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Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.


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Journal ArticleDOI
TL;DR: The experimental investigation shows that the Moiré-lenses have a broad adjustable refractive power range with a high efficiency, which allows one to use them for flexible beam steering and for imaging applications.
Abstract: In an earlier publication [Appl. Opt. 47, 3722 (2008)] we suggested an adaptive optical lens, which consists of two cascaded diffractive optical elements (DOEs). Due to the Moire-effect the combined optical element acts as a Fresnel zone lens with a refractive power that can be continuously adjusted by a mutual rotation of the two stacked DOEs. Here we present an experimental realization of this concept. Four designs of these Moire-DOEs (MDOEs) were fabricated in thin (0.7 mm) glass slides by lithography and subsequent etching. Each element was realized as a 16 phase level DOE designed for 633 nm illumination. Our experimental investigation shows that the Moire-lenses have a broad adjustable refractive power range with a high efficiency, which allows one to use them for flexible beam steering and for imaging applications.

66 citations

Journal ArticleDOI
TL;DR: In this article, a method for line width control in electron beam lithography is presented, which is especially suited for the manufacture of diffractive optical elements, by defocusing the spot of the electron beam writer.

66 citations

Journal ArticleDOI
TL;DR: In this paper, the authors show that the ray theory criteria cannot be relaxed for low frequency Lamb wave tomography to evaluate the maximum depth of corrosion patch in typical pipe inspection problems, and that the likely points of operation in the low frequency regime respect the wavelength condition but not the more stringent Fresnel zone condition.
Abstract: Corrosion is a major problem in the petrochemical industry. Corrosion patches are often inaccessible, and the majority of the conventional nondestructive evaluation techniques are compromised. There is therefore a need for a rapid, accurate, long range inspection technique to measure the remaining thickness in corrosion patches. Low frequency Lamb wave tomography is a potentially attractive technique to rapidly evaluate the thickness of large sections of partially accessible structures. This approach has been used in the past to detect the shape of defects in a range of applications. Time-of-flight straight-ray tomography relies on the dispersive nature of a guided wave mode to reconstruct the depth profile of a corrosion patch. If the frequency is limited to below the cut-off of the higher order modes the interpretation of the signals and the time-of-flight measurement are easier. In order to reconstruct a thickness map with time-of-flight straight-ray tomography the ray theory needs to be valid. There are two validity criteria: the characteristic size of the defect must be larger than the wavelength and larger than the width of the Fresnel zone. For realistic defect sizes, the likely points of operation in the low frequency regime respect the wavelength condition but not the more stringent Fresnel zone condition. The paper demonstrates, with finite element simulations and experiments, that the ray theory criteria cannot be relaxed for low frequency Lamb wave tomography to evaluate the maximum depth of corrosion patch in typical pipe inspection problems.

65 citations

Journal ArticleDOI
J.J. Maciel1, L.B. Felsen
TL;DR: In this article, the authors studied the high-frequency propagation of electromagnetic wavefields from an extended planar aperture distribution through a complicated environment, and showed that narrow-waisted beams can reconstruct the correct transmitted field in these cases, as established independently by numerical evaluation of an exact spectral integral.
Abstract: High-frequency propagation of electromagnetic wavefields from an extended planar aperture distribution through a complicated environment is addressed. Geometric optical ray tracing provides a versatile approximate approach to the class of problems, but it fails in transition regions surrounding shadow boundaries and caustics. The uniformity required there can be established by field tracking with Gaussian beams. The basic theory is summarized and applied to the two-dimensional test problem of transmission of radiation from a finite one-dimensional plane aperture through a planar dielectric layer. For a truncated uniform or focused aperture illumination, tracking the edge or caustic transition regions through the layer when it is located within the Fresnel zone of the aperture poses a problem of substantial complexity. It is shown that narrow-waisted beams, which can be propagated in their far zone as complex ray fields, reconstruct the correct transmitted field in these cases, as established independently by numerical evaluation of an exact spectral integral. >

65 citations

Journal ArticleDOI
T. Chu1
23 May 1983
TL;DR: In this article, a pseudo-frequency independent beam waveguide feed has been designed and built to accommodate a new liquid-helium-cooled millimeter wave radio astronomy receiver in the side cab of the Crawford Hill 7-m antenna.
Abstract: A pseudo-frequency-independent beam waveguide feed has been designed and built to accommodate a new liquid-helium-cooled millimeter wave radio astronomy receiver in the side cab of the Crawford Hill 7-m antenna. This enables the antenna to be tilted without tilting the liquid-filled receiver. Comparison with the old vertex-cab feed indicates very little measured transmission loss through the beam waveguide. The frequency independence is based upon the Fresnel zone imaging principle. Design procedures and practical bandwidth limitations are explained. An explicit expression for the third-order term of an offset reflector surface clarifies the approximation of a lens by an offset reflector.

65 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202326
202249
202137
202052
201965
201878