Showing papers on "Physical optics published in 1995"

Improved PO-MM hybrid formulation for scattering from three-dimensional perfectly conducting bodies of arbitrary shape

Abstract: The method of moments (MM) represents a suitable procedure for dealing with electromagnetic scattering problems of arbitrary geometrical shape in the lower frequency range. However, with increasing frequency both computation time and memory requirement often exceed available computer capacities. Therefore a current based hybrid method combining the MM with the physical optics (PO) approximation suitable for three-dimensional perfectly conducting bodies is proposed in this paper. The hybrid formulation allows a substantial reduction of computation time and memory requirement, while the results are in reasonable agreement with those based on an application of the MM alone. Further improvement can be achieved for flat polygonal parts of the scattering body by a heuristic modification of the PO current density taking into account the effects of edges. As opposed to the physical theory of diffraction (PTD), no additional electric and magnetic line currents along the edges are necessary. >

An iterative physical optics approach for analyzing the electromagnetic scattering by large open-ended cavities

Abstract: A formulation based on the high frequency asymptotic principles of physical optics is developed for analyzing the scattering by relatively arbitrary open-ended waveguide cavities containing complex interior terminations. A magnetic field integral equation (MFIE) is obtained for the equivalent currents on the interior cavity walls and is solved using an iterative physical optics (IPO) algorithm which iteratively applies physical optics to account for multiple reflections inside the cavity. The number of iterations required for convergence is related to the expected number of important reflections. The IPO method is more approximate than a matrix solution of the MFIE, but it is quite accurate for electrically large cavities and is much more efficient. Numerical results are presented which demonstrate the convergence and accuracy of the method by comparison with modal reference solutions. >

A generalized diffraction synthesis technique for high performance reflector antennas

Abstract: Stringent requirements on reflector antenna performances in modern applications such as direct broadcast satellite (DBS) communications, radar systems, and radio astronomy have demanded the development of sophisticated synthesis techniques. Presented in the paper is a generalized diffraction synthesis technique for single- and dual-reflector antennas fed by either a single feed or an array feed. High versatility and accuracy are achieved by combining optimization procedures and diffraction analysis such as physical optics (PO) and physical theory of diffraction (PTD). With this technique, one may simultaneously shape the reflector surfaces and adjust the positions, orientations, and excitations of an arbitrarily configured array feed to produce the specified radiation characteristics such as high directivity, contoured patterns, and low sidelobe levels, etc. The shaped reflectors are represented by a set of orthogonal global expansion functions (the Jacobi-Fourier expansion), and are characterized by smooth surfaces, well-defined (superquadric) circumferences, and continuous surface derivatives. The sample applications of contoured beam antenna designs and reflector surface distortion compensation are given to illustrate the effectiveness of this diffraction synthesis technique. >

Effect of numerical aperture on interference fringe spacing.

Abstract: The effect of numerical aperture on the fringe spacing in interferometry is analyzed by the use of wave optics. The results are compared with published experimental results, and the influence of apodization of the wave front is discussed. The effects of central obscuration and surface tilt are also considered.

Mode-selective dynamic light scattering: theory versus experimental realization.

Abstract: We present a quantitative experimental comparison of fiber-based, single- and few-mode dynamic light scattering with the classical pinhole-detection optics. The recently presented theory of mode-selective dynamic light scattering [Appl. Opt. 32, 2860 (1993)] predicts a collection efficiency and a signal-tobaseline ratio superior to that of a classical pinhole setup. These predictions are confirmed by our experiments. Using single-mode optical fibers with different cutoff wavelengths and commercially available mechanical components, we have constructed a mode-selective detection optics in a simple and compact dynamic light-scattering spectrometer that permits an optimal compromise between signal intensity and dynamical resolution.

Electromagnetic scattering model for a tree trunk above a tilted ground plane

Abstract: An efficient and realistic electromagnetic scattering model for a tree trunk above a ground plane is presented. The trunk is modeled as a finite-length stratified dielectric cylinder with a corrugated bark layer. The ground is considered to be a smooth homogeneous dielectric with an arbitrary slope. The bistatic scattering response of the cylinder is obtained by invoking two approximations. In the microwave region, the height of the tree trunks are usually much larger than the wavelength. Therefore the interior fields in a finite length cylinder representing a tree trunk can be approximated with those of an infinite cylinder with the same physical and electrical radial characteristics. Also an approximate image theory is used to account for the presence of the dielectric ground plane which simply introduces an image excitation wave and an image scattered field. An asymptotic solution based on the physical optics approximation is derived which provides a fast algorithm with excellent accuracy when the radii of the tree trunks are large compared to the wavelength. The effect of a bark layer is also taken into account by simply replacing the bark layer with an anisotropic layer. It is shown that the corrugated layer acts as an impedance transformer which may significantly decrease the backscattering radar cross section depending on the corrugation parameters. It is also shown that for a tilted ground plane a significant cross-polarized backscattered signal is generated while the co-polarized backscattered signal is reduced. >

