Nonlinear acoustic media for implementations of parametric generation of low frequencies has so far been restricted to small values of the parameter B/A, typically between 3 and 13. Parametric amplification, defined as the generation of a low difference frequency signal resulting from the nonlinear interactions of two higher frequency fundamentals is enhanced by medium with a large coefficient of nonlinearity and low sound speed. The acoustic properties of a highly nonlinear medium were estimated and introduced in a numerical model, to evaluate the parametric amplification induced by a thin layer of such material in contact with a spherical transducer. The numerical model predicted a significant enhancement of the sound pressure level for the difference frequency component relative to that obtained when the transducer is driven linearly at the difference frequency. A source was then constructed to compare the theoretical predictions with experimental values and an enhancement of 17dB compared to the linear operation of the transducer was measured. The difference between the parametric amplification achieved with the nonlinear medium and the parametric amplification that would be obtained in water is 73dB.

De nombreuses applications militaires ou civiles, telles que la spectroscopie dans les bandes de transmission de l’atmosphère (bandes 3-5µm et 8-12µm), nécessitent de disposer de sources émettant dans le moyen infrarouge (IR). Les travaux de cette thèse portent sur la génération de rayonnement dans la bande 3-5µm par amplification paramétrique (mélange à quatre ondes) dans les fibres optiques en verres fluorés et en verres de chalcogénures. La première partie de ce travail a été consacrée à l’étude théorique et à la modélisation des conditions d’accord de phase et du gain paramétrique dans des fibres à saut d’indice monomodes en verres fluorés ZBLAN et verres de chalcogénures As2S3 et As2Se3. La nature des résultats obtenus nous a conduit à étudier théoriquement le potentiel de l’accord de phase multimode dans les fibres en verres de chalcogénures. La deuxième partie de ce travail a porté sur la modélisation de l’amplification paramétrique dans des fibres en verres de chalcogénures microstructurées à géométrie hexagonale. Pour ce faire, un modèle simplifié de la propagation dans les fibres microstructurées hexagonales a été développé : le modèle de l’indice effectif de gaine (EIM). Il a ensuite été comparé à une méthode de résolution aux éléments finis. Grâce à cette comparaison, nous avons pu améliorer la précision du modèle EIM en déterminant la valeur de plusieurs paramètres empiriques. Ce modèle nous a alors permis de prédire l’efficacité du processus d’amplification paramétrique dans les fibres microstructurées. L’ensemble de ces études théoriques a permis d’identifier les fibres les plus adaptées à la conversion de fréquence vers la bande 3-5µm. Enfin, nous avons réalisé un banc de mesure de la dispersion chromatique des fibres, ainsi que le dimensionnement d’un convertisseur de fréquence utilisant les fibres identifiées dans l’étude théorique. / Various civil or military applications, such as spectroscopy in the atmospheric transparency windows (3 – 5 µm and 8 – 12 µm ranges), require the use of mid-infrared emitting laser sources.The work presented in this thesis is about light generation in the 3 – 5 µm range by parametric amplification (four-wave mixing) in fluoride and chalcogenide fibers. The first part of the study is devoted to modelizations of phase-matching condition and parametric gain in monomode step-index ZBLAN fluoride fibers as well as As2S3 and As2Se3 chalcogenide fibers. The results obtained in this modelization led to the theoretical study of multimode phase-matching conditions in chalcogenide fibers.The second part of the study presents the modelization of parametric amplification in hexagonal microstructured chalcogenide fibers. A simplified model, called the effective index method (EIM), has been developed and compared to the finite element method. Thanks to this comparison, the accuracy of the EIM model was improved through the determination of several empirical parameters. Using the improved EIM model, we have been able to predict the parametric amplification efficiency in microstructured fibers. Thus, all those theoretical studies allowed us to identify the most adapted fibers for frequency conversion in the 3 – 5 µm range. Eventually, we realized an experimental bench to measure the chromatic dispersion of optical fibers, and we suggested an experimental architecture using the fibers we had indentified in the theoretical study.

