A transfer matrix approach based on local normal modes for coupledwaveguides with periodic perturbations

A simple transfer matrix method is used to analyze power coupling and scattering in optical waveguide structures with periodic index perturbations. The transfer matrix is determined by a mode-matching technique for the local normal modes of the structures. This approach accounts for the change in the field patterns along the waveguide axis and is more rigorous than coupled-mode theory based on ideal modes. The distinct advantages of the method are that (1) it can be applied to structures with relatively strong index perturbations caused by large grating height and/or large index difference; (2) the TM modes whose transverse electric field is perpendicular to the index interface can be properly treated; and (3) the radiation loss due to scattering at the index discontinuities along the waveguide axis can be estimated. Analytical expressions for the power coupling in a grating-assisted codirectional coupler are derived, and numerical results are shown for some typical structures     

Composites based on dielectric materials for microwave engineering

Composites provide possibilities to combine the properties of different materials in order to obtain materials with the necessary parameters (low loss, absorbing, reflective materials, etc.). According to their structure, composite materials are conventionally divided into the microtextures and macrotextures. In turn, these materials may be passive or active. The properties of passive composite materials are invariable, and the properties of active ones can be altered using different effects: electric, magnetic, thermal, etc. In this article we present the results of theoretical and experimental investigations of various microwave dielectric-based composite structures. Particular attention is given to high-Q microwave structures with electrical control.     

A stable solution of time domain electric field Integral equation for thin-wire antennas using the Laguerre polynomials

In this paper, a numerical method to obtain an unconditionally stable solution of the time domain electric field integral equation for arbitrary conducting thin wires is presented. The time-domain electric field integral equation (TD-EFIE) technique has been employed to analyze electromagnetic scattering and radiation problems from thin wire structures. However, the most popular method to solve the TD-EFIE is typically the marching-on in time (MOT) method, which sometimes may suffer from its late-time instability. Instead, we solve the time-domain integral equation by expressing the transient behaviors in terms of weighted Laguerre polynomials. By using these orthonormal basis functions for the temporal variation, the time derivatives can be handled analytically and stable results can be obtained even for late-time. Furthermore, the excitation source in most scattering and radiation analysis of electromagnetic systems is typically done using a Gaussian shaped pulse. In this paper, both a Gaussian pulse and other waveshapes like a rectangular pulse or a ramp like function have been used as excitations for the scattering and radiation of thin-wire antennas with and without junctions. The time-domain results are compared with the inverse discrete Fourier transform (IDFT) of a frequency domain analysis.     

Teaching of advanced wave-propagation phenomena in open-space problems and waveguide devices using MATLAB GUIs

Interactive graphics and animations are fundamental tools for teaching electromagnetic concepts, especially to explain propagation phenomena. For this reason, the study of these phenomena needs advanced analysis tools. This article presents an accurate and efficient hybrid mode-matching method for the analysis of arbitrarily shaped structures with H-plane obstacles and discontinuities. An open-space spectral method is used to model the electric behavior of the H-plane problem. Next, the open-space modes are matched to guided modes, in order to obtain a multimode scattering-matrix representation of the structure. This new method is used to analyze the propagation in passive complex radio-frequency structures. The results from the analysis are integrated into a MATLAB graphical user interface (GUI), which has been designed to improve the teaching/learning process at the Universidad Politecnica of Valencia in Spain. These GUIs are thoroughly explained in the paper, and the improvements in the teaching/learning process are also discussed.     

A microstrip line on a chiral substrate

Right and left circular vector potentials are developed and used in a spectral-domain solution for a microstrip transmission line on a chiral substrate. These vector potentials have properties similar to those of the usual magnetic and electric vector potentials, except that they result in circular rather than linearly polarized fields, thereby simplifying field expansions in chiral media. The chiral microstrip line does not have bifurcated modes like other chiral guided wave structures; however, the chiral substrate causes a significant asymmetry in both the fields and currents     