Improvement of the PO-MoM hybrid method by accounting for effects of perfectly conducting wedges

Abstract: A correction of the conventional physical optics (PO) current close-to-perfectly conducting wedges based on an application of the uniform geometrical theory of diffraction (UTD) is presented. This improved PO current is used in a hybrid formulation in combination with the method of moments (MoM) to deal with three-dimensional scattering bodies of arbitrary shape. The accuracy of this hybrid method is demonstrated by some examples. As opposed to an application of the physical theory of diffraction (PTD), only surface current densities and no fictitious electric and magnetic line currents along the edges are involved which allows a uniform treatment of the MoM and the PO region by expressing the surface current density as a superposition of basis functions defined over triangular patches. >

Aspheric wave-front recording optics for holographic gratings

Abstract: The geometric theory of aspheric wave-front recording optics is extended to include the fourth-order groove parameters that correspond to the fourth-order holographic terms in the light-path function. We derived explicit expressions of the groove parameters by analytically following an exact ray-tracing procedure for a double-element optical system that consists of a point source, an ellipsoidal mirror, and an ellipsoidal grating blank. Design examples of holographic gratings for an in-plane Eagle-type vacuum-UV monochromator are given to demonstrate the capability of the present theory in the design of aspheric wave-front recording optics.

The application of FDTD in hybrid methods for cavity scattering analysis

Abstract: In a previous paper (see ibid., vol.41, p.1560-1569, no.11, 1993), we presented the hybrid ray-FDTD method for analyzing the electromagnetic scattering from two-dimensional cavities with complex terminations. In this paper, we present three hybrid methods for analyzing the scattering from three-dimensional (3-D) inlet cavities. In these hybrid methods, the finite-difference time-domain (FDTD) method is used to determine the reflection matrix associated with the termination. Modal analysis, physical optics (PO), or rays are used to analyze the remaining front section of the cavity. Representative results are presented. >

An exact line integral representation of the physical optics scattered field: the case of a perfectly conducting polyhedral structure illuminated by electric Hertzian dipoles

Abstract: An exact line integral representation of the electric physical optics scattered field is presented. This representation applies to scattering configurations with perfectly electrically conducting polyhedral structures illuminated by a finite number of electric Hertzian dipoles. The positions of the source and observation points can be almost arbitrary. The line integral representation yields the exact same result as the conventional surface radiation integral; however, it is potentially less time consuming and particularly useful when the physical optics field can be augmented by a fringe wave contribution as calculated from physical theory of diffraction equivalent edge currents. The final expression for the line integral representation is lengthy but involves only simple functions and is thus suited for numerical calculation. To illustrate the exactness of the line integral representation, comparisons of numerical results obtained from the surface and the line integral representations are performed. >

Femtolensing: Beyond the semiclassical approximation

Abstract: Femtolensoing is a gravitational lensing effect in which the magnification is a function not only of the position and sizes of the source and lens, but also of the wavelength of light. Femtolensing is the only known effect of 10(exp -13) - 10(exp -16) solar mass) dark-matter objects and may possibly be detectable in cosmological gamma-ray burst spectra. We present a new and efficient algorithm for femtolensing calculation in general potentials. The physical optics results presented here differ at low frequencies from the semiclassical approximation, in which the flux is attributed to a finite number of mutually coherent images. At higher frequencies, our results agree well with the semicalssical predictions. Applying our method to a point-mass lens with external shear, we find complex events that have structure at both large and small spectral resolution. In this way, we show that femtolensing may be observable for lenses up to 10(exp -11) solar mass, much larger than previously believed. Additionally, we discuss the possibility of a search femtolensing of white dwarfs in the Large Magellanic Cloud at optical wavelengths.

Maxwell-Bloch equations: A unified view of nonlinear optics and nonlinear atom optics.

Abstract: The linear and nonlinear properties of propagation of an electromagnetic field through a medium may be derived by the effective elimination of the medium from the coupled Maxwell-Bloch equations. In this paper we suggest, on the other hand, the elimination of the field from the Maxwell-Bloch equations as a means to derive a nonlinear master equation for, e.g., an atomic sample in a laser field. This presents a straightforward and physically intuitive derivation of equations for ``nonlinear atom optics,'' obtained recently by other approaches.

Unification of linear and nonlinear wave optics

Abstract: A new conceptual approach unifies the seemingly disparate fields of linear and nonlinear waves. Linear physics is shown to be the natural way to approach nonlinear wave optics. It generates the fundamental equations, allows for unforeseen generalizations and, through physical insight, motivates novel phenomena.

