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Showing papers on "Diffraction published in 1997"


Journal ArticleDOI
TL;DR: In this paper, the orientation distribution of a textured polycrystalline material has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by x-ray or neutron diffraction.
Abstract: The orientation distribution of a textured polycrystalline material has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by x-ray or neutron diffraction A new method is demonstrated that uses the whole diffraction spectrum, rather than extracted peak intensities, by combining the orientation distribution calculation with the crystallographic Rietveld method The feasibility of the method is illustrated with time-of-flight neutron diffraction data of experimentally deformed polycrystalline calcite It is possible to obtain quantitative information on texture, crystal structure, microstructure, and residual stress from highly incomplete pole figures and from regions of the diffraction spectrum containing many overlapping peaks The approach provides a key for quantitative texture analysis of low symmetry compounds and of composites with complicated diffraction spectra

744 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation in waveguide grating structures.
Abstract: Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure "resonates"; namely, complete interference occurs and no light is transmitted. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation.

664 citations


Journal ArticleDOI
04 Dec 1997-Nature
TL;DR: In this paper, the authors studied the response of a Langmuir-Blodgett multilayer film of cadmium arachidate to laser heating by observing changes in the intensity of one Bragg peak for different delays between the perturbing optical pulse and the X-ray probe pulse.
Abstract: The extension of time-resolved X-ray diffraction to the subpicosecond domain is an important challenge, as the nature of chemical reactions and phase transitions is determined by atomic motions on these timescales. An ultimate goal is to study the structure of transient states with a time resolution shorter than the typical period of vibration along a reaction coordinate (around 100 fs). Biological processes that can be initiated optically have been studied extensively by ultrafast infrared, visible and ultraviolet spectroscopy1. But these techniques probe only electronic states, whereas time-resolved crystallography should be able to directly monitor atomic positions. Here we show that changes in the X-ray diffraction pattern from an organic film heated by a laser pulse can be monitored on a timescale of less than a picosecond. We have studied the response of a Langmuir–Blodgett multilayer film of cadmium arachidate to laser heating by observing changes in the intensity of one Bragg peak for different delays between the perturbing optical pulse and the X-ray probe pulse. A strong decrease in intensity is seen within a picosecond of heating, resulting from disorder introduced to the layers of cadmium atoms before thermal expansion of the film (which ultimately leads to its destruction) has time to occur.

396 citations


Journal ArticleDOI
29 Aug 1997-Science
TL;DR: In this paper, the composition and structure of the β-phase of an aluminum-magnesium-silicon alloy were determined using high-resolution electron microscopy images, and the structure was refined with electron nanodiffraction data (overall R value of 3.1 percent) with the use of a recently developed least squares refinement procedure.
Abstract: Precipitation hardening, in which small particles inhibit the movement of dislocations to strengthen a metal, has long been used to improve mechanical strength, especially of aluminum alloys. The small size of precipitates and the many possible variants of the orientation relation have made their structural determination difficult. Small precipitates in commercial aluminum-magnesium-silicon alloys play a crucial role in increasing the mechanical strength of these alloys. The composition and structure of the β” phase in an aluminum-magnesium-silicon alloy, which occur as precipitates (typically 4 nanometers by 4 nanometers by 50 nanometers) and are associated with a particularly strong increase in mechanical strength, were determined. Element analysis indicates that the composition is Mg 5 Si 6 . A rough structure model was obtained from exit waves reconstructed from high-resolution electron microscopy images. The structure was refined with electron nanodiffraction data (overall R value of 3.1 percent) with the use of a recently developed least squares refinement procedure in which dynamic diffraction is fully taken into account.