降低光子晶体谐振腔Q值的方法分析

该文在分析计算金属光子晶体的正三角形晶格TE模式的色散特性、全局带隙分布图的基础上,针对金属光子晶体结构谐振腔Q值较高的问题,对降低光子晶体谐振腔Q值的方法进行了分析和设计。采用加载介质柱的混合结构和介质微扰两种方法分别对谐振腔的Q值进行有效的控制,并分析了两种方法对谐振腔模式选择性的影响。结果表明,两种方法都能在不改变谐振腔模式选择性和场分布的基础上有效降低Q值,而介质微扰的方法还同时清除了与TE04竞争的两种杂模,提高了模式选择性。     

The graphical reliability evaluation of three-state device networks

A device is said to have three states if it has one successful and two failure modes (i.e., opened and shorted or closed). A typical example of such devices is an electronic diode, an electric switch, a relay, a fluid flow valve, etc.The networks made up of such devices present a complication in the reliability analysis as the networks become more complex.This paper presents a newly developed graphical technique to analyse any type of simple or complex three-state device network with much simplicity, straightforwardness and limited reliability theory knowledge. The theory behind this newly developed technique is briefly explained and the rules to use this technique are established. Some numerical examples are presented and compared with the results obtained from the conventional method. The graphical and conventional results compare quite favourably.     

Recent Advances in Liquid‐Like Thermoelectric Materials

Thermoelectric technology has attracted great attention due to its ability to recover and convert waste heat into readily available electric energy. Among the various candidate materials, liquid‐like compounds have received tremendous research interest on account of their intrinsically ultralow lattice thermal conductivity, tunable electrical properties, and high thermoelectric performance. Despite their complex phase transitions and diverse crystal structures, liquid‐like materials have two independent sublattices in common: one rigid sublattice formed by immobile ions for the free transport of electrons and one liquid‐like sublattice consisting of highly mobile ions to interrupt the thermal transports. This review first outlines the common structural features of liquid‐like thermoelectrics, along with their unusual electron and phonon transport behaviors that well satisfy the concept of “phonon‐liquid electron‐crystal.” Next, some commonly adopted strategies for further improving their thermoelectric performance are highlighted. The main progress achieved in the typical liquid‐like TE materials is then summarized, with an emphasis on their diverse crystal structures, common characteristics, and unique transport properties. The recent understandings on the stability issue of liquid‐like TE materials are also introduced. Finally, an outlook is given for the liquid‐like materials with the aim to boost further development in this exciting scientific subfield.     

Study on the similarity of photonic crystal ring resonator cavity modes and whispering-gallery-like modes in order to design more efficient optical power dividers

In this study, we discuss the wave pattern characteristics of confined cavity modes inside photonic crystal ring resonators (PCRRs). Despite different physical origins, these cavity modes are analogous to the whispering-gallery (WG-like) modes. Because of the absence of perfectly circular symmetry in our PC cavity, the WG-like modes are not degenerate, but they form a close doublet in which the field pattern of each of these doublets repeats itself by \(180^{\circ }/m\) where “m” is the azimuthal index number of WG-like modes. The cavity modes are named according to their symmetric field pattern, and WG-like ones are named by their azimuthal mode numbers “m.” Based on the study of these similarities, we propose a \(1\times 2\) and a \(1\times 4\) T-junction- like power dividers. Through study of the similar properties of these cavity modes with WG-like modes, we have enhanced the output power of both dividers to 49.6 and 24.5 %, respectively. The permittivity of the hexagonal photonic crystal rod is \(\varepsilon _{h}= 12.04\) surrounded by air as the background medium. For our PCRR, the dominant resonating mode is the cavity mode with hexapole field pattern or equivalently WG-like mode with azimuthal mode number of \(m\,=\,6\). The normalized frequencies for this mode and its doublet are \(a/\lambda = 0.348066\) and 0.348301, respectively. The photonic band structure, PC waveguide guided mode and electric field patterns of the confined cavity modes inside the PCRR are calculated using the PWE method, and the transmission spectra are calculated by means of 2D-FDTD simulation method.     

