Variations of mode amplitudes in a range-dependent waveguide

The ray method of calculating the mode amplitudes is used to analyze the sound fields in deep-water acoustic waveguides with two types of inhomogeneities of the refractive index: (i) weak inhomogeneities that cause small perturbations of ray trajectories and (ii) strong inhomogeneities with large spatial scales. Simple analytical relations are derived for describing the variations of the mode structure of the sound field in the presence of the aforementioned inhomogeneities. A new criterion defining the validity of the adiabatic approximation is formulated. To illustrate and test the results obtained, a numerical simulation of the sound fields is performed on the basis of the parabolic equation method.

     

水平变化双层波导声传播问题的耦合简正波解

为了验证现有模型的精度,导出了全反射下边界双层波导中简正波耦合矩阵的解析表达式,并将其应用到全局矩阵耦合简正波模型(Direct Global Matrix Coupled-Mode)中,使得该模型可以提供水平变化双层波导问题的标准解。文中首先利用COUPLE的简正波及耦合矩阵数值解验证了该简正波及耦合矩阵解析表达式的正确性;其次,采用改进的DGMCM模型求解了双层波导海山声传播损失,结果表明,改进后的DGMCM模型可以非常精确地求解水平变化双层波导问题,可作为求解此类问题的标准模型使用。     

领结形中空表面等离子体波导的传输特性

设计了一种领结形中空表面等离子体波导.采用频域有限差分法,对这种波导所支持的基模的能流密度分布、有效折射率、传播长度和模式面积随几何结构参数和工作波长的依赖关系进行了分析.结果表明,沿纵向的能流主要分布在两个上下突起所形成的中间区域.通过调整几何参数及工作波长,可以调节模式的有效折射率、传播长度和模式面积.在工作波长确定的条件下,有效折射率随突起半径的增大呈减小趋势,而传播长度和模式面积则随着突起半径的增大呈增大趋势,四个角上的圆弧半径对波导的传输特性有微调作用,左右扇形区域的半径对波导的传输特性有较明显 关键词： 集成光学 光波导 表面等离子体波导     

一种水平变化可穿透波导中声传播问题的耦合简正波方法

通过利用标准简正波程序KRAKEN计算本地简正波解及耦合矩阵, 进一步发展了求解水平变化波导中声场的全局矩阵耦合简正波方法(Luo et al., "A numerically stable coupled-mode formulation for acoustic propagation in range-dependent waveguides," Sci. China-Phys. Mech. Astron. 55, 572 (2012)), 使得该方法可以处理具有可穿透海底及随深度变化声速剖面等实际问题, 并提供声场的完全双向解. 本文还给出了双层波导中耦合矩阵的解析表达式, 并利用其验证了本方法中耦合矩阵数值算法的精度. 最后, 利用改善后的全局矩阵耦合简正波模型(DGMCM)计算了美国声学学会(ASA)提出的可穿透楔形波导标准问题, 将所得数值解与参考解比较, 结果表明DGMCM方法可以精确处理水平变化波导中声传播实际问题. 关键词： 耦合简正波理论 全局矩阵方法 可穿透楔形波导     

Propagation properties of a modified surface plasmonic waveguide with an arc slot

We introduce a modified surface plasmonic waveguide with an arc slot. The dependences of distribution of energy flux density, effective index, propagation length and mode area of the symmetric mode supported by this waveguide on geometrical parameters and working wavelength are analysed by using the finite-difference frequency-domain (FDFD) method. Results show that the energy flux density distributes mainly in four corners which are formed by two arcs, and the closer to the corners it is, the stronger the energy flux density will be. The effective index, the propagation length and the mode area are influenced by geometrical parameters, including the width, the thickness and the arc radius of the surface plasmonic waveguide, as well as the working wavelength. It has been shown that the surface plasmonic waveguide with an arc slot has better propagation properties than the surface plasmonic waveguide with a straight slot. This work may be helpful for applying the slot surface plasmonic waveguide to integrated photonics.     

双椭圆纳米金属棒构成的表面等离子体波导的传输特性分析

设计了一种双椭圆纳米金属棒表面等离子体波导,采用频域有限差分法,对这种波导所支持的基模的能流密度分布、有效折射率和传播长度随几何结构参数和工作波长的依赖关系进行了分析.结果表明,沿纵向的能流主要分布在两个椭圆金属棒所形成的中间区域,且越靠近金属棒的弧形边,沿纵向的能流越大.通过调节两个金属棒的中心距离以及它们的两个半轴的大小,可以调节模式的有效折射率和传播长度.在工作波长确定的条件下,相对于a=b的情形来说,在a<b时,场与金属表面接触的面积较大,场 关键词： 集成光学 光波导 表面等离子体波导     

梳状波导结构中石墨烯表面等离子体的传播性质

理论上研究了介质/石墨烯/介质梳状波导结构中表面等离子体的传播性质. 波导中表面等离子体模的有效折射率随着石墨烯费米能级的提高而减小, 随着介质折射率的增加而增加. 分析和仿真结果表明, 基于这种梳状波导可以在中红外波段实现新型的纳米等离子体滤波器, 器件的尺度在几百纳米的范围. 通过改变梳状分支的长度, 石墨烯的费米能级, 介质的折射率和波导中石墨烯的层数, 很容易来调节带隙的位置. 另外, 滤波带隙的宽度随着梳状分支数的增加而增加. 这种滤波性质将在可调的高集成光子滤波器件中具有潜在的应用.     

