A Thulium Germanate thin disk laser prototype is developed and its potential applications are discussed. Unfortunately, the thin disk gain material for the CW prototype was unable to lase due to thermal limitations within the disk. However, a CW output power model and a physical pump chamber module have been developed, along with the supporting Zemax models and alignment procedures so other gain materials and future improvements can be tested.

Galactic formation models have long sought to reproduce the observed chemical and kinematical properties of the Milky Way's stellar halo and disk. Recently it is the so-called "intermediate population", the stellar thick disk, that is driving advances in our understanding of the formation of spiral galaxies. The thick disk is kinematically more like the thin disk than the halo, for all the thick disk has a velocity dispersion twice that of the thin disk and rotates ~40 km/s more slowly. It is generally accepted that the thick disk's metallicity distribution function peaks at a lower metallicity than the thin disk but at higher metallicity than the halo. The lower bound of the thick disk is still uncertain, as many observational studies have found only a few thick disk candidate stars or clusters that are more metal poor than (Fe/H)=1. Beers et al. (2002) have so far proposed the largest sample of metal poor thick disk candidates, presenting 9 stars at (Fe/H)= -1.2 or lower and 46 more stars at (Fe/H)= -1 or lower, all of which are believed to belong to the thick disk. Beers et al. (2002) present possible thick disk stars as metal poor as (Fe/H)~ -2.5, roughly 1 dex lower than is suggested by current Galactic formation models (Brook et al., 2005). This study is a high-resolution spectroscopic follow-up of 29 of the stars Beers et al. (2002) and Chiba & Beers (2000) identify as potential metal poor members of the thick disk and an additional 40 stars from the cannonical thick disk, halo, and thin disk. None of the very metal-poor stars identified by Beers et al. (2002) can be confirmed as members of the thick disk and many are not metal poor at all. Only two stars more metal poor than (Fe/H)= 1.2 retain their thick disk membership. These two stars exhibit some of the chemical characteristics of the cannonical thick disk: high α-element abundances and a relatively low s--/r-- process element ratio. Also of interest are six stars with thin disk kinematic signatures but thick disk α-element abundances. That only a small number of metal poor thick disk stars could be confirmed in this study indicates that the thick disk is neither as populous nor as metal poor as has been proposed by Beers et al. (2002). / text

