Computations and measurements of transient stress waves and electrical voltages generated by transmitting ultrasonic piezoelectric transducers

the Mason model is used to compute the transient stress generated by a thickness mode piezoelectric transducer excited by a step voltage for various backings or loadings. The predicted stress pulse is experimentally shown to conform with the plane wave component of the radiation field. Verification is effected after visualizing the real radiation field and isolating its plane wave component and after applying surface transduction for detection.From the transmitting transducer a generated voltage is brought to attention. This quantity is closely related to the width of the initial pulse in the received ultrasonic signal display. The theoretical induced voltages are verified experimentally.     

Acoustic impedance matching of medical ultrasound transducers

Ultrasonic transducers for pulse-echo systems are studied both theoretically and experimentally. For the theoretical calculations the Mason model for thickness-mode disc transducers with and without backing and matching layers is used. By building several of the theoretically investigated transducer configurations it is shown that theory and experiment agree well. Thus the properties of a transducer can be predicted to a good approximation before its experimental realization. To find transducers with good sensitivity and short pulses, the pulse shape and frequency response for the following classes of transducers were studied both theoretically and experimentally: transducers with backing only, transducers with heavy backing and front matching layers, and air-backed transducers with front matching layers.     

Compression and amplification of an ultrasonic pulse reflected from a one-dimensional layered structure

Compression of ultrasonic pulses reflected from layered structures is studied. A short pulse is emitted into water towards a structure consisting of solid plates backed with an air layer. Due to multiple reflections in the structure, the signal is elongated. The reflected signal is received by the same transducer and digitized. After that, the wave is reversed in time and emitted towards the layered structure for the second time; then, the reflected signal is received. Due to the invariance of the processes under the time reversal, the pulse is compressed by the structure: the reflected signal becomes shorter and acquires the waveform of the initial pulse. The possibility of an efficient compression of signals is demonstrated experimentally. Numerical simulations show that the use of more complex structures can considerably increase the compression ratio and produce short signals of a much higher amplitude than that emitted by the transducer. An efficient compression algorithm is proposed.     

充水有限长圆柱薄壳声散射:Ⅱ.实验

从实验和理论上研究了充水有限长圆柱薄壳的声散射。实验采用指向性尖锐的宽带收发合置换能器,发射宽带调频信号和单频窄脉冲信号进行测量,所得结果分别在时域和频域上与理论计算结果相对比,证明其吻合程度很好。理论和实验的结果表明,内部充水圆柱壳在频域上增加了一组由内部流体负载引起的附加波的共振;而在时域上增加了一系列由于声波在壳体内来回反射产生的"回音廊"式回波。频率-角度图中"碗"形亮线出现分簇的原因解释为前几种模式"回音廊"式回波叠加的结果。      

Acoustic scattering from fluid-filled finite cylindrical shell in water:Ⅱ.Experiment

Acoustic scattering from the submerged fluid-filled finite cylindrical shell insonified by an incident plane wave is studied experimentally and theoretically.A monostic broadband transducer with the sharp directivity is used in the experiment.The broadband LFM signal and the single-frequency narrow pulse are used to measure the backscattering field of the cylindrical shell.The measured results have a good agreement with the theory both in time and frequency domain.The theoretical and experimental results show that the resonances of several additional waves which are caused by the internal fluid are presented in the frequency domain.And a series of ’whispering gallery’ waves produced by the waves reflected back and forth in the internal fluid filled in the cylindrical shell are added.The reason for the clustering of the bowl-shape resonance curves in the frequency-angle spectrum is explained as the superposition of the first several modes of ’whispering gallery’ waves.     

Improvement of echo shape in low impedance materials

Multiple internal reflections within an air-backed transducer lead to a deterioration of pulse shape and consequent errors in pulse echo measurements. This effect is particularly pronounced in low impedance materials. It is demonstrated both by calculations and experimentally that using a quarterwave buffer having a specific acoustic impedance higher than either transducer or sample considerably reduces this echo distortion.     

