New approaches to the linear propagation of acoustic fields

New algorithms are described that provide insight into linear field propagation and offer significant reductions in computational complexity. The developments presented here include the usage of a recently developed discrete Hankel transform to implement two single step, planar propagation algorithms for baffled, radially symmetric, acoustic pressure or velocity fields; an update on the single step approaches that reduce computational complexity through geometrically determined spatial frequency limitations; and algorithms for extending to multistep propagation. Two equivalent means of introducing arbitrary medium attenuation into the above schemes are presented. Finally, a planar boundary crossing algorithm that accounts for refraction and reflection (but not multiple reflections) is added to one of the multistep propagating algorithms. The resulting algorithm is then used to examine the differences between the corresponding fields of a focused piston source operating in water and in a layered fat/liver (biomedical imaging) medium. The results yield computationally efficient algorithms that can be used for linear propagation of focused or unfocused beams in attenuating, multilayer media, and also provide the basis for a novel nonlinear propagation algorithm.     

Transmission mode time-reversal super-resolution imaging

The theory of time-reversal super-resolution imaging of point targets embedded in a reciprocal background medium [A. J. Devaney, "Super-resolution imaging using time-reversal and MUSIC," J. Acoust. Soc. Am. (to be published)] is generalized to the case where the transmitter and receiver sensor arrays need not be coincident and for cases where the background medium can be nonreciprocal. The new theory developed herein is based on the singular value decomposition of the generalized multistatic data matrix of the sensor system rather than the standard eigenvector/eigenvalue decomposition of the time-reversal matrix as was employed in the above-mentioned work and other treatments of time-reversal imaging [Prada, Thomas, and Fink, "The iterative time reversal process: Analysis of the convergence," J. Acoust. Soc. Am. 97, 62 (1995); Prada et al., "Decomposition of the time reversal operator: Detection and selective focusing on two scatterers," J. Acoust. Soc. Am. 99, 2067 (1996)]. A generalized multiple signal classification (MUSIC) algorithm is derived that allows super-resolution imaging of both well-resolved and non-well-resolved point targets from arbitrary sensor array geometries. MUSIC exploits the orthogonal nature of the scatterer and noise subspaces defined by the singular vectors of the multistatic data matrix to form scatterer images. The time-reversal/MUSIC algorithm is tested and validated in two computer simulations of offset vertical seismic profiling where the sensor sources are aligned along the earth's surface and the receiver array is aligned along a subsurface borehole. All results demonstrate the high contrast, high resolution imaging capabilities of this new algorithm combination when compared with "classical" backpropagation or field focusing. Above and beyond the application of seismo-acoustic imaging, the time-reversal super-resolution theory has applications in ocean acoustics for target location, and ultrasonic nondestructive evaluation of parts.     

Error control in the perfectly matched layer based multilevel fast multipole algorithm

The development of efficient algorithms to analyze complex electromagnetic structures is of topical interest. Application of these algorithms in commercial solvers requires rigorous error controllability. In this paper we focus on the perfectly matched layer based multilevel fast multipole algorithm (PML-MLFMA), a dedicated technique constructed to efficiently analyze large planar structures. More specifically the crux of the algorithm, viz. the pertinent layered medium Green functions, is under investigation. Therefore, particular attention is paid to the plane wave decomposition for 2-D homogeneous space Green functions in very lossy media, as needed in the PML-MLFMA. The result of the investigations is twofold. First, upper bounds expressing the required number of samples in the plane wave decomposition as a function of a preset accuracy are rigorously derived. These formulas can be used in 2-D homogeneous (lossy) media MLFMAs. Second, a more heuristic approach to control the error of the PML-MLFMA’s Green functions is presented. The theory is verified by means of several numerical experiments.     