Depuis les années 1970, le développement des sources lasers de puissance fonctionnant autour de 2 µm basées sur des cristaux dopés Thulium Tm3+ et/ou Holmium Ho3+est un domaine de recherche actif du fait de ses multiples applications. L’émission autour de 1.9 µm correspond à une transition du niveau excité 3F4 vers le niveau fondamental 3H6 dans un ion trivalent Tm3+, alors que l’émission à 2 µm et au-delà correspond à la transition entre le niveau 5I7 et le niveau fondamental 5I8 de l’ion Ho3+. Ces transitions laser font aujourd’hui l’objet de nombreuses applications dans domaines très variés. nous avons étudié les propriétés spectroscopiques de cristaux de fluorures dopés terres rares en vue d’une émission autour de 2 µm. À partir des spectres d’absorption obtenus, l’analyse de Judd-Ofelt a permis de calculer les durées de vie radiatives ainsi que les rapports de branchement dans les ions Tm3+ et Ho3+. En comparaison avec d’autres cristaux tels que les oxydes, les durées de vie des niveau 3F4(Tm3+) et 5I7(Ho3+) dans les cristaux de fluorures sont plus longues ce qui favorise un fonctionnement laser en régime impulsionnel. Une étude sur la dynamique de fluorescence a été réalisée afin de caractériser l’émission autour de 1.9µm de l’ion Tm3+ et celle autour de 2µm de l’ion Ho3+. Les spectres d’émission obtenus offrent de larges bandes, assez structurées dans la matrice LiYF4 et plus larges et moins structurées dans la matrice CaF2. Dans un second temps, en mesurant les durées de vie du niveau émetteur 3F4 pour différentes concentrations en ions Tm3+, nous avons calculé les paramètres de transferts. Les résultats obtenus ont montré que le processus de relaxation croisée est beaucoup plus efficace dans le cristal CaF2 que dans le LiYF4. Nous avons calculé également les coefficients de transfert et de transfert inverse entre les ions Tm3+ et Ho3+ suite à une excitation de l’ion Tm3+. Nous avons montré que la matrice LiYF4 offre une meilleure efficacité de transfert en vue d’une émission autour de 2µm. Le fonctionnement laser de couches minces en configuration « disque mince » a été étudié. Une première démonstration d’un laser en disque mince LiYF4 : Tm émettant à 1.9 µm réalisé à partir d’une couche épitaxiée par LPE a été faite. En simple passge, une puissance de sortie de 306 mW et une efficacité laser de 36% en puissance absorbée sont atteintes pour une couche de 240 μm d'épaisseur.nous avons étudié un oscillateur laser en guide d’onde LiYF4 :Tm en géométrie planaire et linéaire. Ces guides sont caractérisés par de faibles pertes de propagation grâce à la qualité du procédé de fabrication. Une puissance maximale de 470 mW et un rendement de 14% en puissance incidente ont été obtenus. Une modélisation numérique de l’amplification optique montre que le guidage des faisceaux pompe et sonde permet d’atteindre des gains importants. Le modèle est ensuite adapté pour le régime de saturation afin de calculer le rendement d’extraction et de déterminer le potentiel des guides LiYF4 :Tm3+ en tant comme amplificateur autour de 1.9 µm / Since the 1970s, the development of power laser sources operating around 2 μm based on Thulium Tm3 + and / or Holmium Ho3 + doped crystals is an active field of research because of its multiple applications. The emission around 1.9 μm corresponds to a transition from the excited level 3F4 to the fundamental level 3H6 in a trivalent ion Tm3 +, whereas the emission at 2 μm and beyond corresponds to the transition between the level 5I7 and the fundamental level 5I8 of the Ho3 + ion. These laser transitions are today the subject of many applications in a wide variety of fields. We have studied the spectroscopic properties of rare earth doped fluoride crystals for emission around 2 μm. From the absorption spectra obtained, the Judd-Ofelt analysis made it possible to calculate the radiative lifetimes as well as the branching ratios in the Tm3 + and Ho3 + ions. In comparison with other crystals such as oxides, the lifetimes of the 3F4 (Tm3 +) and 5I7 (Ho3 +) levels in the fluoride crystals are longer, which favors pulse-mode laser operation. A study on fluorescence dynamics was carried out in order to characterize the emission around 1.9 μm of the Tm3 + ion and that around 2 μm of the Ho3 + ion. The emission spectra obtained have broad bands, fairly structured in the LiYF4 matrix and wider and less structured in the CaF2 matrix. In a second step, by measuring the lifetimes of the 3F4 emitter level for different Tm3 + ion concentrations, we calculated the transfer parameters. The results obtained showed that the process of cross relaxation is much more efficient in the CaF2 crystal than in LiYF4. We also calculated the transfer and inverse transfer coefficients between Tm3 + and Ho3 + ions following excitation of the Tm3 + ion. We have shown that the LiYF4 matrix offers a better transfer efficiency with a view to emission around 2 μm. The laser operation of thin layers in "thin disc" configuration has been studied. A first demonstration of a thin-film LiYF4: Tm laser emitting at 1.9 μm made from an epitaxial layer with LPE was made. In single passge, an output power of 306 mW and a laser efficiency of 36% in absorbed power are reached for a 240 μm thick layer. We have studied a laser oscillator in LiYF4: Tm waveguide in planar geometry. and linear. These guides are characterized by low propagation losses thanks to the quality of the manufacturing process. A maximum power of 470 mW and a yield of 14% in incident power were obtained. A numerical modeling of the optical amplification shows that the guidance of the pump and probe beams makes it possible to achieve significant gains. The model is then adapted for the saturation regime in order to calculate the extraction efficiency and to determine the potential of the LiYF4: Tm3 + guides as an amplifier around 1.9 μm

High power lasers have been adapted to material processing, energy, military and medical applications. In the Laser Plasma Laboratory at CREOL, UCF, high power lasers are used to produce highly ionized plasmas to generate EUV emission. This thesis examines the quality of a recently acquired high power thin disk laser through thermal modeling and beam parameter measurements. High power lasers suffer from thermally induced issues which degrade their operation. Thin disk lasers use an innovative heat extraction mechanism that eliminates the transverse thermal gradient within the gain medium associated with thermal lensing. A thorough review of current thin disk laser technology is described. Several measurement techniques were performed on a high power thin disk laser. The system efficiencies, spectrum, and temporal characteristics were examined. The laser was characterized in the far-field regime to determine the beam quality and intensity of the laser. Laser cavity simulations of the thin disk laser were performed using LASCAD. The induced thermal and stress effects are demonstrated. Simulated output power and efficiency is compared to those that have been quantified experimentally.

Pirmoje šio darbo dalyje pristatomi temperatūros pasiskirstymo aktyviojoje lazerio terpėje kompiuterinio modeliavimo rezultatai. Skaičiavimai atlikti dviem aktyviosios terpės geometrijoms – plono disko ir plokščiojo kompozitinio elemento. Apžvelgiama skirtingų aktyvios terpės ir kaupinimo parametrų įtaka temperatūros pasiskirstymui. Antroje darbo dalyje pristatomi lazerio pluošto bangos fronto iškraipymų, atsirandančių dėl nevienodo temperatūros pasiskirstymo aktyvioje terpėje ir paviršiaus deformacijų, skaičiavimo rezultatai. / This work presents results of numerical simulation of temperature distribution inside laser active medium. Calculations were performed for two different active medium geometries. First one thin disk geometry and second - composite slab. Influence of different pump and active medium parameters on temperature distribution are presented. Second part of this work presents results of wave front aberrations calculations. Those aberrations were resulted by irregular temperature distribution and surface deformations.

