Extended Finite-difference Time-domain Research Articles

Acoustic wave rectification in viscoelastic materials

We have numerically investigated the propagation of acoustic waves in a periodic array of triangular holes embedded in a matrix composed of viscoelastic materials, such as polymers, using an extended finite-difference time-domain method. Although the viscoelasticity of the substrate in acoustic wave rectifiers smears out the fine structures observed at the transmission rate, the rectifying effect of acoustic waves survives. Moreover, the transmittance for longitudinal wave incidence broadly shows a vivid rectification effect in a wide frequency range when the difference between the velocities of longitudinal and transverse waves is large. In addition, the extreme sensitivity of shear rate with respect to time (or frequency) gives rise to the marked modulation of the frequency dependence of the transmission rate.

An Extended FDTD Method With Inclusion of Material Dispersion for the Full-Vectorial Analysis of Photonic Crystal Fibers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An extended finite-difference time-domain (FDTD) method with inclusion of material dispersion for the full-vectorial analysis of photonic crystal fibers (PCFs) is presented in this paper. In order for this extended FDTD method to obtain a fast convergence and high accuracy, a constitutive-parameter averaging technique is employed. Numerical results for a typical PCF verify the validity of the extended FDTD method. With this FDTD mode solver, a new birefringent PCF is also investigated, and interesting results are obtained in this paper. </para>

Extended Finite Difference Time Domain Analysis of Induced Phase Modulation and Four-Wave Mixing between Two-Color Femtosecond Laser Pulses in a Silica Fiber with Different Initial Delays

Experimental results of nonlinear propagation with the induced-phase modulation (IPM) effect of two-color 120 fs laser pulses with three different initial delays in a silica fiber were analyzed with finite-difference time domain (FDTD) calculations, including self-phase modulation (SPM), IPM, and degenerate four-wave mixing (DFWM) in the fused-silica fiber. Initial delay dependence by the FDTD qualitatively agrees with the previous experimental results. An ultrabroad spectrum induced by IPM mostly agrees with the previous experimental results that had the optimum initial delay. A Fourier transform of this spectrum yields a 4 fs optical pulse, which agrees with the previous experimental results. The spectral phases of two different pulses with three different initial delays are connected continuously. The IPM spectral phase obtained by the extended FDTD shows that the DFWM occurs. To the best of our knowledge, this is the first observation of DFWM with IPM by the FDTD simulation.

Observation of Slowly Varying Envelope Approximation Breakdown by Comparison between the Extended Finite-Difference Time-Domain Method and the Beam Propagation Method for Ultrashort-Laser-Pulse Propagation in a Silica Fiber

Conventionally, the beam propagation method (BPM) for solving the generalized nonlinear Schrödinger equation (GNLSE) including the slowly varying envelope approximation (SVEA) has been used to describe ultrashort-laser-pulse propagation in an optical fiber. However, if the pulse duration approaches the optical cycle regime (<10 fs), this approximation becomes invalid. Consequently, it becomes necessary to use the finite difference time domain method (FDTD method) for solving the Maxwell equation with the least approximation. In order to both experimentally and numerically investigate nonlinear femtosecond ultrabroad-band-pulse propagation in a silica fiber, we have extended the FDTD calculation of Maxwell's equations with nonlinear terms to that including all exact Sellmeier-fitting values. We compared the results of this extended FDTD method with the solution of the BPM that includes the Raman response function, which is the same as in the extended FDTD method, up to fifth-order dispersion with the SVEA, as well as with the experimental results for nonlinear propagation of a 12 fs laser pulse in a silica fiber. Furthermore, in only the calculation, pulse width was gradually shortened from 12 fs to 7 fs to 4 fs to observe the breakdown of the SVEA in detail. Moreover, the soliton number N was established as 1 or 2. To the best of our knowledge, this is the first observation of the breakdown of the SVEA by comparison between the results of the extended FDTD and the BPM calculations for the nonlinear propagation of an ultrashort (<12 fs) laser pulse in a silica fiber.

Numerical Analysis of Induced Phase Modulation between Two Ultrashort Pulses in Silica Fiber by the Extended Finite-Difference Time-Domain Method

We have analyzed the induced phase modulation (IPM) for ultrashort (74 fs) two-pulse propagation in a silica fiber by the extended finite-difference time-domain (FDTD) method, considering all exact Sellmeier-fitting values and nonlinear polarization PNL involving the Raman response function. We show that nonlinear polarization causes several phenomena in spectral characteristics of propagated pulses, such as self-phase modulation (SPM), self-steepening, Raman response and IPM, by the extended FDTD method. To the best of our knowledge, this is the first IPM calculation by the extended FDTD method for the simultaneous propagation of two ultrashort (74 fs) laser pulses in a silica fiber.

Some aspects of stability and numerical dissipation of the finite-difference time-domain (FDTD) technique including passive and active lumped elements

This paper presents a stability analysis of the extended finite-difference time-domain method including passive and active devices. An explicit, implicit, and semi-implicit incorporation of lumped elements is investigated and the eigenvalues of the resulting discrete system are discussed. With the underlying assumption that the domain is homogeneously loaded with lumped elements, stability criteria are derived on the basis of a resistance, a conductance, and an inductance. Applying a fully implicit method, a parasitic resistance can be observed when reactive devices are included. For an inductance, this numerical dissipation is characterized in detail and an equivalent circuit is given. As an example, the impact on the quality (Q) factor of a cavity loaded with an inductance is shown and compared to the theoretical derivation.

