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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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TL;DR: In this paper, a holographic optical tweezers system is constructed using time-shared multiplexing to generate multiple optical traps using computer generated Fresnel zone plates to produce the required laser focuses for standard optical traps while helical zone plates are used to generate Laguerre-Gaussian laser modes that produce doughnut shaped focuses.
Abstract: A holographic optical tweezers system is constructed using time-shared multiplexing to generate multiple optical traps. Computer generated Fresnel zone plates are used to produce the required laser focuses for standard optical traps while helical zone plates are used to generate Laguerre-Gaussian (LG) laser modes that produce doughnut shaped focuses. Standard Fresnel zone plates are used for trapping non-biological matter whereas LG modes are used in trapping delicate biological matter that is susceptible to damage from excessive laser radiation. A reflective high speed ferroelectric spatial light modulator (SLM), which is used to display the zone plate images, is capable of multiplexing a maximum of 24 binary images at a refresh rate of up to 1440Hz. By programming the SLM to display one binary hologram per bit plane, a commercial 24 bit colour SLM is used to provide 24 multiplexed traps. The ferroelectric holographic system enables multiple independently movable traps using time-shared multiplexing without the need for mechanical movement within the system. The software developed to display the zone plates uses Open GL acceleration to allow fast smooth movement in real time. Open GL utilises the graphics processing unit (GPU) common on many computers today which greatly increases the frame rate of the images displayed on the SLM. Use of the apparatus is demonstrated by the trapping and manipulation of colloidal particles and yeast cells. Preliminary results indicate that the use of LG zone plates for trapping biological cells results in significantly less damage than standard Fresnel zone plates.
1 citations
09 Jun 2007
TL;DR: In this article, a new formulation for backscattering from distributed targets has been developed that expresses the backscattered signal received by the radar as a weighted coherent sum involving the antenna spatial spectrum and the back scattered radiation.
Abstract: A new formulation for backscattering from distributed targets has been developed that expresses the backscattered signal received by the radar as a weighted coherent sum involving the antenna spatial spectrum and the backscattered radiation. While the general formulation is applicable to scatterers in the antenna Fresnel zone, the specific focus in this study was on the far field region.
1 citations
01 Nov 2012
TL;DR: In this article, the authors present methods to verify line-of-sight path, Fresnel zone clearance and propagation loss, starting from a digital topographic map and using different interpolation methods.
Abstract: This paper presents some methods to verify line-of-sight path, Fresnel zone clearance and to calculate propagation loss, starting from a digital topographic map and using different interpolation methods
1 citations
01 Jan 1987
TL;DR: In this article, it was shown that the carbon material from which the absorbing zones are formed is semitransparent to soft x-rays and this lack of opacity results in a reduction of focusing efficiency of about a factor of ten compared to the theoretical figure.
Abstract: The resolution obtainable with an x-ray microscope which uses a Fresnel zone plate as the focusing element is primarily dependent on the width of the finest ring of the zone plate and the accuracy of the zone plate pattern (Simpson, 1984). High accuracy zone plates with outermost zone widths of only a few hundred angstroms have been made by carbon contamination e-beam lithography at Kings College London (Buckley et al., 1985). Whilst these zone plates have the finest zone widths yet produced, the fabrication of one zone plate takes ten hours and the carbon material from which the absorbing zones are formed is semitransparent to soft x-rays. This lack of opacity results in a reduction of focusing efficiency of about a factor of ten compared to the theoretical figure. The reduced efficiency increases the length of time required to form an image in a scanning x-ray microscope (Kenney et al., this volume). The time taken to form an image in an x-ray microscope should be as short as possible to minimize specimen deterioration and maximize throughput. For this reason, the replication of carbon zone plates in a higher contrast material, to improve efficiency and minimize imaging times, is highly desirable.
1 citations
10 Apr 2006
TL;DR: In this article, the effect of the ground curvature by using Fock's theory of diffraction was considered, and the effects of radiowave scattering from ground surfaces with various roughness (large, median, and small) with respect to the wavelength.
Abstract: Based on Green's theorem and the Huygens's principle, this chapter first introduces the Fresnel zone presentation that will be used for the description of terrain and other obstruction effects, such as reflection and diffraction on radio channels. It presents the main formulas for path loss prediction for a free‐space communication link. Next, the chapter describes the reflection phenomena due to a flat terrain. It then deals with the electromagnetic (EM) aspects of radiowave propagation above a rough terrain. Here, the three methods of mathematical derivation of the field strength are presented to obtain the effects of radiowave scattering from ground surfaces with various roughness (large, median, and small) with respect to the wavelength. The chapter considers the effect of the ground curvature by using Fock's theory of diffraction. FInally, it describes diffraction phenomena caused by a single obstruction placed at the flat ground surface.
1 citations