Showing papers on "Physical optics published in 1983"

Scattering from arbitrarily oriented dielectric disks in the physical optics regime

Abstract: A solution has been developed for electromagnetic-wave scattering from a dielectric disk of arbitrary shape and orientation The solution is obtained by approximating the fields inside the disk with the fields induced inside an identically oriented dielectric slab having the same thickness and dielectric constant The fields inside the disk excite conduction and polarization currents, which are in turn used to calculate the fields scattered from the disk This computation has been executed for observers in the far field of the disk for arbitrarily polarized incident waves, and the solution has been expressed in the form of a dyadic scattering amplitude The results apply when the minimum dimension of the disk’s cross section is large compared with both wavelength and disk thickness, although the thickness need not be small compared with wavelength Examples of the dependence of the scattering amplitude on frequency, relative dielectric constant, and disk orientation are presented for disks of circular cross section

Hybrid solutions for scattering from perfectly conducting bodies of revolution

Abstract: A current-based hybrid formulation is developed for predicting the electromagnetic scattering from conducting bodies of revolution (BOR). The electric field integral equation (EFIE) formulation of the problem is solved by incorporating the Fock solution for the surface currents on the scatterer into the method of moments (MM) solution. To treat oblique illumination, the Fock results are extended to arbitrary surfaces with torsion. The formulation is illustrated for spheres and conespheres with smooth and discontinuous joins. The analysis includes nonspecular phenomena such as creeping wave effects. Application of the physical optics (PO) approximation in this hybrid formulation is discussed. The formulation is shown to be accurate even for scatterers in the near-resonance range (i.e., ka \simgtr 7.5 ).

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.

Geometrical optics reviewed: A new light on an old subject

Abstract: The traditional view of geometrical optics as the ray-tracing procedure for the design of mirror, prism, and lens systems of uniform optical components, has become enlarged in recent years. This has been brought about by the applications of electromagnetic systems at ever decreasing wavelengths on the one hand, and by new optical components, such as optical fibers and integrated optical elements, on the other. The range of geometrical optics now includes such concepts as optics-in-the-large, wide-angle and aspheric designs, geodesic optics, and the optics of nonuniform media. The introduction of electromagnetic field theory in this area involves effects such as diffraction, fields near caustics, and evanescent fields. These are also being discussed in a ray optical interpretation, usually presented as the first-order term in an asymptotic expansion. The optical processes have, in turn, introduced some new geometrical theorems and matrix methods which have applications in the overall geometrical optics science. This paper sets out to review these methods and, in view of the commonly expressed compatibility between the two aspects, to investigate the applications of the geometrical theory to the electromagnetic field description. Indications are given as to possible methods available for the definition of a geometrical electromagnetic field theory which would contain the geometrical theories of the optical field in a covariant transformation theory of the electromagnetic field. It is shown that this is compatible with current methods being applied to the unity of all physics in which geometrical optics is the first-order approximation occupying a central position.

Computations of scattering cross sections for composite surfaces and the specification of the wavenumber where spectral splitting occurs

Abstract: The scattering cross sections for composite random rough surfaces are evaluated using the full wave approach. They are compared with earlier solutions based on a combination of perturbation theory which accounts for Bragg scattering, and physical optics which accounts for specular point theory. The full wave solutions which account for both Bragg scattering and specular point scattering in a self-consistent manner are expressed as a weighted sum of two cross sections. The first is associated with a filtered surface, consisting of the larger scale spectral components, and the second is associated with the surface consisting of the smaller scale spectral components. The specification of the surface wavenumber that separates the surface with the larger spectral components from the surface with the smaller spectral components is dealt with in detail. Since the full wave approach is not restricted by the limitations of perturbation theory, it is possible to examine the sensitivity of the computed values for the backscatter cross sections to large variations in the value of the wavenumber where spectral splitting is assumed to occur.