330 citations


Journal ArticleDOI
TL;DR: In this paper, the elastic macrostrains calculated from lattice parameter changes in Rietveld refinements are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source.
Abstract: Macrostrain variations in engineering components are frequently examined using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement time. At reactors, macrostrain behavior is inferred from a single lattice reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordingly it is natural to fit the entire diffraction spectra using a Rietveld [J. Appl. Cryst. 2, 65 (1969)] refinement. This means that an idealized crystal structure is fit to the measured distorted crystal structure, which includes deviation of the measured lattice reflections from the ideal due to elastoplastic strain anisotropies, which are dependent on the particular lattice reflection (hkl) considered. We show that elastic macrostrains calculated from lattice parameter changes in Rietveld refinements (without accounting for hkl dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source. Moreover good refinements on the complete pattern are obtained with short measurement times compared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expected in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these arguments is demonstrated by analysis of a uniaxial tensile load test and a reanalysis of previously reported data taken on a deformed stainless steel ring. The plastic strain predictions compare favorably with a finite element model.

301 citations


Journal ArticleDOI
26 Jun 1997-Nature
TL;DR: In this article, self-trapping of a white-light beam from an incandescent source was observed in both dimensions transverse to the beam when diffraction effects are balanced exactly by self-focusing in the host photorefractive medium.
Abstract: Optical pulses—wave-packets—propagating in a linear medium have a natural tendency to broaden in time (dispersion) and space (diffraction). Such broadening can be eliminated in a nonlinear medium that modifies its refractive index in the presence of light in such a way that dispersion or diffraction effects are counteracted by light-induced lensing1,2. This can allow short pulses to propagate without changing their shape2,3, and the ‘self-trapping’ of narrow optical beams1 whereby a beam of light induces a waveguide in the host medium and guides itself in this waveguide, thus propagating without diffraction4. Self-trapped pulses in space and time have been investigated extensively in many physical systems and, as a consequence of their particle-like behaviour, are known as ‘solitons’ (ref. 5). Previous studies of this phenomenon in various nonlinear media6,7,8,9,10,11,12 have involved coherent light, the one exception being our demonstration13 of self-trapping of an optical beam that exhibited partial spatial incoherence. Here we report the observation of self-trapping of a white-light beam from an incandescent source. Self-trapping occurs in both dimensions transverse to the beam when diffraction effects are balanced exactly by self-focusing in the host photorefractive medium. To the best of our knowledge, this is the first observation of self-trapping for any wave-packet that is both temporally and spatially incoherent.

282 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of the x-ray powder diffraction patterns obtained from K Cl and K Br is used to illustrate how both the angles and the intensities of diffracted beams are determined by the structure of a solid.
Abstract: Typical derivations of the Bragg equation in introductory texts do not adequately explain why x-rays are reflected as if by a mirror by crystal planes, and often lead to the mistaken idea that lattice points can be identified with the centres of atoms or ions in the structure. The treatment offered here is a little more demanding, but to compensate for this also has the advantage that it can easily be extended to provide a basic understanding of the use of XRD in structure determination. Comparison of the x-ray powder diffraction patterns obtained from K Cl and K Br is used to illustrate how both the angles and the intensities of diffracted beams are determined by the structure of a solid. A brief discussion of the effects of crystal size on the broadening of diffraction lines is included both for its intrinsic interest, and because it emphasizes that the sharp diffracted beams which are often observed result from the cooperative effect of very large numbers of scattering centers.

267 citations


Journal ArticleDOI
TL;DR: In this article, the geometrical and resolution corrections that occur in the measurement of the integrated intensities of surface diffraction rods for the case of a six-circle diffractometer are derived.
Abstract: The geometrical and resolution corrections are derived that occur in the measurement of the integrated intensities of surface diffraction rods for the case of a six-circle diffractometer. Since the six-circle geometry entails as special cases the five-circle and the z-axis diffractometers, the results are valid for these geometries as well. The derivations are valid for any incoming or outgoing angle of the X-ray beam, and are particularly important for measurements at large perpendicular momentum transfer. Expressions are derived for the integrated intensity from rocking scans, from stationary measurements and from reflectivity data. With all correction factors known, it is possible to derive the structure factors with a common scale factor from all these types of scans. It is expected that area detectors combined with stationary measurements will find widespread use in the surface X-ray diffraction community.

