Showing papers on "Fresnel zone published in 2000"

Fresnal Zones in Wireless Links, Zone Plate Lenses and Antennas

Abstract: From the Publisher: The first book devoted solely to Fresnel zones in microwave/mm-wave electromagnetics, this comprehensive survey covers theory, design and practical applications. Whether you are an experienced electromagnetics specialist or a newcomer to the field, whether you need practical design guidelines or a clear and concise exposition of the fundamentals, you'll find this book invaluable. Written by an acknowledged expert, the volume includes a detailed description of the application of Fresnel zones to microwave wireless communication links as well as a comprehensive description of the state-of-the-art in newly created zone plate lens and antenna technologies. It also contains an overview of the basic electromagnetic equations for time harmonic fields, and a thorough treatment of the Fresnel-Kirchoff diffraction theory. If you are a researcher, practising engineer or student in wireless/electromagnetics, this authoritative resource will help you to understand Fresnel zones, zone plate devices and their practical uses, in particular the analysis and design of microwave communication links, Fresnel zone antennas for DBS, Fresnel zone plate lenses for radio telescopes, and conformal curvilinear zone plate antennas. The work is generously illustrated.

Effects of volcano topography on seismic broad‐band waveforms

Abstract: Volcano seismology often deals with rather shallow seismic sources and seismic stations deployed in their near field. The complex stratigraphy on volcanoes and near-field source effects have a strong impact on the seismic wavefield, complicating the interpretation techniques that are usually employed in earthquake seismology. In addition, as most volcanoes have a pronounced topography, the interference of the seismic wavefield with the stress-free surface results in severe waveform perturbations that affect seismic interpretation methods. In this study we deal predominantly with the surface effects, but take into account the impact of a typical volcano stratigraphy as well as near-field source effects. We derive a correction term for plane seismic waves and a plane-free surface such that for smooth topographies the effect of the free surface can be totally removed. Seismo-volcanic sources radiate energy in a broad frequency range with a correspondingly wide range of different Fresnel zones. A 2-D boundary element method is employed to study how the size of the Fresnel zone is dependent on source depth, dominant wavelength and topography in order to estimate the limits of the plane wave approximation. This approximation remains valid if the dominant wavelength does not exceed twice the source depth. Further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions. Furthermore, the deployment strategy of seismic instruments on volcanoes, as well as the direct interpretation of the broad-band waveforms in terms of pressure fluctuations in the volcanic plumbing system, are discussed.

Beam combining optical element

Abstract: An optical combiner with a Fresnel lens having a Fresnel surface coated with a thin film spectrally selective reflective coating configured to reflect light within a first wavelength range, and transmit light within a second wavelength range is provided. The optical combiner also includes a covering which covers the Fresnel surface and the reflective coating to provide a generally planar surface which is substantially parallel to a similarly planar exterior surface of the Fresnel lens. The covering may take the form of an optical adhesive (or of a second Fresnel lens) with an index of refraction which substantially matches the index of refraction of the Fresnel lens.

New “Fresnel-Zone” estimates for shear-wave splitting observations from finite-difference modeling

Abstract: We employ full finite-difference modeling to study effects of teleseismic shear-wave splitting in 2D upper-mantle models with lateral variations of anisotropy. In our models the anisotropy is confined to a layer at variable depth within the top 550 km of the upper mantle. We consider propagation of plane shear wavefronts and investigate the effects of lateral variations in symmetry-axis orientation of orthorhombic olivine. Splitting parameters obtained from the synthetic waveforms exhibit characteristic variations as functions of frequency and polarization of the incident shear wave. We estimate the effective Fresnel zones (sensitivity range) from the lateral variations of splitting parameters. This leads to constraints for the extent of homogeneous anisotropic domains and for the depth of inhomogeneous regions.

