<p> The characteristics of small-signal gain in a TE CO₂ laser amplifier are investigated using a new technique based on gain measurements of the sequence, hot and regular CO₂ laser bands. This new technique enables us, for the first time, to determine accurately the rotational and vibrational temperatures characterizing the CO₂ laser system. The gain ratio of the sequence band to the regular band provides a simple and accurate determination of the ν₃ mode vibrational temperature. The variation of this ν₃ mode vibrational temperature with discharge energy enables us to determine the net pumping efficiency to the ν₃ mode levels as a function of input energy. It is found that the ν₃ mode vibrational temperature saturates at high input energy. This saturation sets an upper limit to the gain attainable in TE CO₂ laser amplifiers. Once this saturation occurs, increasing background gas temperature causes a reduction in gain at high input energy. </p> <p> As we can measure all the characteristic temperatures relevant to the gain medium, a comparison between the calculated and experimental gain can be carried out with no adjustable parameters. The result of such a direct comparison confirms both the validity of the conventional "mode temperature" model for CO₂ laser dynamics and the validity of our measurement technique for vibrational temperatures. </p> <p> The results of the present study have shown the existence of a de-excitation mechanism occurring in the discharge, which reduces drastically the pumping efficiency to the ν₃ mode at high discharge energy. It is therefore essential to incorporate such a de-excitation mechanism in the accurate modeling of CO₂ laser dynamics. The present study contributes to a better understanding of CO₂ laser dynamics at high discharge energies. </p> / Thesis / Master of Science (MSc)

A model relating the gain of a Nd³⁺ -glass laser amplifier to its input pumping energy is developed in this thesis. This model, which is based on the Nd³⁺ rate equations, is tested experimentally, using a giant pulse as the input to the amplifier. The results of these experiments conform well to the model. / Thesis / Master of Science (MSc)

The mid-infrared (MIR) system presented in the thesis is based on a sub-100-fs erbium-doped fiber laser operating at 1.55 µm. The output of the laser is split in two, each arm seeding an erbium-doped fiber amplifier. The output of the first amplifier is sent to a grating-based stretcher to be stretched to 50 ps before seeding the optical parametric chirped-pulse amplifier (OPCPA). The output of the second amplifier is coupled to a highly nonlinear fiber to generate the 1 µm needed to seed the a neodymium-doped yttrium lithium fluoride (Nd:YLF) system. This work represents the first time this synchronization scheme is used, and the timing jitter between the two arms at the OPCPA is reduced to 333 fs. The pump laser for the OPCPA is a regenerative amplifier producing 1.6 W followed by a double-pass amplifier, for a final output power of 2.5 W at 1 kHz. Etalons were inserted into the cavity of the regenerative amplifier to stretch the pulses to 50 ps The OPCPA consists of two potassium titanyl arsenate crystals in a noncollinear configuration. With three passes, the gain is 3.8 · 10 6 . Using a grating compressor, the pulse duration is reduced to 140 fs, with a power of 300 mW. Because of the reduction of the timing jitter, the amplitude stability is 1 %, which is a great improvement compare to existing systems. To generate ultrafast light in the MIR, an optical parametric amplifier is used, pumped ii by the output of the OPCPA and seeded with its 3-µm idler. Two crystals were tested, both in a single-pass configuration. For the first crystal, a 4-mm thick silver thiogallate, an efficiency of 7.4 % was reached, with 8.76 mW in the signal and 7.2 mW in the idler. For the second crystal, a 2-mm thick lithium gallium selenide, the efficiency was higher, reaching 10.8 %. The power for the signal was 11.5 mW, and for the idler, 11.11 mW. Using this new scheme, energies on par with current systems are achieved with much higher efficiencies.

The mid-infrared (MIR) system presented in the thesis is based on a sub-100-fs erbium-doped fiber laser operating at 1.55 µm. The output of the laser is split in two, each arm seeding an erbium-doped fiber amplifier. The output of the first amplifier is sent to a grating-based stretcher to be stretched to 50 ps before seeding the optical parametric chirped-pulse amplifier (OPCPA). The output of the second amplifier is coupled to a highly nonlinear fiber to generate the 1 µm needed to seed the a neodymium-doped yttrium lithium fluoride (Nd:YLF) system. This work represents the first time this synchronization scheme is used, and the timing jitter between the two arms at the OPCPA is reduced to 333 fs. The pump laser for the OPCPA is a regenerative amplifier producing 1.6 W followed by a double-pass amplifier, for a final output power of 2.5 W at 1 kHz. Etalons were inserted into the cavity of the regenerative amplifier to stretch the pulses to 50 ps The OPCPA consists of two potassium titanyl arsenate crystals in a noncollinear configuration. With three passes, the gain is 3.8 · 10 6 . Using a grating compressor, the pulse duration is reduced to 140 fs, with a power of 300 mW. Because of the reduction of the timing jitter, the amplitude stability is 1 %, which is a great improvement compare to existing systems. To generate ultrafast light in the MIR, an optical parametric amplifier is used, pumped ii by the output of the OPCPA and seeded with its 3-µm idler. Two crystals were tested, both in a single-pass configuration. For the first crystal, a 4-mm thick silver thiogallate, an efficiency of 7.4 % was reached, with 8.76 mW in the signal and 7.2 mW in the idler. For the second crystal, a 2-mm thick lithium gallium selenide, the efficiency was higher, reaching 10.8 %. The power for the signal was 11.5 mW, and for the idler, 11.11 mW. Using this new scheme, energies on par with current systems are achieved with much higher efficiencies.

