Link between the laws of geometrical optics and the radiative transfer equation in media with a spatially varying refractive index

We proposed in a previous paper [Opt. Commun. 228, 33 (2003)] a modified radiative transfer equation to describe radiative transfer in a medium with a spatially varying refractive index. The present paper is devoted to the demonstration that this equation perfectly works in the non-absorbing / non-scattering limit, what was contested by L. Mart\'i-L\'opez and coworkers [Opt. Commun. 266, 44 (2006)]. The assertion that this equation would imply a zero divergence of the rays is also commented.Comment: 14 pages, 3 figure

Time-resolved measurements from speckle interferometry

We present time-resolved measurements by speckle interferometry of the light scattered by a liquid medium. Measurements were performed by use of reflectance geometry and are compared with results obtained in the same conditions with a femtosecond laser and a streak camera. The setup was also tested in vivo on the forearm of a human volunteer to demonstrate the potential utility of such a setup for biomedical applications

Mesures résolues en temps de transit de la lumière diffuse avec une caméra CCD

Ma thèse s'inscrit dans la continuité de recherches développées au sein de l'équipe OPTIMA du Laboratoire de Physique des Lasers concernant une nouvelle méthode interférométrique de mesure résolue en temps de la lumière diffuse pour des applications biomédicales. Mon objectif était d'étudier de nouvelles solutions technologiques pour améliorer les performances de cette méthode et pouvoir envisager à terme une expérimentation clinique. Le premier point sur lequel nous nous sommes concentrés est la source lumineuse, qui doit être modulée spectralement à une fréquence relativement élevée. Nous avons ainsi montré que les diodes laser à réseau de Bragg intégré permettent d'atteindre les fréquences de modulation nécessaires aux applications biomédicales de cette méthode, et qu'il n'y a donc pas ici de limitation pour la méthode interférométrique. Le deuxième point étudié, qui constitue le coeur de mon travail de thèse, concerne le système de détection et la possibilité d'augmenter le rapport signal sur bruit en multipliant le nombre de détecteurs. L'idée consiste à travailler avec des caméras CCD au lieu des simples photodiodes utilisées précédemment, les pixels de ces caméras étant autant de détecteurs : de par le très grand nombre de pixels d'une caméra CCD, on peut ainsi s'attendre à une amélioration considérable du rapport signal sur bruit. Avec l'utilisation d'une caméra rapide et d'une caméra standard, ce travail a ainsi permis de montrer la pertinence et les limites de cette technique.My thesis is a continuation of the research developed within the OPTIMA team of the "Laboratoire de Physique des Lasers". It concerns a new interferometric method to perform time-resolved measurement of diffuse light for biomedical applications. My goal was to explore new technologies in order to improve the performance of this method and to envision futures clinical trials. The first point which we consider is the light source, which must be spectrally modulated at a relatively high frequency. We showed that laser diodes with integrated Bragg grating can achieve the frequency modulation required for biomedical applications, so that there is no limitation here for the interferometric method. The second point, which constitutes the core of this work, relates to the detection system and the possibility to increase the signal to noise ratio by multiplying the number of detectors. The idea is to work with CCD cameras instead of the simple photodiodes previously used, the pixels of these cameras being as many detectors : from the very large number of pixels of a CCD camera, we can then expect a considerable improvement of the signal to noise ratio. With the use of both a fast camera and a standard camera, this work has demonstrated the relevance and limitations of this technique.PARIS13-BU Sciences (930792102) / SudocSudocFranceF

Scattering spot imaging for the determination of optical and dynamical properties of viscoelastic media

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Sharp eyes for medical imaging

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Reconstruction of bi-dimensional images in Fourier-Transform Acousto-Optic Imaging

International audienceWe present a new method to perform acousto-optic imaging based on a spatio-temporal structuration of long-duration acoustic plane waves. This approach is particularly relevant when using detectors with long integration time. We show how it is possible to reconstruct an image by measuring its two-dimensional Fourier components. A proof of concept is presented using a photorefractive detection scheme, demonstrating equal performances to direct imaging. The overall acquisition time is compatible with medical monitoring applications

Suppression of the Talbot effect in Fourier transform acousto-optic imaging

We report on the observation and correction of an imaging artifact attributed to the Talbot effect in the context of acousto-optic imaging using structured acoustic waves. When ultrasound waves are emitted with a periodic structure, the Talbot effect produces π -phase shifts of that periodic structure at every half of the Talbot distance in propagation. This unwanted artifact is detrimental to the image reconstruction, which assumes near-field diffraction is negligible. Here, we demonstrate both theoretically and experimentally how imposing an additional phase modulation on the acoustic periodic structure induces a symmetry constraint leading to the annihilation of the Talbot effect. This will significantly improve the acousto-optic image reconstruction quality and allows for an improvement of the reachable spatial resolution of the image

A New Insight into Biospeckle Activity in Apple Tissues

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