Neural networks and chaos: construction, evaluation of chaotic networks, and prediction of chaos with multilayer feedforward networks

Many research works deal with chaotic neural networks for various fields of application. Unfortunately, up to now, these networks are usually claimed to be chaotic without any mathematical proof. The purpose of this paper is to establish, based on a rigorous theoretical framework, an equivalence between chaotic iterations according to Devaney and a particular class of neural networks. On the one hand, we show how to build such a network, on the other hand, we provide a method to check if a neural network is a chaotic one. Finally, the ability of classical feedforward multilayer perceptrons to learn sets of data obtained from a dynamical system is regarded. Various boolean functions are iterated on finite states. Iterations of some of them are proven to be chaotic as it is defined by Devaney. In that context, important differences occur in the training process, establishing with various neural networks that chaotic behaviors are far more difficult to learn.     

激光跟踪测量系统自校正方法研究

建立了测量系统运动学模型。针对以往在球坐标激光跟踪系统中参考点只能借助于外部更高精度的仪器来校正的缺点,提出了平面约束自校正方法,并对系统进行了自校正研究。仿真结果表明了该方法的有效性。     

Vector-phase characteristics of acoustic fields: Statistical analysis and measurement algorithms

Statistical characteristics of an acoustic field that are based on the data of simultaneous measurements of pressure and acoustic velocity vector are investigated. Conditions of the formation of vector-phase characteristics of acoustic field are formulated in relation to the dispersion properties of the medium. Crosscorrelation functions of the components of the vector field are presented. Expressions for the characteristic functionals of vector-phase relationships in acoustic fields and, in particular, for the acoustic energy flux are derived with the use of functional methods. Algorithms of space-time processing of the energy flux vector and the optimum measurement algorithm for a Gaussian vector-phase field are considered. The signal-to-noise ratio is determined as the quality index of vector reception algorithms, and its relation to the corresponding parameter of scalar pressure field measurements is revealed. Indices of relative efficiency of vector algorithms are determined depending on the dispersion characteristics of the medium (the flux algorithm) and the dimension of the input vector of observations (the optimum algorithm).     

基于双核DSP的点目标立体视觉定位

介绍了立体视觉系统对点目标的四种定位算法:齐次方法,直接解正规方程的非齐次方法,利用广义逆求解的非齐次方法,解析几何方法。提出了基于双核DSP的并行交互运行流程以实现对目标的2D位置校正和3D定位,通过实验分析了以上四种定位算法的实时性和精度。实验结果表明,采用直接求解正规方程的非齐次方法处理一帧含有4个点目标的图像所带来的延迟仅为9.21ms,定位精度为0.15mm RMS,系统的运行速度和精度可以得到兼顾。     

New method for calculating the spectra and radiation losses of leaky waves in multilayer optical waveguides

The efficiency of a new method for calculating the spectrum and attenuation coefficient of leaky electromagnetic modes is demonstrated with multilayer planar optical waveguides the guiding properties of which are determined by antiresonant reflection from the multilayer cladding (antiresonant reflecting optical waveguides) rather than by total internal reflection from the core-cladding interface as in standard optical waveguides. The new method applies to calculation of both electromagnetic modes in dielectric waveguides and electron quantum states in multibarrier semiconductor heterostructures. The characteristics of multilayer waveguides calculated by the new method are compared with published data obtained from a complex dispersion relation by the transfer matrix method. As an example, the wavelength dependence of the radiation losses for the first TE mode of a planar optical waveguide containing 52 pairs of layers is calculated.     

多层膜体折射率弱不均匀度反演计算的近似模型及性能分析

实际应用中的薄膜或多或少存在体折射率不均匀缺陷,其准确表征对于镀膜工艺参数调校、低损耗薄膜设计与制备分析等具有影响。从Schr-der近似出发,推导了体折射率弱不均匀薄膜的膜系特征矩阵,通过特征矩阵法推广,建立了体折射率弱不均匀多层膜光谱特性计算的近似模型。利用基于有效介质理论的多层均匀膜近似方法,讨论了上述近似模型的有效性、计算精度和时间消耗特性。结果表明,上述模型矩阵计算中引入体折射不均匀度这一参数,为多层膜体折射率不均匀缺陷反演提供了一种有效的手段,为基于宽带光谱测量数据拟合的膜层缺陷数值反演应用奠定了基础。     

基于光谱分类的端元提取算法研究

目前成熟的端元提取算法是基于单形体几何学的像元纯度指数(PPO)算法,N-FINDR,VCA等算法.这些算法从图像所有像元中提取纯光谱,具有提取速度慢、精度不高的缺点;部分算法需要进行光谱降维,不利于小目标信息的提取.该文提出先利用基于空间特征的光谱分类算法进行分类,将格个图像划分成空间相邻、光谱相似的若干类,每一类的...     

