被动迭代时间反转镜研究

抗除多途信道的干扰,优化被动检测性能是声纳信号处理关注的问题之一。通过仿真揭示了多途信道中单水听器时间反转镜的聚焦效应,研究了被动迭代时间反转镜技术,对其多目标定位选择性、多目标选择聚焦性能作了研究。结果表明利用了海洋信道的相干多途特性,被动时间反转镜可实现多个目标的空间匹配滤波：利用迭代算法,被动迭代时间反转镜可在抑制环境噪声干扰的同时,在信道输出总能量强的目标处实现选择性聚焦。     

聚焦波束形成声图法误差分析

聚集波束形成声图测量方法可以实现近场目标的被动定位．利用该方法接收数据或进行数据处理的过程中必定会有各种误差产生,本文从聚焦峰尺度、阵元偏差、波长等方面对聚焦波束形成声图测量方法的影响进行了分析,经仿真研究与实验数据分析可以得出聚焦波束形成声图法受距离与波长的影响较大,但对于阵元的位置要求不高,利用该方法可以实现近程目标的高精度定位。     

空域反转阵导向最小方差波束形成

文章研究了基于空域反转阵（Spatial Reversion Array,SRVA）的导向最小方差（Steered Minimum Variance,STMV）波束形成方法。根据空域反转阵的原理将均匀线阵列的快拍进行空间反转,再与原阵列快拍卷积,得到孔径扩大近一倍的空域反转阵,提高阵列性能。将同样的方法用于构造空域反转阵的导向向量,实现其导向最小方差（SRVA-STMV）波束形成。仿真和海试试验结果表明,相较于常规的STMV波束形成,空域反转阵STMV波束形成能够获得更窄的主瓣、更低的旁瓣、更高的输出信干噪比以及更好的多目标分辨能力。     

改进的被动时间反转阵处理水下通信

时间反转技术由于不受环境制约,能自适应实现信道多途压缩和对指定点的空间聚焦,降低多用户通信过程中的码间干扰和同道干扰,广泛应用在水声通信领域。利用垂直阵加权处理的方法,在被动时间反转的基础上增加了一抵消项,抑制其中一个用户在接收阵的接收幅度以达到对另一用户指定通信的目的。模拟仿真显示该方法比单独只用被动时间反转处理,误码率能降低一半。海上试验数据处理结果与模拟仿真基本一致。     

基于等效孔径相等的体积阵波束形成

利用自适应方法对"米"字形体积阵进行波束形成,发现在预成的多波束中奇偶号波束效果差别较大,影响了检测性能、定位精度.从预成波束方向阵等效孔径入手,解释了奇偶号波束不一致的原因,通过改变预成波束方向、构造阵等效孔径相等措施来改进体积阵波束形成.计算机仿真、水池实测验证了这种方法可以有效地提高体积阵波束形成的整体效果.     

线阵声诱饵对抗具有尺度识别能力鱼雷的分析与仿真

为了研究线阵声诱饵对抗具有尺度识别能力鱼雷的性能,以分裂波束和方位走向法为例,分析了鱼雷对尺度目标回波的识别方法,以此为基础探讨了线阵声诱饵在对抗具有尺度识别能力鱼雷时的若干技术问题,通过仿真验证了线阵声诱饵对抗尺度识别能力鱼雷的有效性。本文证明了线阵式的声诱饵可以有效模拟舰船目标的尺度特征,在对抗有尺度识别能力的鱼雷时是很有意义的发展方向。本文所给出的仿真方法,除了建立线阵诱饵在诱骗有尺度识别功能鱼雷的仿真模型外,还可以作为物理模型应用在诱饵工作程序中,提供各阵元的延迟时间等声学参数。     

拖船辐射噪声特性分析及实用抵消

拖船辐射噪声严重影响拖线阵声纳的探测性能,该噪声是成分复杂、多途传播的近程强干扰,如何抵消该噪声是拖线阵声纳面临的一个难题。在讨论了拖船噪声的成因、谱特性以及多途传播路径等问题后,通过对拖船辐射噪声的特性分析及逆波束形成(Inverse Beamforming,IBF)算法的理论研究,并经仿真表明:基于IBF算法的拖船噪声干扰抵消技术能够有效抵消拖船辐射噪声。海试数据验证了抵消后目标信号所在方位更清晰,且能识别出弱目标信号,其抗多途效果好。该方法易于实现,抵消效果显著,具有一定的实用性和可行性。     

