital holography system for obtaining superresolution is proposed and experimentally demonstrated in which, with only a single laser pulse divided into two trains of subpulses, angular multiplexing can be applied to both the reference beam for different carrier frequency and the object beam for off-axis and on-axis illuminations. Furthermore, in the recording plane, multiple subholograms covering different spatial frequency ranges diffracted from the object can be recorded in a single frame of a CCD. Our analysis and results show that the resolution of the reconstructed image synthesized upon the reconstructed complex amplitudes of the subholograms can be improved effectively.     

NIR, MWIR and LWIR quantum well infrared photodetector using interband and intersubband transitions

This paper presents the design, fabrication and characterization of a QWIP photodetector capable of detecting simultaneously infrared radiation within near infrared (NIR), mid wavelength infrared (MWIR) and long wavelength infrared (LWIR). The NIR detection was achieved using interband transition while MWIR and LWIR were based on intersubband transition in the conduction band. The quantum well structure was designed using a computational tool developed to solve self-consistently the Schrödinger–Poisson equation with the help of the shooting method. Intersubband absorption in the sample was measured for the MWIR and LWIR using Fourier transform spectroscopy (FTIR) and the measured peak positions were found at 5.3 μm and 8.7 μm which agree well with the theoretical values obtained 5.0 μm and 9.0 μm for the two infrared bands which indicates the accuracy of the self-consistent model. The photodetectors were fabricated using a standard photolithography process with exposed middle contacts to allow separate bias and readout of signals from the three wavelength bands. The measured photoresponse gave three peaks at 0.84 μm, 5.0 μm and 8.5 μm wavelengths with approximately 0.5 A/W, 0.03 A/W and 0.13 A/W peak responsivities for NIR, MWIR and LWIR bands, respectively. This work demonstrates the possibility of detection of widely separated wavelength bands using interband and intersubband transitions in quantum wells.     

Digital holographic image fusion for a larger size object using compressive sensing

Digital holographic imaging fusion for a larger size object using compressive sensing is proposed. In this method, the high frequency component of the digital hologram under discrete wavelet transform is represented sparsely by using compressive sensing so that the data redundancy of digital holographic recording can be resolved validly, the low frequency component is retained totally to ensure the image quality, and multiple reconstructed images with different clear parts corresponding to a laser spot size are fused to realize the high quality reconstructed image of a larger size object. In addition, a filter combing high-pass and low-pass filters is designed to remove the zero-order term from a digital hologram effectively. The digital holographic experimental setup based on off-axis Fresnel digital holography was constructed. The feasible and comparative experiments were carried out. The fused image was evaluated by using the Tamura texture features. The experimental results demonstrated that the proposed method can improve the processing efficiency and visual characteristics of the fused image and enlarge the size of the measured object effectively.     

X-ray interferometric Fresnel holography

The X-ray interferometric Fresnel holography was proposed and theoretically investigated. It was shown that under definite conditions the recorded interference pattern formed at the output surface ofthe analyzer crystal (the third block of interferometer) is the X-ray interferometric hologram (generally the Fresnel one) of the object under investigation. Further reconstruction of an image is performed with the help of visible light or a numerical method. As an example the recording of a Fresnel hologram of the simplest case of one-dimensional object, a narrow slit, and the reconstruction of an image by means of visible light were considered. This method may be used in X-ray microscopy and realized using synchrotron sources ofX-ray radiation, as well as X-ray free electron lasers.     

利用相位模板实现数字全息超分辨成像

为了简化数字全息超分辨记录系统,分别在其物光和参考光部分引入一块相位模板,以获得垂直和倾斜方向照明物体的光束和具有不同载波频率的参考光束.当这些具有不同照射方向的光透过物体后,可以使CCD在位置固定的情况下记录到携带低频和高频信息的物体衍射场,不同载波频率的参考光则保证了高频和低频信息在复合全息图的频谱面上能够相互分离.实验结果证明,通过将记录到的物体高频和低频信息合成,可以获得超出系统衍射极限分辨率的再现像.     

Off-axis phase-only holograms of 3D objects using accelerated point-based Fresnel diffraction algorithm

A method for calculating off-axis phase-only holograms of three-dimensional (3D) object using accelerated point-based Fresnel diffraction algorithm (PB-FDA) is proposed. The complex amplitude of the object points on the z-axis in hologram plane is calculated using Fresnel diffraction formula, called principal complex amplitudes (PCAs). The complex amplitudes of those off-axis object points of the same depth can be obtained by 2D shifting of PCAs. In order to improve the calculating speed of the PB-FDA, the convolution operation based on fast Fourier transform (FFT) is used to calculate the holograms rather than using the point-by-point spatial 2D shifting of the PCAs. The shortest recording distance of the PB-FDA is analyzed in order to remove the influence of multiple-order images in reconstructed images. The optimal recording distance of the PB-FDA is also analyzed to improve the quality of reconstructed images. Numerical reconstructions and optical reconstructions with a phase-only spatial light modulator (SLM) show that holographic 3D display is feasible with the proposed algorithm. The proposed PB-FDA can also avoid the influence of the zero-order image introduced by SLM in optical reconstructed images.     