254 citations


PatentDOI
TL;DR: A novel interferometer design suitable for highly accurate measurement of wave-front aberrations over a wide range of wavelengths, from visible to x ray is described, applicable to at-wavelength testing of many optical systems, including short-wa wavelength projection lithography optics.
Abstract: Disclosed is a point diffraction interferometer for evaluating the quality of a test optic. In operation, the point diffraction interferometer includes a source of radiation, the test optic, a beam divider, a reference wave pinhole located at an image plane downstream from the test optic, and a detector for detecting an interference pattern produced between a reference wave emitted by the pinhole and a test wave emitted from the test optic. The beam divider produces separate reference and test beams which focus at different laterally separated positions on the image plane. The reference wave pinhole is placed at a region of high intensity (e.g., the focal point) for the reference beam. This allows reference wave to be produced at a relatively high intensity. Also, the beam divider may include elements for phase shifting one or both of the reference and test beams.

247 citations


Patent
Edward W. Conrad1, David P. Paul1
31 Oct 1997
TL;DR: In this article, a line profile or topographical cross-section of repeating lines on a substrate is provided, including line thickness, line width, and the shape of the line edge.
Abstract: A method for nondestructively determining the line profile or topographical cross-section of repeating lines on a substrate is provided, including line thickness, line width, and the shape of the line edge. A substrate having a repeating structure, such as a grating, is illuminated with broad-band radiation. Diffracted radiation is collected, measured, and recorded as a function of wavelength to provide an intensity versus wavelength curve. An initial model of the line profile of the grating, a model of the broad band radiation shined on the grating, and a model of the interaction of the radiation with the model grating is provided to a data processing machine. The machine uses Maxwell's equations to calculate a model diffracted intensity versus wavelength curve, and the measured intensity curve is then compared with this modeled intensity versus wavelength curve. The line profile in the model is then adjusted and the model intensity curve recalculated to improve agreement between the measured and calculated intensity curves. The model is repeatedly adjusted and the intensity recalculated until the best agreement between the two intensity versus wavelength curves is achieved, thereby providing the line profile. The method similarly provides composition profiles, such as doping depth profiles and optical coating profiles by taking advantage of the relationship between index of refraction and composition.

236 citations


Journal ArticleDOI
01 Mar 1997-Nature
TL;DR: In this article, the quantum-mechanical analogue of the classical phase-space distribution function was used to show that the motion of atoms behaves in a strongly non-classical manner.
Abstract: Beams of atoms can exhibit interference and diffraction phenomena just like waves of light. For a coherent beam of helium atoms in a double-slit experiment, measurements of the quantum-mechanical analogue of the classical phase-space distribution function show that the motion of atoms behaves in a strongly non-classical manner.

Journal ArticleDOI
TL;DR: In this paper, a new formulation of the coupled-wave method for two-dimensional gratings is proposed, based on mathematical and physical results recently obtained for one-dimensional grating.
Abstract: A new formulation of the coupled-wave method for two-dimensional gratings is proposed. It is based on mathematical and physical results recently obtained for one-dimensional gratings. Numerical evidence obtained for many different diffraction problems, including dielectric, metallic, volume, and surface-relief gratings, shows that the new formulation outperforms the conventional one in terms of convergence rates. The specific case of gratings with very small thickness, for which opposite conclusions on the convergence performance are obtained, is studied and explained. The methodology can be applied to other numerical techniques that rely on Fourier expansions of the electromagnetic fields and on grating parameters such as the permittivity and the permeability.