基于水平阵的柱面声源高分辨时反聚焦定位方法

针对浅海条件下利用水平阵对水下目标进行噪声源近场高分辨定位会受海面、海底形成的多途效应影响,致定位结果具有严重的背景起伏甚至出现虚假目标问题,在建立水平阵近场柱面聚焦测量模型基础上,分别研究适用于单频线及宽带连续谱信号的水平阵柱面聚焦定位方法,即将幅度补偿引入MVDR(Minimum Variance Distortionless Response)近场聚焦波束形成中,充分利用声波幅度信息及相位信息,并据基于射线理论的虚源法建立水声相干多途信道模型与静态介质中波传播的互易性及时反不变性,生成与水声信道特征信息相匹配的空间聚焦导向矢量。结果表明,该方法可有效克服多途效应影响,实现柱面声源高分辨定位。     

时间反转镜DOA-AT成像算法

超声成像中广泛应用的DOA算法容易受到介质的不均匀性和多途效应的影响,纵坐标分辨率较差。在时间反转法理论的基础上,提出在随机媒质中采用时间反转镜超声成像的DOA-AT新算法。新算法通过对散射中接收信号的到达时间和响应矩阵频域奇异值进行分解,将时域内目标函数DOA估计与到达时间估计结合起来,使成像目标的纵坐标估计得到明显改善,对目标检测能力增强。     

基于声压时间反转镜的水下目标被动定位技术

文章围绕基于时间反转镜技术的水下运动目标被动定位问题,讨论了被动声压反转镜（Pressure Time Reversal Mirror,PTRM）定位的基本原理,在此基础上,对PTRM进行了定位仿真分析,并对湖上试验数据进行了处理,试验结果与理论结果基本一致。结果表明,利用PTRM技术可以有效地对水下运动目标进行被动定位。     

Time reversal of ultrasonic fields. I. Basic principles

Time reversal of ultrasonic fields represents a way to focus through an inhomogeneous medium. This may be accomplished by a time-reversal mirror (TRM) made from an array of transmit-receive transducers that respond linearly and allow the incident acoustic pressure to be sampled. The pressure field is then time-reversed and re-emitted. This process can be used to focus through inhomogeneous media on a reflective target that behaves as an acoustic source after being insonified. The time-reversal approach is introduced in a discussion of the classical techniques used for focusing pulsed waves through inhomogeneous media (adaptive time-delay techniques). Pulsed wave time-reversal focusing is shown using reciprocity valid in inhomogeneous medium to be optimal in the sense that it realizes the spatial-temporal matched filter to the inhomogeneous propagation transfer function between the array and the target. The research on time-reversed wave fields has also led to the development of new concepts that are described: time-reversal cavity that extends the concept of the TRM, and iterative time-reversal processing for automatic sorting of targets according to their reflectivity and resonating of extended targets.     

线阵相控换能器阵列参数对聚焦性能的影响分析

针对超声相控阵检测中广泛使用的一维线阵换能器,通过建立其声场数学模型和引入指向性函数,仿真分析了阵列参数对声束聚焦性能的影响,提出了改善换能器声场性能的措施。最后对优化参数的阵列换能器声场进行实验测试,测试结果表明其具有良好的声束聚焦性能,为超声相控阵缺陷检测中换能器的选择提供了参考。     

Parabolic mirror optics for collimation of a crescent blue laser beam radiated from channel waveguide Čerenkov second-harmonic generation

Parabolic mirror optics to collimate the crescent-shaped blue laser beam that radiates from ?erenkov second-harmonic generation (SHG) in a channel waveguide configuration is proposed. Mirror collimation optics has a large tolerance to the variations of SHG element parameters, such as the laser source wavelength, as well as to mirror displacement. The anisotropy of a nonlinear crystal in which the waveguide is fabricated has been taken into account. The optimum mirror alignment to obtain a collimated blue laser beam is evaluated in terms of Marechal's criterion. The minimum wave-front aberration with beam intensity weighted is 0.054 λSH. The convergence of the collimated beam is less than 1.6 mrad, and, by using an objective lens, the collimated beam can be focused to a diameter of less than 1 μm, which is 1.27 times the diffraction-limited focusing point.     