This work explores the effects of weak nonlinearity on harmonic oscillators.Two particular systems are studied experimentally: A superconductingresonator formed from a coplanar waveguide that oscillates at microwave frequencies,and the cantilever of an atomic force microscope (AFM) vibratingat ultrasonic frequencies. Both of these systems are described in the introduction,followed by a theory chapter giving a general theoretical framework for nonlinear oscillators. Basic properties of nonlinear oscillators, such asbifurcation and intermodulation, are explained using simple models. Experimental methods, including cryogenic and microwave measurement techniques,are described in some detail. The nonlinear superconducting resonator is studied for use as a parametric amplifier. A strong drive tone, called the pump, drives the oscillator nearthe point of bifurcation. A second, much weaker drive signal that is slightlydetuned from the pump, will cause energy to move from the pump to the signal, giving signal amplification. We have measured a signal gain greaterthan 22 dB in a bandwidth of 30 kHz, for a resonator pumped at 7.6 GHz.This type of amplifier is phase-sensitive, meaning that signals in phase withthe pump will be amplified, but signals in quadrature phase of the pump will be deamplified. Phase-sensitivity has important implications on the amplifier’snoise properties. With a parametric amplifier, a signal can be amplified without any additional noise being added by the amplifier, something that is fundamentally impossible for a standard amplifier. The vibrating AFM cantilever becomes a nonlinear oscillator when it is interacting with a surface. When driven with two frequencies, the amplitudeand phase of the cantilever’s response will develop mixing products, or intermodulation products, that are very sensitive to the exact form of the nonlinearity. Very small changes in the surface properties will be detectable when measuring the intermodulation products. Simultaneously measuring many intermodulation products, or acquiring an intermodulation spectrum,allows one to reconstruct the tip-surface interaction. Intermodulation AFM increases the sensitivity of the measurement or the contrast of the acquiredimages, and provides a means of rapidly measuring the nonlinear tip-surface interaction. The method promises to enhance the functionality of the AFM beyond simple topography measurement, towards quantitative analysis of the chemical or material properties of the surface. / <p>QC 20100812</p>

This thesis explores the use of engineered nonlinear crystals from the KTiOPO4 (KTP) family as the gain material in optical parametric amplifiers (OPAs), with the aim to achieve more knowledge about the benefits and limitations of these devices. The work aims further at extending the possible applications of OPAs by constructing and investigating several efficient and well performing amplifiers. An OPA consists of a strong pump source, which transfers its energy to a weak seed beam while propagating through a nonlinear crystal. The crystals employed in this work are members of the KTP family, which are attractive due to their large nonlinear coefficients, high resistance to damage and wide transparency range. The flexibility of OPAs with respect to different wavelength regions and pulse regimes was examined by employing various dissimilar seed and pump sources. The possibility to adapt an OPA to a specific pump and seed wavelength and achieve efficient energy conversion between the beams, originates from quasi-phasematching, which is achieved in periodically poled (PP) nonlinear crystals. Quasi-phasematched samples can be obtained by changing the position of certain atoms in a ferroelectric crystal and thereby reversing the spontaneous polarisation. In this thesis several material properties of PP crystals from the KTP family were examined. The wavelength and temperature dispersion of the refractive index were determined for PP RbTiOPO4, which is essential for future use of this material. Another experiment helped to increase the insight into the volumes close to domain walls in PP crystals Further, several OPAs were built and their ability to efficiently amplify the seed beam without changing its spectral or spatial properties was studied. Small signal gains of up to 55 dB and conversion efficiencies of more than 35 % were achieved for single pass arrangements employing 8 mm long PPKTP crystals. Apart from constructing three setups, which generated powerful nanosecond, picosecond and femtosecond pulses, the possibility to amplify broadband signals was investigated. An increase of the OPA bandwidth by a factor of approximately three was achieved in a noncollinear configuration. / QC 20101013

