Showing papers on "Fresnel zone published in 1992"

Generation of optical phase singularities by computer-generated holograms.

Abstract: Laser beams that contain phase singularities can be generated with computer-generated holograms, which in the simplest case have the form of spiral Fresnel zone plates.

Fresnel volume ray tracing

Abstract: The concept of “Fresnel volume ray tracing” consists of standard ray tracing, supplemented by a computation of parameters defining the first Fresnel zones at each point of the ray. The Fresnel volume represents a 3-D spatial equivalent of the Fresnel zone that can also be called a physical ray. The shape of the Fresnel volume depends on the position of the source and the receiver, the structure between them, and the type of body wave under consideration. In addition, the shape also depends on frequency: it is narrow for a high frequency and thick for a low frequency. An efficient algorithm for Fresnel volume ray tracing, based on the paraxial ray method, is proposed. The evaluation of the parameters defining the first Fresnel zone merely consists of a simple algebraic manipulation of the elements of the ray propagator matrix. The proposed algorithm may be applied to any high‐frequency seismic body wave propagating in a laterally varying 2-D or 3-D layered structure (P, S, converted, multiply reflected, et...

Use of fresnel zone plates for material processing

Abstract: The apparatus for machining and material processing includes an excimer laser and a Fresnel zone plate array (FZP) positioned parallel to the workpiece, with the distance between the FZP and the workpiece being the focal length of the FZP. For each hole to be formed on the workpiece a corresponding Fresnel zone is patterned onto the FZP. Each Fresnel zone may be patterned directly centered over the desired hole location or in high density patterns it may be located off-center from the hole with deflection being accomplished by the formation of finer circular arcs on the side of the Fresnel zone opposite the desired direction of deflection. A beam scanner is included to provide a more uniform illumination of the FZP by the laser beam. The scanning eliminates non-uniformity of intensity. The alignment mechanism uses a helium-neon laser, the beam from which is projected onto a surface relief grating on the workpiece. The reflected light from the surface relief grating is filtered to create interference fringes which, when aligned, provide maximum light intensity projected through a transmission grating on the Fresnel zone plate.

Diffraction by a circular aperture: a generalization of Fresnel diffraction theory

Abstract: A generalization of the Fresnel approximation in diffraction theory is proposed. The phase term in the diffraction integral is approximated by a paraboloidal variation, not by a binomial expansion but rather by a matching at the critical points in asymptotic evaluation of the integral. The method provides a correction to the optical coordinates of the Fresnel diffraction theory that extends its region of validity. It is applied to diffraction by a circular aperture of a plane wave or focused beam, including effects caused by a large numerical aperture, finite Fresnel number, off-axis illumination, and the presence of aberrations. The method may also be used with other geometries: It is readily applied to cylindrical focusing.

Fresnel zone inversion for lateral heterogeneities in the earth

Abstract: We propose a different kind of seismic inversion from travel-time or waveform inversion for lateral heterogeneities in the earth: Fresnel zone inversion. Amplitude and phase delay of data in several frequency ranges are inverted for model space around ray paths with a width corresponding to the considered frequency so that primary effect of finiteness of wavelength be included. For vertically heterogeneous media, Frechet derivatives for inversion are obtained very efficiently using the paraxial ray approximation, with nearly similar amounts of computation compared to travel-time inversion. As an example, Frechet derivatives are computed for a teleseismic observation system for a three-dimensional structure in the lithosphere beneath an array of seismic stations. Even if the used frequency is around 2 Hz, the width of Frechet derivatives cannot be neglected, particularly near the bottom of the lithosphere. Sensitivity of model parameters to observations is, moreover, different in our approach from conventional travel-time inversion: it is zero along ray paths but large slightly away from them. Some model calculations show that travel-time inversion, particularly with models divided into very fine meshes or blocks, might give misleading results. An example of inversion for a simple Camembert model, in the event that travel-time inversion gives no reliable results, shows how this technique works with much smaller data sets and computation than waveform inversions.