有界波电磁脉冲模拟器下短线缆效应的理论和实验研究

 研究了有界波电磁脉冲模拟器下短线缆效应的理论建模和实验方法。基于传输线模型计算了线缆在有界波电磁脉冲模拟器辐照下的电流响应。建立了有界波电磁脉冲模拟试验环境,其前沿时间小于5 ns,脉冲半高宽约200 ns。开展了短线缆的效应实验验证研究,短线缆负载端响应电流的测量和计算结果吻合得很好,表明应用改型有界波电磁脉冲模拟器开展短线缆效应实验在理论和实验上都是可行的。这种线缆实验方法具有效应实验空间电磁场分布规范均匀、参数指标可控、监测技术成熟等优点。     

粗晶材料晶粒散射波的实验鉴别

在利用超声波束对粗晶材料样品或工件进行检测时，接收回波中，除伤波、底波外，还存在晶粒散射波及其它干扰杂波。本文对晶粒散射波主要特征：晶粒散射波随传播时间出现的最大值位置；晶粒散射波最早到达时间；以及传播过程中晶粒散射波的频谱特性进行了实验验证。其中，用脉冲频谱合成方法，计算了双探买发．收复合声场声轴上声压分布，据此对最大值的出现位置给予了物理解释。并用短时傅立叶变换分析了声波传播过程中晶粒散射波的频谱特性。获得了一些有用结果。     

Radiation field calculations of pulsed ultrasonic transducers: Part 2: spherical disc- and ring-shaped transducers

The exact equation to describe the propagation of ultrasonic pressure waves in a lossless, homogeneous medium was evaluated assuming pulse-excited radiators.Acoustical field patterns were studied using the dimensions and shape of the transducer, the ultrasonic frequency and number of cycles within the pulse as parameters. The results emphasize the influence of the piston shape on interference phenomena within the near — and far-field and on beam-narrowing effects in the intermediate range between near- and far-field. The following transducer shapes are studied: spherical disc and spherical ring.     

Coherent Brillouin spectroscopy in a strongly scattering liquid by picosecond ultrasonics

In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.     

A spatial impulse response based method for determining effective geometrical parameters for spherically focused transducers

This paper proposes a novel method for determining effective geometrical parameters (GPs) of spherically focused transducer. Theoretical basis of the method is the spatial impulse response method (SIRM) that is a powerful tool to calculate transient fields from a piston-like transducer. Experimentally, the method is implemented by measuring arrival times of the pulse signals (either transmission or pulse echo signals) and using them to determine the time intervals between the direct and edge wave pulses. Since the time intervals for a given measurement position depend on the transducer's GPs, then the effective GPs can be obtained. The measurements are carried out in the near-field so that the time intervals can be resolved. The proposed method has been applied to a spherically focused transducer operating in pulse echo mode. The results have shown that the effective GPs used in the SIRM give a closer agreement between the theory and the measurements than the nominal GPs. The method is best suited for characterizing broadband transducers with short pulse excitations. With certain modification it can be also applied to narrowband transducers. The method can be also applied to transducers of other shapes, e.g., planar circle, and planar or curved rectangle provided the piston approximation is appropriate.     

Experimental study of underwater transmission characteristics of high-frequency 30 MHz polyurea ultrasonic transducer

In this paper, we present the transmission characteristics of a polyurea ultrasonic transducer operating in water. In this study, we used a polyurea transducer with fundamental resonance at approximately 30 MHz. Firstly, acoustic pressure radiated from the transducer was measured using a hydrophone, which has a diameter of 0.2 mm. The transmission characteristics such as relative bandwidth, pulse width, and acoustic sensitivity were calculated from the experimental results. The results of the experiment showed a relative bandwidth of 50% and a pulse width of 0.061 μs. The acoustic sensitivity was 0.60 kPa/V with good linearity, where the correlation coefficient R in the fitting calculation was 0.996. A maximum pressure of 13.1 kPa was observed when the transducer was excited at a zero-to-peak voltage of 21 V. Moreover, we experimentally verified the results. The results of the pulse/echo experiment showed that the estimated diameters of the copper wires were 458 and 726 μm, where the differences between the actual and measured values were 15% and 4%, respectively. Acoustic streaming was also observed so that a particle velocity map was estimated by particle image velocimetry (PIV). The sound pressure calculated from the particle velocity obtained by PIV showed good agreement with the acoustic pressure measured using the hydrophone, where the differences between the calculated and measured values were 12–19%.     

