Fresnel zone

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

Blazed Fresnel zone lenses approximated by discrete step profiles: effects of fabrication errors

Abstract: Blazed Fresnel zone lenses for 1.5 j.m wavelength were fabricated in quartz glass by means of microstructuringtechnology. The blazed profile in each zone of the lenses was approximated by 2, 4, and 8 discrete levels. The effects offabrication errors, such as depth and alignment errors, on the diffraction efficiency of the different Fresnel zone lenses were investigated. Further the location and intensity of the parasitic foci appearing due to the discrete level approximation are calculated. Theoretical results along with experimental measurements are presented.1. INTRODUCTIONDiffractive optical lenses are beginning to play an important role in a variety of applications'. This is especially true for those lenses that can be fabricated with microstructuring techniques that are adapted from microelectronic technology. The most common of these diffractive lenses are Fresnel zone lenses that are recorded as surface reliefgratings. In order to obtain the maximum diffraction efficiency, the shape of the grating profiles must be blazed.Unfortunately, such blazed profiles are difficult to fabricate, so they must be approximated by a staircase-like profilehaving a number of discrete levels.Multilevel zone plate lenses can reach high efficiencies depending on the number of discrete levels that are used. Thefabrication of these elements involves multiple steps of repeated pattern transfer and either anisotropic etching orthin film deposition3 to build up the desired surface profile. During these steps a variety of fabrication errors can beintroduced that degrade the optical quality and reduce the diffraction efficiency significantly. Some of these errors andtheir effects have been considered in applications with visible and far infra-red radiations2'.

Features in phase-contrast images of micropipes in SiC in white synchrotron radiation beam

Abstract: An interesting feature in phase-contrast images of micropipes in silicon carbide in white synchrotron radiation beam was experimentally studied and theoretically explained. This feature consists in that a change in micropipe cross-section sizes does not lead to changes in its image sizes, but has an effect only on the contrast. The experiment was performed on the synchrotron radiation source in Pohang, South Korea. On the one hand, this effect is explained by a small phase progression caused by the micropipe, and, on the other hand, by satisfying the conditions for Fraunhofer diffraction, when the transverse micropipe size is smaller than the first Fresnel zone diameter. As a rule, the near-field conditions are satisfied in X-ray optics when only object edges are imaged. However, micropipes are so small that the standard edge theory is inapplicable. A universal intensity distribution profile was obtained for micropipes with very small cross sections.

Fresnel volume and interface Fresnel zone for reflected and transmitted waves from a curved interface in anisotropic media

Abstract: The Fresnel volume and the interface Fresnel zone (IFZ) concepts play an important role in seismic exploration because the IFZ largely contributes to the formation of the reflection and transmission wavefields at an observation point. We derived analytic expressions based on traveltime approximations to evaluate the IFZ size for converted and nonconverted waves reflected (or transmitted) by a curved interface between two homogeneous general anisotropic media, and more specifically for dip-constrained transversely isotropic homogeneous media. The reflectors are of anticline, syncline, or saddle type, and their principal curvature axes may not lie in the incidence plane. As in an anisotropic medium the isochron in most cases assumes a nonelliptical shape, the size and the shape of the IFZ for reflected waves are predominantly dependent on the curvatures of the isochrons together with the curvatures of the interface. The IFZ shapes also exhibit large variation with interface curvature and incidence angle. In addition, the difference between the Thomsen anisotropy parameters ϵ and δ is found to control the size of the IFZ for P-P and P-S reflections. The IFZ for anisotropic media with curved interface can be much larger than that for equivalent isotropic media, and more specifically for positive values of ϵ − δ. The spatial resolution of unmigrated seismic data in anisotropic media would consequently be different from that determined for the same configuration for isotropic models and a planar interface.

Phase Diversity Implementation in Fresnel Incoherent Holography

Abstract: A new adaptive implementation of the Fresnel incoherent correlation holography (FINCH) is presented. By implementing phase diversity mechanism, we allow wavefront error compensation without the need of a guide star. Simulation results present significant resolution improvement.

Optical coupling structure between light source and light guide circuit component

Abstract: PURPOSE:To simplify an optical coupling structure and to attain size reduction and economization by using only a Fresnel zone lens formed on one end surface of the substrate of a light guide circuit component as an optical coupling element between a light source and the light guide circuit components. CONSTITUTION:The light guide circuit component 12 has the substrate 13 and a light guide 14 extending from its one end surface 13a along one flank 13b. Further, the Fresnel zone lens 15 which converges and guides light emitted by the light source 11 to the light guide 14 is formed on one end surface 13a of the substrate 13. At this time, the Fresnel zone lens 15 is formed larger than the size of the end surface of the light guide 14, so the light emitted by the light source 11 is guided through the Fresnel zone lens 15 into the light guide not only from the end surface of the light guide 14, but also from its periphery to perform optical coupling with high coupling efficiency. Consequently, the optical coupling structure which is simple and formed at low cost to small size is obtained between the light source and light guide circuit component.