Imaging systems based on the encoding of optical coherence functions

An imaging scheme is described that is based on the transmission of image-forming information encoded within optical coherence functions. The scheme makes use of dynamic random-valued encoding-decoding masks placed in the input-output planes of any linear optical system. The mask transmittance functions are complex conjugates of each other, as opposed to a similar coherence encoding scheme proposed earlier by two of this paper's authors that used identical masks. [Rhodes and Welch, in Euro-American Workshop on Optoelectronic Information Processing, SPIE Critical Review Series (SPIE, 1999), Vol. CR74, p. 1]. General analyses of the two coherence encoding schemes are performed by using the more general mutual coherence function as opposed to the mutual intensity function used in the earlier scheme. The capabilities and limitations of both encoding schemes are discussed by using simple examples that combine the encoding-decoding masks with free-space propagation, passage through a four-f system, and a single-lens imaging system.     

Spatial correlation properties and correlation vortices of partially coherent vortex beams diffracted by an aperture

Taking the Gaussian–Schell model vortex beam as a typical example of partially coherent vortex beams, the spatial correlation properties and correlation vortices of partially coherent vortex beams diffracted by an aperture are studied. It is shown that the off-axis displacement and spatial coherence affects the spectral degree of coherence. The number and position of correlation vortices depend on the off-axis displacement, spatial coherence, aperture truncation and propagation distance, where the effect of aperture diffraction on the correlation vortices is stressed. The number of correlation vortices decrease as the truncation parameter increases. The correlation vortices in the diffracted field result from the vortex embedded in partially coherent beams at the source plane rather than from the aperture diffraction. The correlation vortices in the diffracted field appear even when the vortex core is stopped by the aperture.     

基于HFSS的孔阵机壳近场屏蔽效能分析

机载电子设备之间的间距很小,而且通常有用于散热的功能性孔阵,容易造成电磁泄漏,必须进行近场屏蔽效能分析.对比分析了采用有限元法的HFSS和传输线法计算孔阵近场屏蔽效能的结果,理论分析和计算结果均表明,传输线法不适合用于近场屏蔽效能计算.基于HFSS仿真分析,研究了相关参数对近场屏效的影响规律,提出了提高近场屏效的设计方...     

基于地震模型土-不规则建筑-AMTMD相互作用系统的最优性能

基于地震模型下建立的土-不规则建筑-主动多重调谐质量阻尼器(AMTMD)相互作用系统的动力方程,定义AMTMD的最优参数准则为:设置AMTMD结构的最大平动和扭转角位移动力放大系数的最小值的最小化。定义AMTMD的有效性准则为:设置AMTMD结构的最大位移动力放大系数的最小值的最小化与未设置AMTMD结构的最大位移动力放大系数的比值。基于定义的二个准则,研究不规则建筑标准化偏心系数(NER)、扭转对侧向频率比(TTFR)、土对结构的刚度比(SRSS)和地震卓越频率系数(EDFR)对AMTMD最优频率间隔和有效性的影响,为软土地基不规则建筑的地震振动控制设计与实现提供参考。     

Spatial coherence of the nonlinearly generated second harmonic portion of backscatter for a clinical imaging system

Correlation-based approaches to phase aberration correction rely on the spatial coherence of backscattered signals. The spatial coherence of backscatter from speckle-producing targets is predicted by the auto correlation of the transmit apodization (Van Cittert-Zernike theorem). Work by others indicates that the second harmonic beam has a wider mainlobe with lower sidelobes than a beam transmitted at 2f. The purpose of this paper is to demonstrate that the spatial coherence of backscatter for the second harmonic is different from that of the fundamental, as would be anticipated from applying the Van Cittert-Zernike theorem to the reported measurements of the second harmonic field. Another objective of this work is to introduce the concept of the effective apodization and to verify that the effective apodization of the second harmonic is narrower than the transmit apodization. The spatial coherence of backscatter was measured using three clinical arrays with a modified clinical imaging system. The spatial coherence results were verified using a pseudo-array scan in a transverse plane of the transmitted field with a hydrophone. An effective apodization was determined by backpropagating these values using a linear angular spectrum approach. The spatial coherence for the harmonic portion of backscatter differed systematically and significantly from the auto correlation of the transmit apodization.     

