Showing papers on "Fresnel zone published in 2005"

Finite-frequency effects in global surface-wave tomography

Abstract: SUMMARY We compare traditional ray-theoretical surface-wave tomography with finite-frequency tomography, using 3-D Born sensitivity kernels for long-period, fundamental-mode dispersion measurements. The 3-D kernels preserve sidelobes beyond the first Fresnel zone, and fully account for the directional dependence of surface-wave scattering, and the effects of timedomain tapering and seismic source radiation. Tomographic inversions of Love and Rayleigh phase-delay measurements and synthetic checkerboard tests show that (1) small-scale S-wave velocity anomalies are better resolved using finite-frequency sensitivity kernels, especially in the lowermost upper mantle; (2) the resulting upper-mantle heterogeneities are generally stronger in amplitude than those recovered using ray theory and (3) finite-frequency tomographic models fit long-period dispersion data better than ray-theoretical models of comparable roughness. We also examine the reliability of 2-D, phase-velocity sensitivity kernels in global surface-wave tomography, and show that phase-velocity kernels based upon a forwardscattering approximation or previously adopted geometrical simplifications do not reliably account for finite-frequency wave-propagation effects. 3-D sensitivity kernels with full consideration of directional-dependent seismic scattering are the preferred method of inverting long-period dispersion data. Finally, we derive 2-D boundary sensitivity kernels for lateral variations in crustal thickness, and show that finite-frequency crustal effects are not negligible in long-period surface-wave dispersion studies, especially for paths along continent‐ocean boundaries. Unfortunately, we also show that, in global studies, linear perturbation theory is not sufficiently accurate to make reliable crustal corrections, due to the large difference in thickness between oceanic and continental crust.

Depth-graded multilayers for application in transmission geometry as linear zone plates

Abstract: Fresnel zone plates for x-ray focusing optics are typically made using lithographic techniques. To achieve optimum efficiency for hard x rays, a depth of several microns is required, which limits the minimum zone width and hence minimum focal spot size achievable using lithography. We are exploring the fabrication of zone plates by an alternative technique that surmounts these limitations: the growth of a multilayer film to be used in transmission (Laue) diffraction geometry, in which the thickness of consecutive layers gradually increases according to the Fresnel zone formula; the film is sectioned after growth to the required depth. For a planar multilayer, this produces a linear zone plate that can focus x rays in one dimension. Here we report the growth and characterization of a depth-graded multilayer suitable for use as a zone plate for hard x-ray focusing. The multilayer has a total of 470 alternating layers of WSi2 and Si with thicknesses increasing monotonically from 15 to 60 nm, for a total thic...

Electrically switchable Fresnel lens using a polymer-separated composite film.

Abstract: A Fresnel lens with electrically-tunable diffraction efficiency while possessing high image quality is demonstrated using a phase-separated composite film (PSCOF). The light scattering-free PSCOF is obtained by anisotropic phase separation between liquid crystal and polymer. Such a lens can be operated below 12 volts and its switching time is reasonably fast (~10 ms). The maximum diffraction efficiency reaches ~35% for a linearly polarized light, which is close to the theoretical limit of 41%.

Self-organization of planar microlenses by periodic precipitation

Abstract: Arrays of planar, Fresnel-like microlenses are prepared by a spontaneous chemical process of periodic precipitation (PP) occurring in a thin layer of a dry gel, and initiated by wet stamping. The PP lenses focus white light more efficiently than the conventional Fresnel zone plates of similar dimensions. Nanoscale topographies of the micropatterned gels can be replicated into transparent elastomers, and used for focusing based on optical path differences. Experimental observations for both types of structures are in agreement with the Fresnel diffraction calculations.

