Dissertations / Theses on the topic 'Fibre Laser'
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Magne, Sylvain. "Etat de l'art des lasers à fibre : étude d'un laser à fibre dopée ytterbium et spectroscopie laser de fibres dopées." Saint-Etienne, 1993. http://www.theses.fr/1993STET4027.
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Cette thèse montre l'intérêt de l'amplification optique et des lasers à fibres dopées par des ions de terres-rares pour l'instrumentation et les capteurs à fibres optiques. Nous détaillons les avantages et les inconvénients d'une telle technologie placée dans son contexte d'optique intégrée, ainsi que ses perspectives d'utilisation. Apres rappel de la théorie de la propagation guidée dans les fibres optiques, les technologies permettant de réaliser des fibres optiques dopées sont décrites. Une étude comparative des méthodes d'analyses de préformes et de fibres dopées permettant de déterminer leurs profils de guidage et de dopage est également présentée. Le comportement amplificateur des fibres optiques dopées est ensuite décrit théoriquement en insistant sur les limites fixées par l'élargissement inhomogène de la transition laser. L'évolution du gain intégré de la fibre est mise en évidence en fonction des paramètres influants (longueur, rayon de dopage. . . ) en soulignant l'intérêt d'un rayon de dopage optimisé. Le seuil d'accrochage du laser est modélisé par des équations et des abaques sans dimension déterminant la longueur de fibre optimale en fonction des pertes de la cavité laser. La conception générale du laser à fibre est ensuite synthétisée et les technologies des composants d'optique intégrée adaptés à la fibre amplificatrice le constituant sont comparées. Les techniques expérimentales concernent principalement l'étude de l'accordabilité par excitation sélective de sites et de la limitation du gain par émission stimulée sur la transition de pompage induisant une absorption résiduelle non saturable ; l'étude des effets de transferts dans les fibres dopées par luminescence coopérative : l'étude d'un procédé de changement de valence par irradiation y conduisant à une réduction des ions uranium placés en matrice verre fluoré (zblan) ; la démonstration d'un effet laser à trois niveaux, fonctionnant en régime continu et en contre-réaction externe par réseau
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Leung, Ian Kin-Hay Electrical Engineering & Telecommunications Faculty of Engineering UNSW. "Development of composite cavity fibre lasers for fibre laser hydrophone systems." Publisher:University of New South Wales. Electrical Engineering & Telecommunications, 2008. http://handle.unsw.edu.au/1959.4/41248.
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In this thesis, my main focus was to establish a novel composite-cavity fibre laser (CCFL) and to apply it in sensing, particularly in the hydrophone application. The CCFL that I have proposed is formed by writing three wavelength matched fibre Bragg gratings directly into a continuous length of doped fibre. I have also examined the relative advantages and disadvantages of interferometric and intensity-based hydrophone systems, and have established a hydrophone system that can be switched between the two modes of operation, by making use of digital signal processing. I have established a theoretical model to study the lasing and spectral characteristics of the CCFL. My analysis showed that whilst the CCFL have significantly different phase and threshold conditions from the common semiconductor diode lasers with external cavity, the CCFL also have mode-limiting properties that are often sought after. Through simulations, I was able to identify that a non-uniform straining scheme, that is, when one of the sub-cavities of the CCFL is restrained from strain, can improve the sensitivity with respect to existing single cavity fibre lasers, in both the frequency and intensity domains. My simulations also showed that the sensitivity of such a straining scheme can be optimised by tuning the reflectivity of the gratings, sub-cavity lengths, doping concentration and pump power. I have fabricated multiple CCFLs using the in-house grating writing facilities, and have experimentally assessed their power and spectral related lasing characteristics. Whilst having a significantly longer total cavity length compared to typical fibre lasers, the CCFLs demonstrated stable single longitudinal mode operation and narrow linewidth in the order for a few tens of kHz. Asymmetric output power and frequency as a result of unequal sub-cavity lengths were also examined. Finally, I conducted sensing experiments by applying the CCFLs in strain monitoring and intensity-based hydrophone. My results showed that the non-uniform straining scheme had significantly improved the intensity response of the CCFL, and that the acoustic pressure and frequency can be determined by directly sampling and applying Fourier transform to the output intensity of the fibre laser.
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Kabeya, David. "Montée en brillance des réseaux de lasers à fibre : Nouvelle approche par diagnostic à contraste de phase dans une boucle d’optimisation." Thesis, Limoges, 2016. http://www.theses.fr/2016LIMO0116/document.
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Les méthodes de combinaison cohérente sont rapidement apparues comme très prometteuses dans la course à la puissance des sources lasers. Cela s’explique par le fait que la puissance autour de l’axe de propagation évolue selon une loi quadratique avec le nombre de faisceaux combinés. Mes premiers travaux ont porté sur la montée en puissance de pompage dans les systèmes de mise en phase passive par auto-organisation. Pour la première fois, nous avons mis en évidence à la fois expérimentalement et numériquement, qu’au-delà du seuil laser, le filtrage spectral intracavité dû à la structure interférométrique du système laser, est un des principaux facteurs limitant l’obtention de qualités de phasage élevées. L’augmentation du nombre d’émetteurs accentue la dégradation de l’efficacité de combinaison avec la montée en puissance, montrant l’incapacité de ce type de méthode à combiner efficacement un grand nombre d’émetteurs lasers de forte puissance. Par la suite, mes travaux ont porté sur l’étude d’une méthode innovante de phasage actif, mise au point à XLIM. Le principe de cette méthode associe un filtrage optique de type contraste de phase, à un algorithme d’optimisation réduisant les écarts de phases entre émetteurs. Les calculs et expériences ont mis en évidence la très faible sensibilité de la méthode au nombre d’émetteurs mis en jeu. Les démonstrations de combinaison cohérente de 7 à 37 émetteurs fibrés délivrant jusqu’à 5W chacun ont été faites. Ce dernier résultat constitue aujourd’hui un record en termes de nombre d’émetteurs combinés de manière active. L’efficacité de combinaison en champ lointain a été estimée à une valeur élevée de 94%, correspondant à une erreur de phase résiduelle d’environ λ/25. Le faible nombre d’itérations d’algorithme nécessaires pour converger a permis de corriger les fluctuations de phase sur une bande d’environ 1kHzCoherent laser beam combining techniques rapidly appeared highly promising in the field of ultra-high power laser sources. Indeed, the combined intensity around the propagation axis follows a quadratic law with the number of combined emitters. The first part of my work has been focused on passive phasing techniques, based on self-organization properties of coupled lasers. We have shown, both numerically and experimentally, that the intracavity filtering function due to the interferometric nature of the set-up, is an intrinsic reason for combining efficiency decrease far above laser threshold. The decrease goes steeper when the number of combined laser increases, making that kind of system inappropriate for coherently combining a large number of lasers delivering high power. The second part of my work consisted in studying an innovative active phasing method that associates a phase-contrast like filter with an optimization algorithm reducing phase errors between emitters. Both simulations and experiments showed the weak sensitivity of this method to the number of combined emitters. We demonstrated the phasing of 7 to 37 fiber lasers, delivering up to 5W each. To the best of our knowledge, this last result is the highest number of fiber lasers combined with an active phasing method. The combining efficiency has been estimated around 94%, corresponding to a residual phase error of λ/25. The weak number of algorithm iterations needed to reach the in-phase regime offered a bandwidth of approximately 1kHz
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Pannell, C. N. "Fibre-optic laser Doppler velocimetry." Thesis, University of Kent, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383370.
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Hashemzadeh, Majid. "Investigations into fibre laser cutting." Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/14057/.
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Fibre laser cutting of mild steel using oxygen and nitrogen is widely used in industries throughout the world. An IPG YLR-2000 Ytterbium fibre machine with a maximum power of 2 kW and a wavelength of 1.06 µm is used throughout this research. The effects of oxygen and nitrogen as assist gases on the feature of laser cutting process are different in terms of kerf width, surface roughness, heat affected zone and striation pattern. The kerf width in oxygen laser cutting is wider than that for nitrogen. The striation pattern on oxygen cut edge is smoother than that for the nitrogen cut edge. When using oxygen, the cut edge is covered by a fragile oxide layer while this feature is not seen on the nitrogen cut edge. After laser cutting with oxygen, the cut edge is dross free whilst nitrogen cut edge is drossy. Laser piercing is used to generate a starting point for laser cutting. The pierced hole is normally larger than the kerf width, which means that it cannot lie on the cutline. An experimental programme investigating the piercing process as a function of laser and assist gas parameters is presented. Oxygen and nitrogen were used as assist gases, with pressures ranging from 0.3 to 12 bar. The sizes, geometries and piercing time of the holes produced have been analysed. The pierced hole size decreases with increasing gas pressure and increasing laser power. Oxygen assist gas produced larger diameter holes than nitrogen. A new technique is presented which produces pierced holes no larger than the kerf with and would allow the pierced hole to lie on the cut line of the finished product – allowing better material usage. This uses an inclined jet of nitrogen when piercing prior to oxygen assisted cutting. Specific point energy (SPE) is a concept that has been successfully used in laser welding where SPE and power density determine penetration depth. This analysis allows welding carried out by different laser systems to be directly compared. This work investigates if the SPE concept can be applied to laser cutting. Laser cutting of various thicknesses of mild steel, two different optical set ups and three different delivery fibres with a range of powers and translation speeds is done to gain results for numerous different parameter combinations. It is found that the SPE concept is applicable to laser cutting and the following effects noted: for given material thickness and any given value of SPE, cost is decreased by using a larger beam diameter; for given cut sheet thickness, cutting efficiency increases with SPE; for given value of SPE, cutting efficiency increases as material thickness decreases.
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Guionie, Marie. "Lasers à fibres bifréquences bipolarisations : stabilisation et montée en fréquence du battement." Thesis, Rennes 1, 2020. http://www.theses.fr/2020REN1S059.
