Les travaux de la présente thèse concernent l’étude théorique et la réalisation d’un Capteur réparti à Fibres Optiques fondé sur la réflectométrie fréquentielle et la rétrodiffusion de Rayleigh. La genèse de cette méthode remonte aux années 80 pour les mesures distribuées. Il y a un peu plus d’une dizaine d’années qu’elle a été mise en œuvre secrètement pour les meures réparties (continument sensibles) par la société américaine « Luna Innovations ». Cette approche permettant de réaliser des profils continus le long de la fibre des paramètres environnants (température et déformation principalement) offre une très bonne résolution spatiale ainsi qu’une résolution de mesure fine et présente donc un grand intérêt pour de nombreuses applications, venant ainsi en complément des techniques reposant sur la diffusion Raman ou Brillouin. L'aspect théorique de cette méthode dite OFDR Rayleigh a été analysé. Pour effectuer une mesure profilométrique il est nécessaire de déterminer simultanément deux informations : la distance physique allant de l’entrée de la fibre jusqu'au point où un paramètre physique à mesurer a été appliqué, et le paramètre physique proprement dit. Pour réaliser cette mesure un lien entre l'influence du paramètre d’influence et le changement d'indice du cœur de la fibre sous test doit être exploité. Il s’agit donc des sensibilités de mesure, mais aussi de l’influence de l’environnement sur le chemin optique parcouru, et donc sur la localisation précise des points de mesure. Ces différents aspects ont donc été analysés et rapportés dans le présent mémoire. Pour mettre en œuvre cette technique fréquentielle, et en particulier son traitement de signal, relativement complexe, deux méthodes différentes ont été proposées : la première dite « corrélationnelle » (mise en œuvre par « Luna Innovations » mais très peu documentée) et la méthode « tensorielle » proposée uniquement dans le cadre de la présente étude. Etant fondée sur les mesures répétitives des décalages spectraux induits par le paramètre physique à mesurer le long de la fibre, la méthode corrélationnelle est plus universelle, s’adressant à tous types de paramètres d’influence. Pourtant, elle s’avère moins précise du fait d'une erreur systématique dans le calcul des décalages spectraux. La méthode tensorielle, quant à elle, fournit un résultat plus précis, cependant elle n'est applicable que pour les mesures de déformations, puisque reposant sur l'effet élasto-optique dans le cœur de la fibre. Une procédure de traitement de signal a ensuite été élaborée pour les deux méthodes précitées, la corrélationnelle et la tensorielle. Les performances capacités de l'appareil dont nous disposons, Luna OBR 4600, ont également été montrées, tant en ce qui concerne les mesures de température que de déformations. Un banc expérimental mettant en œuvre cette méthode de mesure OFDR-Rayleigh a également été conçu est mis au point au laboratoire. Un logiciel de traitement des données a de fait été élaboré est réalisé. Ce programme a été testé à l'aide d'une simulation et grâce à l’utilisation des données brutes délivrées par l’appareil Luna OBR 4600. Plusieurs tests du programme de traitement des données brutes de cet instrument ont été réalisés. Les résultats de ces tests peuvent être considérés plutôt comme satisfaisants, étant globalement en bon accord avec les résultats fournis directement par l'instrument commercial OBR 4600. Au-delà, de nombreux exemples d'applications innovantes de la méthode Rayleigh OFDR ont été présentés et illustrés. Parmi ceux-ci, citons, le système de détection de fuites de sodium liquide pour la surveillance de fonctionnement des circuits de refroidissement des réacteurs de génération IV que nous avons élaboré, grâce à cette méthode Rayleigh OFDR, et un prototype industriel d'un tel système proposé. Enfin, l’analyse des points pouvant faire l’objet d’améliorations et/ou de marges de progrès a été menée, et est rapportée en fin de mémoire. / The present PhD thesis have been carried out at the Laboratoire de Mesures Optiques (Optical Measurement Laboratory) based at CEA Saclay Nuclear Research Center, France. It concerns theoretical analysis and experimental