The experimental characterization of wave propagation systems: II. Continuous systems and the effects of dispersion

This paper forms the second part of a study of the wave propagation approach and its application to the experimental characterization of structural/acoustic systems. It is shown that the measurement of time delays and attenuation factors by correlation techniques offers a useful complement to the measurement of frequency response in system characterization. Transient excitation is used and although the rapid frequency sweep is suitable for measurements in continuous systems, it is shown that the dispersion encountered in flexural wave propagation increases the difficulty in the interpretation of correlation data. To overcome this problem, another transient excitation signal is developed which has essentially single frequency content but which yields clear correlation functions, permitting the accurate measurement of group velocities at a single frequency.     

Proposal for Fourier-Transform Phase-Modulation Fluorometer

I propose a concept of a novel Fourier-transform phase-modulation fluorometer by which a fluorescence decay waveform can be obtained. In the fluorometer, the modulation frequency of the excitation light source is swept continuously from a start frequency f_min to an end frequency f_max with a time duration T. The resultant fluorescence signal waveform is Fourier-transformed to obtain amplitude and phase spectra. The ratio of the amplitude spectrum and the difference of the phase spectrum over those of the reference spectra that are obtained from a non-fluorescent material are calculated, respectively, and the pair of both spectral data is inverse-Fourier-transformed again to obtain the fluorescence decay waveform. To verify and demonstrate the effectiveness of the concept, I carried out (1) numerical simulations, (2) determination of a time constant of a passive resistor-capacitor (RC) differential circuit, and (3) measurement of a fluorescent decay waveform of YAG materials packed in Nichia’s white LED.     

Point source equalised by inverse filtering for measuring ground impedance

A point source for measuring ground impedance which complies with the omnidirectionality requirements of the ANSI S1.18 standard is presented. The source consists of a small aperture connected to a conventional loudspeaker through a cone. Whilst this source radiates with deviations lesser than ±1 dB within ±45°, its frequency response exhibits strong peaks arising from resonances of air inside the cone. To equalise these resonances, inverse filtering is applied. Inverse filters pre-emphasise the electrical signal driving the sound source so that zero-phase (or minimum-phase) cosine-magnitude signals are radiated. Equalisation by inverse filtering has two main advantages in the ground impedance measurement. On the one hand, flattening the frequency response avoids large fluctuations of the excess attenuation curve, which can make difficult its inversion to ground impedance data. On the other hand, inverse filtering shorten the time response of the sound source, this in turn making easier the positioning of a time window to separate ground reflection from reflections/diffractions coming from nearby objects usually present in the acoustic scenario.     

Application of time delay spectrometry for rough surface characterization

This paper describes the design and performance of an ultrasound measurement system, utilizing a swept frequency excitation signal, for analyzing the backscattered signal from planar rough surfaces. The implementation is in the form of a time delay spectrometry (TDS) system operating in reflection mode whose advantages are improved signal-to-noise ratio even with low peak power relative to conventional pulse-echo methods. Because of simultaneous transmission and reception, the TDS system requires two transducers. The TDS measurement system uses a swept frequency spectrum analyzer as the central analog processing unit. Both planar piston and focused transducers in the low MHz range were used for the measurements. Due to the statistical nature of rough surface backscatter, the mean of several statistically uncorrelated measurements is required to characterize the scattering behavior of a given rough surface. Backscatter data are obtained for a series of planar rough surfaces, in the form of scattering magnitude vs. frequency and vs. incident (=backscattered) angle. Analysis of the results reveals a good correlation between the root-mean-square (RMS) height and mean backscatter magnitude at 0 degrees incident angle, and between the ratio of RMS height to correlation length and the difference in mean backscatter magnitude between 0 degrees and 5 degrees.     

Surface plasmon polaritons frequency-blue shift in low confinement factor excitation region

Surface plasmon polaritons' (SPPs') frequency blue shift is observed in finite-difference time-domain (FDTD) simulation of parallel electron excitation Au bulk structure. Comparing with cold dispersion of SPPs, an obvious frequency blue shift is obtained in low confinement region excitation simulation results. Then, according to SPPs' transverse attenuation characteristics, the excited frequency mode instead of cold dispersion corresponding frequency mode matches it. Thence, this excited mode is confirmed to be SPPs' mode. As is well known the lower the frequency, the smaller the confinement factor is and the lower the excitation efficiency, the wider the bandwidth of excited SPPs is. And considering the attenuation in whole structure, the excited surface field contains attenuation signal. In a low confinement factor region, the higher the SPPs' frequency, the higher the excitation efficiency is, while broadband frequency information obtained in attenuation signal provides high frequency information in stimulation signal. Thence, in the beam-wave interaction, as the signal oscillation time increases, the frequency of the oscillation field gradually increases. Thus, compared with cold dispersion, the frequency of excited SPP is blueshifted This hypothesis is verified by monitoring the time domain signal of excited field in low and high confinement factor regions and comparing them. Then, this frequency-blue shift is confirmed to have commonality of SPPs, which is independent of SPPs' material and structure. Finally, this frequency-blue shift is confirmed in an attenuated total reflection (ATR) experiment. Owing to frequency dependence of most of SPPs' devices, such as coherent enhancement radiation and enhancement transmission devices, the frequency-blue shift presented here is of great influence in the SPPs applications.     

