Radio waves propagating through tunnels are strongly attenuated in the presence of discontinuities such as bends and branches. The useful structural modifications are requested to get better circumstances for radio waves in tunnels. In this paper, we propose several modifications arranged in a conventional T-junction of two-dimensional tunnels and analyze the transmission characteristics of radio waves by using the finite volume time domain (FVTD) method.

Optical couplers which are composed of three channel waveguides arranged two-dimensionally are investigated numerically. The mode-matching method that matches the boundary conditions in the sense of least squares is applied to this problem, using the hybrid-modal representation. The precise numerical results of the dispersion relations and field distributions are presented for the lowest three modes near the cutoff. The arrangement of three waveguides can be optimized so as to satisfy the condition for maximum power-transfer efficiency.

Electromagnetic crystals formed by vertical full posts stacked in a rectangular waveguide are analyzed using the image theory and the lattice sums technique. It is shown that the frequency response of the crystals consisting of circular posts can be obtained by a simpler matrix calculus based on the one-dimensional lattice sums, the T-matrix of a circular cylinder in free space, and the generalized reflection and transmission matrices.

The power transfer characteristics of a symmetric nonlinear directional coupler (NLDC) are analyzed rigorously using the beam propagation method based on the finite difference scheme. The NLDC consists of two linear waveguides separated by a Kerr-like nonlinear gap layer. The change of nonlinear refractive index along the coupler is precisely evaluated by making use of the second-order iteration procedure with respect to a small propagation length. For the incidence of TE0 mode of the isolated linear waveguide, the highly accurate numerical results are obtained for the behavior of power transfer, and the coupling length and critical power for optical switching. The dependencies of the coupling length and critical power on the width of the gap layer and the input power levels are discussed, compared with those predicted by the coupled-mode approximations.

A semi-analytical method for a planar periodic array formed by a finite number of magnetized ferrite circular cylinders is presented using a model of layered cylindrical structures. The method uses the T-matrix approach and the extraction of the reflection and transmission matrices based on the cylindrical harmonic mode expansion. Based on the proposed method, plane wave scattering by the finite number of magnetized ferrite circular cylinders is numerically studied from the viewpoint of realization the electronic switching and electronic scanning effects by varying the applied magnetic field.

Cylindrical dielectric waveguide with a periodically varying radius are investigated numerically. The mode-matching method that matches the boundary conditions in the sense of least squares in applied to this problem, using the hybrid-modal representation. The accurate numerical results of the dispersion relations and field distribution are presented for the HE11 mode. It is shown that the attenuation rate in the stopband is significantly smaller than that of TE01 mode, whereas the HE11 mode suffers an additional power leakage due to the coupling with EH11 and HE12 modes of radiation region.

Electromagnetic scattering and radiation in cylindrical electromagnetic bandgap (EBG) structure is analyzed. The radiated field from a line source placed inside the eccentric configuration of the cylindrical EBG structure and plane wave incident on the cylindrical EBG structure is numerically studied based on the method proposed by the authors in their early papers. Using the developed formulation, it is shown first time that when the cylindrical EBG is illuminated by plane wave of particular resonance frequencies, the field are strongly enhanced or shaded inside the cylindrical EBG structure and this effect depends on the angle of incidence of the plane waves. We give a deep physical insight into explanation of this phenomenon based on the Lorentz reciprocity relation for cylindrical structures.

A rigorous and simple method is proposed for analyzing guided modes of metallic electromagnetic crystal waveguides. The method is a combination of generalized reflection and transmission matrices and the mode-matching technique. Fast convergence, low computer cost, and high calculating precision are main advantages of the proposed method. This method can easily avoid the relative convergence phenomena than a classical mode-matching method, and the proposed formulation is very suitable to analyzing multilayered problems with very low computer cost. The existence of H-polarized modes in metallic electromagnetic crystal waveguides has been verified.

A coupled-mode analysis of a symmetric planar nonlinear directional coupler (NLDC) is presented by using a singular perturbation scheme. The effects of linear coupling and nonlinear modification of refractive index are treated to be small perturbations, and the modal fields of isolated linear waveguides are employed as the basis of propagation model. The self-consistent first-order coupled-mode equations governing the transfer of optical power along the NLDC are obtained in analytically closed form. It is shown that tha critical power for optical switching derived from the coupled-mode equations is in close agreement with that obtained by the numerical analysis using the finite difference beam propagation mathod.