Scattering cross sections for composite surfaces that cannot be treated as perturbed-physical optics problems

Abstract: Perturbation and physical optics theories have traditionally been used to derive the scattering cross sections for composite surfaces that can be regarded as small-scale surface perturbations that ride on filtered, large-scale surfaces. In this case, perturbation theory accounts for Bragg scattering, while physical optics theory accounts for specular point scattering. However, for a more general class of composite surfaces that cannot be decomposed in such a manner, the perturbed–physical optics approach cannot be used. In these cases, it is shown, using the full wave approach, that the specular scattering associated with a filtered surface (consisting of the larger-scale spectral components) is strongly modified and that Bragg scattering and specular point scattering begin to blend with each other. Since the full wave solution accounts for Bragg scattering as well as specular point scattering in a self-consistent manner, it is not necessary to filter (decompose) the composite surface to evaluate the scattering cross sections in the general case. However, filtering the composite surface enhances one's physical insight as to the validity (or lack thereof) of the perturbed-physical optics decomposition and also facilitates the numerical evaluation of the scattering cross sections.

Wave propagation phenomena

Abstract: The phenomenon of wave propagation is encountered frequently in a variety of engineering disciplines. For the design of antennas the interaction with electromagnetic waves has to be known. For earthquake analysis the elastodynamic wave propagation is essential. Knowledge of surface waves of liquids is necessary for the design of harbours and dams, and for the design of pressure vessels and piping networks in several branches of industry pressure transient analyses are required. What these wave propagation phenomena have in common is that they describe the motion of a disturbance with a definite speed (velocity of light, velocity of sound). The specific properties of these waves are reflection,diffraction and interference,-well known phenomena in physical optics.

Light scattering by large spheroids in the Physical Optics Approximation: numerical comparison with other approximate and exact results

Abstract: Physical Optics Approximation is used to compute scattering efficiency factors forward- and back-scattering intensities, angular distributions of intensity and depolarization by large dielectric or absorbing spheroids. The results are compared with those obtained by exact theories or other approximate calculations. If the radius of curvature at any point of the illuminated part of the scatterer is greater than about a few wavelengths, that approximation is generally the best one and in good agreement with the exact theories. This curvature requirement depends on the size of the spheroid but also on its orientation with respect to the incident light direction.

Generalized prism dispersion theory

Abstract: The case of dispersion in multiple‐prism arrays is considered. A general expression is derived, using geometrical optics, for the dispersion of multiple‐prism assemblies arranged either in the additive or the compensating‐pair configurations.

Nonlinear Optics and Spectroscopy

Abstract: ing's vector it follows that the light amplitude at the focal spot would reach 108 volts per centimeter, comparable to the electric field internal to the atoms and molecules responsible for the binding of valence electrons. These are literally pulled out of their orbits in multiphoton tunneling processes, and any material will be converted to a highly ionized dense plasma at these flux densities. It is clear that the familiar notion of a linear optical response with a constant index of refraction, that is, an induced polarization proportional to the amplitude of the light field, should be dropped at much less extreme intensities. There is a nonlinearity in the constitutive relationship which may be expanded in terms of a power series in the electric field components

Generalized pencils of rays in statistical wave optics

Abstract: The recently introduced generalized pencil of Sudarshan which gives an exact ray picture of wave optics is analysed in some situations of interest to wave optics. A relationship between ray dispersion and statistical inhomogeneity of the field is obtained. A paraxial approximation which preserves the rectilinear propagation character of the generalized pencils is presented. Under this approximation the pencils can be computed directly from the field conditions on a plane, without the necessity to compute the cross-spectral density function in the entire space as an intermediate quantity. The paraxial results are illustrated with examples. The pencils are shown to exhibit an interesting scaling behaviour in the far-zone. This scaling leads to a natural generalization of the Fraunhofer range criterion and of the classical van Cittert-Zernike theorem to planar sources of arbitrary state of coherence. The recently derived results of radiometry with partially coherent sources are shown to be simple consequences of this scaling.