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Cette étude porte sur les lasers à fibres bifréquences bipolarisations. Le contrôle de la différence de fréquence de ces lasers représente un enjeu important en photonique microonde. En effet, maîtriser le battement permettrait la réalisation de sources compactes et à faible bruit visant des applications de métrologie ou télécom. Nous travaillons ici sur des sources émettant à 1,5 μm, de type DFB ou DBR, soit dopés Er soit co-dopés Er-Yb. Ils présentent une fréquence de battement d'environ 1 GHz pour les DFB, et 100 MHz pour les DBR. Les objectifs de la thèse ont été, d'abord de stabiliser le battement, puis d'explorer différentes méthodes pour modifier la biréfringence de la fibre et ainsi accroître la fréquence de battement. Nous avons d'abord utilisé la méthode de stabilisation par boucle à verrouillage de phase pour asservir le battement sur une fréquence de référence. En nous servant de la diode de pompe comme actuateur, nous avons réussi à stabiliser des battements entre 300 MHz et 10 GHz pendant plusieurs jours. Ensuite, nous avons mis en place une méthode de stabilisation par réinjection optique décalée en fréquence. Un modèle théorique basé sur des équations-bilan couplées a été utilisé et a permis de retrouver les observations expérimentales. En stabilisant le battement sur une référence extérieure, différents régimes dynamiques ont été observés. Dans la zone stable, on réduit le bruit de phase jusqu'à −100 dBc/Hz à 1 kHz de la porteuse. Puis, nous avons couplé le montage de la réinjection optique a une ligne à retard, afin de stabiliser efficacement le battement sur lui-même. Enfin, nous avons étudié plusieurs pistes pour augmenter la fréquence de battement des lasers DBR pour répondre aux besoins des applications. Nous avons pu suivre en temps réel la modification de biréfringence photo-induite par un faisceau UV. Nous avons aussi étudié une méthode de modification réversible de la biréfringence, en exploitant l'effet élasto-optique. Quelque soit la méthode employée, nous avons observé un accroissement de la fréquence de battement depuis 100 MHz jusqu'à plus de 10 GHz dans le meilleur des casThis work is about dual-polarization dual-frequency fibers lasers. The control of the frequency difference of these lasers is a major challenge in microwave photonics. Controlling the beat frequency could allow compact and low-noise sources, in order to develop applications in metrology or telecom. Here, we focus on 1.5 μm sources, in either DFB or DBR configurations, made of Er-doped or co-doped Er:Yb silica fibres. Their beat frequency is about 1 GHz for DFB lasers, and 100 MHz for DBR lasers. This thesis investigates several methods to stabilize the beat note, then to modify the fiber birefringence, to increase the beat frequency. First, an optical phase-locked loop method is used to lock the beat note on a frequency reference. By using the pump diode as an actuator, we have successfully stabilized beat frequencies between 300 MHz and 10 GHz for days. We then study a stabilization method by frequency-shifted optical feedback. A theoretical model based on rate equations model is used and allows to retrieve the experimental observations. We observe different dynamical regimes by locking the beat note on a reference frequency. In the stable area, we reduce the phase noise to −100 dBc/Hz at 1 kHz from the carrier. Next, we have mixed the set-up of the injection-locking to a delay line, in order to effectively stabilize the beat note on itself. Finally, various approaches have been explored to increase the beat frequency of DBR lasers, to the needs of applications. It has been possible to continuously monitor the impact of a UV beam on the fiber birefringence. We also studied a method for reversible modification of the birefringence, exploiting the elasto-optical effect. Regardless of the method used, we observed an increase in the beat frequency from 100 MHz to more than 10 GHz in the best case
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Castaneda, Juan Carlos Hernandez. "Ytterbium fibre laser cutting of wood." Thesis, University of Manchester, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.520761.
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This Ph.D. project provides for the first time the basic characteristics of fibre laser interaction with pine wood in dry and wet state. Fibre laser has gained importance in the laser material processing market as it presents an advanced technology. It is compact, reliable and more efficient than CO2 and Nd:YAG lasers. The high quality laser beam generated is currently in the kilowatt power level suitable for cutting operations. Furthermore, this laser system needs low maintenance and it has very long working life. It also presents the flexibility of fibre delivery, up to 200 meters from the laser source. Three major experiments were performed: laser cutting of dry pine wood with multiple passes of the laser beam; the laser cutting of wet and dry pine wood; analysing the effect of single and dual assisting gas jet(s) in the fibre laser cutting process. In each experiment, the main process parameters and their interactions affecting the process were discussed. Furthermore, the application of a desirability equation provided the optimal combination of the parameters involved (within the experimental window tested) to maximise the yield and the quality of the laser cutting operation. The effect of the assistant gas jet(s) was further analysed with a computational fluid dynamic (CFD) model, which information clarified its/their action in ejecting fumes and combusted materials in the process. Finally, an application study of Ytterbium fibre laser cutting of hardwood branches was performed; the results obtained were compared with CO2 laser cutting results reported in the literature.
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Carey, Christian. "Laser forming of fibre metal laminates." Thesis, University of Liverpool, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.511073.
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Hanna, Marc. "Sources laser femtoseconde à fibre optique." Habilitation à diriger des recherches, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00806296.
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Les sources laser à fibre sont de plus en plus utilisées dans des domaines très divers car elles possèdent des avantages majeurs : qualité de faisceau, intégration, possibilité d'émettre une puissance moyenne élevée, efficacité, robustesse. Nous présentons des travaux de recherche visant à étendre les performances des sources ultrarapides basées sur les fibres en utilisant des effets d'optique non linéaire et en proposant de nouvelles architectures laser.
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Mandal, Jharna. "Fibre laser development for sensor applications." Thesis, City University London, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.435942.
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Ji, Junhua. "Cladding-pumped Raman fibre laser sources." Thesis, University of Southampton, 2011. https://eprints.soton.ac.uk/186035/.
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In this thesis, I investigate cladding-pumped Raman fibre lasers and amplifiers. Such devices, offering a novel way to generate Raman gain, combine the advantages of the hugely successful cladding-pumped rare-earth doped fibre lasers with those of stimulated Raman scattering. They not only inherit most advantages of conventional fibre devices, such as flexibility, high efficiency, compactness, and robustness, but also provide their own advantages and distinct properties relative to conventional fibre sources, i.e., wavelength flexibility and nearly instantaneous gain without energy storage. Cladding-pumped Raman fibre laser sources utilise double-clad Raman fibres as the gain medium. These are similar to a rare-earth doped double-clad fibre except that there is no laserion doping of the core. With double-clad fibres, the high-power output from low-cost multimode pump sources can be converted into diffraction-limited signal beams, e.g., through stimulated Raman scattering. Thus, cladding-pumped Raman fibre laser sources are a kind of brightness enhancers. In the beginning of this thesis, I theoretically analyse various factors that limit the brightness enhancement of such devices. One of the limits is unwanted 2nd-Stokes generation, which restricts the area ratio between the inner cladding and core. By designing a new DCRF with a W-type core, I successfully relax this restriction by nearly five times. Combined with other factors, i.e., core damage threshold, walk-off, numerical aperture, and background loss, a brightness enhancement of more than 3500 for the designed fibre could be achieved in such devices shown by a model with right pump sources and parameters. Secondly, I focus on the conversion efficiency of such devices. A well-designed fibre with inner-cladding-to-core area ratio around six was used as a double-clad Raman fibre, pumped by a source with nearly rectangular pulse shapes. The nearly rectangular pulses were obtained from an erbium and ytterbium co-doped master optical power oscillator through prepulse shaping. A sufficiently short piece was chosen to reduce the background loss and walk-off. The highest peak power conversion into the 1st Stokes was 75% and the energy conversion efficiency was over 60% in a pulsed cladding-pumped Raman fibre amplifier. Thirdly, I study the power scalability. Theoretically, I analyse the achievable power of such devices. The core size turns out to be a critical factor in most cases. The ultimately output power is limited to around 24 kW by thermal lensing if the core is large enough and enough pump power available. Experimentally, in collaboration with co-workers, a 100 W claddingpumped Raman fibre laser was demonstrated at 1116 nm. The output beam was nearly diffraction-limited. It shows the potential of power scalability of such devices and the ability of generating high power diffraction-limited sources at wavelengths outside the conventional range that rare-earth doped fibres offer. Since a large core size is a critical factor for power scaling, new double-clad Raman fibres with large-mode areas were introduced. They were experimentally demonstrated to work as efficiently as the previous fibre. An Nd:YAG laser was used to pump one of these fibres, and a 1 mJ Raman fibre source with good beam quality was thus demonstrated. This shows that double-clad Raman fibres offer another approach to obtaining high-brightness high-energy sources. In addition, based on a cladding-pumped Raman fibre converter, a simple and efficient method was proposed to generate supercontinuum sources.
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Chen, Hui-Chi. "Fibre laser welding of dissimilar materials." Thesis, University of Manchester, 2010. https://www.research.manchester.ac.uk/portal/en/theses/fibre-laser-welding-of-dissimilar-materials(de9ec14d-4201-4e65-b4f8-9a40bfd7a883).html.
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Joining technology has played an important role in manufacturing since the industrial revolution. Welding methods are under constant development in response to real demands. Laser welding is considered an effective joining method that can provide high quality and cost effective results to bring economical benefits to industry. Nowadays, fibre lasers have the capability to fill some of the roles of the CO2 and Nd:YAG lasers in industrial welding applications because of their excellent characteristics such as higher energy density and superior beam quality. However, up to now, few quantitative evaluations of its performance against more traditional lasers have been conducted in laser material processing. This thesis presents an investigation into the fibre laser welding of dissimilar materials processes. The challenges in the welding of dissimilar materials are mainly related to the large differences in the physical and chemical properties of the welding materials. These differences readily cause residual stresses, intermetallic phases and chemical composition gradients. The aim of this work is to understand and explain mechanisms occurring in single mode fibre laser welding of dissimilar materials. The first part of this work addressed fibre laser butt welding of Ti-6Al-4V titanium alloy to Inconel 718 nickel alloy. Here, the weld quality was evaluated in terms of the weld geometry, microstructures, hardness distributions and the formation of intermetallic phases. Results showed that the offset position of the laser beam was an important factor affecting the weld quality. Furthermore, the thermal history of the weld was simulated using analytical modelling analysis and this was used to identify a parameter window for crack-free welding. The second part of this work focused on fibre laser lap welding of Zn-coated steel to Al alloy with different laser power delivery modes (pulsed wave and continuous wave). The relationship between the weld quality and process parameters, such as: pulse frequency, laser power, welding speed, the shielding gas type and number of welding passes, were investigated. The mechanical properties, metallurgical effects and corrosion performances of welds were analysed. Results showed that the shielding gas type and the number of welding passes were key factors in controlling the weld quality in the fibre laser welding of Zn-coated steel to Al alloy process. Finally, the common features, characteristics and the potential of fibre laser welding of dissimilar materials are presented.
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Phillips, Mark Wayne. "An investigation of laser action in single transverse mode fibre lasers." Thesis, University of Southampton, 1989. https://eprints.soton.ac.uk/396542/.
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This thesis deals primarily with the study of pulsed output operation in single transverse mode fibre lasers resulting from active modulation of the laser inversion and the intracavity field. Sinusoidal modulation of the pump power offers an efficient method for producing energetic output pulses of a few microseconds duration. For certain ranges of pump parameters the fibre laser exhibits a highly nonlinear dynamic response to the pump signal and can even become chaotic. Modelocked operation has been demonstrated using both active amplitude and phase modulation schemes. Bandwidth-limited modelocked pulses with a sech2 profile and 20 psec duration are reported. These are the shortest duration pulses yet observed in a doped fibre laser system. The effects of self phase modulation and group velocity dispersion are shown to have a major influence on the modelocking process, resulting in significant performance differences between AM modelocked and FM modelocked fibre lasers. FM modelocking appears most favourable. Detuning the phase modulation frequency has allowed investigation of continuous-wave FM operation in fibre lasers. Laser spectra and temporal behaviour are characteristic of a single FM oscillation. Modelocking has also been demonstrated in a system where the phase modulation occurs within the fibre itself, induced by a transverse acousto-optic standing wave. The laser oscillates over a 500 GHz bandwidth capable of supporting sub-picosecond duration modelocked pulses. Incomplete locking of the axial modes leads to output pulses of 60 psec duration. The low loss resonator configuration allows efficient end-pumping by a low power laser diode. This system appears a likely candidate for enhanced modelocking through nonlinear mode-coupling techniques. We also discuss schemes for narrowing the fibre laser bandwidth, either by intracavity etaloning or by extracavity frequency-selective feedback.
