广义兰姆波的积累二次谐波发生研究

借助色散导波的部分波分析方法、二阳微扰理论及同的声非线性反射处理技术,本文对声波导中广义兰姆波的积累二次谐波发生进行了理论研究。结果表明,在一定条件下,源于声波导介质的体非线性、由文义兰姆波的部分波非线性相互作用所发生的二次谐波具有随传播距离积累增长的性质,文中给出了广义兰姆波的积累二次谐波发生所要满足的条件,并求解出相应的积累二次谐波之解析式。     

复合固体层中兰姆波的积累二次谐波发生与传播研究

文章提出一种分析复合固体层中兰姆波的积累二次谐波发生与传播的方法。基于导波的部分平面波分析理论，建立了复合固体层中非线性兰姆的理论模型。在分析过程中，将非线性作为线性情形的二阶微扰进行处理的方法。尽管复合固体层中兰姆波具有频散性质，分析结果表明，对于一些特定条件，二次谐波的振幅可以随传播距离积累增长。根据边界条件和初始激发条件，得到积累二次谐波的形式解，在此基础上进行了数值分析，给出一些有趣的声场分布。     

反向脉冲二次谐波增强研究

1引言 自从1997年T.Christopher提出使用超声波在传播过程中产生的谐波成分进行医学成像[1]以来,谐波成像方法和技术的研究得到重视,并逐步应用到医学诊断中.由于传声介质的非线性,声波在传播过程中发生畸变,并滋生谐波,谐波的阶次越高,谐波的能量越小,信号幅度越低.二次谐波的衰减较小,具有良好的指向性,因此作为医学谐波成像的首选信号.提高二次谐波信号是二次谐波医学成像的关键技术.传统的方法使用二次谐波带通滤波器,由于滤波器的带宽和品质因素的原因,实际效果不能令人满意.本文用反向脉冲技术消除基波,增强二次谐波,提高二次谐波成像的对比度,为二次谐波成像提供有效的方法.     

《声学技术》2003年总目录

论文与报告·色散介质中积累二次谐波的发生邓明晰 (0 3.1.1)……………………………………………………………水中目标强度的一致性时域有限差分计算冯玉田 ,王荣庆 ,王朔中 (0 3.1.4 )………………………………一种采用两个相干累加器提高自适应谱线增强器性能的方法王　彦 ,马章勇 ,黄建人 (0 3.1.8)…………HIFU聚焦探头声场功率测量的凹锥面反射靶设计分析　　　　　…………………………………………　　霍彦明 ,吴　敏 ,宋文章 ,叶兆雄 ,王智彪 (0 3.1.11)………………………………………………………用超声脉冲反射谱评价圆管厚度…     

大振幅平面声波在多层介质中的传播

本文用逐级近似法就一维情况下分析了大振幅平面声波在多层介质的界面上发生反射和透射的传播特性。在二级近似下计算了精确到二阶小量的二至三个界面各层的反射波和透射波的二次谐波，并推广至几个界面，得到一些规律性的结果。从拉格朗日坐标下的一级近似和二级近似的波动方程式出发，假定声波在多层介质中传播在各个界面仍是连续的，即各层界面的边界条件由界面两侧的压力平衡和质点速度相等来决定，由此求得各层界面在二级近似下的二次谐波的反射波和透射波。计算结果表明，二级近似下的反射波和透射波均由两部分组成，一部分是由一级反…     

二次谐波在二维材料结构表征中的应用

二次谐波作为非线性光学的重要分支, 逐渐成为表征晶体结构的重要手段之一。在众多表征方法中, 二次谐波因其无损检测、高稳定性、可调谐性、超快响应、偏振敏感性、通用性、操作简单等特点被广泛应用于二维材料结构表征, 为二维材料的物性研究和功能应用提供了重要信息, 大大推动了二维材料基础研究的快速发展。本文综述了近几年二次谐波在二维材料结构表征中的研究, 简述了二次谐波产生原理, 介绍了飞秒激光器接入共聚焦拉曼光谱仪产生二次谐波测试装置, 分别讨论了二次谐波在二维材料的层间堆垛层数、层间堆垛角度、单层二维材料晶界及晶体取向表征方面的应用。同时, 本文还介绍了采用二次谐波强度直接、灵敏地检测晶体中应变幅度以及通过二次谐波信号变化跟踪材料中的缺陷变化, 接着讨论了二次谐波与拉曼光谱、光致发光的多维度关联分析在材料全面深度表征方面的重要性。最后展望了二次谐波未来在材料结构表征中的潜在研究方向。     

