Cette thèse traite des dégradations induites par la pompe (photo-noircissement) et les radiations ionisantes externes (radio-noircissement) dans les fibres optiques en silice dopées ytterbium (FDY) utilisées en environnement sévère. Au travers de caractérisations expérimentales et de modélisations inédites, elle analyse leur interaction et en tire les conséquences quant à la tenue des FDY aux radiations sous pompe. La première partie porte sur l’identification des défauts induits (centres colorés) et leurs mécanismes de formation/guérison. Elle s’appuie sur un ensemble de caractérisations post-irradiation (RPE, ARI, TL) réalisées sur des échantillons de préformes et sur leur corrélation originale (guérison thermique, couplage TL et ARI). L’étude systématique en fonction de la composition met en évidence l’influence des co-dopants (Al, Ce) sur la capture des charges libérées lors des processus d’ionisation. La seconde partie analyse le noircissement se développant sous l’effet simultané de la pompe et de l’irradiation ionisante. A partir d’un banc de mesures autorisant le suivi de la dégradation en temps réel, on montre que photo- et radio-noircissements résultent des mêmes centres colorés blanchis par la pompe. Ce résultat, ajouté aux mécanismes préalablement identifiés, permet de proposer un modèle physique local de la dégradation photo-radio-induite. La confrontation des simulations issues de ce modèle à une large variété d’observations originales faites « en ligne » conduit à sa validation. Il est ainsi démontré que, pour des débits de dose inférieurs à une valeur critique, la dégradation des FDY pompées et irradiées ne peut excéder leur niveau de photo-noircissement. / This thesis deals with the degradation induced by the pump (photodarkening, PN) and ionizing radiations (radiodarkening, RN) in ytterbium-doped optical fiber (YDF) used in harsh environments. Through original experimental characterizations and modeling, it analyses the interplay between PN and RN and reveals important and novel properties of the radiation resistance of pumped YDF. The first part investigates induced defects (color centers) together with their creation/recovery mechanisms. It used a set of post-irradiation characterizations (ESR, RIA, TSL) conducted on preform samples and benefited from their original correlation (thermal recovery protocols coupling TSL and RIA). A systematic study as a function of composition reveals the influence of co-dopants (Al, Ce) on the trapping of carrier freed during ionization processes. The second part examines the darkening build-up under the simultaneous action of the pump and an ionizing irradiation. By using a measurement bench that allowed us to follow the real-time “on line” degradation of fiber samples, we showed that photo- and radio-darkening both arise from the same color centers that can be bleached by the pump. On the basis of this finding and of the preceding identified mechanisms, we propose a local physical model of the photo-radio-induced darkening. The latter is thoroughly validated by further successful comparisons of simulated degradation with a wide variety of “on line” original observations. Then, we notably demonstrate that for dose rates lying below a critical value (explicited by our theory), the degradation of pumped and irradiated YDF never exceeds the photo-darkening level.

In 1920, Leon Brillouin discovered a new kind of light scattering – Brillouin scattering – which occurs as a result of the interaction of light with a transparent material’s temporal periodic variations in density and refractive index. Many advances have since been made in the study of Brillouin scattering, in particular in the field of fiber optics. An in-depth investigation of Brillouin scattering in optical fibers has been carried out in this thesis, and the theory of stimulated Brillouin scattering (SBS) and combined Brillouin gain and loss has been extended. Additionally, several important applications of SBS have been found and applied to current technologies. Several mathematical models of the pump-probe interaction undergoing SBS in the steady-state regime have emerged in recent years. Attempts have been made to find analytical solutions of this system of equations, however, previously obtained solutions are numerical with analytical portions and, therefore, qualify as hybrid solutions. Though the analytical portions provide useful information about intensity distributions along the fiber, they fall short of describing the spectral characteristics of the Brillouin amplification and the lack of analytical expressions for Brillouin spectra substantially limits the utility of the hybrid solutions for applications in spectral measurement techniques. In this thesis, a highly accurate, fully analytic solution for the pump wave and the Stokes wave in Brillouin amplification in optical fibers is given. It is experimentally confirmed that the reported analytic solution can account for spectral distortion and pump depletion in the parameter space that is relevant to Brillouin fiber sensor applications. The analytic solution provides a valid characterization of Brillouin amplification in both the low and high nonlinearity regime, for short fiber lengths. Additionally, a 3D parametric model of Brillouin amplification is proposed, which reflects the effects of input pump and Stokes powers on the level of pump wave depletion in the fiber, and acts as a classification tool to describe the level of similarity between various Brillouin amplification processes in optical fibers. At present, there exists a multitude of electro-optic modulators (EOM), which are used to modulate the amplitude, frequency, phase and polarization of a beam of light. Among these modulators, phase modulation provides the highest quality of transmitted signal. As such, an improved method of phase-modulation, based on the principles of stimulated Brillouin scattering, as well as an optical phase-modulator and optical phase network employing the same, has been developed. Due to its robustness, low threshold power, narrow spectrum and simplicity of operation, stimulated Brillouin scattering (SBS) has become a favourable underlying mechanism in fiber-based devices used for both sensing and telecommunication applications. Since birefringence is a detrimental effect for both, it is important to devise a comprehensive characterization of the SBS process in the presence of birefringence in an optical fiber. In this thesis, the most general model of elliptical birefringence in an optical fiber has been developed for a steady-state and transient stimulated Brillouin scattering (SBS) interaction, as well as the combined Brillouin gain and loss regime. The impact of the elliptical birefringence is to induce a Brillouin frequency shift and distort the Brillouin spectrum – which varies with different light polarizations and pulse widths. The model investigates the effects of birefringence and the corresponding evolution of spectral distortion effects along the fiber, and proposes regimes that are more favourable for sensing applications related to SBS – providing a valuable prediction tool for distributed sensing applications. In recent years, photonic computing has received considerable attention due to its numerous applications, such as high-speed optical signal processing, which would yield much faster computing times and higher bandwidths. For this reason, optical logic has been the focus of many research efforts and several schemes to improve conventional logic gates have been proposed. In view of this, a combined Brillouin gain and loss process has been proposed in a polarization maintaining optical fiber to realize all-Optical NAND/NOT/AND/OR logic gates in the frequency domain. A model describing the interaction of a Stokes, anti-Stokes and a pump wave, and two acoustic waves inside a fiber, ranging in length from 350m-2300m, was used to theoretically model the gates. Through the optimization of the pump depletion and gain saturation in the combined gain and loss process, switching contrasts of 20-83% have been simulated for different configurations.