Dyadic Green Function for an Electromagnetic Medium Inspired by General Relativity

The dyadic Green function for a homogeneous electromagnetic medium inspired by the spatiotemporally nonhomogeneous constitutive equations of gravitationally affected vacuum is derived.     

加速神经网络势能面的构建:一种杂化的训练算法

Machine learning approaches have been promising in constructing high-dimensional potential energy surfaces (PESs) for molecules and materials. Neural networks (NNs) are one of the most popular such tools because of its simplicity and efficiency. The training algorithm for NNs becomes essential to achieve a fast and accurate fit with numerous data. The Levenberg-Marquardt (LM) algorithm has been recognized as one of the fastest and robust algorithms to train medium sized NNs and widely applied in recent NN based high quality PESs. However, when the number of ab initio data becomes large, the efficiency of LM is limited, making the training time consuming. Extreme learning machine (ELM) is a recently proposed algorithm which determines the weights and biases of a single hidden layer NN by a linear solution and is thus extremely fast. It, however, does not produce sufficiently small fitting error because of its random nature. Taking advantages of both algorithms, we report a generalized hybrid algorithm in training multilayer NNs. Tests on H+H₂ and CH₄+Ni(111) systems demonstrate the much higher efficiency of this hybrid algorithm (ELM-LM) over the original LM. We expect that ELM-LM will find its widespread applications in building up high-dimensional NN based PESs.     

基于空间特征的光谱分类算法研究

随着成像光谱仪器的广泛应用,利用数据立方体进行物质分类与识别成为一项重要的研究内容,分类算法对最终的目标识别准确度与精度具有很大的决定作用。目前常见的分类算法主要利用了光谱维信息,从光谱匹配的角度进行物质分类。由于仪器探测的物质反射光谱不仅反映了物质种类,还与物质表面的几何结构,表面粗糙度等有关,因此仅仅利用物质的反射光谱进行物质分类识别具有一定的误差。该文在利用可见光反射光谱进行分类的基础上,结合图像空间特征,对分类过程进行控制,达到提高分类准确度的目的。利用该分类算法进行真假叶片识别,结果表明其具有较好的空间连续性,很大程度上克服了"麻点"效应,验证了算法的有效性。     

Measurement of acoustic absorption coefficient with phase-conjugate ultrasonic waves

Experimental results on measurements of the acoustic absorption coefficient in test objects that were obtained with two methods, i.e., a standard insert-substitution method and a modification thereof using phase-conjugate waves, are given. Samples of gelatin and biological tissue in vitro (porcine muscle fibers) were used as test objects. Gelatin objects were manufactured that were both homogeneous and with inhomogeneities in the form of a rough surface or inclusions (air bubbles) distributed over the volume. A rough surface leads mainly to phase distortions of a probe beam, while bubble inclusions cause additional field scattering. For all homogeneous samples, both compared methods produce identical results. In the case of inhomogeneous samples including biological tissues, absorption measurement by a standard method may lead to significant errors. It is demonstrated that the use of properties of phase-conjugate waves provides an opportunity to eliminate almost completely the measurement error connected with phase distortions and reduce the error in the case of a medium with scatterers.     