0.14 THz高功率太赫兹脉冲的参数测量

给出了0.14 THz高功率单次脉冲信号的频率和功率的测量方法及实验结果。针对高功率太赫兹脉冲频率高、峰值功率高和脉宽短的特点,实验中采用了截止波导滤波法与谐波混频法相结合的方式准确测定了脉冲信号频率,利用辐射场功率密度积分法获取了辐射脉冲的远场功率分布,并给出了单次脉冲的辐射功率。某实验条件下的测量结果表明,0.14 THz高功率太赫兹脉冲的频率为0.146 3 THz,脉宽约为1.5 ns,功率不小于0.5 MW。     

Pulse mode of operation of a spherical piezoceramic transducer filled with liquid and having a correcting electric circuit

By means of a computational method, the possibility of radiating a short acoustic pulse by a transducer in the form of a piezoceramic sphere internally filled with liquid is investigated. An electric inductive-resistive circuit is connected to the electric input of the transducer. Solution is obtained based on scheme-analogs theory for piezoceramic transducers, and spectral Fourier transform theory. The values of parameters of the system, providing minimal durations of radiated signals, are determined. Computation was carried out for different values of relative thicknesses of the transducer wall. The estimates of durations and amplitudes of the acoustic signals radiated into the external medium are obtained.     

Spin-echoes in nitrogen-14 quadrupolar spin-system with axially symmetric electric field gradient tensor

The behaviour of the magnetisation in the spin-1 system with the axial symmetry of the electric field gradient (EFG) tensor was analysed theoretically. The behaviour of the NQR signals in hexamethylenetetramine (C6H12N4) was also studied experimentally using the pulse NQR technique and sensitive equipment. It was shown that the axial symmetry of the EFG in a sample could not be the reason why spin-echo signals were not observed in conventional pulse experiments for the same type of sample. A very short spin-spin relaxation time T2 seems to be the reason of that. The signal-to-noise ratio was sufficient for measuring T2 in C6H12N4, which had not been reported earlier. Experimental results of applying certain pulse sequences to the 14N NQR in a sample are also presented.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

超声换能器辐射场应力定量测量方法研究

提出了一种基于动态光弹观测系统的超声换能器在透明固体中辐射场应力定量测量新方法。该方法关注到激光束穿过非均匀应力区时的双折射累积效应和Senarmont补偿法均匀应力假设与真实应力非均匀分布之间的差异,利用滤波反投影算法重建出应力场;根据应力与换能器激励电压的线性关系,用4种激励电压动态校准重建后的应力数据。实验定量测量了圆形晶片超声换能器在固体内部辐射的瞬态纵波场的应力分布,结果表明应力幅值与激励电压成正比,符合线性规律。     

介质参数对受激布里渊散射特性的影响

 采用一维瞬态模型对受激布里渊散射（SBS）过程进行了数值模拟，研究了介质参数对SBS特性的影响规律；选取FC-72、FC-75、CCl₄、CS₂、丙酮、苯以及CCl₂/乙醇的混合溶液等作为散射介质，在Nd∶YAG调Q激光系统下进行了实验研究，比较了各种介质所产生的SBS脉宽、能量反射率和波形之间的差异，由此讨论了介质参数对SBS特性的影响。结果表明，介质增益系数越大，吸收系数越小，SBS能量反射率就越大；吸收系数越大，产生的SBS脉宽就越窄；声子寿命越短，SBS波形前沿就越陡，并且越容易出现调制现象。     

Cavitation field from a horn—A new model

The cavitation field from a horn-type transducer is experimentally probed by the optical method.Based on these observations,we propose a model of the field which hypothesizes that most violent caviation bubbles originate from the vibrating surface and there-from fast drift to the near liquid region.These bubbles are chiefly responsible for the practical applications of cavitation for a large power input to the transducer.During migration they become weakened.Cavitation bubbles are also produced in the liqui...     

Coupled vortex oscillations in spatially separated permalloy squares

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations.