Showing papers on "Physical optics published in 1985"

A progression of high-frequency RCS prediction techniques

Abstract: Since the widespread use of radar in World War II, engineers have grappled with the problem of calculating the radar echo characteristics of a wide variety of bodies, and over the years methods have steadily improved. This paper surveys a progression of methods, not necessarily in chronological order, each of which offers its own peculiar advantage. We begin with the caveat that large, complex targets are of prime current interest, hence the scattering calculations are for the "high-frequency" regime. As such, the method of moments, although an extremely powerful tool, is not suitable for routine computations for large bodies. Geometric optics, perhaps the oldest method in use, is a simple prescription, but gives the wrong answer for flat and singly curved surfaces, and no answer at all if the specular point is not on the surface. Physical optics does yield results in those cases, but fails by progressively wider margins as the scattering direction swings farther from the specular direction. The theories of Keller and Ufimtsev introduce edge-diffraction terms that improve the prescription, but the diffraction coefficients are poorly behaved in the transition regions of the shadow and reflection boundaries. The uniform theory of Kouyoumjian and Pathak provides the proper behavior of the transition regions, but the scattering direction is constrained to lie on the Keller cone. A method of equivalent currents suggested by Ryan and Peters, extended by Knott and Senior, and refined by Michaeli allows the scattering direction to be arbitrary, but the equivalent currents are nonphysical because they depend on the direction of the observation. Mitzner developed an "incremental length diffraction coefficient" which extends Ufimtsev's theory much as Michaeli's equivalent currents extend Keller's theory. None of these methods can handle the surface traveling wave, a significant echo mechanism for long, smooth bodies, because the methods treat localized scattering phenomena, while the surface traveling wave involves the entire surface. However, as demonstrated by Ross, the repeated application of edge-diffraction theory to account for multiple interactions between pairs of parallel edges comes close to giving the correct result. Unfortunately, there has been no extension of this application to the routine calculation of the surface traveling wave contribution for arbitrary surface geometries. Moreover, practical schemes developed for re-entrant structures and for the interaction between scattering elements consume a great deal of computer time. Thus while our repertory of useful computation methods has greatly expanded in the last 40 years, there remain some scattering mechanisms we still cannot treat in a routine fashion.

Resolved forward Brillouin scattering in optical fibers.

Abstract: Forward light scattering by the thermally excited acoustic eigenmodes of an optical fiber produce numerous narrow lines not predicted by the bulk-interaction theory of Brillouin scattering. Optical heterodyne detection has been used to resolve the scattering spectrum which begins at about 20 MHz and extends to the detection limit.

Bistatic RCS of Complex Objects near Forward Scatter

Abstract: A technique is presented for predicting the bistatic radar cross section (RCS) of an opaque, arbitrarily shaped object at angles near forward scatter. Babinet's principle is employed to approximate the object's scattering pattern as the two-dimensional Fourier transform of its silhouette. Calculations for spheres and right-circular cylinders are shown to agree with predictions based on exact Mie theory, method of moments, and physical optics. Restrictions on the applicability of the method are presented.

Interference of atoms in separated optical fields

Abstract: We consider the interference of atoms caused by the wave nature of their motion. Coherently interfering beams of particles may be obtained by using the diffraction of atoms from separated standing waves resonant with the transition between the ground and excited states of the atoms. Interference arises with and without spatial separation of the diffracted atomic beams. A problem concerning two-quantum perturbation of the wave function of an atom in the ground state by a standing-wave field has been solved. Both amplitude and phase perturbation are taken into account. We show that in spite of the absence of coherence in the incident beam and a rapid decay of the coherence induced by the standing wave for scattered spatially overlapping beams, the interference can be observed for the conditions of an echo. We first consider the phenomenon of an echo under the quantum-mechanical action of light on the translational degrees of freedom. We show that owing to this phenomenon phase memory can be transferred through the ground state over unlimitedly large distances. Owing to the interference of the density harmonics, a periodic structure in the spatial distribution of the atomic density with a period of less than one wavelength of the light is localized at distances of the order of the distance between the fields. The harmonic amplitude is calculated for instantaneous excitation in the case in which the duration of the excitation is comparable with the backward Doppler linewidth in the beam and for scattering under Bragg conditions. In the latter case we show that the effective temperature of the atoms participating in the interference is less than one recoil energy.

Scattering from thin dielectric disks

Abstract: A solution for scattering from thin dielectric disks has been obtained by approximating the currents induced inside the disk with the currents which would exist inside a dielectric slab of the same thickness, orientation and dielectric properties. This procedure yields an electrostatic approximation when the disk thickness T is small compared to the wavelength of the incident radiation and yields a conventional physical optics approximation when the dimension A , characteristic of the geometrical cross section of the disk, is large compared to wavelength. When the ratio A/T is sufficiently large one or the other of these approximations applies, regardless of the frequency of the incident radiation. Consequently, the solution provides a conventional approximation for the scattered fields at all kA . As a check on this conclusion, a comparison has been made between measurements of the radar cross section of thin dielectric disks and the cross sections predicted using this theory. Agreement was found for thin disks with both large and small values of kA .

Realization of First Order Optical Systems Using Thin Lenses

Abstract: A first order optical system is investigated in full generality within the context of wave optics. The problem is reduced to a study of the ray transfer matrices. The simplest such systems correspond to axially symmetric propagation. Realization of such systems by centrally located lenses separated by finite distances is studied. It is shown that, contrary to the commonly held view, the set of first order systems that can be realized using axially symmetric thin lenses exhausts the entire SL(2, R) group; at most three lenses are needed to realize any element of this group. In particular, the inverse of free propagation can be so realized. Among anisotropic systems it is again shown that every element of the lens group Sp(4, R) can be realized using a finite number of thin lenses.

Scattering matrix approach to thermal wave propagation in layered structures

Abstract: In this paper we describe a new technique, based on Fourier optics, to explain the propagation, as well as loss, of three‐dimensional thermal waves in isotropic, homogeneous materials. Using this, the dependence of temperature distribution at any arbitrary infinite parallel plane on the aperture distribution is derived. In addition, the temperature distribution at the aperture plane, due to a given perpendicular source is formulated, by applying the boundary conditions in the spatial frequency domain. A scattering matrix theory is developed to analyze the propagation of thermal waves in multilayered structures. This directly relates the heat source characteristics to the temperature distribution at any level. The contrast mechanism in the subsurface mode of operation is explained, and the dependence of the response of a typical system on depth is explored. In addition, the theoretical amplitude and phase images of a cylindrical void are presented; the results are in good agreement with the published exper...

Effects of deterministic surface distortions on reflector antenna performance

Abstract: Systematic distortions of reflector antenna surfaces can cause antenna radiation patterns to be undesirably different from those of perfectly smooth reflector surfaces In this paper, a simulation model for systematic distortions is described which permits an efficient computation of the effects of distortions in the reflector pattern The model uses a vector diffraction physical optics analysis for the determination of both the co-polar and cross-polar fields An interpolation scheme is also presented for the description of reflector surfaces which are prescribed by discrete points Representative numerical results are presented for reflectors with sinusoidally and thermally distorted surfaces Finally, comparisons are made between the measured and calculated patterns of a slowly-varying distorted offset parabolic reflector

Radiation pattern analysis of reflector antennas

Abstract: A method is proposed for accurately calculating the radiation pattern of reflector antennas in the entire angular range from the main beam direction to the off axis region. The principle of the method is similar to the physical optics diffraction theory, and consists of the result obtained by conventional physical optics corrected with the contribution from the edge current calculated by the geometrical theory of diffraction. However, the edge current is expressed in a simple form by means of a local coordinate system defined by the peripheral line and the mirror normal at each edge point and of the incident electromagnetic vector components in reference to this coordinate system. Therefore, it is possible to treat an arbitrarily shaped mirror surface and an antenna with a primary radiator using an arbitrary pattern. It is confirmed that the results obtained by the present method approach those obtained by the physical optics method near the main beam and those obtained by the geometrical theory of diffraction at the off axis. Accuracy is tested by applying the present method to the problem of diffraction from a circular disk for which a rigorous solution exists. The present method makes unnecessary the distinction of the use of physical optics (main beam) and the geometrical theory of diffraction (off axis) performed so far, in an empirical manner. It is possible systematically to obtain the pattern near the first-third side lobes where two methods may be connected.

Generalized Tomography as a Unified Approach to Linear Inverse Scattering: Theory and Experiment

Abstract: Conventional X-ray Computer Tomography dwells upon the mathematical formulation of the Fourier Slice Theorem relating the multidimensional spatial Fourier transform of an object to be reconstructed, i. e. its K-space, with the onedimensional Fourier transform of its projections1. Problems arise if this K-space — due to a specific experimental situation — cannot be filled by X-ray data; the utilization of acoustic, elastodynamic or electromagnetic waves requires the solution of the pertinent inverse scattering problem instead of the projection formulation2. This can be achieve by the linearized inversion of Huygens’ principle under the Physical Optics or Born approximation. A first heuristic approach is the pseduinversion in terms of Generalized Holography and its Ravleigh-Sommerfeld-specialization within a monofrequent migration formalism3 yielding only poor axial resolution in the resulting image. Therefore, broadband approaches like Systhetic Aperture Radar (SAR) for microwaves4 and Synthetic Aperture Forcussing Techniques for acoustic wave5 have been suggested, which, at elast in the farfield, can be shown to be special time-domain versions of the well-known POFFIS-identity6 resulting from linear inversions of Kirchhoff’s integral (POFFIS for Physical Optics Farfield Inverse Scattering); this monostatic algorithm can be extended to bistatic illumination, yielding a procedure called FIFFIS (Frequency Independent Farfield Inverse Scattering)7, which fills K-space in a bistatic boradband manner8.

