Modified null field method for elastic wave scattering from a partially debonded fiber-SH-waves

Debonded fibers influence the macro-mechanical behavior of fiber-reinforced composite materials. Debonded fibers contribute to the initiation and growth of cracks at the fiber/matrix interface. To examine such problems, the scattering of elastic SH-waves (problem of anti-plane strain) from debonded fibers is studied with a numerical method which can handle the mixed boundary conditions on the partially bonded fiber. A modification of the null field of T-matrix method is developed for this purpose. The modification is achieved by the introduction of a mathematical surface. The simultaneous solution of the integral representations of the field scattered by the mathematical surface and the actual fiber surface give rise to sufficient equations that permit solution of the debonded fiber problem. The scattering cross-section and the far field amplitude are calculated as a function of frequency, fiber properties and debonding area. This will be of interest in structural applications where such cross-sections can be used to compute the dynamical effective properties of damaged composites.     

采用解析法对Nd:YAG单晶光纤的温度场研究

采用解析法对Nd:YAG单晶光纤激光器热效应相关的光纤温度场分布进行研究。建立了Nd:YAG单晶光纤热模型,在单晶光纤所满足的边界条件下通过解析求解热传导方程,得到在高功率808 nm泵浦光抽运下产生946 nm激光的单晶光纤温度场分布,并与传统Nd∶YAG激光晶体的温度场进行比较,然后分别与同泵浦条件下的有限元数值方法的分析结果进行研究对比,最后分析泵浦光参数、单晶光纤参数等对温度场的影响。结果表明,功率为86 W的泵浦光入射至单晶光纤端面的最高温升仅为30.98℃,明显优于同泵浦条件下传统Nd∶YAG晶体的端面温升结果94.37℃,与有限元数值法得到的Nd:YAG单晶光纤19℃温升结果相比,该解析法结果更接近于实验的测量值31℃,能够更精确描述晶体光纤温度场。本文可对单晶光纤激光器热效应的精确研究提供一定参考,进而有利于提高单晶光纤激光器的性能。     

1 550 nm单模锥形光纤模场的演化特性

根据光纤模式传输理论,结合单模锥形光纤的结构,使用有限差分光束传播法(FD-BPM)仿真了1 550 nm波长下光纤模式的传输,分析包层内模场和纤芯内模场之间的相互影响。仿真结果表明:因为包层模的存在,随着包层内模式数的减少和纤芯对模场约束状态的变化,锥形光纤纤芯内不同区域模场分布变化明显,模场的均方根宽度在不同区域有不同程度的起伏。通过数值计算得到了光纤内模场能量的分布,发现因为包层模的存在,纤芯内模场能量减小速度比没有包层模时的速度要慢。     

An efficient shooting method for fiber amplifiers and lasers

A simple shooting method is proposed for the design of distributed multi-pumped Raman fiber amplifiers (RFAs) and Yb-doped double-clad fiber lasers (DCFLs). Using the proposed method a distributed RFA with 10 pump sources and 1700 mW total input pump power is simulated and high-power Yb-doped DCFL rate equations are solved numerically. The numerical simulation results show that the proposed method has good convergence in the condition of increasing numbers and power of input pump sources.     

1 550 nm单模锥形光纤模场的演化特性

根据光纤模式传输理论,结合单模锥形光纤的结构,使用有限差分光束传播法(FD-BPM)仿真了1 550 nm波长下光纤模式的传输,分析包层内模场和纤芯内模场之间的相互影响。仿真结果表明：因为包层模的存在,随着包层内模式数的减少和纤芯对模场约束状态的变化,锥形光纤纤芯内不同区域模场分布变化明显,模场的均方根宽度在不同区域有不同程度的起伏。通过数值计算得到了光纤内模场能量的分布,发现因为包层模的存在,纤芯内模场能量减小速度比没有包层模时的速度要慢。     

Experimental verification of improved effective index method for endlessly single mode photonic crystal fiber

We report the experimental verification of Improved Effective Index Method (IEIM) for Endlessly Single Mode Photonic Crystal Fibers (ESM PCFs) from far field intensity measurements. In this paper all the key parameters i.e. V-Number, effective refractive index of the cladding, radius of the core and numerical aperture have been obtained from the experimentally measured far-field intensity pattern of ESM-PCFs. It is shown that the transmission characteristics of ESM-PCFs match with the simulation result obtained using IEIM and also with the core diameter of PCF obtained from scanning electron microscope (SEM) within the experimental limit.     

