Off-axis compressed holographic microscopy in low-light conditions

This Letter reports a demonstration of off-axis compressed holography in low-light level imaging conditions. An acquisition protocol relying on a single exposure of a randomly undersampled diffraction map of the optical field, recorded in the high heterodyne gain regime, is proposed. The image acquisition scheme is based on compressed sensing, a theory establishing that near-exact recovery of an unknown sparse signal is possible from a small number of nonstructured measurements. Image reconstruction is further enhanced by introducing an off-axis spatial support constraint to the image estimation algorithm. We report accurate experimental recovering of holographic images of a resolution target in low-light conditions with a frame exposure of 5 μs, scaling down measurements to 9% of random pixels within the array detector.     

Digital holography for quantitative phase-contrast imaging

We present a new application of digital holography for phase-contrast imaging and optical metrology. This holographic imaging technique uses a CCD camera for recording of a digital Fresnel off-axis hologram and a numerical method for hologram reconstruction. The method simultaneously provides an amplitude-contrast image and a quantitative phase-contrast image. An application to surface profilometry is presented and shows excellent agreement with contact-stylus probe measurements.     

基于滤波成像的大视角数字全息技术

为提高数字全息再现像视角,提出一种基于滤波成像的数字全息技术来实现大视角的三维物体面型测量.利用离轴像面数字全息技术,通过在4F相干图像处理系统的空间频谱面处放置可移动的低通滤波器,使满足CCD分辨率的物光波与参考光波干涉形成全息图,并控制低通滤波及成像区域分别记录不同谱段的子全息图.再现时,首先对子全息图进行数字傅里叶变换,重构对应频谱段,并对频谱段进行拼接形成完整的物光频谱|而后通过数字再现获得大视角的数字全息再现像.利用该方法测量了圆柱形表面（光滑的缝纫针）的三维形貌,并取得了较好的实验结果.     

基于旋转照明光的数字全息相干噪声抑制

提出一种数字全息相干噪声抑制方法。首先在照明光方位角给定的前提下,通过在极向平面内连续旋转照明光,实现在多角度照明条件下记录离轴数字全息图。然后对所获得的全息图分别重构,同时利用倾斜相差补偿算法和数字图像自相关配准方法分别补偿再现像的相位倾斜及横向偏移。最后将重构像进行非相干叠加,从而有效抑制再现像的相干噪声。实验结果及相关统计评价数据皆验证了该方法的有效性。     

Imaging interferometric microscopy

We introduce and demonstrate a new microscopy concept: imaging interferometric microscopy (IIM), which is related to holography, synthetic-aperture imaging, and off-axis-dark-field illumination techniques. IIM is a wavelength-division multiplex approach to image formation that combines multiple images covering different spatial-frequency regions to form a composite image with a resolution much greater than that permitted by the same optical system using conventional techniques. This new type of microscopy involves both off-axis coherent illumination and reinjection of appropriate zero-order reference beams. Images demonstrate high resolution, comparable with that of a high-numerical-aperture (NA) objective, while they retain the long working distance, the large depth of field, and the large field of view of a low-NA objective. A Fourier-optics model of IIM is in good agreement with the experiment.     

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.     

Digital holography with ultimate sensitivity

We propose a variant of the heterodyne holography scheme that combines the properties of off-axis and phase-shifting holography. This scheme makes it possible to filter off numerically the zero-order image alias and the technical noise of the reference. It is then possible to record and reconstruct holographic images at an extremely low signal level. We show experimentally that the sensitivity of the method is limited only by the quantum nature of photons.     

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.     

Electron holography of biological samples

In this paper, we summarise the development of off-axis electron holography on biological samples starting in 1986 with the first results on ferritin from the group of Tonomura. In the middle of the 1990s strong interest was evoked, but then stagnation took place because the results obtained at that stage did not reach the contrast and the resolution achieved by conventional electron microscopy.

To date, there exist only a few (12) publications on electron holography of biological objects, thus this topic is quite small and concise. The reason for this could be that holography is mostly established in materials science by physicists. Therefore, applications for off-axis holography were powerfully pushed forward in the area of imaging, e.g. electric or magnetic micro- and nanofields. Unstained biological systems investigated by means of off-axis electron holography up to now are ferritin, tobacco mosaic virus, a bacterial flagellum, T5 bacteriophage virus, hexagonal packed intermediate layer of bacteria and the Semliki Forest virus. New results of the authors on collagen fibres and surface layer of bacteria, the so-called S-layer 2D crystal lattice are presented in this review. For the sake of completeness, we will shortly discuss in-line holography of biological samples and off-axis holography of materials related to biological systems, such as biomaterial composites or magnetotactic bacteria.     

基于离轴数字全息改善散斑自相关重建效果

针对在成像物体没有超出散射介质记忆效应范围的情况下,提出一种结合数字离轴全息术减少散射介质成像中散斑自相关噪声的方法。当成像目标经过散射介质时,使用自相关技术结合相位恢复算法能够从散斑中重建成像目标。但在实际成像的过程中,为了有效抑制环境噪声和热噪声等对重建效果的影响,设计利用离轴全息中的相移法消除噪声项中静态噪声项的干扰,再利用散斑自相关与相位恢复算法重建去噪后效果更好的成像目标。采用结构相似度对重建效果进行定量评估,仿真结果表明对于给出的成像目标,去噪前后的结构相似度从0.8796提高到0.9875,验证了该方法的有效性。说明所提出的方法能够改善散斑自相关法重建效果。     

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.     

