水声无源材料回声降低测量时反聚焦方法

提出了利用时间反转(时反)聚焦技术的水声无源材料回声降低测量方法。首先实现有界水声环境下有、无试样接收信号的时反聚焦,然后利用聚焦信号测量试样反射系数,最后通过标准试样对反射系数进行修正,从而得到试样的回声降低测量值。在小型水池中进行了铝板试样和钢板试样回声降低的测量,铝板试样的尺寸为1.1m×1.0×0.005m,测量频率范围为3~20kHz,钢板试样尺寸与铝板试样相同,测量频率范围为0.5~20kHz,测量结果经修正后与平面波理论计算值基本一致,和理论计算值的相对误差小于10%,扩展不确定度小于1.5dB。本方法采用时反原理实现了接收信号的空时聚焦,提高了测量信混比,因此适用于非自由场环境下无源材料回声降低的测量试验,尤其适用于低频的回声降低测量。      

水声无源材料插入损失测量时反聚焦方法

提出了利用时间反转(时反)聚焦技术的水声无源材料插入损失测量方法。通过无试样及有试样情况下实现接收信号的时反聚焦,并对聚焦信号进行透射系数计算,获得试样的插入损失。由于时反原理可实现接收信号的空时聚焦,从而提高测量信混比,因此本方法适用于非自由场环境下材料声学参数的测量,尤其适用于低频条件下的声学参数测量。波导水池试验开展了对两块试样的测量,试样尺寸为1.1m×1.0m×5mm,测量频率范围为1—20kHz,通过测量值与理论值的对比验证了该方法的有效性。      

水声材料透声性能测量技术的改进

本文对传统的水声无源材料透声性能测量技术提出了几点改进,以便在小型消声水池中测量有限尺寸的大面积材料的插入损失（透射系数）随频率的变化规律。材料样品的典型尺寸为 １×１ｍ２左右,测量频率范围为 ２－２０ ｋＨｚ．文中对标准样品进行了声学性能测量,测量结果和平面波理论模型计算值有较好的吻合。     

The technique of coherent echo processing in the approximation of a small number of point scatterers

In this paper, we aim at studying the structure of individual samples of coherent echo signals and determining signal features that permit one to detect coherent backscatter and also improve the methods of signal processing. We offer a model of the received signal in the form of a square pulse with duration of the order of the sounding pulse length and filled with a sinusoidal signal of arbitrary frequency. It is shown that this model is quite effective to interpret coherent echo signals in some cases. There are two ways of practical use of the model. One is a technique which singles out the sounding sessions in which strong coherent echo signals are distinguished against the background of other signals received by the Irkutsk incoherent scatter radar. Another is a technique of the spatial resolution improvement.     

太赫兹波段豆油光谱的优化测量

太赫兹时域光谱技术是基于飞秒超快激光技术的有效的光谱检测技术,太赫兹波独特的优势使其成为一种有效的无损检测手段,并被广泛地应用到各个领域。然而在样品检测尤其是液体样品检测过程中,由于Fabry-Perot效应的存在,太赫兹波在样品、样品容器、以及光学元件之间的多次反射,使时域信号产生回波,样品的吸收光谱在频域内产生振荡,有可能会隐藏一些重要的吸收特征。为了解决这一问题,对解卷积算法进行改进,在传统计算模型的基础上,考虑系统中液体池窗片和光学元件对太赫兹波的非线性吸收,将包含回波的太赫兹时域信号描述为太赫兹主脉冲与一系列冲击信号和非线性传递函数的卷积。通过分析,有效去除回波引起的频谱振荡,进一步提高太赫兹波段豆油光学参数的测定精度。实验对比了改进前后0.2～2 THz波段豆油的频谱及吸收谱,实验结果证明,与传统的主脉冲截取法相比,本算法不仅能有效去除回波引起的频谱振荡,且在相同检测条件下,可将太赫兹波段豆油样品的频率分辨率由50 GHz有效提高至10 GHz。该算法不受被测对象参数的影响,同样适用于其他液体的太赫兹时域光谱测量。最后对吸收谱中残余的频谱振荡进行了深入分析。     

Preparation of inverted medium and processing in the inverted medium

The processing of weak optical signals in spatial-spectral holographic (SSH) materials coherently inverted with optical frequency chirped pulses were investigated. Simulations and experimental studies in Tm³⁺:YAG were conducted to characterize the parameters of the frequency chirped laser pulse used to invert the SSH material in order to obtain high photon echo efficiency for SSH lidar processing. Collinear and angled beam geometries and single shot and accumulated processes were investigated. Echo efficiencies as high as 450% were measured, significantly higher than the typical stimulated photon echo efficiency of 10%.     