Journal ArticleDOI
TL;DR: In this article, the influence of defects in a photonic crystal was studied using a finite set of parallel cylinders and the results showed that a slight local change in the crystal period can be used for the realization of devices that radiate energy in a very narrow angular range.
Abstract: We use a rigorous method for diffraction by a finite set of parallel cylinders to study the influence of defects in a photonic crystal. The method allows us to give an accurate description of all the characteristics of the electromagnetic field (near-field map, scattered field, and energy flow). The localized resonant modes can also be computed. We show some of their symmetry properties and the influence of coupling between two neighboring defects. Finally, an example is given, which shows that a slight local change in the crystal period can be used for the realization of devices that radiate energy in a very narrow angular range.

Journal ArticleDOI
TL;DR: In this paper, the authors present quantitative measurements of the interaction between a guided optical wave and a two-dimensional photonic crystal using spontaneous emission of the material as an internal point source.
Abstract: We present quantitative measurements of the interaction between a guided optical wave and a two-dimensional photonic crystal using spontaneous emission of the material as an internal point source. This is the first analysis at near-infrared wavelengths where transmission, reflection, and inplane diffraction are quantified at the same time. Low transmission coincides with high reflection or in-plane diffraction, indicating that the light remains guided upon interaction. Also, good qualitative agreement is found with a two-dimensional simulation based on the transfer matrix method. [S0031-9007(97)04591-2]

Journal ArticleDOI
TL;DR: In this article, an array of bottom-mounted circular cylinders is analyzed under the assumptions of linear theory, where the cylinders are identical, and equally spaced along the array, and a second trapped wave is established, corresponding to Dirichlet boundary conditions on the channel walls.
Abstract: Water wave diffraction by an array of bottom-mounted circular cylinders is analysed under the assumptions of linear theory. The cylinders are identical, and equally spaced along the array. When the number of cylinders is large, but finite, near-resonant modes occur between adjacent cylinders at critical wavenumbers, and cause unusually large loads on each element of the array. These modes are associated with the existence of homogeneous solutions for the diffraction by an array which extends to infinity in both directions. This phenomenon is related to the existence of trapped waves in a channel. A second trapped wave is established, corresponding to Dirichlet boundary conditions on the channel walls, as well as a sequence of higher wavenumbers where ‘nearly trapped’ modes exist.

Journal ArticleDOI
TL;DR: In this paper, a new ray picture model based on the multiple interference of light waves in dielectric resonant grating-waveguide structures is presented, which clearly elucidates the phase relationship between the incident plane wave and the waves diffracted from the grating structure that is responsible for the interference of these waves.
Abstract: A new ray picture model based on the multiple interference of light waves in dielectric resonant grating–waveguide structures is presented. The model clearly elucidates the phase relationship between the incident plane wave and the waves diffracted from the resonant grating structure that is responsible for the interference of these waves. As a result of this interference process the incident wave can be totally reflected at a certain wavelength and orientation angle. The model is used to describe and analyze this resonance behavior of the grating–waveguide structures as a function of wavelength and incidence angle. The analysis is verified experimentally with semiconductor (InGaAsP/InP) structures at wavelengths of 1.55 μm and also with dielectric (silicon nitride/SiO2) structures at wavelengths of 0.6 μm. All of the structures were formed by electron beam lithography and chemical vapor deposition. The measured results reveal that subnanometer resonance bandwidths and finesses as large as 6000 can be achieved at contrast ratios of 50 with relatively compact structures.

Journal ArticleDOI
TL;DR: In this paper, a new concept based on a Fabry-Perot interferometer for the generation of nondiffracting Bessel beams is described and proposed for potential applications in microlithography such as the fabrication of small isolated patterns.
Abstract: A new concept based on a Fabry–Perot interferometer for the generation of nondiffracting Bessel beams is described and proposed for potential applications in microlithography such as the fabrication of small isolated patterns. It was experimentally demonstrated that the depth of focus can be increased by a factor of about 2, and simultaneously the transverse resolution improved by a factor of 1.6, when using this technique to image contact holes. The properties of simultaneous imaging of two contact holes were also investigated. It was shown experimentally that, even in the most critical case (when the first diffraction rings overlap), undesirable interference effects between the adjacent contact holes can be eliminated by means of a phase shifting technique.