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Boulnois, Arnaud. "Étude en théorie des lasers : application au laser à fibre optique et au laser multimode longitudinal." Lille 1, 1997. http://www.theses.fr/1997LIL10203.
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Les travaux presentes au cours de cette these ont pour objet l'etude theorique de la dynamique du laser a fibre dopee au neodyme. Afin de decrire le comportement tres complexe de ce systeme, nous avons mis en oeuvre deux methodes d'analyse distinctes associees chacune a des proprietes physiques particulieres. Nous consacrons notre premiere approche a la description d'un laser oscillant simultanement sur deux etats de polarisation orthogonaux. La modelisation de ce systeme necessite un redeveloppement complet des equations du laser sur la base de proprietes tres generales de l'interaction rayonnement-matiere. Nous obtenons ainsi un nouveau modele pour le laser de classe b a fibre dopee qui a conduit aux resultats suivants : - la mise en evidence de l'existence de deux seuils independants associes a chaque direction de polarisation du champ - la demonstration de l'influence de chacune de ces polarisations sur la croissance de l'autre - la caracterisation de l'effet de la polarisation et de l'orientation du champ de pompe par rapport aux axes propres de la fibre sur la repartition de l'intensite du champ laser emis - la mise en evidence numerique de regimes periodiques, quasi-periodiques et du phenomene d'antiphase dans des regimes chaotiquesDans la seconde partie, nous privilegions l'aspect multimode comme source d'instabilites. Pour cette nouvelle approche, nous utilisons le modele de bloch-maxwell pour un laser multimode et desaccorde en frequence. L'etude complete de ce systeme ne pouvant etre conduite par les methodes habituelles, nous mettons en oeuvre des theories plus originales. La premiere consiste a redefinir l'approximation du laser de classe b afin de faire apparaitre un petit parametre. Puis, par des operations arithmetiques, nous transformons le modele dans le but d'en extraire une solution oscillante particuliere et de determiner son seuil d'existence. Enfin, nous reconstruisons les solutions a partir de ce point de bifurcation a l'aide d'un developpement perturbatif et d'une definition de nouvelles echelles de temps et d'espace
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Shalaby, Badr Mohamed Ibrahim. "Source laser à fibre pour application à l’environnement « Laser à fibre multimodales et multicoeurs : cavité à contrôle modal"." Limoges, 2011. https://aurore.unilim.fr/theses/nxfile/default/743045ad-1b87-4e2d-8049-0a1692337c72/blobholder:0/2011LIMO4005.pdf.
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La montée en puissance dans les lasers ou amplificateurs à fibres uni-modales s’accompagne d’effets non linéaires, de dommage sur les faces de la fibre à cause d’une petite aire effective dans les guides amplificateurs classiques. L’objectif de ma thèse a été d’étudier l’exploitation, au sein de lasers, de fibres amplificatrices de large section, telle qu’une fibre multimodale ou une fibre à coeurs multiples tout en assurant la génération de faisceau de bonne qualité spatiale. Tout d’abord, j’ai étudié numériquement et expérimentalement le processus de l’auto-imagerie dans une fibre de grande aire modale dans une configuration laser en anneau unidirectionnel. J’ai successivement considéré une fibre multimodale (FMM) puis une fibre à coeurs multiples (FMC). Cette configuration réalise un filtrage spatial sur le champ proche en utilisant une boucle de rétroaction par une fibre monomode. Nous avons aussi étudié numériquement et expérimentalement l’excitation du mode fondamental et la combinaison linéaire de plusieurs modes guidés dans une architecture de laser qui réalisait un filtrage spatial sur le champ lointain. La dernière partie, consiste en une étude expérimentale de la combinaison cohérente de deux fibres multicoeurs parallèles en utilisant à la fois une excitation sélective et un filtrage du champ lointainThe rise of power delivered by lasers and amplifiers based on standard single mode fiber is accompanied by non linear effects and damage on fibers output face. The goal of my thesis research was to generate high quality beam from lasers based on fibers with large cross-section, such as multimode fibers (MMFs) or multicore fibers (MCFs). First, I numerically and experimentally studied the process of self-imaging in MMF and MCF in a ring fiber laser configuration. The investigated configuration performs spatial filtering on the near field using a feedback loop by a single mode fiber. I have also investigated the excitation of the fundamental mode and the linear combination of several guided modes in a laser architecture where a spatial filtering was achieved in the far field. The last part dealt with experimental study of coherent combination of two parallel multicore fibers using of both selective excitation and far field spatial filtering
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Di, Francesco Mattia. "Laser-assisted automated fibre placement process development." Thesis, University of Bristol, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.752749.
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Yao, Tianfu. "Fibre laser sources with low quantum defect." Thesis, University of Southampton, 2014. https://eprints.soton.ac.uk/372830/.
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High power fibre lasers with efficient amplification of optical signals have been widely used for various applications for many years. A great advantage of fibres is their thermal properties, which are so good that fibres are often quoted as being immune to thermal degradation. However, powers keep increasing and the fibre geometry is becoming more similar to their “bulk” (non-waveguiding) counterparts. Thermal problems are severe in bulk lasers, and are, unsurprisingly, becoming more important also for fibres. Heating is inevitable in the laser cycle due to the energy difference(quantum defect) between pump and signal photons. Nevertheless, there is substantial room to reduce the heating by minimising the quantum defect, with pump and signal wavelengths as close as possible. In this thesis, I demonstrate low-quantum-defect fibre amplifiers and lasers based on two different energy conversion processes, i.e., in high-brightness (tandem) pumped ytterbium-doped fibre and in short-wavelength-pumped Raman fibre. The latter approach increases the photon energy relative to the thermal energy and vibrational energy of the host. Firstly, as it comes to tandem-pumping of ytterbium-doped fibre amplifiers, the challenge is to make the pump and signal wavelengths as close as possible, while keeping the signal gain and pump absorption sufficiently high. With the optimum average ytterbium excitation level and high pump brightness, the quantum defect can be as small as 0.6% when pumped at 1030 nm, according to theoretical calculations. Subsequently, an experimental amplifier core-pumped by a single-mode laser source is presented. A 2% quantum defect is reached, with pump and signal wavelengths at 1030 nm and 1050 nm, respectively. The slope efficiency reaches 95% to 96%. Initial investigations show low photodarkening with tandem-pumping, with some dependence on the dependence on ytterbium ions concentration. Secondly, Raman conversion of pulses in a diode-pumped highly nonlinear fibre is studied in a ring-laser cavity configuration. The quantum defect is 3.5% with 806 nm pump and 835 nm Stokes wavelengths. A slope efficiency of 65% is obtained with 600 m long fibre and 100 ns pulse width. Then, I study experimentally and theoretically 975 nm continuous-wave-pumped fibre Raman lasers based on a graded-index and a double-clad fibre. Both lasers emit at 1019 nm (4.3% quantum defect) with improved brightness. A record laser output of 6 W and 19% slope efficiency from the double-clad Raman fibre and 20 W from the graded-index fibre shows further scaling of singlemode power is possible with improved cavity and fibre design.
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Alcock, Ian Peter. "Laser action in neodymium doped silica fibre." Thesis, University of Southampton, 1988. https://eprints.soton.ac.uk/404730/.
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This thesis describes an investigation into the suitability of neodymium doped monomode optical fibre as a gain medium for miniature laser systems. Characterisation of the material and parameters required for quantitative analysis of such laser systems are presented in a spectroscopic analysis carried out in the first part of the thesis. Measurements performed in this section also indicate that at room temperature the wide fluorescence bands of the neodymium doped silica fibre are spectrally broadened by a homogeneous process. The behaviour of a longitudinally pumped continuous-wave fibre laser is modelled in the next section followed by the design and realisation of a practical system. Efficiencies of approximately 6% and threshold pump powers of 8-12 mW have been obtained, even under narrow linewidth operation. Tunability over ranges of 45-60nm has also been demonstrated. The thesis is concluded by experiments on pulsed fibre lasers. Q-switching the laser has achieved peak powers of several watts in pulses 180ns wide with good agreement between the results obtained and predicted values. Shorter pulses 450ps wide have been realised by mode-locking a fibre laser. Suggestions for reducing the pulse width further and the effects of material dispersion are also given. Finally modulated pump sources have been investigated and it is shown that synchronous pumping with short pulses has limitations due to dispersion, while resonantly pumping relaxation oscillations is a simple means of obtaining a pulsed output.
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AZAMI, NAWFEL. "Laser hybride fibre-guide accordable electro-optiquement." Nice, 1998. http://www.theses.fr/1998NICE5263.
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L'accordabilite de la longueur d'onde du laser a fibre optique dopee erbium a l'aide d'un interferometre de type mach-zehnder integre sur un substrat de niobate de lithium (linbo 3) conjugue les talents respectifs de l'amplificateur a fibre dopee erbium et de l'optique integree sur linbo 3. Les forts gains atteints par la fibre dopee erbium ajoutes a un coefficient electro-optique du niobate de lithium des plus eleves demontrent une accordabilite spectrale continue de 24 nm de la longueur d'onde laser dans la troisieme fenetre des telecommunications optique pour des tensions variant entre 0 et 7 volts. Le laser est caracterise par un faible seuil d'oscillation de 5 mw de puissance pompe couplee a 980 nm et par un rendement de 4%. Les proprietes polarimetriques de la fibre laser associee a un ou deux polariseurs intracavites permettent, par une configuration laser et une birefringence adequates, d'accorder mecaniquement la longueur d'onde laser de la fibre dopee erbium sur une plage spectrale de 28 nm autour de 1550 nm par rotation de la lame quart d'onde d'un controleur de polarisation a fibre intracavite. Le laser est caracterise par un seuil d'oscillation de 4mw de puissance pompe couplee a 980 nm et un rendement de 20%. L'association des deux effets de filtrage, interferometrique accordable electro-optiquement a l'aide d'un interferometre de mach-zehnder fabrique par echange protonique et polarimetrique, demontre l'accordabilite electro-optique du laser a fibre dopee erbium dans une plage spectrale de 30 nm autour de 1550 nm pour des tensions variant entre 11 et +7 volts et pour deux positions differentes de la lame quart d'onde du controleur de polarisation a fibre. Le laser est caracterise par un seuil de 5 mw de puissance pompe couplee a 980 nm et un rendement de 5%.
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Lacot, Eric. "Dynamique d'un laser à fibre dopée Er3+." Grenoble 1, 1992. http://www.theses.fr/1992GRE10202.
