Optical coherence tomography benefits from the high brightness and bandwidth, as well as the spatial coherence of supercontinuum (SC) sources. The increase of spectral power density (SPD) over conventional light sources leads to shorter measuring times and higher resolutions. For some applications, only a portion of the broad spectral range can be used. Therefore, an increase of the SPD in specific limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the SPD of SC sources by amplifying the excitation wavelength inside of a nonlinear photonic crystal fiber (PCF). An ytterbium-doped PCF was manufactured by a nanopowder process and used in a fiber amplifier setup as the nonlinear fiber medium. The performance of the fiber was compared with a conventional PCF that possesses comparable parameters. Finally, the system as a whole was characterized in reference to common solid-state laser-based photonic SC light sources. An order-of-magnitude improvement of the power density was observed between the wavelengths from 1100 to 1350 nm.

Les sources de paires de photons constituent un bloc de base pour les technologies de traitement et transmission de l'information quantique. Une source consistant en une fibre microstructurée à coeur liquide permet à la fois une réduction du bruit de diffusion Raman, une adaptation simple et efficace aux réseaux de télécommunication quantique, et l'ajustement de ses propriétés d'émission par ingénierie de la microstructure et choix du liquide non linéaire. Ces recherches se concentrent sur l'étude de l'émission de paires de photons d'une telle source, et du mélange à quatre ondes à leur origine. Nous soulignons le manque d'une description quantitative correcte des phénomènes non linéaires à l'origine des paires dans les modèles existants, et en proposons un se basant sur le champ D pour y parvenir. Nous mettons expérimentalement en évidence l'inconsistance avec la forme de spectre usuellement attendue les sources de paires de photons. Pour l'expliquer, nous développons un modèle rendant compte de la non-uniformité du guide, soit la variation de ses propriétés de propagation sur sa longueur. Par une approche analytique initiale simple de cette caractéristique, nous exposons l'étalement du spectre et la diminution du taux maximum d'émission de paires. Une description numérique par morceaux apporte une description plus proche de la réalité et met en lumière la très forte sensibilité du spectre à la non-uniformité. Un autre effet de cette dernière se traduit par la différenciation du spectre selon le sens de propagation de la lumière dans le guide. Lors de l'intrication en polarisation des paires dans un dispositif de type boucle Sagnac, cette non-réciprocité dégrade la visibilité des paires. Pour compenser cet effet, nous proposons une solution simple de symétrisation du profil des fibres à leur fabrication, appuyée par de premiers résultats encourageants. Cette étude ouvre la voie à la prise en compte des non-uniformités inhérentes aux guides réels, impactant fortement leur émission de paires de photons. / Photon-pair sources are a basic block for implementation of quantum information and telecommunication. A microstructured fibered source with liquid core induce a Raman scattering noise reduction, and at the same time allows a simple and lossless coupling to telecom network, with an engineering of its emission properties through the structure and liquid choices. This work focus on four-wave mixing leading to photon pairs emission in such a source. As existing models lack a correct emph{quantitative} description of nonlinear phenomena for pairs emission, we propose here one based on the D field to do so. We show a mismatch between the spectrum form usually expected and the experimental one. To explain this, we develop a model describing the effects of guide nonuniformity, meaning variation of its propagation properties along itself. Through an initial and simple analytical approach, we demonstrate the spectrum spreading and the diminution of the maximum of emission pairs rate. With a piece-wise numerical description for real guides, we highlight the very strong sensitivity of the emission spectrum towards nonuniformity. Another effect arising from this feature is the spectrum differentiation depending on the propagation direction within the guide. Upon pairs polarization entanglement by inserting the guide into a Sagnac loop interferometer, such nonreciprocity induces a deterioration of pairs visibility. In order to counteract this effect, we propose, based on first encouraging results, a simple solution involving a symmetrization of fibers profile during their manufacture. This study paves the way for taking into account inherent nonuniformity of real waveguides, which strongly impacts their photon pair emission.