Ce travail de thèse a pour objectif de développer une nouvelle technologie afin de fournir une solution bas coût et simple d’utilisation pour les raccordements de fibres optiques entre elles. Cette technologie fibre-fibre pourra être étendue à des raccordements fibre-composant d’optique intégrée.Cette nouvelle technique est basée sur la photo-polymérisation d’une solution chimique, utilisée pour reconstruire une structure d’indice guidante entre les deux éléments de guides optiques à connecter. Pour mettre au point cette technologie nous nous sommes concentrés sur la réalisation d’un raccordement de deux fibres optiques.Après une analyse des technologies existantes et des causes de pertes lors des raccordements entre deux fibres optiques, nous détaillons les différentes solutions de photo-polymères employés. Nous abordons le fonctionnement de la polymérisation radicalaire, ainsi que la composition des solutions utilisées, puis nous présentons les performances attendues. Pour cela nous utilisons un environnement de simulation FDTD (Finite Différence Time Domaine), ce qui nous permet de mettre en évidence que cette technique de raccordement a le potentiel d’être équivalente aux techniques employées dans les télécoms de nos jours. Toujours dans l’objectif d’étudier ce nouveau type de connexion, et d’en optimiser les performances, nous avons mis en place une méthode d’analyse de la répartition de l’indice de réfraction au cœur même des connexions polymère. Pour cela nous employons une méthode de tomographie interférométrique, que nous avons adapté à la mesure de fibre et de petits objets en extrémité de fibre. Cette nouvelle technique d’analyse, développée en partenariat avec le département mécatronique de l’Université de Varsovie, permet de reconstruire une cartographie en trois dimensions de la répartition de l’indice de réfraction. Nous avons ainsi pu analyser des connexions polymères réalisées avec différentes solutions photochimiques, et observer les défauts d’homogénéité d'indice. Ceci dans le but de reproduire le plus parfaitement possible la structure d’indice des fibres optiques, et par là même réduire au maximum les pertes optiques au niveau de la connexion. A l’aide de ces analyses, nous sommes arrivés à l’application final de cette technique pour le marché des Télécommunication optiques : Nous avons ainsi réaliser des prototypes de raccordement de deux fibres optiques, ainsi que de tous les outils nécessaires à leur réalisation sur le terrain, et avons démontré que leur performances étaient équivalente aux produits aujourd’hui présent sur le marché. / The aim of this thesis is to develop a new technology providing a low cost and “easy to use “solution for interconnections of optical fibers. This fiber-optic technology can be extended to connections between fiber and integrated optical component. This new technique is based on photo-polymerization of a chemical solution used to rebuild a guiding index structure between two elements of optical waveguides. With the aim to study this new type of connection, and optimize it’s performances, we have implemented a method for analysing the distribution of the refractive index within the polymer connexions. For this we use an interferometric tomography method, that we adapted to measure the fiber and small items at fiber end. This new analysis technique, developed in partnership with the Department of Mechatronics of Warsaw University, allows to reconstruct a three-dimensional mapping of the refractive index distribution. We are able to analyse polymers connections made with different photochemical solutions and observe defects in the index homogeneity. This is done in order to replicate as possible the index structure of optical fibers, and thereby minimize optical losses at connection. Using these analysis, we set to the final application of this technique, at the optical Telecommunications market: we have developed prototypes of splicing between two optical fibers, as well as designed all necessary tools for their implementation in the field, and we have demonstrated that their performance is equivalent to products now available on the market.