Le bulbe Galactique, est cruciale pour comprendre les processus physiques responsables de la formationde la galaxie. L'étude spectroscopique des étoiles vieilles de faible masse permettre de caractériser endétail la chimie et la cinématique du bulbe. Dans cette thèse, nous avons utilisé des données provenantdu Gaia-ESO survey pour mener une étude détaillée du système du disque ainsi que du bulbeGalactique. La distribution de métallicité du bulbe est bimodale. La population riche en métaux montreune cinématique typique de la barre. Elle présente une caractéristique de double RC et recouvre laséquence du disque mince à haute métallicité dans le plan [Mg/Fe] vs. [Fe/H]. Nous associons cesétoiles avec celles de la barre formée à la suite de l'évolution séculaire du disque mince primordial.D'autre part, la population pauvre en métaux présente une cinématique chaude et ne participe pas à laforme en X du bulbe. Ces étoiles semblent imiter la distribution de celles du disque épais dans le plan[Mg/Fe] vs. [Fe/H]. Quand nous comparons la position en métallicité du genou de cette distribution,qui se trouve à [Fe/H]=-0.37+/-0.09 dex, elle est plus élevée de 0.6 dex par rapport au disque épais. Unmodèle d'évolution chimique permet de bien ajuster cette distribution pour les étoiles du bulbe ensupposant un épisode de formation stellaire rapide (<1 Gyr) et intense. L'origine du bulbe pauvre enmétaux reste encore relativement incomprise, mais divers projets futurs devraient permettre de faire ladistinction entre les processus violents ou ceux liés à une évolution séculaire qui ont pu contribuer à saformation / The Galactic bulge, as a massive and old Galactic component, is key to understand the physicalprocesses responsibles for the formation of the Galaxy. The spectroscopic study of long lived low massstars represents an opportunity to characterize the detailed chemical and kinematical patterns of theeventual mix of stellar populations building up the bulge. In this thesis we made use of data comingfrom the Gaia-ESO survey to conduct a detailed analysis of the disk system as well as bulge stellarpopulations. The bulge metallicity distribution function is bimodal. The metal-rich population exhibitsbar-like kinematics, displays the double RC feature and overlaps the metal-rich end of the thin disksequence in the [Mg/Fe] vs. [Fe/H] plane. We associate these stars with the bar X-shape bulge formedas the product of secular evolution of the early thin disk. On the other hand, the metal-poor populationpresents isotropic hot kinematics and does not participate in the X-shaped bulge. When compared to thethick disk, bulge stars seem to mimic their distribution in the [Mg/Fe] vs. [Fe/H] plane. Whencomparing the metallicity position of the so called ``knee'', that of the bulge is found to be at [Fe/H]=-0.37+/-0.09 dex, being 0.6 dex higher than that of the thick disk. A chemical evolution model suitablyfits the whole bulge sequence by assuming a fast (<1 Gyr) intense burst of star formation taking place atearly epochs. The origin of the metal-poor bulge still remains unconstrained, but further research shouldallow to distinguish between violent processes or secular evolution for its origin

Lasers which operate at both high average power and energy are in demand for a wide range of applications such as materials processing, directed energy and EUV generation. Presented in this dissertation is a high-power 1 μm ytterbium-based hybrid laser system with temporally tailored pulse shaping capability and up to 62 mJ pulses, with the expectation the system can scale to higher pulse energies. This hybrid system consists of a low power fiber seed and pre-amplifier, and a solid state thin-disk regenerative amplifier. This system has been designed to generate high power temporally tailored pulses on the nanosecond time scale. Temporal tailoring and spectral control are performed in the low power fiber portion of the system with the high pulse energy being generated in the regenerative amplifier. The seed system consists of a 1030 nm fiber-coupled diode, which is transmitted through a Mach-Zehnder-type modulator in order to temporally vary the pulse shape. Typical pulses are 20-30 ns in duration and have energies of ~0.2 nJ from the modulator. These are amplified in a fiber pre-amplifier stage to ~100 nJ before being used to seed the free-space Yb:YAG thin-disk regenerative amplifier. Output pulses have maximum demonstrated pulse energies of 62 mJ with 20 ns pulse after ~250 passes in the cavity. The effects of thermal distortion in laser and passive optical materials are also. Generally the development of high power and high energy lasers is limited by thermal management strategies, as thermally-induced distortions can degrade laser performance and potentially cause catastrophic damage. Novel materials, such as optical ceramics, can be used to mitigate thermal distortions; however, thorough analysis is required to optimize their fabrication and minimize thermal distortions. iv Using a Shack-Hartmann wavefront sensor (SHWFS), it is possible to analyze the distortion induced in passive and doped optical elements by high power lasers. For example, the thin-disk used in the regenerative amplifier is examined in-situ during CW operation (up to 2 kW CW pump power). Additionally, passive oxide-based optical materials and Yb:YAG optical ceramics are also examined by pumping at 2 and 1 μm respectively to induce thermal distortions which are analyzed with the SHWFS. This method has been developed as a diagnostic for the relative assessment of material quality, and to grade differences in ceramic laser materials associated with differences in manufacturing processes and/or the presence of impurities. In summation, this dissertation presents a high energy 1 μm laser system which is novel in its combination of energy level and temporal tailoring, and an analysis of thermal distortions relevant to the development of high power laser systems.