L'objet de cette thèse est l'étude de la sélection des structures transverses stationnaires dans les lasers. Nous nous attachons à décrire expérimentalement et à expliquer numériquement à l'aide d'une méthode de type Fox--Li les observations de modes transverses qui peuvent être réalisées dans un laser à état solide pompé longitudinalement. Ainsi, nous sommes amenés à explorer en profondeur le comportement des cavités non--dégénérées dans différentes situations de pompage, ainsi que les notions de dégénérescence exacte et de dégénérescence partielle. Nous sommes également amenés à étudier, bien que de manière assez qualitative, le rôle important des effets thermiques dans la formation des modes transverses dans les zones de dégénérescence de la cavité. Ces questions ont fait l'objet de multiples investigations depuis l'avènement du laser, mais nous mettons toutefois en évidence que de sérieux problèmes d'interprétations d'observations expérimentales persistent dans la littérature scientifique contemporaine. Une idée fausse assez répandue stipule par exemple qu'il est possible d'exciter un mode Laguerre--Gauss ou Ince--Gauss d'ordre élevé en utilisant un faisceau de pompe suffisamment petit afin qu'il recouvre parfaitement le lobe principal du mode visé. Nous montrons analytiquement, numériquement et vérifions expérimentalement que ceci est impossible dans une situation non--dégénérée, et expliquons comment certaines observations réalisées proches d'une dégénérescence peuvent contribuer à propager cette idée erronée. Bien que le modèle que nous utilisons n'élude pas les questions importantes concernant le rôle de la dynamique temporelle dans la formation des modes transverses dans un laser, sa pertinence réside dans sa simplicité et sa capacité à reproduire de manière très satisfaisante toutes les observations expérimentales. Ainsi, toutes les observations de modes transverses qu'il est possible de réaliser dans un laser peuvent être comprises à travers ce modèle, qui ne nécessite comme ingrédients principaux qu'une cavité, un gain saturé, des effets thermiques et éventuellement des effets de diaphragme. De plus, le modèle simple que nous présentons peut s'étendre sans difficulté à l'étude de cavités instables ou de cavités à miroirs asphériques qui offrent des possibilités nouvelles pour l'excitation de modes transverses originaux. / The main concern of this thesis is the selection of stationary transverse structures in lasers. Throughout the manuscript, we get involved into describing the observations that can be realized in a longitudinally pumped solid--state laser, from both experimental and numerical perspectives using Fox--Li simulations. We explore in detail, on the one hand, the particular behavior of non--degenerate cavities in various pumping situations, and on the other hand the exact and partial degeneracy situations. We also investigate the very influential role played by thermal effects concerning the formation of transverse structures in degeneracy regions, although this study is limited to a more qualitative description. These problems have been of great interest since the early development of laser, however we show in this manuscript that the contemporary scientific litterature still suffers from misinterpretations concerning experimental observations of transverse modes that can occur in lasers. For instance, there still remains the widespread belief that it is possible to excite a high-order Laguerre--Gaussian or Ince--Gaussian mode by strongly focusing the pump beam inside the active medium so that it matches the principal lobe of the targeted mode. We demonstrate analytically, numerically and experimentally that this idea is unfounded and show how some experiments that occur near a cavity degeneracy are misleading and can contribute to spread this misconception. Even if the model we use is unable to deal with some very significant issues concerning the mode formation in the time domain and the build--up of the laser oscillation, we believe that its strength lies into its simplicity and its ability to reproduce every experimental observation in a very accurate way. Hence, we managed to explain and interpret all the experimental observations we have realized given a very few assumptions including the presence of a cavity, a saturated gain, thermal effects and eventually diaphragm effects. Moreover, the simple model that we present can be extended without any difficulty to the case of unstable resonators, or to resonators built from graded--phase mirrors which offer new possibilities towards the excitation of original transverse modes.