Finite element analysis of MMIC structures and electronic packagesusing absorbing boundary conditions

In this paper, a three-dimensional finite element method (FEM) is employed in conjunction with first and second-order absorbing boundary conditions (ABC's) to analyze waveguide discontinuities and to derive their scattering parameters. While the application of FEM for the analysis of MMIC structures is not new, to the best of the knowledge of the authors the technique for mesh truncation for microstrip lines using the first and higher-order ABC's, described in this paper, has not been reported elsewhere. The scattering parameters of a microstrip discontinuity are computed in two steps. As a first step, the field distribution of the fundamental mode in a uniform microstrip is obtained by exciting the uniform line with the quasi-static transverse electric field, letting it propagate, and then extracting the dominant mode pattern after the higher order modes have decayed. In step two, the discontinuity problem is solved by exciting the structure by using the fundamental mode obtained in step one. The scattering parameters based on the voltage definition are calculated by using the line integral of electric fields underneath the strip. Numerical solutions for several waveguide discontinuities and electronic packages are obtained and compared with the published data     

Terahertz Spectroscopic Analysis of Peptides and Proteins

Spectroscopic analysis using the Terahertz frequencies between 0.1-15 THz (3–500?cm^?1) has been underutilised by the biochemistry community but is starting to yield some scientifically interesting information. Analysis of structures from simple molecules like N-methylacetamide, to polyamides, peptides and relatively complex proteins provides different types of information dependant on the molecular size. The absorbance spectrum of small molecules is dominated by individual modes and specific hydrogen bonds, peptide spectra have peaks associated with secondary structure, while protein spectra are dominated by ensembles of hydrogen bonds and/or collective modes. Protein dynamics has been studied using Terahertz spectroscopy using proteins like bacteriorhodopsin, illustrating a potential application where this approach can provide complementary global dynamics information to the current nuclear magnetic resonance and fluorescence-based techniques. Analysis of higher-order protein structures like polyomavirus virus-like particles generate quite different spectra compared to their constituent parts. The presence of an extended hydration layer around proteins, first postulated to explain data generated using p-germanium spectroscopy may present a particularly interesting opportunity to better understand protein’s complex interaction with water and small solutes in an aqueous environment. The practical aspects of Terahertz spectroscopy including sample handling, the use of molecular dynamics simulation and orthogonal experiment design are also discussed.     

Assessment of Rational Approximations for Square Root Operator in Bidirectional Beam Propagation Method

Accurate rational approximations to the square root operators are crucial for the bidirectional beam propagation method in simulation of strongly reflective structures. An assessment and comparison of various commonly used rational approximations is performed for both transverse electric (TE) and transverse magnetic (TM) waves. The range of validity and level of accuracy for the different approximations are studied by investigating the structures with varying reflection intensities. Guidelines for accurately modeling of evanescent and propagating modes are provided.     

A study of the fields associated with vertical dipole sources in stripline circuits

A prototype, integral-equation-based, full-wave layered interconnect simulator (UA-FWLIS) has recently been developed. In this simulator, the method of moments (MoM) reaction integrals are evaluated analytically, thereby dramatically improving the computational efficiency compared with standard numerical integration techniques. The closed-form representation for the reaction elements takes the form of a modal series expansion. Since the reaction between closely-spaced elements requires the dominant mode and a significant number of higher-order modes, an investigation of the behavior of the fields associated with both the propagating and higher-order evanescent wave modes will be beneficial for the further development of the simulator. The simulator can currently handle homogeneously-filled stripline structures only. In this paper, the electric field analysis is extended to an inhomogeneously-filled stripline structure. The knowledge gained from these studies will allow for the extension of the simulator to the analysis of inhomogeneously-filled stripline structures. The spectral domain technique is first used to obtain the field expressions in the spectral domain.     

A scalar variational conformal mapping technique for weakly guiding dielectric waveguides

A novel approach of combining the finite-element method with the conformal mapping technique is proposed for solving the scalar variational formulas for weakly guiding dielectric waveguides. This approach avoids spurious modes and gives satisfactory results even for modes near cutoff, requiring less computer memory and time. Various specific dielectric structures, such as the rectangular guide, channel guide, and rib guide, are considered to estimate the error associated with the scalar formulation relative to the vectorial formulation. The efficiency of this approach is demonstrated with an analysis of a directional coupler whose coupling properties are described by means of numerical results such as propagation constants, field distributions, etc.     