Two-dimensional crystals and quasicrystals in nonlinear optics.

Abstract: We report the observation of two-dimensional periodic and quasiperiodic structures in the transverse profile of an optical beam circulating in a loop which contains a nonlinear medium. The symmetries of the patterns are imposed by an image rotation within the loop. We propose a simple model which shows the existence of two unstable bands of transverse wave vectors. Close to threshold, the predictions of the model are qualitatively and quantitatively confirmed by the experiment.

XPATCH: a high-frequency electromagnetic scattering prediction code using shooting and bouncing rays

Abstract: This paper describes an electromagnetic computer prediction code for generating radar cross section (RCS), time domain signatures, and synthetic aperture radar (SAR) images of realistic 3-D vehicles. The vehicle, typically an airplane or a ground vehicle, is represented by a computer-aided design (CAD) file with triangular facets, curved surfaces, or solid geometries. The computer code, XPATCH, based on the shooting and bouncing ray technique, is used to calculate the polarimetric radar return from the vehicles represented by these different CAD files. XPATCH computes the first-bounce physical optics plus the physical theory of diffraction contributions and the multi-bounce ray contributions for complex vehicles with materials. It has been found that the multi-bounce contributions are crucial for many aspect angles of all classes of vehicles. Without the multi-bounce calculations, the radar return is typically 10 to 15 dB too low. Examples of predicted range profiles, SAR imagery, and radar cross sections (RCS) for several different geometries are compared with measured data to demonstrate the quality of the predictions. The comparisons are from the UHF through the Ka frequency ranges. Recent enhancements to XPATCH for MMW applications and target Doppler predictions are also presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Theories between theories: Asymptotic limiting intertheoretic relations

Abstract: This paper addresses a relatively common “scientific” (as opposed to philosophical) conception of intertheoretic reduction between physical theories. This is the sense of reduction in which one (typically newer and more refined) theory is said to reduce to another (typically older and “coarser”) theory in the limit as some small parameter tends to zero. Three examples of such reductions are discussed: First, the reduction of Special Relativity (SR) to Newtonian Mechanics (NM) as (v/c)2→0; second, the reduction of wave optics to geometrical optics as λ → 0; and third, the reduction of Quantum Mechanics (QM) to Classical Mechanics (CM) asħ→0. I argue for the following two claims. First, the case of SR reducing to NM is an instance of a genuine reductive relationship while the latter two cases are not. The reason for this concerns the nature of the limiting relationships between the theory pairs. In the SR/NM case, it is possible to consider SR as a regular perturbation of NM; whereas in the cases of wave and geometrical optics and QM/CM, the perturbation problem is singular. The second claim I wish to support is that as a result of the singular nature of the limits between these theory pairs, it is reasonable to maintain that third theories exist describing the asymptotic limiting domains. In the optics case, such a theory has been called “catastrophe optics”. In the QM/CM case, it is semiclassical mechanics. Aspects of both theories are discussed in some detail.

X-ray focusing optics. I. Applications of wave optics to doubly curved crystals with a point x-ray source

Abstract: A wave optics approach to the calculations of the intensity distribution of an x‐ray point source image with a two‐dimensional (2D) focusing geometry is presented. Analytical formulas are derived to calculate the intensity distribution at the focal plane. This approach has taken into account the effects of x‐ray scattering within a 2D bent crystal, which includes the x‐ray refraction and absorption inside the crystal, and the effects of elastically deformed crystals described by the anisotropic elasticity theory. Based upon the elastic bending model, the modified Bragg law and 2D lens equations are discussed. In addition, the x‐ray extinction distance for curved crystals is found to be dependent on the size and the bending radius of bent crystals. For a monochromatic x‐ray point source, calculation of the intensity distribution with a 2D bent silicon crystal is given for both perfect and misaligned sources. The spatial resolution and the size of the image are determined.

Range-finding method using diffraction gratings.

Abstract: A model in geometric optics, along with some preliminary experimental results for a new range-finding method that exploits near-field diffraction phenomena found with plane gratings, is presented. Among the characteristics investigated is a magnification effect applicable to three-dimensional microscopy. A variety of embodiments of the method is disclosed, including an off-axis illumination model and a method of near-field focus compensation that takes advantage of the Scheimpflug condition.

Optics in the Age of Euler: Conceptions of the Nature of Light, 1700–1795

Abstract: According to received historiography, the fundamental issue in eighteenth-century optics was whether light could be understood as the emission of particles, or as the motion of waves in a subtle medium. Moreover, the emission theory of light was supposed to have been dominant in the eighteenth century, backed by Newton's physical arguments. This picture is enriched and qualified by focusing on the origins, contents and reception of the wave theory of light, published by Leonhard Euler in 1746, studied in depth in this 1995 book. Contrary to what has been assumed, the particle–wave debate only starts with Euler. When the emission view of light suddenly became dominant in Germany around 1795, it was new chemical experiments that proved crucial. Reflecting on the mathematical, experimental and metaphysical aspects of physical optics, a general picture of early modern science is outlined in the epilogue to the book.