Fiber based ultrashort pulse laser sources are desirable for many applications; however generating high peak powers in fiber lasers is primarily limited by the onset of nonlinear effects such as self-phase modulation, stimulated Raman scattering, and self-focusing. Increasing the fiber core diameter mitigates the onset of these nonlinear effects, but also allows unwanted higher-order transverse spatial modes to propagate. Both large core diameters and single-mode propagation can be simultaneously attained using photonic crystal fibers. Thulium-doped fiber lasers are attractive for high peak power ultrashort pulse systems. They offer a broad gain bandwidth, capable of amplifying sub-100 femtosecond pulses. The longer center wavelength at 2 ?m theoretically enables higher peak powers relative to 1 [micro]m systems since nonlinear effects inversely scale with wavelength. Also, the 2 [micro]m emission is desirable to support applications reaching further into the mid-IR. This work evaluates the performance of a novel all-fiber pump combiner that incorporates a thulium-doped photonic crystal fiber. This fully integrated amplifier is characterized and possesses a large gain bandwidth, essentially single-mode propagation, and high degree of polarization. This innovative all-fiber, thulium-doped photonic crystal fiber amplifier has great potential for enabling high peak powers in 2 [micro]m fiber systems; however the current optical-to-optical efficiency is low relative to similar free-space amplifiers. Further development and device optimization will lead to higher efficiencies and improved performance.

At the Dawn of the 21st century, the field of specialty optical fibers experienced a scientific revolution with the introduction of the stack-and-draw technique, a multi-steps and advanced fiber fabrication method, which enabled the creation of well-controlled micro-structured designs. Since then, an extremely wide variety of finely tuned fiber structures have been demonstrated including novel materials and novel designs. As the complexity of the fiber design increased, highly-controlled fabrication processes became critical. To determine the ability of a novel fiber design to deliver light with properties tailored according to a specific application, several mode analysis techniques were reported, addressing the recurring needs for in-depth fiber characterization. The first part of this dissertation details a novel experiment that was demonstrated to achieve modal decomposition with extended capabilities, reaching beyond the limits set by the existing mode analysis techniques. As a result, individual transverse modes carrying between ~0.01% and ~30% of the total light were resolved with unmatched accuracy. Furthermore, this approach was employed to decompose the light guided in Large-Mode Area (LMA) fiber, Photonic Crystal Fiber (PCF) and Leakage Channel Fiber (LCF). The single-mode performances were evaluated and compared. As a result, the suitability of each specialty fiber design to be implemented for power-scaling applications of fiber laser systems was experimentally determined. The second part of this dissertation is dedicated to novel specialty fiber laser systems. First, challenges related to the monolithic integration of novel and complex specialty fiber designs in all-fiber systems were addressed. The poor design and size compatibility between specialty fibers and conventional fiber-based components limits their monolithic integration due to high coupling loss and unstable performances. Here, novel all-fiber Mode-Field Adapter (MFA) devices made of selected segments of Graded Index Multimode Fiber (GIMF) were implemented to mitigate the coupling losses between a LMA PCF and a conventional Single-Mode Fiber (SMF), presenting an initial 18-fold mode-field area mismatch. It was experimentally demonstrated that the overall transmission in the mode-matched fiber chain was increased by more than 11 dB (the MFA was a 250 ?m piece of 50 ?m core diameter GIMF). This approach was further employed to assemble monolithic fiber laser cavities combining an active LMA PCF and fiber Bragg gratings (FBG) in conventional SMF. It was demonstrated that intra-cavity mode-matching results in an efficient (60%) and narrow-linewidth (200 pm) laser emission at the FBG wavelength. In the last section of this dissertation, monolithic Multi-Core Fiber (MCF) laser cavities were reported for the first time. Compared to existing MCF lasers, renown for high-brightness beam delivery after selection of the in-phase supermode, the present new generation of 7-coupled-cores Yb-doped fiber laser uses the gain from several supermodes simultaneously. In order to uncover mode competition mechanisms during amplification and the complex dynamics of multi-supermode lasing, novel diagnostic approaches were demonstrated. After characterizing the laser behavior, the first observations of self-mode-locking in linear MCF laser cavities were discovered.

