Fresnel zone

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

Refractive and diffractive neutron optics with reduced chromatic aberration

Abstract: Thermal neutron beams are an indispensable tool in physics research. The spatial and the temporal resolution attainable in experiments are dependent on the flux and collimation of the neutron beam which remain relatively poor, even for modern neutron sources. These difficulties may be mitigated by the use of optics for focusing and imaging. Refractive and diffractive optical elements, e.g. compound refractive lenses and Fresnel zone plates, are attractive due to their low cost, and simple alignment. These optical elements, however, suffer from chromatic aberration, which limit their effectiveness to highly monochromatic beams. This paper presents two novel concepts for focusing and imaging non-monochromatic thermal neutron beams with well-known optical elements: (1) a fast mechanical transfocator based on a compound refractive lens, which actively varies the number of individual lenses in the beam path to focus and image a time-of-flight beam, and (2) a passive optical element consisting of a compound refractive lens, and a Fresnel zone plate, which may focus and image both continuous and pulsed neutron beams. (C) 2014 Elsevier B.V. All rights reserve (Less)

Diffraction patterns in the shadows of disks and obstacles.

Abstract: We compare the Fresnel diffraction pattern of a thin circular disk with that of a square obstacle, specifically evaluating the on-axis field strength. Photographs of the diffraction patterns reveal some curious features for the square obstacle. Second, the precise electric and magnetic fields behind a conducting circular disk are evaluated without invoking the Fresnel approximation and contrasted with the rigorous electromagnetic result for a metal sphere. The calculations show that the two cases differ only slightly in the Fresnel region. In the near-field new computational results for the sphere are analyzed.

Enhancement of modal feedback in unstable resonators using mirrors with a phase step.

Abstract: Numerical calculations using the Prony method have shown that the feedback coefficient in unstable laser resonators can be increased by factors of as much as 5 over the geometrical value using a mirror with a phase step at its center. A phase shift of close to π over an area of an equivalent Fresnel number of 0.5 leads to minimum output losses. Experiments with a TEA CO2 laser confirm the prediction. The results can be attributed to a cancellation of the output wave by destructive interference, which confines the laser beam around the resonator axis.

Focussing optics for a synchrotron-based X-ray microprobe

Abstract: X-ray microprobe experiments at third-generation synchrotron sources will provide trace element analysis of samples with 1 μm × 1 μm spatial resolution and femtogram sensitivity. For these experiments to be possible. X-ray optical elements need to be developed to focus the beam from a hard X-ray undulator to a micron spot size. In June 1991 several different optical elements were tested during a dedicated undulator run at CHESS. The undulator produced radiation similar to that which will be available at third-generation sources like the Advanced Photon Source. Both Fresnel zone plates and multilayer-coated spherical mirrors were tested. With the Fresnel zone plate a spot size of 8.5 μm × 30 μm was achieved in the first order and 6 μm × 20 μm in the second order. With a Kirkpatrick-Baez multilayer mirror system a spot size of 4 μm × 9 μm was achieved. Based on these results, some of the requirements for an optical system suitable for a dedicated microprobe beamline are given.