An investigation of coded aperture imaging for small animal SPECT

Abstract: Coded apertures provide a substantial gain in detection efficiency compared with conventional collimation and are well suited to imaging small volumes. Here, the authors investigated several aspects of coded aperture design for a small animal SPECT system, including aperture/detector configuration, sampling requirements, and susceptibility to scatter. They simulated various source distributions and detection systems which included one, two, and four stationary detectors placed around the object, each with a pinhole array or a Fresnel zone plate in front of the detector. Image volumes were reconstructed using an iterative successive over-relaxation algorithm with a penalized weighted least squares cost function. Multiple pinhole arrays performed better than Fresnel zone plates in terms of reconstructed mean squared error and signal-to-noise. The authors' design goals of 1% detection efficiency can be achieved with a four-detector system with arrays of 100 pinholes per detector and the scatter fraction for a 2-cm diameter object is <5%. It is concluded that a coded aperture design shows great promise for small animal SPECT.

High-resolution lenses for sub-100 nm x-ray fluorescence microscopy

Abstract: We report on the design, fabrication, and testing of Fresnel zone plates for high-resolution x-ray fluorescence microscopy using the scanning x-ray microscope at the European Synchrotron Radiation Source. The germanium lenses were optimized for operation near the sulphur absorption edge at 2472 eV photon energy. The high measured diffraction efficiencies of up to 9.6% and the good match to the spatial coherence of the undulator beam resulted in a photon flux of about 4×108 photons per second within the bandwidth of a silicon 〈111〉 monochromator. Using a test object consisting of zinc sulphide nanostructures, we were able to image features in sulphur x-ray fluorescence mode with lateral dimensions down to below 100 nm.

Method for modeling seismic acquisition footprints

Abstract: A model seismic image of a subsurface seismic reflector is constructed, wherein a set of source and receiver pairs is located, and a subsurface velocity function is determined. Specular reflection points are determined on the subsurface seismic reflector for each of the source and receiver pairs. A Fresnel zone is determined on the subsurface seismic reflector for each of the specular reflection points, using the subsurface velocity function. One or more seismic wavelets are selected and a set of image points is defined containing the subsurface seismic reflector. A synthetic seismic amplitude is determined for each of the image points by summing the Fresnel zone synthetic seismic amplitude for all of the Fresnel zones that contain the image point, using the seismic wavelets. The model seismic image of the subsurface seismic reflector is constructed, using the synthetic seismic amplitudes at the image points.

Sensitivity Kernels for Time-Distance Inversion

Abstract: Inversion of local-area helioseismic time-distance data has so far only been done in the ray approximation (Kosovichev, 1996) Since this is a high-frequency approximation its applicability can be questioned for the solar case (1997) showed that for a simple solar model the localized wave packets do follow, but are not confined to, the ray path We use an approximation based on the first Fresnel zone that has been developed in geophysics by (1996) to go beyond the ray approximation in the inversions We have calculated sensitivity kernels using both approximations To test them we use a finite-difference forward modeling of the whole wave field in an acoustic medium reminiscent of the Sun We use the finite-difference modeling to calculate sensitivity kernels for the full wave field and compare this with the other kernels The results show that the Fresnel-zone-based kernels are in good agreement with the sensitivity obtained from the modeling Thus these new kernels represent a significant step forward in the inversion of time-distance data

Gaussian imaging transformation for the paraxial Debye formulation of the focal region in a low-Fresnel-number optical system

Abstract: The Debye formulation of focused fields has been systematically used to evaluate, for example, the point-spread function of an optical imaging system. According to this approximation, the focal wave field exhibits some symmetries about the geometrical focus. However, certain discrepancies arise when the Fresnel number, as viewed from focus, is close to unity. In that case, we should use the Kirchhoff formulation to evaluate accurately the three-dimensional amplitude distribution of the field in the focal region. We make some important remarks regarding both diffraction theories. In the end we demonstrate that, in the paraxial regime, given a defocused transverse pattern in the Debye approximation, it is possible to find a similar pattern but magnified and situated at another plane within the Kirchhoff theory. Moreover, we may evaluate this correspondence as the action of a virtual thin lens located at the focal plane and whose focus is situated at the axial point of the aperture plane. As a result, we give a geometrical interpretation of the focal-shift effect and present a brief comment on the problem of the best-focus location.

