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A novel implementation of the stretched coordinate perfectly matched layer (PML) is presented for the two-dimensional (2D) transmission line modelling (TLM) method. The formulation offers a unified approach and is based on the mapping of the TLM node to a complex stretched domain for which the resulting transformation of the constituent RLC transmission line components is elaborated. The transformation is shown to modify the TLM connect-scatter algorithm. The absorption performance is demonstrated by simulating a canonical waveguide test case. Unlike the existing split-field based TLM-PML implementations, which are better suited to lossless media, the numerical results obtained show the proposed PML formulation is effective in the termination of both lossy and lossless media.

Large-scale three-dimensional (3D) electromagnetic simulations are commonly used as design and investigation tools in a wide variety of technological fields. It is not uncommon for both excitations and observation quantities to be expressed in terms of particular field profiles of feed waveguides. These may then be used to evaluate, e.g. scattering parameters. These field profiles must be obtained as a pre-processing task before the main simulation. Use of a theoretical field profile as an excitation to a discretised structure will typically cause a non-physical reflection. It is therefore more desirable in practice to use a field profile that is consistent with both the discretisation of the geometry and the 3D method of simulation. The authors present an approach to extracting these 2D field profiles from large-scale 3D unstructured meshes which are to be simulated with the unstructured transmission line modelling method. Discretised slices from the 3D mesh are extracted and incrementally extruded into a form suitable for consistent pre-processing. The impacts of all the parameters of the approach are investigated. Benchmarking is undertaken on both coaxial cable and microstrip waveguide feed structures showing that good quality results can be obtained straightforwardly.

In this paper, the Unstructured Transmission Line Modelling (UTLM) method based on a tetrahedral mesh is applied to model the electromagnetic coupling into wire looms and bundles with multiple cores that are typical of an aircraft system, when they are exposed to plane wave illuminations. The impact on the electromagnetic coupling into wires of both bundle configuration and the positioning of the bundle relative to simple structures are investigated using the UTLM method with explicit meshing of the wires. The work not only confirms that UTLM method as a powerful tool for dealing with wire looms and bundles but provides invaluable information on the margins to be expected in key experimental waveform parameters such as peak amplitude and frequency response.

Resonant frequency extraction from time signal simulations of optical resonators is often required to resolve closely spaced resonances in addition to estimating high Q factors. Traditional Fourier transform-based fitting methods thus require very long time signals, limiting their usage. Complex exponential fitting-based methods such as matrix pencil, modified Prony and harmonic inversion provide better accuracy, but each has its own advantages as well as disadvantages. In this study, it is shown that the matrix pencil method is limited by the number of time samples, but provides accurate results, whereas the harmonic inversion method provides less accurate results with high efficiency. The difference formulation of the modified Prony method implemented with multiple precision, while providing the best accuracy, is the least efficient approach and requires careful adjustment of its parameters to obtain a solution.

Transmission-line modelling (TLM) is a valuable technique for numerical simulation of electromagnetic fields. Recently, investigations have been successfully carried out into the use of the TLM approach on unstructured meshes However, as the local characteristics of mesh geometry will clearly make an impact upon the fidelity of simulations, it is necessary to examine suitable techniques for mesh optimisation. Three different methods have been investigated here to find optimal meshes for subsequent TLM simulations: the centroidal Voronoi Tessellation method, the optimal Delaunay triangulation method and a spring relaxation method. These different techniques are assessed in terms of maximum efficiency of the subsequent TLM simulations and the computational cost that is incurred to identify them. Several different canonical examples of mesh are considered to determine the generality of the conclusions drawn.

In some recent publications, formulations of a time-domain beam propagation method (TD-BPM) were developed to incorporate dispersive media with complex permittivity described by a single first-order pole or a modified version thereof. However, for many emerging optical applications, procedures are needed for general linear dispersive media with dispersions described by rational functions, such as a combination of multiple second-order poles. The authors present an efficient frequency-dependent full-band TD-BPM formulation for the analysis of general linear dispersive media. In this method, the complex time-domain convolution of the dispersive media is evaluated efficiently via a Z-transform. The formulation is validated by simulating dispersive media with complex permittivity described by single and multiple second-order poles at microwave and optical frequencies. It is shown that the proposed formulation can accurately simulate broadband electromagnetic responses using a much larger time-step size than those required by other conventional numerical techniques. Extension of this formulation to the treatment of any general material, such as dispersive non-linear material, is straightforward.

To date, the transmission line modelling method has been applied to structured, usually Cartesian, meshes. The flexibility offered by using 2D and 3D unstructured meshes, which is invaluable for practical simulation tasks, is now combined with the transmission line modelling algorithm to yield a powerful tool for computational electromagnetics.