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La dynamique d'un laser a fibre dopee er#3#+ a ete analysee aussi bien en regime de pompage continu qu'en regime de pompage module. En pompage continu, lorsque l'on augmente la puissance de pompe, le laser atteint un second seuil et une bifurcation de hopf apparait menant le laser d'un regime de fonctionnement continu a un regime de fonctionnement pulse. Une etude de l'influence des parametres aussi bien intrinseques (dopage, longueur,. . . ) que extrinseques (pompage, cavite,. . . ) nous a permis de determiner dans un espace de parametres propres au laser la frontiere entre les differents regimes de fonctionnement. Nous avons aussi pu observer, aussi bien en regime de fonctionnement continu qu'en regime de fonctionnement pulse, que le comportement transitoire de l'intensite totale du laser est en fait la somme des comportements transitoires de deux etats orthogonaux de polarisation. En regime transitoire (reponse a un pompage en echelon) le laser a fibre presente des oscillations de relaxation. C'est en modulant la puissance de pompe de maniere sinusoidale au voisinage de la frequence de relaxation que nous avons pu observer dans le comportement dynamique du laser a fibre une cascade de doublement de periodes conduisant a un fonctionnement chaotique. Nous avons developpe un modele semi-classique decrivant de la dynamique du laser a fibre dopee. Cette dynamique est parfaitement decrite par un modele de deux lasers couples pompes de maniere coherente. La transition entre le regime continu et le regime pulse est bien decrite ainsi que les deux etats propres de polarisation et le comportement chaotique
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Nyairo, Kennedy Obare. "The multichannel grating cavity laser." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240058.
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Xu, Mengrong. "Advances in hollow core fibres and application to mid-infrared fibre gas lasers." Thesis, University of Bath, 2018. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760959.
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Anti-resonant hollow core fibre is a new kind of optical fibre waveguide in which light is trapped in a hollow core surrounded by the capillary formed microstructured cladding. This fibre exhibits high damage threshold, low dispersion and ultra-low nonlinearity with relatively low loss of a few tens of dB/km. Its intrinsic feature of multimode delivery limits the applications with high requirements of single mode transmission. In my thesis, I demonstrate how the design of hollow core fibre can be improved with single mode guidance. S2 imaging measurement was used to analyse the mode content of the solid core fibres. In my research, I established S2 measurement to measure the mode contents in hollow core fibres for the first time. Two hollow core fibres with 8 capillaries and 7 capillaries in their claddings were fabricated in same fashion and showed differences in low attenuations. By comparing the mode contents in both of the fibres via S2 imaging measurement, 7-capillary HCF was demonstrated to give better performance on single mode guidance. Among the applications of the HCF, the property of delivering high power in HCF makes the gas filled HCF laser possible. In my research, a continuous-wave mid-infrared acetylene filled hollow core laser was built with a slope efficiency of 33% and an output power of over 1 watt at the wavelength region of 3.1~3.2 μm. The pump source is an Erbium-doped fibre amplified tunable laser diode which works at C-band wavelength. The fibre without the gain medium has two transmission bands with low attenuation of 0.037 dB/m and 0.063 dB/m at pumping and lasing wavelengths respectively. This laser system works in either cavity-based configuration or single pass ASE configuration. The latter configuration shows a better performance in high output power and high slope efficiency. The optimized laser system was studied experimentally with the proper fibre length and gas pressure. This laser system could be extended to be filled with other molecules to longer wavelengths and has potential for high power output.
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Scurria, Giuseppe. "High power 2 μm fiber laser for mid-infrared supercontinuum generation in fluoride fibers." Thesis, Bordeaux, 2019. http://www.theses.fr/2019BORD0342.
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Le développement de sources optiques ayant une forte brillance et un large spectre est nécessaire pour différentes applications telles que les contremesures optroniques, les systèmes LIDAR et la spectroscopie. Ce travail de thèse est consacré à l'étude de la génération de supercontinuum de forte puissance dans le domaine spectral allant de 2 à 5 μm. Un laser à fibre dopé thulium, émettant à 2 μm, a été réalisé et caractérisé en régimes continu, déclenché et à verrouillage de modes déclenchés. En régime continu, une puissance de 45 W a été atteinte avec un rendement différentiel de 58%. L'intégration de deux embouts en quartz fondu, fusionnés aux extrémités de la fibre active, a permis une amélioration drastique du contrôle de la température et de la stabilité générale du système dans tous les régimes de fonctionnement précédemment mentionnés. En régime combiné, de verrouillage de modes déclenchés, la puissance moyenne maximale en sortie de laser était de 40 W à une fréquence de déclenchement de 150 kHz. Pour une puissance moyenne en sortie de 20 W et une fréquence de déclenchement de 50 kHz, l'impulsion verrouillée la plus énergétique avait une énergie de 88 μJ et une puissance crête estimée de 60 kW. Dans tous les régimes de fonctionnement mentionnés, le facteur de propagation du faisceau M2 était proche de la limite de diffraction avec une excellente valeur de 1,1. Le faisceau de ce laser fibré a été utilisé pour pomper des fibres optiques en fluorure pour la génération de supercontinuum dans le domaine spectral de 2 à 5 μm. Les travaux expérimentaux ont été focalisés sur les matériaux en fluorure que sont le ZBLAN et le fluorure d’indium (InF3). Avec la fibre en ZBLAN, plus de 10 W ont été générés sur l'ensemble du domaine spectral avec une limite supérieure de domaine spectrale allant jusqu'à 4,4 μm. L'efficacité de conversion du supercontinuum a été mesurée à 35%, 28%, 15% et 8% pour un faisceau généré de longueur d'onde supérieure à 2,15 μm, 2,65 μm, 3,1 μm et 3,5 μm, respectivement. Concernant la fibre en InF3, la réalisation d'un nouveau système d'injection (combinant une fibre ZBLAN large coeur et un coupleur commercial fibre à fibre) a permis d'atteindre une grande stabilité thermo-mécanique de la fibre pendant les expériences à haute puissance. La génération de supercontinuum dans cette fibre s'est concrétisée par un faisceau de sortie dont le spectre atteignait la longueur d'onde de 4,7 μm et une puissance de 7 W sur l'ensemble du domaine spectral. À notre connaissance, ceci est le premier faisceau supercontinuum, ayant une puissance de l'ordre de quelques watts, généré dans une fibre en InF3 pompée par un oscillateur uniqueHigh brightness and broad spectrum optical sources in the mid-infrared (mid-IR) are promising for different applications such as optronic countermeasures, LIDAR systems and spectroscopy. This thesis research work is dedicated to the investigation of high power supercontinuum generation in the 2-5 μm range. A thulium-doped fiber laser emitting at 2 μm has been built and characterized in continuous wave, Q-switching and Q-switched mode-locking regime. In continuous wave operation, as much as 45 W have been reached with a slope efficiency of 58%. The implementation of two fused-quartz end-caps fusion spliced at the extremities of the active-fiber improved the thermal management and the overall stability of the entire system in all mentioned regimes of operation, allowing for higher pump powers. In Q-switched mode-locking, the maximum average output power level was 40 W, for a Q-switch repetition rate of 150 kHz. At the average output power level of 20 W and 50 kHz of Q-switch repetition rate, the most energetic mode-locked pulse had an energy of 88 μJ and an estimated peak power of ~60 kW. In all the mentioned operation regimes, the measured beam parameter M2 of the fiber laser was 1.1, close to the diffraction limit. This laser has been used to pump fluoride optical fibers (ZBLAN and InF3) for supercontinuum generation in the 2-5 μm range. In ZBLAN, more than 10 W in all spectral bands have been obtained, with an output spectrum extending up to 4.4 μm. A conversion efficiency of 35%/28%/15%/8% has been measured for wavelengths longer than 2.15 μm/2.65 μm/3.1 μm/3.5 μm, respectively. For the InF3 fiber, a new design of an injection system, consisting of a large core diameter ZBLAN optical fiber and a commercial fiber-to-fiber coupler, allowed to enhance the thermo-mechanical stability of the fiber. The supercontinuum radiation generated in InF3 showed an output spectrum spanning up to around 4.7 μm with an output power level of 7 W in all spectral bands. To the best of our knowledge, this was the first Watt-level supercontinuum radiation in an InF3 fiber pumped by a singleoscillator
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Barnier, Fabien. "Fibre Bragg grating techniques." Thesis, University of Hull, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.322570.
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Salama, Adel. "Laser machining of carbon fibre reinforced polymer composite." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/laser-machining-of-carbon-fibre-reinforced-polymer-composite(7310ed95-b876-480b-a8b4-2033b4309cb6).html.
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Carbon fibre reinforced polymer (CFRP) composites have found a wide range of applications in the aerospace, marine, sports and automotive industries owing to their lightweight and acceptable mechanical properties compared to the commonly used metallic materials. The currently dominating method of machining CFRP is by mechanical means that has found many problems including extensive tool wear, fibre pull-out and delamination. Lasers as non-contact tools have been widely applied for cutting and drilling materials. However, machining of CFRP composites using lasers can be challenging due to inhomogeneity in the material properties and structures, which can lead to thermal damage such as charring, heat affected zones (HAZs), resin recession and delamination. In previous studies, Nd:YAG, diode pumped solid state (DPSS), CO2 (continuous wave), disk and fibre lasers were used in machining CFRP composites and the control of damage such as the size of heat affected zones (HAZ) and achieving comparable material removal rate with the mechanical processes remain a challenge. Most reported work showed a typical heat affected zone of 0.2-1.2 mm. The availability of short pulsed transversely excited atmospheric (TEA) CO2 lasers and ultra-short laser pulse sources such as picosecond lasers make it possible to improve the laser machining quality of CFRP materials. In this research, the machining of CFRP composites using a microsecond pulsed TEA CO2 laser, a state of the art high power picosecond laser and a 1 kW single mode fibre laser system was investigated. The yielded heat affected zone was less than < 25 µm for the TEA CO2 and the picosecond laser machining, although the material removal rate was low. Additionally, it has been shown that the pulsed fibre laser improved the machining quality compared to that with the continuous mode. A potential application of the fibre laser for composite repair and remanufacturing was investigated. The interactions between picosecond laser beam and CFRP composite were studied in more detail including understanding the self-limiting effect in single and multiple parallel tracks drilling/machining through both experimental and theoretical studies. Furthermore, a sequential laser and mechanical drilling of CFRP was investigated to improve the machining rate. The work performed in this PhD was driven by aerospace industry needs, with the collaboration of Rolls-Royce plc and BAE Systems as industrial partners.
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Stratoudaki, Theodosia. "Laser generated ultrasound in carbon fibre reinforced composites related to laser wavelength." Thesis, University of Warwick, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.396957.
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Tang, Mincheng. "Etude du comportement dynamique des sources laser ultrarapides à base de fibres actives fortement dispersives." Thesis, Normandie, 2017. http://www.theses.fr/2017NORMR117/document.