[pt] No presente trabalho, demonstramos o uso de fibras de cristais fotônicos (PCF) em diferentes configurações para sensoriamento. Investigamos o uso de fibras PCF com alta birrefringência para sensoriamento de pressão hidrostática e deformação, explorando a baixa dependência térmica. Onde a interferência entre os modos que se propagam em uma fibra birrefringente é analisada. Monitoramos o deslocamento do padrão de interferência do espectro de reflexão, com a variação da pressão ou deformação aplicada à fibra. Demonstramos uma técnica de criar cavidades Fabry-Perot dentro de uma fibra óptica, bem como uma técnica para controlar seu comprimento, e consequentemente, o período das franjas no padrão de interferência no espectro de reflexão. O interferômetro Fabry-Perot (FPI) investigado, possuí um altíssimo contraste nas franjas do padrão de interferência, acima dos 30 dB, sendo no momento, o recorde para o valor do contraste das franjas para FPI silica-ar. O dispositivo foi investigado quanto à resposta a deformação. Um robusto encapsulamento foi feito, possibilitando investigar a resposta do dispositivo à vibração externa. Um estudo da relação da sensibilidade com o comprimento do FPI é também investigado. Construímos um outro interferômetro a partir de um pedaço de fibra PCF e de duas regiões de colapso de suas micro-estruturas. Estas regiões permitem a excitação e recombinação dos modos da fibra. Um dos modos que participa da interferência, é o modo de casca da fibra PCF, o qual é sensível a mudança de índice de refração do meio. O dispositivo apresenta contraste acima dos 40 dB, e é investigado quanto à resposta as mudanças de índices de refração do meio externo em contato com a fibra. Com a mesma montagem, funcionalizamos um pedaço de 2,0 cm de PCF para monitoramento de respiração humana. A partir disso, desenvolvemos um dispositivo capaz de monitorar a respiração do ser humano. / [en] In this work, we have demonstrated the use of Photonic Crystal Fiber (PCF), in different configurations, for sensing applications. The high birefringence and low temperature dependence characteristics of the PCF were explored for sensing hydrostatic pressure and deformation in a reflection configuration. Sensing was based on the analysis of the interference patterns between the modes that propagate in the birefringent fiber under the variation of pressure and deformation applied to the fiber. We have also demonstrated a technique to manufacture a Fabry-Perot Interferometer (FPI) cavity within an optical fiber with control of the cavity length and thus the control of the period of the fringes in the interference pattern. The Fabry-Perot Interferometer investigated presented a very high fringe contrast, above 30 dB, and showed a record value of the contrast of the fringes for FPI silica-air. The FPI device integrated within the fiber was tested as a strain sensor and also as a device to monitor vibration. A study of the relative sensitivity of the length of the FPI was also investigated. Another interferometer was built from a piece of standard fiber and a PCF with two regions of collapsed microstructures. These regions allowed the excitation and recombination of the fiber modes. This device was tested as a refractive index sensor, presenting a fringe contrast above 40 dB. An application of this device was the development of a humid sensor to monitor human breathing.

