相控阵高强度聚焦超声高阵元驱动效率相位独立控制方法

针对如何简化上百阵元高强度聚焦超声相控阵控制模块的问题,本文提出高阵元驱动效率的相位独立控制方法。利用该方法计算了一种256阵元中心开孔球面相控阵的聚焦声场,并与传统的幅度相位综合控制方法进行了分析和比较。计算和实验结果表明该方法在简化相控阵驱动控制模块、提高阵元驱动效率的同时,可以有效实现相控阵的空间三维单焦点聚焦。由于阵元驱动控制的简化而引起的焦域能量偏差,可以通过适当调整相控阵输入总声功率进行有效的补偿。      

Optimization of sparse synthetic transmit aperture imaging with coded excitation and frequency division

An effective aperture approach is used for optimization of a sparse synthetic transmit aperture (STA) imaging system with coded excitation and frequency division. A new two-stage algorithm is proposed for optimization of both the positions of the transmit elements and the weights of the receive elements. In order to increase the signal-to-noise ratio in a synthetic aperture system, temporal encoding of the excitation signals is employed. When comparing the excitation by linear frequency modulation (LFM) signals and phase shift key modulation (PSKM) signals, the analysis shows that chirps are better for excitation, since at the output of a compression filter the sidelobes generated are much smaller than those produced by the binary PSKM signals. Here, an implementation of a fast STA imaging is studied by spatial encoding with frequency division of the LFM signals. The proposed system employs a 64-element array with only four active elements used during transmit. The two-dimensional point spread function (PSF) produced by such a sparse STA system is compared to the PSF produced by an equivalent phased array system, using the Field II simulation program. The analysis demonstrates the superiority of the new sparse STA imaging system while using coded excitation and frequency division. Compared to a conventional phased array imaging system, this system acquires images of equivalent quality 60 times faster, when the transmit elements are fired in pairs consecutively and the power level used during transmit is very low. The fastest acquisition time is achieved when all transmit elements are fired simultaneously, which improves detectability, but at the cost of a slight degradation of the axial resolution. In real-time implementation, however, it must be borne in mind that the frame rate of a STA imaging system depends not only on the acquisition time of the data but also on the processing time needed for image reconstruction. Comparing to phased array imaging, a significant increase in the frame rate of a STA imaging system is possible if and only if an equivalent time efficient algorithm is used for image reconstruction.     

Maximum repeaterless transmission of lightwave systems imposed by stimulated Brillouin scattering in fibres

To aid in designing high-speed optical networks, the fundamental limitations of lightwave transmission systems are investigated from the viewpoint of fibre nonlinearities, particularly of stimulated Brillouin scattering (SBS). When SBS occurs, the fibre launch power is usually restricted; thus, SBS is detrimental to lightwave systems. The effects of SBS on signals transmitted in fibres are clarified for several modulation schemes, especially coherent modulation schemes. A generalized model based on the maximum power spectrum density in the signals is proposed to estimate the degree of SBS generation. The maximum repeaterless transmissions for various bit rates, laser diode linewidths, and Brillouin gains of optical fibres are presented. The degradation of transmission due to SBS is clarified experimentally, focusing on the bit error rate, fibre input power, and spectra of signals transmitted in fibres. In addition, various techniques for suppressing the SBS effects are proposed. A repeaterless transmission experiment conducted over 364.3 km using optical booster amplifiers is presented to verify the effectiveness of the SBS suppression technique.     

A post-compression based ultrasound imaging technique for simultaneous transmit multi-zone focusing

The compression error of post-compression based coded excitation techniques increases with decreasing f-number, which causes the elevation of side-lobe levels. In this paper, a post-compression based coded excitation technique with reduced compression errors through dynamic aperture control is proposed. To improve the near-field resolution with no frame rate reduction, the proposed method performs simultaneous transmit multi-zone focusing using two mutually orthogonal complementary Golay codes. In the proposed method, the two mutually orthogonal sequences of length 16 are simultaneously transmitted toward two different focal depths, which are separately compressed into two short pulses on receive after dynamic focusing is performed. After carrying out the same transmit-receive operation for the same scan line with the complementary set of the orthogonal Golay codes, a single scan line with two transmit foci is obtained.The computer simulation results using a linear array with a center frequency of 7.5 MHz and 60% 6 dB bandwidth show that the range side-lobe level can be suppressed below −50 dB, when f-number is maintained not smaller than 3. The performance of the proposed scheme for a smaller f-number of 2 was also verified through actual experiments using a 3.85 MHz curved linear array with 60% 6 dB bandwidth. Both the simulation and experimental results show that the proposed method provides improved lateral resolution compared to the conventional pre-compressed and post-compression based coded excitation imaging using Golay codes.     

