Fresnel zone

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

Synthesis and analysis of a frequency-scanned planar waveguide array focused in the Fresnel zone

Abstract: A 2D array of radiators excited by the modes of a planar array of parallel waveguides and focused in the Fresnel zone is considered. The problem of determining the positions of array radiators for focusing the field at a finite distance from the array aperture is solved for the cases of plane and cylindrical fronts of the initial distribution of the phases of waveguide modes. A phenomenological model of the slotted-waveguide array is used to determine the position of the focal line, the aberration of the scattering spot, and the frequency scanning sector.

Measurements based specular reflection formulation for point cloud modelling

Abstract: Describing the environment using a point cloud is a promising method for both propagation predictions and channel simulations. Recently, a specular reflection model has been proposed and verified through measurements to account for reflections from large reflecting surfaces. Here, the formulation for the specular reflections for point cloud (SRPC) is improved based on a few canonical measurements in an anechoic chamber. The specular reflection image theory for large surfaces, the Radar Cross Section equation for small surfaces, the Fresnel-Kirchhoff integral formula, and measurements are investigated. Unexpectedly to the authors, the results show that, when neglecting the diffraction effects, the contributions to the specular reflected field occur mainly from the illumination of about a third of the first Fresnel zone. A prediction of the SRPC formula is the appearance of “radio flares” due to electrically small reflecting surfaces.

Simulation of paraxial pinhole photon sieves model based on Fresnel zone plates

Abstract: Soft X-rays has applications in many fields because of its special characters. Therefore the focusing and imaging is important to those applications. The traditional refractive lens to focus soft x-rays has some shortcomings. Fresnel zone plates can overcome these shortcomings. However, it presents new problems, that is, the resolution is limited by the width of the outermost zone. Kipp et al2 developed a novel diffractive optical element called the photon sieve. It can overcome the limitations of Fresnel zone plates. According to Fresnel-Kirchhoff diffraction integral, one can present the far field model3 and obtain the radius and the central position of pinholes on the conditions of far field and near field correction. We show the simulated figures and the diffraction pictures when the number of pinholes is 20, 40 and 80 respectively by imitating this model. Then we compare some diffraction pictures of the photon sieves with different number of pinholes to monochromatic light, and analyze the influence of the number of pinholes to the ability of focusing. The result indicates that the pinhole photon sieve has higher precision focusing ability than zone plates. This model is simple in theory and it has facility in operation.

A generalization of the cornu spiral for lossy media

Abstract: The classical problem of Fresnel diffraction by a straightedge is extended to allow for loss in the medium. The loss modifies the usual Cornu spiral solution because the argument of the Fresnel integral becomes complex. For high loss the contributions of the outer Fresnel zones are highly attenuated.

Spectral alternatives for the synthesis of short-pulse wavefields in waveguides

Abstract: For the synthesis of short-pulse 2D wavefields in simple waveguiding configurations, various alternative spectral field representations can be constructed. In the Spectral Theory of Transients (STT), a wavefield is expanded in terms of non-dispersive time-domain plane wave constituents. The STT furnishes an appropriate setting for the description of short-pulse fields within the Fresnel zone in a waveguide. In the far zone a ray-field expansion is not very effective, since the constituents cease to be well resolved in space and time. Therefore, a global field expansion in terms of a frequency spectrum of time-harmonic modes is more appropriate, but does not yield sufficient information about the time behavior of the field. Fortunately, the scope of the STT can be extended through the construction of an angular spectrum of global dispersive time-domain spectral-mode (TDSM) constituents, which exploits the advantages of both synthesis options. The field representation in terms of a spectral integral of a series of TDSM constituents proves to be equivalent to the representation in which the field is assembled with contributions from a periodic infinite array of image sources. The use of analytic signals allows for the complication of the space-time source coordinates in the resulting expressions for the field, thus yielding the 2D complex-source pulsed-beam (CSPB) solutions, associated with a class of directive sources. The asymptotic evaluation of the remaining spectral integral leads to an expression for the modal field in terms of instantaneous frequencies and instantaneous angles of propagation, through which a cogent description of the space-time-domain wave phenomena is obtained.