Journal ArticleDOI
TL;DR: In this paper, boundary integral analysis is applied to the analysis of diffraction from both conductive and dielectric diffractive optical elements, and the boundary element method is used to solve the boundary integral equations and validate its implementation by comparing with analytical solutions.
Abstract: We apply boundary integrals to the analysis of diffraction from both conductive and dielectric diffractive optical elements. Boundary integral analysis uses the integral form of the wave equation to describe the induced surface distributions over the boundary of a diffractive element. The surface distributions are used to determine the diffracted fields anywhere in space. In contrast to other vector analysis techniques, boundary integral methods are not restricted to the analysis of infinitely periodic structures but extend to finite aperiodic structures as well. We apply the boundary element method to solve the boundary integral equations and validate its implementation by comparing with analytical solutions our results for the diffractive analysis of a circular conducting cylinder and a dielectric cylinder. We also present the diffractive analysis of a conducting plate, a conducting linear grating, an eight-level off-axis conducting lens, an eight-level on-axis dielectric lens, and a binary dielectric lens that has subwavelength features.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that a CCA of dyed particles embedded in a poly acrylamide hydrogel acts as a nanosecond optical Bragg diffraction switching device.
Abstract: Monodisperse, highly charged colloidal particles in low ionic strength solutions self-assemble into bcc or fcc crystalline colloidal arrays (CCAs) due to interparticle repulsive interactions. We demonstrate that a CCA of dyed particles embedded in a poly acrylamide hydrogel acts as a nanosecond optical Bragg diffraction switching device. Under low light intensities the CCA is refractive index matched to the medium and does not diffract. However, high intensity excitation within the dye absorption band heats the spheres within nanoseconds to decrease their refractive index. The array ``pops up'' to diffract light within 2.5 ns. These intelligent CCA hydrogels may have applications in optical limiting, computing, and nanosecond fast optical switching devices, etc.

Journal ArticleDOI
TL;DR: In this article, the coupled wave theory of Kogelnik was extended to the case of moderately absorbing anisotropic materials with grating vector and medium boundaries arbitrarily oriented with respect to the main axes of the optical indicatrix.
Abstract: The coupled wave theory of Kogelnik [H. Kogelnik, Bell Syst. Tech. J. 48, 2909 (1969)] is extended to the case of moderately absorbing thick anisotropic materials with grating vector and medium boundaries arbitrarily oriented with respect to the main axes of the optical indicatrix. Dielectric and absorption modulation with common grating vector and of arbitrary relative phase shift is considered. Solutions for the wave amplitudes, diffraction efficiencies, and angular mismatch sensitivities are given in transmission and reflection geometries. The main difference of the new results with respect to the expressions valid for isotropic media arise due to the walk-off between the wave-front and energy propagation directions. The difference is particularly important in materials with large birefringence, such as organic crystals, ordered polymers, and liquid crystalline cells. The special case of Bragg diffraction and two-beam coupling at holograms recorded in optically inactive photorefractive crystals is analyzed in detail. It is found that the two-beam coupling gain is influenced substantially by an absorption anisotropy.

Journal ArticleDOI
TL;DR: A genetic algorithm based method for solving crystal structures directly from powder diffraction data has been developed in this article, which is based on fitting the diffraction samples generated from trial structures against the measured diffraction measurements.
Abstract: A genetic algorithm (GA) based method for solving crystal structures directly from powder diffraction data has been developed. The method is based around fitting the diffraction data generated from trial structures against the measured diffraction data and has the ability to handle flexible molecules and multiple fragments. It is computationally highly efficient and takes full advantage of the implicit parallelism of the GA. The method is illustrated with the solutions of three crystal structures of varying complexity.