声管中的宽带脉冲法的水声材料吸声系数测量

现有的水声管吸声系数测量的脉冲法,由于水声管高度的限制,存在低频限制。提出了基于"后置""逆滤波的宽带脉冲声测试方法,在测量获得系统的传递函数后,换能器发射宽频短脉冲信号,然后对接收到的标准反射体和待测样本的反射信号分别进行逆滤波处理,恢复未经传递系统"污染"的反射信号,计算待测样品的反射系数和吸声系数。仿真实验分析了"后置"逆滤波相对于传统"前置"逆滤波的在低频测试方面优势。对橡胶材料样品进行了实验测试,无论在低频段还是中高频段,宽带脉冲法和CW (Continuous Wave)脉冲法测试结果均吻合较好。宽带脉冲法是一种有效的测试方法,其低频测试能达到350 Hz,能有效拓展低频测试范围。      

Flaw-to-grain echo enhancement by split-spectrum processing

A split-spectrum processing technique for an ultrasonic flaw detection system has been developed which improves the flaw-to-grain echo ratio in large-grained materials. The enhancement is achieved by partitioning a wide-band received spectrum to obtain frequency shifted bands, which are then processed to suppress the grain echoes with respect to the flaw echo, using a novel signal minimization algorithm. Experimental data for titanium and stainless steel samples are presented which show superior flaw detection capabilities for the minimization algorithm with respect to frequency averaging techniques.     

声表面波检测铝板表面微缺陷深度实验研究

为了探究声表面波与不同深度微裂纹缺陷相互作用的关系,将脉冲激光作用于一系列不同缺陷的试件铝板上进行线光源激励,激发激光超声波。用超声传感器接收在铝板中传播的激光超声信号,通过数字荧光示波器采集激光超声在铝板中的传播数据。对采集到的反射波数据进行分离谱分离过程得到的铝板中激光超声的时域分布和透射波数据进行频域分析。实验发现:缺陷深度影响着反射回波两峰值特征点到达时间差,两者之间近似线性关系,也影响着透射波的截止频率且二者呈现递减关系。     

A multiple function sonar rangefinder for divers

The rangefinder described here is a self-contained instrument for hand-held operation by a diver for searching, tracking, navigation and surveying applications. It has four modes: 1 — to measure range to a passive target directly to ± 1 m; 2 — to measure range to a passive target indirectly to ± 0.1 m; 3 — to measure range to a coded transponder to either ± 1 m or ± 0.1 m; 4 — to indicate the presence of multiple targets downrange where each target occurs in a selected range interval. The pulse echo principle is employed, with pulses typically 250 μs long. The carrier frequency of 300 kHz can be altered easily if required by changing the transducer array.     

Scheme for nonlinear frequency modulated signal cancelling system

A cancelling system for the four nonlinear frequency modulated (NLFM) signal (i.e. Taylor window, Tangent-based, Combination linear frequency modulation (LFM) and tan-FM, Stepped NLFM) is presented. It is mainly composed of a digital radio frequency memory (DRFM) and a field programmable gate array (FPGA) chip etc. The received signals are stored and reproduced by DRFM, and the system delay time is controlled by the FPGA chip. According to the target's radar cross section (RCS), the radar echo cancelling wave will be generated by the FPGA and DRFM on signal processing. The effect of error on the cancelling wave is analysed and the method for reducing nonlinear phase errors is presented. Theoretical analysis and simulation show that the system effectively reduces the signal power received by the radar receiver. Numerical simulation results show that about 11.3 dB target gain reduction can be achieved under the condition of large deviation.     

Effect of the domain structure on ferromagnetic resonance in easy-axis ferromagnets

The results of experimental and theoretical investigations of ferromagnetic resonance (FMR) in ferrite single crystals having hexagonal structure and magnetocrystalline anisotropy of the easy-axis type are reported. Experiments are carried out on disk-shaped samples of materials differing in anisotropy strength. The values of the effective anisotropy fields and the gyromagnetic ratios (g-factors) of the materials are determined from measured frequency dependences of the FMR field along the easy and hard magnetization directions for homogeneously magnetized samples. It is shown that if the FMR spectra are also measured for the same samples in the presence of a domain structure, then one can experimentally determine not only the above-mentioned parameters but also the saturation magnetization of uniaxial magnetic materials. It is shown that the theory of the FMR frequency spectrum of a partially magnetized sample with a simple domain structure in the form of a system of plane-parallel layers is in good agreement with the experiment.     

宽频带雷达散射截面缩减相位梯度超表面的设计及实验验证

针对相位梯度超表面在隐身技术中的应用,提出通过表面波耦合和异常反射两种机制复合实现宽频带后向雷达散射截面(RCS)缩减,采用开口谐振环进行相位梯度设计,实现了一种二维极化无关相位梯度超表面,在10 GHz附近,超表面通过将垂直入射电磁波耦合为表面波实现RCS缩减,而在大于11 GHz的频率范围内,相位分布的不均匀性使垂直入射的电磁波发生漫反射或者异常反射,降低后向RCS,制作了厚度为2 mm的超表面样品,测试了其反射率曲线和后向RCS,并与相同尺寸的金属板进行了对比,实验结果表明,在宽频段内(9.5—17.0 GHz),超表面在垂直入射情况下可将后向RCS缩减至少10 dB,由于厚度薄、重量轻、频带宽,RCS缩减超表面在隐身新材料和新技术方面具有很大的应用潜力。     