Zone plate designs for terahertz frequencies (Invited Paper)

Abstract: The phase-correcting Fresnel zone plate provides lens-like focusing and imaging of electromagnetic waves by means of diffraction instead of refraction, often referred to as diffraction optics The zone plate has seen extensive investigation and use at microwave and millimeter-wave frequencies, and recently has been applied in the terahertz region These cases have dealt principally with large-angle designs, where the focal length (F) and diameter (D) are comparable (F/D = 03 to 25), unlike the typical optical examples The planar zone plate, in particular, offers the advantages of simplicity of design and construction, low loss, low weight, and low cost, while giving performance similar to that of a refractive lens As one goes to terahertz frequencies, ease of construction becomes more difficult The attenuation in conventional low-loss materials increases at higher frequencies, and dimensional tolerances become smaller, making fabrication more difficult Although earlier designs employing polystyrene have been built and tested at frequencies up to 280 GHz, higher frequency designs are simpler to fabricate and have lower loss if low dielectric constant materials are used This investigation addressed designs for terahertz frequencies The optimization of the zone plate has also been examined, and improvement has been found for radial compression, where the radii of the zone boundaries are slightly shortened

Multipath Fresnel zone routing for wireless ad hoc networks

Abstract: This paper describes a novel multipath routing algorithm, Fresnel zone routing (FZR), for wireless ad hoc networks. First, the concept of Fresnel zones in an ad hoc network is introduced to classify intermediate nodes according to their capacity and efficiency for forwarding traffic. Second, multiple paths via different Fresnel zones are constructed efficiently using a hybrid of proactive and on-demand approaches. By dispersing traffic into multiple zones according to network load and congestion conditions, FZR can alleviate congestion at intermediate nodes and achieve better transport layer throughput. The operations of FZR are implemented and verified with discrete-event simulation models. Simulation experiments show that FZR can provide significant performance improvement over existing unipath and multipath routing protocols for both TCP and UDP traffic.

Fresnel lens combination

Abstract: A focusing unit includes a Fresnel lens combination, where the Fresnel lenses are oriented to reduce shadowing losses. Shadowing is a scattering of light from reflection at the facet walls that separate adjacent Fresnel zones on a given Fresnel lens. Two substantially adjacent Fresnel lenses make up the focusing unit, which can be used as a condenser that collects light from a source in a projection system. Both Fresnel lenses have non-faceted sides that face the light source. The first Fresnel lens collimates the light from the source. The second Fresnel lens receives the collimated beam, with a range of incident angles determined by the spatial extent of the source. Components such as reflective polarizers and anti-reflection coatings can be used between the Fresnel lenses and can be applied to the non-faceted side of the second Fresnel lens.

Focusing characteristics of curvilinear half-open Fresnel zone plate lenses: plane wave illumination

Abstract: A diffraction field-focusing equation based on a specific conical-segment linearization procedure is derived for the Fresnel zone plate (FZP) lens of arbitrary curved profile and is applied for contrasting plane, spherical, parabolic and conical zone plate lenses, convex-side illuminated by a paraxial plane wave front. Two sets of 100-GHz curvilinear and plane FZP lenses are studied numerically with regards to their dimensions, axial focusing intensity and footprint, and frequency bandwidth. For the first set , where the curvilinear and plane lenses share a common lens base aperture and have equal focal lengths, the following new finding has resulted: regardless of their different in shape profiles the FZP lenses have equal zone numbers and produce similar axial focusing. The second set also consists of plane, spherical, parabolic and conical lenses. They share a common apex, and have equal in diameter base apertures and focal lengths but different thicknesses. For such disposition and proportions, the FZP lenses possess different zone numbers and focusing parameters (gain, efficiency, footprint and bandwidth). The belief that the curvilinear FZP have superior (or inferior) electromagnetic characteristics, compared to those of the plane FZP lens with equal number of zones is not in general true. Their relative focusing qualities can vary significantly depending on the lens positioning and dimensions.

Axial irradiance of a focused beam

Abstract: The principal maximum of axial irradiance of a focused beam with a low Fresnel number does not lie at its focal point; instead it lies at a point that is closer to the focusing pupil. It has been shown by the numerical example of a weakly truncated Gaussian beam that its value increases and its location moves closer to the pupil when spherical aberration is introduced into the beam. Such an increase has been referred to as “beyond the conventional diffraction limit.” Similarly, an increase in the value and a shift in the location of the principal maximum of axial irradiance of a uniform beam toward the pupil by the introduction of some spherical aberration has been characterized as an unexpected result. We explain why and how such a result comes about and that it neither invalidates any diffraction limit nor is it unexpected. We illustrate this for uniform as well as Gaussian beams of various truncation ratios. Both focused and collimated beams aberrated by spherical aberration or astigmatism are considered.

