材料非线性衰减系数的二次谐波测量方法研究

采用有限幅值法测量材料在基波和非线性引起的二次谐波作用下的衰减系数:利用准线性下的KZK方程推导基波和二次谐波的声压分布,并提取波束修正系数;采用短纯音信号进行非线性实验,对检测得到的基波和二次谐波声压进行衍射修正处理,有效抑制衍射对衰减系数测量的不利影响,继而通过线性拟合的方法计算得到更精确的基波和二次谐波的衰减系数。以水为例进行实验,研究了实验测量所得衰减系数的频率依赖关系,结果表明在非线性条件下水的衰减系数与频率间存在较强的线性关系,而线性条件下衰减系数随频率呈现二次方增长的特性则不适用于非线性条件。该研究提出了准确测量非线性声波衰减系数的方法,为更有效地应用非线性超声检测提供理论依据。      

RLC串联电路暂态过程衰减系数的谐波分析

分析了RLC串联电路欠阻尼振荡过程衰减系数α的理论值与实验测量值偏离的原因.将阻尼振荡波按傅里叶级数展开为基波与谐波之和,分别以基波与各次谐波代替阻尼振荡波,测出电感和电容上的损耗电阻,从而对衰减系数α的理论值进行了修正,使得α的理论修正值与实验测量值相一致.     

TDLAS波长调制法中调制深度与高次谐波中心幅值关系的研究

可调谐半导体激光光谱技术（TDLAS）是近年来发展十分迅速的光谱检测技术,相较于其他光谱检测技术,它具有高灵敏度、高分辨率、实时监测、便携性好、小型化等优点,在工业环保、医疗检测、气象监测等领域得到了广泛的应用。TDLAS波长调制法中谐波信号易受气压影响,经研究发现气压的影响是调制深度对谐波信号的影响,基于TDLAS技术谐波法的原理,研究了各次谐波与调制深度的关系,通过计算四次谐波与二次谐波中心幅值比,利用调制深度函数推算当前气压环境的调制深度,调整调制频率幅度,使得调制深度接近各次谐波最佳调制深度值,使谐波信号信噪比最佳,提高检测精度。实验通过国瑞智GRZ5031湿度发生器产生固定为1 000 ppm的水汽,调节气阀控制密封箱内不同的气压环境,采用TDLAS水汽检测系统获得了10.2~177.9 kPa气压条件下的二次谐波和四次谐波信号,并进行了仿真与实验分析。仿真结果显示：四次谐波与二次谐波中心幅值比的理论值和仿真值最大相对误差为-1.44%,调制深度的理论值与仿真值最大相对误差为1.78%,说明了仿真下的调制深度函数曲线与理论一致。实验结果显示：根据调制深度函数推算调制深度值,当m=2.226 7时,实测的二次谐波中心频率幅值达到最大值,当m=4.061 0时,实测的四次谐波中心频率幅值达到最大值,与理论结果一致;在30.2 kPa<p<177.9 kPa时,调制深度与气压乘积mp值相对误差较小,最大相对误差不超过±3.2%,说明了此气压条件下的mp值波动不大,通过调制深度函数推算的调制深度值与实际值近似,验证了调制深度函数理论的准确性。     

非线性声场中的参量激发

本文基于求解单频声波方程近似解的方法,得到了非线性声场中谐波的声压与介质性质、初始声压幅值及频率之间的定量关系.并对两列相对声压幅值和相对频率不同情况下的声场分布进行了研究.通过分析单、双频声源辐射场中的谐波分布和传播规律发现:在非线性声场中会不断地出现新的谐波,激发的各阶谐波随着声波传播距离的增大逐渐增强而后减弱.在声源的附近,谐波的声压随基波声压振幅的增大而增大;但在基波的频率增大时反而会减小.在输入总声能相同的情况下,与单频声场相比双频声源辐射场的声能量分布较均匀,声的传播距离较大,远场中的谐波含量较大.结果表明,基波的频率越高,衰减得越快,谐波的积累越缓慢;声压的极值越大,基波声能量转移得越多,产生的谐波越多,基波的衰减越快,声压对远场声能的负效应增大;如果改用多频声源,并适当地控制输入声波的组成成分,可以达到改善声场分布均匀性、增大声辐射距离的效果.     