Numerical reconstruction and twin-image suppression using an off-axis Fresnel digital hologram

In this work we proposed a novel approach for the numerical reconstruction of digital holography that uses only an off-axis Fresnel digital hologram and can easily be carrying out in a personal computer. From the presented off-axis holographic principle, the reference wave accompanies to the numerically reconstructed image, also makes the separation to the zero-order and twin images. Therefore, the concept can be applied to suppress the blurring of numerical reconstruction by adding the extra reconstruction plane and the formularized numerically reference wave in the reconstruction procedure. As expected, the blurring images can be separated out and removed from the reconstructed image. Then, by performing the back propagating to the exact reconstruction plane, one can obtain the pure object image without the blurring. In contrast to the temporal phase-shifting digital holographic-based scheme that employs multiple exposures the technique can be more effectively and easily overcome the blurring problem, also provide the practical feasibility in the digital holographic application. PACS 42.40.-I; 42.30.Wb; 42.40.Ht     

Twin-image elimination in optical scanning holography

We propose a novel multiplexing technique to solve the twin image problem in optical scanning holography without the use of a spatial carrier, as commonly used in conventional off-axis holography. The technique involves simultaneously acquiring sine and cosine Fresnel zone-lens plate coded images by optical scanning. A complex addition of the two coded images will then be performed and decoded to give a twin-image rejection reconstruction. Computer simulations will be presented to demonstrate the validity of the idea.     

An introduction to off-axis electron holography

The technique of off-axis electron holography is introduced and reviewed. The history of the topic is discussed briefly looking at the origin of electron holography and the reasons for its growth in recent years around the world. The formation and numerical reconstruction of the hologram is discussed in detail on a theoretical and practical level. The spread of holography is illustrated through a number of applications ranging from ultra high resolution studies of crystal structures, the imaging of magnetic and electric fields, the determination of sample composition and morphology and the use of holography for imaging biological specimens.     

Digital holography at millimetre wavelengths

In this paper, we describe a method for wavefront reconstruction at millimetre wavelengths using off-axis holography, a frequently used image recording technique at visible wavelengths first demonstrated in the 1960’s. Millimetre radiation has been highlighted recently in the imaging of non-conducting materials and objects, which often show significant transparency at these long wavelengths, even though opaque at visible and infrared wavelengths. Holography provides a method for recording a lens-less image of an object thus reducing loss of spatial frequency information, which is important for obtaining maximum resolution at long wavelengths. An experimental arrangement based on a simple form of Fresnel off-axis holography is described, with the object illumination and reference beams derived using two radiating horn antennas fed by a single coherent source (a Gunn diode oscillator operating at 100 GHz – wavelength 3 mm) via a 3 dB cross-guide coupler. The process is discussed in the context of imaging and in the validation of millimetre wave radiation sources (horns). We show that the entire fields from these and other components can be analysed using computational optics with data derived from a single measurement obtained using inexpensive equipment.     

粒子场相移全息术

本文利用相移全息术记录的双曝光粒子场全息图,以改进再现图像的SNR、分辨率和对比度。并发展了这种技术的理论,讨论了相移的引入方法,给出了实验结果,与通常的同轴和离轴双曝光结果进行了比较。     

Comparison of off-axis and in-line electron holography as quantitative dopant-profiling techniques

Many different dopant-profiling techniques are available for semiconductor device characterization. However, with length scales shrinking rapidly, only transmission electron microscopy (TEM) techniques promise to fulfil the spatial resolution required for the characterization of future device generations. Here, we use three advanced TEM techniques, off-axis electron holography, Fresnel imaging (in-line electron holography) and Foucault imaging, to examine a focused ion beam-prepared silicon p–n junction device. Experiments are carried out on electrically unbiased samples and with an electrical bias applied in situ in the TEM. Simulations are matched to experimental data to allow quantitative conclusions to be drawn about the underlying electrostatic potential distributions. The off-axis electron holography and Fresnel results are compared to assess whether the techniques are consistent, and whether they can be used to provide complementary information about dopant potentials in semiconductor devices.     

Phase-shifting digital holography

A new method for three-dimensional image formation is proposed in which the distribution of complex amplitude at a plane is measured by phase-shifting interferometry and then Fresnel transformed by a digital computer. The method can reconstruct an arbitrary cross section of a three-dimensional object with higher image quality and a wider viewing angle than from conventional digital holography using an off-axis configuration. Basic principles and experimental verification are described.     

用Stokes参量法实现数字同轴偏振全息的研究

提出了一种基于Stokes参量的数字同轴偏振全息方法.在实验中用一束线偏振光和一束椭圆偏振光作为参考光, 分别与物光进行干涉,通过拍摄在两个相互垂直方向上的全息图,计算出物光在这两个方向的振幅和相位信息, 从而得到物光的Stokes参量和物体的全偏振信息,实现对各向异性物体偏振态空间分布的图像重建. 实验结果表明,该方法可用于物体的全偏振特性的测量.这种方法在求出物光Stokes参量的同时, 也可消除零级像和共轭像的干扰,因此也可用于同轴或离轴全息.     

Digital off-axis holography with a synthetic aperture

The use of a synthetic aperture is demonstrated as part of an off-axis setup for digital holography. The aperture was constructed from camera recordings at nine different positions. An algorithm to reconstruct object sections, which requires only moderate computer memory, was used. The result shows strongly improved resolution.