Validation of ray and wave theoretical travel times in heterogeneous random media

Abstract: [1] We investigate the validity of three theories of seismic tomography by numerical comparison of cross-correlation travel time shifts of acoustic waves with their theoretical predictions in 3-D heterogeneous random media. Whenever the characteristic heterogeneity scale length is shorter than half of the first Fresnel zone width, ray-theoretical travel times yield large discrepancies from the cross-correlation measurements. Born-Frechet kernel theory is valid for small-scale structures but restricted to weak velocity perturbations. With increasing perturbation strength, neglecting the nonlinearity due to wave path bending in linearized ray and Born theories is no longer eligible. General ray theory accounts for the detour least-time paths in strongly heterogeneous media, which prominently improves the predicted travel times for intermediate to large scale velocity variations.

Computer-generated holograms for 3D objects using the fresnel zone plate

Abstract: A new fast algorithm using the “host” Fresnel zone plate was proposed to improve the computational efficiency of computer-generated hologram (CGH) for 3D objects. By reading .3DS files, the spatial position information of each point of the 3D object was obtained directly. With the illumination of plane wave, the “host” Fresnel zone plate of a single point could be equal to all points located in the same depth plane - as the Fresnel zone plate was translated and superimposed along the horizontal and vertical axes. Consequently, the hologram of a 3D object could be built up by superimposing different Fresnel zone plates in the corresponding depth planes. For a digital object composed of 1060 points, it cost about 83s to generate a hologram of 1024*768 pixels. The CGH of 3D objects with the results of the reconstruction was presented in this paper, which proved the feasibility of this algorithm.

Flat and conformal zone plate antennas with new capabilities

Abstract: We call attention to the existence of a free parameter in the design of Fresnel zone plates. Historically, zone plates have been designed with a specific choice for this parameter, which can be taken as a type of phase reference. We present two methods of interpreting the parameter, either in terms of a reference radius or equivalently a reference phase. Importantly, the parameter can be chosen to have non-standard values which are shown to improve important aspects of antenna performance and to add new functionality to zone plate antennas

Fresnel zone imaging system and method

Abstract: A method for Fresnel zone imaging is provided. The method comprises identifying a plurality of constructive regions and a plurality of destructive regions in an energy transmitting device and converting the destructive regions to the constructive regions by using an apodization profile. The apodization profile comprises apodization values for each constructive region and destructive region and the apodization values comprise real numbers.

Axial intensity distribution behind a Fresnel zone plate

Abstract: An analytical expression based on an improved Rayleigh–Sommerfeld diffraction formula with evanescent term is derived for analyzing the axial light intensity distribution throughout the whole space behind a Fresnel zone plate. The effects of the number of Fresnel zones and the size of aperture on the axial intensity distribution are calculated for two kinds of Fresnel zone plate with larger and smaller aperture. The validity of the general formulae for calculating the focal lengths and the relative intensities of the foci of a Fresnel zone plate is analyzed.

Diffraction by Cantor fractal zone plates

Abstract: The paper reports studies, both experimental and using numerical simulation, of the Fresnel diffraction by recently introduced fractal zone plates associated with triadic and quintic Cantor sets. The evolution of the intensity patterns at planes transversal to the propagation direction is presented. A series of conventional and doughnut-like foci are observed around the principal focus. The position, depth and size of the foci depend on similarity dimensions and the fractal level of the encoded fractal structures, both directly related to the number of the corresponding Fresnel zones.