A full vector, semi-analytical, yet accurate, analysis of coated optical fibre facets is presented, based upon the free space radiation mode method. This yields a simple final expression for the modal reflectivity. A highly efficient algorithm has been developed to implement this analysis. Example results for the reflectivity of the fundamental HE11 mode supported by typical single-mode fibres are given, which are compared to those obtained from an equivalent scalar investigation. Results show that correct modelling of the low reflectivities of coated facets requires a full vector analysis.

The properties of a tapered semiconductor laser cavity, which can be used to generate a high-brightness optical beam, are studied. The cavity consists of a narrow straight waveguide section and a tapered power amplifier section. The influence of the input waveguide width and length, as well as of the length and taper angle of the power amplifier section, on the overall performance of the cavity are discussed. Finally, the application of beam spoilers is studied. In order to solve the wave equation, the wide-angle-beam propagation method incorporating a higher-order finite-difference scheme is applied.

We propose and analyse a new optical three-dimensional waveguide having the function of a single-mode polarisation-maintaining waveguide or TE/TM mode splitter. In the optical waveguide proposed, a multilayer film composed of two dielectric materials with different refractive indices provides confinement in the height direction, and strong mode separation between TE- and TM-like modes, whilst a strip-loaded or rib-type structure provides confinement in the lateral direction. A full analysis of the structure based on a mode-matching method and a familiar equivalent circuit representation of 3D waveguides is presented. We also discuss several fundamental properties of the guided waves supported by the proposed waveguide for various structural parameters.

The authors present a comparison between different estimates of the shielding effectiveness (SE) of cavities containing apertures and predictions made using a rigorous variational solution. Although the rigorous solution is computationally intensive, the method can be used as a standard against which more practical approximations, numerical methods and laboratory measurements can be compared. SE predictions from three methods are compared to the rigorous solution. These are: electromagnetic time domain modelling using the transmission line modelling (TLM) method, experimental measurements in a screened room, and an approximate solution based on the interaction of the lowest order waveguide mode and the aperture field. These comparisons of shielding effectiveness evaluations were made for a rectangular cavity with a range of aperture sizes and for frequencies in the range 200 MHz to 1.0 GHz. The results show that the measurements compare well with the rigorous solution, time domain simulations have a relatively long computation time and may still have noticeable discrepancies while the approximate solutions are computationally very efficient but may be significantly in error in particular for observation points close to the aperture.

The spectral index method (SIM) for analysis of the rib waveguide in air is improved by including the singular behaviour of the principal field component in the vicinity of the reentrant corners, thereby giving the singular SIM (SSIM). This generally improves the value of the modal propagation constant as compared with more exact solutions. The new approach is successfully extended further to the rib waveguide coupler.

The generalised Fourier variational (GFV) method is implemented for the analysis of optoelectronic structures incorporating two new features: multiple layers and complex refractive indices. This extended method is validated by calculating complex propagation constants for commonly occurring types of index-guided laser geometries. The method produces highly accurate complex results by simple means, and is very fast.

The proven free space radiation mode method is extended for the first time to the important case of multilayered structures. Simple transcendental equations are obtained which can be solved in a matter of seconds, accurately yielding both propagation constants and field profiles. Further, the inclusion of lossy materials and active regions allows the approach to be used for the first time to model buried laser structures. A variety of practical examples are examined, and results are compared with those from complex finite element and finite difference calculations, demonstrating excellent agreement and a substantial reduction in computational effort.

New and accurate semianalytical expressions, based on the free-space radiation mode method, are derived for the reflectivity of a polarised guided mode of a slab waveguide incident on a multicoated laser facet at an arbitrary angle. Solutions may be obtained simply and with minimum computational effort. Results presented confirm that the coated angled facet can reduce the guided-mode reflectivity to near zero for both polarisations. The far-field radiation pattern is also significantly modified by angling the waveguide to the facet.

It is shown that the finite difference method may be extended to study the propagation characteristics of optical waveguides whose cross-sections contain regions of complex local refractive index, representing gain or loss. Complex propagation constants are calculated for a range of stripe geometry laser structures and compared with those obtained from other methods. When used in conjunction with a simultaneous finite difference solution of the carrier rate equation, the complex optical waveguide method provides a powerful self-consistent laser model.

A complete mathematical analysis of the dispersion in a symmetrical supercondensed node is presented. Comparisons show that this node exhibits superior dispersion characteristics over a stub-loaded node in addition to its established higher computational efficiency.

A fast and near exact modal analysis is presented to obtain the propagation constants and electric field profiles for dielectric multilayered channel (rib) waveguides. Calculations are performed for a particular channel configuration and found to be in excellent agreement with those obtained using the finite difference method.