大振幅驻波的实验研究Ⅱ：驻波场谐波的饱和

大振幅驻波在激波形成前的过程中，谐波声压级随基波声压级（Ｌｐ１）增长而出现饱和现象。在Ｌｐ１＜１５３ｄＢ时，ｍ次谐波声压振幅ｐｍ与基波声压幅值ｐ１；呈ｐｍ～ｐｍ１关系。基波升高１ｄＢ，ｍ次谐波升高ｍｄＢ。之后，各次谐波均开始趋于饱和。次数越高，饱和越快，ｐｍ～ｐｍ１的关系逐渐变为ｐｍ～ｐ１的趋向。在频率域内，变化过程大致可划分为三个区域：Ｉ．线性区；Ⅱ．变化区；Ⅲ．激波区。三个区域对应了时域的激波形成。Ｌｐ１＞１６０ｄＢ后，即可认为激波已经形成。基波与电信号的关系也呈饱和现象，但与频率有关。上述结果有助于理论研究工作的进一步深入。     

超高速碰撞产生等离子体的频谱响应特征

 针对探针与接地极板间的阻抗匹配问题,通过对超高速碰撞产生等离子体特征参量诊断系统的理论分析,获得了超高速碰撞产生等离子体的频率组成特征;基于该理论特征建立了一种适用于超高速碰撞产生等离子体的探针测量系统,并利用该探针测量系统进行了LY12实心球形铝弹丸超高速碰撞LY12铝靶实验,得到了该实验中超高速碰撞产生等离子体的功率谱特征。理论分析及实验获得的功率谱结果均表明超高速碰撞产生的等离子体具有低频为主的频谱特征：当频率低于5.8 kHz时,功率谱线比较平滑且幅值较小;频率达到11.0 kHz时,功率谱线的峰值和功率全谱峰值相近。因此频率在5.8~11.0 kHz范围的低频频段对功率谱线的峰值贡献较大,频率超过11.0 kHz时,功率谱线的大幅度抖动对功率谱线的峰值贡献较小。理论及实验结果证明了实验系统的可靠性和实现过程的合理性。     

兰姆波非线性效应的实验观察(Ⅱ)

基于Ritec-SNAP系统对固体板中传播的兰姆波的非线性效应进行了实验观察。根据导波的模式展开分析方法和兰姆波的频散曲线,简述了兰姆波的积累二次谐波发生条件。采用一定倾角的斜劈换能器在固体板表面激发和接收兰姆波的基波和二次谐波时域信号,阐述了兰姆波的基波、二次谐波时域脉冲包络的积分振幅的物理意义,在固体板表面分别测量了不同传播距离的兰姆波的基波、二次谐波的幅频曲线。在兰姆波具有非线性效应的频率值附近,分析了兰姆波的二次谐波振幅随传播距离的变化关系。实验结果进一步证明了兰姆波在一定条件下具有强烈的非线性效应,其二次谐波表现出随传播距离积累增长的性质。     

双层散射介质的单次后向散射光谱分析

从上皮组织的结构特点出发,基于米氏(Mie)散射理论,建立了双层散射介质的单次后向散射光谱的理论模型,该模型通过偏振门屏蔽来自下层的噪声背景,只保留来自上层的单次散射光。计算分析了粒子的形态学参量如平均尺寸及其分布、相对折射率变化时,单次散射光谱的特征。并用傅里叶波形分析法研究了这些参量对单次后向散射光谱曲线形状及其谐波幅值的影响。结果表明,这些只经历了表层粒子单次散射的光谱信号对表层粒子的平均尺寸及其分布、相对折射率具有灵敏性。对光谱曲线波纹结构的幅值、频率,散射强度,光谱谐波的幅值有直接的影响。研究结果对早期癌症的散射光谱特征识别及其特征提取有重要的实用价值。     