基于模式分离的兰姆波逆时偏移成像

应用频率域逆时偏移方法实现各向同性和各向异性板中缺陷的兰姆波成像.由于缺陷引起的多模态散射信号会在重建图像中形成伪像,根据基本导波模式振动对称性的差别进行了模式分离预处理.基于多元阵列超声技术,开展了铝板和复合板内缺陷频率域逆时偏移超声成像方法的数值仿真研究.首先,建立有限元模型,采用环形传感器数值采集由缺陷引起的兰姆波散射信号,然后,将采集到的多模式散射信号进行模式分离处理,再将模式分离后的兰姆波散射信号经时间反转后并在相应的接收器处重新激励,在频域中运用格林函数反向传播兰姆波散射信号,获取监测区域的声场信息,与正向传播声场进行互相关,重建缺陷图像.首先对铝板中单缺陷以及复合材料板中相邻的两个相同缺陷进行数值仿真,对比有无模式分离处理的缺陷逆时偏移成像效果,体现出模式分离的重要性.在此基础上,采用逆时偏移方法对复合板材内位置邻近、深度不同的双缺陷进行识别.数值结果表明,模式分离预处理后的缺陷重建图像能够有效去除多模式干扰产生的伪像.文中提出的成像方法对各向同性板和各向异性板内缺陷的检测和成像具有很好的发展潜力,可以准确地探测多个缺陷的形状、尺寸和深度.     

Nonlinear elastic imaging using reciprocal time reversal and third order symmetry analysis

This paper presents a nonlinear imaging method for the detection of the nonlinear signature due to impact damage in complex anisotropic solids with diffuse field conditions. The proposed technique, based on a combination of an inverse filtering approach with phase symmetry analysis and frequency modulated excitation signals, is applied to a number of waveforms containing the nonlinear impulse responses of the medium. Phase symmetry analysis was used to characterize the third order nonlinearity of the structure by exploiting its invariant properties with the phase angle of the input waveforms. Then, a "virtual" reciprocal time reversal imaging process, using only one broadcasting transducer and one receiving transducer, was used to insonify the defect taking advantage of multiple linear scattering as mode conversion and boundary reflections. The robustness of this technique was experimentally demonstrated on a damaged sandwich panel, and the nonlinear source, induced by low-velocity impact loading, was retrieved with a high level of accuracy. Its minimal processing requirements make this method a valid alternative to the traditional nonlinear elastic wave spectroscopy techniques for materials showing either classical or non-classical nonlinear behavior.     

基于Fizeau干涉仪的激光雷达测温方法研究

气温是描述大气状态的基本参数之一,温度的准确测量对天气预报、气候预测及其他气象参数的反演都至关重要。激光雷达作为一种遥感仪器,已经用于气象要素的探测中（风、温度、气溶胶的光学厚度等）。目前,测温激光雷达主要有拉曼激光雷达（振动和转动）、共振荧光激光雷达和Rayleigh散射激光雷达等,拉曼激光雷达需要大功率的激光器和复杂的背景滤波器;共振荧光激光雷达无法探测平流层内的温度;基于Rayleigh散射的测温激光雷达多应用于温度的相对测量,反演温度时需要建立响应函数和校准程序;基于固体腔扫描F-P干涉仪测量大气Rayleigh散射光谱来反演温度的方法,时间分辨率较低,并且该方法在测量过程中需要运动部件,所以不利于星载。在大气低层,分子的Rayleigh散射光谱会受到Brillouin散射的影响,两种散射信号叠加形成的Rayleigh-Brillouin散射光谱不再服从Gaussian分布,直接通过测量散射光谱的半高全宽来反演温度,会产生误差。基于回波能量的方法会受到气溶胶Mie散射信号的影响,所以在对流层中该方法并不适用。为了实现对流层内温度的高精度和高时间分辨率的测量,提出利用Fizeau干涉仪和PMT阵列对对流层内分子的Rayleigh-Brillouin散射光谱进行测量,并通过插值的方法来对回波信号中气溶胶Mie散射信号进行抑制,从而使Mie散射信号对温度反演的影响较小,最后将测量光谱和理论光谱进行全光谱匹配来实现温度的反演。除此之外,还对Fizeau干涉仪的自由光谱区、固体腔几何长度、腔体反射率、扫描间隔等参数进行了优化设计。为了验证本文提出方法的可行性,利用Matlab软件建立了一套仿真模型,通过模拟表明,在不考虑云、风和水汽含量的影响时,利用该方法测量对流层内的大气温度时,测量误差小于1 K。该测温方法可以对对流层内的大气温度廓线实现高精度、高时间分辨率的测量, 在测量过程中不需要使用运动部件,有较高的使用价值,并对同类高光谱激光雷达分光系统的研究具有借鉴意义, 为我国高光谱激光雷达陆基及星载应用提供了一套可行的技术方案和温度反演方法。     

