Teng-Tai Hsu

High-frequency fields excited by truncated arrays of nonuniformly distributed filamentary scatterers on an infinite dielectric slab: parameterizing (leaky mode)-(Floquet mode) interaction

In previous studies, we have developed and tested observable-based parameterizations (OBP) of time-harmonic wavefield scattering by periodic or aperiodic finite arrays of planar strip and filament scatterers. The resulting algorithm is based on truncated Floquet modes and Floquet-modulated edge diffractions due to the truncations of the array. The corresponding robust wave processes link features (observables) in scattering data with geometrical features in the model configuration in such a manner as to be useful for subsequent application to target classification and identification. The present study extends these investigations to a finite array of filamentary scatterers located on the surface of an infinitely extended dielectric slab, thereby parameterizing (Floquet mode)-(leaky mode) interaction as a classifier of the more complicated phenomenology in this composite configuration. The outcome is an OBP with two separate constituents that can be interpreted, respectively, as slab-modified Floquet scattering by the truncated array and as truncated-Floquet-induced excitation of slab-guided leaky waves. This new OBP for the composite problem is validated by comparison with reference solutions generated numerically, its relevance to wave-oriented data processing is demonstrated in the companion paper.

FDTD analysis of plane-wave diffraction from microwave devices on an infinite dielectric slab

A 2-D (two-dimensional) Huygens surface is developed for the finite difference time domain (FDTD) algorithm, allowing the investigation of pulsed plane-wave scattering from arbitrary 2-D structures placed on or in an infinite dielectric slab. Example results are presented for scattering from a perfectly conducting strip on an infinite dielectric slab, and the results are compared with data computed via the method of moments. Additionally, the diffraction from a finite, saw-tooth dielectric grating on an infinite dielectric slab is investigated.

Frequency-domain scattering by nonuniform truncated arrays: wave-oriented data processing for inversion and imaging

We previously presented an asymptotic diffraction theory for time-harmonic and transient scattering by arbitrarily illuminated truncated nonuniform thin-wire gratings [ J. Opt. Soc. Am. A11, 1291 ( 1994)]. We parameterized and interpreted the results in terms of scattered truncated Floquet modes (FM’s) and Floquet-modulated edge diffraction, which generalize the constructs of the conventional geometric theory of diffraction (GTD). We also demonstrated that numerical implementation of the FM–GTD algorithm yields results that compare very well with data computed from rigorously based numerical reference solutions. We enlarge the previous frequency-domain numerical data base for gratings to scattering by truncated arrays whose elements are arbitrarily oriented strips rather than thin-wire filaments and also to arrays whose element locations depart from truncated periodicity in a random rather than an orderly manner. We show that the FM–GTD parameterization of the scattered field remains applicable under these generalized conditions. With a view toward inversion and imaging, our principal purpose is the application of space-wave-number phase-space processing techniques to extract the footprints of truncated nonuniform periodicity from the scattered-field data. Because the processing is tied to the wave physics, we refer to this procedure as wave-oriented data processing. Implementation involves projection onto appropriate phase-space subdomains and the generation of space–wave-number phase-space distributions by windowed Fourier transforms. It is found that this form of processing in the frequency domain highlights effects of truncation and perturbed periodicity but is not very sensitive to the structure of the array elements (i.e., wires versus strips). In a companion paper [ J. Opt. Soc. Am. A11, 2685 ( 1994)] we perform phase-space processing in the time domain, show how the time-domain FM-GTD phenomenology is revealed through time-frequency distributions, and show also how short-pulse excitation enhances the sensitivity with respect to element structure by means of spatial–temporal resolution.

Short-pulse propagation in a hollow waveguide: analysis, optoelectronic measurement, and signal processing

An asymptotic analysis is performed for short-pulse propagation in a hollow waveguide. It is demonstrated that each time-domain mode supported by the guide is characterized by a time-dependent frequency which, as time proceeds, approaches the modal cutoff frequency. This phenomenon is demonstrated experimentally by performing short-pulse optoelectronic measurements for the case of rectangular waveguide. In these measurements a short-pulse laser is used to switch planar antennas photoconductively, generating freely propagating waveforms with instantaneous bandwidth from 15-75 GHz. Time-frequency signal processing is performed on the measured data, the results of which are in close agreement with the predictions of the asymptotic analysis. >

Short pulse sound wave scattering by a fluid shell model target near a randomly distorted bottom interface: Wave oriented data processing for target‐clutter discrimination