Au cours de cette thèse, nous nous intéressons à deux applications nécessitant des sources laser impulsionnelles : l'imagerie active et l'usinage laser. L'imagerie active nécessite des sources laser efficaces émettant dans la gamme de sécurité oculaire (entre 1,5 µm et 1,7 µm) à des cadence de l'ordre du kilohertz et produisant des énergies par impulsion de plusieurs millijoules. Les sources efficaces émettant dans la gamme de sécurité oculaire utilisent l'ion erbium. Cependant la structure électronique complexe de l'ion erbium entraîne de nombreux effets parasites qui limitent fortement l'énergie accessible lors d'un fonctionnement à haute cadence. Pour diminuer l'influence de ces effets parasites nous avons utilisé le concept de fibres cristallines dans le cadre d'une collaboration entre le Laboratoire Charles Fabry et l'entreprise Fibercryst. La géométrie des fibres cristallines, combinant les propriétés des cristaux massifs et les avantages des fibres en verre nous a permis de dépasser les limites des sources actuelles. L'usinage de matériaux requière des sources laser impulsionnelles émettant dans le proche infrarouge (1 µm) et alliant forte énergie, forte puissance crête et forte puissance moyenne. Pour réaliser de telles sources, il est nécessaire d'utiliser des milieux à gain permettant de limiter les phénomènes thermiques et les effets induit par la puissance crête (effets non-linéaires). Pour cela nous avons utilisé à nouveau le concept de fibre cristalline, dopée cette fois ci par l'ion néodyme. Ces fibres cristallines ont été utilisées comme amplificateur de puissance pour amplifier des micro-lasers fonctionnant à haute cadence (de 1 à 100 kHz) et produisant des impulsions courtes (< 1 ns). / In this thesis we investigate two different pulsed laser sources for two specific applications : remote sensing and material processing. On the first part, remote sensing require efficient laser source emitting in the eye-safe range (1.5 – 1.7 µm) and producing several millijoules per pulse at a few kilohertz. Efficient eye-safe laser sources use erbium doped gain media. Nevertheless, the complex electronic structure of the erbium ion leads to several parasitic effects which limit the energy at high repetition rate. In order to minimize those effects we have used the concept of single crystal fibers developed in a close collaboration between the Laboratoire Charles Fabry and the company Fibercryst. The specific geometry of single crystal fibers, merging the advantages of bulk crystals and optical fibers, allows us to overcome limits of current laser sources. On the other part, material processing require near-infrared pulsed laser sources (1 µm) with high pulse energy, high average power and high peak power. In such laser sources, the amplifying medium must be design to avoid both thermal effects and non liner effects. For this purpose, we used neodymium doped single crystal fibers as power amplifier to enhance the performance of passively q-switched microlasers operating at high repetition rate (1 to 100 kHz) and emitting short pulses (< 1 ns).