Identification of scattering mechanisms from measured impulse response signatures of simple conducting objects

Abstract: : The backscattered impulse response of several perfectly conducting objects has been measured and qualitatively compared to several theoretical predictions. The value of the impulse response waveform as a time history of scattering behavior is due to its unique relationship to geometry of the scatterer. This relationship is explored for local diffraction mechanisms, multiple diffraction between separate points on the scatterer, creeping waves, and natural resonances. Finally, it is demonstrated that a time domain view of a high frequency solution such as the Uniform Geometrical Theory of Diffraction offers a physically intuitive explanation for individual scattering mechanisms and multiple interactions. (Author)

Geometrical optics i

Abstract: THIS is a second edition of a work which appeared first in 1872, and which was designed to meet the requirements of students reading for the first three days of the Mathematical Tripos. The new edition does not differ greatly from the old, except in form. The type is larger and clearer, and in this respect the book is considerably improved.An Elementary Treatise on Geometrical Optics.By W. Steadman Aldis (London: Deighton, Bell, and Co., 1886.)

Strong fluctuations in the spinor wave optics. Mutual coherence function

Abstract: In this paper, we apply the spinor wave optics to light beam propagation in a strong turbulent medium. Using the Markov technique, we give the equations for the mutual coherence 4‐vector Γμ(z; ρ1, ρ2), μ=0,1,2,3, whose third component Γ3(z; ρ1, ρ2) is the usual mutual coherence function (the three other components have to do with the Poynting vector). Then, scaling transverse and longitudinal distances by the beam waist ω0 and diffraction length l, respectively, an expansion procedure in powers of ω0l−1 is developed. The zeroth‐order approximation is discussed, and it is shown that this approximation gives the same mutual coherence function as the paraxial optics. The case of a plane wave is also considered.

Geometrical optics in weakly anisotropic media

Abstract: The propagation of polarized waves in weakly anisotropic media is considered. The geometric optics approximation is used and the ray paths are determined in the approximation that the medium is isotropic. The transport of polarized waves along these rays is described by the first-order approximation; this yields a system of two coupled ordinary differential equations. The effects of birefringence, dispersion and diffraction are given by the second-order approximation.

Image compensation in the presence of thermal blooming.

Abstract: The effect of image compensation in viewing extended targets through thermal blooming is discussed. A wave-optics propagation code simulating multiple point sources and a low-bandwidth return-wave adaptive optics system is used to determine the steady-state thermally induced phase distortions and wave-front correction through various Zernike modes. Incoherent point spread functions for the isoplanatic regions are generated and convolved with the appropriate object field to reconstruct the extended target image. Image distortion, degradation in peak irradiance, and adaptive optics loop stability are discussed with respect to degree of correction and wavelength sensitivity.

Turbulence Induced Adaptive Optics Performance Degradation: Evaluation In The Time Domain

Abstract: Normally one of the two approaches is used to evaluate the performance of an adaptive optics system. In the first approach the normalized antenna gain (or Strehl ratio) associated with a variety of degrading effects acting alone is evaluated. The normalized antenna gain of the system when degraded by a combination of these effects is then found by forming the product of the individual antenna gains involved. The second approach is to evaluate system performance by simulation on a large mainframe computer. In the work presented here, physical optics formulas and elementary statistical concepts are used to develop an approach that shares some of the advantages of both of these previous approaches. By working in the time domain, relatively simple formulas are developed that shed insight into the factors that degrade adaptive optics system performance. In addition, the impact of several degrading factors acting simultaneously can be evaluated. In this study the normalized antenna gain is evaluated for a system degraded by turbulence, anisoplanatism, a finite servo bandwidth and a combination of anisoplanatism and a finite servo bandwidth. In the finite servo bandwidth study, system performance is evaluated as a function of diameter illustrating that the resulting antenna gain decays from the diffraction limited antenna gain to the large diameter asymptotic limit antenna gain first predicted by Greenwood. The diameter dependence of this effect is similar to that due to anisoplanatism but not as severe at intermediate diameters. The last study evaluating the combined degradation associated with anisoplanatism and a finite servo bandwidth illustrates the important role played by high temporal frequency phase information. Introducing a finite bandwidth to a system already degraded by anisoplanatism can actually improve performance slightly, in certain cases, because highly distorted high frequency information is lost.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Comparison of geometric and physical optics closed-cavity calculations with HF experiments.