A finite difference time‐domain (FDTD) code has been used to assemble a comprehensive data base of realizations for short‐pulse plane wave scattering by a fluid shell model target submerged in the presence of a randomly distorted bottom interface. The entire problem is two dimensional with the scattered acoustic pressure observed along an elevated track above the target and parallel to the mean bottom boundary. Two bottom topographies are examined: (1) a smoothly deformed interface between the wated column and a fluid bottom; (2) a collection of soft randomly pitched thin flat strips. The database furnished by the ensemble of realizations for the randomly irregular bottom, in which the deterministic target remains unchanged, is subjected to wave‐oriented data processing that yields space‐wave number and time‐frequency phase space distributions extracted via windowed transforms and refined by locally applied high resolution algorithms. The processing is applied to the target alone, the cluster alone, and t...

High frequency radiation from truncated arrays of filamentary sources on a dielectric slab

Previously explored radiation phenomenologies pertaining to individual line sources on a dielectric slab and to finite arrays of line sources in free space are combined to synthesize and parametrize radiation from finite line source arrays located on a dielectric slab surface. From rigorous analysis and subsequent high frequency asymptotics for the prototype periodic case with linear interelement phasing, the authors obtain an observable-based parametrization (OBP) which connects features in reference data with structural features of the (slab)-(line array) configuration via wave processes with different physical content. The OBP is derived by applying the Poisson summation formula to collectively restructuring leads to fields which are parametrized in terms of Floquet mode (FM) beams, slab-guided leaky modes (LM) peaked by phase matching to the truncated FM, and edge diffractions from the ends of the truncated array. The OBP algorithm is validated by comparison with numerical reference data, and the parametrization provides choices for design and classification.

Frequency domain wave-oriented data processing of scattering by truncated periodic strip gratings

In a companion paper (see ibid., p.668, 1994), we have investigated wave-oriented processing techniques which extract from frequency domain (FD) scattering data for truncated periodic strip gratings the wave phenomenology that ties features in data to scattering mechanisms responsible for these features. The present paper explores time domain (TD) wave-oriented processing techniques for scattering by such gratings due to a pulsed incident plane wave. Time (t)-frequency (/spl omega/) phase space distributions implemented via windowed transforms now extract from the TD data new phenomenologies which are interpreted in terms of novel TD Floquet modes (FM) and TD-FM edge diffractions from the grating aperture truncations. This is in agreement with the previously developed analytic TD scattering models.<<ETX>>

Phase‐space footprints of truncated periodicity: Time domain wave‐oriented data processing

A companion paper [L. B. Felsen et al., paper 3aSAb8] has dealt with phase space processing techniques that extract frequency domain (FD) Bragg‐modulated phenomenologies from scattering data pertaining to truncated periodic arrays of filaments. Displayed in subdomains of the (x,z,kx,kz)ω phase space accessed by windowed and global transforms, the footprints of the dispersive truncated Bragg modes are of special interest. These processing techniques are now extended and applied to short‐pulse time domain (TD) synthetic scattering data from the filament arrays. The phase space is enlarged to (x,z,t,kx,kz,ω), with a correspondingly enriched choice of subdomain processing options and visualizations. Again, special attention is given to the truncated TD Bragg modes whose behavior is governed by an instantaneous dispersion relation extracted by the processing. Specific plots include the subdomains (t,ω), (t,px), and (ω,kx), which complement the FD results (px≡kx/ω). [Work supported by AFOSR.]

Effects of departures from periodicity on the frequency domain and short pulse acoustic scattering from finite soft and rigid flat strip arrays.

Many structures of interest contain regularly spaced elements that leave their imprint on the acoustic scattered field. For periodic arrangements, even a few elements may imprint on the scattered field the signature of the periodic structure Floquet modes that characterize an infinite array [L. Carin and L. B. Felsen, J. Acoust. Soc. Am. 90, 2355(A) (1991)]. Explored here, for various collections of flat, soft, and rigid strip scatterers, is how departures from periodicity due to array truncation, as well as displacement, change in size, etc., of individual and groups of elements affects the time‐harmonic and short pulse plane wave scattered response. Reference solutions obtained numerically by direct integration of a (spectral domain)–(moment method) procedure are interpreted phenomenologically through asymptotic reductions to yield truncation edge diffracted contributions, local Floquet modes from the overall bulk of the array, and individual scatterings from displaced elements. The contracting behavior...