This diploma thesis is focused on measurement of the physical quantities by fiber optic sensors based on multimode interference. The objective is to evaluate the appropriate method of signal provided by the spectrum analyzer. Firstly, a review of optics, optical phenomena and fibers is performed. The basic theory necessary for the realization of measurements and the principal of fiber optic sensors and SMS (singlemode–multimode–singlemode) structures are discussed. Finally, the automated measuring workplace for absorption spectrum measurement is created and described. Given data are finally evaluated by an artificial neural network and a sensor concept for practical utilization is designed.

This work encompasses a study of fibre optic devices and micro-resonator cavities and their use as strain sensors, interferometers, and eventually as light sources. We demonstrate the use of a pi-shifted fibre Bragg grating as a strain sensor and quantify its performance through several metrics relevant to its operation in a frequency-locked system. We then discuss the use of waveguide devices as refractive index sensors in a detailed and critical analysis of most of the literature on fibre optic-based interferometry where we define new metrics of assessment that are aimed to be device-independent. We then categorize and study the performance of the literature on fiber-optic refractive index sensors and highlight few key devices of each category based on their performance or design. We relate the performance of these interferometers to various design parameters to outline a way for future researchers to create high-performance devices. Finally, we study light emission in dye-loaded droplet micro-resonators, which are a promising new platform to generate low-threshold lasing action excited by lasers coupled to the whispering gallery modes of the resonator. We characterize multiple types of emissions in these cavities and pave the way for future microdroplet-based lasers. / Graduate / 2022-08-10

Diploma thesis deals with problems of industrial networks using optical fiber as the main transmission lines. The task of the diploma work is to study and summarize the issues of industrial networks as well as the ways to increase their transmission capacity. In telecommunications and computer networks are generally used multiplexes, thanks to which we are able to achieve the desired transmission capacity. Consequently, the following work deals with the selection of the adequate solutions using described multiplexes as well as other transmission methods. The aim of the diploma work is to compare various renovation techniques of an old industrial network. Using this network as an example the work should demonstrate the individual problems and their solutions while considering different types of methods used. Based on the price, the performance and the difficulty to install other devices and transmission media we are able to select the appropriate solution for optical link.

Ph. D.

Les sources lasers femtosecondes sont utilisées dans grand nombre d’applications (industrielles, médicales, de recherche fondamentale) avec un besoin croissant d’impulsions très énergétiques à haut taux de répétition. Bien que la technologie Ti:Saphir fournisse des impulsions PetaWatt, son taux de répétition s’avère limité. Une alternative est l’utilisation de la technologie fibrée. Cependant, l’énergie extractible d’une seule fibre est intrinsèquement limitée.Une solution prometteuse est alors de réaliser une combinaison de fibres (jusqu’à plus de 10 000 fibres pour l’accélération de particules). La combinaison de fibres par méthode interférométrique (avec un record de 64 fibres combinées en régime continu) a prouvé qu’elle était un excellent candidat pour la combinaison d’un grand nombre de fibres.La collaboration XCAN entre l’Ecole Polytechnique et Thales, vise à réaliser un démonstrateur de combinaison cohérente de 61 fibres amplifiées en régime femtoseconde. Les travaux menés au cours de cette thèse s’inscrivent dans ce projet.Dans un premier temps, afin d’étudier les points durs inhérents au régime femtoseconde tout en s’affranchissant des difficultés liées à l’amplification, la méthode interférométique en régime femtoseconde a été étudiée sur un démonstrateur passif, c’est-à-dire sans amplification, de 19 fibres. Une fois la méthode de mise en phase validée celle-ci a pu être testée avec succès sur le démonstrateur avec amplification du projet XCAN.Nous présentons également les travaux menés afin d’augmenter un paramètre clé des systèmes de combinaison de faisceaux à savoir : l’efficacité de combinaison du système laser. Pour cela, nous avons réalisé une mise en forme de faisceaux issus des fibres de la tête optique. Cette mise en forme, gaussien vers super-gaussien, est réalisée à l’aide de deux réseaux de lames de phase dont nous présenterons le calcul des profils asphériques. Afin de valider expérimentalement nos simulations et après réalisation des lames de phase nous avons pu tester celles-ci sur le démonstrateur passif, démontrant une augmentation de 14 %.Les travaux présentés dans ce manuscrit présentent ainsi les premiers par vers la réalisation d’une nouvelle architecture laser massivement parallèle, capable de délivrer à la fois une haute puissance crête et une haute puissance moyenne. / Femtosecond fiber sources are used in a large number of applications (industrial, medical, fundamental physics) with a growing need in high energy pulses at high repetion rate. Although Ti: Saphirre technology provides energies up to PetaWatt, its repetion rate is low (up to 1 Hz). An alternative is to use an amplified fiber. However, the extractable energy of a single fiber is intrinsically limited.A solution is then to combine several fibers (up to 10 000 fibers for particle acceleration). Coherent beam combining of fibers with an interferometric method (with a record of 64 fibers combined in the cw regime) has proven to be an excellent candidate to combine a large number of fibers.The XCAN project, a collaboration between l'Ecole polytechnique and Thales, aims to realize a demonstrator of 61 fibers coherently combined in the femtosecond regime.The works presented in this thesis are part of this project.In order to study the hard points inherent to the femtosecond regime and to free from the amplification issues, the interferometric method has been implemented on a passive demonstrator, meaning without amplification, of 19 fibers. Once the interferometric method validated, it has been succesfully tested on the amplified XCAN demonstrator.We present also the works done to increase a key parameter of beam combining systems : the combining efficiency. To do this, we have realized a beam shaping of the fiber array output beams. This beam shaping, gaussian to super-gaussian, is done with two arrays of phase plates. The aspherical profiles calculation is described. In order to validate our simulations we have tested the phase plates on the passive demonstrator by getting an increase of 14 %.The works presented in this manuscript are the first steps towards a new massively parallel laser architecture, able to provide both high peak power and high average power.