Attenuation lengths of low-energy electrons in solids

A compilation is presented of measured attenuation lengths of low-energy electrons in solids in the energy range (40 to 2000 eV) normally employed in X-ray photoelectron and Auger-electron spectroscopy. The techniques used to obtain electron attenuation lengths are summarized, and it is pointed out that the accuracy of measurement needs both to be defined adequately and to be improved for more meaningful intercomparisons of data and theory. An approximate expression is derived to predict attenuation lengths using either dielectric data (derived from optical or electron-energy-loss data) or average excitation energies estimated from electron binding energies for given materials at electron energies greater than about 500 eV. Good agreement is found between the predictions of this formula and some measured attenuation lengths (e.g. for Al, C, Mo, W) but further work is required to validate the formula and to extend it to lower electron energies.     

Using ultrasonic attenuation to monitor slurry mixing in real time

Staff at Pacific Northwest National Laboratory have developed and applied a simple ultrasonic attenuation measurement to measure slurry concentration in real time during suspension of solids settled in a large tank. This paper presents a simple single frequency ultrasonic measurement technique that demonstrates the ability of ultrasonic sensors to measure slurry concentration. Sensor calibration data show that in this attenuation regime ultrasonic signal attenuation is proportional to the applied frequency and to the slurry volume fraction. Real-time measurements of ultrasonic signal attenuation were used to track the process of slurry mixing using single sensors and sensor arrays. Results from two experiments show the use of real-time measurements of ultrasonic signal attenuation to track the process of slurry mixing in situ and to track the ability to maintain a well-mixed steady state condition. Comparison of concentration means of the ultrasonic measurements with concentration means obtained from discrete extractive measurements show that the distributions overlap and cannot be statistically distinguished. The real-time ultrasonic sensor can be used as a primary measurement method or to reduce reliance upon extractive methods to measure slurry density and solids concentration.     

Fabry–Pérot filter cavities for wide-spaced frequency combs with large spectral bandwidth

We use low-finesse Fabry–Pérot cavities in series to generate frequency combs with a large mode spacing in a way that allows its application to a large optical bandwidth. The attenuation of laser modes closest to the pass bands of the cavity exceeds 70 dB for a filter ratio of m=20 relative to the resonant modes centered within the pass bands. We also identify the best cavity geometry to suppress spurious transmission of higher order transversal modes. Such a thinned out frequency comb can be used to calibrate traditional spectrographs for precision astronomy. In the time domain mode filtering generates a pulse train with a multiplied repetition rate. High-fidelity filtering, as described here, implies small variations of the pulse energies.     

Control of modulated vibration using an enhanced adaptive filtering algorithm based on model-based approach

Conventional adaptive filtering algorithms, typically limited to the control of single or multiple sinusoids, are not appropriate to control modulated vibrations, especially in the presence of rich side band structures. To overcome this deficiency, a new control algorithm is proposed that introduces a feedback loop with the model predictive sliding mode control (MPSMC) in the adaptive filtering system. Several amplitude and frequency modulation cases are first computationally studied, and conventional and proposed methods are comparatively evaluated in terms of estimation error, performance in time and frequency domains, stability, and uncertainty in the reference signal. To experimentally validate the proposed algorithm, an active strut (with longitudinal vibrations) is constructed. Overall, the proposed adaptive algorithm yields superior reductions at the main frequencies and at side bands; also, good attenuation is found on a broadband basis.     

Estimation of shear wave velocity in gelatin phantoms utilizing PhS-SSOCT

We report a method for measuring shear wave velocity in soft materials using phase stabilized swept source optical coherence tomography (PhS-SSOCT). Wave velocity was measured in phantoms with various concentrations of gelatin and therefore different stiffness. Mechanical waves of small amplitudes (??10 ??m) were induced by applying local mechanical excitation at the surface of the phantom. Using the phase-resolved method for displacement measurement described here, the wave velocity was measured at various spatially distributed points on the surface of the tissue-mimicking gelatin-based phantom. The measurements confirmed an anticipated increase in the shear wave velocity with an increase in the gelatin concentrations. Therefore, by combining the velocity measurements with previously reported measurements of the wave amplitude, viscoelastic mechanical properties of the tissue such as cornea and lens could potentially be measured.     

