Measurement of Thermo-Elastic Deformation of an Optic using a Polarization Based Shearing Interferometer

A shearing interferometer is presented which uses polarization control to shear the wavefront and to modulate the interference pattern. The shear is generated by spatial walk-off in a birefringent crystal. By adjusting the orientation of the birefringent crystal, the components of the wavefront gradient can be independently measured to allow determination of the full wavefront vector gradient as well as reconstruction of the wavefront. Further, the monolithic nature of the crystal used for shearing allows the interferometer to be setup without need for precise alignment of any components. An algorithm incorporating homodyne detection is presented which analyzes the modulated interferograms to determine the components of the wavefront gradient, from which the wavefront is reconstructed. The thermal deformation of a mirror subject to heating from absorption of a Gaussian pump beam was accurately observed with a sensitivity better than \lambda/160. We show that this sensitivity is scale invariant, and present a method to account for the non-uniform spatial frequency response of the interferometer

Al-Badîl, ou L’alternative. Récit d’une expérience à la croisée entre journalisme et engagement militant

Le processus de démonopolisation de l’espace médiatique égyptien en cours depuis la fin des années 1990 se caractérise, entre autres, par l’émergence d’une presse privée qui se dit indépendante à la fois de l’Etat et du champ politique traditionnel. Bien que son impact sur l’espace public national ait été vite perçu par nombre d’observateurs, ce phénomène n’a pas fait l’objet d’études particulières. Cet article se propose de retracer l’histoire d’un journal privé ‘indépendant’ qui paraît sur la scène médiatique en juillet 2007 : le quotidien Al-Badîl (L’Alternative). Saisie dans une perspective diachronique, et par les acteurs individuels et collectifs dont elle est à chaque fois le produit, l’expérience d’Al-Badîl révèle une profonde imbrication entre les univers du journalisme et de l’engagement militant. Le journal est un lieu de confrontation symbolique entre deux manières différentes de concevoir le journalisme et l’engagement militant et un espace où s’opère le passage d’une génération intellectuelle et militante à une autre.The Egyptian media system has been affected by a clear process of de-monopolization since the late 1990’s. This led to the appearance, among other things, of a new private press that claimed to be independent from both the State and traditional political field. Although its impact on the national public sphere was quickly perceived as relevant by many observers, this phenomenon in itself has not been studied. This article seeks to trace the history of an ‘independent’ private newspaper that first appeared on the media scene in July 2007: the daily paper Al-Badîl (The Alternative). Studied in a diachronic perspective and through its individual and collective actors, Al-Badîl’s ‘experiment’ reveals a deep overlap between the worlds of journalism and political activism. The newspaper is a space of symbolic confrontation between two different ways of perceiving journalism and political activism and the site of a switch process from an intellectual and political generation to another

Quantitative two-dimensional shadowgraphic set-up for high-sensitivity measurement of low-density laser-plasmas

We present a quantitative shadowgraphic method which can measure the density of a laser-generated plasma in air with sensitivity and resolution comparable or better than traditional interferometric techniques. Simultaneous comparison of both shadowgraphy and interferometry has been carried out allowing the experimental evaluation of the reliability of the shadowgraphic method

Statistical mechanics approach to the phase unwrapping problem

The use of Mean-Field theory to unwrap principal phase patterns has been recently proposed. In this paper we generalize the Mean-Field approach to process phase patterns with arbitrary degree of undersampling. The phase unwrapping problem is formulated as that of finding the ground state of a locally constrained, finite size, spin-L Ising model under a non-uniform magnetic field. The optimization problem is solved by the Mean-Field Annealing technique. Synthetic experiments show the effectiveness of the proposed algorithm

Efficient Phase Unwrapping Architecture for Digital Holographic Microscopy

This paper presents a novel phase unwrapping architecture for accelerating the computational speed of digital holographic microscopy (DHM). A fast Fourier transform (FFT) based phase unwrapping algorithm providing a minimum squared error solution is adopted for hardware implementation because of its simplicity and robustness to noise. The proposed architecture is realized in a pipeline fashion to maximize throughput of the computation. Moreover, the number of hardware multipliers and dividers are minimized to reduce the hardware costs. The proposed architecture is used as a custom user logic in a system on programmable chip (SOPC) for physical performance measurement. Experimental results reveal that the proposed architecture is effective for expediting the computational speed while consuming low hardware resources for designing an embedded DHM system

Spatially resolved stress measurements in materials with polarization-sensitive optical coherence tomography: image acquisition and processing aspects

We demonstrate that polarization-sensitive optical coherence tomography (PS-OCT) is suitable to map the stress distribution within materials in a contactless and non-destructive way. In contrast to transmission photoelasticity measurements the samples do not have to be transparent but can be of scattering nature. Denoising and analysis of fringe patterns in single PS-OCT retardation images are demonstrated to deliver the basis for a quantitative whole-field evaluation of the internal stress state of samples under investigation.Comment: 10 pages, 6 figures; Copyright: Blackwell Publishing Ltd 2008; The definitive version is available at: www.blackwell-synergy.co

Materials for phantoms for terahertz pulsed imaging

Phantoms are commonly used in medical imaging for quality assurance, calibration, research and teaching. They may include test patterns or simulations of organs, but in either case a tissue substitute medium is an important component of the phantom. The aim of this work was to identify materials suitable for use as tissue substitutes for the relatively new medical imaging modality terahertz pulsed imaging. Samples of different concentrations of the candidate materials TX151 and napthol green dye were prepared, and measurements made of the frequency-dependent absorption coefficient (0.5 to 1.5 THz) and refractive index (0.5 to 1.0 THz). These results were compared qualitatively with measurements made in a similar way on samples of excised human tissue (skin, adipose tissue and striated muscle). Both materials would be suitable for phantoms where the dominant mechanism to be simulated is absorption (similar to ∼100 cm(-1) at 1 THz) and where simulation of the strength of reflections from boundaries is not important; for example, test patterns for spatial resolution measurements. Only TX151 had a frequency-dependent refractive index close to that of tissue, and could therefore be used to simulate the layered structure of skin, the complexity of microvasculature or to investigate frequency-dependent interference effects that have been noted in terahertz images

Zero and First-Order Phase Shift Correction for Field Map Estimation with Dual-Echo GRE Using Bipolar Gradients

A simple phase error correction technique used for field map estimation with a generally available dual-echo gradient-echo (GRE) sequence is presented. Magnetic field inhomogeneity maps estimated using two separate GRE volume acquisitions at different echo times are prone to dynamic motion errors between acquisitions. By using the dual-echo sequence, the data are collected during two back-to-back readout gradients in opposite polarity after a single radio frequency pulse, and interecho motion artifacts and alignment errors in field map estimation can be factored out. Residual phase error from the asymmetric readout pulses is modeled as an affine term in the readout direction. Results from phantom and human data suggest that the first-order phase correction term stays constant over time and, hence, can be applied to different data acquired with the same protocol over time. The zero-order phase correction term may change with time and is estimated empirically for different scans.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85843/1/Fessler31.pd

On-Line Metrology with Conoscopic Holography: Beyond Triangulation

On-line non-contact surface inspection with high precision is still an open problem. Laser triangulation techniques are the most common solution for this kind of systems, but there exist fundamental limitations to their applicability when high precisions, long standoffs or large apertures are needed, and when there are difficult operating conditions. Other methods are, in general, not applicable in hostile environments or inadequate for on-line measurement. In this paper we review the latest research in Conoscopic Holography, an interferometric technique that has been applied successfully in this kind of applications, ranging from submicrometric roughness measurements, to long standoff sensors for surface defect detection in steel at high temperatures