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Les lasers ultra-rapides fibrés sont aujourd’hui incontournables dans de nombreuses applications industrielles et scientifiques du fait de leur stabilité, de leur compacité et des hautes puissances disponibles. Les performances actuelles, rendues accessibles par le développement de fibres à larges aires modales et le concept d’amplification à dérive de fréquence, sont toutefois complexes à mettre oeuvre et limitées par l’utilisation de composants massifs pour les étapes de compression et d’étirement des impulsions. Ces travaux de thèse, à la fois expérimentaux et numériques, avaient pour objectif d’explorer des régimes dynamiques originaux basés sur l’utilisation de fibres actives spécifiques combinant large aire modale et propriétés dispersives adéquates pour la génération d’impulsions ultra-courtes de haute énergie. Les études numériques ont ainsi permis de montrer que des régimes impulsionnels à haute dispersion normale pouvaient être atteints en exploitant les phénomènes de résonnance et de couplage de modes dans des fibres de Bragg ou à profil en W. L’étude de l’influence des paramètres de la cavité laser sur le mécanisme de verrouillage de modes a permis d’identifier des configurations attractives pour la montée en puissance. La mise en oeuvre expérimentale de ces concepts a notamment permis le développement d’une source laser à soliton dissipatif produisant des impulsions énergétiques (38 nJ, 700 fs après compression) à des longueurs d’ondes autour de 1560 nm, record pour ce type d’oscillateur. La réalisation expérimentale de sources ultra-rapides basées sur des fibres actives spécifiques combinées au phénomène de couplage de mode ont permis d’identifier les potentialités et limitations de ces architectures originales à fortes dispersions totales pour la montée en énergieUltrafast fiber lasers represent today a ubiquitous technology in various industrial and research applications thanks to their inherent advantages such as compactness, stability and high power. The best performances to date, mostly relying on large mode area fibers and chirped pulse amplification, however require complex experimental developments and are limited by the use of bulk components for pulse stretching and compression. The experimental and numerical work presented in this PhD thesis aimed at exploring original dynamical regimes based on specific active fibers combining large mode area and high dispersions for the generation of high-energy ultra-short pulses. The numerical studies then showed that pulsed regimes with high normal dispersions could be reached by exploiting resonance and mode-coupling phenomena in Bragg or W-type fibers. Studying the influence of the cavity parameters on mode-locking mechanisms allowed to target attractive configurations for energy scaling. The experimental implementation of this concept allowed the development of a dissipative soliton source delivering record high-energy chirped pulses (38 nJ, 700 fs after compression) at 1560 nm. The realization of ultrafast sources based on specific active fibers combined to mode-coupling phenomena then brought the possibility to identify the potentiality and limitations of these particular architectures with high dispersions for energy scaling
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Chouli, Souad. "Structures optiques dissipatives en cavité laser à fibre." Phd thesis, Université de Bourgogne, 2011. http://tel.archives-ouvertes.fr/tel-00692049.
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Cette thèse concerne l'étude de la dynamique des structures optiques dissipatives observées dans une cavité à gestion de dispersion utilisant l'évolution non linéaire de la polarisation comme technique de blocage de modes. Nous avons montré expérimentalement l'existence d'une transition graduelle entre le régime de fonctionnement continu et le régime de fonctionnement multi-impulsionnel. Nous nous sommes intéressés à l'état intermédiaire où il nous a été possible d'obtenir divers régimes inédits et d'étudier ainsi le comportement collectif des solitons dissipatifs en présence d'un fond continu. La dynamique de "la pluie de solitons" est une manifestation complexe et fascinante constituée de trois composantes de champ : le fond continu, les solitons de dérive et la phase condensée. Elle s'accompagne d'une circulation d'énergie à travers ces trois composantes. Le mouvement relatif des solitons de dérive ainsi que l'asymétrie temporelle présentent les caractéristiques majeures qui distinguent cette dynamique des autres. D'autres types d'auto-organisation ont été observés et étudiés, comme "le relargage des solitons de la phase condensée" ou bien encore "la vobulation du train de solitons". Nous nous sommes intéressés aussi à la propagation d'une seule impulsion dans la cavité. Pour la première fois, une importante dynamique de respiration spectrale a été prédite dans une cavité à gestion de la dispersion. Nous avons montré qu'une compression temporelle de l'impulsion est accompagnée d'un élargissement spectral d'une grande ampleur dans la partie passive de la cavité et que la largeur de l'impulsion peut dépasser la largeur de la bande passante du milieu amplificateur. Nous avons étudié la dynamique de la respiration spectrale, l'extraction et l'optimisation du signal laser en fonction des paramètres de la cavité et nous avons présenté les caractéristiques d'une cavité qui permet la génération d'une impulsion dont sa largeur spectrale est supérieure à la largeur de la bande passante de l'amplificateur d'un facteur de 2.4. Les dynamiques présentées dans cette thèse témoignent de la complexité et de la richesse de la dynamique dissipative des lasers à fibre fonctionnant en régime de blocage de modes passif par évolution non linéaire de la polarisation.
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Al-Mashikhi, Said Omar. "Fibre laser cutting of thin section mild steel." Thesis, University of Nottingham, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.523004.
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Midgley, J. H. "The semiconductor laser applied to fibre optic sensors." Thesis, University of Kent, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304186.
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Vas, Prashanth. "Laser Doppler Imager Flare (LDIflare) small fibre function." Thesis, University of Warwick, 2017. http://wrap.warwick.ac.uk/98579/.
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Distal sensorimotor peripheral neuropathy (DSPN), the classical length dependent symmetrical neuropathy of diabetes, can affect up to 50% of those with diabetes leading to significant morbidity, mortality and healthcare costs. There is increasing recognition that small nerve fibres, mediating pain, temperature and autonomic function, are involved early in the course of diabetic neuropathy, preceding large fibre involvement. However, assessment small fibre neuropathy (SFN), continues to be a significant challenge. The currently available options are either invasive, subjective with poor reproducibility, may not directly assess the region of interest or are still in a research phase. Thus, there is an ongoing need for simple, non-invasive and reproducible techniques for the evaluation of SFN. The laser Doppler imager flare (LDIflare) is one such novel, non-invasive technique of assessing small fibre function. It has been shown to be a reliable indicator of small fibre neuropathy, even when other SFN markers are either inconclusive or normal. However, the original methodology, took over an hour to complete, which limited its use as a clinical tool. The LDIflare methodology was therefore modified to overcome this limitation by incorporating an accelerated acclimatisation phase and a shorter duration of skin heating but at a higher final temperature reducing the total procedure time to under 30 minutes. The size of the resultant flares was nearly twice as large compared to the older method while demonstrating similar group differences in those with and without clinical neuropathy. Assessment of the LDIflare in healthy volunteers (n=94) demonstrated significant inverse relationship of LDIflare size to age (r=-0.42, p < 0.0001) but not with other anthropometric or metabolic factors except for fasting triglycerides (r=-0.36, p < 0.0001). Furthermore, the LDIflare possessed a sensitivity of 77%, specificity of 90%, positive predictive value (PPV) of 82% and negative predictive value of (NPV) 87% for the detection of clinical neuropathy. Recent observations, exploring into the aetiopathogenesis of diabetic neuropathy, have suggested that triggers for neuropathy development in the two main forms of diabetes may be different. Small fibre function (SFF) in individuals with type 1 diabetes with (MV+, n=24) and without (MV-, n=24) renal and retinal microvascular disease, but all without clinical neuropathy was assessed using the LDIflare. The finding of abnormal SFF only in the MV- group suggests that direct microvascular damage is an early aetiopathogenic factor in type 1 diabetes. Furthermore, in another study of normal glucose tolerant individuals not meeting the criteria for metabolic syndrome, SFF was observed to be abnormal with increasing fasting triglyceride concentrations. This is suggestive that hypertriglyceridaemia may play a pathogenic role in the development of neural dysfunction, and may partly explain the presence of neuropathy early in the course of type 2 diabetes, when significant hyperglycaemia is not a factor. The LDIflare in its current modification is a novel, reliable, non-invasive measure and objective method of detecting small fibre neuropathy. It has good reproducibility and offers excellent accuracy for the detection of clinical neuropathy. The age based normative values allow for a clear distinction of abnormal results. While further comparative studies between the LDIflare and modern markers of SFN are desired, the studies included in this submission support the use of the LDIflare technique to investigate abnormalities in the peripheral nervous system, in particular small nerve fibres, in research but also in clinical domains.
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Schmidt, Castellani Carlos Eduardo. "All-fibre wavelength versatile short pulsed laser sources." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/24135.
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Pulsed lasers operating in the picosecond or femtosecond regimes find a wide range of applications in optical sciences, such as spectroscopy, laser surgery, material processing and optical communications. Among the existing sources of short-pulses, mode-locked fibre lasers play an important role mainly due to their robust and compact nature, and also due to their ability to generate outputs over a wide range of repetition-rates, pulse durations, pulse shapes, peak powers and optical wavelengths. Considering the case of wavelength versatility, Raman amplification can be used to fill the spectral gaps that are not covered by the emission band of traditional rare-earth doped elements such as ytterbium and erbium, allowing the generation of light at unconventional wavelengths. Additionally, another contribution has come from the recent development of new nanomaterials such as graphene and carbon nanotubes that can be used as saturable absorbers over a broadband wavelength range. The experimental work reported in this thesis is mainly focused in combining the wavelength versatility allowed by Raman gain and carbon nanotubes and graphene to generate short-pulsed fibre lasers at different wavelengths. High power ytterbium and erbium lasers and also a high power Raman laser operating at 1450 nm are used as pump sources to seed the Raman gain and carbon nanotubes and graphene are the saturable absorbers used as mode-lockers. All the fibres utilized in the oscillators are highly non-linear single mode silica fibres doped with GeO2. The lasers operate in the dissipative soliton regime, generating chirped pulses with durations on the order of hundred of picosecond that are suitable for external compression. We demonstrate for example an erbium-pumped Raman oscillator generating 500 ps pulses that are linearly compressed to 2 ps. The results presented in this document are a contribution towards making fibre based lasers more universal devices in terms of wavelength operation.
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Hadeler, Oliver. "Distributed feedback fibre laser strain and temperature sensors." Thesis, University of Southampton, 2002. https://eprints.soton.ac.uk/46100/.
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This thesis presents the development of two new types of polarimetric distributed feedback (DFB) fibre laser sensors for simultaneous strain and temperature measurements. These fibre Bragg grating (FBG) based sensors offer strain and temperature measurement accuracies of ±0.3 - ±15 με and ±0.04 - ±0.2°C which are suitable for many applications. The main advantage of these DFB fibre laser sensors over other FBG based sensors is the simplicity of their interrogation system. The first type of sensor operates stably in a single longitudinal mode which splits into two orthogonally polarised modes. This sensor utilises the wavelength of one polarisation mode and the RF beat frequency between the two polarisation modes. The system complexity is reduced to a minimum in the dual longitudinal mode polarirnetric DFB fibre laser sensor which utilises the RF beat frequencies between two longitudinal modes and their associated orthogonal polarisations, therefore requiring only a simple and cost effective frequency counter. -ions and pump excited state absorption into account. An extended version of this model incorporates, for the first time, self-heating in DFB fibre lasers which is caused by non-radiative decays. The performance of DFB fibre lasers employed in telecommunication applications is likely to benefit from these modelled results, which are also verified by experimental data.
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Renaud, Cyril C. "High-power diode-pumped ytterbium-doped fibre-laser." Thesis, University of Southampton, 2001. https://eprints.soton.ac.uk/38210/.