Radiation pattern analysis of reflector antennas

Abstract: A method is proposed for accurately calculating the radiation pattern of reflector antennas in the entire angular range from the main beam direction to the off axis region. The principle of the method is similar to the physical optics diffraction theory, and consists of the result obtained by conventional physical optics corrected with the contribution from the edge current calculated by the geometrical theory of diffraction. However, the edge current is expressed in a simple form by means of a local coordinate system defined by the peripheral line and the mirror normal at each edge point and of the incident electromagnetic vector components in reference to this coordinate system. Therefore, it is possible to treat an arbitrarily shaped mirror surface and an antenna with a primary radiator using an arbitrary pattern. It is confirmed that the results obtained by the present method approach those obtained by the physical optics method near the main beam and those obtained by the geometrical theory of diffraction at the off axis. Accuracy is tested by applying the present method to the problem of diffraction from a circular disk for which a rigorous solution exists. The present method makes unnecessary the distinction of the use of physical optics (main beam) and the geometrical theory of diffraction (off axis) performed so far, in an empirical manner. It is possible systematically to obtain the pattern near the first-third side lobes where two methods may be connected.

Out-of-band response of reflector antennas

Abstract: The response of reflector antennas to out-of-band frequencies has been analyzed using physical optics. A simple approximate expression has been obtained for the effective aperture, and this expression yields both the receiving pattern and the frequency dependence of the on-axis gain. The theory has been compared with published out-of-band measurements, and the pattern agreement is good, but the measured gain falls below the theory. This discrepancy is caused by mismatch loss in the coax-to-waveguide adapter.

The Principles of Nonlinear Optics

Abstract: (1985). The Principles of Nonlinear Optics. Optica Acta: International Journal of Optics: Vol. 32, No. 1, pp. 1-2.

The Effect of Varying the Internal Angle of a Dihedral Corner Reflector

Abstract: : Small deviations from orthogonality can reduce drastically the baskscattering radar cross section of dihedral corner reflectors. In this paper this effect is studied using the method of physical optics with emphasis on the reduction of RCS achievable for modest departures from orthogonality.

Finite Element Methods For Evaluating Optical System Performance

Abstract: Many aspects of geometric optics and wave optics are compatible with the finite element method of analysis. This fact provides a new and powerful tool in the fields of optomechanical design and optical systems engineering. Using special features available in some commercially available finite element codes, it is possible to include optical system parameters as a portion of the finite element model. The simultaneous solution of the optical and mechanical problems provides higher accuracy and consistency of the results, efficient calculation of many different load cases, and solutions to optics design problems which are difficult or impossible to handle in traditional lense design codes. This paper shows results possible in commercially available finite elements codes used to calculate optical system parameters, including surface figure changes, generalized optical ray tracing and wavefront error computation. The paper is illustrated with examples drawn from recent practice.

Hybrid solutions at internal resonances

Abstract: The use of hybrid solutions for integral equation (IE) formulations in electromagnetics is illustrated at frequencies where a perfectly conducting scatterer exhibits internal resonances. Hybrid solutions, incorporating the Fock theory and physical optics Ansatzes, and the Galerkin representation, are compared with the method of moments (MM) solutions of the electric, magnetic, and combined field formulations at such frequencies. Numerical results are presented for spheres and a right circular cylinder.

Limits to concentration by passive means.

Abstract: On etend le theoreme de Liouville et son analogue optique dans le domaine des ondes. Limites des performances des concentrateurs passifs

Optimal boundary impedance for the minimization of reflection: (I) Asymptotic solutions by the geometrical optics method

Abstract: In the design of surfaces which absorb waves, the impedance boundary condition is used as an effective means of diminishing the reflection. In this paper, we use the geometrical optics method to approximate the optimal impedance value which minimizes the reflected field for the scalar wave equation with a monochromatic source. Our treatment yields good results for optimal impedance in the asymptoticity region of the geometrical optics solution.