No description available.

The invention of chirped-pulse amplification (CPA) in 1985 led to a tremendous increase in obtainable laser pulse peak intensities. Since then, several table-top, Ti:sapphire-based CPA systems exceeding the 100 TW-level with more than 10 W average power have been developed and several systems are now commercially available. Over the last decade, the complementary technology of optical parametric chirped-pulse amplification (OPCPA) has improved in its performance to a competitive level. OPCPA allows direct amplification of an almost-octave spanning bandwidth supporting few-cycle pulse durations at center wavelengths ranging from the visible to the mid-IR. The current record in peak power from a table-top OPCPA is 16 TW and the current record average power is 22 W. High energy, few-cycle pulses with stabilized carrierenvelope phase (CEP) are desired for applications such as high-harmonic generation (HHG) enabling attoscience and the generation keV-photon bursts. This dissertation conceptually, numerically and experimentally describes essential aspects of few-cycle OPCPA, and the associated pump beam generation. The main part of the conducted research was directed towards the few-cycle OPCPA facility developed in the Laser Plasma Laboratory at CREOL (University of Central Florida, USA) termed HERACLES. This facility was designed to generate few-cycle pulses in the visible with mJ-level pulse energy, W-level average power and more than 100 GW peak power. Major parts of the implementation of the HERACLES facility are presented. The pump generation beam of the HERACLES system has been improved in terms of pulse energy, average power and stability over the last years. It is based on diode-pumped, solid-state amplifiers with picosecond duration and experimental investigations are presented in detail. A iii robust system has been implemented producing mJ-level pulse energies with ~100 ps pulse duration at kHz repetition rates. Scaling of this system to high power (>30 W) and high peak power (50-MW-level) as well as ultra-high pulse energy (>160 mJ) is presented. The latter investigation resulted in the design of an ultra-high energy system for OPCPA pumping. Following this, a new OPCPA facility was designed termed PhaSTHEUS, which is anticipated to reach ultra-high intensities. Another research effort was conducted at CELIA (Univeristé de Bordeaux 1, France) and aimed towards a previously unexplored operational regime of OPCPA with ultra-high repetition rates (10 MHz) and high average power. A supercontinuum seed beam generation has been established with an output ranging from 1.3 to 1.9 µm and few ps duration. The pump beam generation has been implemented based on rod-type fiber amplifiers producing more than 37 W average power and 370 kW peak power. The utility of this system as an OPCPA pump laser is presented along with the OPA design. The discussed systems operate in radically different regimes in terms of peak power, average power, and repetition rate. The anticipated OPCPA systems with few-cycle duration enable a wide range of novel experimental studies in attoscience, ultrafast materials processing, filamentation, LIBS and coherent control