High resolution X-ray diffraction using a high brilliance source, with rapid data analysis by auto-fitting

In a production environment in particular, fast data collection and analysis, which are also highly reliable, are desirable. Measurement can be speeded up by increasing the diffracted intensity, thus reducing the time required to measure it reliably. Increased intensity with a smaller beam footprint at the sample have been achieved in a double-crystal diffractometer by the use of a novel ellipsoidal mirror working by total external reflection, positioned before the reference crystal. To optimise the performance of the mirror and provide high brightnesses, an X-ray source with a very small focal spot is required. Such a high brightness source has been made that uses electromagnetic focusing of the electron beam onto the target. Rapid data analysis is achieved by the use of an auto-fitting program that employs a genetic algorithm and the full dynamical theory of X-ray diffraction. Choice of an appropriate error function produces a deep global minimum while the genetic algorithm avoids convergence on local minima. From the model that produces the best fit, samples parameters such as layer thickness and alloy composition are extracted with quantified goodness of fit.     

Three-dimensional Dammann vortex array with tunable topological charge

We describe a kind of true 3D array of focused vortices with tunable topological charge, called the 3D Dammann vortex array. This 3D Dammann vortex array is arranged into the structure of a true 3D lattice in the focal region of a focusing objective, and these focused vortices are located at each node of the 3D lattice. A scheme based on a Dammann vortex grating (DVG) and a mirror is proposed to provide a choice for changing the topological charge of the 3D Dammann vortex array. For experimental demonstration, a 5×5×5 Dammann vortex array is implemented by combining a 1×7 DVG, a 1×5 Dammann zone plate, and another 5×5 Dammann grating. The topological charge of this Dammann vortex array can be tuned (from -2 to +2 with an interval of +1) by moving and rotating the mirror to select different diffraction orders of the 1×7 DVG as the incident beam. Because of these attractive properties, this 3D Dammann vortex array should be of high interest for its potential applications in various areas, such as 3D simultaneous optical manipulation, 3D parallel vortex scanning microscope, and also parallel vortex information transmission.     

Efficient dynamic focus control for three-dimensional imaging using two-dimensional arrays

An efficient dynamic focus control scheme for a delay-and-sum-based beamformer is proposed. The scheme simplifies dynamic focus control by exploiting the range-dependent characteristics of the focusing delay. Specifically, the overall delay is divided into a range-independent steering term and a range-dependent focusing term. Because the focusing term is inversely proportional to range, approximation can be made to simplify dynamic focus control significantly at the price of minimal degradation in focusing quality at shallow depths. In addition, the aperture growth controlled by a constant f//sub number/ can also be utilized to devise a nonuniform quantization scheme for the focusing delay values. Efficacy of the proposed scheme is demonstrated using simulated beam plots of a fully sampled, two-dimensional array. Design procedures are also described in detail. One design example shows that, with the proposed dynamic focus control scheme, a 4096-element array only requires 227 independent controllers for the range-dependent focusing term. Moreover, only 28 non-uniform quantization levels are required to achieve the same focusing quality as that of a conventional scheme with 784 uniform quantization levels. The beam plots of a fully sampled array show that sidelobes are slightly increased below the -30 dB level for imaging depths less than 3 cm. At greater depths, there is no observable degradation.     

成像声呐中聚焦波束形成技术研究及实现

介绍了聚焦波束形成的基本原理,分析了一种基于半圆阵的相位补偿方法。通过Matlab仿真得出聚焦波束形成的波束图,相比远场方法,波束宽度减小,旁瓣得到抑制。设计了一种基于FPGA的数字聚焦波束形成器的实时处理结构,使用8组加权系数即可完成成像声呐近场范围内分辨力的改进。通过乒乓操作和并行结构提高处理速度,实时产生72个波束。实验结果表明,所设计的聚焦波束形成器使某型成像声呐近场分辨力得到了提高。