亚稳态氪原子束流诊断系统结构设计与优化

亚稳态氪原子具有较高的内能,在金属表面碰撞时会激发产生电子,利用电子透镜对退激发电子进行成像与检测,即可实现亚稳态氪原子束流强度以及空间分布的表征。基于该原理设计了一种亚稳态氪原子束流成像系统,采用有限元分析方法研究了电极间距、厚度与电势对退激发电子束流运动的影响规律。结果表明:成像弥散随着电子透镜聚焦能力的提升而减小。其亚稳态氪原子束流各物点成像误差在2mm以内,成像大小为 16.5mm,平均误差为3.71%,最大误差为9.38%。     

A Study of Electric Charging Using Low-Magnification Electron Holography

As experimentally found by low-magnification holography, both the size of the condenser aperture and the excitation of the objective lens strongly modify the object wavefunction of an electron wave. For example, the phase by electrically charged polystyrene latex spheres attached to a copper grid increases with decreasing excitation of the objective lens and/or decreasing diameter of the condenser aperture. It was also found that a smaller diameter of the condenser aperture increases the phase shift by a carbon foil or magnetic memory cells. In addition, the phase of the image wave is modified by the lateral position of the aperture, whereas different electron densities of the imaging beam have only minor influence on the detected phase shifts.     

Graded-index fiber lens proposed for ultrasmall probes used in biomedical imaging

The quality and parameters of probing optical beams are extremely important in biomedical imaging systems both for image quality and light coupling efficiency considerations. For example, the shape, size, focal position, and focal range of such beams could have a great impact on the lateral resolution, penetration depth, and signal-to-noise ratio of the image in optical coherence tomography. We present a beam profile characterization of different variations of graded-index (GRIN) fiber lenses, which were recently proposed for biomedical imaging probes. Those GRIN lens modules are made of a single mode fiber and a GRIN fiber lens with or without a fiber spacer between them. We discuss theoretical analysis methods, fabrication techniques, and measured performance compared with theory.     

Behavior of the point-spread function in photon-limited confocal microscopy

We study the behavior of the point-spread function (PSF) of the confocal scanning optical microscope (CSOM) when the available optical energy density from the sample plane is low (<7.5 microJoule/micrometers2). The PSF profile is analyzed under three photon-limited imaging conditions: (1) reflection-type CSOM with a weak source and a perfectly reflecting sample, (2) reflection-type CSOM with a strong illumination source and a weak sample, and (3) fluorescence CSOM with a weak fluorescent sample. Linfoot's image quality criteria of fidelity, structural content, and correlation quality are used to assess the reproducibility of the PSF profile as a function of the photon number. Low photon numbers yield a PSF profile that is difficult to maintain from one location in the sample plane to another. The optical sectioning capability of the CSOM was found to deteriorate more quickly against light power reduction than its transverse resolving power. The signal-to-noise ratio of the scanned CSOM image improves exponentially with the photon number from the sample plane. The noise that is generated by an unstable PSF has an average amplitude that decreases exponentially with the photon number and is significant only at low photon numbers. The CSOM image quality deteriorates because of spurious high-frequency components, degradation in the PSF dynamic range, and varying resolving power.     

A compact,automated and long working distance optical tweezer system

We demonstrate a compact, automated, long working distance optical tweezer system using a novel mechanism for controlling the position of the optical trap. Our system uses a single focusing lens with a working distance of 4.5?mm and the trapping beam is steered by moving the lens with a miniature coil-magnet assembly. The sample is imaged through a 100×?microscope objective and a CCD camera captures the magnified image. A custom image processing software detects the position of the laser beam and identifies the sample objects. This information is used to generate appropriate electrical signals to drive the coils which move the focusing lens along the desired path. The system is fairly simple and power efficient due to minimal usage of optical elements in the laser path; hence our setup is simple, low-cost and requires low optical power. Computer-generated arbitrary trapping paths and time-shared trapping patterns are successfully demonstrated. Efficient trapping of micron size spheres with laser powers as low as 1.5?mW is observed.     