realizations of a “distributed Optical Fiber Sensor” based on Optical Frequency Domain Reflectometry using the Rayleigh backscattering phenomena. As the OFS community know, such technology has been developed by the US company Luna Innovations Inc., which keeps carefully secret any details of both the technology involved and the signal processing of such instruments, named OBR (Optical Backscattered Reflectometer). Advantageously, the present Rayleigh-OFDR method provides a very high spatial resolution (in comparison with other methods based on reflectometry, i.e. OTDR, DTS Raman, Brillouin BOTDR and BOTDA…) and so remains extremely sought for many applications, especially for Structural Health Monitoring. In the present PhD thesis the theoretical aspects of the Rayleigh OFDR method has been presented. As a reminder, in order to measure a physical parameter profile along the fiber, it is required to determine two subjects simultaneously: the distance at which the physical parameter is applied (i.e. the sensor address) to the fiber under test, and the amplitude of this parameter (i.e. “the measurement” itself). A relation between the influence of any environment parameter (temperature, strain, etc.) and the fiber core refractive index changes has to be used. These relations have been shown, as far as the hypothesis linked to the theoretical aspects. Two different methods for such measurement have been proposed: first of all the “correlation method” (used by Luna) and second “a tensor-based method”, fully innovative as developed during the present PhD thesis. The correlation method may be considered as more “universal” as based on the measurement of spectral shifts caused by the physical parameter to measure. But, in the same time, we consider it is less accurate, as a systematic error may somehow fail the spectral shift calculation algorithm. The tensor method do not suffer of such systematic error, but it remains only applicable to strain sensing, as based on the elasto-optic effect. Signal processing procedure has been elaborated and tested for both methods, and of course discussed in the manuscript. In the same time, performances of the existing Luna OBR 4600 reflectometer has been tested at Lab. and are reported for both temperature and strain measurements. An optical mock-up for Rayleigh OFDR method has also been developed. A software, dedicated to signal processing has been fully developed either. This program has been tested for simulated sensing signals and for raw data delivered by the Luna OBR 4600 reflectometer. Results of such tests could be considered as satisfactory, even some questions to be solved remains. Finally, some practical examples of Rayleigh OFDR industrial applications have been shown and detailed. Moreover, in such context of real applications on field, an innovative Rayleigh OFDR system devoted to liquid sodium leakage detection around secondary cooling circuitry of generation IV nuclear reactors has been developed and tested on a nuclear installation. An industrial prototype of such measurement system has been elaborated. Finally, way of progress are analyzed and discussed for future R&D activities.

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Fast, sensitive, quantitative, and low-cost analysis of environmental pollutants is highly valuable for environmental monitoring. Due to its single-molecule sensitivity and fingerprint specificity, surface-enhanced Raman spectroscopy (SERS) has been widely employed for heavy metal, organic compound, and pathogen detection. However, SERS quantitation is challenging because 1) analytes do not stay in the strongest enhancing region ("hot spots") and 2) SERS reproducibility is poor. In this dissertation, gold nanoparticle/bacterial cellulose (AuNP/BC) substrates were developed to improve SERS sensitivity by increasing hot spot density within the laser excitation volume. Environmentally relevant organic amines were fixed at "hot spots" by lowering solution pH below the analyte pKa and thus enabling SERS quantitation. In addition, a new SERS internal standard was developed based upon the electromagnetic enhancement mechanism that relates Rayleigh (elastic) and