Some experimental investigation of the effect of railway tunnels on mobile communications in Western India

Establishing of mobile communications in guided structures like railway tunnels, coal mines, etc., is gaining increasing importance due to the passage of high speed and high density of trains in tunnels and for monitoring of information on safety of workers and day-to-day operations in various mines. The present study reports investigation of mobile communication experiments in ultra high frequency band in railway tunnels of western India conducted in a moving train both in uplink and downlink directions. The observed signal fluctuations are discussed in terms of tunnel characteristics and different modes of propagation in tunnels and compared with model deduced values.     

Analysis of a CPW on electric and magnetic biaxial substrate

A formulation of the spectral domain technique (SDT) for the analysis of the general case of uniaxial/biaxial, electric/magnetic anisotropic multilayer planar structures is presented. As an illustration of the capabilities of this formulation it is applied to the analysis of shielded coplanar waveguide (CPW) structures. The dispersive properties of the fundamental and higher-order modes in various cases of electric/magnetic anisotropy as well as the induction electric and magnetic lines are calculated. Such results, and particularly the induction lines, may be used to predict the behavior of the structure studied as well as to point out its sensitive geometrical and electrical parameters. Some general rules which lead to a better understanding of the effect of anisotropy are discussed     

Signal-Flow-Based Graphs for Failure-Mode Analysis of Systems with Control Loops

Control loops make failure-mode analysis via fault trees extremely difficult. This paper proposes a new approach based on signal flow graphs to model systems with control loops. Mason's Rule is applied to assess the effect of the loops. The top event of the system is defined by an inequality on a node-variable of the signal flow graph. Basic failures are modeled by source variables. Cut-off failures of the control loops are also considered. The method is useful for uncovering failure modes leading to the top event in complicated systems with control loops. General steps to apply the method to a system are: 1. Draw a SFG for the system. 2. Model basic failures by source variables. 3. Select a node-variable to define a top event. 4. Represent the top event in terms of the source variables, using Mason's Rule. 5. Discretize the source variables. 6. Classify loop states. 7. For each loop state, obtain system failure modes, using a search tree like Fig. 5. 8. Review the failure modes by more accurate simulation models. Any model is an approximation of an actual system. Thus, the resulting failure modes like those in Table 3 should be examined again, using past experience, more accurate simulation models, etc. The method should be viewed primarily as useful tool for uncovering failure modes in which complicated systems with the control loops fail.     

计算周期性导波结构的时域有限差分方法

本文提出一种计算周期性导波结构的时域有限差分方法.由Floquet定理建立边界条件,在电场边界和磁场边界上两次使用Floquet定理,从而将计算域限制在一个周期结构内,并且在导波结构侧面引入吸收边界条件,保证了计算精度.通过预先给出传播常数,经FDTD迭代计算,其谐振频率就是该传播常数所对应的工作频率.     

含液晶缺陷层阶梯型双周期结构的电场方向调控特性

利用传输矩阵法研究了含向列相液晶缺陷层的一维阶梯型Double-period第四代准周期结构缺陷模的电场方向调控特性。分析了外加电场方向和正入射波方向间夹角θ与缺陷模的位置、品质因子、空间位置光场分布的变化关系,最后探讨了缺陷模的品质因子对光场强度的影响。结果表明：随着夹角θ的逐渐增大,缺陷模的位置向短波方向移动,缺陷模波长的调控量为84nm;随着夹角θ的逐渐增大,缺陷模的品质因子逐渐增大;缺陷模在空间位置的光场分布呈现出局域现象,随夹角θ的增大,空间位置的光场强度逐渐增强;缺陷模的品质因子与光场强度成正比关系。     

Direct analytical solution for the electric field distribution atthe conductor surfaces of coplanar waveguides

A new analytical method of evaluating the electric field distribution across the conductor surfaces of coplanar waveguides (CPWs) is presented. Here, a series of conformal mappings are used to transform the CPW's geometry and field distribution into a uniform image domain, to facilitate a direct field solution. The cumulative electric flux distribution across each conductor surface within the dielectric substrate is studied, and its effects on coupling and propagation modes are described. Direct solutions for the quasi-static normal electric field components are presented together with their graphical representations. Numerical computations show how the total electric flux terminating on the CPW's conductor surfaces varies in terms of the CPW's geometry and substrate parameters