A new multichannel time reversal focusing method for circumferential Lamb waves and its applications for defect detection in thick-walled pipe with large-diameter

This paper proposes a new multichannel time reversal focusing (MTRF) method for circumferential Lamb waves which is based on modified time reversal algorithm and applies this method for detecting different kinds of defects in thick-walled pipe with large-diameter. The principle of time reversal of circumferential Lamb waves in pipe is presented along with the influence from multiple guided wave modes and propagation paths. Experimental study is carried out in a thick-walled and large-diameter pipe with three artificial defects, namely two axial notches on its inner and outer surface respectively, and a corrosion-like defect on its outer surface. By using the proposed MTRF method, the multichannel signals focus at the defects, leading to the amplitude improvement of the defect scattered signal. Besides, another energy focus arises in the direct signal due to the partial compensation of dispersion and multimode of circumferential Lamb waves, alongside the multichannel focusing, during MTRF process. By taking the direct focus as a time base, accurate defect localization is implemented. Secondly, a new phenomenon is exhibited in this paper that defect scattered wave packet appears just before the right boundary of truncation window after time reversal, and to which two feasible explanations are given. Moreover, this phenomenon can be used as the theoretical basis in the determination of defect scattered waves in time reversal response signal. At last, in order to detect defects without prior knowing their exact position, a large-range truncation window is used in the proposed method. As a result, the experimental operation of MTRF method is simplified and defect detection and localization are well accomplished.     

Analytical investigation of response of birefringent fiber Bragg grating sensors in distributed monitoring system based on optical frequency domain reflectometry

When Fiber Bragg gratings (FBGs) are used as strain sensors, both longitudinal and lateral strain can be applied uniformly or non-uniformly over the length of the FBGs. In order for the demodulation of such FBG signal, this paper investigates the response of birefringent FBGs which are monitored by distributed measurement system based on optical frequency domain reflectometry. A numerical model of the distributed measurement system is built based on piece-wise uniform approach, which considers polarization states of propagating lights. The numerical model simulates analytical response of birefringent FBGs especially when birefringence induces power fluctuations in the distributed spectra, which can be noise or new opportunity for sensitive monitoring of birefringence. Simulation results show the relationships between the power fluctuations and the polarization states of the propagating lights. Consequently, appropriate methods of polarization control for sensitive distributed birefringent FBG monitoring are discussed.     

Electrical field analysis of metal‐surface plasmon resonance using a biaxially strained Si substrate

Electrical field components of metal‐surface plasmon resonance were analyzed in detail. Both longitudinal optical (LO) and transverse optical (TO) phonon modes of a biaxially strained Si layer can be excited by surface‐enhanced Raman spectroscopy (SERS). The z to y polarization ratio in SERS measurements was calculated to be 0.78 using the intensity ratio of TO to LO phonon modes. The electrical field components of SERS were also calculated by the finite‐difference time‐domain method. Copyright © 2014 John Wiley & Sons, Ltd.     

前后光栅周期对于双光栅结构薄膜太阳能电池光俘获效应的影响

本文用时域有限差分法对硅层等效厚度为100 nm的具有不同前后光栅周期的介质/金属双光栅结构薄膜太阳能电池进行了模拟分析,比较了三角形最佳相同与不同周期光栅结构的吸收光谱特性,分析了光栅高度、填充比、硅吸收层厚度对最佳相同和不同周期光栅结构光吸收特性的影响,以及相应结构中导致光吸收增强的共振模式.结果表明前后光栅周期为1:1的共形双光栅结构中存在光泄漏现象,偏离1:1后的光栅结构可有效地抑制低级次衍射光的泄漏,前光栅周期小于后光栅周期的结构光吸收性能的提高来自于平面波导模式在吸收层中的有效激发和传播,而前光栅周期大于后光栅周期的结构光吸收性能的提高则来自于后光栅界面上所激发的等离子体极化模式.在较厚的硅吸收层厚度,前后光栅周期比为1:2和1:3的电池结构也会出现光泄漏现象,从而使具有最大光吸收效率的结构偏离这些周期比结构的位置.     

独立函数元:模型、方法和应用

针对常规线性变换获得的多分量成分可能不相干,但通常不满足统计独立的特点,提出了一种基于独立函数元的信号分解和重构的方法.该方法不仅继承了线性变换的诸多优点,而且还有统计域表征的优点.讨论了独立函数元的模型、定义和获取方法,详细分析了心音独立函数元在心音信号处理方面的应用.实验验证了所述方法的有效性和实用性.     