Optical effect of diffractive multifocal focusing of radiation on a bicomponent diffraction system.

Abstract: It is shown for the first time, to our knowledge, that when a plane wave illuminates a certain type of bicomponent optical system, consisting of two plane screens with circular apertures on a given optical axis, a multifocal diffractive focusing effect can appear. Here the diffraction picture in the focal planes represents the circular nonlocal bands of the Fresnel zones with a bright narrow peak at the center, whose intensity can exceed by 6–10 times the value of the incident-wave intensity. The detected optical effect is observed across a wide range of wavelengths, λ = 0.4–103 µm, and ratios of the aperture diameters d1 ≥ 2d2 = 25–1000λ, and it is also insensitive to changes in the medium of the wave propagation. For the large diameters of input holes, d1 = 2d2 > 100λ, or for wavelengths in the radio-frequency region of the spectrum, the bicomponent diffraction system acts as a long-focus lens with a high-intensity Gaussian distribution of radiation, at times exceeding the initial intensity, persisting at large distances (z = 1–100 cm) from the diffraction system.

Computational study of diffraction patterns for near-field Fresnel and Gabor zone plates

Abstract: Near-field designs of Fresnel and Gabor zone plates are computationally analyzed by using versions that allow the foci to be brought closer to the plate than in the usual far-field applications. It is found that the Fresnel plate has a dominant primary conjugate pair of foci well inside the far-field region and a more intense primary focus and smaller off-focal-plane sidelobes than the near-field Gabor systems, thus yielding a superior imaging performance.

Electron beam lithography of Fresnel zone plates using a rectilinear machine and trilayer resists

Abstract: We describe the use of a commercial e-beam lithography system (JEOL JBX-6000FS) to fabricate Fresnel zone plates for x-ray microscopy. The machine is capable of controlling the pitch of optical gratings with sub-nanometer precision, so its beam placement properties are more than adequate for zone plate fabrication. The zone plate pattern is written into a thin top layer (PMMA or Calixarene) of a trilayer resist, and transferred into thick nickel zones using reactive ion etching (RIE) followed by electroplating. Zone plates with outermost zone widths of 30 nm have exhibited efficiencies up to 10.0% at a 390 eV photon energy and with diameters in the range 80 to 120 μm. Zone plates with outer zones of 18 to 20 nm were also fabricated in thinner Ni with correspondingly lower efficiencies of 2.6%. Zone plates with outermost zone widths of 45 nm have been fabricated with larger diameters up to 160 μm. All results reported were obtained with a 50 kV system with 80 μm field deflection size; future efforts will make use of a 100 kV, 500 μm field size system.

Fresnel diffraction as an aperture edge integral

Abstract: The standard Fresnel diffraction wave field from an aperture in a plane screen involves an integral over the aperture area. Here it is reduced to an aperture edge integral instead. The reduction really amounts to performing one integral but can be considered as the reduction of the ‘Maggi-Rubinovics’ edge formula from Kirchhoff optics to paraxial (i.e. Fresnel) optics.

Fresnel number of a regular polygon and slit.

Abstract: The concept of the Fresnel number is generalized to make it possible to describe the regular polygon and slit illuminated by a homogeneous plane wave. The generalization is based on nonlinear regression of the axial intensity distribution curve. A useful analytical expression for the Fresnel number is presented. A simple experiment to show the different Fresnel numbers in one observation plane is illustrated.

Fresnel zone plate lens and antennas for millimeter waves: history and evolutions of developments and applications

Abstract: This paper summarized some of the growth in investigations for Fresnel zone plate lens and antennas in millimeter-wave regions.