Radio propagation measurements and modelling for line-of-sight microcellular systems

Abstract: A knowledge of radio propagation characteristics in the microcell environment is recognized to be essential for future frequency allocation and system implementation of proposed personal communications services (PCS). A comprehensive radio propagation measurement program in the San Francisco Bay area is discussed. Measurements were performed using three transmitting antenna heights of 3.2 m, 8.7 m, and 13.4 m and two frequencies in the 900 MHz and 1900 MHz bands. Five test settings were chosen in urban, suburban, and rural areas in order to study propagation in a variety of environments. For all LOS measurements even in different environments, the variation of signal strength with distance was found to show distinct ear and far regions separated by a break point. It was also found that the location of the break point for different frequencies and antenna heights could be calculated based on first Fresnel zone clearance. The regression analysis shows a slope that is less than two before the break point, while it is greater than two after the break point. This break distance can be used to define the size of the microcell and to design for fast hand-off. >

A millimeter-wave integrated-circuit antenna based on the Fresnel zone plate

Abstract: A new type of millimeter-wave integrated-circuit antenna is based on a quasi-optical design is investigated. It consists of a Fresnel zone plate on one side of a dielectric substrate and a resonant strip dipole antenna at the focus of the zone plate on the opposite side of the substrate. All of the components are made using simple integrated-circuit fabrication techniques: thin-film metal depositions on planar dielectric substrates. Another features of this design is the short focal length of the zone plate; the focal length/diameter (f/d) for the zone plates studied ranges from 0.1 to 0.5. The antennas described are for a frequency of 230 GHz ( lambda /sub 0/=1.3 mm); however, the design is easily scaled to other millimeter-wave or submillimeter-wave frequencies. Measured results are reported for four different focal length zone plates. Moderate gains, above 20 dB, are obtained. A theory is developed which predicts the on-axis gain, beamwidth, and sidelobe levels. Design graphs are given to aid in the selection of the geometrical parameters to achieve a desired gain from the integrated-circuit zone-plane antenna. >

On-axis conoscopic holography without a conjugate image.

Abstract: We present a method for removing the conjugate image in an incoherent-light holographic technique, namely, on-axis conoscopic holography. The point-spread function that we obtain is that of a complex Gabor zone pattern, which thus should allow good-quality reconstructions of objects. Experimental results are also presented, which confirm the validity of this method.

Composite ray-mode approximations for backscattered sound from gas-filled cylinders and swimbladders

Abstract: The normal mode solution for sound backscattered by a finite fluid (gas)‐filled cylinder in water is replaced by a Kirchhoff approximation for ka>0.15 and the normal mode solution for ka<0.15 where a is cylinder radius and k is the wave number. The composite solution was tested for density and sound‐speed ratios g and h in the ranges of 0.0012

Dual focal point electro-optic lens with a Fresnel-zone plate on a PLZT ceramic.

Abstract: A new dual focal point electro-optic lens that is switchable to focusing and unfocusing is proposed and successfully demonstrated. This electro-optic lens is constructed by coating transparent fine electrodes in the Fresnel-zone plate onto a PLZT ceramic plate. Its focal length changes from 1.25 m to binary at 515 nm with the external voltage of 210 V.

Unified and generalized Fresnel numbers

Abstract: The concept of Fresnel number is discussed and expressions are derived for misaligned optical systems. For the case of perfectly aligned optical elements, the usual number,N, based on a Fresnel zone concept is found to be given byN = (a 2/λ)(D 1/B 1 +A 2/B 2), whereB 1,D 1 andB 2,A 2 are the transfer matrix elements of the optical systems before and after a circular aperture of radiusa respectively. A modified definition of the Fresnel number is proposed for Gaussian beam propagation. This parameterN′ G, is related to the complex beam parameter and may be represented by the angleθ = tan−1 N′ G, found in the familiar Collins chart and its dual representation. A general relation for the transformation of this Fresnel number is found. The expressions for Gaussian beam transformation are thus simplified, since the waist-waist transform is given byN′ G1 =N′ G2 = 0. Finally, these two kinds of Fresnel numbers are written as tensors when applied to cases involving elliptical apertures, astigmatic beams and nonsymmetrical systems.