Raman (in-elastic) scattering. Rayleigh scattering arising from the amplified spontaneous emission of the excitation laser was employed as a normalization factor to minimize the inherent SERS signal variation caused by the heterogeneous distribution of "hot spots" across a SERS substrate. This highly novel technique, hot spot-normalized SERS (HSNSERS), was subsequently applied to evaluate the efficiency of SERS substrates, provide in situ monitoring of ligand exchange kinetics on the AuNP surface, and to reveal the relationship between the pKa of aromatic amines and their affinity to citrate-coated AuNPs (cit-AuNPs). Finally, colloidally stable stable pH nanoprobes were synthesized using co-solvent mediated AuNP aggregation and subsequent coating of poly(ethylene) glycol (PEG). These nanoprobes were applied for pH detection in cancer cells and in phosphate buffered aerosol droplets. The latter experiments suggest that stable pH gradients exist in aerosol droplets. / PHD / Traditional analytical methods, such as gas chromatography/mass spectroscopy, liquid chromatography/mass spectroscopy, etc., cannot meet the demand for rapid screening of target environmental pollutants in drinking water. This issue arises due to the requirements for time-consuming sample pre-treatment, well-trained experts, complex instrumental parameter optimization, and scale challenges that limit onsite measurement. Surface-enhanced Raman spectroscopy is a promising approach to overcome these limitations. To improve SERS quantitation, surface-enhanced elastic scattering was developed as a novel internal standard to account for the SERS signal variation caused by substrate heterogeneity (“hot spot” normalization). Compared with traditional SERS internal standards, using scattered light as an internal standard reduces cost, time, interference, and experimental complexity for SERS detection. With this novel approach, the kinetics of adsorption/desorption of guest ligands/citrate onto/from the AuNP surface were quantified in situ and in real time. In addition, the SERS intensities of organic amines acquired at different solution pH values were differentiated using “hot spot” normalization, which revealed the relationship between aromatic amine pK_a and their affinity to the AuNP surface. Finally, the chemistry in confined aqueous environments, such as aerosol droplets, membrane channels, and cells, is challenging to probe using conventional analytical tools due to their inaccessible small volumes. To address this problem, SERS pH nanoprobes were synthesized and used to detect the pH inside cancer cells and micrometer-sized aerosol droplets.

Doctor of Philosophy / Filtered Rayleigh scattering (FRS) is a laser-based measurement technique that makes use of the scattering of light off particles that are much smaller than the wavelength of light that hits them (i.e., Rayleigh scattering of air molecules). The scattered laser light is altered after encountering particles in predictable ways that can be related to changes in velocity, temperature, and density. However, other sources of scattered light interfere with the pure Rayleigh scattering signal such as Mie and background scattering. Mie scattering is the scattering of light off particles that are much bigger than the wavelength of light that hits them (i.e., dust particles suspended in air). Background scattering is the laser light scattered off physical objects that reflect back into the region of interest. The different types of scattering are accounted for with intensive modeling and iterative fitting schemes where the error between simulated data and experimental data is minimized. This fit allows for velocity, temperature, and density information to be extracted from the measured scattered light. This iterative scheme is then applied to experimental measurements on the ground with mini turbojet engines as well as full-scale turbofan engines. A data grouping technique is derived such that the total measured force using FRS can be divided into individual contributions from different parts of the engine. These developed techniques have laid the foundation for future in-flight measurements of engine forces.