Lasers de estado sólido sintonizáveis com uma estreita largura de linha de emissão na região do vermelho são uma alternativa conveniente aos lasers de corante para aplicações em espectroscopia de alta resolução. Nesse trabalho, foram investigados lasers contínuos de Nd:YLiF4 e Nd:YVO4 operando em frequência única na região de 1,32 - 1,34&mu;m, assim como a geração de segundo harmônico (GSH) desses lasers usando cristais de BiB3O6 (BiBO) e KTiOPO4 com inversão periódica de domínios (ppKTP), para a obtenção da emissão no vermelho (0,65 - 0,67&mu;m). Utilizando um laser de Nd:YVO4 operando em frequência única em uma configuração em anel com um cristal não linear BiBO do tipo I, demonstrou-se o recorde de 680mW no vermelho em 671,1nm, sem a utilizaçãde nenhum elemento seletivo. Uma intonia em todo o ganho (~4 nm) foi obtida atravésda inserção de um etalon com filme refletor (R = 40%) e com 100&mu;m de espessura, o que reduziu a potência de saída no vermelho para 380mW no comprimento de onda de maior ganho (671,15 nm). Em 1342nm foi demonstrada uma potência de saída de 1,5W em frequência única quando utilizado um espelho de saída com transmissão de 2%. Foi demonstrado ainda uma ótima eficiência de conversão de segundo harmônico em um laser em anel de Nd:YLF na polarização &pi; (&lambda; = 1321,5nm) quando usando um cristal de ppKTP. Este laser forneceu 1,4W em frequência única no vermelho em 660,5nm. Essa potência é a máxima que pode ser extraída desse laser no segundo harmônico e no fundamental quando utilizado um espelho com transmissão ótima. Utilizando um etalon com filme refletor (R = 25%), o laser pôde ser sintonizado em &Delta;&lambda; ~ 1,6nm. / All solid-state continuous-wave (cw) narrow emission linewidth and tunable red lasers are convenient alternative sources to bulky and expensive dye-lasers for highprecision laser spectroscopy. Single-frequency operation of diode-pumped Nd:YLiF4 and Nd:YVO4 cw ring lasers were investigated in the 1.32 - 1.34&mu;m range, together with their intracavity second-harmonic generation (SHG) to the red spectral range (0.65 - 0.67&mu;m) using either BiB3O6 (BiBO) or periodically-poled KTiOPO4 (ppKTP) crystals. We report on such a single-end diode-pumped Nd:YVO4 unidirectional red ring laser containing a type-I cut BiBO nonlinear crystal, yielding a record of 680 mW of single-longitudinal mode (SLM) red output power at 671.1nm without any intra-cavity etalon. For smooth SLM wavelength tuning over the full gain bandwidth (4 nm), a partially-coated (R = 40%) 100m-thin etalon was found necessary, reducing the maximum SLM power (at 671.15 nm) to 380 mW. At 1342.5nm and with a T = 2% transmission output coupler, the laser provided an optimal 1.5W of single-frequency power. We demonstrate also optimal intracavity SHG of a Nd:YLF ring laser in the &pi; - polarization ( &lambda;= 1321.5nm) using a ppKTP. The laser yielded 1.4 W of singlefrequency red power at 660.5 nm, as much as the maximum fundamental power that can be extracted from the resonator using an optimal output coupler. With a partiallycoated (R = 25%) thin etalon, the laser was tunable over &Delta;&lambda; ~ 1.6nm.

Au cours de cette thèse, nous étudions de nouveaux concepts et architectures lasers pour la réalisation de lasers solides pompes par diode de forte puissance convertis dans l’ultraviolet (UV). Ce type de laser est de plus en plus utilise pour de nombreuses applications d’usinage et de spectroscopie. Nous démontrons d’abord une architecture laser originale a 236,5 nm basée sur la conversion de fréquence d’un oscillateur laser impulsionnel nanoseconde utilisant une fibre cristalline de Nd :YAG émettant a 946 nm. En étudiant les diverses limitations de ce type de laser et en optimisant les deux étages de conversion de fréquence, nous sommes parvenus a démontrer des performances record dans l’ultraviolet, bien au-dessus de l’état de l’art. Puis, dans le cadre du consortium ANR ≪ UV-Challenge ≫, nous avons étudié deux nouveaux cristaux non-linéaires pour la conversion de fréquence vers l’UV : le Ca5(BO3)3F (CBF), un nouveau cristal non-linéaire non-hygroscopique pour la génération de troisième harmonique a 343 nm, ainsi qu’un cristal bien connu, le BaB2O4 (BBO), mais réalise avec une méthode de croissance non-standard, la méthode Czochralski. Nous avons démontré les premiers résultats de génération d’ultraviolet a 343 nm avec le CBF, et montre que la méthode de croissance Czochralski permettait d’obtenir des cristaux de BBO plus efficaces pour la génération de quatrième harmonique a 257 nm a forte puissance moyenne. Enfin, nous avons proposé et démontré un nouveau concept pour la conversion de fréquence, l’accord