Focussing optics for a synchrotron-based X-ray microprobe

Abstract: X-ray microprobe experiments at third-generation synchrotron sources will provide trace element analysis of samples with 1 μm × 1 μm spatial resolution and femtogram sensitivity. For these experiments to be possible. X-ray optical elements need to be developed to focus the beam from a hard X-ray undulator to a micron spot size. In June 1991 several different optical elements were tested during a dedicated undulator run at CHESS. The undulator produced radiation similar to that which will be available at third-generation sources like the Advanced Photon Source. Both Fresnel zone plates and multilayer-coated spherical mirrors were tested. With the Fresnel zone plate a spot size of 8.5 μm × 30 μm was achieved in the first order and 6 μm × 20 μm in the second order. With a Kirkpatrick-Baez multilayer mirror system a spot size of 4 μm × 9 μm was achieved. Based on these results, some of the requirements for an optical system suitable for a dedicated microprobe beamline are given.

Physical and numerical model study of diffraction effects on seismic profiles over simple structures

Abstract: SUMMARY 2-D scale models and synthetic seismograms have been used to study diffraction artifacts and interpretation pitfalls on seismic profiles over two classes of simple-shaped structures: a vertical fault model and a rectangular mound model. The fault throw was varied from one eighth of a wavelength (λ/8) to two wavelengths (2λ). For throws greater than λ/2, the structure is resolved in the x-t domain but diffractions which are present convey misleading structure. Amplitude and wavelet anomalies are observed for small fault displacement (λ/2) which serve to detect but not to define the exact fault position and extent. Migration improves but does not necessarily overcome the problem. Mound structures having heights of λ/8 to 2λ, and length of one half to twice the Fresnel zone radius, were investigated. Lateral resolution is improved as the length of the structure increases, but only for mounds which are vertically resolved (height greater than λ/2). For low relief mounds, resolution deteriorates with increasing mound length. The resulting edge diffractions, which interfere with the reflection events, serve to detect and identify the target. Amplitude anomalies are observed across the structures, but it is not possible (without calibration) to interpret the edges of the model. Even for thick mounds the diffraction patterns complicate the seismic picture and can lead to false interpretation. The migration aperture needs to be about 10 times the Fresnel zone radius to produce satisfactory results. However, for vertically unresolved structures, migration may not significantly help. Complicating factors with post-stack migration are lack of knowledge of the true velocity function and inadequacy of the CMP zero-offset equivalence assumption.

Determination of Fresnel Zones From Traveltime Measurements

Abstract: For a horizontally stratified (isotropic) earth, the rms-velocity of a primary reflection is a key parameter for common-midpoint (CMP) stacking, interval-veloc­ ity computation (by the Dix formula) and true-ampli­ tude processing (geometrical-spreading compensa­ tion), As shown here, it is also a very desirable parameter to determine the Fresnel zone on the reflec­ tor from which the primary zero-offset reflection re­ sults, Hence, the rms-velocity can contribute to eval­ uating the resolution of the primary reflection. The situation that applies to a horizontally stratified earth model can be generalized to three-dimensional (3-D) layered laterally inhomogeneous media. The theory by

The Backscattering from Anisotropic Turbulent Irregularities

Abstract: The problem of scattering of electromagnetic waves from anisotropic irregularities, generated by turbulence in the atmosphere, was considered in Born approximation. A valid solution is found not only for the Fraunhofer zone but also for the Fresnel zone of the antenna. In this case no limiting conditions on correlation length of refractive index irregularities appear and there are only limitations on radar range. It is shown that the scattered power, measured at the output of the receiving antenna, mainly depends on the degree of anisotropy and angular width of the antenna pattern. Received power increases with the increasing of anisotropy, but then saturates at a level which depends on the antenna parameters.

Kirkpatrick-Baez microscope based on a Bragg-Fresnel x-ray multilayer focusing system.

Abstract: We demonstrate how to use a cylindrical Bragg–Fresnel multilayer focusing system instead of spherical mirrors for two-dimensional focalization. We performed our tests at 8 keV with a collimated x-ray tube having an apparent anode source size of 75μm × 100 μm. These tests demonstrate that focalization can be obtained within <20 μm. This offers new possibilities in optical systems for collimators and fine focusing of x-rays, especially for synchrotron radiation sources.