Field strength influences on gradient recalled echo MRI signal compartment frequency shifts

Different trends of echo time dependent gradient recalled echo MRI signals in different brain regions have been attributed to signal compartments in image voxels. It remains unclear how variations in gradient recalled echo MRI signals change as a function of MRI field strength, and how data processing may impact signal compartment parameters. We used two popular quantitative susceptibility mapping methods of processing raw phase images (Laplacian and path-based unwrapping with V-SHARP) and expressed values in the form of induced frequency shifts (in Hz) in six specific brain regions at 3T and 7T. We found the frequency shift curves to vary with echo time, and a good overlap between 3T and 7T mean frequency shift curves was present. However, the amount of variation across participants was greater at 3T, and we were able to obtain better compartment model fits of the signal at 7T. We also found the temporal trends in the signal and compartment frequency shifts to change with the method used to process images. The inter-participant averaged trends were consistent between 3T and 7T for each quantitative susceptibility pipeline. However, signal compartment frequency shifts generated using different pipelines may not be comparable.     

Increasing the speed of relaxometry-based compartmental analysis experiments in STEAM spectroscopy

In this work we present a method for improving the speed of spin-spin relaxation time (T2) measurements for compartmental analysis in stimulated echo localized magnetic resonance spectroscopy without reducing the sampling density. The technique uses a progressive repetition time (TR) to compensate for echo time (TE) dependent variations in saturation effects that would otherwise modulate the received signal at short TRs. The method was validated in T2 studies on 10 young healthy subjects in spectroscopic voxels localized along either the right or left Sylvian fissure (2 x 2 x 1.5 cm3, 10 ms mixing time (TM), 2048 data points, 819.2 ms acquisition time). The TR was automatically adjusted so that TR-TM-TE/2 was kept constant as the TE was incremented. Compared to long TR T2 experiments, the progressive TR technique consistently replicated the T2 relaxation times and reference signals of the tissue water compartment while reducing the data acquisition time by more than 50%. The percent error was on average less than 2% for estimates of T2 and S(0) for the tissue water, an indication that the progressive TR technique is a useful method for determining the tissue water signal for internal referencing.     

Femtosecond primary and stimulated photon echoes in a dye-doped polymer film at room temperature

The signals of both primary and stimulated femtosecond photon echoes are observed and investigated in a dye-doped polymer film at room temperature using a modernized femtosecond echo-spectrometer. It should be noted that stimulated photon echo in the solid-state sample is observed for the first time at such a high temperature. Experimentally obtained decay curves of these signals have a nonexponential character. The spectra of these echo signals are also measured. It is found that the spectrum of the primary photon echo is short-wave shifted with respect to the spectrum of excitation. This can be used for the coherent laser cooling of a sample. The spectrum of the stimulated photon echo is also shifted to the short-wave range relative to the spectrum of excitation, but its shift is much less than that of the primary photon echo. The experiment shows that the femtosecond echo signals at room temperature are excited via the phonon-side band of the absorption line.     

轴向B-dot束流偏角探测器信号的频域处理北大核心CSCD

提出一种针对轴向B-dot束流偏角探测器信号的频域处理方法,从探测器本身的频域响应出发,并从探测器信号的频谱中提取有效信息,能够一定程度上消除束流横向偏移对偏角测量的干扰。该方法是对时域分析法处理轴向B-dot信号的一种补充,相比于时域法的一阶近似,对信号频谱的处理更接近对复杂真实情况的近似,但并未从根本上解决由安装误差等原因导致束流横向偏移而带来的测量不准确。从标定试验台测试结果来看,该方法的测试精度与时域法相当,约1mrad。     

A model for domain and domain wall NMR signals in magnetic materials

Spin echo NMR signals in magnetic materials (simple metals, alloys or intermetallic compounds) generally result from mixed contributions of distinct magnetic regions of the sample, the magnetic domains and the domain walls. The amplitude of the signal is proportional to the so-called enhancement factor which in most of the cases greatly differs in these two regions, depending upon the wall mobility, the magnetic anisotropy, etc. The experimental access to domain and domain walls is possible, in principle, by a careful control of the RF power applied to the sample. In this paper a simple superposition model is proposed which includes both contributions to the NMR signal. We calculate the amplitude of the spin echo in magnetic powder samples and compare it with experimental situations where it has been possible to separate different contributions to the signal. This has been done in some RCo₂ magnetic rare-earth intermetallic compounds by analyzing the spectral line widths and the curve of the spin echo amplitude versus the applied RF field. Despite its simplicity, the present model allows the understanding of the main features of the NMR spectra and the dependence of the echo amplitude with the RF power in these compounds.     

High-resolution interference acoustic location

A method of forward-scattering location on the basis of resolving the arrival times of direct and scattered signals is proposed. The method is specific in that it allows one to use a long sinusoidal probing signal. The required time resolution of the signals produced by moving targets is achieved by processing the spectrum of the received echo signals so as to broaden its effective bandwidth. The method is tested in both numerical and full-scale acoustic experiments. For the first time, the forward-scattering location is implemented with the use of long probing pulses in a narrow frequency band.