Experimental study of volume speckle in four-wave mixing arrangement

We study a high frequency modulated speckle pattern in a four-wave mixing arrangement. The three-dimensional nature of the speckles is considered in the phase conjugate reflectivity evaluation which implies to analyze its dependence on the average speckle size. Also, the reflectivity is analyzed in terms of the external applied field, the probe beam ratio, and the pupil aperture diameter of the imaging system that generates the subjective speckle beam

The scale of predictability

We introduce a new stylized fact: the hump-shaped behavior of slopes and coefficients of determination as a function of the aggregation horizon when running (forward/backward) predictive regressions of future excess market returns onto past economic uncertainty (as proxied by market variance, consumption variance, or economic policy uncertainty). To justify this finding formally, we propose a novel modeling framework in which predictability is specified as a property of low-frequency components of both excess market returns and economic uncertainty. We dub this property scale-specific predictability. We show that classical predictive systems imply restricted forms of scale-specific predictability. We conclude that for certain predictors, like economic uncertainty, the restrictions imposed by classical predictive systems may be excessively strong

High technology for aphasia rehabilitation

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Estudio de un conversor óptico fotorrefractivo y sus aplicaciones

Objetivos de la tesis: - estudiar el registro y reconstrucción de distribuciones unidimensionales (redes de Ronchi) de baja frecuencia, registradas incoherentemente en un conversor óptico fotorrefractivo, sin soporte holográfico y en configuración transversal. En particular, se pretende caracterizar el estado de polarización del haz de lectura producido por la modulación de birrefríngencia inducida en el medio. El interés, en este estudio, es controlar el contraste y así optimizar la imagen reconstruida a la salida del sistema. - implementar los efectos Talbot y Lau a partir de una red registrada en un cristal fotorrefractivo. Esto ha permitido visualizar estos efectos con características novedosas, propias de la naturaleza del registro y en esta modalidad efectuar un tratamiento unificado de los fenómenos analizados. A su vez, estas características han motivado la implementación fotorrefractiva de los interferómetros Talbot y Lau.Doctor en Físic

Modulated speckle simulations based on the random-walk model

The random-walk model is employed to simulate modulated speckle patterns. We demonstrate that the geometrical image approximation fails to describe the modulated speckle pattern. A new approach to analyzing this phenomenon is proposed. The validity of the approximations employed is verified by comparison of the simulation with the experimental results. Speckle metrological applications and phase measurement techniques could be improved by taking advantage of this model

Speckle photography with different pupils in a multiple-exposure scheme

The use of different multiple aperture pupils for recording each image in speckle photography is proposed. The introduction of suitable spatial frequency carriers, by internally modulating imaged speckles, allows one to selectively isolate or combine the spectral content of different images into spatially separated regions in the Fourier plane. Theoretical and experimental results extend the speckle photography technique to the depic tion of several specklegrams of multiple uniform in plane displacements. In this case, because different pu pils are considered for recording, the cross correlation functions for the amplitudes and intensities in the image plane are calculated on the basis of the statistical properties of the object. Also, the ensemble average inten sity in the Fourier plane is analytically derived, and fringe visibility is investigated

Phase-object analysis with a speckle interferometer

We describe the characteristics of a double-exposure specklegram obtained through a double-aperture system, by introduction of a wedge in front of one aperture in one exposure. It is assumed that a uniform displacement of the diffuser is produced between exposures. The average intensity distribution and visibility of the interferometric fringes in the Fourier plane are analyzed. An alternative interferometric technique for phase-object detection is proposed

Regional probabilities of precipitation change: A Bayesian analysis of multimodel simulations

Tebaldi et al. [2005] present a Bayesian approach to determining probability distribution functions (PDFs) of temperature change at regional scales, from the output of a multi-model ensemble, run under the same scenario of future anthropogenic emissions. The main characteristic of the method is the formalization of the two criteria of bias and convergence that the REA method [Giorgi and Mearns, 2002] first quantified as a way of assessing model reliability. Thus, the General Circulation Models (AOGCMs) of the ensemble are combined in a way that accounts for their performance with respect to current climate and a measure of each model's agreement with the majority of the ensemble. We apply the Bayesian model to a set of transient experiments under two SRES scenarios. We focus on predictions of precipitation change, for land regions of subcontinental size. We highlight differences in the PDFs of precipitation change derived in regions where models find easy agreement, and perform well in simulating present day precipitation, compared to regions where models have large biases, and/or their future projections disagree. We compare results from the two scenarios, thus assessing the consequences of the two alternative hypotheses, and present summaries based on their averaging