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This thesis details the work I have done on double cladding ytterbium-doped fibre lasers. The two main subjects developed were the study of an efficient compact-launching system, and the operation of the double cladding compact fibre as a continuous-wave laser and a Q-switched laser.During this work, highly efficient side launching systems were developed. These allowed coupling of more than 70% of the pump light into the doped fibre and gave as good lasing efficiency as end-pumping system. The pump and the signal were also well separated in two different guides at the ends of the device. Efficient lasing at 976 nm (85% slope efficiency) was demonstrated too, by using small inner cladding area and double passed pump. Finally, pulse-energy as high as 7.7 mJ was achieved with a large core cladding pumped ytterbium doped Q-switched fibre laser.
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Matsas, Victor Jacob. "Passively and actively mode-locked fibre laser devices." Thesis, University of Southampton, 1993. https://eprints.soton.ac.uk/399129/.
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This thesis examines a number of passively and actively mode-locked fibre laser devices. The passive systems that are described incorporate the fast, saturable absorber-like action of a Kerr-type nonlinear optical switch to provide the passive mode-locking mechanism. Two such all-fibre nonlinear switches are described and analysed, namely the nonlinear optical loop mirror and the nonlinear optical gate. The mode-locked fibre laser systems based on the performance of these devices, informally known as the "Figure-of-eight" laser and the "nonlinear polarisation evolution" laser are described and characterised with respect to their modes of operation, self-starting thresholds and resonator parameters. These lasers provide a unique and convenient method for generating fundamental solitons 0.1-5 ps long. A theoretical analysis shows that the resonant loss mechanism, which is responsible for the formation of sidebands in the optical spectrum of the solitons produced by these systems, imposes a limit on the soliton pulsewidth that can be derived from a given cavity. This limit has a square-root dependence on the product of the intracavity dispersion and resonator length. The actively mode-locked fibre laser configurations that are described are 100-1000m long and form part of an investigation into their use for distributed temperature sensing. The sensing method is based on the production of spontaneous Raman backscattered radiation from the mode-locked pulse circulating within the cavity. An extended version of the Kuizenga-Siegman (K-S) analysis of active mode-locking has shown that in systems with large dispersion-length-(bandwidth)2 products there is a significant increase in the pulse durations that can be achieved. However, the accuracy of the K-S theory is also shown to be questionable when used for cavity lengths >50m. Optical time-domain reflectometry measurements on a 4km mode-locked ring laser provided strong supporting evidence for the successful use of long fibre lasers for distributed temperature sensing.
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Barber, Paul Richard. "Blue laser action in Tm-doped fluoride fibre." Thesis, University of Southampton, 1995. https://eprints.soton.ac.uk/399096/.
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This thesis describes the development of a fibre laser capable of converting infra-red light into coherent blue light in a simple and efficient way. This was achieved in a fluoride fibre, ZBLAN, doped with the rare-earth ion Tm3+ which has broad emission around 480nm when pumped by infra-red light with a wavelength near 1140nm. Multiple absorption of pump photons has been proposed as the upconversion process. The spectroscopy of Tm3+-doped ZBLAN glass has been investigated. Specific attention has been paid to the absorption strength near 1140nm from both the ground state and the excited states, 3H4, and 3F4, of the Tm3+ ion. This spectroscopy has allowed a rate equation model of the process to be produced which aided the selection of the optimum pump wavelength and enabled the small-signal gain and laser threshold to be calculated. The most efficient blue laser fabricated had a slope efficiency of 31% and a laser threshold of 90mW. both with respect to 1140nm pump power launched into the fibre core. By variation of the laser cavity parameters the laser threshold could be lowered at the expense of the slope efficiency; the lowest threshold achieved was 11 mW, which demonstrates the suitability of this laser for diode pumping. These results were all obtained with fibre doped to 1000 ppm by weight with Tm3+ ions. Fibres of a higher doping level displayed a photodarkening effect when pumped with infra-red light; this has also been investigated. A laser source with a broad gain bandwidth, 1100 to 1150 nm, has been produced to pump the upconversion fibre laser. This was based on Yb3+-doped germanosilicate fibre and utilised fibre grating technology to select the laser wavelength within the gain bandwidth. The knowledge of Yb3+-doped germanosilicate glass obtained was further used in a short investigation into fabricating a single frequency laser, operating at 1083nm, based on a short fibre laser cavity formed with fibre grating reflectors. An output power of approximately 5mW was achieved in a single frequency.
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Nowell, Mark Charles. "Push-pull directly modulated laser diodes." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339501.
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Girondin, Victor. "Etude et modélisation d'un modulateur à électro-absorption pour des applications de transmission radio sur fibre de signaux ULB aux fréquences centimétriques. Simulation système et caractérisation d'une liaison radio sur fibre à modulation externe." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066127/document.
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Ce travail de thèse, consiste à étudier les performances d'une liaison Radio sur Fibre simplifiée mettant en ¿uvre une modulation extérieure de la source optique avec des signaux OFDM. La source laser est couplée sur une même puce avec un modulateur à électro-absorption, développée par Alcatel III-V LaB été modélisée à l'aide du logiciel ADS( Advanced Design System) et caractérisée au laboratoire.Dans une première partie, nous présenterons une introduction de la technologie Radio sur Fibre avec ses applications, et son intérêt pour les technologies futures.Dans une seconde partie, nous nous intéresserons à la modélisation électrique du modulateur à électro-absorption utilisée pour effectuer la modulation numérique de type OFDM.Dans une troisième partie, sera proposée un modèle complet de la chaine de transmission intégrant tous les composants de la liaison, de manière à effectuer une analyse comportementale du système.La quatrième partie, présentent les performances mesurées réalisées au laboratoire sur le banc de caractérisation de la chaine en modulation numérique OFDM Ultra Large Bande et simulées sous ADS et les corrélations entre les résultatsThis thesis is to study the performance of a radio link over fiber simplified implementing an external modulation of the optical source with OFDM signals. The laser source is coupled on a single chip with an electro-absorption modulator, developed by Alcatel III-V LaB been modeled using software ADS (Advanced Design System) and characterized in the laboratory. In the first part, we will present an introduction to the Radio on Fiber technology with its applications, and its interest in future technologies.In the second part, we will focus on the electrical modeling of the electro-absorption modulator used to make the digital OFDM type. In the third part, will be offered a complete model of the transmission chain incorporating all components of the link, so as to perform behavioral analysis system. The fourth part, have measured performance in the laboratory on the chain of characterization bench in digital OFDM Ultra Wideband and simulated in ADS and correlations between the results
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Aubourg, Adrien. "Sources laser à fibre cristalline YAG dopée erbium et pompée par diode." Thesis, Palaiseau, Institut d'optique théorique et appliquée, 2014. http://www.theses.fr/2014IOTA0008/document.
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Parmi les nombreuses applications des sources laser, certaines nécessitent une propagation du faisceau dans l'atmosphère sur plusieurs kilomètres : télémétrie, désignation, ou encore imagerie active. Pour éviter tout risque oculaire tout en proposant de plus grandes portées, ces applications doivent faire appel à des sources laser émettant une longueur d'onde dans la gamme à sécurité oculaire autour d'un minimum local d'absorption de l'atmosphère (1550-1650 nm). De telles sources existent déjà commercialement, mais ne répondent pas aux exigences militaires de compacité, de consommation électrique, de performance et de fonctionnement sur une large gamme de température (-40°C/+60°C).Mes travaux de thèse tentent d'apporter une réponse à l'ensemble de ces exigences. Avec l'aide des partenaires industriels Fibercryst et Cilas, ils portent sur la réalisation de sources laser compactes et efficaces à fibre cristalline Er3+:YAG directement pompée par une diode laser pour des applications militaires.A l'aide d'un algorithme de simulation d'un laser Er3+:YAG déclenché passivement élaboré et affiné durant la thèse, plusieurs sources sont réalisées expérimentalement. L'étude autour des absorbants saturables pour le déclenchement passif a permis une amélioration notable des caractéristiques du faisceau.Ces recherches, dont les résultats peuvent déjà présenter un certain intérêt commercial, ouvrent la voie vers de nouvelles techniques et architectures autour des sources laser à cristaux dopés aux ions erbium pour la conception de futurs prototypes plus performantsAmong the several applications of laser sources, some requires kilometers range propagation in the atmosphere : telemetry, guidance system or active imagery. High pulse energy improves the range of the system, but may cause permanent blindness to an observer's eyes. Hence, these applications must use laser beam which wavelength are located in the eye-safe region, ideally at the local minimum of the atmosphere absorption (1550-1650 nm). Such laser sources are already commercially available, but are not suited for the demanding military needs : compacity, electrical consumption, performance and large operating temperature range (-40°C/+60°C).My work aims to develop a laser source filling these specifications. Thanks to the collaboration with the industrial partners Fibercryst and Cilas, it focuses on the design of a compact, efficient, directly diode-pumped Er3+:YAG single cristal fiber laser for military applications.With a homemade numerical simulation of a passively Q-switched Er3+:YAG laser source, many laser emitters are experimentally designed and compared. Further studies around saturable absorbers allowed sensible improvements of the output pulse energy.This work, whose results may already be commercially interesting, may lead to new technics and architectures of erbium doped solid-state laser for better prototypes
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Xiang, Dao. "Study of Random Fibre Lasers and Applications." Thesis, Université d'Ottawa / University of Ottawa, 2015. http://hdl.handle.net/10393/32767.
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The properties of two novel random fibre lasers, in which stimulated Brillouin scattering supplies the effective gain mechanism and Rayleigh scattering along the standard telecommunication optical fibre provides random distributed feedback, are characterised. Firstly, ultra-narrow microwave signals with a Dirac delta function profile are successfully created by beating two random-lasing near-Gaussian beams, arising from the synchronization of optical modes from two Stokes signals with random phase accumulated over the ultra-long optical fibre. This finding provides a completely new approach to synthesise high spectral purity microwave signals from Brillouin fibre lasers with randomised feedback. In addition, we also develop a theoretical model of the random fibre Fabry-Pérot resonator based on the fact that the pump depletion effect naturally selects out the effective Rayleigh feedback regions localised in both ends of this long fibre. A narrow random-laser output with the linewidth of ~860 Hz is experimentally demonstrated and is employed to characterise the linewidth of the pump light. Furthermore, the random laser dynamics is studied and one application towards the physical entropy source is eventually achieved.
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Dargier, de Saint Vaulry Claire. "Conception et réalisation d'instruments biomédicaux : cathéter à fibre optique modifiée pour laser visible : guides d'onde diélectriques creux pour laser à CO2." Lille 1, 1991. http://www.theses.fr/1991LIL10126.
