Fresnel zone

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

Nanometer-scale ablation with a table-top soft x-ray laser

Abstract: Ablation of holes with diameters as small as 82 nm and very clean walls was obtained in poly(methyl methacrylate) focusing pulses from a Ne-like Ar 46.9 nm compact capillary-discharge laser with a freestanding Fresnel zone plate diffracting into third order. These results demonstrate the feasibility of using focused soft x-ray laser beams for the direct nanoscale patterning of materials and the development of new nanoprobes.

Determination of fresnel zones from traveltime measurements

Abstract: For a horizontally stratified (isotropic) earth, the rms‐velocity of a primary reflection is a key parameter for common‐midpoint (CMP) stacking, interval‐velocity computation (by the Dix formula) and true‐amplitude processing (geometrical‐spreading compensation). As shown here, it is also a very desirable parameter to determine the Fresnel zone on the reflector from which the primary zero‐offset reflection results. Hence, the rms‐velocity can contribute to evaluating the resolution of the primary reflection. The situation that applies to a horizontally stratified earth model can be generalized to three‐dimensional (3-D) layered laterally inhomogeneous media. The theory by which Fresnel zones for zero‐offset primary reflections can then be determined purely from a traveltime analysis—without knowing the overburden above the considered reflector—is presented. The concept of a projected Fresnel zone is introduced and a simple method of its construction for zero‐offset primary reflections is described. The pr...

The Accuracy of the Born and Ray Approximations in Time-Distance Helioseismology

Abstract: Time-distance helioseismology measures the time for acoustic wave packets to travel, through the solar interior, from one location on the solar surface to another. Interpretation of travel times requires an understanding of their dependence on subsurface inhomogeneities. Traditionally, time-distance measurements have been modeled in the ray approximation. Recent efforts have focused on the Born approximation, which includes finite-wavelength effects. In order to understand the limitations and ranges of validity of the ray and Born approximations, we apply them to a simple problem—adiabatic acoustic waves in a uniform medium with a spherical inclusion—for which a numerical solution to the wave equation is computationally feasible. We show that, for perturbations with length scales large compared to the size of the first Fresnel zone, both the Born and first-order ray approximations yield the same result and that the fractional error in the travel time shift, computed by either approximation, is proportional to the fractional strength of the sound speed perturbation. Furthermore, we demonstrate that for perturbations with length scales smaller than the first Fresnel zone the ray approximation can substantially overestimate travel time perturbations while the Born approximation gives the correct order of magnitude. The main cause of the inaccuracy of the Born approximation travel times is the appearance of a diffracted wave. This wave, however, has not yet been observed in the solar data.

Traveltimes of waves in three-dimensional random media

Abstract: SUMMARY We present the results of a comprehensive numerical study of 3-D acoustic wave propagation in weakly heterogeneous random media. Finite-frequency traveltimes are measured by cross- correlation of a large suite of synthetic seismograms with the analytical pulse shape representing the response of the background homogeneous medium. The resulting 'ground-truth' traveltimes are systematically compared with the predictions of linearized ray theory and 3-D Born-Frechet (banana-doughnut) kernel theory. Ray-theoretical traveltimes can deviate markedly from the measured cross-correlation traveltimes whenever the characteristic scalelength of the 3-D heterogeneity is shorter than half of the maximum Fresnel zone width along the ray path, i.e. whenever a < 0.5(λL) 1/2 , where a is the heterogeneity correlation distance, λ is the dominant wavelength of the probing wave, and L is the propagation distance. Banana-doughnut theory has a considerably larger range of validity, at least down to a ≈ 0.1(λL) 1/2 in sufficiently weakly heterogeneous media, because it accounts explicitly for diffractive wave front healing and other finite-frequency wave propagation effects. In this paper, we conduct a comprehensive numerical investiga- tion of the validity of both linearized ray theory and 3-D Born- Frechet kernel theory in weakly heterogeneous random media. We use a pseudospectral method to solve the 3-D acoustic wave equation in a suite of Gaussian and exponentially correlated random media, characterized by their root-mean-square slowness variation and their correlation scalelength. Finite-frequency traveltime shifts are mea- sured at a variety of source-receiver distances, by cross-correlation of the numerically computed synthetic seismograms with the cor- responding analytical response of the background homogeneous medium. This study extends the analyses of Nolet & Dahlen (2000) and Hung et al. (2001), who investigated the wave front healing effects downstream of an isolated, slow or fast, spherically sym- metric slowness anomaly using the parabolic approximation and the pseudospectral method, respectively. In a statistically homoge- neous random medium, the diffractive healing of wave front corru- gations produced by near-source slowness anomalies is continually being augmented by new corrugations produced by more distant anomalies, as the wave propagates away from the source. Because of this, a ray-theoretical skeptic might argue that conclusions based upon the study of a 'lonely bowling ball' are not pertinent to seis- mic wave propagation in the Earth's mantle. Spetzler & Snieder's (2001b) study of picked traveltimes of plane waves in 2-D random media shows that, for their case, Rytov scattering theory predicts traveltimes more accurately than ray theory. Similarly, we use our 'ground-truth' numerical results to place empirical constraints upon the validity of both linearized ray theory and 3-D Born-Frechet ker- nel theory in spatially extended 3-D random heterogeneous media.

3-D Printed Circularly Polarized Modified Fresnel Lens Operating at Terahertz Frequencies

Abstract: In this paper, a novel 3-D printed circularly polarized (CP) modified Fresnel lens antenna operating at 300 GHz is introduced By virtue of the superior geometric flexibility of the 3-D printing technique, a modified Fresnel lens consisting of subwavelength discrete dielectric posts in the odd-numbered Fresnel zones is proposed It is further demonstrated that by integrating dielectric anisotropic metamaterial, the modified Fresnel lens can realize CP radiation fed by a simple linearly polarized (LP) open-ended waveguide (OEWG) The 3-D printing approach is also investigated to push the performance envelops of the 3-D printer for realization of the terahertz CP lens The measured results show that the axial ratios (ARs) of the fabricated antenna prototype are smaller than 3 dB from 265 to 320 GHz Moreover, the modified Fresnel lens has a maximum gain of 274 dBic, which is 09 dB larger than that of the conventional Fresnel zone plane antenna (FZPA) These validate the concept, the design, and the fabricated prototype