Fresnel zone

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

Holographic grating scanners with aberration corrections.

Abstract: Optically recorded Fresnel zone plates have proper spatial frequency characteristics for use as a laser scanner. Such grating scanners can easily be made and can scan over large cone angles with, however, severe aberrations of the diffracted laser beam at large deflection angles. Optically recorded Fresnel zone plates for laser beam scanning are investigated, and their aberration properties are analyzed. A method for removing the wavefront error caused by this type of zone plate is discussed, and the feasibility of the method is demonstrated.

Analytic expression for Fresnel diffraction

Abstract: An analytic formula for the light field amplitude (and therefore also intensity) behind a circular aperture illuminated by spherical or plane waves in the Fresnel limit is given, and the essential difference between Fresnel diffraction and Fraunhofer diffraction is discussed.

Untangling the incoherent and coherent scattering components in GNSS-R and novel applications

Abstract: As opposed to monostatic radars where incoherent backscattering dominates, in bistatic radars, such as Global Navigation Satellite Systems Reflectometry (GNSS-R), the forward scattered signals exhibit both an incoherent and a coherent component. Current models assume that either one or the other are dominant, and the calibration and geophysical parameter retrieval (e.g., wind speed, soil moisture, etc.) are developed accordingly. Even the presence of the coherent component of a GNSS reflected signal itself has been a matter of discussion in the last years. In this work, a method developed to separate the leakage of the direct signal in the reflected one is applied to a data set of GNSS-R signals collected over the ocean by the Microwave Interferometer Reflectometer (MIR) instrument, an airborne dual-band (L1/E1 and L5/E5a), multi-constellation (GPS and Galileo) GNSS-R instrument with two 19-elements antenna arrays with 4 beam-steered each. The presented results demonstrate the feasibility of the proposed technique to untangle the coherent and incoherent components from the total power waveform in GNSS reflected signals. This technique allows the processing of these components separately, which increases the calibration accuracy (as today both are mixed and processed together), allowing higher resolution applications since the spatial resolution of the coherent component is determined by the size of the first Fresnel zone (300–500 meters from a LEO satellite), and not by the size of the glistening zone (25 km from a LEO satellite). The identification of the coherent component enhances also the location of the specular reflection point by determining the peak maximum from this coherent component rather than the point of maximum derivative of the incoherent one, which is normally noisy and it is blurred by all the glistening zone contributions.

Fresnel diffraction in an optical system containing lenses

Abstract: A convenient expression describing the location of Fresnel diffraction patterns when a diffracted spherical wave is focused by a lens is given. Experimental results confirm the predicted positions of 26 axial extrema corresponding to integer numbers of Fresnel half-period zones. Experimental radial intensity profiles are presented for some of these positions.

Characterization and correction of spherical aberration due to glass substrate in the design and fabrication of Fresnel zone lenses

Abstract: As with a conventional lens, a Fresnel zone lens (FZL) can be used to image objects at infinity or nearby In the latter case, the FZL converts a diverging spherical wavefront into a converging spherical wavefront The glass substrate on which the FZL is fabricated introduces spherical aberration resulting in a shift of the image plane and blurring of the image Two novel schemes for correction of this spherical aberration are proposed and studied in this paper To demonstrate them, FZLs are designed with and without aberration correction They are fabricated using electron beam direct writing The devices are evaluated and the accuracy of the proposed aberration correction schemes is validated