二维非线性单原子正方晶格色散摄动分析

声子晶体的色散关系决定弹性波在其中的传播方式。从二维无限周期结构的波动方程出发,提出了一种分析非线性离散型声子晶体的色散关系的一阶近似摄动法。通过引入Bloch理论与小参数摄动展开法,得到了一阶近似的色散关系与频散曲线,以分析不同方向上的阻抗配置与非线性系数对频散及群速度的影响。以二维单原子正方晶格为例,得到了其一阶频散曲线。色散结果显示带隙及传播方向与波幅相关。最后模拟了晶格对点谐波激励的响应,以验证摄动分析有效性。     

超声导波混频表征和定位早期局部损伤的研究进展

超声导波具有远距离传输的特性，能够快速、有效地大范围检出薄板中的损伤或缺陷。非线性超声导波相较于传统超声导波，主要研究基波与材料中微观组织演化相互作用而产生的高阶谐波，对尺寸远小于基波波长的损伤或缺陷比较敏感。其中，超声导波的二次谐波相对容易激发，已被用于定量评估早期损伤。但是，超声导波的二次谐波容易受到测量系统非线性的干扰，并且无法定位材料中的局部损伤。超声导波混频在频率、模式、传播方向的选择上具有一定的灵活性，克服了二次谐波的缺点。目前，超声导波混频在理论、模拟和实验上取得了一定的进展，已被用于表征和定位金属材料中处于早期阶段的疲劳、热老化、微裂纹、冲击损伤和局部塑性变形等。高频段超声导波混频、兰姆波相向混频和非共线混频中差频谐波或和频谐波的传播性，以及更多类型损伤的定位和表征仍有待进一步研究。     

穆村站光电互感器畸变波形分析

通过穆村数字化变电站光电互感器试验时二次波形发生畸变,经过对波形的分析发现齐次谐波分量过高,针对生成谐波的主要因素,分析故障发生的原因并进行处理。     

高压直流输电系统换相失败谐波分析研究

本文利用pscad分析了高压直流输电过程中发生换相失败后非特征谐波的变化以及谐波对保护算法影响,并提出了相关的解决措施来避免谐波的影响并探讨了不稳定谐波的控制措施,为今后直流输电工程的设计、调试和运行提供有用的参考和技术积累。     

Second Harmonic Reflected Light

A review is given of the laws of non-linear optical reflection which govern the direction, polarization and intensity of second harmonic light generated in reflection. The complex non-linear susceptibility of III-V and II-VI piezoelectric compounds has been measured over a range of frequencies and shows characteristic dispersive properties. Both the amplitude and the phase can be determined. When the medium has a centre of inversion, the reflected second harmonic production is several orders of magnitude smaller. The effect has magnetic dipole and electric quadrupole character and has been observed in silicon, germanium and several metals and alloys. In metals the contribution from core electrons and the conduction electron plasma have the same order of magnitude.     

Suppression of propagating second harmonic in ultrasound contrast imaging

Contrast agents, such as bubbles, are used in ultrasound to enhance backscatter from blood. To increase contrast between these agents and tissue, nonlinear methods such as harmonic imaging can be used. Contrast is limited, however, by tissue second harmonic signals. We show that a major source of this signal is nonlinear propagation through tissue. In addition, we present methods to suppress this second harmonic generation. One simple approach is to decrease the f/number of the imaging system. Simulations show that doubling the size of the array, while keeping total power output constant, decreases propagating second harmonic generation. A second approach uses active noise cancellation to suppress second harmonic generation. A specific method, the harmonic cancellation system (HCS), is developed and presented as an example. In simulations, HCS decreased second harmonic generation by over 30 dB. Using such methods, contrast can be improved between tissue and bubbles in harmonic imaging.     