Abstract: Geometrical and physical optics models for calculating the closed-cavity power of a cw supersonic diffusion chemical laser are presented. The mixing and kinetic gain medium formulation employed in these calculations is described along with its anchoring to HF small signal gain data. Mixing parameters thus established are used to compute the closed-cavity power and spectral distribution with the two models, which agree reasonably well with experimental data. The reasonably good agreement between the two models in their computed spectra, intensity, and loaded gain distributions indicates that in many applications the use of the more economical geometric model may be adequate for extensive closed-cavity power computations and performance analyses of chemical lasers.

Unstable resonator misalignment in ring and linear toroidal resonators

Abstract: Optical axis motion in a ring resonator is investigated as a function of resonator mirror misalignment by constructing an equivalent paraxial model and applying the ray matrix formalism Analytical expressions are derived for the optical axis motion The paraxial model of the ring is shown to imply a linear resonator as a specific case, and the ring resonator expressions collapse to the familiar Krupke-Sooy results for that case Using this method, new misalignment expressions are determined for more complex linear resonators Uncorrectable misalignment conditions caused by toroidal mirror parameter errors are studied analytically and with a geometric optics code, and resulting phase front errors are given for a special case These results are also examined as a basis for toroidal mirror quality specifications

Self-imaging effect in physical radiometry

Abstract: A new type of self-imaging effect connected with Fourier imaging of the radiance function in physical radiometry is analyzed.

Transient Current Density Waveforms on a Perfectly Conducting Sphere

Abstract: : The time-dependent surface current density waveforms at various points on the surface of a perfectly conducting sphere illuminated by a plane electromagnetic wave are presented The incident plane wave has an impulsive or shock-type time dependence Current density waveforms in the E-plane (strong creeping wave) and H-plane on both the lit and shadowed sides of the sphere are shown On the illuminated side of the sphere, removal of the Kirchhoff or physical optics approximation for the surface current density permits a detailed examination of the deficiencies of this approximation for short times (high frequencies) From these results the form of a first order correction to the Kirchhoff current is suggested On the shadowed side of the sphere, the changing character of the current waveform is noted

Physical optics approach for scattering by thin helical wires

Abstract: A physical optics formulation of scattering by thin wire helices of finite and infinite extent is presented. The high frequency limit of the integral representation of the physical optics field is obtained by the saddle point technique and is shown to be identical to the field computed by the geometrical optics approximation. For thin wire helices of finite extent, the physical optics approximation to the current should be supplemented by current waves traveling along the wire that are excited by the discontinuities at the ends of the wire. This modified physical optics approach is found to be a useful approximation to compute scattering by nonresonant helices that have a larger pitch than approximately 5 wavelengths. A hybrid method composed of the modified physical optics technique and the method of moments is applied, for the degenerate case of a linear wire, in order to extend the region of validity to cover the resonant cases. The modified physical optics approach is typically 5–10 times faster than Galerkin's method, and most important, it provides physical insight into the scattering phenomenon.

On the numerical analysis of the subreflector of an offset Cassegrain microwave antenna

Abstract: The physical optics analysis of the scattering from the subreflector of an offset Cassegrain microwave antenna is formulated in a novel way using the geometrical properties of the conic sections. The resulting integral is computed by means of an algorithm involving quadratic approximation of the phase and polynomial approximation of the remainder of the integrand. The formulation allows for displacement of the phase center of the feed pattern representation. Examples are presented which provide a comparison with previously reported results obtained using the geometrical theory of diffraction rather than physical optics.

Physical Optics Modeling Of Systems With Multiple Lasers

Abstract: Many laser systems use several laser beams which may interact either coherently or incoherently. Most physical optics codes model the propagation of a single laser beam. Some codes can analyze the kinetics of multiple laser lines. A powerful computer code, called LOTS2, has been developed at the Los Alamos National Laboratory by UNM which allows the simultaneous treatment of different laser beam trains. These beams may have different wavelengths, apertures, divergences, substantially different Fresnel numbers, and need not be colinear. Multiphoton phenomena such as Raman processes and isotope separation may be studied as well as conventional single photon phenomena. LOTS2 has a virtual memory feature that allows computer arrays to be larger than the central memory of mainframe machines such as the CDC 7600 and Cray 1. The separability of the diffraction propagation mathematics has been exploited to allow cylindrical lenses and high aspect ratio beam cross sections to be modeled. Considerable effort has been devoted to making the code easy to use.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.