Locating where Transient Signals Travel in Inhomogeneous Media

Abstract: Locating where transient signals travel between a source and receiver requires a final step that is needed after using a theory of diffraction such as the integral theorem of Helmholtz and Kirchhoff. Introduced here, the final step accounts for interference between adjacent apertures on a phase screen by adaptively adjusting their phase and amplitude, yielding a hierarchy of energy contributions to any desired window of signal travel time at the receiver. The method allows one to check errors in ray theory at finite wavelengths. Acoustic propagation at long distance in the oceanic waveguide (50-100 Hz, 0.05 s resolution) has significant deviations from ray theory. The boundary condition of zero pressure at the surface of the ocean appears to cause sound to travel in a nearly horizontal trajectory for a much greater distance near the surface than predicted by rays. The first Fresnel zone is an inappropriate scale to characterize where transient sounds travel near a ray path as assumed by a standard scattering theory. Instead, the Fresnel zone is too large by an order of magnitude for cases investigated here. Regions where sounds travel can have complicated structures defying a simple length scale. These results are applicable to the physics of underwater sound, optics, radio communication, radar, geophysics, and theories of wave-scattering.

Electron holography used to realize Young's double-hole experiment together with the mapping of magnetic fields

Abstract: This paper is intended to give an overview of the working principle of electron holography. Taking advantage of the potential of this technique, Young's well-known double-hole experiment with electrons has been realized with an original approach. Moreover, the Fresnel diffraction through a circular aperture which reveals the existence of the Fresnel zones for electrons is demonstrated. An important application of electron holography for the investigation of microscopic magnetic fields is also described. Pictorial maps are shown, which represent the magnetic field distribution in matter.

Improving the resolution of a solid immersion lens optical system using a multiphase Fresnel zone plate

Abstract: We propose a method to improve the resolution of near-field optical system with a solid immersion lens by using a multiphase level Fresnel zone plate. The analyses are based on scalar angular spectrum theory. The results show that the multiphase Fresnel zone plate can not only decrease the spot size but also decrease the sidelobe intensity and enhance greatly the diffractive efficiency compared with annular amplitude filter or binary 2-, 3-, 4-zone phase filter.

Optimized 3D VSP Survey Geometry Based On Fresnel Zone Estimates

Abstract: Summary We present a method to optimize the acquisition geometry of 3D VSP data, that is, seismic data recording surface sources in large-aperture borehole receiver arrays. By estimating the Fresnel volume of specular rays at the target depth of interest, we are able to adapt the shooting geometry to the varying size of the Fresnel zone with offset from the well. Such an adaptive pattern of varying shot spacing helps to reduce illumination artifacts close to the receiver well and, at the same time, reduces the overall number of shotpoints, and thus, effort required to obtain the same image volume. We show a real data example where our approach has successfully been applied to optimize the shotpoint geometry.

3D CRS processing: a better use of Pre-stack data

Abstract: The 3D ZOCRS operator is based on a second order approximation of traveltimes in midpoint and half-offset coordinates (Cristini 2003). The stacking surface in the timespace domain, used to compute the CRS volume, depends on eight parameters that describe the shape and direction of normal ray and wavefront curvatures for Normal (N) wave and Normal Incidence Point (NIP) wave. The eight parameters are obtained with only two searches (a three-parameters followed by a five-parameter search) determined by a coherency analysis procedure. The method does not require the exact knowledge of the velocity model, an a priori knowledge of the near surface constant velocity being sufficient. However in presence of multiples it is possible to constrain the parameters’ space using an input velocity field. These boundaries can be obviously relaxed depending on the reliability of the velocity field. Moveout corrections are stretch-free, computed automatically for every sample and not limited to single CMP locations, but are extended, in a physically consistent manner, to super-gather defined on the projected Fresnel zones. This implies a better focusing of the events and a dramatic increase of fold. Adding to the previous ones the contribution of the structural term derived from the N-wavefront leads a superior imaging quality. Moreover, considering this approach as a data driven technique, the auxiliary outputs (e.g. projected Fresnel zones and structural term) may provide useful information for inversion and interpretation. 3D CRS Processing: A Better Use of Pre-stack Data

