Plane-wave analysis of the near field of light diffracted by ultrasound.

Although the phenomenon of light diffraction by ultrasound has been studied very extensively during the last 40 years, almost all investigations were concentrated on the individual far field (Fraunhofer) diffraction orders. In the present paper, the basic theory is developed for studying the near field (Fresnel region) of light diffracted by an arbitrary plane ultrasonic wave and the fundamental periodicity properties are stated. The general plane-wave theory of Raman-Nath has been taken as a starting point. From the analysis, the near field of the diffracted light is seen to be highly sensitive to variations of the ultrasonic amplitude and this feature provides a useful technique for observing weak ultrasonic waves. In particular, for the specific case of Raman-Nath-type diffraction, a procedure is presented allowing the reconstruction of the time waveform of the ultrasonic wave from the diffracted light intensity signal.     

部分时滞诱发Watts-Strogatz小世界神经元网络产生随机多共振

已有研究显示时滞可诱发神经元网络产生随机多共振,但它们主要讨论了神经元间的耦合都存在时滞的情形.然而实际中,有些神经元间的信息传递是瞬时的或时滞很小可以忽略的,即神经元网络中只有部分神经元间的耦合具有时滞,简称部分时滞(若神经元网络内共有l条耦合边,其中有l1条耦合边是具有时滞的,而剩余的耦合边的时滞为零,则我们称这类时滞为部分时滞).本文以Watts-Strogatz小世界神经元网络为研究对象,主要讨论部分时滞对该神经元网络系统响应强度的影响.研究结果指出,系统响应强度随部分时滞的增加呈现多峰的变化态势,即部分时滞可诱发随机多共振现象;而且使系统响应强度达到最优水平的部分时滞的取值区间随随机时滞边概率的增加渐渐变窄,当随机时滞边概率足够大时,系统响应强度只有在时滞位于外界信号周期的整数倍附近才会达到最优.此外,我们还分析了随机连边概率和神经元网络中边的总数对部分时滞诱发的随机多共振现象的影响.结果显示,部分时滞诱发的随机多共振现象对随机连边概率具有一定的鲁棒性,而神经元网络中边的总数对部分时滞诱发的随机多共振的影响则较大.     

Theory of transit time ultrasonic flowmeters

A theory of transit time ultrasonic flowmeters for clean fluids is developed from the equations of fluid mechanics applied simultaneously to the fluid and the sound vibrations. These equations are linearized (weak sound) and use is made of the electroacoustic reciprocity theorem to give a relation between the voltages and currents at the transducer terminals and the fluid velocity. The technique of “reciprocal operation” of a transit time ultrasonic flowmeter is described and the way this technique eliminates zero drift is explained. The theory can be applied to meters with broad sound beams (which provide a better average over velocity profiles) or meters in which the wavelength of sound is not necessarily small compared with the duct diameter. Small modificaition of the sound field (due to flow) is assumed and the resulting phase (or amplitude) shift of the received signal is expressed as an integral throughout the fluid of the dot product of the fluid velocity and a weight vector defined in terms of the sound fields in the stationary fluid. Simple flowmeter designs which approach the ideal of complete immunity to velocity distribution are described.     

微通道板中电子时间倍增特性的数值模拟

 对微通道板的动态特性进行了数值模拟,得到了电子的渡越时间与入射时刻的关系曲线。获得了光电子的倍增次数随入射时刻而变化,并在考虑了入射电子为一高斯电子脉冲的情况下,分析了选通脉冲的幅度、宽度和波形对选通特性的影响。结果表明：随着倍增次数增多,渡越时间越大;当电压幅度不同时,增益曲线的峰值所对应的电子入射时刻也不同。     

Three-dimensional images generated by quadrature interferometry

Quadrature detection techniques have been applied to images obtained from a Mach-Zehnder interferometer with differently polarized beams to yield the real and the imaginary parts of the diffracted fields simultaneously. This approach eliminates the need for phase retrieval by providing complete information on the complex amplitude of the diffracted signal. We present results in which we demonstrate our ability to reconstruct two- and three-dimensional microscopic objects from their complex diffraction patterns.     

Medical ultrasound imager based on time delay spectrometry

A reflection mode proof-of-concept medical ultrasound imager based on time delay spectrometry has been developed and tested. The system uses a broad band swept-frequency signal operating up to 10 MHz. Signal processing using a fast Fourier transform (FFT) permits extraction of range information. The imager has a higher signal-to-noise ratio than pulse-echo systems which allows high resolution at greater depths. The time delay spectrometry (TDS) spread spectrum operates at lower peak intensities than pulse-echo and permits more control of the spectral content and amplitude of the signal. At present, the system is non-real time which degrades in vivo imaging because of averaging over several cardiac cycles and tissue movement.     

