Дисертації з теми "Ultra-short optical pulses"

In this thesis, the possibility of integrating linear pulse shapers into various all-optical signal processing devices for applications in high speed optical communication systems, to enhance the overall system performance, is investigated. The linear pulse shaping is performed using superstructured fibre Bragg gratings, which seem very promising passive devices for such application due to their compactness, easy integrability and compatibility with fibre based devices. At the same time, optical switching is facilitated by nonlinear effects in state of the art highly nonlinear fibres. The generation and manipulation of waveforms with specific shapes also requires suitable techniques for their precise characterization. For this reason, optical sampling oscilloscope or linear and nonlinear frequency resolved optical gating techniques are presented in this thesis. As a first example of all optical signal processing using pre-shaped pulses, the incident noisy data pulses are expanded into rectangular pulses at the input port of a nonlinear optical switch (nonlinear optical loop mirror). The flat top of the shaped pulses allows for the mitigation of any mistiming of the original signal across a time window defined by their width, simply by switching them with shorter clean clock pulses. By using a nonlinear switch with full regenerative properties as well, it is demonstrated that amplitude noise reduction as well as timing jitter reduction can be achieved in a single nonlinear switch. In a different switch configuration, where cross-phase modulation is utilized in a single-pass configuration, retiming is obtained by preshaping clean control pulses into pulses with a parabolic shape. XPM induced by such pulses can provide linear frequency-shifting to shorter mistimed data pulses across a temporal window corresponding to the full width of the parabolic pulses. This frequency-shift is proportional to the relative pulse displacement from the control bit-slot centre. Propagation in a suitable length of a dispersive medium can then be used to correct for the mistiming. The parabolic pulse shape is also very interesting for nonlinear propagation in normal-dispersion fibres, since it can propagate at high peak powers without undergoing deleterious pulse distortion (avoiding wave-breaking effects). It is demonstrated that the nonlinearly broadened parabolic pulse spectrum is highly flat and coherent, with a high spectral density, as required for spectral slicing or pulse compression applications. Finally, the use of Bismuth oxide highly nonlinear fibre is investigated in order to enhance the compactness and stability of the switching system. 2R-regeneration at 10- and 40-Gb/s is demonstrated using just 2 m of this fibre.

2006/2007
The need of coherent and intense pulsed radiation is spread among many research disciplines, such as biology, nanotechnology, physics, chemistry and medicine. The synchrotron light from traditional sources only partially meets these characteristics. A new kind of light source has been conceived and developed in the last decades: the Free-Electron Laser (FEL). The FEL process relies on the interaction between a relativistic electron beam and an electromagnetic wave in presence of a static and periodic magnetic field, produced by a device called undulator. This interaction generates coherent radiation at a fundamental frequency and its higher harmonics. In the standard configuration, the electron beam is generated by a linear accelerator and the interaction occurs in a single passage through one or several undulators. An alternative configuration can be obtained if the electrons are supplied by a storage ring. This work has been carried out at the Elettra laboratory within the ``new light sources'' group. My thesis focuses on both numerical and experimental issues about the generation of coherent harmonics on storage-ring FELs. The Elettra SRFEL has been originally designed to operate in ``oscillator configuration'' where the radiation is stored in an optical cavity (made of two mirrors). This process also drives the emission of radiation in the harmonics. In this work, different experimental methods have been implemented at Elettra to concentrate the power in giant pulses, both for the fundamental wavelength and its harmonics. Using this technique, it has been possible to generate fundamental radiation at 660 nm and 450 nm with (intra-cavity) power of few mJ and third harmonic radiation at 220 nm and 150 nm with few nJ of power. This process has been studied numerically by using a tri-dimensional simulation which also accounts for the re-circulation of the beam. The results of simulations are in good agreement with experimental measurements and allow to investigate the inner structure of the light below the picoseconds