基于谱线频率特征的调谐二极管激光吸收光谱参数分析

调谐激光二极管吸收光谱(TDLAS)技术因其高分辨率、高灵敏度和快速测量等优点在工业生产、环境污染监测等方面受到广泛应用。波长调制光谱（wavelength modulation spectrum, WMS）的二次谐波信号经常用作气体浓度反演的检测信号。TDLAS检测性能与系统参数,如锁相放大器的时间常数、扫描幅度、扫描频率、调制幅度、调制频率等的选取紧密相关,实际测量中各参数的选择多以谱线形态特征为依据,参数之间的关联性未能得到较好体现。由于信号的采样与处理均在频域对谱线产生作用,各参数之间的作用相互关联。然而很少有研究参数对谱线频域的影响,针对此问题,在一定的理论基础上通过实验分别观察各调制参数对二次谐波信号的影响。通过保持其他参数不变,只改变一个参数的方法,得出各个参数对信号线型、频率特征及噪声引入的影响规律,继而分析并验证了多参数联合变化对谱线频带的决定作用。与基于时域特征的传统方法相比,基于谱线频率特征分析一方面具有与谱线信号采集检测处理机理相近的优点,另一方面可以直观得到各参数对主频带的影响和不同频率信号的衰减趋势。总结出基于频率特征的各参数的基本选取方法,以谱线频带和截止频率相互关系为判定标准,截止频率的大小由锁相放大器时间常数决定。通过设置合适的时间常数和扫描参数使信号频带与截止频率相近但不相交,使谱线频带内频率分量不产生衰减,频带外噪声得到最大抑制;再根据锁相放大器的性能和信号信噪比来确定调制参数,使谱线主频幅度最大;最后根据系统需求确定采样率。单周期采样点不变时,低扫描频率时检测精度相对提高但耗时较长;反之,扫描频率提高,速度变快但检测精度下降。通过联合影响规律调整关联参数,减小硬件限制对参数最优值选取造成的影响。可在考虑系统检测需求与硬件条件限制的前提下,通过参数选择得到最优二次谐波信号,为此技术的实际应用提供了参数优化的实验依据与参考方法。     

基于TDLAS-WMS的痕量甲烷气体检测仪

甲烷是一种无色、无味、易燃、易爆的气体,不仅造成煤矿作业的重大安全隐患,而且又是温室效应的重要气体之一,对于甲烷气体的监测具有极其重要的意义。采用混合可调谐二极管激光吸收光谱(TDLAS)与波长调制光谱(WMS)的检测技术,利用甲烷的2v3(第二泛频带)带R(3)支带吸收谱线,设计并研制出痕量甲烷气体检测仪。通过调谐系数-0.591 cm-1·K-1,采用改变DFB激光器工作温度的方式来获得甲烷在1.654 μm处的最佳吸收谱线。待DFB激光器激射中心谱线选择后,通过调节其注入电流幅值来获得合适的发光强度。同时,结合频率调制技术将待测信号频率移至高频区,减小1/f噪声。在光学结构方面,采用有效光程为76 m的herriott气室,确保对痕量甲烷气体进行检测。利用该痕量甲烷气体检测仪,在被测气体浓度为50～5 000 μmol·mol-1的范围内,对二次谐波信号进行了提取,并利用最小均方误差准则分别对气体浓度、信噪比的关系、谐波峰值信号与气体浓度的关系进行了线性拟合,最低检测限达到了1.4 μmol·mol-1。实验表明,谐波波形对称性良好,未观察到强度调制现象,消除强度调制等因素对谐波检测的影响。     