Whittaker方程的场方法

用场方法来求解Whittaker方程.将一个场变量取作为其余场变量和时间的函数并对这个函数建立基本偏微方程.如能求得它的完全积分，那么Whittaker方程的解可由解代数方程来得到. 关键词： 场变量 基本偏微分方程 场方法 积分     

熔拉和腐蚀单模阶跃光纤的模场分析

考虑到包层厚度和包层外介质的影响 ,在单模光纤的高斯近似理论基础上 ,导出了单模阶跃光纤模场半径方程 ,并就光纤在传感等领域应用中常见的熔拉和腐蚀处理两种情况 ,进行了相应的数值计算 ,给出了近似解析解。引入特性转换点 ,对模场特性作了分析。     

实现散射场强整形的微散射体阵列逆向设计方法

本文提出一种基于全介电微散射体的散射阵列结构逆向设计方法,用以实现散射电场强度整形.该方法基于空域傅里叶变换与角谱变换,从给定目标区域处期望实现的散射场强分布出发,逆向求得所需的感应源,再利用电磁感应源理论,逆向设计微散射体阵列,且只需通过解析计算便能够快速地求取出微散射体阵列的电磁参数值分布.基于提出的逆向设计方法,文中提供了三维情况下的具体算例,实现了给定方形面目标区域的散射场强整形.理论计算结果与全波仿真结果符合良好,表明本文提出的逆向设计方法具有可行性与有效性.     

基于滤波法的光纤光栅传感解调方案

FBG传感器对应变和温度的敏感体现在其反射光中心波长的变化上，因此如何测量波长的变化就成为FBG传感器的关键。在光纤光栅的多种解调方法中，因滤波法的测量器件制作简单，测量系统简易、方便直观而被广泛应用。主要介绍了8种基于滤波法的解调方法，对它们各自的成本、检测精度、测量范围、器件制作以及适用的环境进行了详细的比较，分析了它们的优缺点，以便于在不同的检测条件下选用最佳的解调方案。重点介绍了光学小波滤波解调法，探讨了其测量系统的原理，并指出该方法可用于微弱信号的检测与处理。     

147 fs碳纳米管倏逝场锁模全光纤掺铒光纤激光器

利用化学腐蚀法在光纤包层表面成功制备了调制深度为3.9%的单壁碳纳米管饱和吸收体.组建了环形腔结构的全光纤掺铒光纤激光器,以制备的单壁碳纳米管薄膜为锁模元件,利用倏逝场锁模实现了锁模输出.锁模脉冲的中心波长为1556 nm,3 dB光谱带宽为24 nm,脉冲宽度为147 fs,重复频率为150 MHz.在520mW抽运功率下,平均输出功率为21 mW,相应的单脉冲能量为0.14 nJ.     

非自由声场中目标声场还原与重建的等效源方法

为消除在非自由声场中重建声场时干扰声源对重建效果的影响,提出一种采用单个测量面上的声压和质点振速作为输入、等效源法作为分离和重建算法的非自由声场中目标声场还原与重建方法。该方法首先利用单面声压-质点振速测量和基于等效源法的声场分离技术将测量的混合声场分离为来自目标声源的向外传播的声场和来自干扰声源的向内传播的声场,然后利用向内传播的声场和目标声源的边界条件计算出干扰声在目标声源表面产生的散射声场,并将其从向外传播的声场中去除,还原出目标声源在自由声场条件下的辐射声场,最后利用还原的声场实现目标声场重建。通过数值仿真和实验检验了该方法的有效性和必要性。仿真和实验的结果表明,该方法可以在非自由声场的测量条件下,有效地去除干扰声的影响,实现目标声场的准确重建。     

高功率双包层光纤激光器受激拉曼散射和热效应的理论研究

受激拉曼散射和热效应会限制光纤激光器功率的提高。利用高功率光纤激光器的速率方程和热传导方程,理论研究了双端泵浦和分布泵浦下双包层光纤激光器的受激拉曼散射和热效应,得到了光纤中的泵浦光、激光和斯托克斯光的功率分布,光纤激光器的输出特性以及光纤中的温度分布。分析表明,当泵浦功率增大到一定值时,光纤激光器中出现SRS,一部分激光功率会转移给斯托克斯光,影响激光功率进一步提高;与双端泵浦方式相比,分布泵浦下光纤激光器的斜率效率和最大输出功率相差不大,但是,光纤中的温度分布被有效地降低,因此,分布泵浦方式更为有效。     

T-matrix method for electromagnetic scattering from scatterers with complex structure

We describe a T-matrix program for light scattering calculations from particles with complex structure. The code treats the cases of homogeneous, layered and composite scatterers. These results are combined with basic results concerning the scattering by inhomogeneous scatterers and aggregates to apply to more general types of scatterers. Some numerical simulations are presented.     