Journal ArticleDOI
TL;DR: The focusing of light by a high-aperture lens into a stratified medium is considered, and the solution is obtained in terms of plane waves, and it satisfies Maxwell's equations.
Abstract: We consider the focusing of light by a high-aperture lens into a stratified medium. The solution is based on our previously obtained results [J. Opt. Soc. Am. A12, 325 (1995)], where we represented the illumination incident upon a plane interface between media with mismatched refractive indices as a sum of plane waves. The present solution is obtained in terms of plane waves, and it satisfies Maxwell’s equations. The diffraction integrals are obtained in a form that is readily computable. We present numerical examples for some practical cases.

Journal ArticleDOI
TL;DR: Theoretical and experimental advances in determination of three-phase invariants by multiple-beam X-ray diffraction are reviewed in this article, where the fundamental physics and mathematical analyses are explained.
Abstract: Theoretical and experimental advances in determination of three-phase invariants by multiple-beam X-ray diffraction are reviewed. The fundamental physics and mathematical analyses are explained. Plane-wave dynamical theory for the interpretation of multiple-beam interference is summarized. New results of its applications to the solution of the enantiomorphism problem of light-atom structures and to the solution of the structure of macromolecules by means of measured phases in conjunction with statistical methods are reported. Practical aspects of applying the three-beam diffraction technique to proteins are emphasized.

Patent
Masahiro Ogusu1, Shigeru Oshima1
21 Feb 1997
TL;DR: An optical multiplexing and demultiplexing device of a type using a lens and a diffraction grating, which incorporates a waveguide array having waveguide channels with narrow spacings for facilitating high resolution, is described in this article.
Abstract: An optical multiplexing and demultiplexing device of a type using a lens and a diffraction grating, which incorporates a waveguide array having waveguide channels with narrow spacings for facilitating high resolution, where the narrow spacings are realized by forming an etching groove between each adjacent waveguide channels and providing a measure for preventing optical coupling among the waveguide channels in the form of an air gap, or a layer having a refractive index lower than a cladding layer, or a layer for absorbing/reflecting light, which is provided in the etching groove. The device also incorporates a reflection mirror for reflecting the initially diffracted lights from the diffraction grating back to the diffraction grating, such that the diffracting grating diffracts the diffracted lights reflected back from the reflection mirror again to produce wavelength division multiplexed/demultiplexed optical signals in high resolution.

Journal ArticleDOI
TL;DR: In this paper, the Rietveld program XRS-82 and the profile extraction program ALLHKL have been modified to allow flat imaging-plate powder diffraction data to be refined.
Abstract: The combination of intense X-ray synchrotron radiation and an imaging-plate area detector makes it possible to extract real-time structural information using Rietveld refinement of individual time slices. Powder diffraction data from capillary samples were collected using a flat imaging plate mounted perpendicular to, or at an oblique angle to, the incoming beam. Owing to the geometry of the experiments, several factors must be taken into account when Rietveld refinement of powder diffraction data obtained using a flat imaging plate is performed: (i) nonequal step sizes are obtained when the diffraction profiles are extracted; (ii) the refined zero-point correction is different from that for conventional diffraction geometry; (iii) the sample-detector distance is not constant, requiring a modified geometric term in the Lorentz factor; (iv) the geometric contribution to the instrumental resolution function must be considered. The geometric factors have been derived and are discussed. The Rietveld program XRS-82 and the profile extraction program ALLHKL have been modified to allow flat imaging-plate powder diffraction data to be refined. The Rietveld refinement of powder diffraction data of α-quartz collected using a 30 s exposure is presented. LaB6 powder diffraction data were collected using various settings. The profile variation with angle and the resolution function are described. Data were collected up to sin θ/λ = 1.29 A−1 (d values down to 0.39 A) using high energy and with the imaging plate inclined at an angle to the incoming beam.