通信波段金属包覆波导传播特性研究

采用等效折射率的方法,以金、银、铜作为金属材料,计算了入射波长为1550nm时,2种不同结构金属包覆波导的几何色散关系、场分布特征及损耗情况。采用金属为衬底的非对称包覆结构进行计算,结果表明TE模的特点与全介质波导的情况类似。采用双面金属对称包覆结构进行计算,计算表明TM0模由金属-介质界面处的表面模对称结合而成,其等效折射率不在导模的范围之中,TE模和TM1模等效折射率的范围大于全介质波导的情况,TM1模由金属-介质界面处的表面模反对称结合而成。     

水平变化浅海声波导中模态特征频率与声源距离被动估计

针对水平变化浅海声波导中声源宽容性被动测距问题,理论分析了海底地形水平缓变浅海波导中卷绕变换基础上的低频声场特征频率.推导了绝对硬海底时水平变化波导中声场模态时频到达结构以及模态瞬时相位的表达式,由该表达式给出了特征频率与收发距离的变化关系,进而提出了水平变化浅海波导中声源距离被动估计的修正方法,通过仿真和实验对理论与方法进行了验证.     

Propagation of electromagnetic waves in a waveguide with a modulated anisotropic insert

The propagation of transverse electric (TE) and transverse magnetic (TM) waves in a waveguide of arbitrary cross section with an anisotropic periodically space-and time-modulated insert is considered. Under the assumption that the modulation indices are small, analytical expressions for the TE and TM fields in the domain of weak interaction between the signal wave and modulating wave are derived. The frequency near which strong interaction between these waves occurs is found, and specific features of the fields in this domain are revealed.     

Propagation of electromagnetic waves in a rectangular waveguide with periodic lattices

The problem of propagation of electromagnetic waves in a waveguide with one or two periodic lattices of simple or special form is solved in strict formulation. The formulas obtained are studied by computer, and the numerical results obtained analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1834–1839, December, 1978.The author considers it her pleasant duty to thank V. P. Shestopalov and L. I. Belousov for their constant interest and assistance in the study.     

The ray approach for analyzing the modal structure of the sound field in a range-dependent waveguide

Simple ray-approximation formulas are obtained for the mode amplitudes in a waveguide with range-dependent parameters. The idea of the proposed approach is based on the mode expansion of the complex field amplitude determined using the geometrical-optics approximation. A specific example of calculating mode amplitudes is analyzed for a deep-water sound channel with a sound speed profile nonadiabatically varying with distance. The results of the calculation are compared with the numerical solution obtained for the same problem by the parabolic equation method.     

Propagation characteristics of a circular waveguide filled with a chiroferrite medium

A chiroferrite is made from chiral objects immersed in a magnetically biased ferrite. A waveguide including a chiroferrite is anticipated to possess some novel features due to the presence of chirality in combination with anisotropy. In this paper, the propagation characteristics of a circular metal waveguide filled with a chiroferrite medium are investigated by an analytical procedure. The equations of relations between the transverse components and the longitudinal ones of electromagnetic fields in general chiroferrite waveguides are derived in a cylindrical coordinate system. The coupled wave equations for the longitudinal electric and magnetic fields are decoupled into two Helmholtz equations which are then solved analytically. The characteristic equation is formulated for the circular chiroferrite waveguide. Propagation constants varying with the structural and material parameters have been calculated for several lower chiral modes. A number of interesting features have been revealed by the numerical results.     

Propagation of Bose-Einstein condensates in a magnetic waveguide

Gaseous Bose-Einstein condensates of 2-3 x 10(6) 23Na atoms were loaded into a microfabricated magnetic trap using optical tweezers. Subsequently, the condensates were released into a magnetic waveguide and propagated 12 mm. Single-mode propagation was observed along homogeneous segments of the waveguide. Inhomogeneities in the guiding potential arose from geometric deformations of the microfabricated wires and caused strong transverse excitations. Such deformations may restrict the waveguide physics that can be explored with propagating condensates. Finer perturbations to the guiding potential fragmented the condensate when it was brought closer to the surface.     

Propagation properties of the terahertz waveguide using a metallic nanoslit narrower than skin depth

A terahertz(THz) waveguide using a metallic nanoslit whose width is much smaller than the skin depth is analytically investigated. By taking some important physical properties into account, we derive a simple,yet accurate, expression for the effective index. We also study the changes in modal field and the attenuation coefficient in the whole THz region, and find some interesting physical properties. Finally, we verify that these theoretical analyses coincide with the rigorous numerical simulations. This research can be useful for various applications of THz waveguides made of metallic nanoslits.