Wideband wave propagation measurements for local multipoint distribution services (LMDS) at 26 GHz

Abstract: The results of a wideband measurement campaign at 26 GHz in the inner city of Karlsruhe, Germany, are presented. The measurements are performed using the Marconi (former Bosch Telecom) Digital Multipoint System (DMS). Therefore, the results can be directly adopted for planning purposes. The measurements are carried out for two different base station heights with several terminal station positions. Thus, a large number of different propagation scenarios is investigated, such as free Fresnel zone (FFZ), obstructed Fresnel zone (OFZ), obstructed line of sight (OLOS) and non-line of sight (NLOS) scenarios. In order to characterise polarisation effects both vertical and horizontal polarisation are utilised. Furthermore, the influence of vertical misalignment of the terminal station antenna is examined in NLOS scenarios. Using strong multipath components to build up stable links in NLOS scenarios is a critical issue in LMDS planning. Therefore, closer investigations on this issue are presented. As a last point the impact of vegetation obstructing the line of sight (LOS) path is analysed. For all scenarios the delay spread and coherence bandwidth are derived from the measured average power delay profile (PDP).

Fresnel zones and spatial resolution for P- and SH-waves in transversely isotropic media

Abstract: In an elastically anisotropic medium where the seismic wave velocity is a function of direction, the wavefront shape is nonspherical and, in most cases, assumes a nonelliptical shape. Numerical modelling techniques have been used to calculate the Fresnel‐zone diameter for compressional (P) and shear (SH) waves in transversely isotropic and isotropic media, respectively. The size of the Fresnel zone is found to be predominantly dependent on the curvatures and wavelength of the wavefront as well as the dip angle of the reflector. In addition, the anisotropic elastic parameters δ* (critical near‐vertical anisotropy), e (the P-wave anisotropy), and γ (the SH-wave anisotropy) are found to significantly affect the size of the Fresnel zone. Numerical modeling results show considerable differences between the Fresnel zones for anisotropic and isotropic velocity functions at various reflector dips. In addition, the Fresnel‐zone dimensions for anisotropic media exhibit asymmetry and considerable change with dip. By...

Zone-plate-array lithography using synchrotron radiation

Abstract: We present our results in the design and development of a multiple-beam x-ray pattern generator based on an array of Fresnel zone plates (FZPs). Exposures have been carried out using a synchrotron source at SuperACO (France), a PC-controlled piezoelectric scanning stage, and an array of FZPs fabricated by high resolution electron-beam lithography and reactive-ion etching of a tungsten absorber. In particular, we have implemented an array of apertures using a self-aligned process in order to minimize background intensity coming from diffraction orders other than the positive first order. We show that even with polychromatic synchrotron radiation and relatively low resolution FZPs, patterns of various geometries could be successfully written in poly(methylmethacrylate) resist and submicrometer resolution was obtained after metal liftoff.

Micro-Optical Platform for All-Optical Demultiplexing Switch Array

Abstract: As it is free from electrical parasitics, the use of the all-optical switch array is an attractive approach for achieving very high-speed signal processing for optical communication and interconnection beyond the Gbit/s range. However, two incident beams and one outgoing beam for each switch may require a complex geometry, thus the module assembly procedure remains a serious obstacle for its realization. The present work reports the design, test fabrication, and characterization of an optical platform dedicated to all-optical demultiplexing at 1.55 µm, using asymmetric Fabry-Perot saturable absorber switches as active devices, and off-axis binary Fresnel zone lenses as the focusing optics. Two different platform configurations were fabricated by a substrate stacking technique achieving spot sizes of 13 µm and 22.5 µm at the switch plane, with reasonable overlap between the pump and signal beams. The influence of wavelength variation, fabrication tolerance and throughput are discussed.

A Theoretical Analysis of Multiple Diffraction in Urban Environments for Wireless Local Loop Systems

Abstract: The simulation technique of Walfisch [1] is used to examine multiple diffraction in wireless local loop systems. The simulations results show that the average propagation characteristic is described by a two slope model. In the immediate vicinity of the basestation the propagation loss is found to have a distance dependence of 20 dB per decade. At greater distances the slope increases to approximately 40 dB per decade.