A guided wave technique for the characterization of highly attenuative viscoelastic materials

The measurement of the acoustic properties of highly attenuative materials such as bitumen is very difficult. One possibility is to use measurements of the extent to which filling a cylindrical waveguide with the material affects the dispersion relationship of the cylinder. Torsional modes have been excited using piezoelectric transducers placed at one end of the cylinder, while the phase velocity and attenuation spectra have been measured by means of laser scanning. At each frequency, under the hypothesis of linear viscoelasticity, the phase velocity and attenuation of the fundamental torsional mode have been calculated as a function of the bulk shear velocity and the bulk shear attenuation of the inner core at that frequency. The resulting phase velocity and guided wave attenuation contour plots have been employed for deriving the unknown shear properties from the measured velocity and attenuation of the guided wave. The monochromaticity of the approach has not required a particular frequency dependence of the material properties to be assumed. Results for bitumen are given.     

HLS-Ⅱ储存环工作点测量系统的开发（英）

介绍了在合肥光源重大维修改造工程期间,开发的一套基于扫频激励法的工作点测量系统。测量系统硬件包括基于条带BPM的信号拾取电极、前端信号调理模块、一台带跟踪输出的信号分析仪N9000AEP、功率放大器以及用于激励束流的条带激励器。系统控制软件采用EPICS开发,基于EPICS Stream Device设计的输入输出控制器(soft IOC)运行在服务器虚拟机中,通过VXI11协议进行通信实现对信号分析仪的控制和数据获取;用户操作界面采用EPICS平台下的EDM开发,通过Channel Access与IOC通信。在HLS-II储存环上对工作点测量系统进行了在线的调试。调试结果说明,在扫频范围为2.1 MHz到4.1 MHz时,工作点测量系统的统计分辨率在水平方向上好于0.000 3,在垂直方向上好于0.000 2。     

Improved pulsed broadband ultrasonic spectroscopy for analysis of liquid-particle flow

The paper contains an investigation of broadband pulsed ultrasonic spectroscopy techniques, intended for testing of suspensions, such as a liquid-particle flow containing small diameter particles. Influence of traditional and novel broadband pulse shapes on quality of frequency spectra is analysed, as well as pulse design aspects leading to an optimal shape of an ultrasonic excitation wave. Effects that may influence signal quality and method reliability in a given setup, in particular resonances and noise are discussed. Solutions for signal acquisition and averaging techniques are presented, as well as results of testing of instrumentation limits and overall performance. Results of acoustic spectroscopy measurement of a concentrated liquid-particle flow are provided. A number of experimental and numerical examples, together with comprehensive explanations, show a potential for ultrasonic attenuation spectroscopy to be a successful methodology for an on-line measurement of fluid-particle suspensions and composite non-homogeneous materials in general.     

衰减匹配的超声Barker码激励方法

为增加超声穿透高声衰减介质的能力,提出了一种衰减匹配的超声Barker码激励方法。基于换能器高斯响应与材料非频散线性衰减的假设,得到了Barker码激励的信号模型,求解旁瓣抑制滤波后脉冲压缩的信噪比表达式可知,该方法仅需要根据材料衰减特性与轴向分辨率的要求,分别调整Barker码的中心频率与时长,便可以获得更高的信噪比。取衰减系数为1.4 Np/(MHz·cm)、厚度为5 cm的橡胶为试样进行验证。当与方波激励方法的轴向分辨率相近时,衰减匹配的Barker码激励方法比传统Barker码激励方法的信噪比增益提高接近5 dB;当牺牲一定轴向分辨率时,信噪比增益提高接近11 dB。结果表明,衰减匹配的Barker码激励方法可以降低依频率衰减对脉冲压缩的影响,有效提高衰减回波的信噪比。      

Characterization of carbon nanotube filters and other carbonaceous materials by Raman spectroscopy—II: study on dispersion and disorder parameters

Recently, we have reported on the characterization of various carbonaceous materials including multiwalled carbon nanotube (MWCNT) filters, which have specific molecular filtering capabilities and good mechanical strength and can be produced in bulk as highly aligned arrays of bundles of CNTs. We have extended our studies using Fourier transform‐Raman spectroscopy with 1064 nm excitation wavelength and a rotating sample holder in the region 1000–2800 cm⁻¹, in addition to 532 and 785 nm, which were used for Raman excitation in our previous study. Raman spectra were analyzed for band positions and line shape with special emphasis on the D‐, G‐ and G′‐ bands. For the single‐walled species, Carbotrap and graphite spectra were also recorded with 488 nm excitation. A dispersion study has been made from the Raman data available with the different excitation wavelengths. Slight band shifts and band broadening could be observed under the two sample conditions, one with the stationary sample and the other with sample rotation. The spectral changes are related to the excessive heating caused in a stationary sample by laser irradiation. Based on our findings in this study combined with our earlier study, we can state that only a careful line shape analysis and study of intensity pattern of the D‐ and G‐Raman bands under well‐defined measurement conditions lends itself as a good measure of degree of alignment in the MWCNT bundles. Copyright © 2010 John Wiley & Sons, Ltd.     

