Fresnel zone

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

Theoretical Aspects of the Fresnel Focusing Technique

Abstract: Recent developments of real time echographic device have shown the interest of multi transducer focusing arrays. Large focusing aperture must be synthetised to obtain a good resolution. The use of delay lines, allowing a converging cylindrical lens to be simulated, is one of the classical approach of the focusing problem in the echographic mode. A continuous variation of the focal length of such a lens allows a good focusing on all the echographic range. However, if the dimension of the lens aperture is small enough, the sequential use of a discrete number of focal lengths is sufficient to cover the entire depth of field.

Fresnel zones and spatial resolution for P- and SH-waves in transversely isotropic media

Abstract: In an elastically anisotropic medium where the seismic wave velocity is a function of direction, the wavefront shape is nonspherical and, in most cases, assumes a nonelliptical shape. Numerical modelling techniques have been used to calculate the Fresnel‐zone diameter for compressional (P) and shear (SH) waves in transversely isotropic and isotropic media, respectively. The size of the Fresnel zone is found to be predominantly dependent on the curvatures and wavelength of the wavefront as well as the dip angle of the reflector. In addition, the anisotropic elastic parameters δ* (critical near‐vertical anisotropy), e (the P-wave anisotropy), and γ (the SH-wave anisotropy) are found to significantly affect the size of the Fresnel zone. Numerical modeling results show considerable differences between the Fresnel zones for anisotropic and isotropic velocity functions at various reflector dips. In addition, the Fresnel‐zone dimensions for anisotropic media exhibit asymmetry and considerable change with dip. By...

Expansions for irradiance distribution near the focus in systems of different Fresnel numbers

Abstract: It is shown in an earlier paper dealing with flat-topped light beams [Opt. Lett.27, 1007 (2002)] that the profile of flat-topped beams can be expressed in the form 1−[1−exp(−xi2)]M, where xi is a dimensionless parameter and M is a nonnegative number. The expansion of the proposed expression is a finite series containing only the lowest-order Gaussian modes. This situation provides the possibility of reformulating the scalar theory of diffraction at an aperture in an opaque screen if the Gaussian mode expansion is employed to describe the boundary values of the light incident on the screen. As an example of this effort, an asymptotic model is established for three-dimensional irradiance distributions near the focus in systems of different Fresnel numbers. The proposed expansions contain only elementary functions and permit all elementary operations; therefore no special functions or special algorithms are needed in the evaluation of either irradiance distributions or the integrated energy in a focused field.

On the radiation characteristics of Fresnel zone plate antenna

Abstract: This paper presents some numerical results of radiation patterns of the Fresnel zone plate antenna (FZPA) based on the vectoral Kirchhoff diffraction integral. The results of beamwidth, sidelobe level and efficiency vs zone type, focal length and feeder illumination are given as design criteria. The experimental data for a FZPA fed by a E-sectoral horn are provided for verifying the availability of the theoretical analysis.