Fresnel zone

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

Focusing high energy X‐rays with stacked Fresnel zone plates

Abstract: Stacking technique was developed in order to increase focusing efficiency of Fresnel zone plates at high energies. Two identical Si chips each of which containing Fresnel zone plates were used for stacking. Alignment of the chips was achieved by on-line observation of the moire pattern from the two zone plates. The formation of moire patterns was studied theoretically and experimentally at different experimental conditions. To provide the desired stability Si-chips with zone plates were bonded together with slow solidification speed epoxy glue. Technique of angular alignment in order to compensate a linear displacement in the process of gluing was proposed. Two sets of stacked FZPs were produced and experimentally tested to focus 15 and 50 keV X-rays. Gain in the efficiency by factor 2.5 was demonstrated at 15 keV. Focal spot of 1.8 µm vertically and 14 µm horizontally with 35% efficiency was measured at 50 keV. Forecast for the stacking of nanofocusing Fresnel zone plates was discussed.

X-ray submicrometer phase contrast imaging with a Fresnel zone plate and a two dimensional grating interferometer

Abstract: The application of a two dimensional (2D) grating interferometer–Fresnel zone plate combination for quantitative submicron phase contrast imaging is reported. The combination of the two optical elements allows quick recovery of the phase shift introduced by a sample in a hard X-ray beam, avoiding artifacts observed when using the one dimensional (1D) interferometer for a sample with features oriented in the unsensitive direction of the interferometer. The setup provides submicron resolution due to the optics magnification ratio and a fine sensitivity in both transverse orientations due to the 2D analysis gratings. The method opens up possibilities for sub-micro phase contrast tomography of microscopic objects made of light and/or homogeneous materials with randomly oriented features.

Frequency and spatial correlation functions in a fading communication channel through the ionosphere

Abstract: Equations for the two-frequency two-position mutual coherence functions are derived under the usual parabolic and Markov approximations. These equations are then solved numerically. It is shown that the mutual coherence functions occur naturally in the study of pulse distortion through a random communication channel and in the investigation of signal correlations. Contour plots of correlation functions show the possibility of having equal values at two frequency separations for a given spatial separation. This behavior is explainable in terms of overlapping Fresnel zones. Recently it has been observed that radio signals from communication satellites at a frequency as high as several GHz may experience the scintillation phenomenon when received on the ground [Pope and Fritz, 1971; Skinner et al., 1971; Sessions, 1972; _ Craft and Westerlund, 1972; Taur, 1973]. This came as a surprise since scintillation was not anticipated to occur at such a high frequency. After some experimentation it is now believed that the scintillation is caused by the electron density irregularities in the ionosphere. As is well known, for a given electron density fluctuation, th e rms fluctuation in the refractive index of the ionospheric medium is inversely proportional to the square of the signal frequency. Therefore, under conditions when GHz scintillation occurs, signals with a lower frequency will experience even more severe fading. This implies that the effects of multiple scattering on signal statistics are very important under such conditions. To obtain their statistics correctly, it is desirable to develop a scintillation theory that takes into account the multiple scattering effects. Thanks to the recent advances in the theory of wave propagation in random media, it is now possible to develop such a theory [Liu et al., 1974a; Yeh et al., 1975]. In this paper, we shall apply a similar technique to investigate the frequency and spatial correlations of signals passing through the irregularity slab. In satellite communications the ability to transmit wideband data is limited primarily by the inter-symbol interference which is closely

In-depth resolution from multifrequency Born fields scattered by a dielectric strip in the Fresnel zone.

Abstract: The achievable depth resolution in reconstructing the permittivity profile of a dielectric strip under the Born approximation when data are collected in the Fresnel zone is studied. We consider a rectilinear measurement aperture and an orthogonal and centered rectilinear investigation domain. The roles of the aperture extent and frequency diversity are highlighted.

A Systematic Study of Varying Reference Phase in the Design of Circular Fresnel Zone Plate Antennas

Abstract: A circular Fresnel zone plate antenna was simulated using a finite difference time domain software to study the effect of changing the reference phase from the standard 0deg to any value between 0deg and 180deg. Several prototypes were built to validate the simulations using four different focal-length-to-diameter (F/D) ratios. It was found that a significant improvement in the level of the first sidelobe, up to about 9 dB according to the simulations, could be achieved without sacrificing gain or cross-polarization levels