Second harmonic generation in the strong absorption regime

Abstract

A detailed theoretical analysis of the second harmonic generation in the strong absorption regime (great absorption index K _2w) is presented. Inside the nonlinear medium, it is found that only the harmonic wave travelling in the same direction as the fundamental one survives. Outside the nonlinear medium, the transmitted and reflected intensities of the harmonic waves show a peculiar dependence on k _2w- The ratio between transmitted and reflected intensities ranges from 5 to 75. Reflected and transmitted harmonic waves appear to be generated in the outer layers within a thickness of a fraction of wavelength. Experimental support is reported with application to a proton-exchanged LiNbO₃ waveguide.     

Impact Damage Detection in Composite Structures Considering Nonlinear Lamb Wave Propagation

To detect micro-structural damages like fiber/matrix cracks and delaminations in composite materials accurately new methods are developed. Previous works have investigated and shown that the acoustical nonlinearity parameter is a good tool to detect micro-structural damages like plasticity in metal and fatigue damages in metal and composites. In this work, the second harmonic generation is used to analyze composite specimens for impact damages. Therefore, Lamb waves are launched and detected by a piezoelectric actuator and sensor, respectively, at a certain frequency to generate cumulative second harmonic modes. The excitation frequency has to meet special conditions. The signal processing is done by using the wavelet transform to avoid misinterpretations that may occur using the short-time Fourier transform. The Morlet wavelet is used as the mother wavelet. The results of the relative acoustical nonlinearity parameter are compared to the development of the group velocity due to impact damages. It is shown that the relative acoustical nonlinearity parameter is more sensitive to impact damages than the development of the group velocity.     

Squeezing and photon antibunching in second harmonic generation: an analytical approach

With the help of second-order nonlinear interactions and hence Hamiltonian, we construct the equations of motion corresponding to pump and signal (second harmonic) fields in a second harmonic generation. The corresponding coupled differential equations involving the non-commuting field operators are not solvable in closed analytical forms. With the help of a perturbation method, we obtain analytical solutions of these field operators up to cubic orders in the interaction constant. In an appropriate limit (truncating the solution up to the second order in the interaction constant), these solutions lead to the existing solutions under the short-time approximation. The present analytical solutions are exploited to investigate the squeezing and the antibunching of photons of the input coherent light coupled to the said second-order nonlinear medium. We report the squeezing and antibunching effects of the pump field even for solutions up to the second order and hence the present results are consistent with earlier results. However, for the second harmonic mode, we report the squeezing involving the leading cubic term in the interaction constant.     

Numerical simulation of cascaded third harmonic generation for both phase matched and mismatched schemes

Numerical solutions to the nonlinear coupling-wave equations of second harmonic (SH) and third harmonic (TH) generators are investigated for both phase matched and phase mismatched configurations. For phase mismatched TH generation, several kinds of schemes (the phase mismatch either in second harmonic generation (SHG) or sum-frequency generation (SFG) process) are considered and analyzed. The physical nature corresponding to the different ratios of the coupling coefficients is discussed.     

On the possibility of obtaining attosecond pulse trains during second harmonic generation by high-intensity femtosecond pulses

The possibility of obtaining attosecond pulse trains during second harmonic generation by high-intensity femtosecond pulses is demonstrated by means of computer simulation. The attosecond pulses are formed at the basic frequency with a low efficiency (≤8%) of the energy conversion from first to second harmonic. The regimes of attosecond pulse generation at the double frequency are considered.     

Progress in the realization of a frequency standard at 192.1 THz(1560.5 nm) using 87Rb D2-line and second harmonicgeneration

Second harmonic generation of a 192.1 THz semiconductor distributed feedback (DFB) laser is achieved using a KNbO₃ crystal in a resonant ring cavity. Optical feedback from this cavity is used to stabilize the laser frequency and reduce its linewidth. A second harmonic power of 5.5 μW is generated with 38 mW incident on the cavity. We use the second harmonic signal to observe saturated absorption lines and orientation signals in rubidium vapor. Injection-locking of a 780 nm Fabry-Perot laser using the second harmonic signal is also demonstrated. With this scheme, we observe saturated absorption lines in rubidium