基于光外差技术的超宽带频率响应测量系统

介绍了一种利用光外差技术测量光电探测器超宽带频率响应的测试方法。将一个可调谐外腔激光器和一个固定波长的分布反馈激光器（DFB-LD’s）产生的激光输入到光电探测器进行混频。通过对可调激光器腔长的控制,可以在光电探测器产生从DC到上百GHz的拍频信号,在无需额外校准光源的情况下就可以进行光电探测器超宽带频率响应特性的测试,这是该方法最突出的优点。实验证明该方法比较准确、简便、易于操作。在实验中,对两个不同的InGaAs p-i—n探测器进行测量,得到器件的3dB带宽分别为14．4GHz和40GHz。该测量方法对同类实验的研究和应用都具有实用意义。     

A sliding Goertzel algorithm for adaptive passive neutralizers

A common method used in tuning adaptive–passive tuned vibration neutralizers is to adjust its resonance frequency to match the excitation frequency, which has the characteristic that the phase angle between its vibrating mass and its support is ?90°. A sliding-Goertzel algorithm is presented and demonstrated for extracting the vibration signals at the frequency of interest. The benefit of using the sliding Goertzel algorithm compared to other methods when used in vibration environments with multiple tones is that additional band-pass notch filtering is not required. This algorithm could also be used for adaptive tuned mass dampers, adaptive Helmholtz resonators, and adaptive quarter-wave tubes.     

High-speed terahertz time-domain spectroscopy based on electronically controlled optical sampling

We demonstrate high-speed terahertz (THz) time-domain spectroscopy based on electronically controlled optical sampling (ECOPS). The ECOPS system utilizes two synchronized Ti:sapphire femtosecond lasers with a 100 MHz repetition frequency. The time delay between the two laser pulses is demonstrated to be rapidly swept at a scan rate of 1 kHz on a time delay window of 77 ps by using an external offset voltage applied to a locking electronics. It is shown that a THz pulse can be exactly measured by ECOPS, as is done by asynchronous optical sampling (ASOPS), and the measurement time is shortened by a factor of 50 by using ECOPS compared with ASOPS in the case of employing 100 MHz repetition-rate lasers.     

Vibration characteristics of bone conducted sound in vitro

A dry skull added with damping material was used to investigate the vibratory pattern of bone conducted sound. Three orthogonal vibration responses of the cochleae were measured, by means of miniature accelerometers, in the frequency range 0.1-10 kHz. The exciter was attached to the temporal, parietal, and frontal bones, one at the time. In the transmission response to the ipsilateral cochlea, a profound low frequency antiresonance (attenuation) was found, verified psycho-acoustically, and shown to yield a distinct lateralization effect. It was also shown that, for the ipsilateral side, the direction of excitation coincides with that of maximum response. At the contralateral cochlea, no such dominating response direction was found for frequencies above the first skull resonance. An overall higher response level was achieved, for the total energy transmission in general and specifically for the direction of excitation, at the ipsilateral cochlea when the transducer was attached to the excitation point closest to the cochlea. The transranial attenuation was found to be frequency dependent, with values from -5 to 10 dB for the energy transmission and -30 to 40 dB for measurements in a single direction, with a tendency toward higher attenuation at the higher frequencies.     

噪声对等光频采样调频连续波激光雷达测距精度的影响分析及实验验证

调频连续波激光雷达具有测量范围大,精度高,无需合作目标等优点,在计量学和工业现场测量中具有重要作用。简单介绍了等光频重采样调频连续波激光雷达的基本结构和测距原理,分析了系统辅助干涉信号和测量干涉信号中存在的主要噪声及其特点。当系统辅助干涉信号中存在噪声时,会造成极值点不准确并且引入测量误差。随后,使用Cramér-Rao下界定理评估测量干涉信号的噪声对测量结果的影响。为了提高测量的准确性和稳定性,基于经验模态分解的小波阈值滤波和汉宁窗带通滤波结合小波滤波的自适应滤波方法分别用来去除了辅助干涉信号和测量干涉信号中的噪声。实验中多次测量了平面镜和多种粗糙度样块,并使用精密导轨验证测量结果的准确性。实验结果表明,当被测物位于3.9 m左右时,使用自适应滤波方法去除噪声后,系统对反射镜和其他粗糙度样块的测量不确定度为20 μm和几十微米(K取值2),远小于使用小波阈值滤波的方法(120 μm和几百微米)。同时,通过对比精密导轨位移数值和系统的测量结果,证明了所提出的方法能够有效提高系统的测量准确性。