Optical information system focal point displacement detecting means having Fresnel zone plates with rectilinear grating

Abstract: This invention pertains to a displacement detecting device for an optical head for detecting focal point displacement of irradiation light. The displacement detecting device has optical means for converting reflected rays from an irradiated face into an astigmatic bundle of rays and a photodetector for detecting a change in the the focused shape. The displacement detecting device thus detects focal point displacement of irradiation light. The optical means is obtained by integrally forming a plurality of Fresnel zone plates having rectilinear gratings, focusing reflected rays from the irradiated face, and having different focal lengths.

Precise alignment through thick wafers using an optical copying technique

Abstract: Submicrometer alignment of two mask patterns on the top and the bottom surface of a thick glass substrate is possible by using optical copying. The optical copying step transfers an alignment pattern from one surface to the other by shadow casting or Fresnel propagation. In a demonstration experiment, the alignment between the two surfaces of a 3-mm-thick quartz glass substrate was achieved with a precision of 0.5–0.7 μm.

Mössbauer-Fresnel zone plate

Abstract: We present a novel application of the Fresnel zone plate geometry in which the Mossbauer effect is used to focus nuclear‐resonant x‐ray photons and thereby to monochromatize synchrotron radiation. The calculated efficiency and energy bandpass of the Mossbauer–Fresnel zone plate monochromator compare well with those of other Mossbauer‐effect monochromators currently in use. The unique capability of focusing Mossbauer radiation into a small (few μm) spot may have applications in spatially resolved and high‐pressure Mossbauer‐effect studies.

Multi-focus camera

Abstract: PURPOSE:To provide a multi-focus camera capable of changing the depth of focus by controlling liquid crystal. CONSTITUTION:The multi-focus camera is provided with an optical system consisting of an optical lens 3 and a liquid crystal plate 2 composed of plural picture element areas arranged in a grid pattern, and liquid crystal control means 4, 5, 6 which form plural Fresnel zones on the liquid crystal plate 2 by applying a voltage to each picture element area of the liquid crystal 2 and give a specified optical phase difference to adjacent Fresnel zones so as to make up a Fresnel zone plate.

System for locating fresnel reflections along an optical fiber

Abstract: A system for locating Fresnel reflections along an optical fiber, including an emission circuit for emitting optical pulses which are coupled into the input face of the fiber; a reception and transformation circuit (20) for receiving signals reflected by the fiber and transforming them into electrical signals; a selection circuit (20) for taking the electrical signals and selecting therefrom only those electrical signals which correspond to Fresnel reflections; a comparison circuit (23, 25, 26) coupled to the selection circuit (20) and receiving a comparison reference signal expressing the rank of an arbitrarily-chosen one of the Fresnel reflections; and an evaluation circuit (13, 21, 22, 24, 26) coupled to the comparison circuit (23,25,26) and to the selection circuit (20) so as to locate the point along the fiber that gave rise to the arbitrarily-chosen Fresnel reflection

X-Ray Holography Using Photoresists: High Resolution Lensless Imaging

Abstract: X-ray microscopy using two types of holography (Gabor and Fourier transform) is considered. The advantages and drawbacks of Gabor holography using photoresists are considered in some detail. Recent results in Gabor holography using undulator radiation are presented, including twin image noise reduction, the imaging of 3D objects, and the results of initial attempts at imaging hydrated specimens.

Optical systems making use of phase type fresnel zone plates

Abstract: The optical system of the invention includes a phase type Fresnel zone plate 5. The zone plate 5 includes thereon a central zone C located around the optical axis, through which light flux 2 emanating from an object and having a relatively small divergence angle passes, and a peripheral zone P located on the outside thereof. The central and peripheral zones C and P have given Fresnel pitches and blazing angles, so that the peripheral zone P can make use of diffraction higher in order than that used by the central zone C to obtain the desired refractive power. Although the minimum pitch of the phase type Fresnel plate zone is thus relatively larger in the peripheral zone P, the optical system is not only satisfactory in terms of the effect on correction of chromatic aberration and refractive power but ensures a good image surface contrast as well.