Use of the focal point matrix for CFP operator traveltime estimation

Abstract: Summary We introduce the focal point matrix, that is constructed from the data-driven determined traveltime operators found using Common focus Point (CFP) technology. This matrix contains all the confocal reflectivity information connected to one point in the subsurface. From this matrix we determine three major features: The area of image contribution per common shot record, surface projected Fresnel zone, to the image. Mapping function: relates communicating beams for surface source positions to surface receiver positions, an actual coupling of beams with the same incident as reflection angle at the target. QC for the area over which the traveltime operator can be

Maskless nanolithography with diffractive optics: Zone- plate-array lithography (ZPAL)

Abstract: We present zone-plate-array lithography (ZPAL), a new scheme for nanolithography, which uses a massively parallel array of Fresnel zone plates and micromechanical multiplexing to print patterns in a dot-matrix manner on a substrate.

Performance of hard x-ray zone plates at the advanced photon source

Abstract: Fresnel zone plates have been highly successful as focusing and imaging optics for soft x-ray microscopes and microprobes More recently, with the advent of third-generation high-energy storage rings, zone plates for the hard x-ray regime have been put to use as well The performance of zone plates manufactured using a combination of electron-beam lithography and x-ray lithography is described

Photo mask for verifying adjustment of illumination system and method thereof

Abstract: PURPOSE: A photo mask, comprising a fresnel zone target pattern, for verifying an adjustment of a precis ion projector illumination system and its method are provided to achieve the image of high contrast and high resolution. CONSTITUTION: In a photo mask for verifying an adjustment of a precis ion projector illumination system, a fresnel zone target pattern(20) is formed on a test photo mask(12'). The fresnel zone target pattern(20) is generally formed by the alternation of the phases of concentric substantially circular patterns. The concentric substantially circular patterns forms the fresnel zone target pattern(20) by the alternation of a transparent area(22) and an opaque area(24).

Fresnel zone plate coded imaging

Abstract: WT5”BZ]This paper introduced the principle of the Fresnel zone plate coded imaging, and design a Fresnel zone plate coded image camera with high spatial resolution. A experimental simulation with two aperture visible light source is made. The result shows that this coded image camera can reveal the source distribution truly. [WT5”HZ]

Near-Field Scattering Characteristics of Spherical Wave from the Fractal Rough Surface

Abstract: A study on the near-field scattering of Spherical Wave from the rough surface has been made.The analysis and calculation are started from path-different rough surface case.The rough surfaces are modeled by the band limit fractal function. Results show the predicted Fresnel zone interfering phenomenon. The physical mechanism for the change of the near-field backscattering coefficients with the different platform altitude is explained from a lot of different aspects.

Nickel zone plates for soft X-ray microscopy

Abstract: Soft X-ray microscopes require high-spatial-resolution Fresnel zone plates with high and uniform diffraction efficiency as imaging optics. The combination of the zone height for optimal diffraction efficiency with the fact that the first-order spatial resolution of a zone plate scales linearly with its outermost zone width leads to high aspect ratios of the nanostructures. In the water window wavelengths region nickel is a well suited zone plate material, yielding high diffraction efficiency at moderate aspect ratios of the zones. Since it cannot be structured by reactive ion etching a tri-layer process making use of electrodeposition techniques was developed. All nickel zone plates presented in this work have demonstrated excellent first-order diffraction efficiencies at 2.4 nm wavelength. Zone plates with outermost zone widths of 40 nm, 30 nm and 25 nm were fabricated with the tri-level process yielding 21.6%, 18.0% and 14.7% first-order groove efficiency, respectively. These measured efficiencies correspond to maximum obtainable efficiencies determined by the galvanoform height of 100%, 95% and 85%.