Physical optics

About: Physical optics is a research topic. Over the lifetime, 5342 publications have been published within this topic receiving 101388 citations. The topic is also known as: wave optics.

Physical Optics Imaging of 3-D PEC Objects: Vector and Multipolarized Approaches

Abstract: The problem of imaging 3-D perfect electric conducting objects from scattered field measurements is addressed. Plane waves at a fixed angle of incidence and varying frequency provide the illuminating radiation whereas the scattered field is collected in far-field zone. The physical optics approximation is adopted to simplify the scattering model and the scatterers' shapes (i.e., their contour surfaces) are described as the support of ? distributions. This leads to a linear integral relationship between the scattered field and the unknown distributions, the linear distributional model, which is inverted by employing its singular value decomposition. Emphasis is placed on the study of the dyadic operator to be inverted which links the vector unknown to the scattered field vector and on the role of the polarization diversity. In particular, three different approaches are presented and compared from the computational as well as the resolution point of view. The analysis is performed numerically by means of synthetic data generated by a finite-element-method-based forward solver.

The application of the principles of physical optics to crystal-structure determination

Abstract: The analogy between the diffraction of light by a set of holes, and the diffraction of X -rays by a set of atoms can be exploited in solving many different types of X -ray diffraction problem . A special instrument has been built to obtain diffraction patterns rapidly, making possible several different approaches to the determination of crystal structures. Of particular importance is that involving the ‘optical transform’ of a molecular representation. Consideration of the optical transform, which is closely related to the Fourier transform, can, for simple space groups, lead directly to the crystal structure. For more complicated space groups, optical transforms of molecules in different orientations have to be combined, and some rules governing their combination are given. The discussion includes several examples of structures which have been determined by optical-transform methods. These methods require the presentation of the X -ray data in the form of a weighted reciprocal lattice, from which information can often be obtained directly. For several new structures such information as the separation of two similarly oriented molecules, or the orientation of hexagonal arrangements of atoms, has been deduced by considering the weighted reciprocal lattice alone. In some cases this led to the complete structure; in others it was a valuable supplement to the optical transform method. Despite the lack of quantitative accuracy, optical methods can be of use in refining approximate structures by ordinary Fourier methods. Examples are given of the deter­mination of the relative phase angles of the reflexions of both centrosymmetric and non-centrosymmetric structures. The relationships between optical methods and more conventional methods of structure determination are discussed.

The SAR Image of Short Gravity Waves On a Long Gravity Wave

Abstract: A SAR imaging model appropriate to oceanographic applications is derived, unifying fundamental models of hydrodynamics, rough surface scattering, and SAR imaging of time-variant scenes. The sea surface is a sinusoidal long gravity wave upon which short gravity waves propagate and are modified by the long wave in accordance with a recent theory of Phillips; the electromagnetic scattering is described by the two-scale approximation appropriate to long wave and short wave ensembles that are, respectively, smoothly varying and not too rough with respect to the radar wavelength. The resulting model, accurate to first order in the long wave slope, for the first time fundamentally characterizes the nonlinear hydrodynamic and scattering interactions of the long and short waves and their effect, along with temporal variation, on the SAR image. Of particular importance, the long wave enters (among other ways) as a phasemodulated waveform that, when filtered by the SAR system, can be, for large-amplitude long waves, the principal determinant of the image nature. The numerical analysis of the model is discussed and an approximation describing the image of a delimited scene area is derived and exemplified. (1) When the small waves are a range-directed ensemble and the long wave is azimuth directed, the latter’s temporal variation “blurs,” in azimuth, the image due, primarily, to the SAR system’s narrowband filtering of the aforementioned phase- modulated waveform and, secondarily, the nonlinear hydrodynamic interaction; it is shown that this “blurring” is, at higher long wave amplitudes, due to a quadratic phase error proportional to the phase velocity of the long wave. That part of the short wave ensemble allowed influential by SAR system (wavenumber) filtering is approximately nondispersive during the SAR azimuth integration time, its concerted effect being a rigid azimuth image translation, proportional to the short wave’s mean phase velocity. (2) More briefly treated, when the long wave is ranged directed and the short wave ensemble is simply a single range-directed sinusoid (“Bragg-matched”), the image is solely range variant and its nature is primarily determined by the aforementioned narrowband filtering effect, the secondary effects of nonlinear hydrodynamics and physical optics (i.e., surface slope) being evident. Therefore, the present model, as thus far elaborated, contradicts predictions of models based on the SAR response to a point scatterer in motion in accordance with the “orbital motion” of the long wave: e.g., no “azimuth bunching” attributed to such motion is observed.