A study for the special fresnel lens for high efficiency solar concentrators

Abstract: Design a Fresnel lens for a concentrator to collect more sunlight onto the solar cell due to the efficiency and cost. Since 1970, the non-imaging concentrator was used for solar energy; most of them were reflecting mirrors. The non-imaging optical system provides large aperture and forgiving imaging requirements. The Fresnel lens used in non-imaging optical system was usually called non-imaging Fresnel lens. In this research, the Fresnel lenses were refracting optical elements but diffracting ones. According to the method of Ralf Leutz and Akio Suzuki [2], using minimum deviation and minimum dispersion to design a non-imaging Fresnel lens, which obeys the edge ray principle. Use optical software TracePro to simulate the non-imaging Fresnel lens, and each pitch size was 0.3mm and 200mm focus distant. Discusses the losses of non-imaging Fresnel lens and find out the relation of efficiency and F-Number. The optical concentration ratio could reach 15X (2-D) and 230X (3-D).

Large-Size Local-Domain Basis Functions with Phase Detour and Fresnel Zone Threshold for Sparse Reaction Matrix in the Method of Moments

Abstract: Locality in high frequency diffraction is embodied in the Method of Moments (MoM) in view of the method of stationary phase. Local-domain basis functions accompanied with the phase detour, which are not entire domain but are much larger than the segment length in the usual MoM, are newly introduced to enhance the cancellation of mutual coupling over the local-domain; the off-diagonal elements in resultant reaction matrix evanesce rapidly. The Fresnel zone threshold is proposed for simple and effective truncation of the matrix into the sparse band matrix. Numerical examples for the 2-D strip and the 2-D corner reflector demonstrate the feasibility as well as difficulties of the concept; the way mitigating computational load of the MoM in high frequency problems is suggested.

Focusing of a train of pulses in phase opposition through a linear dispersive medium

Abstract: There exists a well known analogy between paraxial or one-dimensional Fresnel diffraction and the propagation of pulses in linear dispersive medium with negligible attenuation. Under this analogy, the envelope of a pulse is equivalent to the distribution of complex amplitude of the light in diffraction. In this context, we study the propagation of a train of identical Gaussian chirped pulses arranged in time in the same way as the Fresnel zones of a phase zone plate, in a highly dispersive guiding medium. From this study we find that the input train concentrates into a single pulse for certain values of total dispersion. We establish the focusing condition and characterize the output signal through its width and peak intensity, showing their dependences on the parameters that define the input train and the dispersive device.

Communication area inspecting device

Abstract: PROBLEM TO BE SOLVED: To inspect the communication area between opposite two stations for simply performing radio communication on a desk. SOLUTION: When an inspection condition input section 11 inputs inspection conditions including the position information of first and second stations to be communicated, a Fresnel zone calculating section 12 calculates the Fresnel zone between the first and second stations, based on the inputted inspection conditions; a site information searching section 13 for searching a site directly below the calculated Fresnel zone; a building information searching section 14 searches the building information of a building in the searched site; and an obstruction determination section 15 determines the presence or absence of a building that may be within the Fresnel zone, based on the searched building information. A result output section 16 outputs a result in which a communication trouble occurs, and a result in which no communication troubles occur, when there are buildings that may be within the Fresnel zone and when there are no buildings within the Fresnel zone, respectively. COPYRIGHT: (C)2005,JPO&NCIPI

Study of Optical Properties of X-ray System Based on Two Zone Plates

Abstract: We presents the results of study of focusing and imaging properties of double-lens system for hard x-ray radiation consisting of two Fresnel zone plates (ZP) made from silicon. We demonstrate for the first time the phenomenon of focusing by two crystal ZP located at significant distance from each other. We investigate by both theoretically and experimentally the peculiarities of intensity distribution at the focal plane during a scan by second ZP normally to the optical axis of the system. We investigate as well the intensity distribution along the optical axis for our double-lens system from crystal ZP. We realize experimentally a registration of the focused image of the object by means of double-lens system based on ZP. Measurements are performed on the beam line BM-5 of the European Synchrotron Radiation Facility (ESRF) at the x-ray energy 9.4 keV. We elaborate a computer program for theoretical simulation of the optical properties of x-ray double-lens system based on ZPs. A calculation is made by convolution of transmission function and Kirchhoff propagator in paraxial approximation by means of Fast Fourier Transformation.