<p>This thesis explores the use of engineered nonlinear crystals from the KTiOPO4 (KTP) family as the gain material in optical parametric amplifiers (OPAs), with the aim to achieve more knowledge about the benefits and limitations of these devices. The work aims further at extending the possible applications of OPAs by constructing and investigating several efficient and well performing amplifiers.</p><p>An OPA consists of a strong pump source, which transfers its energy to a weak seed beam while propagating through a nonlinear crystal. The crystals employed in this work are members of the KTP family, which are attractive due to their large nonlinear coefficients, high resistance to damage and wide transparency range. The flexibility of OPAs with respect to different wavelength regions and pulse regimes was examined by employing various dissimilar seed and pump sources.</p><p>The possibility to adapt an OPA to a specific pump and seed wavelength and achieve efficient energy conversion between the beams, originates from quasi-phasematching, which is achieved in periodically poled (PP) nonlinear crystals. Quasi-phasematched samples can be obtained by changing the position of certain atoms in a ferroelectric crystal and thereby reversing the spontaneous polarisation.</p><p>In this thesis several material properties of PP crystals from the KTP family were examined. The wavelength and temperature dispersion of the refractive index were determined for PP RbTiOPO4, which is essential for future use of this material. Another experiment helped to increase the insight into the volumes close to domain walls in PP crystals</p><p>Further, several OPAs were built and their ability to efficiently amplify the seed beam without changing its spectral or spatial properties was studied. Small signal gains of up to 55 dB and conversion efficiencies of more than 35 % were achieved for single pass arrangements employing 8 mm long PPKTP crystals. Apart from constructing three setups, which generated powerful nanosecond, picosecond and femtosecond pulses, the possibility to amplify broadband signals was investigated. An increase of the OPA bandwidth by a factor of approximately three was achieved in a noncollinear configuration.</p>

L'objectif de cette thèse est d'étudier une technique de combinaison cohérente de faisceaux : l'amplification paramétrique optique (OPA) à multiple pompes. Cette technique permet de transférer instantanément l'énergie de nombreuses pompes en un unique faisceau signal sans stockage d'énergie, et ainsi s'affranchissant d'effets thermiques dans le milieu amplificateur. Ceci peut s'avérer intéressant pour combiner l'énergie de multiples lasers à fibre et réaliser l'amplification à forte cadence de lasers très énergétiques ou d'impulsions à spectre large. A l'aide d'un code de calcul général et d'une étude expérimentale utilisant comme cristal non linéaire du BBO ou du LBO, nous calculons dans un premier temps la localisation des pompes autour du signal à amplifier, ainsi que les tolérances angulaires correspondantes qui déterminent la criticité d'alignement d'une telle configuration. Nous nous intéressons ensuite aux mécanismes de recombinaisons parasites entre une pompe et l'idler correspondant à une autre pompe. Après avoir démontré expérimentalement que ces recombinaisons peuvent dégrader les caractéristiques spatiales et spectrales du signal amplifié, nous montrons qu'il est possible d'éliminer ces risques de couplages néfastes en écartant suffisamment les pompes entre elles. Une modélisation originale de l'OPA multi-pompes suggère de relier ces phénomènes parasites aux effets des réseaux résultant des interactions entre les différentes pompes. La dernière partie présente l'expérience d'OPA à 5 pompes qui nous a permis d'atteindre un rendement de transfert énergétique des pompes vers le signal de 27%, et obtenir ainsi un signal plus énergétique que chaque pompe prise séparément. / This work deals with a technique of combination of coherent beams: Optical Parametric Amplification (OPA) with Multiple Pumps. This technique is used to instantly transfer the energy of several pumps on one beam, without energy storage and thus avoiding thermal effects in the amplifying media. It can be useful to combine energy of numerous fiber lasers and to amplifiy with a high repetition rate very high energy lasers or broadband pulses. With a numerical and experimental study using BBO and LBO as nonlinear crystal, we determine how to dispose the pumps around the signal and the corresponding angular tolerances of such set up. Then we focus our attention on recombining mechanisms between a pump and a non-corresponding idler. We demonstrate experimentally that these cascading effects may decrease the spatial and spectral quality of the amplified signal, and that these phenomena can be avoided with a minimum angle between the different pumps. A novel modelling of multi-pumps OPA links these cascading effects to the gratings generated by the interaction between the pumps. The last part presents a 5 pump OPA experiment. We achieve a pump-to-signal efficiency of 27% and so that a signal more powerful than each pump is obtained.

No description available.