用于毫米波焦面阵成像系统的扩展半球介质透镜

毫米波成像是近年来毫米波领域的一个研究热点,而焦面阵成像因其具有实时成像的优点更加受到重视。中分析可用为面阵成像的扩展半球介质透镜。这种焦面阵成像结构将集成天线阵贴附在透镜背面接收透镜聚焦的功率,消除了集成天线工作在毫米波频段时存在的表面波对天线性的影响,具有尺寸紧凑、损耗小的特点。采用Stratton-Chu公式和射线追迹分析了电磁波入射到扩展半球透镜上时在其背面的场分布,即透镜的焦区场分布,以获得透镜用于焦面阵成像时的性能。为验证分析方法的正确性,对平面波垂直入射和会聚高斯束入射两种情形进行了实验验证,实验结果与理论分析吻合较好。该透镜天线还可用来消除常规集成毫米波系统中抛物面天线与集成前端之间的过渡,以降低损耗,改善系统性能,也可用于与准光系统的连接或耦合。该结果将对上述应用提供理论指导。     

图像传感器尺寸与镜头焦距转换系数

数码相机图像传感器尺寸随相机的不同而不同,同样焦距的镜头在不同尺寸的传感器上,成像的视角也不同。所以仅仅以镜头的真实焦距,无法比较不同相机的拍摄范围(成像视角)。要想将不同尺寸图像传感器上成像的视角,转化为135相机上同样成像视角所对应的镜头焦距,就要了解镜头焦距转换系数的相关概念。     

Speckle-motion artifact under tissue shearing

Research has shown that, for a rotating phantom, the speckle pattern may not replicate the phantom motion, rather it may show a large lateral translation component in addition to rotation. This translation effect was labeled speckle-motion artifact. An image formation model has been shown to explain the phenomenon, pointing to the curvature of the imaging system point spread function (PSF) at the origin of this effect. The present paper extends this analysis and proposes a model, which predicts that a lateral motion artifact also would occur with shear motion. In the model, the artifact is found to be proportional to the shear angle and dependent of shear orientation, being maximal for shear that runs parallel to the axial direction; as for rotation, the artifact increases with frequency and beamwidth. This would mean that, when viewing a parabolic flow in the far field or with a highly curved PSF, an apparent contraction/expansion pattern in the direction of the vessel wall would be superimposed to the real velocity profile. In elastography, when viewing an inclusion subjected to an axial strain, four motion artifact regions are expected near the inclusion. The model is developed using the Fourier domain representation of the speckles for tissue-motion compensated signals, also called Lagrangian speckle. It can explain the artifact in terms of a simple spectral translation of a parabolic phase profile; given this, it is shown the artifact would be proportional to the lateral derivative of the axial displacement field. The spectral representation of Lagrangian speckle, for shear, also provides a simple geometrical interpretation for speckle decorrelation in terms of the shear strength and orientation, and in terms of the beam characteristics, i.e., the axial and lateral bandwidth.     

Improvement of transcutaneous fluorescent images with a depth-dependent point-spread function

The point-spread function (PSF) for transcutaneous fluorescent imaging was obtained as an analytical solution in a closed form. It is applicable to cases in which the optical property of the image-blurring turbid medium is considered to be fairly homogeneous. We proposed a technique to improve a transcutaneous image by using depth-dependent PSF. Contrast of the fluorescent image was improved for depths of 1-15 mm in a scattering medium (micro(s)' = 1/mm). The visible depth was more than doubled with this technique. An experiment with a rat demonstrated considerable improvement of a transcutaneous image of the cerebral vein at a specified depth. The spread image of the heart was reduced to the correct size by use of the PSF with the actual depth of the heart.     

