Rigorous analysis of form birefringence of weakly fused fiber-opticcouplers

Based on a rigorous vectorial formulation, the surface integral equation method is used to solve the vectorial mode patterns and the coupling characteristics of weakly fused fiber-optic couplers. Form birefringence versus aspect ratio of the coupler cross-section for different normalized frequencies V are calculated. A dramatic feature-that there is a local maximum of form birefringence for the weakly fused couplers with large V-values-is found. The dependence of the aspect ratio at which the coupler is isotropic on the V-value is also presented. The fused coupler with large V-value is isotropic as the aspect ratio is close to 1.8, but its polarization effect is still significant when the aspect ratio is 2.0     

Polarisation analysis of strongly fused and weakly fused tapered couplers

The polarisation properties of both strongly fused and weakly fused single-mode tapered fibre couplers have been analysed. A comparison with experimental results shows excellent agreement with our analysis.     

Analysis of the coupling characteristics of a tapered coupledwaveguide system

Optical coupling characteristics between one uniform waveguide and one tapered waveguide are discussed. This coupling structure is analyzed by utilizing coupled-mode theory and system step approximation. Based on numerical investigation, various coupling characteristics are presented. It is shown that the proposed structure can be used as a waveguide-type optical power divider by selecting an appropriate taper slope for the tapered waveguide or by separating the coupled waveguides properly for a desired power-dividing ratio     

Variational analysis of fused tapered couplers

A variational technique is used to evaluate the coupling ratio of a fused coupler. The technique can be used for both strongly and lightly fused couplers. Simple formulas are presented for use in regions of high V-value and are shown to be in agreement with the more lengthy beam propagation calculations. The importance of the fibre core in determining the field shape is highlighted. A realistic coupler model containing a gradual taper is presented and the strong wavelength response of the coupling ratio is confirmed by numerical means.     

熔锥型单模光纤耦合器模型的优化研究

在弱导和弱耦合近似条件下,对光纤耦合器的几何形状做近似处理,并采用连续函数进行描述.根据Snyderd的局部模式理论,采用三角波与高斯波的变系数叠加函数对光纤耦合器纵向各区域的光模场分布进行描述.利用局部模耦合理论和变分法计算分析光纤耦合器纵向各区域的传播常数和耦合系数,讨论耦合系数与结构参数和波长的关系.计算表明:光纤耦合器熔锥区对光纤耦合器中的光功率耦合行为的影响不可忽略,光纤纤芯半径和包层半径对耦合区和熔锥区的耦合系数的影响成相反趋势.因此,可以通过合理控制耦合器的结构参数,制作出宽带平坦光纤耦合器.     

Perturbation analysis of fused tapered single-mode fibre couplers

By treating a fused tapered single-mode fibre coupler as a perturbation of the same coupler with the fibre cores ignored, simple analytical expressions are derived for the coupling coefficients of the coupler. These new expressions highlight the effects of the fibre cores on the characteristics of the coupler.     

电磁脉冲对电缆耦合问题的理论研究

为克服传输线方程求解电磁脉冲电缆耦合问题的不足,提出了有限差分求解电场积分方程的方法,解决了架空电缆电磁脉冲耦合效应的数值模拟难题;引入了基于细线散射的时域有限差分法,解决了地面铺设电缆电磁脉冲耦合效应的数值模拟难题。利用时域电场积分方程方法研究了长度、线径等电缆参数变化及脉宽、极化方向等馈源参数变化对其电磁脉冲耦合效应的影响。给出了电缆耦合电流的波形特征、沿线分布规律。研究表明:在电磁脉冲作用下,自由放置的电缆,其感应皮电流为衰减振铃波形;电流最大值出现在电缆的中心处;随着电缆长度的增加,电缆上的感应皮电流非线性变大;电缆线径的增加、脉冲宽度的改变对其耦合电流峰值影响较小;电场的极化方向对电缆感应皮电流的影响较大。     

基于FDTD的环形孔缝电磁耦合规律研究

为提高电子设备及系统在复杂电磁环境下工作的稳定性,应用时域有限差分方法对不同脉宽电磁脉冲通过带有不同形状（方形环,圆环,矩形环）环形孔缝屏蔽腔体的耦合规律进行了分析.研究结果表明：电磁脉冲通过环形孔缝耦合现象明显,其通过圆环的耦合能量最小;对于矩形环,当入射波极化方向与环形孔缝的短边平行时,若矩形环孔缝纵横比越大,则耦合能量也越大,当极化方向与短边垂直时,则纵横比越大,耦合能量越小;入射电磁脉冲脉宽越短,电磁脉冲越容易耦合进入环形孔缝;腔体壁的反射及谐振会增强耦合效应.     

Time-domain modeling of electromagnetic field coupling to finite-length wires embedded in a dielectric half-space

The transient behavior of a single straight line embedded in a dielectric half-space and illuminated by the electromagnetic pulse (EMP) is analyzed directly in the time domain using the wire antenna approach. The formulation is based on the corresponding space-time Hallen integral equation. The effects of a two-media configuration are taken into account via the Fresnel reflection and transmission coefficient, respectively. The space-time Hallen integral equation is handled by the time-domain version of the indirect Galerkin-Bubnov boundary element method. The transient response obtained using the direct time-domain approach is compared with the results obtained via an indirect frequency-domain analysis method. Some illustrative numerical results are presented in the paper. Numerical results obtained via the different approaches agree satisfactorily, i.e., the maximum deviation between the results is around 6%.     