Mode field diameter and cut-off wavelength measurements of single mode optical fiber standards used in OTDR calibrations

This paper describes determination of the mode field diameter (MFD), and the cut-off wavelength of the single mode optical fiber standards used in OTDR calibrations, which was constructed in Optics Laboratory of National Metrology Institute of Turkey. Measurements of MFD were performed using computer controlled stepper motor unit with far field scanning (FFS) Method. For the same standard fibers, cut-off wavelength measurement was performed employing bend reference method, which is one of the transmitted power techniques. For both measurements, a nitrogen-cooled 5 mm InGaAs radiometer was used to detect optical flux. Detailed uncertainty analyses were presented for the MFD and cut-off measurements. While the total expanded uncertainties in MFD data at 1306.7 nm and 1550 nm were calculated as 0.8% and 1.0% (k=2), respectively, the total expanded uncertainty at determination of cut-off wavelength was found as 0.318% (k=2).     

一种分析楔形光纤的等效矩形近似-阶梯串联法

楔形光纤(WSF)是实现平面光波光路芯片入出端口与光纤高效连接的核心部件. 采用数值模拟方法分析其中光波传输演化过程,是优化设计光子器件耦合结构的重要基础. 提出基于等效矩形近似的三维阶梯串联法(ERA-SCM),将楔端沿传输方向细分,引入矩形波导近似,给出了细分后各段近似矩形波导的等效折射率;在此基础上,建立了阶梯串联法分析模型,分级给出WSF中光波传输过程与模场演变. 数值分析结果表明,ERA-SCM比有限差分束传播法(FD-BPM)能够更精确地分析非对称光纤和波导结构,描述其中光模场的演化. WSF出射光场实测结果表明,ERA-SCM数值模拟结果与实验结果的误差为1.9%,而FD-BPM的误差为4.5%. ERA-SCM是分析非对称光波导光波传输与模场演变的有效方法. 关键词： 平面光波光路 楔形光纤 等效矩形近似 阶梯串联法     

用于缺陷定量的超声散射信号去相关分析及贝叶斯反演方法

针对结构中缺陷定量识别问题,建立了椭圆缺陷超声散射仿真模型,研究了缺陷偏转角度及尺寸对散射信号去相关特性的影响。结果表明,超声信号去相关系数矩阵与缺陷几何状态(如尺寸及角度)存在很大关联性,但并非线性相关。结合参数化有限元分析与贝叶斯理论,发展了一种基于超声散射信号去相关分析的缺陷反演方法,开展了缺陷反演数值仿真及检测实验。结果表明,发展的缺陷反演方法可以很好实现铝块与铝板缺陷位置及尺寸反演。其中铝块开口缺陷尺寸(深度)反演误差约为激励波长0.2倍,位置误差约为激励波长0.6倍;铝板圆形缺陷尺寸反演误差约为激励波长0.04倍,位置误差约为激励波长0.4倍。研究工作为结构中缺陷定量识别与评价做了有益探索。     

基于Pockels效应的高压电场光纤测量与Labview软件的实现

采用基于Pockels效应的高压电场测量方法,建立了电光效应电压测量系统。从理论上分析了单片BGO晶体测量电压的极限,提出了多片BGO晶体间隔排列测量高压的解决方案。通过引入虚拟仪器技术,以Labview作为软件开发平台编写软件程序和对采集信号进行处理与分析,提高了系统的稳定性和准确性,取得了良好的效果。     

Shielded method for fabricating the long-length phase-shifted distributed feedback fiber lasers

The successful fabrication of phase-shifted distributed feedback (DFB) fiber lasers is very difficult for a long time. The paper introduced the shielded method for fabricating the long-length phase-shifted DFB fiber lasers. The method is simple, convenient, effective and efficient for it can be finished one-off.     

Self-consistent field method and non-self-consistent field method for calculating the positron lifetime

Many methods are used to calculate the positron lifetime,these methods could be divided into two main types.The first method is atomic superposition approximation method and the second one is the so called energy band calculation method.They are also known as the non-self-consistent field method and self-consistent field method respectively.In this paper,we first introduce the two basic methods and then,we take Si as an example and give our calculation results,these results coincide with our latest experimental results,finally,we discuss the advantages and disadvantages of the two methods.