Gradient-index human lens as a quadratic phase transformer

The correspondence between the linear integral transform and the ray-transfer matrix of a first-order optical system is used to evaluate the transmittance function of gradient-index (GRIN) human lens regarded as a quadratic phase transformer. The size of the GRIN crystalline lens has been considered for redefining the effective transmittance function by the pupil function. The role that the GRIN nature of the human lens plays in the retinal image quality using the point spread function (PSF) is commented on. The simulation results show that the effective radius of the output face of the lens decreases with increasing thickness and that it is higher for flat end surfaces than for curved end faces. On the other hand, the simulation results also show, for small pupil sizes, that the GRIN nature of the human lens is a retinal image degradation source producing the spread of the PSF and that the curved end surfaces of the lens constitute a retinal image quality improve factor contributing to the narrowing of the PSF.     

Voigt modelling of size-strain analysis: Application to α-Al2O3 prepared by combustion technique

A comprehensive analysis of size and strain broadened profile shapes in X-ray diffraction line broadening analysis is presented. Both size and strain broadened profiles were assumed to be Voigtian and the derived microstructural parameters (size and strain) were found to be in close agreement with those calculated from model independent Warren-Averbach method. The method is applied to three different alumina samples viz. micron size α-alumina (α-Al₂O₃) prepared by the combustion of aluminium nitrate and urea mixture, annealed samples and commercial α-Al₂O₃ sample. It is likely from the present analysis that a significant Gaussian size contribution is related to narrow size distribution observed from the analysis. It has been concluded that present Voigtian analysis is more reliable and may largely replace the earlier simplified integral breadth methods of analysis often used in line broadening analysis.     

Hybrid imaging using linear retroreflectors

It has been observed that retroreflective materials can be used in combination with beam splitters to produce real images. This is practical on a large size scale but has a maximum optical efficiency of 25%. Conversely, curved reflectors efficiently produce real images, but their cost increases very rapidly with size. We introduce a new imaging method, which combines the advantages of both systems, through the use of a linear retroreflective film. This material is retroreflective in one plane and conventionally reflective in the perpendicular plane. The net result is an efficient real image system that can have unlimited extent in one transverse direction, and which can be inexpensively manufactured on a large scale.     

Detection of the Object Velocity Using Doubly-scattered Dynamic Speckles Under Gaussian Beam Illumination

Abstract

The dynamic properties of doubly-scattered speckles produced at the image plane have been investigated using a scattering model that consists of a coherent Gaussian beam, a cascade of two moving diffusers and a single imaging lens. Dependence of the fluctuation speed of time-varying image speckles on the velocities of the diffusers is found to change with the size of illuminating speckles and the width of the point-spread function of the imaging lens. When the point-spread function and the focal position satisfy certain conditions, three kinds of velocity information—the velocity of each diffuser, the average velocity of the diffusers and the velocity difference between the diffusers—can be obtained by measuring a temporal correlation length of the time-varying speckle intensity. Experimental results confirm the theoretical predictions.     

Numerical calculation of the sound field focused by acoustic lens with an arbitrary axisymmetric sound speed distribution

The expressions that describe the sound field focused by an acoustic lens with different sound speed distribution (SSD) in lens are given. Numerical calculation results are presented, in which the axial and lateral sound pressure distributions and the -3 dB sound beam profile along the focused field are given out for three types of the acoustic focused lens with revolving-arc curved outer surface and four types' axisymmetric SSD. It is shown that, when sound beam is focused by a lens with SSD, the best focal point moves away from the lens and achieves a large focal region compared with the no-SSD case. The maximum axial focused intensity at the best focal point is smaller than that of the no-SSD case, but the focused beam width at the best focal point is larger than that of the no-SSD case. The larger the coefficient of the SSD, the larger the focused beam width and the larger focal region, but the divergent angle is almost the same.     

Improved field scanner incorporating parabolic optics. Part 2: Experimental verification and potential for volume scanning

We investigated the experimental performance of an afocal scan engine employing two off-axis parabolic reflectors and it was found not to introduce astigmatism when compared to a freely propagated beam. The performance of the new afocal engine is very similar to an ideal single-mirror scan engine in terms of spot size and beam spot profile (or point spread function) and has an improved flatness of field over other two-dimensional laser scan engines. The parabolic scan engine is contrasted with a comparable spherical mirror arrangement and found to produce superior performance at the intermediate image plane when focused through a scan lens. Further modeling and experimentation point toward volume scanning applications. The significant performance improvement provided by this design, now verified experimentally, will result in superior image quality for fast scanning confocal and two-photon microscopy in particular.     