An implementation of an improved all-link line symmetrical condensed node for the TLM method is presented. The new node has the advantage of containing no stubs but is still capable of modelling inhomogeneous media on a generally graded mesh; it requires less storage and run time than the stub-loaded and hybrid nodes, can operate on a higher time-step than previous nodes and it is shown to have a reduced velocity error for axial propagation.

The authors report what they believe to be the first successful fabrication of an InGaAs/GaAs vertical-cavity surface emitting laser (VCSEL) using optically transparent indium-tin-oxide (ITO) ohmic contacts to p- and n-type regions of the laser. Threshold current densities J/sub th/ are similar to those lasers fabricated from the same wafer using conventional ohmic contacts.

A new method of operating ferrite line pulse sharpeners which significantly improves achievable rise time is described. The essence of the method is to subject the line to an axial magnetic field of varying amplitude so enhancing the interaction between the pulse and processional phenomena in the ferrite.

An elegant and accurate technique of interfacing segments of a uniform computational mesh with different spatial resolution is described. The technique is illustrated in the time-domain for a TLM mesh. Results show that a seamless interface is achieved between different mesh areas, thus allowing multi-scale problems to be accurately mapped onto the mesh.

A simple and intuitive TLM algorithm has been developed to model nonlinear optical phenomena in photonic bandgap structures such as fibre Bragg gratings. The method is used to simulate periodic nonlinear media having a Kerr nonlinearity. Many phenomena occuring in these devices, such as bistable switching, soliton propagation and limiting, are modelled and discussed. The numerical time-domain approach gives valuable insight into the behaviour of these devices.

The simulation of beam-wave interaction devices usually requires calculation of macroscopic source terms to allow for the contribution of electrons to the electromagnetic wave. In certain circumstances this approach may be problematic. This can be overcome if the beam-wave interaction is modelled using a TLM technique.

An exact Fourier operator transform method (F-OPT) is presented for finding the polarised mode propagation constants of a rib waveguide with any number of supporting layers and an arbitrary cladding. The implementation is of the order of one hundred times faster than the finite difference method for the same accuracy of the normalised propagation constant.

An all-link line symmetrical condensed node capable of modelling inhomogeneous materials is described. It is shown that it has significant advantages in storage and run time over the conventional stub-loaded symmetrical condensed node.

A method of interfacing field problems described by TLM to lumped parameter circuits is presented. The technique permits the study of the interaction of complex circuits with components described by a full field model. The procedure is illustrated by results obtained for a capacitive circuit where the capacitor is described by a two-dimensional TLM model.

Recent years have seen the rapid evolution of silicon-based optoelectronics. The absorption properties of Si1−xGex alloys make them excellent materials for the realisation of photodetectors operating at the near-infrared wavelengths of 1.3 and 1.55 μm which are important for fibre optic communication systems. An important design consideration for such systems is the efficient coupling of optical fibres to optoelectronic integrated circuits. A Si1−xGex rib waveguide spot-size transformer is presented that performs two essential roles: the transformation of the large spot fibre to a small spot waveguide mode, and subsequently the transfer of the mode from a passive Si waveguiding region to an active Si1−xGex region where optical absorption may occur. This transformation is accomplished by the use of a tapered waveguide structure. An analysis technique based on the spectral index method is used to investigate the local modal behaviour of the structure and experimental results confirm its successful operation.

New results are presented for the reflectivity of a buried waveguide at a realistic depth incident on a coated facet. Diffractive effects are shown to be significant enough to prevent previously reported two-layer coatings meeting the practical specifications for high performance semiconductor optical amplifiers.

With ever increasing safety and operating restrictions being imposed on high-performance military aircraft, space vehicles, missile technology and satellites, for example, it is of increasing importance to measure and quantify the performance of experimental procedures. All experiments are subject to some inherent inaccuracy or loading and a detailed knowledge of experimental repeatability can assist in determining the levels of acceptable experimental error. Validation of the results of one experimental technique against another using the method detailed, allows the potential benefit of assessing and, more importantly, locating common errors in both methods, thus providing a rigorous framework for confidence building. The validation technique can also be used to identify best practice in a situation where there are ill defined test plans. The authors illustrate how sets of experimental results can be compared and their differences quantified. Furthermore, the interpretation of these results allows rational decisions to be made regarding the quality of measurement techniques or facilities, allowing them to be used with a measured level of confidence.

The authors report a new self-aligned method for the fabrication of porous silicon optical waveguides which is simple and avoids the typical processing problems associated with the chemically-reactive nature of the porous silicon internal surface. The method is compatible with high temperature oxidation, enabling waveguides operating between visible (633 nm) and infra-red (1.3 /spl mu/m) to be fabricated.

A new spectral method is proposed and applied to the analysis and design of integrated optical spotsize transformers. Results presented are of the same accuracy as those from a finite difference method. The new method is simple, efficient and gives propagation constants and fields over a wide range of structures in seconds, making it well suited to the class of optical waveguide problems involving interacting rib and buried waveguides.