A coupled FDTD-artificial neural network technique for large-signal analysis of microwave circuits

We propose a first-order global modeling approach of Monolithic Microwave Integrated Circuits (MMIC) by modeling the active device with a neural network based on a full hydrodynamic model. This neural network describes the nonlinearities of the equivalent circuit parameters of an MESFET implemented in an extended Finite Difference Time Domain mesh to predict large-signal behaviors of the circuits. We successfully represented the transistor characteristics with a one-hidden-layer neural network, whose inputs are the gate voltage Vgs and the drain voltage Vds. The trained neural network shows excellent accuracy and dramatically reduces the computational time in comparison with the hydrodynamic model. Small-signal simulation is performed and validated by comparison with HP-Libra. Then large-signal behaviors are obtained, which demonstrates the successful use of the artificial neural network. © 2002 John Wiley & Sons, Inc. Int J RF and Microwave CAE 12: 25–36, 2002.

Analysis of crosstalk between finite-length microstrip lines: FDTD approach and circuit-concept modeling

Two approaches, one based on the circuit concept and the other based on field theory, are used to model the crosstalk between two microstrip lines of finite length and arbitrary orientation. In the circuit-concept modeling, a set of equations for the line voltages and currents has been derived from a modified telegrapher's equation. A four-port network expression is resultantly obtained by solving the equations, thus the crosstalk can be predicted by applying terminal conditions to the network expression. On the other hand, the extended finite-difference time-domain (FDTD) method has been used to model the terminal resistors and the feeding resistive voltage source in the crosstalk analysis. Several physical models have been fabricated and experiments performed. The calculated results are compared to measurements. In our experiment, for microstrip lines of finite length and arbitrary orientation, there are short line-sections or vias at each of the four ports, which should be incorporated into the crosstalk analysis. This effect has been investigated numerically and experimentally.

A practical large-signal global modeling simulation of a microwave amplifier using artificial neural network

We present a new technique to obtain large-signal global modeling simulation of a MMIC amplifier. The active device is modeled with a neural network trained with data obtained from a full hydrodynamic model. This neural network describes the nonlinearities of the equivalent circuit parameters of a MESFET implemented in an extended Finite Difference Time Domain (FDTD) mesh. We successfully represented the transistor characteristics with a one-hidden-layer neural network whose inputs are the gate voltage V/sub gs/, and the drain voltage V/sub ds/. Small-signal simulation is performed and validated by comparison with HP-Libra. Then, the large signal behavior is obtained, which demonstrates the successful use of artificial neural network (ANN) in the FDTD marching time algorithm.

Analysis and design of integrated active circulator antennas

A study on the analysis and design of active integrated antennas based on active quasi-circulators is reported in this paper. The antenna consists of a novel hybrid active circulator and a short-circuited quarter-wavelength microstrip antenna, which combine to form an active antenna with transmit and receive action at the same frequency. A full-wave model of the configuration using the extended finite-difference time-domain method is devised to analyze its operation, to study parasitic electromagnetic coupling effects, and to derive design guidelines. Experimental results for a hybrid model are also presented.

Full-wave design and optimization of mm-wave diode-based circuits in finline technique

This paper presents a full-wave design and optimization of a quasi-planar frequency doubler and a balanced subharmonic mixer in finline technique by applying the extended finite-difference time-domain (FDTD) method. The structures are based on the junction of a coplanar waveguide and a finline using two Schottky diodes mounted across this junction. The diodes are represented by their large-signal device circuit model. The specific problem of embedding the lumped elements in the FDTD mesh at millimeter-wave frequencies is discussed. A new method for the inclusion of the device into the grid is developed, avoiding nonphysical reactances. The frequency doubler is designed for optimal conversion loss at 0-dBm input power in a frequency band from 20 to 25 and 40 to 50 GHz, respectively. With the subharmonic mixer, matching structures at both the local-oscillator (LO) port and the radio-frequency (RF) port have been employed so that a conversion loss of 14.8 dB could be achieved with only 5-dBm LO power. The operation frequencies are 18 GHz for the LO and 56 GHz for the RF. The simulation results are validated by measurements.

Full-wave analysis of packaged microwave circuits with active and nonlinear devices: an FDTD approach

This paper presents a comprehensive full-wave analysis of packaged nonlinear active microwave circuits by applying the extended finite-difference time-domain (FDTD) method. Based on the approach of using equivalent sources, the device-wave interaction is characterized and incorporated into the FDTD time-marching scheme. As a consequence, analysis of linear and nonlinear properties, including harmonic generation and intermodulation, can be accomplished by employing a large-signal device circuit model. The implementation is first validated by comparing results of FDTD and HP MDS simulation of the circuit without the packaging structure. The analysis then goes beyond the capability of the circuit simulator to include the packaging effect. This analysis is useful in circuit design involving electromagnetic compatibility/electromagnetic interference (EMC/EMI) problems.

Modeling of microwave active devices using the FDTD analysis based on the voltage-source approach

This letter describes a voltage-source-based formulation of the extended finite-difference time-domain algorithm for the purpose of modeling microwave devices. The device-wave interaction is fully characterized by replacing the lumped devices with equivalent voltage sources in the device region, which in turn generate electromagnetic fields according to Faraday's law. This formulation is applied to the analysis of a typical microwave amplifier, which includes a three-terminal active MESFET device. Simulation results are in good agreement with measured data.