Alors que la science attoseconde connaît un développement fulgurant, le besoin de nouvelles sources laser adaptées à la génération d'impulsions attosecondes uniques est apparu. Grâce à ses propriétés d'accordabilité en fréquence et d'amplification de spectres ultra-larges à même de supporter des durées d'impulsions ultracourtes, conjuguées à la possibilité de stabiliser passivement la phase sous l'enveloppe (CEP) du champ électrique associé à l'impulsion laser, l'amplification paramétrique (OPA) s'est imposée comme un des outils incontournables pour la réalisation de telles sources.De plus, un intérêt croissant se manifeste pour la montée en cadence des sources d'harmoniques d'ordre élevé (HHG), en tirant parti des avancées des laser à fibre. Récemment fut démontrée la génération d'impulsions ultracourtes à très haute cadence, stabilisées en phase, dans la partie visible du spectre. Décaler la bande d'amplification vers l'infrarouge présenterait des avantages certains du point de vue de la génération d'harmoniques. En effet, travailler avec une source laser infrarouge permet d'étendre le spectre d'harmoniques et donc de réduire la durée des impulsions attosecondes générées. Jusqu'à présent, l'amplification paramétrique large bande dans l'infrarouge à haute cadence était rendue impossible par la difficulté à générer un signal à ces longueurs d'onde directement à partir d'un laser à fibre.Les travaux exposés ici décrivent la réalisation de nouvelles sources paramétriques, spécifiquement conçues en fonction des exigences de la génération d'impulsions attosecondes uniques, aussi bien en régime de forte énergie qu'à des cadences élevées.Nous présentons tout d'abord le développement d'un OPA avec stabilisation passive de la CEP, capable d'amplifier un spectre d'une largeur de 700 nm centré à 1,75 µm et délivrant une énergie de 450 µJ à la cadence de 10 Hz. Puis, nous détaillons une architecture originale d'amplification paramétrique à haute cadence pompé par un laser à fibre, qui nous a permis de générer des impulsions stabilisées en phase d'une durée inférieure à trois cycles optiques à la longueur d'onde centrale de 2,2 µm, avec une énergie de 5 µJ à la cadence de 100 kHz.Enfin, nous explorons la possibilité d'accroître la puissance de sortie des OPA infrarouges large bande à des niveaux de plusieurs dizaines de watts, grâce à la technique de combinaison paramétrique de plusieurs faisceaux de pompe fibrés. / While attosecond science reaches new frontiers in physics, the need for innovative primary sources suited for the generation of single attosecond (as) pulses emerges. Featuring high tunability, ultra-broadband amplification bandwidth and the ability of passively stabilizing the random Carrier-Envelope Phase (CEP) of any pump laser, Optical Parametric Amplification (OPA) has proven to be one of the most effective tools to meet the stringent requirements of High-Order Harmonics (HHG) driving sources.Moreover, there is a growing interest for higher repetition rate HHG sources, pumped by Ytterbium-doped fiber lasers. High-repetition rate, CEP-stable, few cycle pulses have been successfully generated by OPAs operating in the visible part of the spectrum. Shifting the amplified bandwidth towards longer wavelengths would be clearly profitable. In fact, the shorter harmonic wavelength cut-off will allow significantly extending the harmonics spectrum and consequently shorten as pulse durations. Until know, generation of CEP-stable, few-cycle pulses in the infrared at ultra-high repetition rates was impossible due to the issue of generating a broadband infrared seed directly from a fiber laser. This thesis describes the implementation of new supercontinuum-seeded parametric sources, specifically designed for isolated attosecond pulses generation with high energy or high repetition rate.The development of a CEP-stable three-stages OPA source is reported, amplifying a 700 nm broad spectrum at a central wavelength of 1,75 µm with an energy of 450 µJ at a 10 Hz repetition rate. Then, a new architecture based on a two-stage cascaded OPA pumped by a home-made fiber laser is presented, which allowed us to generate CEP-stable 3-cycles pulses at the central wavelength of 2,2 µm, with an energy of 5 µJ at 100 kHz. Finally, we discuss the possibility of increasing the output power of parametric amplifiers to several tens of watts with broadband parametric combination of several fiber-pump beams.

