Dynamic Time Domain Research Articles

Photoelastic studies for composite dynamics

The orthotropic birefringent composites suitable for the study of dynamic photoelasticity are investigated and the determination of residual birefringence in the materials is briefly described. A stress-optic law for dynamic photoelastic analysis of orthotropic birefringent composites is postulated based on the static stress-optic law of Hyer and Liu. Subsequently, practical methods of calibrating dynamic mechanical constants and dynamic stress-fringe values are proposed. With dynamic strain measurements and time domain BEM for anisotropic media, three calibration specimens (0°, 90° and 45°) are used to verify the proposed stress-optic law in uniaxial-stress fields and a plate of unidirectional fiber-reinforced birefringent composite under impact loading, with the loading direction parallel, perpendicular and at 45° to the fiber direction, is analyzed to verify the proposed stress-optic law in biaxial-stress fields. Results show that the dynamic stress-optic law for orthotropic birefringent composites is valid in the two cases.

Dynamic beam propagation method for flared semiconductor power amplifiers

In this paper, we present the results of a dynamic two-dimensional time-domain beam propagation method (two spatial dimensions plus time) for simulating travelling wave semiconductor optical amplifiers with nonuniform transverse section in pulsed regime. Using the effective refractive index method, the paraxial approximation and a moving timeframe, we reduce the scalar wave equation to a nonlinear parabolic partial differential equation for the slow complex envelope of the electric field. Diffraction, refractive index dispersion up to the second order, gain saturation and gain dispersion are included in the model. We numerically solve the equation through a locally one-dimensional (LOD) method employing both finite-difference techniques and FFT-based pseudospectral methods.

Mode selection in complex-coupled semiconductorDFB lasers

Mode suppression in complex-coupled DFB lasers with both in-phase and antiphase gain gratings is examined through a novel use of power balance. The theory is then backed up by more detailed arguments using a dynamic time domain model. The phase of the gain grating in relation to the index grating determines which mode of the DFB is suppressed. Longitudinal variations in the gain coupling coefficient caused by spatial hole burning provide additional side-mode suppression mechanisms.

Formulation of a three-dimensional distinct element model—Part II. Mechanical calculations for motion and interaction of a system composed of many polyhedral blocks

A three-dimensional formulation of the distinct element method is embodied in computer program 3DEC, which has been adapted to run on a personal computer. This formulation is based on a dynamic (time domain) solution algorithm which solves the equations of motion of a three-dimensional block system by an explicit finite-difference scheme. The scheme is well suited in rock mechanics studies to determine if a discontinuous rock mass will fail under a given set of applied loads. Part II of this paper describes the mechanical calculations of rigid body motion and block interaction. Part II also presents the technique used in 3DEC to generate a three-dimensional model for rock mechanics analysis. Several examples are provided which demonstrate the capabilities of 3DEC and the size of problem that can be analyzed on a personal computer. The examples emphasize the use of microcomputer graphics to assess model results.

Crosslink architectures for a multiple satellite system

Several candidate crosslink communications architectures are described for a packet-switched, low-altitude, multiple satellite system (MSS). One architecture represents a frequency-division approach, in which a cellular concept is combined with dynamic time-domain allocation within a cell. A second architecture uses an unsynchronized space-time division approach consisting of directional antennas and random accessing. A third architecture uses toroiclal antennas with unsynchronized random accessing. The recommended architecture uses a synchronized space-time division approach, consisting of directional antennas and contention-based pair-wise scheduling. This architecture, called PRS (Pseudo-random Scheduling) is found to have significantly reduced power and bandwidth requirements relative to the other approaches.