Dual reflector synthesis for specified aperture power and phase

Abstract: A dual reflector is synthesised to achieve a specified aperture power and phase distribution, allowing generally shaped radiation patterns to be achieved using a simple feed Geometrical optics is used to shape the reflectors A separate physical optics analysis confirms the success of this synthesis

Generalized Rays in Statistical Wave Optics: Application to Anisotropic Gaussian Schell-Model Beams

Abstract: —Propagation characteristics of beams generated by anisotropic Gaussian Schell-model soruces are studied using the generalized pencils of rays. It is found that in every section normal to the beam axis the cross-sepctral density is anisotropic Gaussian Schell-model, modulo a quadratic phase-front with non-degenerate principal radii of curvature. The ratio of the transverse coherence length to the beam width is the same for every transferse section of the beam. For a broad choice of the source parameters there exists a plane normal to the beam axis wherein the intensity distribution exhibits rotational invariance about this axis. Condition on the source parameters for obtaining anisotropic Gaussian Schell-model beam with spherical phase-front is derived. The ray pattern describing the angular distribution of rays at a point has, at every point, its peak normal to the phase-front and tangential to a family of hyperboloids similar to the one familiar in the context of the coherent Gaussian beams. The far zone limit of our result for the intensity distribution is in agreement with the recent result of Li and Wolf.

Theoretical and Experimental Scattering by Hyperbolic Loaded Dishes

Abstract: Experiments are presented on electromagnetic scattering by hyperbolic reflectors peripherally loaded by epoxid resin melted to graphite powder, made in the X-bande range in an anechoic electromagnetic room. The results are compared with the theoretical prediction carried out using both the Uniform Theory of Diffraction (UTD) extended to surface impedances and Physical Optics (PO). The effectiveness of the behaviour of a lossy material layer as a surface impedance as well as the effectiveness of such loading technique applied to the subreflector in order to control the radiation diagram, have been verified. The good agreement between experimental and computed data confirms the conclusions obtained theoretically.

Evaluation of reflected fields at caustic regions using a set of GO equivalent line currents

Abstract: A set of equivalent electric and magnetic line currents is derived which supplements the geometrical optics (GO) solution in the far zone whenever one of the surface principal radii becomes very large. These hypothetical currents lie along the specular line of the surface and are shown to produce the same result as the stationary phase contribution of the physical optics integral. An example of a systematic application of such equivalent currents for the computation of the scattered field from a complex structure is also demonstrated.

Secondary pattern computation of an offset reflector antenna

Abstract: Reflector antennas are widely used in communications satellite systems because they provide high gain at low cost. In analyzing reflector antennas the computation of the secondary pattern is the main concern. A computer program for calculating the secondary pattern of an offset reflector has been developed and implemented at the NASA Lewis Research Center. The theoretical foundation for this program is based on the use of geometrical optics to describe the fields from the feed to the reflector surface and to the aperture plane. The resulting aperture field distribution is then transformed to the far-field zone by the fast Fourier transform algorithm. Comparing this technique with other well-known techniques (the geometrical theory of diffraction, physical optics (Jacobi-Bessel), etc.) shows good agreement for large (diameter of 100 lambda or greater) reflector antennas.

The radar cross section of perfectly-conducting rectangular flat plates and rectangular cylinders: A comparison of physical optics, GTD and UTD (Geometrical Theory of Diffraction and the uniform geometrical theory of diffraction) solutions

Abstract: : The method of Physical Optics, the Geometrical Theory of Diffraction and the Uniform Geometrical Theory of Diffraction are applied here to electromagnetic scattering from flat plates and rectangular cylinders. The results are compared with experimental measurements to establish the domains of validity of the theoretical techniques. Keywords include: Optics; Diffraction; Electromagnetic scattering; and Backscattering. (Australia).

Quantum Effects in the Interference of Light

Abstract: Interference effects produced by two sources of light are discussed within the framework of the quantum electrodynamic interpretation. Interference should disappear when all the source atoms are in the fully excited state, and this prediction may distinguish between quantum mechanics and various guided wave theories. The introduction of a light amplifier into an interferometer arm cannot unambiguously determine the path of the photon, because of spontaneous emission effects. Quantum mechanics, like classical wave optics, therefore predicts only a reduction of visibility in such interference experiments. In the special case of an interference experiment in which the two light sources consist of two single atoms, two photons cannot be found simultaneously at two positions that are separated by an odd number of half interference fringes. This quantum mechanical prediction amounts to another kind of EPR paradox, and should lend itself to another experimental test of the theory.

Gaussian beams and their generalizations.

Abstract: Boundary-value problems for the reduced wave equation that give rise to Gaussian and generalized Gaussian beams in paraxial regions are studied via complex ray geometrical optics. Expressions for the field valid both within and outside the paraxial region are obtained, and the results are compared with those given by paraxial wave optics and evanescent-wave theory.