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Le laser est un outil de plus en plus utilisé en milieu médical. Son usage nécessite une instrumentation adaptée à chaque application chirurgicale. Nous avons conçu et réalisé deux types d'instruments. Dans une première partie du mémoire, nous présentons un cathéter à fibre optique modifiée prévu initialement pour répondre à un cahier des charges correspondant à celui de l'angioplastie coronaire. Cet instrument, couplé à un laser à argon ionisé devrait trouver des applications dans d'autres spécialités telles que par exemple la gynécologie (débouchage des trompes utérines). Dans la deuxième partie du mémoire, nous nous sommes intéressés à la mise au point de guides d'onde creux pour transmettre le rayonnement d'un laser à CO2. Ces guides doivent servir à étendre les possibilités de traitement en arthrochirurgie du genou et en coëliochirurgie chez la femme. Pour chaque instrument, nous expliquons d'abord, les choix qui ont été faits au niveau des matériaux et des paramètres optogéométriques des constituants en fonction des résultats d'une étude théorique des effets physiques liés à l'utilisation de l'outil. Nous décrivons ensuite, les procédés de fabrication et d'assemblage de l'instrument puis, les expériences de caractérisation physique de celui-ci. Nous présentons alors, les premiers résultats expérimentaux
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Nguyen, Minh-Hoang. "PROPELA - procédé de perçage laser pour l'aéronautique." Thesis, Bordeaux, 2017. http://www.theses.fr/2017BORD0577/document.
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Le perçage est un procédé de fabrication répandu dans le milieu industriel. Le secteur aéronautique a recours à ce procédé pour usiner de nombreuses pièces. Les chambres de combustions en sont un exemple. Exposées à des cycles thermiques sévères, elles sont percées d’une multitude de trous de refroidissement. Chez Safran Helicopter Engines, motoriste spécialisé dans la conception de turbines à gaz pour hélicoptères, ce perçage est réalisé à l’aide d’un procédé d’usinage unique : le perçage par percussion laser à la volée. Ce procédé phare leur permet de réaliser des milliers de trous inclinés dans des tôles en matériaux réfractaires. Safran Helicopter Engines cherche à maintenir son avance dans la production de turbines en s’appuyant sur les innovations constantes du procédé de perçage. Ce travail de thèse s’est articule autour du développement d’un nouveau procédé de perçage laser. De manière à définir un procédé fiable et efficace, plusieurs stratégies de perçage sont étudiées via les possibilités offertes par un laser à fibre milliseconde de haute puissance. Les résultats obtenus laissent entrevoir des perspectives intéressantes en termes d’augmentation de cadence de production à qualité d’usinage équivalente. L’amélioration des performances a été démontrée expérimentalement sur un banc d’essais représentatif des machines de production. Des trous de 400 μm de diamètre incliné à 60o ont pu être réalisés avec un cycle de perçage raccourci au minimum d’un facteur 2. Cette thèse présente également des éléments de compréhension des phénomènes de formation d’un trou. Pour cela, une modélisation des processus thermo hydrodynamiques impliqués dans le procédé de perçage laser a été développée. Ce modèle basé sur une approche par éléments finis repose sur l’étude thermique au cours de l’interaction entre le faisceau laser et la matière. Une partie du modèle inclut les phénomènes de propagation du faisceau dans la cavité en cours de formation afin de traduire de manière plus réaliste le dépôt d’énergieDrilling is a well-known manufacturing process. The aeronautic industry uses this process to machine a lot of elements. Among them, we can mention turbine blades, fuel injectors, combustion chambers. The latter are exposed to serious thermal stresses. To limit their impacts, combustion chambers are drilled with multiples cooling holes. At Safran Helicopter Engines, engines manufacturer specialized in engines design for helicopters, drilling is performed with a unique machining process : on the fly laser percussion drilling. This flagship process allows drilling thousands of tilted holes on refractory materials. Safran Helicopter Engines seeks to preserve its leading position in turbo engines manufacturing by looking for steady innovative drilling process. This thesis is structured around the development of a new laser drilling process. To define a reliable and efficient process, several drilling strategies were studied through the possibilities offered by a high-power millisecond fiber laser. The presented results allow catching a glimpse of the possibilities of production improvement. The performance enhancement was established through experimentation with a test bench. Holes of 400 μm diameter with an angle of 60o were drilled with a reduced processing time thanks to this new drilling process. In this thesis, we also propose a theoretical approach describing the hole formation. A mathematical model, accounting for all the physical process taking place during the drilling process is introduced. This model is based on finite elements method and considers the laser beam propagation within the drilled holes by solving Maxwell equations
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Hey, Tow Kenny. "Laser Brillouin à fibre microstructurée en verre de chaleogénure." Phd thesis, Université Rennes 1, 2013. http://tel.archives-ouvertes.fr/tel-00860216.
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Souvent considéré néfaste dans le domaine de la télécommunication car il limite la puissance d'un système de transmission optique, l'effet Brillouin peut être utilisé pour la réalisation de lasers. Un laser à fibre Brillouin peut potentiellement être très cohérent et très peu bruité ; ce qui incite son utilisation dans plusieurs domaines incluant la défense, la métrologie et les télécommunications. L'objectif de cette thèse, qui s'insert dans le cadre du projet ATOS (Antenne acoustique en technologie Tout Optique pour la Surveillance), est d'obtenir un laser Brillouin à la fois compact et avec un seuil laser relativement bas. Pour respecter ces deux conditions, il est nécessaire de disposer d'une fibre avec un très fort coefficient de gain Brillouin gB et ayant une très petite aire effective de manière à concentrer la puissance optique dans le cœur de la fibre. Nous avons ainsi choisi d'utiliser une fibre faite à partir de verres en chalcogénure, qui ont un gB de deux ordres de grandeurs supérieures au gB d'une fibre monomode silice classique avec une microstructure dans le coeur. Ces travaux de recherche contribuent donc, d'une part, à démontrer qu'il est expérimentalement possible de réaliser des lasers Brillouin compacts, bas seuils et exhibant des caractéristiques remarquables en termes de bruit et de cohérence avec des fibres microstructurées en verre de chalcogénure et, d'autre part, à étudier la potentialité de ces cavités lasers dans le cadre du projet ATOS tout en proposant d'autres applications possibles pour la métrologie, l'instrumentation et les télécommunications.
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Malleville, Marie-Alicia. "Évaluation du potentiel des fibres apériodiques à très large aire modale pour la réalisation de sources laser impulsionnelles." Thesis, Limoges, 2020. http://www.theses.fr/2020LIMO0009.
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Ces travaux de thèse s’inscrivent dans une collaboration long-terme entre le laboratoire Xlim et l’entreprise EOLITE Systems dans le but de développer de nouvelles fibres optiques à très grande aire modale typiquement capables de délivrer une puissance moyenne supérieure à 200 W et une puissance crête de l’ordre de 1 MW avec une émission monomode transverse à 1030 nm. Pour cela, des fibres optiques à microstructuration apériodique (FA-LPF) ont été développées en vue d’améliorer les performances des fibres commerciales, en repoussant en particulier le seuil des instabilités modales transverses. Une étude expérimentale sans précédents a été menée pour mettre en évidence l’influence de la structure de la fibre, de l’architecture de la source et du diamètre de champ modal sur le seuil d’apparition du phénomène. Par ailleurs, la fibre commerciale a été remplacée par une FA-LPF dans un prototype laser industriel, et a donné lieu à une validation de principe, tant du point de vue des performances pures que du vieillissement. La faisabilité d’une fibre micro-structurée courte et efficace (de 50 cm de long), a également été étudiée, en passant par une augmentation de la concentration en ions ytterbium dans le coeur de la FA-LPF ou encore par l’amélioration du ratio coeur/gaine. Finalement, un concept de fibre à coeur enterré a permis d’atteindre des dimensions de coeur supérieures à 110 μm tout en maintenant une émission monomode transverseThese Ph.D work is conducted in the context of a long-term collaboration between Xlim laboratory and the company EOLITE Systems in order to develop new large mode area fibers typically capable of providing up to 200 W of average output power and a peak power of 1 MW while maintaining a transverse single-mode emission at 1030 nm. For that purpose, optical fibers with an aperiodic microstructuration (FA-LPF) were developed in order to improve the performances of commercial fibers, mainly by pushing further the transverse mode instabilities power threshold. An unprecedented experimental study has been conducted to investigate the influence of the fiber structure, the laser source architecture and the mode field diameter. Furthermore, by replacing the commercial fiber by a FA-LPF in an industrial laser prototype, as a proof-of-concept, the FA-LPF permits to obtain a laser source with at least similar properties as those of the laser sources of the company regarding the laser efficiency or the lifetime test. The feasibility of a microstructured fiber shorter and still efficient (50 cm-long), has also been studied, by increasing the ytterbium ions concentration in the FA-LPF core or by improving the core to clad ratio. Finally, a new concept of fiber with a depressed-index core led to core diameters higher than 110 μm while maintaining a transverse single-mode emission
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Guesmi, Khmaies. "Etude d’un laser à fibre microstructurée en forme de huit et développement de sources à 1.6 μm." Thesis, Angers, 2015. http://www.theses.fr/2015ANGE0030/document.
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Les travaux de recherche, rapportés dans ce manuscrit, portent sur l’étude d’un laser à fibre en forme de huit et le développement de sources à 1.6 µm. En premier temps, nous avons étudié la dynamique impulsionnelle d’un laser à fibre micro-structurée en forme de huit. L’objectif est de montrer l’impact des propriétés de la fibre micro-structurée sur le comportement impulsionnel du laser. Nous avons également étudié le phénomène d’hystérésis dans cette cavité. Nos résultats numériques ont permis de démontrer l’universalité de ce phénomène dans les cavités lasers. Autrement, il est indépendant de la technique de verrouillage de modes. En second lieu, nous avons développé une source laser émettant à 1.6 µm à partir d’un amplificateur fonctionnant dans la bande C. La méthode que nous avons explorée est basée sur la gestion des pertes linéaires. L’émission, en continu et en verrouillage de modes, a été démontrée dans deux configurations différentes. Enfin et en se basant sur ce concept, nous avons rapporté des sources accordables sur une large fenêtre spectrale. Nous avons également étudié différentes formes des régimes harmoniques autour de 1.6 µmDuring our research, we are interested in studying of the figure of eight fiber laser based on the microstructured optical fiber and developing a 1.61 µm mode locked fiber laser from a C-band double-clad Er : Yb doped fiber amplifier. In the first step and based on a theoretical model, we have investigated the multi-pulse emission of a microstructured figure-of eight fiber laser operating in passive mode-locking. The proposed laser is mode locked by the nonlinear amplifying loop mirror (NALM). We further study the hysteresis dependence and the number of pulses in steady state as a function of both the small signal gain and the nonlinear coefficient of microstructured fiber. Our results demonstrate that the nonlinear coefficient of microstructured fiber plays a key role in the formation of multi-soliton. In the second step and based on the control of the linear losses of the cavity, we demonstrate the possibility to achieve filter less laser emission above 1.6 μm, from a C-band double-clad Er: Yb doped fiber amplifier, using a figure-of-eight geometry and a unidirectional ring cavity. We also reported a widely tunable mode locked fiber laser and harmonic mode locking of twin and third pulse around 1.61 µm
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Papachristos, Thalis. "Generation of microwaves using a mode locked fibre ring laser for radio-over-fibre systems." Thesis, University of Kent, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.344148.
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Adnan, Muhammad. "Experimental platform towards in-fibre atom optics and laser cooling." Thesis, Limoges, 2017. http://www.theses.fr/2017LIMO0109/document.