Differential geometrical methods in the study of optical transmission (scalar theory). I. Static transmission case

Abstract: Static optical transmission is restudied by postulation of the optical path as the proper element in a three-dimensional Riemannian manifold (no torsion); this postulation can be applied to describe the light–medium interactive system. On the basis of the postulation, the behaviors of light transmitting through the medium with refractive index n are investigated, the investigation covering the realms of both geometrical optics and wave optics. The wave equation of light in static transmission is studied modally, the postulation being employed to derive the exact form of the optical field equation in a medium (in which the light is viewed as a single-component field). Correspondingly, the relationships concerning the conservation of optical fluid and the dynamic properties are given, and some simple applications of the theories mentioned are presented.

Detection of moving objects embedded within scattering media by use of speckle methods.

Abstract: Speckle-pattern subtraction methods are used for the detection of moving objects embedded in scattering media. Results show that the ability to detect small nonstationary objects is greatly enhanced.

Computation of Maps for Particle and Light Optics by Scaling, Splitting, and Squaring

Abstract: New methods are presented for the integration of autonomous flows, with an emphasis on the Hamiltonian case. The Hamiltonian results are expected to have important applications for charged-particle optics (including accelerator design) and for graded-index light optics.

High‐frequency analysis of Gaussian beam scattering by a two‐dimensional parabolic contour of finite width

Abstract: A high-frequency analysis of the reflection and diffraction of well-focused electromagnetic Gaussian beams (GBs) by a perfectly conducting parabolic surface with an edge is presented for the two dimensional case. The fields are evaluated analytically via the physical optics (PO) approximation, and only the reflected field is expressed as a GB. The GB procedure developed here is expected to be highly efficient in applications involving large reflector antennas, since it avoids the conventional time-consuming numerical evaluation of the PO integrals. Some numerical results are provided to indicate the accuracy of the analytical expressions obtained. The three-dimensional case will be reported separately.

Dual-reflector synthesis based on analytical gradient-iteration procedures

Abstract: Syntheses of shaped dual-reflector systems under the assumptions of physical optics usually involve some iterative optimisation procedure whereby a penalty function is minimised. It is shown that, when an analytical gradient procedure is compared with other approaches, it is very efficient. To illustrate its efficiency the procedure generates a dual-offset shaped-reflector system for the EUTELSAT II European-coverage pattern in 30 min on modern personal microcomputer.

The physical-optics integral and computer graphics

Abstract: Hardwired graphics processors offer an alternative for doing physical optics (PO) scattering. We present an improved expression to the one obtained by Rius et al. (1993) for the PO integral. We do this by taking advantage of existing information about the normal to the surface at the pixel level. The resulting PO formula reflects not only the distance to the surface at the pixel level, but also the orientation of the surface with respect to the viewing axis.

A fully polarimetric multiple scattering model for agricultural fields

Abstract: A discrete model has been developed at Tor Vergata University, which describes vegetation as an ensemble of discs and cylinders overlying a homogeneous half-space with a rough interface. The model predicts the backscatter coefficient including multiple scattering effects. The first version, which was based on a scalar approach (2 Stokes components) has now been improved in order to get a fully polarimetric approach (4 Stokes components). Soil scattering is represented, for each pair of incidence and scattering directions, by a 4/spl times/4 scatter matrix, computed by means of the small perturbation approximation (for smooth soils) and the geometrical optics approximation (for rough soils). The vegetation layer is subdivided into many thin layers, each filled with discs and/or cylinders. The scattering and transmission properties of a thin layer are represented, for each pair of incidence and scattering directions, by two 4/spl times/4 scatter matrices (for upper and lower half-spaces respectively) and, for each incidence direction, by a 4/spl times/4 extinction matrix. Matrices are computed by means of Rayleigh-Gans or physical optics approximations, depending on frequency. Finally, the contributions of the various thin layers and that of the soil are combined by means of the matrix doubling algorithm. In this paper, the authors' recent modeling advances are described. In particular, the differences between the scalar approach and the polarimetric approach are emphasized. The polarimetric model results are compared with experimental radar data obtained over agricultural fields of the Italian Montespertoli site during the MAC-91 Campaign. In particular, the crop biomass effects are investigated at L-band in the case of sunflowers. Some polarimetric features, like backscatter coefficients in linear and circular polarizations, and phase difference between VV and HH channels are considered.