Fresnel zone

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

Temporal-coherence effects of a moving phase screen illuminated by spatially coherent quasi-monochromatic light

Abstract: We consider how the coherence of the observed field is affected by propagation when a nonabsorbing phase screen is moved at constant velocity in front of an aperture illuminated by quasi-monochromatic plane-wave illumination. It is shown that when the rms value of the profile is not too large, there is always a component of transmitted light whose temporal coherence is unaffected by the motion of the phase screen. Closed-form expressions for the temporal coherence of the observed field are obtained for the following cases: (1) a point aperture, (2) the Fraunhofer region, and (3) limiting cases of Fresnel diffraction for relatively large- and small-grain scatterers. Numerical results are presented.

Focusing of an elliptic vortex beam by a square Fresnel zone plate

Abstract: We propose a new (to our knowledge) method for performing beam shaping. Calculations show that beam patterns including diamond, rhombus, and hollow beam with diamond-shaped dark core can be generated in the focal plane of a square Fresnel zone plate by choosing appropriate parameters of an incident elliptic vortex beam. Numerical simulations have demonstrated the validity and effectiveness of this method.

Fresnel zone lenses for an electro-optical 2 x 2 switch

Abstract: An optical two by two exchange bypass module formed by a combination of static and dynamic Fresnel zone lenses is proposed. The basic concept of this switching element is to cascade pairs of dynamic Fresnel zone lenses and pairs of static Fresnel zone lenses, located on each side of a 1.5 mm thick quartz-glass substrate. Two types of elements were designed, to operate at wavelengths of 1.52 micrometers and 0.633 micrometers . The dynamic Fresnel zone lenses on the frontside of the substrate are off-axis lenses which are made switchable by filling them with a liquid crystal after covering the Fresnel structure with a transparent electrode. The second electrode is deposited on a face down mounted cover-glass. The static lenses on the backside of the substrate are conventional on-axis Fresnel zone lenses. The dynamic and the static Fresnel zone lenses, whose blazed profiles have been approximated by multi-levels, have been fabricated by means of micro-structuring techniques. Static and dynamic, on-axis and off-axis Fresnel zone lenses with different focal lengths and of different circular diameters ranging from 0.2 mm to 2.0 mm have been realized. The measured spot-sizes of the Fresnel zone lenses were close to eighth diffraction limited values and the diffraction efficiency of the eight-level lenses was higher than 80%. The switching times of the dynamic lenses were in the range of several milli-seconds and the switching contrast was about 1:10.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Metal layer Bragg–Fresnel lenses for diffraction focusing of hard x-rays

Abstract: A thin-film Bragg–Fresnel lens (BFL) was developed for diffractive focusing of hard x-rays into submicron to nanometer spots for scanning x-ray spectromicroscopy. The lens is made of metal-layer Fresnel zones deposited on an x-ray reflective substrate. The use of a high-density lens structure reduces the thickness of the lens and simplifies the fabrication process. Linear and elliptical lenses made of a 200-nm-thick Au film were fabricated using e-beam lithography and a metal deposition process. The focusing capabilities of the Au layer BFLs were demonstrated at the Advanced Photon Source.

Temporal effects in ultrashort pulsed beams focused by planar diffracting elements

Abstract: The pulse envelope of an ultrashort pulsed beam is evaluated on the focal points of a Fresnel zone plate. The description of the field dynamics is given in terms of a diffraction-induced pulse train. Within these terms we follow an analytical procedure to characterize the temporal broadening observed at the principal focus, which is significant if the number of Fresnel zones exceeds the number of cycles in the pulse. For Gaussian-type envelopes, the focal field may be accurately expressed in a simple closed form. This expression has a flat-top shape at the principal focus and other odd-order foci, and a two-peak envelope in the case of a low-integer even-order focus. Finally, extremely high orders present a time-domain evolution that emulates a train of uniform pulses with temporal characteristics equivalent to those of the incident beam.