Differential Phase Contrast Imaging in the Scanning Transmission X-Ray Micro scope,

Abstract: : Calculations of the contrast transfer functions for differential phase contrast imaging are presented and compared to those for absorption imaging in the x-ray microscope. In the scanning transmission x-ray microscope (STXM) the specimen is scanned in a raster by an x-ray probe formed with a Fresnel zone plate. To achieve near diffraction limited resolution it is necessary to have a coherent source, even when forming an incoherent brightfield image by measuring the x-ray intensity transmitted by the object. This has so far been the only imaging mode used in the STXM and for hydrated biological specimens is well suited to soft x-ray wavelengths within the water window (2.33 to 4.36 nm) where carbon absorbs much more strongly than water. However, by the use of phase contrast rather than amplitude contrast, it is possible to form images at wavelengths where the absorption is low, resulting in lower radiation dose for the same level of contrast. Calculations have demonstrated very clearly the advantages of phase contrast imaging at wavelengths outside the water window.

Acousto-optical reconstruction of a pulsed ultrasonic signal

Abstract: In the Fresnel zone of laser light diffracted by a sound wave, the light intensity is modulated in both time and space, and the pattern of this modulation contains all the information about the time waveform of the diffracting sound wave. A method is proposed to recompose the sound wave from the optical signal, recorded during one ultrasonic pulse period in a single point in the Fresnel region of the diffracted light. The technique can be considered as a real-time reconstruction and is applicable to both continuous and pulsed ultrasonic waves for physical parameters within the range of Raman-Nath-type diffraction.

Lessons for pulsed-array radar from quantum light detection

Abstract: Antenna arrays are potentially useful for both the transmission and the reception of trains of electromagnetic pulses. In this paper, we formulate an aiming problem pertaining to an RKA- driven launcher of microwave pulses, and draw on a study of quantum measurements to develop an approach to aiming. Appendix A reviews the behavior of microwave radiation focused at a target in the Fresnel zone. Appendix B derives the antenna current for optimal angular sensitivity of a monopulse radar for a target in the Fresnel zone, very close to the axis.

2-D X-Ray Focusing by Circular Phase Bragg-Fresnel Lenses at a Bragg Angle Close to π/2

Abstract: Two-dimensional focusing of hard X-rays has been accomplished for the first time by means of a phase Bragg-Fresnel lens with efficiency close to 40%. Images of the radiation region of a conventional X-ray tube and of a storage ring of synchrotron radiation have been obtained.

Fresnel reflections locating system along an optical fibre

Abstract: System for locating Fresnel reflections along an optical fibre, characterised in that it comprises: - means of emitting optical pulses coupled to the entrance face of the fibre, -means (20) of receiving and transforming into electrical signals the signals reflected by the fibre, -means (20) of selecting from the electrical signals just the electrical signals corresponding to Fresnel reflections, -comparison means (23, 25, 26) coupled to the selection means (20) and receiving a comparison reference signal conveying the rank of one of the arbitrarily chosen Fresnel reflections, -evaluation means (13, 21, 22, 24, 26) coupled to the comparison means (23, 25, 26) and to the selection means (20) for positioning on the fibre the point which gave rise to the arbitrarily chosen Fresnel reflection.

Flat mirror antenna

Abstract: PURPOSE:To improve the manufacturing efficiency of the antenna so as to reduce the manufacturing cost and time by providing a metallic diffraction ring, metallic reflecting plate which is arranged so that the reflecting plate can face the diffraction rig at an interval of about 1/4 wavelength, and pickup antenna section which is arranged near the focusing position of the diffraction ring. CONSTITUTION:A dielectric reinforcing plate which supports and fixes a diffraction ring 1 and reflecting plate 2 is provided between the ring 1 and plate 2. A pickup antenna section 3 for reception is supported at the focusing position of the ring 1 by means of a supporting arm 8. In a Fresnel zone formed in a radio wave propagating path between a transmitting and receiving points, same-phase components are converged to the antenna section 3 provided at the focusing position of the ring 1 after the components are reflected by the ring 1 and, at the same time, opposite-phase components passed through the ring 1 are converged to the antenna section 3 as same-phase components after the components are reflected by the reflecting plate 2 provided at an interval of about 1/4 wavelength from the ring 1 and their phases are matched.