Journal ArticleDOI
TL;DR: In this article, the orientation distribution of a textured polycrystal has been determined from a few individual pole figures of lattice planes hkl, measured by X-ray or neutron diffraction.
Abstract: The orientation distribution of a textured polycrystal has been traditionally determined from a few individual pole figures of lattice planes hkl, measured by X-ray or neutron diffraction. A new method is demonstrated that uses the whole diffraction spectrum, rather than extracted peak intensities, by combining ODF calculation with Rietveld crystal structure refinement. With this method, which is illustrated for a synthetic calcite texture, it is possible to obtain quantitative texture information from highly incomplete pole figures and regions of the diffraction spectrum with many overlapping peaks. The approach promises to be advantageous for low-symmetry compounds and composites with complicated diffraction spectra. The method is particularly elegant for time-of-flight neutron diffraction, saving beam time by using small pole-figure regions and many diffractions.

Journal ArticleDOI
TL;DR: In this article, a 3D propagation model for path loss prediction in a typical urban site is presented, based on geometrical optics and uniform theory of diffraction (UTD), which takes into account numerous rays that undergo reflections from ground and wall surfaces and diffraction from the corners or rooftops of buildings.
Abstract: This paper presents a three-dimensional (3-D) propagation model for path-loss prediction in a typical urban site, based on geometrical optics (GO) and uniform theory of diffraction (UTD). The model takes into account numerous rays that undergo reflections from the ground and wall surfaces and diffraction from the corners or rooftops of buildings. The exact location of the reflection and diffraction points is essential in order to calculate the polarization components of the reflected and diffracted fields and their trajectories. This is accomplished by local ray-fixed coordinate systems in combination with appropriate dyadic reflection and diffraction coefficients. Finally, a vector addition of the received fields is carried out to obtain the total received field strength and, subsequently, the path loss along a predetermined route. The model computes the contributions of various categories of rays, as selected, in a flexible manner. Several results-path loss versus distance and power-delay profile-are given, and comparisons with measured data are presented.

Patent
31 Jul 1997
TL;DR: An optical device includes a plurality of picture elements the phase and/or amplitude transmission of which vary in the lateral direction of the optical device reducing the level of diffraction caused by the device.
Abstract: An optical device includes a plurality of picture elements the phase and/or amplitude transmission of which vary in the lateral direction of the optical device reducing the level of diffraction caused by the device.

Journal ArticleDOI
TL;DR: The application of the angular spectrum method to other kinds of boundary conditions is discussed, as is the relationship between wave backpropagation, phase conjugation, and the time-reversal mirror.
Abstract: Wave backpropagation is a concept that can be used to calculate the excitation signals for an array with programmable transmit waveforms to produce a specified field that has no significant evanescent wave components. This concept can also be used to find the field at a distance away from an aperture based on measurements made in the aperture. For a uniform medium, three methods exist for the calculation of wave propagation and backpropagation: the diffraction integral method, the angular spectrum method, and the shift-and-add method. The boundary conditions that are usually implicitly assumed by these methods are analyzed, and the relationship between these methods are explored. The application of the angular spectrum method to other kinds of boundary conditions is discussed, as is the relationship between wave backpropagation, phase conjugation, and the time-reversal mirror. Wave backpropagation is used, as an example, to calculate the excitation signals for a ring transducer to produce a specified pulsatile plane wave with a limited spatial extent.

Journal ArticleDOI
TL;DR: A new formulation for obtaining the absolute backscatter coefficient from pulse-echo measurements is presented, and the final correction is a weak function of frequency when the scattering volume is near the focal area, rather than the frequency squared dependence proposed by earlier investigators.
Abstract: A new formulation for obtaining the absolute backscatter coefficient from pulse-echo measurements is presented. Using this formulation, performing the diffraction correction and system calibration is straightforward. The diffraction correction function for the measurement of backscatter coefficient and the acoustic coupling function for a pulse-echo system are defined. Details of these functions for two very useful cases are presented: a flat disk transducer and a spherically focused transducer. Approximations of these functions are also provided. For a flat disk transducer, the final formulation appears as a modification to the established Sigelmann-Reid formulation. For a focused transducer, the final correction is a weak function of frequency when the scattering volume is near the focal area, rather than the frequency squared dependence proposed by earlier investigators.