Fresnel zone

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

Investigation of Beam Width Shaping of a Ku-band Horn Antenna using a Diffractive Optic Element and an Electromagnetic Wave Absorber

Abstract: The feasibility of beam-shaping of a Ku-Band horn as a transmitting (Tx) antenna by mounting two different versions of lenses which are kind of Fresnel Zone Plates (FZP) is studied. The designed and fabricated geometrical structures offer a simpler approach building Fresnel Zone Plates by using an electromagnetic wave absorber and diffractive optic material. The diffractive optic material, paraffin, forms a set of alternating open and opaque annular zones on a flat surface, based on the design principles of Fresnel Zone Plates. An electromagnetic wave absorber covers the top surface of the formed paraffin, but not including the grooves. Thereafter, the Fresnel Zone plates are suitably attached in front of the transmitter horn antenna, located in the far-field region of a receiving antenna. The half-power beam-widths for the horn antenna (unloaded) and with two types of lenses are investigated. The results indicate that Fresnel zone plate structures can play a role suppressing side lobes in H-plane so that the effective radiation is to be significantly concentrated.

Liquid crystal devices based on photoalignment and photopatterning materials

Abstract: Liquid crystal (LC) display and photonics devices based on photo-alignment and photo-patterning LC cells are developed. A fast switchable grating based on ferroelectric liquid crystals and orthogonal planar alignment by means of photo alignments. Both 1D and 2D gratings have been constructed. The proposed diffracting element provides fast response time of around 20 μs, contrast of 7000:1 and high diffraction efficiency, at the electric field of 6V/μm. A switchable LC Fresnel zone lens was also developed with the efficiency of ~42% that can be further improved, and the switching time for the 3 μm thick cell is ~6.7 ms which is relatively fast in comparison of existing devices. Thus, because of the photoalignment technology the fabrication of Fresnel lens became considerably simpler than others. A thin high spatial resolution, photo-patterned micropolarizer array for complementary metal–oxide–semiconductor (CMOS) image sensors was implemented for the complete optical visualization of so called “invisible” objects, which are completely transparent (reflective) and colorless. Four Stokes parameters, which fully characterized the reflected light beam can be simultaneously detected using the array of photo-patterned polarizers on CMOS sensor plate. The cheap, high resolution photo-patterned LC matrix sensor was developed to be able successfully compete with the expensive and low reliable wire grid polarizer patterned arrays currently used for the purpose.

A boundary diffraction wave in the Fresnel-Kirchhoff theory

Abstract: A method for transformation of the Fresnel-Kirchhoff diffraction surface integral into an integral over the contour bounding this surface is proposed for an observation point in the Fresnel zone. An expression for the boundary wave field is obtained for a parametric representation of the contour. Examples of application of the obtained relations are given. Calculated dependences are compared to the experimental results.

Phase zone plates for hard X-ray focusing

Abstract: The major obstacle for constructing a deep UV or X-ray lens is the low refraction and high absorption of materials in this region. Only a few Fresnel zones are utilized, so the diameter of an X-ray lens is of the order few microns, and the resolution is hardly better than that of a pinhole. The thickness of a phase zone plates (ZP) is determined by the refraction of the material. To provide π phase shift and form a hard x-ray phase ZP a thickness of a few microns is required. Approaches and techniques developed at the Center for X-ray Lithography (CXrL) for producing thick phase ZPs are presented.

An Integral-Equation Approach to ELF Propagation in a Non-Stratified Earth-Ionosphere Waveguide.

Abstract: : This report develops and applies a practical method for analyzing long-wave propagation under conditions where the properties of the earth-ionosphere waveguide change markedly over lateral distances comparable to a wavelength or Fresnel zone. Full-wave solutions are used to describe both the vertical and horizontal dependences of the fields, but certain compromises are made to achieve tractability. The method is thus characterized as quasi-full-wave. Its main limitation is that the equation describing the vertical dependence is assumed to nearly decouple from the equation governing the lateral dependence. The method is valid at any frequency for which waveguide modes describe terrestrial propagation. Nonetheless, its practical utility is probably limited to ELF because the lateral properties of the earth-ionosphere waveguide are usually gradual enough to permit use of the WKB approximation at higher frequencies. To facilitate numerical solution, the lateral equation is transformed into an integral equation that accounts for most full-wave properties, including diffraction around a localized disturbance and reflection from lateral gradients. Numerical solutions based on model disturbances having lateral gradients in the direction of propagation reveal a standing wave pattern in front of the disturbance.