Finite-difference Time-domain FDTD Method Research Articles

A non-conforming multi-element DGTD method for the simulation of human exposure to electromagnetic waves

The great majority of numerical calculations of electric energy deposition in human tissues exposed to microwaves are performed using the finite-difference time-domain FDTD method and voxel-based geometrical models of the tissues. The straightforward implementation of the method and its computational efficiency are among the main reasons for FDTD being currently the leading method for numerical assessment of human exposure to electromagnetic waves. However, the rather difficult departure from the commonly used Cartesian grid and cell size limitations regarding the discretization of very detailed structures of human tissues are often recognized as the main weaknesses of the FDTD method in this application context. In particular, interfaces between tissues where sharp gradients of the electromagnetic field may occur are hardly modeled rigorously in these studies this is generally referred as the staircasing effect. We present here an alternative numerical dosimetry methodology that is based on a DGTD method designed to work with non-conforming cubic-tetrahedral meshes for more flexibility in the discretization process of complex propagation scenes. For example, in the context of head tissues exposure to mobile phone radiation, the computation domain is divided in two regions: an unstructured tetrahedral mesh-based heterogeneous model of head tissues and an orthogonal cartesian mesh of the vacuum part of the propagation domain. We present numerical results comparing this approach with strategies based on fully Cartesian and fully tetrahedral meshes of the whole computational domain. Copyright © 2013 John Wiley & Sons, Ltd.

Numerical assessment of the combination of subgridding and the perfectly matched layer grid termination in the finite difference time domain method

The finite-difference time-domain FDTD method is one of the well-established numerical techniques to model electromagnetic problems. In order to be able to efficiently include the finer geometrical details, subgridding is used to avoid the use of a fine mesh over the whole region of interest. Our work builds on and extends the systematic framework provided by Chilton in his PhD H-refinement, P-refinement, and T-refinement strategies for the FDTD method developed via finite element principles, Ohio State University, 2008.

A time-domain near- to far-field transformation for FDTD in two dimensions

This paper proposes an algorithm based on the equi alence principle to calculate, straightforwardly in the time domain, the transient far field response of a two-dimensional structure. The algorithm implementation in ol es the numerical e aluation of a time integral along the nearto far-field con ersion contour of the equi alent currents at the ( ) Huygens surface, obtained with the finite-difference time-domain FDTD method. 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 427 432, 2000.

A finite-difference time-domain electromagnetic field solver for vertical-cavity lasers

ABSTRACT: We present a full- ¤ ector Maxwell equation sol ¤ er for theanalysis of ¤ ertical-ca ¤ ity surface-emitting lasers VCSELs with circular()symmetry using the finite-difference time-domain FDTD method. We()apply this model to the air-post index-guided 1.55 m m VCSEL structure,and determine the resonant wa ¤ elengths and the quality factor forselected lowest order modes. The results of this study enable quantitati ¤ eand qualitati ¤ e descriptions of the trans erse-mode competition andemission spectra of VCSELs. Q 1998 John Wiley & Sons, Inc.Microwave Opt Technol Lett 18: 385]387, 1998. Key words: electromagnetic field sol ¤ er; finite-difference time-domainmethods; ¤ ertical-ca ity lasers Vertical-cavity surface-emitting lasers VCSELs demonstrate.many properties desirable for optical communications andinterconnects, such as low-beam divergence and single-modeoperation which result in superior coupling efficiency intosingle-mode fibers 1 , the possibility of wafer-level testing,wxand ease for two-dimensional array fabrication 2 . In manywxapplications, and for long-haul optical communications, inparticular, single-transverse-mode operation is desired. Forthis reason, a number of theoretical analyses have beenrecently reported in an attempt to understand the transverse-mode competition and the emission spectra of these lasers.The scalar beam-propagation 4 and the effective index 5wx wxmethods have been successfully used, but since they neglectthe vector nature of the electromagnetic field, these methodsare unable to model the full transverse-mode spectrum ofthese lasers. The transverse modes of vertical-cavity lasersare rather complicated, while their spatial shape and polar-ization properties depend on the type of mode definitionscheme. Index-guided, gain-guided, and oxide-apertured lasersshow distinctly different transverse modes and their depen-dence on injection current 6, 7 . Because these cavities arewxsmall on the order of a few wavelengths in all directions , the.application of beam-propagation methods and effective-indexmethods is unable to provide a sufficiently accurate descrip-tion of the optical field in the cavity. Moreover, the beam-propagation method does not account for multiple reflectionsin quarter-wave mirrors. Particularly difficult structures tomodel are the index-guided air-post devices and the oxide-apertured devices in which the large lateral refractive indexvariations severely add to the complexity of the problem 8 .wxFor this reason, it is necessary to include the vector nature ofthe electromagnetic field to accurately analyze vertical-cavitylasers. In this paper, we present an analysis of the passivecavity of a 1.55 mm air-post index-guided VCSEL using afull-vector finite-difference time-domain FDTD method..The analyzed VCSEL structure is a 1550 nm double-fusedcavity that consists of an InGaAsP cavity sandwiched betweentwo AlGaAs AlGaAs.rGaAs quarter-wavelength distributedBragg reflectors DBR . A detailed description and the physi-.cal and optical structure parameters of this device are givenin 9 , while the simplified structure is shown in Figure 1 a .wx .The transverse modes of this resonator are determined byindex guiding in the top mirror, while the cavity loss occursdue to diffraction of the wave in the unguided sections of theactive layer and the bottom mirror. In a high-quality res-onator, such as the VCSEL, one may assume that the modesare not significantly perturbed by free-space propagation, andthe approximate cavity spectrum may be obtained by model-ing the VCSEL as a perfect cylindrical dielectric resonatorwx10 . However, this model does not provide the quality factorof each individual mode, namely, the discrimination betweenthe modes. For this purpose, one has to resort to a moreinvolved analysis: we use a full-vector finite-difference time-domain FDTD applicable to cylindrical high-quality open.resonators with dielectric andror metal boundaries. The pro-gram is used to determine the resonance wavelengths, thequality factor, and the mode spatial distribution for selectlowest order modes in the VCSEL cavity.The principle of operation of the FDTD program is basedon determining the passive-resonator temporal response tonarrowband noise delivered by an antenna source within thecavity. The resonator temporal response, detected at several