Ces dernières années, le développement des lasers femtosecondes s’est massivement orienté vers des sources à fortepuissance moyenne pour des applications autant scientifiques - par exemple la génération d’impulsions XUV - qu’industrielles. Cettethèse a consisté à développer une source laser permettant d’amplifier des impulsions ultra-brèves à très haute cadence par un laserà fibre avec d’un côté le développement d’un laser de pompe femtoseconde fibré (<600fs) à haute cadence (100kHz), et de fortepuissance (50W) et de l’autre des amplificateurs paramétriques optiques ultrabrefs (<10fs) pompés par cette même source. Pour cela,au sein d’une architecture à dérive de fréquence de fort étirement (2ns), nous avons exploité les propriétés des fibres photoniques à trèsgros coeur dopé à l’Ytterbium qui, pour approcher des énergies proches de 1mJ, nécessitent d’effectuer des études d’endommagement etde préparation des fibres. Nous avons alors pu démontrer une puissance maximale de 90W mais d’excellents résultats ont été établis dansun régime stable et robuste pour lequel ce laser délivre une puissance de 60W avec des impulsions de durées inférieures à 400fs. Aprèsdoublage en fréquence, ce laser a permis alors d’amplifier en deux étages dans des cristaux de BBO des bandes spectrales supérieuresà 300nm centrées autour de 800 nm avec une énergie par impulsion de 19[mu] J (1.9W). Avec un système d’étirement et de compressionbasé sur la combinaison de lames de silice et de miroirs à dérive de fréquence, ces impulsions ont pu atteindre une durée finale de 9.7fs.Ainsi, ces deux sources permettent d’ouvrir la voie à de vastes champs d’investigation en physique moléculaire et atomique. / In recent years, the development of femtosecond lasers has been heavily oriented towards high average power sources forboth scientific experiments - such as XUV pulses generation - as well as for industrial applications. This work has been devoted to developa laser source able to amplify ultra-short pulses at a very high repetition rate. In one hand, we develop a high average power (50W)pump laser based on a Fiber Chirped Pulse Amplification (FCPA) technology delivering 400 fs pulses at a high repetition rate (100kHz).In the other hand, a multistage ultrafast optical parametric amplifiers (<10fs) pumped by this source has been then implemented. Toachieve this, we took the benefits of the Ytterbium-doped large-core photonics fibre’s properties in order to approach energies closeto 1mJ. Even in a highly stretched chirped pulse architecture (2ns), using this kind of technology, required to perform studies overdamage and preparation processes of fibers. Thereby, we demonstrated a maximum output power of 90W, but excellent results havebeen obtained in a stable and robust regime in which this laser delivers 60W with pulse durations shorter than 400fs. After frequencydoubling, this laser was sent as a pump into a two-stages - non collinear parametric amplifier made with BBO crystals and a spectrumdelivered a by a CEP-Stable-6fs Ti :sa oscillator has been amplified around 800nm over a spectral bandwidth larger than 300nm witha pulse energy of 19[mu] J (1.9W). By using a stretching and compression scheme based on the combination of silica wedges and chirpedmirrors, the final pulses have been then recompressed down to 9.7fs. These laser systems can be now used to pave the way for vast fieldsof investigation in molecular and atomic physics.

1 Title: High-average power picosecond mid-IR source Author: Michal Vyvlečka Department: Department of Chemical Physics and Optics Supervisor: Ing. Ondřej Novák, Ph.D., Hilase centre, Institute of Physics of CAS Abstract: High average power wavelength tunable picosecond mid-IR source based on optical parametric generation (OPG) and optical parametric amplification (OPA) is being developed. The conversion system is pumped by an Yb:YAG thin-disk laser delivering 100 W of average power at 100 kHz repetition rate, 1030 nm wavelength, and 2-3 ps pulse width. Part of this fundamental beam pumps an OPG process in a PPLN crystal. The generated wavelength is determined by PPLN's poling period and temperature. Tunability of the signal wavelength between 1.46 µm and 1.95 µm was achieved, the signal beam of 20 mW was generated at 2 W of pump power, when double pass of the beams through PPLN crystal was used. The corresponding idler wavelengths were in range 2.18-3.50 μm. The signal beam was further amplified by OPA process in two KTP crystals, which was pumped by the fundamental beam. The signal beam was amplified up to 2 W at pumping of 38 W. Tuning of the output wavelength was realized by change of the phase-matching angle in KTP crystals. Tunability between 1.70-1.95 µm for signal and 2.18-2.62 µm for idler was...