平面自旋压缩态的产生与原子干涉的机理

在玻色-爱因斯坦凝聚体中实现自旋压缩和量子纠缠, 对于提高原子干涉测量相位灵敏度和原子钟精度有着非常重要的意义. 基于一种新的平面自旋分量的不确定性关系, 介绍了如何利用两分量玻色-爱因斯坦凝聚系统中原子间相互作用提供的非线性效应和原子内部能级间线性耦合, 实现量子平面自旋压缩(挤压)和模式纠缠. 描述了一项关于平面压缩态的理论工作, 该工作利用哈密顿量的精确对角化求解系统基态, 优化非线性作用和线性耦合强度比值, 使得包含平均自旋方向在内的两个正交自旋分量的不确定度同时压缩, 因此在平面上所有相位角度的涨落都受到压制, 而在与该平面垂直的第三个自旋分量方向反压缩. 利用传统自旋压缩判定纠缠, 只能判断多个不可分辨的原子处于纠缠态, 而平面自旋压缩可以检测两个可区分模式(比如, 原子内态)间的纠缠, 从而在不同模式间进行量子信息处理. 同时, 为实现超越标准量子极限的原子干涉相位精密测量, 传统方式是利用单个自旋分量压缩, 但需要对待测相位角度有很好的估计, 或者可以进行多次测量以逐渐逼近可获得的最大压缩极限, 这就要求量子态可以被精确的重复制备. 而利用平面自旋压缩, 对任意未知相位角度只需要测量两个垂直自旋分量就可以实现高的相位测量灵敏度.     

基于脉冲时延估计的荧光寿命表征方法

荧光检测和分析技术中,荧光寿命的精确测量具有重要意义。针对纳秒级荧光寿命在流式细胞分析系统中无法直接测量的问题,提出一种基于互相关算法的脉冲时延估计荧光寿命表征方法。该方法将改进的线性调频Z变换算法与相关峰内插算法相结合,并与标准FFT算法仿真做对比,通过对哺乳动物细胞的荧光寿命进行测量,将所得数据进行处理,验证线性调频Z变换(MCZT)和相关峰内插(FICP)算法性能。实验结果表明,使用该算法降低了FFT计算带来的栅栏效应,可以提高互相关函数的分辨能力,测量荧光寿命的相对误差提高了4.344 3%。     

The motion of a tagged particle and nonhomogeneous media in R1

We analyze the asymptotic behavior of a tagged particle inside an infinite system of identical elastic point masses. The main objective is to study very nonhomogeneous media—particles which are more and more dispersed far from the origin. We suggest that the limit motion of a tagged particle may serve to classify media in the nonhomogeneous case as well as in the homogeneous case.     

Multilayer optical disc system using homodyne detection

A write/read system using high-productivity multilayer optical discs was developed. The recording medium used in the system consists of planar recording layers and a separated guide layer, and is fabricated by web coating and lamination process. The recording layers in the medium are made of one-photon-absorption material, on which data can be recorded with a normal laser diode. The developed system is capable of focusing and tracking on the medium and amplifying readout signals by using phase-diversity homodyne detection. A highly layer-selective focusing method using homodyne detection was also proposed. This method obtains stable focus-error signals with clearly separated S-shaped curves even when layer spacing is quite narrow, causing large interlayer crosstalk. Writing on the medium and reading with the signal amplification effect of homodyne detection was demonstrated. In addition, the effectiveness of the method was experimentally evaluated.     

分布式反馈激光器动态扫描光纤布拉格光栅波长解调系统

通过对该系统采集到的法布里-珀罗(F-P)标准具透射谱和光纤布拉格光栅(FBG)传感器反射谱进行寻峰算法以及拟合算法的研究,采用C语言编程和LabVIEW编程相结合的方式,实现了FBG波长信号的解调。其中,由于系统采集到的F-P透射谱和FBG反射谱线时域信号数据都由离散点构成,且在3dB带宽内均符合高斯曲线分布,采用高斯拟合对采集到的信号数据进行寻峰处理,提高系统精度;又由于分布式反馈(DFB)激光器的波长扫描存在着一定的非线性,采用二项式拟合对DFB激光器的波长扫描曲线进行拟合,以降低其非线性导致的误差。另外,设置一路标准FBG传感通道用于波长校准。实验研究表明该系统稳定性良好,波长测量范围为1 550.012~1 554.812nm,分辨力为1pm,精度为±10pm,验证了该系统可用于FBG波长信号检测的可行性。