Fresnel zone

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

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.

Modeling of radiative transfer in optical fiber drawing processes with fresnel interfaces

Abstract: Radiative heat transfer is the most dominant mode of heat transfer in an optical fiber drawing process, and its accurate prediction is the key to a realistic simulation of flow and heat transfer in the system. In this study, the finite-volume method (FVM) is developed to model radiative heat transfer in a gas enclosure as well as inside a glass preform with a Fresnel boundary at the gas-glass interface. Unlike diffuse boundaries, the reflections and transmissions at Fresnel boundaries are governed by Fresnel's relation and Snell's law, and they are strongly dependent on the angular direction. During the implementation of the FVM in semitransparent media, control-angle overlap may occur, and it is treated by assuming that the flux of radiant energy across a control angle is determined only by the discrete direction over this control angle and the corresponding radiative intensity. At the Fresnel boundary, a reflective or refractive intensity may not coincide with any discrete direction, and it is approximated by an intensity whose discrete direction makes the smallest angle with the corresponding reflective or refractive intensity. To validate the present FVM in semitransparent media with Fresnel boundaries, two benchmark problems are examined and the present solutions are found to be very accurate with a fine angular discretization. After that, the present model is applied to investigate an optical fiber drawing process with different boundary conditions and temperature distributions. For the diffuse boundary at the gas-preform interface, the results from the FVM are very close to those from the zonal method in each case. For the Fresnel boundary, the FVM provides a solution that tends to be higher in the upper neck-down region and lower in the low neck-down region than that from the same method with the diffuse boundary. The maximum difference between two solutions varies and may reach 10-15% at some cases.

Notes on semiempirical terrestrial wave propagation modeling for macrocellular environments - comparisons with measurements

Abstract: Due to their low computational requirements, two-dimensional semiempirical wave propagation algorithms are still indispensable tools for terrestrial mobile communications and radio broadcasting coverage predictions in the very high and ultrahigh frequency bands. In this paper, we discuss various improvements of an algorithm based on digital terrain data. The algorithm combines the basic empirical propagation curves of Okumura et al. (1968) with terrain adaptive propagation curves derived from Fresnel zone clearance analyses of several terrain subsections. Also, a multiple knife-edge diffraction algorithm with approximate evaluation of the Kirchhoff diffraction integrals is applied when the line of sight is obstructed. Prediction quality is further improved by a robust algorithm for determination of effective transmitter antenna heights, and land usage along the terrain profile different from "open terrain" is considered by a multilevel Fresnel zone blockage evaluation. Prediction errors with standard deviations of about 6 to 7 dB were found for a great variety of measurements in flat and moderately undulating terrain. Transmitter stations with very high antennas (more than 200 m) caused larger standard deviations of the prediction errors. However, improvements of the predictions could be achieved in these cases by evaluating street orientations with respect to the direct path.

Fresnel approximation for off-axis illumination of a circular aperture

Abstract: Various formulations have been proposed for the Fresnel approximation for diffraction by a circular aperture. These formulations can describe the effects of convergent illumination, finite value of Fresnel number, and off-axis illumination. The retention of a further term, which is dependent on the coordinates of the focus and the observation point, is proposed. This results in a redefined axial optical coordinate, giving improved prediction of the diffracted field for off-axis points.

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.