Vector wave analysis for nonnormal incident rays in epimicroscopic refractive index profile measurements

We have investigated the effects of nonnormal incident rays in calculating the refractive index profile of a dielectric sample using the reflectance measurement data obtained with a scanning confocal epimicroscope and also by solving three-dimensional vector wave equations for linearly polarized light. The numerically calculated reflection data of tightly focused Gaussian beams with different numerical apertures (NAs) on planar surfaces with various refractive indices confirm that the reflectance increases with an increase in the NA of a focusing objective lens. This is due to the nonnormal incident ray components of a Gaussian beam. We have found that the refractive index obtained with the assumption of a normal incident beam is far from the real value when the NA of a focusing lens becomes larger than 0.5, and thus the variation in the reflectance for different angular components in a Gaussian beam must be taken into consideration while using a larger NA lens. Errors in practical refractive index calculation for an optical fiber based on a normal incident beam in reflectance measurements can be as large as 1% in comparison to real values calculated by our three-dimensional vector wave equations.     

声子晶体平板组合成像特性分析

用多重散射方法分析了声子晶体单平板和多平板的成像特性。发现：在特定频率范围内,引入等效负折射率后,单板负折射成像可用反Snell定律描述;多层声子晶体平板成像服从依次成像规律,可类比传统几何光学中共轴球面系统的成像,引入实物实像、虚物虚像等概念;平板透镜不改变出射波的出射方向,仅使出射位置产生平移;多平板透镜的累加成像与等厚度的单一平板透镜成像具有相同的效果。平板透镜的这些成像特点,使其在成像质量、加工方法、系统组合等方面比球面透镜更具优势。     

Real-time ultrasound process tomography for two-phase flow imaging using a reduced number of transducers

In this paper a novel ultrasound tomography imaging system is presented. It employs a relatively small number of transducers that produce fan-shaped beam profile to effectively insonify the cross-section. The impact on the image quality due to the reduction of the number of transducers is discussed, and different approaches such as multiple receiver data acquisition and nonlinear thresholding are explored. A prototype of a tomographic imaging system with only 36 transducers has been constructed, and processing rates of up to 100 frames per second have been achieved using a parallel processing technique. Good image reconstructions based on simulations and real objects also are provided to confirm the principles of the theoretical analysis.     

Two-dimensional point spread matrix of layered metal-dielectric imaging elements

We describe the change of the spatial distribution of the state of polarization occurring during two-dimensional (2D) imaging through a multilayer and in particular through a layered metallic flat lens. Linear or circular polarization of incident light is not preserved due to the difference in the amplitude transfer functions for the TM and TE polarizations. In effect, the transfer function and the point spread function (PSF) that characterize 2D imaging through a multilayer both have a matrix form, and cross-polarization coupling is observed for spatially modulated beams with a linear or circular incident polarization. The PSF in a matrix form is used to characterize the resolution of the superlens for different polarization states. We demonstrate how the 2D PSF may be used to design a simple diffractive nanoelement consisting of two radial slits. The structure assures the separation of nondiffracting radial beams originating from two slits in the mask and exhibits an interesting property of a backward power flow in between the two rings.     

Focusing of synchrotron radiation

Focusing of the synchrotron radiation from a storage ring using a convex lens is geometrically analysed and tested. The source radiation is supposed to have a bivariate normal distribution in its phase space both vertically and horizontally. Its modification caused by a lens is calculated as a function of distances among the source, the lens and the image plane. It is shown that the horizontal image becomes sharpest when the source is focused on the image plane. The vertical image, however, is not sharpest under this condition. The vertical distribution has more information than the horizontal; we can derive the orbit dependence of the vertical profile and the angle distribution of the radiation changing focusing.