Harmonic spatial coherence imaging: an ultrasonic imaging method based on backscatter coherence

We introduce a harmonic version of the short-lag spatial coherence (SLSC) imaging technique, called harmonic spatial coherence imaging (HSCI). The method is based on the coherence of the second-harmonic backscatter. Because the same signals that are used to construct harmonic B-mode images are also used to construct HSCI images, the benefits obtained with harmonic imaging are also obtained with HSCI. Harmonic imaging has been the primary tool for suppressing clutter in diagnostic ultrasound imaging, however secondharmonic echoes are not necessarily immune to the effects of clutter. HSCI and SLSC imaging are less sensitive to clutter because clutter has low spatial coherence. HSCI shows favorable imaging characteristics such as improved contrast-to-noise ratio (CNR), improved speckle SNR, and better delineation of borders and other structures compared with fundamental and harmonic B-mode imaging. CNRs of up to 1.9 were obtained from in vivo imaging of human cardiac tissue with HSCI, compared with 0.6, 0.9, and 1.5 in fundamental B-mode, harmonic B-mode, and SLSC imaging, respectively. In vivo experiments in human liver tissue demonstrated SNRs of up to 3.4 for HSCI compared with 1.9 for harmonic B-mode. Nonlinear simulations of a heart chamber model were consistent with the in vivo experiments.     

A collocation mesh-free method based on multiple basis functions

Two dominant shape functions are used to approximate scattered points in mesh-free methods, e.g. the interpolating radial basis function (RBF) and the approximating moving least squares (MLS). In the present paper, a new shape function is developed as a linear interpolating function of both MLS and RBF. This function inherits the properties of both MLS and RBF and is regularized by a control parameter μ, which takes different values in the domain [0,1]. Based on the proposed shape function, the collocation method is applied to solve initial and boundary value problems in one and two dimensions. The present method gives good results and achieves good convergence trends for different values of μ, compared with MLS and RBF individually, for a large number of nodes.     

Automated interferogram analysis based on an integrated expert system

Interferometric data, either from single-frame fringe-tracking and Fourier-transform techniques or from multiframe phase-shifting techniques, pose a problem of 2π ambiguity, that is, wrapped-phase information. As the degree of noise level increases, especially in high-speed aerodynamics, these techniques encounter difficulties in phase extraction to provide continuous unwrapped-phase information. Here, a new hybrid approach, called the integrated expert system, which is developed primarily for aerodynamic interferogram evaluation, is presented. The integrated expert system utilizes interferometric-specific knowledge rules to compensate for the limitations associated with conventional techniques. It integrates in a single structure an expert system and algorithmic programming to provide, as much as possible, a unified approach for all the interferogram evaluation techniques. This initial attempt may provide a useful groundwork for future development in intelligent interferogram processing.     

基于有限单元法的多间隙耦合齿轮传动系统非线性动态特性分析

针对齿轮传动系统的动态传递误差、单双边冲击状态、脱齿、拍击及混沌现象等复杂非线性动力学问题,在同时考虑齿侧间隙、轴承间隙、时变啮合刚度及齿面摩擦等非线性特性的基础上,首次提出一种基于有限单元法的多间隙耦合齿轮传动系统的非线性动态特性分析方法。以某单级斜齿轮传动系统为例,利用大型通用有限元分析软件AN-SYS/LS-DYNA建立耦合系统动力学模型,分析支撑状态下耦合系统的非线性动态特性,研究了不同转速及负载力矩对耦合系统非线性动态特性的影响规律。结果表明有限单元法能在满足高精度分析的条件下求解各种复杂工况的齿轮系统非线性动力学问题,为进一步研究齿轮传动系统非线性动力学问题提供有力工具。     

Spatial coherence of forward-scattered light in a turbid medium

We study spatially coherent forward-scattered light propagating in a turbid medium of moderate optical depth (0-9 mean free paths). Coherent detection was achieved by using a tilted heterodyne geometry, which desensitizes coherent detection of the attenuated incident light. We show that the degree of spatial coherence is significantly higher for light scattered only once in comparison with that for multiply scattered light and that it approaches a small constant value for large numbers of scattering events.     