基于矩阵变换的光场成像及重聚焦模型仿真

基于光场的两平行平面表示方法,针对光场相机两参考平面间的共轴空间距离变换、离轴空间距离变换、轴旋转和沿轴旋转以及透镜作用等对光线描述方式的影响,给出了相应的光场矩阵变换方程,并利用这些方程,建立了光场相机的成像模型、采样模型及辐射传输模型.从光场共轴空间距离变换的观点出发推导出可实现不同点清晰成像的数字重聚焦公式.利用所建立的模型,计算机仿真生成了目标景物,并用所建立模型对生成景物进行了数字重聚焦,验证了数字重聚焦后的图像与直接聚焦图像的一致性.     

Techniques for recording holographic lenses for infrared wavelengths

Three approaches for recording holographic lenses for infrared wavelengths are discussed. For long wavelengths such as the 10.6 μm line from CO₂ lasers holographic lenses are best made as computer-generated holograms. For the near infrared holographic lenses can be made using visible wavelengths with a computer-generated hologram to provide aberration correction. Finally a two step procedure is described which can be used to make holographic lenses for most infrared wavelengths.     

离轴微粒场全息术中记录介质分辨率要求

本文从离轴微粒场全息记录光路的一般模型出发,导出了离轴微粒场全息术中记录介质分辨率要求的普遍公式。讨论了三种典型形状微粒对记录介质的分辨率要求。结果表明,除了参考光的倾角外,微粒的尺寸和参、物光的波前曲率差及微粒重心偏离光轴的距离也都是影响记录介质分辨率要求的主要因素。     

A parallel implementation of a modified Richardson-Lucy algorithm for image de-blurring

Passive imaging in the millimetre wave region of the spectrum is attractive compared with imaging in the visible and infrared because of its better penetration through cloud and rain. However, the diffraction limited spatial resolution is far worse and computer restoration of millimetre wave images is necessary. The Richardson-Lucy algorithm is frequently used for image restoration because compared with linear algorithms it has reduced sensitivity to error in a priori knowledge, and fewer restoration artefacts. However, it requires many iterations to converge and is very slow compared with linear methods. This paper investigates modifications of the Richardson-Lucy algorithm which speed up the execution of the algorthm. Software techniques investigated include incorporating a speedup parameter, using dual point spread functions and implementation in the Fourier domain. A parallel implementation of the algorithm has also been investigated using a tree topology network of between 1 and 10 transputers. Finally an application of the transputer system to the de-blurring of a millimetre-wave image obtained from a mechanical scanning system is described in the paper.     

基于微透镜阵列的积分视场成像光谱仪前置成像系统分析与设计

作为对天文光谱进行观测的仪器，成像光谱仪有着十分重要的作用。由于传统的狭缝型成像光谱仪的狭缝限制，对面源天体的观测需多次扫面，才能获得完整的面源三维数据立方体(x, y; λ)，这样将会浪费大量的观测时间。为了实现目标物体三维数据立方体的快速扫描，提出了一种基于微透镜阵列的无狭缝、静态化、快速高效的可见光到近红外波段积分视场成像光谱仪结构，并对其基本工作原理进行分析。为了扩展微透镜阵列积分视场成像光谱仪在医学、农业、物探等其他领域的应用潜能，该研究的光谱波段选择可见光到近红外波段。根据视场积分的工作原理，分析和设计了像方远心结构的离轴三反前置成像系统。系统采用视场离轴方式，波段范围400～900 nm，相对口径F/5，主镜、次镜和三镜皆为二次非球面，二次非球面系数分别为：-7.05，-0.92和-1.61。为减小系统体积，在离轴三反系统的焦平面附近放置反射镜。系统在奈奎斯特空间频率60 lp·mm-1处，调制传递函数大于0.75，成像质量接近衍射极限，满足系统要求。     

基于空间广义相干系数加权的医学合成孔径超声成像

为解决广义相干系数用于合成孔径成像中所存在的运算量大,图像对比度改善有限等问题,提出空间广义相干系数加权成像方法。该算法根据单个孔径成像结果之间的相干性来计算相干系数,通过加权空间合成进行成像。采用对FieldⅡ仿真点目标和吸声斑目标的数据进行成像表明,算法不仅使运算量减少N(N为阵元数)倍,而且相对于传统的广义相干系数算法,算法对散射点成像可提高信噪比7 dB,对于吸声斑成像可提高对比度3.2 dB。可见算法可以有效地提高成像速度和成像质量。      

Study on the simplified phase-shifting digital holographic microscopy

In this paper, a new simplified technique for effectively eliminating the zero order and the conjugate virtual image in digital holographic microcopy, which makes use of two-step phase-shifting method of just recording two holograms and an intensity image of object wave, is proposed. Meanwhile, combined with the principle of making full use of spatial bandwidth of the CCD sensor by in-line lens-less Fourier holographic recording geometry, the theory and experimental methods to increase the resolution of the reconstructed image in digital holography by using phase-shifting technique are detailedly analyzed. At end, the validity and availability of this technique has been demonstrated through the off-axis and in-line Fourier transform recording geometry. The study provides some theoretical and experimental guidance for the design and operation of a digital holographic microscopy system.     

X-ray interferometric short-wavelength holography with diffraction focusing

A four-unit off-axis interferometric scheme for recording x-ray short-wavelength holograms of amorphous bodies which are opaque to visible light is proposed. The diffraction spreading of the object wave in a unit is removed by diffraction focusing of the same wave in the next unit. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 9, 681–684 (10 November 1998)