Fresnel zone

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

Modeling the acoustic field of physiotherapy ultrasound transducers using non uniform acoustic pressure distributions

Abstract: In ultrasonic physiotherapy, the therapeutic effect is produced in the first 5 cm, into the Fresnel zone. Models are focused principally in the Fraunhofer zone which is too far from the therapeutic region. More realistic models are required to express the overlapping commonly produced near the transducer face. This paper presents the modeling of the acoustic field of physiotherapy ultrasonic transducers under non-uniform pressure distributions by using the finite element method. Theoretical approaches are compared with measurements to determine their accuracy. Although the nonuniform vibration models were presented years ago, their validation is still not complete. The results show the efficacy of modeling real transducers under free vibrating, simply supported and clamped conditions in relation to the overlapping in the Fresnel zone. In was concluded that the Fresnel zone is not correctly modeled with any of the approaches of this paper which were proposed in the literature.

Integration of Fresnel Zone Plates in the Bulk of Sapphire Crystal by Femtosecond Laser Pulses

Abstract: The goal of this study was to integrate Fresnel zone plates with diameters up to 300 μm in the bulk of sapphire crystal by direct laser writing technique. These zone plates are intended to be integrated directly into GaN LED substrate because of their potential to increase the light extraction efficiency of UV LED devices. For integration task a femtosecond (320 fs) high repetition rate (100 kHz) Yb:KGW laser system was used together with high precision linear positioning stages. By exposing sapphire to the focused femtosecond laser radiation, regions of modified refractive index were induced and required zone plates were patterned. The discussions about optimal fabrication parameters as well as focusing properties of fabricated zone plates are presented. It is shown that sapphire can be a suitable material for microphotonic device integration: working zone plates with diameters up to 300 μm and diffraction efficiencies up to 23% were integrated below (80 μm) the sapphire surface.

Analysis of square Fresnel Zone Plate Lens for dual band detectors

Abstract: Dust emission is used in space applications for star-formation characterization normally acquired by different detectors operating at Infrared (IR) and sub-millimeter wave (sub-mm wave) frequencies. An integrated dual band detector working at both frequency ranges is described in this paper. The proposed configuration implements a spiral antenna working at sub-millimeter wave frequencies whose design is based on Fresnel zones theory. At the same time, this spiral antenna is used as Fresnel lens to focus IR power (12 μm) into an IR detector. The paper describes the influence of implementing a sub-mm wave spiral antenna based on square Fresnel zones. In particular, the effects on the focal distance and the focal gain at IR frequencies and the radiation features at sub-mm wave frequencies are studied. Furthermore, the importance of the number of Fresnel zones used in the design of the sub-mm spiral antenna is also determined.