Depuis leur avènement, les fibres à cristal photonique à cœur creux ont prouvé leur capacité à convertir des fréquences avec une haute efficacité, notamment en jouant sur le phénomène de diffusion Raman stimulée. Dans le cadre d’un contrat CIFRE entre la société GLOphotonics et l’institut de recherche Xlim, ce projet de thèse a consisté à développer ces fibres afin d’améliorer leurs performances optiques pour cibler deux voies d’applications: une industrielle pour proposer un laser compact multi-ligne dans le visible et dans l’UV et une seconde plus fondamentale pour réaliser un synthétiseur d’onde optique. L’amélioration de ces performances repose sur l’exacerbation de l’inhibition du couplage entre le mode du coeur d’air et les modes de silice de la gaine. Pour cela deux types de micro-structures ont été explorées à savoir une maille Kagomé et une maille tubulaire. Plusieurs fibres ont été alors fabriquées démontrant des performances records sur toute une gamme de longueurs d’onde (8,5 dB/km à 1 µm, 7,7 dB/km à 750 nm, 13,8 dB/ km à 549 nm, et autour de 70 dB/km à 355 nm). Concernant la fonctionnalisation de ces fibres, des micro-capsules photoniques ont été conçues et réalisées permettant à la fois de palier au problème de la perméabilité de la silice au gaz (stabilité de la conversion dépassant 12 mois) et de démontrer une conversion de 26 lignes dans le visible. Un produit industriel nommé CombLas a alors été produit puis appliqué à une étude de cytométrie en flux pour étudier l’influence du taux de répétition du laser de pompe. Ce produit a également été étendu à la gamme spectrale de l’UV avec la génération de 24 lignes entre 225-400 nm. Enfin, des travaux plus fondamentaux ont été réalisés consistant à développer un synthétiseur d’onde optique à base de génération Raman dans ces fibres creuses. Une nouvelle dynamique a été observée démontrant le piégeage de molécules d’hydrogène par un réseau optique auto-assemblé de puits de potentiel ultra-profonds et nanométriques. Cela permis de générer un régime Lamb-Dicke de la diffusion Raman stimulée. Des signatures sub-Doppler usuellement vues dans les atomes froids ont été mesurées avec des largeurs de bandes plus étroites de plus de 5 ordres de grandeurs par rapport à ce qui est prédit dans la littérature. Finalement, cette largeur de bande a été optimisée d’un ordre de grandeur en jouant sur la longueur de la fibre et la pression de l’hydrogène. / Since their advent, hollow-core photonic crystal fibers have proved to be highly efficient for frequency conversion, especially via by playing with stimulated Raman scattering. Within the frame work of a CIFRE contract between the firm GLOphotonics and the Xlim research institute, this thesis project has consisted in developing these fibers to enhance their optical performances, in order to target two different field of applications: an industrial one to offer a a compact multi-line laser in the visible and UV and a second more fundamental one to realize a optical wave synthesizer. The amelioration of these performances relies on the exacerbation of the inhibition of the coupling between the air core mode and the silica cladding modes. Two types of micro-structures have been explored, a Kagomé and a tubular lattice. Several fibers have been fabricated demonstrating record performances on all a wavelength range (8.5 dB/km at 1 µm, 7.7 dB/km at 750 nm, 13.8 dB/km at 549 nm, and around 70 dB/km at 355 nm). Concerning the functionalization of the fibers, photonic micro-cells have been designed and realized enabling to overcome the problem the permeability of silica to gas (conversion stability over 12 months) and demonstrate a conversion to 26 lines in the visible. An industrial product coined CombLas has been made and used for flow cytometry in order to study the influence of the repetition rate of the pump laser. This product has also been extended to the UV range with 24 lines generated between 225-400 nm. Also, more fundamental research has been realized consisting in developing an optical wave synthesizer based on Raman generation in hollow core fibres where a new dynamic has been observed demonstrating the trapping of hydrogen molecules by an auto-assembled optical lattice of ultra-deep and nano-metric potential wells. This configuration has enabled to generate a Lamb-Dicke regime of stimulated Raman scattering. Sub-Doppler signatures usually found in cold atoms have been measured with linewidths narrower than 5 orders of magnitude than what is predicted in the literature. Finally, this linewidth has been optmised of an order of magnitude by plaing on the length of the fiber and the pressure of hydrogen.