de phase mécanique. En exerçant de fortes pressions mécaniques sur un cristal non-linéaire, nous sommes parvenus à modifier de façon significative ses propriétés d'accord de phase. / In this thesis, we study new concepts and laser architecture for the development of high power, diode-pumped solid-state lasers frequency converted in the ultraviolet range (UV). Ultraviolet lasers are increasingly used in many manufacturing process and spectroscopic applications. We first demonstrate a novel laser architecture emitting at 236,5 nm using a frequency converted, pulsed nanosecond laser oscillator emitting at 946 nm based on a Nd:YAG single-crystal fiber. We demonstrate state-of-the-art performances by studying the main laser limitations and by optimizing the two frequency conversion stages. Then, we study two new non-linear crystals for frequency conversion in the UV : Ca5(BO3)3F (CBF), a new non-hygroscopic crystal for third harmonic generation to 343 nm, and a well-known crystal BaB2O4 (BBO) grown by an unusual growth method, the Czochralski (CZ) growth. We demonstrate the first UV generation experiment with CBF, and show that BBO crystals grown by the CZ method yield better conversion efficiencies in high average power, fourth harmonic generation to 257 nm experiments. Finally, we propose and demonstrate a new concept for frequency conversion called mechanical phase-matching. We managed to change a non-linear crystal’s properties significantly by applying a strong mechanical compression force on its facets.

Lasers de estado sólido sintonizáveis com uma estreita largura de linha de emissão na região do vermelho são uma alternativa conveniente aos lasers de corante para aplicações em espectroscopia de alta resolução. Nesse trabalho, foram investigados lasers contínuos de Nd:YLiF4 e Nd:YVO4 operando em frequência única na região de 1,32 - 1,34&mu;m, assim como a geração de segundo harmônico (GSH) desses lasers usando cristais de BiB3O6 (BiBO) e KTiOPO4 com inversão periódica de domínios (ppKTP), para a obtenção da emissão no vermelho (0,65 - 0,67&mu;m). Utilizando um laser de Nd:YVO4 operando em frequência única em uma configuração em anel com um cristal não linear BiBO do tipo I, demonstrou-se o recorde de 680mW no vermelho em 671,1nm, sem a utilizaçãde nenhum elemento seletivo. Uma intonia em todo o ganho (~4 nm) foi obtida atravésda inserção de um etalon com filme refletor (R = 40%) e com 100&mu;m de espessura, o que reduziu a potência de saída no vermelho para 380mW no comprimento de onda de maior ganho (671,15 nm). Em 1342nm foi demonstrada uma potência de saída de 1,5W em frequência única quando utilizado um espelho de saída com transmissão de 2%. Foi demonstrado ainda uma ótima eficiência de conversão de segundo harmônico em um laser em anel de Nd:YLF na polarização &pi; (&lambda; = 1321,5nm) quando usando um cristal de ppKTP. Este laser forneceu 1,4W em frequência única no vermelho em 660,5nm. Essa potência é a máxima que pode ser extraída desse laser no segundo harmônico e no fundamental quando utilizado um espelho com transmissão ótima. Utilizando um etalon com filme refletor (R = 25%), o laser pôde ser sintonizado em &Delta;&lambda; ~ 1,6nm. / All solid-state continuous-wave (cw) narrow emission linewidth and tunable red lasers are convenient alternative sources to bulky and expensive dye-lasers for highprecision laser spectroscopy. Single-frequency operation of diode-pumped Nd:YLiF4 and Nd:YVO4 cw ring lasers were investigated in the 1.32 - 1.34&mu;m range, together with their intracavity second-harmonic generation (SHG) to the red spectral range (0.65 - 0.67&mu;m) using either BiB3O6 (BiBO) or periodically-poled KTiOPO4 (ppKTP) crystals. We report on such a single-end diode-pumped Nd:YVO4 unidirectional red ring laser containing a type-I cut BiBO nonlinear crystal, yielding a record of 680 mW of single-longitudinal mode (SLM) red output power at 671.1nm without any intra-cavity etalon. For smooth SLM wavelength tuning over the full gain bandwidth (4 nm), a partially-coated (R = 40%) 100m-thin etalon was found necessary, reducing the maximum SLM power (at 671.15 nm) to 380 mW. At 1342.5nm and with a T = 2% transmission output coupler, the laser provided an optimal 1.5W of single-frequency power. We demonstrate also optimal intracavity SHG of a Nd:YLF ring laser in the &pi; - polarization ( &lambda;= 1321.5nm) using a ppKTP. The laser yielded 1.4 W of singlefrequency red power at 660.5 nm, as much as the maximum fundamental power that can be extracted from the resonator using an optimal output coupler. With a partiallycoated (R = 25%) thin etalon, the laser was tunable over &Delta;&lambda; ~ 1.6nm.