Diffraction of light by an acoustic wave with three frequency components

Collinear light diffraction by three-frequency sound is investigated theoretically. The amplitude distributions of transmitted and diffracted light waves along the cell are calculated for different amplitudes of sound signals. The dependence of the intensity of principal diffraction peaks on the frequency difference between acoustic signal components is studied for different amplitude ratios of these components. It is shown that the character of this dependence for a wave being in synchronism differs substantially from that for two other waves characterized by detuning. The dependence of the amplitudes of principal and parasitic diffraction peaks on the efficiency of acoustooptical interaction is investigated. It is demonstrated that parasitic sideband components in diffracted light can play a considerable role if the diffraction efficiency is sufficiently high and exceeds 80%.     

100 kA微秒导通时间等离子体断路开关研究

 研制了最大导通电流约为100 kA的微秒导通时间等离子体断路开关,开展了该导通电流下的等离子体断路开关性能实验,得到的负载电流上升时间为54-76 ns,最高开关电压为1.38 MV,最高电压倍增系数达到4.9。建立了导通阶段开关区等离子体运动的二维雪耙模型,初步数值模拟结果表明,该模型对目前开展的实验有较好的预估能力。     

Estimation of ultrasound attenuation and dispersion using short time Fourier transform

Determination of the acoustic attenuation and dispersion has important applications in ultrasound tissue characterization and non-destructive material testing. Current signal processing methods Fourier transform of ultrasound signals to get the spectra of amplitude and phase to estimate respectively the attenuation and dispersion of a given medium. These methods are frequency domain method and obsessed with ambiguity issue in the phase unwrapping calculation. Conventional ultrasound velocity measuring method detects the time of arrival of a pulse (or echo) signal, which is a time domain method to compute group velocity (not phase velocity). This paper presents a novel approach based on the short time Fourier transform (STFT)--a time-frequency analysis, to estimate the ultrasonic dispersion and attenuation. Only the amplitude information of the pulse-signal spectra is used. Based on the time-frequency presentation, the attenuation coefficient of the signal is obtained by computing the amplitude decay of pulse spectrum in time domain, while phase velocities are obtained based on the "time-of-flight" (TOF) of the mono frequency component of the pulse signals. As a result, we eliminate the ambiguity issue in phase angle calculation. Furthermore, the proposed method makes the phase velocity pedagogically intuitive for novice users. The paper presents experiments to evaluate demonstrate the performance of the proposed method.     

Effect of Airy disk on schlieren diffraction interferometer

Schlieren is among the oldest and widely used optical techniques for detection of change in refractive index in transparent media. Present work demonstrates the effect of the position of schlieren element in the Airy pattern on the schlieren interferogram. It has been shown that the amplitude of the diffracted wave, starting from schlieren element and serving as an inbuilt reference beam for the schlieren diffraction interferometer, becomes maximum when schlieren element diffracts light from the Airy disk. Effect of F-number of the optical system on schlieren pattern is also demonstrated via observations on the amplitude of diffracted light.     

Optimal estimation of the time instant of occurrence of an impulse disturbance in a random signal in discrete time

Using the methods of the Markov theory of nonlinear filtering, the problem of optimal estimation in discrete time of the time instant of occurrence of a random impulse disturbance in a signal observed against a background of white noise has been solved. It is shown that the optimal estimate in the mean-square sense for the impulse occurrence can be represented as the sum of two conditional estimates calculated in current time with the help of a system of recurrent equations with a posteriori probability of the impulse occurrence and auxiliary probability densities. A simplified approximate estimation algorithm has been developed using a Gaussian approximation of the auxiliary probability densities.Nizhny Novgorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 6, pp. 498–511, June, 1993.     

基于统计学模型的真空绝缘堆闪络概率计算分析

对比了几种不同类型的过电压因子下绝缘堆闪络概率的特点, 考虑了多层均压及圆周渡越时间后得到的闪络概率更能反映绝缘堆耐压水平;简化计算统计学经验公式中矩阵可保持绝缘堆闪络概率计算值准确性并减少过电压因子的静电场计算次数。分析在固定间隙距离下绝缘环个数与电压峰值及电场强度峰值的关系, 计算结果表明:存在最优绝缘环个数承受最高电压峰值与电场强度, 承受最大工作场强的绝缘环个数下, 工作电压幅值已降低很多。在选择绝缘环个数时应综合考虑, 该计算方法可应用于工程绝缘结构设计中合理选取绝缘环个数。     

Interferometry of flashboard and cable-gun plasma opening switcheson Hawk

Interferometry of plasma opening switch (POS) plasmas on the Hawk generator has shown many important features of the plasma evolution during conduction and opening. Opening occurs when a low-density region forms at a radial location determined by plasma redistribution during the conduction phase, consistent with J×B forces and the measured plasma distributions produced by the sources alone. High neutral densities have been detected in the POS region during conduction. Low-density plasma appears between the POS and the load at the time current appears in the load, and high-density plasmas appear there later in time. There are two important differences between the density evolution of POS's utilizing flashboard and cable-gun plasma sources. 1) There is a substantial (two-three times) increase in the electron inventory during conduction using cable guns that is not detected using flashboards. This is attributed, primarily, to ionization of ions and neutrals for the cable-gun case. 2) The conduction scaling with plasma density implies that the cable-gun POS has an effective ion mass/charge ratio about double that for the flashboard POS     