Scholte波识别水池缩比实验

该文基于简正波分离识别Scholte波的方法,分析了之前Scholte波识别水池缩比实验结果中出现较大误差的原因,提出从声场中优先提取其他阶简正波的方法来改善实验结果、降低实验误差,并证实增加合成孔径垂直阵采样点数至119点（一次采样期间首末阵元信号保持良好相关性）,有助于改善水池Scholte波识别误差,从而使水池Scholte波识别缩比使实验结果与理论值更为接近。水池缩比实验的结果证明该方法改善了Scholte波识别实验的结果,降低了声源深度较浅、Scholte波激发较弱时的实验误差,使水池缩比实验简正波分离识别Scholte波的方法更加可靠。     

Correlation-based imaging technique using ultrasonic transmit–receive array for Non-Destructive Evaluation

This paper describes a novel array post-processing method for Non-Destructive Evaluation (NDE) using phased-array ultrasonic probes. The approach uses the capture and processing of the full matrix of all transmit–receive time-domain signals from a transducer array as in the case of the Total Focusing Method (TFM), referred as the standard of imaging algorithms. The proposed technique is based on correlation of measured signals with theoretical propagated signals computed over a given grid of points. In that case, real-time imaging can be simply implemented using discrete signal product. The advantage of the present technique is to take into account transducer directivity, dynamics and complex propagation patterns, such that the number of required array elements for a given imaging performance can be greatly reduced. Numerical and experimental application to contact inspection of isotropic structure is presented and real-time implementation issues are discussed.     

焊缝散射对板中超声水平剪切导波直线合成孔径阵列成像的影响

为探讨焊缝散射与板中超声导波阵列成像之间的关系,提出了将散射渡越时间矩阵作为分析工具用来开展焊缝散射对板中超声水平剪切(Shear horizontal,SH)导波直线合成孔径阵列成像影响的研究。运用散射因子和散射渡越时间矩阵描述焊缝散射,结合分析超声SH导波直线阵列信号所特有的圆锥曲线特征和合成孔径成像算法,从导波阵列信号和阵列成像机理出发,重点研究了焊缝散射因子和散射渡越时间矩阵在阵列成像中的作用。结果显示,焊缝散射易造成同一成像散射体反射声波的阵列信号形成"多峰",焊缝散射因子随之同步变大,散射渡越时间矩阵的秩随之成倍数关系增大,导致导波图像相应出现对应倍数的"条带"伪像,图像椭圆簇轨迹特征复杂化,特别是图像背景噪声增幅可达50%,图像信噪比减小幅度可达10%。成像实验验证了理论分析的有效性,表明散射渡越时间矩阵是研究焊缝散射对超声导波阵列成像影响规律的有力工具,该影响规律的揭示可为深入开展焊接板结构超声导波阵列图像处理和图像识别提供基础。      

A method for the construction of hierarchical generalized space shift keying (GSSK) modulation for unequal error protection

In this paper, we propose a systematic method to achieve two-level unequal error protection (UEP) with generalized space shift keying (GSSK) modulation for multiple-input multiple-output (MIMO) systems. GSSK is a modulation scheme that encodes the source information in the antenna indices. To enable the nonuniform arrangement of the spatial constellation of GSSK modulation, new techniques are needed as new features of GSSK modulation are observed. The proposed method is based on first partitioning the GSSK constellation into subsets and then choosing constellation points inside a subset. The high-priority (HP) bits select the subset and the low-priority (LP) bits select constellation points inside a subset. The proposed subset partitioning procedure developed based on a theoretical study on set partitioning in the multidimensional space guarantees optimal or near-optimal HP protection, enables simple and systematic designs for LP protection, and increases the LP capacity. The proposed systematic design approach applies to systems with any numbers of transmit antennas and can create various hierarchical GSSK schemes with different HP/LP protection capabilities.     

256阵元高强度聚焦超声相控阵系统误差与多焦点模式精确控制

对一种256阵元中心开孔凹球面二维相控阵合成三维多焦点声场进行了系统的误差分析,总结了阵元激励信号的幅度和相位误差对声场参数的影响规律,为上百阵元相控阵驱动控制提供了设计容差依据.分析表明:上百阵元相控阵在声场合成能力方面具有很强的鲁棒性,5位相位量化精度足以保证合成声场的有效性,误差主要影响声场焦域能量的分布,通过提高相控阵发射总声功率等途径降低幅度误差百分比可以有效减弱固定方差的幅度误差的影响.     

Current CONtrolled Transmit And Receive Coil Elements (C2ONTAR) for Parallel Acquisition and Parallel Excitation Techniques at High-Field MRI

A novel intrinsically decoupled transmit and receive radio-frequency coil element is presented for applications in parallel imaging and parallel excitation techniques in high-field magnetic resonance imaging. Decoupling is achieved by a twofold strategy: during transmission elements are driven by current sources, while during signal reception resonant elements are switched to a high input impedance preamplifier. To avoid B(0) distortions by magnetic impurities or DC currents a resonant transmission line is used to relocate electronic components from the vicinity of the imaged object. The performance of a four-element array for 3 T magnetic resonance tomograph is analyzed by means of simulation, measurements of electromagnetic fields and bench experiments. The feasibility of parallel acquisition and parallel excitation is demonstrated and compared to that of a conventional power source-driven array of equivalent geometry. Due to their intrinsic decoupling the current-controlled elements are ideal basic building blocks for multi-element transmit and receive arrays of flexible geometry.     

Aspherical single-lens objective for radio-vision systems of the millimeter-wavelength range

The characteristics of a system for video imaging of objects in the 3-mm-wavelength range are presented. A calculation procedure and characteristics of an aspherical single-lens objective that is used for imaging (point scattering function, resolution, and field of vision) are presented. An 8 × 8 array of antenna elements that are loaded with detectors is used for the detection of signals in the image plane. A digital processing unit is used for the detection, control, and input-output of electric signals that form the image. The processing and data-transfer rates of the unit are presented. Test experiments on imaging of simple objects (one or several metal spheres with different diameters) are performed in the active radar regime using the proposed system and monochromatic signal.     

基于Si-PM阵列的辐射作用位置定位方法研究

在辐射成像中准确估计射线与探测器相互作用的空间位置是保障成像质量的关键步骤。为进一步提升辐射作用事件的定位效果,有效抑制重心法在辐射事件作用位置靠近探测器边缘时造成的定位偏移,针对Si-PM阵列构成的位置灵敏探测系统建立了基于响应函数的定位算法。搭建了基于CsI阵列与Si-PM阵列构成的位置灵敏探测器和基于ASIC的电子学读出系统,使用等效电阻网络简化了Si-PM阵列的输出信号数量, 通过实验获得了辐射事件在Si-PM阵列上的响应函数。实验结果表明,在同一条件下使用响应函数法在最边缘像素散点的FWHM仅为使用重心法获得FWHM的46.2%,与中央像素散点的FWHM相当,基于响应函数的辐射事件方法定位效果明显优于传统重心法的定位效果,可有效克服重心法的边缘效应。     

一种基于编码孔径成像原理的三维成像方法

三维成像技术因其应用广泛而备受关注。根据编码孔径成像的基本原理,提出了一种非相干可见光三维成像方法。这种两步成像方法的第一步采用空间位置编码的照相机阵列对物体拍照,在第二步中,首先将照相机阵列拍照得到的物体照片根据拍照时的位置关系合成为一幅图像,然后采用计算机程序模拟光学反投影解码方法解码再现出物体不同深度的表面分层图像。设计了初步的实验,该实验采用1部照相机依次在各编码位置对物体模型拍照,编码形式是包含9个点的无冗余阵列形式,物体模型只包含2个深度层次,布置在距离照相机阵列1.5m的地方。实验得到了信噪比较高的物体模型的分层解码图像,验证了这种三维成像方法的可行性。     

Integral cross sections for electron impact excitations of argon and carbon dioxide

Electron-impact excitation integral cross sections play an important role in understanding the energy transfer processes in many applied physics. Practical applications require integral cross sections in a wide collision energy range from the excitation threshold to several keV. The recently developed BE-scaling method is able to meet the demands of integral cross sections for dipole-allowed transitions while the prerequisite relies on the accurate generalized oscillator strengths. Fast electron and x-ray scatterings are the conventional experimental techniques to approach the generalized oscillator strengths, and the joint study by both methods can provide credible cross-checks. The validated generalized oscillator strengths can then be used to extrapolate optical oscillator strengths by fitting the data with the Lassettre formula. The fitted curve also enables the integration of generalized oscillator strengths over the whole momentum transfer region to obtain the BE-scaled integral excitation cross sections. Here, experimental measurements by both fast electron and x-ray scattering of argon and carbon dioxide are reviewed. The integral cross sections for some low-lying states are derived from the cross-checked generalized oscillator strengths for the first time. The integral cross sections presented in this paper are openly available at https://doi.org/10.11922/sciencedb.01466.     

基于改进空间频率域采样的天文光干涉望远镜阵列优化

提出一种基于改进空间频率域(UV)采样的阵列评价函数,用于长基线天文光干涉望远镜阵列几何结构的优化。该评价函数将UV采样区域沿径向和角度方向分别进行划分,统计划分所得区域中UV采样点数目并计算UV采样点密度,以UV采样点密度偏离理想高斯分布的大小作为评价依据。在具体的优化技术上,利用遗传算法的全局收敛特性,降低了传统算法对初始结构的依赖,采用该评价函数对6孔径望远镜阵列进行优化设计,并与国际主流天文光干涉阵列CHARA进行了性能对比。分析结果表明:优化所得Array-6阵列UV采样点密度分布具有径向连续覆盖和低频强调的特点,有利于对轮廓信息的恢复;双星模拟成像实验中Array-6阵列重构图像相对于原始图像的误差为21.34,相比CHARA阵列降低了18.16%,具有更高的成像质量。该优化算法具备优化大孔径数目阵列的能力,对于射电波段望远镜阵列的优化设计亦有一定的参考意义。     

Distributed imaging using an array of compressive cameras

We describe a distributed computational imaging system that employs an array of feature specific sensors, also known as compressive imagers, to directly measure the linear projections of an object. Two different schemes for implementing these non-imaging sensors are discussed. We consider the task of object reconstruction and quantify the fidelity of reconstruction using the root mean squared error (RMSE) metric. We also study the lifetime of such a distributed sensor network. The sources of energy consumption in a distributed feature specific imaging (DFSI) system are discussed and compared with those in a distributed conventional imaging (DCI) system. A DFSI system consisting of 20 imagers collecting DCT, Hadamard, or PCA features has a lifetime of 4.8× that of the DCI system when the noise level is 20% and the reconstruction RMSE requirement is 6%. To validate the simulation results we emulate a distributed computational imaging system using an experimental setup consisting of an array of conventional cameras.     

A purpose-built neck coil for black-blood DANTE-prepared carotid artery imaging at 7 T

Atherosclerotic plaques in the bifurcation of the carotid arteries can pose a significant health risk due to possible plaque rupture and subsequent stroke. The assessment of plaques, and evaluation of the risk they pose, can be performed with Black-Blood (BB) vessel wall magnetic resonance imaging. However, resolution at standard clinical field strengths (up to 3 T) is limited, hampering reliable assessment and diagnosis. The aim of this study was to investigate the benefits of 7 T MRI using a BB application that has been successful at clinical field strengths. Therefore, for BB imaging, each sequence was preceded with ‘Delay Alternating with Nutation for Tailored Excitation’ (DANTE) preparation pulses for blood signal suppression. A coil comprising a 4-channel Tx array was designed and built to provide the required excitation coverage for the DANTE train; and a 4-channel Rx array was constructed to target the carotid bifurcation. Human and phantom results showed satisfactory blood suppression and comparable SNR and CNR to 3 T, therefore demonstrating the feasibility of the application at 7 T. However, the imposed SAR restrictions led to long scan times and subsequent motion artifacts. Thus, more accurate local SAR supervision schemes are required which could lead to a further improvement of BB DANTE vessel wall imaging at 7 T.