Completely fused tapered couplers: comparison of theoretical and experimental results

Completely fused single-mode fibre couplers, characterised by an almost circular cross-section at the waist, have been fabricated. An analytic model which takes into consideration the taper and variable aspect ratio of the coupler has been developed. Good agreement is obtained between the theoretical and experimental values.     

矩形金属机壳孔缝电磁耦合特性研究

针对矩形金属机壳上孔缝在高功率电磁环境下的电磁耦合问题,提出了一种使用模式匹配法和基于矩量法求解的混合位积分方程来分析孔缝电磁耦合特性的全波混合算法.该算法考虑了高次模、孔缝形状、孔缝厚度以及入射波极化方向等因素对电磁耦合特性的影响.通过将数值仿真结果与经典孔缝电磁耦合模型的测试结果进行对比,验证了该算法具有较高的准确性,与经典的时域有限差分法相比,该算法具有很高的计算效率.研究结果表明:有孔金属机壳在外界强电磁辐射条件下具有明显的谐振效应,在谐振频率点,耦合进入机壳的电场将大大增强,且孔缝附近以及机壳中心的耦合电场峰值要高于其他位置的耦合电场峰值;随着机壳表面上孔缝厚度的增加,耦合进入其内部的电场也在减弱;当机壳上的孔缝为矩形且入射波的电场极化方向平行于矩形缝隙的短边时,对应于该极化方向的孔缝电场耦合强度是所有极化方向中最强的.     

飞机舵面缝隙缺陷建模及电磁散射特性分析

为了研究飞机舵面缝隙在飞机头向威胁角域的散射特性,提取舵面前端缝隙设计了雷达散射截面(Radar Cross Section,RCS)数值仿真分析模型.采用混合场积分方程和多层快速多极子算法进行了RCS仿真计算,并以RCS峰值、波峰宽度、均值增量三个评价指标分析散射特点.舵面前端缝隙的垂直极化散射明显(不同频率下的均值...     

Design of a single-mode tapered waveguide for low-losschip-to-fiber coupling

The novel waveguide structures described in this paper have nonlinearly tapered shapes that result in low radiation losses despite their relatively short lengths. The core at the waveguide endface connected with the fiber has a very small cross section and an expanded mode field with a non-Gaussian shape. The taper structures are analyzed by using an improved step-transition method. This method is a based on the theory of enclosing a waveguide within electrical walls and that can therefore treat the radiation modes in a tapered waveguide as discrete mode spectra. Analyzing the relationships between the lengths and shapes of the tapers and the radiation loss due to the tapers show that appropriately tapered semiconductor waveguides operating at an optical wavelength of 1.55 μm and having a taper length of less than 0.7 mm can have a radiation loss of only 0.1 dB and a coupling loss with a conventional single-mode fiber of less than 0.5 dB     

熔融拉锥型光纤耦合器损耗的实验研究

结合2×2熔锥型光纤耦合器的制作,实验研究了拉锥速度、耦合长度、火焰位置3个关键制作参数对耦合器的插入损耗和附加损耗影响。当拉锥速度控制在150μm/s时,耦合器的插入损耗和附加损耗可以控制在较低水平;在拉锥长度较短的区间内,插入损耗与拉锥长度基本成线性关系;制作低损耗耦合器,火焰存在最佳高度为5.75 mm。     

High-efficiency coupling of laser diodes in tapered proton-exchanged waveguides

Low-loss coupling between a laser diode and a tapered waveguide fabricated by selective area proton-exchange in LiNbO/sub 3/ is reported. The effectiveness of this waveguide is demonstrated by generating 12.3 mW of second harmonic blue light pulse by Cerenkov radiation.<>     

锥形光纤微透镜耦合特性

文章深入探讨和分析了半导体激光器与单模光纤耦合时采用锥形光纤微透镜后其耦合效率与参数之间的关系。给出了微透镜有效透过率的计算方法和考虑球面像差时耦合效率的计算公式，理论分析与实验结果相吻合。     

Time-domain characteristics of ultra-wideband tapered loop antennas

Presented are the transient characteristics of a tapered loop antenna fed by the composite right/left-handed coplanar waveguide-to-coplanar stripline balun. The antenna operates within 2.5-7.5 GHz and thus is suitable for ultra-wideband applications. The performance of the system comprising two identical tapered loop antennas is characterised by pulse radiation and reception. The fidelity of pulse response is presented to evaluate the signal distortion in the time domain     

Measurement of intermodal coupling in weakly multimode fibre optics

Using selective mode excitation via the cladding of an etched step-index weakly multimode fibre, and spatial mode separation at the output using a similarly etched section, intermodal coupling has been measured, mode-by-mode in the first seven guided modes of the fibre. Experimentally the observation has been made that most coupling terms remain less than -20 dB after 500 m of fibre.<>     

Analytic theory of coupling from tapered fibers and half-blocksinto microsphere resonators

Coupling from tapered fibers and polished half-block couplers into the high-Q whispering gallery modes of microsphere resonators is investigated analytically. Numerous formulas are derived to predict the external coupling Q values, and intrinsic whispering gallery loss, for arbitrary structures, and for any sphere mode. Phase-mismatch due to the differences in propagation constants between input and sphere modes is taken into account. These formulas are strictly mechanical once a simple characteristic equation is solved which relates the spherical mode orders to the resonant wave vector. Results are in very good agreement with values that are calculated by different numerical methods     

Reciprocal but nonsymmetrical transmission characteristics of tapered irises

Both the input impedance and the transmission coefficient of an H plane tapered iris are found to be reciprocal but asymmetrical. The H plane tapered iris behaves similarly to a transformer of nonunity turn ratio.