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Cette thèse décrit la conception et la réalisation d'une plateforme expérimentale pour le refroidissement par laser et le guidage d’atomes de Rb dans les fibres à cristal photonique à cœur creux (HC-PCF). Cette plateforme a pour but de fournir un système polyvalent pour explorer le refroidissement par laser à l’intérieur des fibres avec l'objectif à plus long terme de réaliser une fibre optique constituée d’un cœur rempli d’atomes froids (micro-cellule photonique). La plateforme a été conçue pour héberger plusieurs expériences sur le guidage d'atomes froids et thermiques ainsi que la spectroscopie dans les HC-PCFs pour répondre à plusieurs questions ouvertes liées par exemple à l'effet de la surface interne des HC-PCFs sur la structure énergétique des atomes ainsi que le piégeage et le refroidissement des atomes. La plateforme comprend une chambre spécifique à vide ultra-élevée (UHV) et un ensemble de lasers pour le refroidissement et le guidage des atomes à l'intérieur du HC-PCF hautement adapté. La chambre UHV a été conçue pour accueillir plusieurs HC-PCFs et deux pièges magnéto-optiques (MOT). Les HC-PCFs ont été conçus et fabriqués avec différents diamètres de cœur, contenu modal et post-traités avec des matériaux différents pour la surface interne du cœur. Par exemple, les diamètres du cœur varient de ~ 30 μm à ~ 80 μm traités avec une couche d'aluminosilicate ou une couche de PDMS afin de fournir un grand espace de paramètres pour évaluer l'effet de la surface sur les atomes confinés dans les fibres. Ainsi, le système a été construit et caractérisé. Le laser de refroidissement/repompage a été stabilisé en fréquence, avec une variance d'Allan de σ(τ)=3,8×10^(-11)/√τ. Avec ce système nous avons généré un MOT avec les deux isotopes du Rb, avec une température de refroidissement faible de l’ordre de 7 μK. La plateforme est maintenant opérationnelle pour entreprendre le premier guidage atomique et explorer la faisabilité du refroidissement des atomes à l'intérieur des HC-PCFsThis thesis reports on the design and fabrication of an experimental platform for in-fibre laser cooling of Rb and atom optics. By in-fibre laser cooling, we mean the long term aim of laser cooling thermal Rb atoms of a Photonic MicroCell (PMC), and subsequently developing what would be cold-atom photonic crystal fibre (PCF). The platform was designed to harbor several experiments on cold and thermal atom guidance and in-fibre spectroscopy so to address several open questions related for example to the effect of the core inner-wall surface on the atom energy structure and on selective fibre mode excitation for atom trapping and cooling. The completed platform comprises a specific and large ultra-high vacuum (UHV) chamber and a set of lasers for both atom cooling and atom guiding inside highly tailored hollow-core PCF (HC-PCF). The UHV chamber was designed to accommodate several HC-PCFs and two magneto-optical traps (MOT). The HC-PCF were designed, fabricated and post-processed to exhibit different core diameter, modal content and core inner surface material. For example, the mode field diameters range from ~30 µm to ~80 µm for the fundamental Gaussian-like core mode, and the surface materials include pure silica, a layer of Aluminosilicate or a layer of PDMS so to provide a large parameter space in assessing the effect of surface on the fibre-confined atoms. The system has been constructed and characterized. The cooling/repumping laser was frequency-stabilized, with measured Allan variance deviation of σ(τ)=3.8×10^(-11)/√τ. With the system we generated MOT with both isotopes of the Rb atom, with a cooling temperature as low as 7 µK. The platform is now operational to undertake the first atom guidance and explore the feasibility of atom cooling inside a HC-PCF
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Nevay, Laurence James. "Results from the laser-wire at ATF2 and development of a fibre laser for its upgrade." Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:088adcb4-1e0d-48f4-bebe-7efbb2dc1b80.
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The commissioning and development of a laser-wire transverse electron beam profile monitor at the Accelerator Test Facility 2 in Japan is presented. The experimental setup used including a gigawatt laser system is detailed and characterised. Results from data taking in December 2010 are presented detailing the use of the laser-wire to tune the electron beam. In conjunction with this, the use of a fibre-based laser system as a suitable laser source for a laser-wire is discussed. A test bed fibre laser system was constructed to investigate the suitability of fibre lasers and the results are presented demonstrating high efficiency and excellent spatial quality. From this, a laser system capable of high resolution and high repetition intra-train scanning for demonstration at the Accelerator Test Facility 2 was designed, constructed and characterised. A commercial fibre laser system was extended using a photonic crystal fibre where periodic amplification designed to match the duty cycle of an accelerator was used advantageously to achieve higher than normal pulse energies. The results and techniques developed to measure them are presented.
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Dauliat, Romain. "Advanced microstructured bers design for a robust singlemode high power laser operation." Limoges, 2013. https://aurore.unilim.fr/theses/nxfile/default/a5fd0c92-b555-4d68-a28a-3c6cf3fe34eb/blobholder:0/2013LIMO4015.pdf.
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The development of tailored micro-structured optical fibers has known a real infatuation during the last two decades allowing an outstanding management of optical properties. In particular, this approach was beneficial to the emission of a robust singlemode beam from large mode area fibers whose the core size exceeds 40 μm. This ability has made fiber lasers serious and competitive candidates for widespread applications such as micro-machining or surgery. In this purpose, several fiber structures have been devised (Large Pitch Fibers and Distributed Mode Filtering bers for instance), pushing further the performances of high-power fiber lasers and amplifiers. However, new detrimental phenomena impact their operation at high-power regime. Indeed, a degradation of the beam quality is noticed when the emitted power overcomes a certain threshold, because of the inherent capability of state-of-the-art fibers to sustain several guided modes into their core. In this context, I first investigated thoroughly the behavior of guided modes in state-ofthe- art ultra large-mode-area fibers and highlighted several milestones allowing to get an efficient selective amplication. Hence, I benefited from these statements to propose novel kinds of all-solid micro-structured fibers exhibiting an enhancement of the singlemode robustness. I demonstrated a record modal discrimination in a so-called Spiral LPF whose the inner cladding pattern is totally asymmetry. The first fabrications of these all-solid fibers are also reported, contributing to clearly define the prospect of this work as well as the fabrication hurdlesLe développement de fibres optiques micro-structurées spécifiques a connu un réel engouement au cours des 20 dernières années permettant un contrôle remarquable des propriétés optiques. En particulier, cette approche a été bénéfique à l'émission d'un faisceau monomode robuste à partir de fibres à large aire modale dont le diamètre de coeur excède 40 μm. Cette capacité a promu les lasers à fibres au rang de sérieux et compétitifs candidats pour des applications répandues telles que le micro-usinage ou la chirurgie. Dans ce but, de multiples structures de fibres ont été mise au point (Large Pitch Fibers et les "Distributed Mode Filtering fibers" par exemple), accroissant les performances des lasers et amplicateurs fibrés de fortes puissances. Cependant, de nouveaux phénomènes néfastes impactent leur fonctionnement dans ce régime. En effet, une dégradation de la qualité du faisceau est notable lorsque la puissance émise surpasse un certain seuil, en raison de la capacité inhérente des fibres de l'état de l'art à supporter plusieurs modes guidés dans leur coeur. Dans ce contexte, j'ai premièrement étudié attentivement le comportement des modes guidés dans les fibres à très large aire modale de l'état de l'art et mis en lumière plusieurs principes fondamentaux utiles à l'obtention d'une amplification sélective efficace. Ensuite, j'ai tiré bénéfice de ces constats pour proposer de nouveaux types de fibres microstructurées exhibant une amélioration de la robustesse de leur unimodalité. J'ai ainsi démontré une discrimination modale record dans une fibre appelé LPF Spirale dont le motif de la gaine interne est totalement asymétrique. Les premières fabrications de ces fibres toute solides sont également rapportées, contribuant à clairement définir les perspectives de ces travaux ainsi que les contraintes de fabrication
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De, Mollerat Du Jeu Rémi. "Développement d'architectures de fibres structurées pour l'amplification d'impulsions haute puissance crête." Thesis, Limoges, 2018. http://www.theses.fr/2018LIMO0033/document.
Full textAbstract:
Dans le cadre d'un contrat CIFRE entre la société Thales L.A.S. France et le laboratoire XLIM (UMR 7252 du CNRS et de l'Université de Limoges), mon projet de thèse consiste à développer des architectures de fibres optiques dopées aux ions d'ytterbium pour l'amplification d'impulsions avec une haute puissance crête à 1 µm dans l'optique de réaliser une combinaison cohérente de faisceau afin d'obtenir une source laser avec une très haute puissance crête et une haute cadence de répétition. Basé sur un design de fibres à large cœur appelées FA-LPF, deux axes de développement sont abordés. Le premier concerne l'amélioration des tolérances à une courbure externe appliquée sur les FA-LPFs. A l'aide d'une étude numérique, une stratégie novatrice est mise en place pour mitigé les effets d'une courbure. L'architecture est alors dénommée FA-LPF assistée par saut d'indice. Une émission laser monomode en régime continu d'une puissance optique de 65 W est démontrée avec un rayon de courbure de 60 cm pour un diamètre de mode de 47 µm. Le second axe est l'implémentation du contrôle de la polarisation dans les FA-LPFs. L'objectif est d'obtenir un signal linéairement polarisé. Plusieurs architectures testées numériquement permettent une propagation à polarisation linéaire unique dans des structures passives sur un large spectre optique, avec notamment une propagation monomode à polarisation unique à 1400 nm pour un diamètre de cœur de 140 µm. La meilleure des quatre, appelée FA-LPF 4+2 SAP, est fabriquée avec des dopants actifs d'ytterbium. En raison de défauts de fabrication, les fibres obtenues sont à maintien de polarisation. Un ratio de polarisation de 17 dB est obtenu en configuration amplification en régime continu avec un gain de 24 dB (une puissance émise de 50 W)As part of a CIFRE contract between Thales L.A.S. France and the XLIM laboratory (UMR 7252 of the CNRS and the University of Limoges), my thesis project consists in the development of ytterbium-doped optical fiber architectures for pulses amplification with high peak power at a wavelength of 1 µm in order to achieve a coherent beam combination to obtain a laser source with both a very high peak power and a high repetition rate. Based on a large-core fibre design called FA-LPF, two development axes are addressed. The first one concerns the tolerances improvement to an external bending applied on the FA-LPF. With the help of a numerical study, an innovative strategy is implemented to mitigate the effects of bending. The architecture is then called step-index assisted FA-LPF. A single a singlemode laser emission in continuous regime with an optical power of 65 W is demonstrated with a 60 cm bending radius for a 47 µm mode field diameter. The second axis is the implementation of the polarisation control in FA-LPFs. The goal is to obtain a linearly polarised signal. Several numerically tested architectures allow single linear polarisation propagation in passive structures over a broadband optical spectrum, including a singlemode single-polarisation propagation at 1400nm for a core diameter of 140 µm. The best architecture, called FA-LPF 4+2 SAP, is fabricated with active ytterbium dopants. Due to manufacturing defects, the obtained fibres act as polarisation maintaining fibres. A polarisation extinction ratio of 17 dB is obtained in an amplification configuration in continuous regime with 24 dB of gain (an emitted power of 50 W)