During the DARPA SCENICC program, J. Ford, et al., demonstrated that CFBs provide a compelling route to compact, wide angle imagers. Monocentric lenses readily provide diffraction-limited images over wide field but onto a hemispherically curved image surface. They demonstrated visible CFBs can be tapered, cut and polished to relay curved images to flat sensors. We have shown that this provides a volumetric imaging efficiency a hundredfold larger than bulk optics can produce; a hundred times the resolution in the same volume or a hundred times less volume for the same resolution. Ford's work leveraged commercial fiber bundles available for the visible spectrum based on silica. We have developed hybrid fiber bundles using step-index confinement between chalcogenide glass cores and polymer cladding with high index-contrast. The high contrast is necessary to provide tight confinement to the high-index As-Se core with minimal crosstalk between closely spaced cores. Tight confinement also minimizes absorption losses in the PEI polymer cladding. The high contrast of this system also provides a large NA to optimize coupling into the CFB from fast lenses. We introduce disorder into the core radius as a mechanism to further decouple adjacent cores, reduce crosstalk and increase fill-factor. We present coupled-mode theory, modal crosstalk superposition, and finite-element modelling to quantify coupling losses and crosstalk as a function of geometry and disorder. We fabricated preforms, drew small fiber bundles and characterized optical properties of the bundles to aid scale-up to megapixel MWIR CFBs.

The purpose of this research is to evaluate the influence of through-the-thickness reinforcements on the fracture behavior of stitched sandwich composites and to develop predictive methodologies to aid in simulating their damage-tolerant capability. Sandwich composites are widely used for their high stiffness-to-weight ratio due to their unique material architecture, which is composed of two rigid, outer facesheets that are bonded to a light-weight internal core. However, sandwich composites are limited by their low interlaminar strengths and can develop core-to-facesheet separation when subjected to low out-of-plane loads. In this study, sandwich composites were manufactured with through-the-thickness reinforcements, or stitches, to act as crack-growth inhibitors and to improve interlaminar properties. Stitch processing parameters, such as the number of stitches per unit area (stitch density) and stitch diameter (linear thread density), have considerable influence on the in-plane and out-of-plane behavior of composite structures. A design of experiments (DoE) approach is used to investigate stitch processing parameters and their interaction on the fracture behavior of stitched sandwich composites. Single cantilevered beam (SCB) tests are performed to estimate the required energy to propagate crack growth, or Mode I fracture energy, during the separation of the facesheet from the core. Additionally, embedded optical fibers within the SCB test articles are used to determine the internal crack front variation. During testing, unique fracture morphologies are obtained and show dependency on stitch processing parameters. Furthermore, embedded optical fibers indicate that the internal crack front is approximately 10% greater than visual edge measurements, which is primarily attributed to Poisson’s effect. The DoE approach is then used to develop a statistically informed response surface model (RSM) to optimize stitch processing parameters based on a maximum predicted fracture energy. Novel analytical formulations are developed for estimating the mode I fracture energy using the J-integral approach. The DoE approach is then used to inform and validate finite element models that simulate the facesheet-to-core separation using a discrete cohesive zone modeling approach. The predicted load and crack growth response show good agreement to experimental measurements and highlights the capability of stitching to arrest delamination in stitched sandwich composites.