On Empirical Scrutiny of the Bohmian Model Using a Spin Rotator and the Arrival/Transit Time Distribution

A setup that entails nonuniqueness in the quantum prediction of the arrival/transit time distribution is analysed using the Bohmian model, implying the plausibility of two distinct calculational schemes having empirically distinguishable predictions. One of them agrees with the results obtained from the probability current density based approach that has been suggested for predicting the quantum arrival/transit time distribution.     

Characteristics of microwave magneto-optic modulation based on magnetostatic waves

According to the magneto-optic (MO) perturbation theory, the coupled-mode equations for guided optical waves (GOWs) with microwave magneto-static waves (MSWs) in MO film waveguides are presented, which cannot only be used to analyze the MO modulation induced by MSW pulses, but also explain the optical pulse compression caused by chirped MSWs. The rectangular pulse modulation of continuous guided optical waves by magnetostatic forward volume waves (MSFVWs) excited under normal bias magnetic field is theoretically studied in detail. It is shown that: (1) the diffracted light pulse approximates to an isosceles trapezoid, which is in agreement with the experimental results, and the duration time and the flat-top response time are equal to the sum and difference between the width and transit time of the rectangular MSFVW pulse, respectively; (2) utilizing a small duty factor of MSFVW pulse helps to improve the modulating data rate, which is in inverse proportion to the transit time, and the relative peak intensity of the diffracted light pulse is less than the corresponding Bragg diffraction efficiency when the MSW duty factor is less than 0.5.     

A combination of laser-induced grating and transient-absorption experiments for investigation of laser pulse properties and fast molecular relaxation processes

A repetitive low-power laser-pulse apparatus has been developped which allows both absorption relaxation and light-induced grating experiments without changing geometry or components. The influence of pulse width and coherence time on the diffracted intensity correlation function is discussed for a weak amplitude grating in terms of a simplified theory. From the corresponding absorption relaxation signals including the coherent coupling contribution some easy ways for detecting vibronic-relaxation, intersystem-crossing and orientational-relaxation times are deduced. The advantage of the in-situ measurement of the amplitude grating autocorrelation function leads to a precise zero-delay calibration of the transient absorption equipment. Furthermore one gets the response function for the absorption experiments from the grating experiments, if stable mode-locking operation of the argon laser is reached. This condition can be controlled either by the time course of the absorption or grating signals. A surprisingly short coherence length is detected for the cavity-dumped laser beam. A reliable check of the coherent-coupling theory confirms the theoretical assumptions incorporated into the fast-relaxation signal analysis. Experiments on dye molecules show high triplet yield of heavy-atom substituted dyes and fast rotational diffusion of oblong molecular rotors.     

Combination of differential discrimination and direct discrimination in pulsed laser timeof-flight systems

The output amplitude of the differential circuit is studied for differential discrimination in pulsed laser time-offlight systems. Based on the studies of the probability of detection and the probability of false alarms, the minimum detectable input signal of differential discrimination can be calculated. The results indicate that the differential discrimination detectability of the small signal will be reduced. A combined discrimination is proposed in this Letter to improve the time resolution of the large signal and ensure the probability of detection of the small signal at the same time. A proper value of the circuit parameter is found to balance the time resolutions of the small and large signals.     

多层长渡越时间轴向绝缘堆的闪络概率分析

 对基于J.C.Martin经验公式的多层长渡越时间轴向绝缘堆的闪络概率分析方法进行了分析与总结。在对比各种类似分析方法优劣的基础上，提出了一种较为成熟的针对多层长渡越时间轴向绝缘堆的闪络概率分析方法，为类似绝缘堆结构的设计提供了理论依据。同时，对其中的不确定因素和存在的问题也进行了较为详细的分析和讨论，为闪络概率分析方法的进一步完善奠定了基础。     

Study on probability of detection for fatigue cracks in sonic infrared imaging

Detection reliability of sonic infrared imaging is one of the increasingly important aspects for out of lab applications. And for the detection reliability evaluation, probability of detection (POD) for different defects under given test conditions has been successfully used as an accepted quantitative measurement. In this study, we test a set of C45 ferritic steel plates with artificial fatigue cracks. Experimental results show that the crack heating response increases with the increasing crack length, and the relationship between the logarithmic form of heat response signal and the crack length appears to be linear. Based on the above statistic characteristics, the linear regression analysis and the Wald method are adopted to estimate the POD function and its confidence interval. The study aims to provide a quantitative evaluation method for detection reliability in sonic infrared imaging.