The dispersion characteristics of two nonidentical coupled microstrip lines and N identical coupled microstrip lines are analyzed using the coupled-mode theory combined with Galerkin's moment method in spectral domain. In this approach, the solutions to the original coupled microstrips are approximated by a linear combination of eigenmode solutions associated with the isolated single microstrip, and the reciprocity relation is used to derive the coupled-mode equations. The coupling coefficients are given by the simple overlap integrals in spectral domain between the eigenmode fields and currents of the individual microstrips. It is shown that the numerical results are in very good agreement with those obtained by the direct Galerkin's moment method over a broad range of weak to moderately strong coupling.

A semi-analytical approach for analyzing the electromagnetic radiation of a line source in cylindrical electromagnetic bandgap (EBG) structure is presented. The cylindrical structure is composed of circular rods periodically distributed along concentrically layered circular rings. The method uses the T-matrix of a circular rod in isolation, the reflection and transmission matrices of a cylindrical array expressed in terms of the cylindrical waves as the basis, and the generalized reflection and transmission matrices for a layered cylindrical structure. Using the proposed method, the radiated field from a line source placed inside a three-layered cylindrical EBG structure with defects is investigated. The defects are created by removing the particular circular rods from each circular ring. The structure is prominent from the viewpoint of flexible design of the directive antennas. Numerical examples demonstrate that the cylindrical EBG structures are very effective at forming and controlling the directed beam in the radiated fields.

A two-dimensional radiation by a relativistic sheet electron beam passing over a corrugated conducting surface is investigated. The leakage coeffcient and phase change of space-charge waves due to the radiation is calculated using a perturbation approach based on the multiple scales.

The scattering of electromagnetic plane waves incident obliquely upon a column of hot electron plasma, which is magnetized in the axial direction and is located in free space, is investigated based on a kinetic model for the plasma. This model takes into account a full thermal effect in the axial direction and includes the first-order thermal effect with respect to the transverse direction. The boundary-value problem is solved for both cases of polarizations of the incident wave, i.e., TM wave and TE wave. The numerical results of the back-scattering cross section, the far-field power pattern, and the albedo of the plasma column are presented for several typical values of plasma parameters. The thermal effect of plasma on the characteristics of scattering and absorption are discussed at length. It is shown that when the frequency of incident wave is close to the electron cyclotron frequency, in the case of high-temperature plasma, the energy of incident wave is strongly absorbed through the process of collisionless absorption.

A three-parallel thin-film waveguide is analyzed numerically, using mode-matching method that matches the boundary conditions in the sense of least squares. The precise numerical results of the dispersion relations and field distributions are presented for the lowest three hybrid modes.

Five channel optical waveguides are investigated numerically by using mode-matching method. The precise numerical results of the dispersion relations near the cutoff and the field distributions are presented for the lowest five HE modes. When the geometric parameters of the waveguides can be optimized so as to satisfy the phase-matching condition, it is shown that the five channel optical waveguides operate as power divider.

Three-wave interactions in a linear, active, and lossless system are investigated. The stability properties of the coupled-waves describing the interaction are discussed at length by taking into consideration possible combinations of parities and directions of group velocities of the component waves. Several schematic dispersion diagrams are presented for the classification of existing instabilities in this system. The power gains of traveling-wave amplification in the three-wave interaction are also analyzed. It is shown that when one passive-wave couples to other two active-waves propagating in the same direction, a power gain considerably larger than that of corresponding two-wave interaction can be obtained.

A rigorous semi-analytical approach for the scalar field in a microstructured optical fiber, which is formed of layered cylindrical arrays of circular rods symmetrically distributed on each concentric cylindrical layer, is presented. The method uses the T-matrix of a circular rod in isolation and the generalized reflection and transmission matrices of cylindrical arrays. Numerical examples of the mode index for three-layered hexagonal structure of circular air holes are demonstrated and compared with those obtained by a variational method.