一种新型高透射率高品质因子光子晶体耦合腔波导结构

提出了一种在二维三角晶格光子晶体线缺陷波导中放置椭圆空气孔的耦合腔波导结构。基于平面波展开法,利用MPB对线缺陷波导的能带结构进行了计算并给出能带图。基于时域有限差分法（FDTD）,利用MEEP对椭圆空气孔在波导中的排列方式、数量、尺寸进行优化设计,并对频率位于微腔共振频率处的光波在耦合腔波导结构中的品质因子和传输特性进行研究与比较,给出了电场分布图。仿真结果表明,当椭圆空气孔的长轴方向纵向排列时,相应的微腔共振频率在光子禁带内可获得90%以上的透射率,对应的品质因子Q可达10⁴量级;选择合适的参数,获得的Q可高达10⁷量级,对应共振频率的透射率仍在60%以上。     

1.66 μm和 1.55 μm光源对长距离光纤拉曼温度传感器测量时间影响的理论对比

通过理论分析,在综合考虑长距离光纤拉曼温度传感器中各光学器件波长相关特性的基础上,比较了1.66 μm和1.55 μm光源对传感器测量时间的影响.计算结果表明,由于各光学器件在1.5 μm波段的高性能和最大允许入纤脉冲峰值功率的增加,采用1.66 μm光源的光纤拉曼温度传感器,最高可获得约1.94倍的背向反斯托克斯信号.在相同空间分辨率和温度分辨率的情况下,测量时间可以降低约3/4.     

傅里叶望远镜部分周期信号解调及对成像的影响

为了解决傅里叶望远镜高功率短脉宽激光只能包含部分周期信号的难题,本文提出多脉冲拼接技术并对其基本原理和应用范围进行深入研究.首先介绍高功率短脉宽激光与声光移频器移频带宽之间的矛盾,进而引出利用傅里叶望远镜传统时间解调公式进行部分周期信号解调存在的困难.然后给出多脉冲拼接技术的基本理论,推导出理想情况下移频频率、脉冲重复率和脉宽满足的拼接条件.进而详细分析移频频率稳定性和脉冲重复率稳定性对多脉冲拼接成像结果的影响,并通过计算机仿真研究一般情况下脉宽变化对多脉冲拼接成像结果的影响.最后得出移频频率和脉冲重复率对成像无影响,激光脉宽变化小于25%仍可识别目标轮廓的结论.     

基于石墨烯纳米条波导边耦合矩形腔的等离子体诱导透明效应

为了减小器件尺寸、实现超快速响应和动态可调谐,研究了基于石墨烯纳米条波导边耦合矩形腔的单波段和双波段的等离子体诱导透明(PIT)效应,通过耦合模式理论和时域有限差分法从数值计算和模拟仿真两方面分析了模型的慢光特性.通过调节石墨烯矩形腔的化学势,同时实现了单波段、双波段PIT模型的谐振波长和透射峰值的可调谐性.当石墨烯的化学势增加时,各个波段PIT窗口的谐振波长逐渐减小,发生蓝移.此外,通过动态调谐石墨烯矩形腔的谐振波长,当石墨烯矩形腔的化学势为0.41—0.44 eV时,单PIT系统的群折射率控制在79.2—28.3之间,可调谐带宽为477 nm;当石墨烯矩形腔1, 2, 3的化学势分别为0.39—0.42 eV, 0.40—0.43 eV, 0.41—0.44 eV时,双PIT系统的群折射率控制在143.2—108.6之间.并且,整个系统的尺寸小于0.5μm~2.研究结果对于超快速、超紧凑型和动态可调谐的光传感、光滤波、慢光和光存储器件的设计和制作具有一定的参考意义.     

光电子发射引起的柱腔内系统电磁脉冲的模拟

 用时域有限差分法结合PIC粒子模拟方法，对光电子发射引起的圆柱腔内电磁脉冲现象进行了模拟，并对单能电子发射时电场的空间分布和系统电磁脉冲波形特征进行了分析。利用粒子抽样和间隔时间粒子注入的方法，得到了特定电子发射谱下的计算结果，并与非抽样方法所得的结果进行了比较。计算结果显示，采用该方法后，噪声略有增加，但计算要求的条件大大降低，计算的粒子数有效地减少，适用于3维粒子模拟计算；计算结果还显示，发射电子能谱越高，注量越大，表面电场区与饱和电场区的长度越短。