<p dir="ltr">From transonic to hypersonic regimes, the characterization of high-speed flow dynamics is critical for the development, testing, and improvement of launch and reentry vehicles, boost-glide vehicles, and thermal protection systems. The design of this technology often relies on computational/empirical models for predictions which make quantitative thermodynamic measurements crucial for numerical validation. Laser diagnostic techniques facilitate non-intrusive, <i>in situ</i> measurements of fluid dynamic properties as well as visualization of flows, shocks, and boundary layer interactions. However, many diagnostics rely on seeding the flow with foreign materials to make measurements, such as the application of particle image velocimetry (PIV), Doppler global velocimetry (DGV), and planar laser-induced fluorescence (PLIF). Molecular filtered Rayleigh scattering (FRS) diagnostics are attractive for flow characterization due to the fact that pressure, temperature, density and velocity measurements can be made directly from air or N₂ molecules without the need for seeding materials. The development of the burst-mode laser (BML) has enabled high-energy pulses generated at the rates necessary to resolve phenomena such as instabilities in boundary-layers and shock-wave evolution using Rayleigh scattering methods. The goal of this dissertation is to advance molecular burst-mode FRS for quantitative, high resolution, multi-parameter measurements. For fixed-wavelength FRS measurements, the spectral characteristics of a BML system were investigated and improved by integrating an etalon for spectral-gating. For multi-parameter measurements, two strategies for wavelength-agility, the ability to quickly switch between two or more laser wavelengths, of the BML were explored: frequency-scanning and frequency-shifting. The frequency-scanning FRS (FS-FRS) technique measurement rate was increased to 1 kHz and demonstrated for 1-ms pressure, temperature, and radial velocity measurements in an underexpanded jet flow. Building upon this, an acousto-optic modulator-based method was implemented to generate frequency-shifted pulses. The rapid frequency-shifting increased the effective FRS multi-parameter measurement rate to 25 kHz and planar pressure, temperature, and radial velocity measurements were captured in an overexpanded jet flow. Finally, design tools for the laser configuration of wavelength-agile FRS were developed for the optimization of relative absolute measurement errors.</p>

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Cette thèse a pour objectif l’étude des propriétés optiques non linéaires (ONL) de second ordre de molécules de référence et de composés organiques photochromes, via des calculs de chimie quantique et des expériences de diffusion Hyper-Rayleigh (HRS). L’étude des réponses optiques de second harmonique d’une série de solvants organiques met tout d’abord en évidence la nécessité d’utiliser des méthodes théoriques intégrant les effets de la corrélation électronique, les effets d’environnement, ainsi que des bases très étendues pour évaluer quantitativement les premières hyperpolarisabilités moléculaires. La décomposition du tenseur de première hyperpolarisabilité en contributions dipolaires et octupolaires via le formalisme des harmoniques sphériques permet également d’illustrer l’effet de la symétrie des composés sur leurs réponses ONL. La seconde étude porte sur la caractérisation des effets de solvant sur les propriétés ONL d’un dérivé anil présentant un équilibre entre une forme énol et une forme cétone. Il est démontré que l’équilibre énol/cétone peut être déplacé vers la forme cétone en augmentant la proportion d’éthanol dans le mélange binaire de solvants éthanol/cyclohexane, conduisant à une augmentation de l’intensité HRS du mélange. La dernière partie de la thèse propose une étude théorique systématique des effets de substitution sur les propriétés ONL de sels de flavylium, dans le but d’optimiser le contraste des réponses ONL entre les différentes formes des composés. Il est démontré qu’une combinaison optimale de substituants permet d’obtenir de forts contrastes, démontrant le potentiel de ces composés comme systèmes multi-états pour des applications ONL. / This PhD addresses the nonlinear optical (NLO) properties of the reference organic molecules and molecular switches, by means of quantum chemical calculations and Hyper-Rayleigh Scattering (HRS) measurements. The thorough investigation of the NLO responses of a series of organic solvents first evidences the need of using theoretical methods including environment and electron correlation effects, as well as highly extended basis sets to quantitatively evaluate the molecular first hyperpolarizabilities. Decomposing the first hyperpolarizability tensor into dipolar and octupolar contributions via the spherical harmonics formalism also highlights the impact of the molecular symmetry on the NLO responses. The second part addresses the solvent effects on the NLO properties of an anil derivative, which exhibits an equilibrium between a enol and a keto form. It is shown that the enol/keto equilibrium is shifted towards the keto form when increasing the ethanol ratio in ethanol/cyclohexane solvent mixtures, leading to an increase of the HRS intensity. The last part addresses the effects of the chemical substituents on the NLO properties of flavylium salts, with the aim of optimizing the NLO contrasts between the various forms of the compounds. It is found that optimal combinations of chemical substituents lead to high NLO contrats, which demonstrates the potential of these compounds as multi-states systems for NLO applications