干涉合成孔径声纳新体制研究

提出一种干涉合成孔径声纳新体制：采用多个发射阵和一条接收阵的收发阵结构,通过对正交回波信号进行分离和分别成像处理得到多幅来自不同收发路径的合成孔径图像,将这些合成孔径图像进行干涉处理,实现了干涉合成孔径三维声成像;论证了新体制的可行性;论述了新体制相对于已有体制的优越性。     

Experimental investigation of the performance of an annular aperture and a circular aperture on the same very-small-aperture laser facet

A very-small-aperture laser (VSAL) with a circular aperture has a trade-off between the spot size and the output power. A nanometric annular aperture is fabricated to overcome this difficulty. The advantages of the annular aperture are demonstrated by measuring and comparing its near-field intensity distribution with that of a circular aperture. These apertures are fabricated on the same VSAL to ensure that they are under the same illumination conditions. The experimental results indicate that an annular aperture produces a smaller spot size and a higher peak intensity than a circular aperture. The confinement effect and the enhancement effect are attributed to the convergence of the power flow that passes through the annular aperture. The observed enhancement effect decreases when the distance from the VSAL facet is increased, but it does not vanish even when the distance is as large as 3.5 microm.     

Simultaneous defocusing of the aperture and medium on a spectroholographic storage system

In a spectroholographic storage system the defocusing method is often used to obtain spectrum uniformity and improve the quality of the recorded information. However, defocusing introduces vignette and stronger interpixel cross talk in the marginal field of view. We report a method that defocuses the aperture and medium together. Based on the pixel spread function, two inequalities are introduced to estimate the upper and lower bounds of the energy received at the CCD. We balance the spectrum uniformity with interpixel cross talk and vignette and then allow the designer to select optimal structure values of the defocusing spectroholographic storage system, i.e., the defocusing value, aperture size, and fill factors for the spatial light modulator and CCD.     

A synthetic aperture telescope based on a pair of gratings

A synthetic aperture is an effective approach to enhancement of the resolution of telescopes. In this paper, a new method to improve the resolution by using a pair of gratings is proposed. It allows collection of parts of the light diffracted from the object which could not previously reach the imaging device. This method improves the resolution and light energy utilization ratio of telescopes without introducing new chromatic aberration. An experiment for resolution testing was carried out and the feasibility and availability of the method were verified.     

多维自回归(AR)模型计算地震动空间相干函数的适用性研究

传统周期图法求解地震动相干函数时需要进行功率谱的人为加窗平滑,且仅能考虑记录两两之间的空间相关性.针对以上缺陷,该研究采用多维自回归(autoregressive,AR)模型代替周期图法进行相干函数计算,并从多个方面对其合理性加以评估和验证.首先以中国台湾SMART-1台阵数据为算例,对比AR模型以及周期图法计算相干函...     

Fractal finite element method based shape sensitivity analysis of multiple crack system

This paper presents fractal finite element based continuum shape sensitivity analysis for a multiple crack system in a homogeneous, isotropic, and two dimensional linear-elastic body subjected to mixed-mode (modes I and II) loading conditions. The salient feature of this method is that the stress intensity factors and their derivatives for the multiple crack system can be obtained efficiently since it only requires an evaluation of the same set of fractal finite element matrix equations with a different fictitious load. Three numerical examples are presented to calculate the first-order derivative of the stress intensity factors or energy release rates.     

基于统计能量法的排气管道系统的振动和噪声分析与研究

基于统计能量分析(SEA)原理,结合压缩机排气管道系统的实际结构尺寸建立了SEA模型。采用理论及经验结果讨论了统计能量分析法三个重要参数确定,利用现场测试壳体加速度来确定输入功率。运用AutoSEA2004仿真软件预测了距离管道系统外部一定距离的声压级,并与现场测试所得数据进行了对比分析,结果表明,预测管道系统的总声压级为102.7dB(A),比实测结果低4.2dB(A),在500 Hz以上主要频段上,预测曲线与实测曲线吻合较好,为管道系统噪声治理提供了数值仿真依据。     

Effects of misalignment errors on the optical transfer functions of synthetic aperture telescopes

A segmented or diluted aperture optical system will undergo phase errors as a result of errors in the positioning of the segments. The errors associated with a segmented primary mirror limit the image quality obtainable with synthetic aperture telescopes. Here we study the effects of segmentation errors on image quality, considering both the phase angle and the amplitude of the optical transfer function (OTF). We show that, in these kinds of telescope, phasing and alignment errors among segments reduce the amplitude and distort the phase angle of the OTF.