A novel beam propagation algorithm is presented that eliminates errors caused by modelling tapered optical structures with staircase approximations. In consequence, highly accurate results are obtained with a substantial improvement in computational efficiency. The algorithm is developed from a nonorthogonal co-ordinate system that intrinsically describes the taper geometry and can be implemented using conventional beam propagation techniques.

Scattering in the transmission-line modelling (TLM) method is determined for a variety of nodes using a simple, rigorous and physically satisfying process. The suggested method is simpler to apply, and more advantageous from the methodological and computational points of view compared to the conventional method.

A vector correction is derived to the propagation constants obtained from a polarised analysis of graded-index optical waveguides. This general formula is reduced to a form suitable for use with semiconductor rib and strip-loaded waveguides. Corrected propagation constants calculated for a range of semiconductor rib waveguide structures coincide with available results from a full vector approach. Moreover, the vector correction term is always small, underpinning the accuracy of this simple and fast polarised approach for the first time.

A radiation mode technique, first used in optical fibre theory, is used to study the propagation of waves in practical optoelectronic devices. The method is validated against other techniques for step discontinuities, tapers, air-gaps and Y junctions. The solution deals successfully with reflected waves and is found to be accurate enough to be used for design purposes.

A simple exact implementation of Fourier operators is introduced and used for analysing reflection and transmission at open dielectric waveguide discontinuities, including the laser facet. The method is stable, and readily implemented using standard numerical techniques.

A simple formula is derived which is applied as a quasi-TE- or quasi-TM-mode corrector to the propagation constant values obtained from a scalar treatment of semiconductor rib waveguides. The resulting polarised propagation constants agree favourably with well established benchmark values for both types of mode. The present approach offers advantages in the speed and simplicity of solution.

The use of transmission-line modelling (TLM) as a general tool for designing cavity applications for microwave heating purposes is discussed. This method is used to validate a recent procedure for designing rectangular cavities loaded with an inhomogeneous dielectric layer. The influence of dielectric losses on the cavity resonances is studied.

A saddle point variational principle, for estimating the propagation constant of an optical rib waveguide mode from a discontinuous trial function, is generalised to provide a theoretical underpinning for the familiar effective index method. It is shown that better accuracy can be achieved in a rigorous manner, while maintaining much of the simplicity that makes the original effective index approach so attractive.

A simple vector correction is applied to the polarised solution obtained from a single component wave equation. The resulting propagation constants are shown to be indistinguishable from six-component vector finite element results used as a benchmark. This considerably strengthens the validity of the polarised solution for the accurate modelling of rib and stripe waveguides, and shows that emphasis should be placed on finding quick and reliably accurate methods for the polarised approximation. Only then does the vector correction itself become accurate.

Pulse sharpening using ferrite material was first noticed in the mid-fifties during the development of storage devices for telecommunication and computer systems. Since that time various workers have explained how a switched ferrite can change the propagation characteristics of a transmission line to improve the rise time of an input pulse. In the paper the advantages achievable by imposing electron spin resonance and subsequent frequency enhancement, which has greatly improved the performance of these lines, are described for the first time. Experimental evidence of the benefits of spin conditions is provided and a simple transmission line model introduced to simulate the pulse sharpening effects.

Optical ray theory is used to design a phase-corrected microwave horn antenna of short-length which produces a tapered aperture amplitude distribution in the E-plane. The basis of the design is the creation of a region where the effective refractive index varies with hyperbolic secant (HYSEC) form in a direction normal to the horn axis. In a practical realisation this effective refractive index distribution is achieved in both smooth and stepped form by varying the plate separation of the planar waveguide supporting a TE/sub 01/ mode. Good agreement is found between predicted and experimentally determined aperture and far-field distributions. Finally, the study is extended to the design of a novel antenna incorporating dielectric loading which, in theory, allows for operation at even shorter horn lengths.

A finite-difference method is used to design a sectoral horn antenna that uses corner channel inserts to produce a tapered aperture amplitude distribution in the E-plane. The channel inserts effectively suppress sidelobe levels in the radiation pattern. Good agreement is found between predicted and experimental far-field distributions.

It is shown that ray theory can be used in designing a microwave horn with a hyperbolic secant refractive-index profile. The horn is short in length, has constant phase across the plane aperture and tapered illumination in the E-plane, resulting in a radiation pattern with low sidelobes. Experimental verification has been made on a model operating around a frequency of 10 GHz.

A new design based on ray theory is presented for a phase corrected horn antenna which is shorter than one with a hyperbolic secant refractive index profile. A dielectric load is essential for realistic implementation. With load permittivity decreasing linearly towards the aperture, a smooth flared profile can be obtained. The antenna's compactness makes it useful for marine applications.