The beam quality and the system efficiency of a diode-pumped solid-state laser source are directly related to the thermal profile inside the laser crystal. The thermal profile in a laser crystal should be made uniform in order to reduce the negative effects of the thermal lens. However, the absorbed pump profile that forms a uniform thermal profile inside the gain medium may adversely affect the system efficiency. In this thesis, a computational and empirical method was developed for designing pumping chambers that results in a suitable thermal profile inside the gain medium, and thus desired laser beam quality was achieved while keeping the system efficiency at an acceptable level. Accomplishment of this thesis work will lead to the design of pumping chambers and resonators in high power laser systems operating at even higher thermal loads.

In many fields, scientific or industrial, optical devices that can be tailored in terms of spectral qualities and output power depending on the application in question are attractive. Nonlinear optics in combination with powerful laser sources provide a tool to achieve essentially any wavelength in the electromagnetic spectrum, and the advancement of material technology during the last decade has opened up new possibilities in terms of realising such devices. The main part of the thesis deals with the development of compact functional lasers based on nonlinear interaction utilising diode-pumped solid-state lasers and also laser diodes. Efficient frequency conversion into the visible and ultraviolet part of the electromagnetic spectrum has been achieved, using both Nd:YAG and Nd:YVO4 lasers as well as a semiconductor laser as the fundamental light sources. For the nonlinear conversion, periodically poled potassium titanyl phosphate (PPKTP), bismuth triborate (BiBO) and beta barium borate (BBO) have been employed. In the search for compact and reliable light sources emitting in the visible part of the spectrum, two different approaches have been explored. First, a scheme based on sum-frequency mixing of a diode-pumped solid-state laser and a laser diode of good beam quality. The idea of this approach is to take advantage of the individual strength of each device, which would be the flexibility in terms of wavelength for the laser diode and the possibility to reach high output power from the diode-pumped solid-state laser. Second, by mixing two different solid-state lasers substantially more output power could be generated albeit at a cost of less freedom in the choice of spectral output. As these two light sources had their central wavelength at 492 nm and 593 nm, respectively, they are highly interesting in biomedical applications since they correspond to the peak absorption of several popular fluorophores. In applications such as lithography, material synthesis and fibre grating fabrication, laser sources emitting in the deep-UV spectrum are desired. An all solid-state 236 nm laser source with 20 mW of average power have been designed and constructed, by frequency-quadrupling a passively Q-switched Nd:YAG laser lasing on a quasi-three level transition. Also, a novel concept for miniaturising solid-state lasers has been examined. Using a heat-conductive polymer carrier, a generic approach especially suited for mass-production of functional laser devices is presented. Finally, it has been proven that GRIN lenses can provide a very compact beam shaping solution to standard laser diodes based on the beam twisting approach. This method offers several advantages such as compactness of the beam shaping system, automated assembly in solid-state laser manufacturing due to the shape of these lenses and polarisation preservation of the laser diode output. / QC 20100903

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