In vitro microemboli classification using neural network models and RF signals

Emboli classification is of high clinical importance for selecting appropriate treatment for patients. Several ultrasonic (US) methods using Doppler processing have been used for emboli detection and classification as solid or gaseous matter. We suggest in this experimental study exploiting the Radio-Frequency (RF) signal backscattered by the emboli since they contain additional information on the embolus than the Doppler signal. The aim of the study is the analysis of RF signals using Multilayer Perceptron (MLP) and Radial-Basis Function Network (RBFN) in order to classify emboli.Anthares scanner with RF access was used with a transmit frequency of 1.82 MHz at two mechanical indices (MI) 0.2 and 0.6. The mechanical index is given as the peak negative pressure (in MPa) divided by the square root of the frequency (in MHz). A Doppler flow phantom was used containing a 0.8 mm diameter vessel surrounded by a tissue mimicking material. To imitate gas emboli US behaviour, Sonovue microbubbles were injected at two different doses (10μl and 5μl) in a nonrecirculating at a constant flow. The surrounding tissue was assumed to behave as a solid emboli. In order to mimic real clinical pathological situations, Sonovue concentration was chosen such that the fundamental scattering from the tissue and from the contrast were identical. The amplitudes and bandwidths of the fundamental and the 2nd harmonic components were selected as input parameters to the MLP and RBFN models. Moreover the frequency bandwidths of the fundamental and the 2nd harmonic echoes were approximated by Gaussian functions and the coefficients were used as a third input parameter to the neural network models. The results show that the Gaussian coefficients provide the highest rate of classification in comparison to the amplitudes and the bandwidths of the fundamental and the 2nd harmonic components. The classification rates reached 89.28% and 92.85% with MLP and RBFN models respectively.This short communication demonstrates the opportunity to classify emboli based on a RF signals and neural network analysis.     

基于ip-iq理论的改进谐波检测方法

有源电力滤波器(APF)是电力系统中实现电能质量管理的关键设备,谐波检测作为APF的关键技术之一,检测的精度和响应速度直接决定了APF的整体性能。介绍了传统的基于瞬时无功功率理论的ip-iq谐波电流检测算法; 针对其检测精度受锁相环输出相位误差影响,以及低通滤波器的存在限制了谐波检测系统的响应速度,提出了应用锁频环技术,产生与电网电压同频的单位正余弦信号的一种改进谐波检测算法; 并提出采用滑动平均滤波器(MAF)代替低通滤波器提高其响应速度以及改善谐波检测精度; 对改进的检测算法进行了理论分析,通过MATLAB进行仿真研究,理论分析和仿真结果都证明了该方法的有效性。     

Technique of Broadband Measurements of Frequency Conversion Efficiency for Each Harmonique in Frequency Multipliers up to Terahertz Range

The paper describes technique of quantitative analysis of a millimeter- and submillimeter-wave range multipliers performance in particular the measurement of the frequency conversion efficiency. The output power on each harmonic produced by the multiplier can be measured separately without any changes in the setup even in the case of simultaneous emission of all harmonics including the fundamental in a frequency range up to 1 THz and well above. The measuring technique primarily developed for high resolution microwave spectroscopy is based on absorption of radiation in spectral lines of rarefied sample gas and photo-acoustic method of the absorption detection. Sensitivity independence of the detection method on frequency and well known spectral parameters of the lines provide the technique with broadbandness, high spectral resolution and power measurement reliability. Principal limits of measurable radiation power may vary from nanowatts up to hundreds of watts. The technique permits real time operation convenient, e.g., for multiplier tuning. Practical examples include measurements of harmonic generation efficiency of multipliers of 78-118 GHz fundamental frequency range, and power measurement up to 6-th harmonic of 230 GHz fundamental radiation of phase locked Backward Wave Oscillator (BWO).     

Quantum ratchet driven by broadband perturbation

Motion of an atomic ensemble trapped into a deep optical lattice is considered. We propose a novel approach to construct an atomic ratchet by superimposing two additional lattices whose amplitudes are small and subjected to broadband modulation. The broadband modulation is modeled by means of harmonic noise. Directed atomic transport occurs with the properly chosen phase shift between the signals modulating the amplitudes of the additional lattices. It is shown that efficiency of the ratchet depends non-monotonously on the parameter determining the spectral width of modulating signals.     