Les sources laser à fibre, haute puissance et de faible largeur spectrale sont de plus en plus utiles pour des applications à la fois industrielle et scientifique. Cependant, le développement de ces sources est limité par les seuils d’apparition d’effets non-linéaires tels que la Diffusion Brillouin Stimulée (DBS) du fait d’un fort confinement de la lumière dans le coeur de la fibre. Un premier amplificateur en régime monofréquence de puissance moyenne égale à 50W a été développé à partir d’une diode laser signal de 50 mW. L’étude du bruit d’intensité sur cet amplificateur a montré que l’utilisation de fibres standards (diamètre de coeur < à 20μm) jusqu’à 50W est limitée par l’apparition du phénomène de DBS se traduisant par une dégradation du bruit d’intensité. Pour s’affranchir de ces effets non-linéaires, une solution réside sur l’utilisation de fibres à large aire modale (LMA) dont les diamètres des coeurs est égale à plusieurs dizaines de microns. Les fibres LMA présentent un recouvrement [coeur dopé / onde de pompe] optimisé conduisant à une réduction de la longueur du milieu à gain. Par cette stratégie de la diminution du confinement spatial du faisceau avec une diminution de la longueur d’interaction, le seuil d’apparition des effets non-linéaires est repoussé. Mais paradoxalement, cette stratégie va induire une nouvelle limitation non-linéaire, non plus en termes de puissance crête mais de puissance moyenne. En effet, les coeurs multimodaux des fibres LMA, couplés à une charge thermique par unité de longueur forte (pour une puissance de moyenne de sortie équivalente) vont induire l’apparition d’effets non-linéaires en puissance moyenne : les Instabilités Modales (IM) et la Dégradation Modale de la Fibre (DMF). Ce sont donc les stratégies usuelles utilisées pour repousser les effets non-linéaires qui ont conduit à l’avènement de ces nouveaux effets délétères. Un système d’asservissement du bruit d’intensité efficace (suppression > 30 dB) sur une bande passante d’1MHz permet de supprimer le bruit en excès des diodes de pompe. / High power, narrow linewidth fiber lasers are useful for both industrial and scientific applications. Nevertheless, nonlinear effects like Stimulated Brillouin Scattering (SBS) are main limitations of these laser sources due to high power in fiber core. A first amplifier in single-frequency operation with 50W of output power from a laser diode seeder of 50 mW was developed. Study of intensity noise on this amplifier developed with standard fiber (core diameter less than 20 μm) showed that SBS leads to a degradation of noise properties of the laser. The use of large mode area (LMA) fibers is a solution for suppressing nonlinear effects with core diameters bigger than several tens of microns. LMA fibers show an overlap between doped core and pump wave optimized leading to a reduction of gain medium length. This strategy permits to increase nonlinear effect threshold. Second step of high power amplifiers with LMA fibers allows to obtain 100W in single-frequency regime without DBS. LMA fiber used have a core diameter equals to 40μm. Power scaling from 100W to 200W highlights a new limiting non-linear effect: Fiber Modal Degradation (FMD). Indeed, multimodal cores of these fibers, coupled to high thermal load lead to non-linear effects like Modal Instabilities (IM) and FMD. FMD effect, first described by Ward et al in 2016, is a thermo-optic effect characterized by a beam quality degradation with power transfer from fundamental mode to high order modes. Furthermore, a decrease of output power, synonym of guidance loss of fundamental mode in gain medium is observed. Unlike well-known effect IM, this phenomenon doesn’t act like a threshold phenomenon. In fact, transitory regime in association with FMD is longer than IM caused by photodarkening dynamic. In our study, beam quality at the output of the fiber was degraded after several tens of hours at 200W. For understanding this effect, a photodarkening effect study both in continuous wave (CW) and pulsed regime was carried out. This study shows that for the first time a photodarkening and photobleaching equilibrium on high power amplifiers in pulsed regime. These thermo-induced effects threshold depends on thermal load and are different for both regimes: 120W for CW and 150W for pulsed regime. Finally, a study and a reduction of intensity noise based from a servo-loop were carried out on 100W amplifier. A 1MHz bandwidth with a 30 dB decrease of noise were demonstrated. These results allow to develop high power and low intensity noise lasers at industrial level”

Virgo est un interféromètre de Michelson dont les bras contiennent des cavités Fabry-Perot. Il a été construit pour détecter directement les ondes gravitationnelles. Le projet Advanced Virgo est une amélioration majeure de Virgo pour atteindre une sensibilité encore plus élevée avec laquelle la détection des ondes gravitationnelles deviendra probable. On prévoit un système laser mono-fréquence de 175 Watts de puissance optique présentant des stabilités accrues pour le bruit relatif de puissance et pour le bruit de fréquence. Ce travail de thèse a pour objet la réalisation de ce système laser de haute-puissance et de haute-stabilité basée sur l'utilisation d'amplificateurs à fibre combinés de façon cohérente. Des amplificateurs à fibre disponibles dans le commerce sont caractérisés en termes de qualité de faisceau, de bruit de puissance, de bruit de fréquence, de stabilité de pointé du faisceau, et également en terme de stabilité à long terme sur quelques milliers d'heures. On implémente l'interférométrie de Mach-Zehnder pour la combinaison cohérente de faisceaux. Les techniques de caractérisation de faisceaux laser sont aussi développées en considérant leurs limites ultimes. Hormis un déficit de puissance optique, le système laser développé dans cette étude sur la base de la combinaison cohérente de Master Oscillator Fiber Power Amplifiers, remplit les conditions posées par Advanced Virgo. / Virgo is a cavity-enhanced Michelson interferometer built for the direct detection of gravitational waves. The Advanced Virgo project consists of major upgrades to the Virgo gravitational wave detector for an order of magnitude improvement in differential strain sensitivity, one of which is the tenfold increase in injected laser power to 175 Watts. The use of fiber laser amplifiers and their coherent combination are foreseen to deliver the required high-power low-noise beam. In this thesis work, we review the laser requirements for gravitational wave detectors, introduce the design of the laser system for Advanced Virgo, and develop the means for laser characterization in accordance with the stringent noise specifications. We then present the results to date, notably the quasi-continuous long-term operation of two 40-Watt fiber laser amplifiers over thousands of hours and their coherent combination with Mach-Zehnder interferometry. Although the targeted power for Advanced Virgo is not yet attained, the developed system shows decent noise performance and is promising for further power-scaling efforts.

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