Emulator with inverse trend in first-order and second-order polarisation mode dispersion

We design a polarisation mode dispersion (PMD) emulator through subdividing a polarisation maintaining fibre (PMF) of 22 m in length. The aim of this emulator design is to show that first-order and second-order PMD can be inversely proportional to each other. Furthermore, the emulator is also used to show that the magnitude of PMD is independent to whether its statistics approach theoretical distributions or not, of most importance is the degree of mode coupling. The same (as former) applies to its autocorrelation function (ACF). The PMD control mechanism for the emulator is not in real time.     

Controlling of symmetric and asymmetric mode coupling in long-period fiber gratings singe-side induced by long-pulse CO2 laser

We report a simple fabricating approach to control the mode couplings in long-period fiber gratings (LPFGs) through side exposing fiber to long-pulse-10.6-μm laser from a cheap, internally modulated CO₂ tube. By tuning focused-spot size on fibers, not only circularly symmetric mode coupling but also asymmetric mode couplings can be effectively achieved. Simulation of mode profiles in grating cross-section with Finite Element Method (FEM), and LPFG-cladding etching experiment with hydrofluoric acid (HF), support our explanation that asymmetric mode coupling in LPFGs depends on local refractive-index (RI) change within an azimuthally thin cladding layer, resulted from large-spot method induced deep melt flow on fiber surface during CO₂ laser irradiation.     

Low threshold laser cavities of two-dimensional photonic crystal line defect mode

Two-dimensional (2D) rod-type photonic crystal (PC) line defect waveguide (LDW) laser cavities based on three types of line defect modes with zero group velocity are studied by using finite-difference time-domain (FDTD) method. These laser cavities have high quality (Q) factor, better localization of light, non-uniform gain distribution and small overlap between gain medium and light field. Therefore, they have the advantages over conventional and air-bridge PC cavities with uniform gain, such as low threshold, single mode lasing and effectively avoiding thermal effect. From their comparison, one can find the mode at middle Brillouin zones (BZ) is the best one to be used as lasing mode. Its dynamic lasing process and lasing features are demonstrated by the numerical experiment where the FDTD method coupling Maxwell's equations with the rate equations of electronic population is used.     

A simple technique for evaluation of transmission coefficient at the splice with angular offset and transverse offset for first higher mode propagation in graded index fibers

We present simple and accurate analytical expressions for transmission coefficient at the splice with angular offset and transverse offset in case of first higher mode propagation in graded index fibers. We employ the simple series expression for first higher order modal field for graded index fiber. The evaluation of the concerned parameters based on our formalism requires very little computations. Here, we show that our estimations match excellently well with the exact results in case of step and parabolic index fibers. Further, splice being highly tolerant for longitudinal separation, our analysis is judiciously restricted to the case of transverse and angular mismatches only. The analysis should find application in dual mode graded index fiber in all optical technology.     

灵敏度控制思想在主动膜系设计中的快速实现算法

 基于斜入射薄膜制备实践中镀膜误差对光谱性能的严重退化影响的认识,提出了一种基于灵敏度控制思想的主动膜系设计方法。在深入分析了镀膜中膜层结构参数误差的分布规律的基础上,运用膜系光谱系数关于膜层参数的导数计算的解析模型,建立了膜系灵敏度的定量计算模型和快速实现算法。以一45°入射高精度消偏振增透膜的设计实验为例,探讨了灵敏度控制思想在膜系设计中的可行性、快速性和有效性。结果表明,这一新型设计方法不会显著增加程序时间消耗,能获得具有良好可镀制性能的薄膜,对于正入射和宽角度入射膜同样适用,而且可以避免昂贵的失败试镀和采样,有助于缩短新薄膜的生产周期,特别是对于高精度斜入射薄膜的重复性制备具有重要意义。     

求解波束波导中激励-传输-辐射问题的积分方程方法

针对波束波导系统结构和馈电特性,应用体面积分方程建立了任意结构波束波导中电磁波的激励、传输和辐射问题所对应的物理模型.在激励端,采用模式展开方法描述输入口面场分布,并利用模式匹配方法精确模拟波束波导馈电端的匹配状况.通过对波束波导内部的实际结构的精确拟合,基于积分方程建立了激励-传输-辐射的电磁模型,并以高效数值解法给...     

低过模高功率微波方圆模式转换器设计

 设计了一种结构简单的高功率微波方圆模式转换器,可以实现圆波导TM₀₁模式与矩形波导TE10模式之间的相互转换。转换器工作在C波段,中心频率4.1 GHz,其输入端口和输出端口相互垂直。计算和仿真结果表明:中心频率处该模式转换器的转换效率可达99%,回波损耗小于-20 dB,转换效率大于90%的带宽大于0.2 GHz。转换器整体3维尺寸都只有10 cm左右。     

混响室的概率统计分析方法及其蒙特卡罗模拟

 提出了一种研究混响室问题的理论分析方法,从理想混响室的离散概率模型出发,采用蒙特卡罗模拟来获得室内电磁场量及其相关物理量的概率统计特性。运用该方法获得了混响室内若干场量（均为归一化值）的概率密度函数、室内一段均匀无耗单导体传输线的负载平均耦合截面及负载电流响应模值的概率分布,所得结果与已有文献给出的结果（包括由解析公式计算或测量所得的结果）吻合良好。由于蒙特卡罗方法的普适性,采用该方法可以获得任意待分析参量的全部概率统计特性（只要样本数足够大）。同时该方法无需针对具体混响室进行,只要在其测试区域能较好地与理想混响室的基本概率模型相吻合,就能给出较精确的计算结果,且其计算量远远小于基于全波分析的蒙特卡罗方法。     

相对论速调管放大器杂模振荡的抑制

 研究了相对论速调管放大器（RKA）输入腔和中间腔之间的高阶杂模振荡问题。通过模式分析得知杂模在谐振腔内为TM11模式,而在漂移管中表现为TE11模式,针对该模式能在漂移管中传输的特性,利用漂移管内壁涂覆吸波材料吸收杂模功率的方法进行抑制。通过3维粒子模拟程序,分析了吸波材料的电导率及涂覆长度对抑制杂模增长率的影响。利用模拟分析得到的结果,对漂移管中涂覆吸波材料的RKA输入腔及中间腔结构进行了3维模拟研究,结果显示:合适的吸波材料的引入能够很好地抑制RKA输入腔和中间腔之间的杂模振荡。