小波变换应用于谐波谱线的噪声滤除与基线校正

红外光谱谐波检测系统中的噪声与基线漂移问题一直是光谱处理的热点,提出一种采用小波变换的Mallet分解算法, 解决谐波检测中各种复杂噪声以及基线漂移的问题。选取适当小波函数及分解层次将谐波曲线中含有的噪声和基线漂移与有用信号分解到不同频带;分析频带信息,设定一个检测信息频带, 应用阈值处理及系数置零的方法使频率处于此频带的信息保留下来。小波变换方法可以在一次分解与重构过程中同时去除谐波信号的噪声与基线的双重干扰,从而将谐波信号有效地测量出来。实验证明,应用小波变换进行谐波校正的方法可应用于不同的谐波检测系统,具有普遍适用性。     

托卡马克电源窄带宽近基频间谐波检测方法

为解决托卡马克磁体电源系统中出现的低频谐波检测问题,研究了基于可调参数 Q 的 F42N150 单 频模拟陷波器和椭圆数字带通滤波器的混合检测算法。通过边界优化有效剔除干扰基波及 5 次谐波,实现有效的 频谱分离。利用 MATLAB 优化了为设计过程进行,使用基于 DSP 的数字分析系统进行了试验验证,验证了检测 算法的正确性和有效性。      

Stochastic resonance in an under-damped bistable system driven by harmonic mixing signal

Stochastic resonance(SR) is studied in an under-damped bistable system driven by the harmonic mixing signal and Gaussian white noise. Using the linear response theory(LRT), the expressions of the spectral amplification at fundamental and higher-order harmonic are obtained. The effects of damping coefficient, noise intensity, signal amplitude, and frequency on spectral amplifications are explored. Meanwhile, the power spectral density(PSD) and signal-to-noise ratio(SNR) are calculated to quantify SR and verify the theoretical results. The SNRs at the first and second harmonics exhibit a minimum first and a maximum later with increasing noise intensity. That is, both of the noise-induced suppression and resonance can be observed by choosing proper system parameters. Especially, when the ratio of the second harmonic amplitude to the fundamental one takes a large value, the SNR at the fundamental harmonic is a monotonic function of noise intensity and the SR phenomenon disappears.     

局域基谐波频谱峰值比的超声图像融合

提出一种局域基谐波频谱峰值比(LFHR)的超声图像融合方法。首先,分离超声射频信号基谐波,计算二者的局域频谱峰值比,确定融合权值;然后,加权平均基谐波成分,得到融合射频信号;最后,经过希尔伯特变换、对数压缩、插值/降采样生成B超融合图像。非均匀囊肿组织模型仿真结果表明,LFHR法获取的融合图像较原始图像的对比度、对比度噪声比、组织杂波比分别提高2.44倍、2.80倍及3.22倍,谐波成分谱能量提高83%。人体颈动脉临床实验结果进一步验证了LFHR法的有效性。综上,LFHR法获得了组织轮廓清晰内部细节丰富的B超融合图像,图像质量显著提高。      

Robust and accurate fundamental frequency estimation based on dominant harmonic components

This paper presents a new method for robust and accurate fundamental frequency (F0) estimation in the presence of background noise and spectral distortion. Degree of dominance and dominance spectrum are defined based on instantaneous frequencies. The degree of dominance allows one to evaluate the magnitude of individual harmonic components of the speech signals relative to background noise while reducing the influence of spectral distortion. The fundamental frequency is more accurately estimated from reliable harmonic components which are easy to select given the dominance spectra. Experiments are performed using white and babble background noise with and without spectral distortion as produced by a SRAEN filter. The results show that the present method is better than previously reported methods in terms of both gross and fine F0 errors.     

Continuous-wave single-frequency 532 nm laser source emitting 130 W into the fundamental transversal mode

The nonlinear effect of second-harmonic generation is an efficient way to realize high-power green laser sources. But when scaling up the harmonic power, many setups reported in the literature have been limited by conversion efficiency degradation or the fundamental laser power. Here we report on the generation of 134 W of cw laser light at a wavelength of 532 nm from a fundamental power of 149 W by second-harmonic generation in an external optical resonator comprising a lithium triborate crystal. The external conversion efficiency was 90%. The harmonic light consisted of a single spectral line. At least 97% of it was emitted into the fundamental transversal mode.