scale, where the instrumentation resolution reaches its limit. Structures of hundreds of femtoseconds inside the laser pulse have been found and this implies a higher peak power. Moreover, the numerical results have been confirmed by spectral measurements. By removing the optical cavity and focusing an external laser in the first undulator, a ``seeded single-pass'' configuration has been implemented. In the first undulator, the interaction with the external laser (``seed'') modulates the electron energy which is converted to spatial modulation (``bunching''). A Fourier analysis of the bunched electron-beam shows the presence of components at all harmonics (even and odd) and this explains why electrons in the second undulator can emit at any harmonic. To implement this configuration a design and layout plus tri-dimensional simulations were performed. Followed by the installation of the seed laser (Ti:Sapphire, lambda = 796 nm), the timing and the diagnostics. The commissioning focused on optimizing the spatial overlap and the synchronization between the electrons and the seed laser. Coherent harmonic radiation has been obtained at 265 nm, the third harmonic of the seed laser. After the characterization of this light, the seed frequency has been doubled by means of a nonlinear crystal. With this setup, radiation down to 99.5 nm (the fourth harmonic of the seed) has been generated. The shot-to-shot stability is comparable to the stability of the synchrotron radiation (fluctuations of few %) but the number of photons per pulse (~10^9) is about two-three orders of magnitude bigger than the synchrotron one. Thus this coherent radiation can be used for experiments similar to those suggested for the next generation FELs. Summarizing, the light source developed during my thesis is a unique facility able to generate coherent radiation with variable polarization, variable duration (between 100 fs and 1 ps), with peak power of the order of mega-Watts in a wide spectral VUV range. In the latest implementation, this radiation source has been used for two different kind of experiments, one in gas-phase, the other of solid state. The obtained results demonstrate the appealing of this source for user experiments. In perspective, there is a plan to extend the wavelength range below 100 nm and to improve the tunability of the source.
Vari ambiti della ricerca scientifica, dalla biologia alle nanotecnologie, passando per la fisica, la chimica e la medicina, richiedono per le loro indagini una radiazione spazialmente coerente con un elevato numero di fotoni per impulso. Poiché la radiazione di sincrotrone non possiede queste caratteristiche, negli ultimi anni gli sforzi si sono concentrati nello sviluppo delle cosiddette sorgenti di quarta generazione: i laser a elettroni liberi (LEL). Il processo LEL avviene per l'interazione di un'onda elettromagnetica con un fascio di elettroni relativistici in presenza di un campo magnetico. Tale campo, statico e periodico, viene generato da un dispositivo detto ondulatore. L'interazione produce emissione di luce coerente ad una frequenza fondamentale e alle sue armoniche superiori. La configurazione standard prevede che gli elettroni siano prodotti da un acceleratore lineare e l'interazione si risolve tipicamente in un singolo passaggio attraverso uno o più ondulatori. Una configurazione alternativa si ottiene quando gli elettroni sono forniti da un anello di accumulazione. La tesi si è svolta presso il laboratorio Elettra, nel gruppo che si occupa dello sviluppo di nuove sorgenti di luce. La mia attività di ricerca comprende sia aspetti teorico-numerici che sperimentali relativi alla generazione di armoniche coerenti su LEL installati su anelli di accumulazione. Storicamente il laser a elettroni liberi ad Elettra è nato in ``configurazione oscillatore'' (la radiazione è immagazzinata in una cavità ottica formata da due specchi). Ad ogni passaggio successivo gli elettroni interagiscono con l'onda electtromagnetica amplificandola fino all'instaurarsi dell'effetto laser. Questo processo guida anche l'emissione alle armoniche superiori. Diversi metodi sperimentali possono essere usati per concentrare la potenza in impulsi giganti, sia per la fondamentale che per le armoniche. Questa tecnica, che ho affinato durante il mio lavoro di tesi, ci ha permesso di generare potenze dell'ordine di alcuni mJ per la fondamentale (nella cavità) e di alcuni nJ alla terza armonica di 660 nm e di 450 nm, cioè 220 nm e 150 nm rispettivamente. Dal punto di vista numerico, per studiare questo processo abbiamo modificato un codice per simulare tridimensionalmente la nostra configurazione ed abbiamo aggiunto una parte che propaga gli elettroni lungo l'anello. Le simulazioni sono in ottimo accordo con i dati sperimentali e ci permettono di investigare più nel dettaglio l'impulso, nella scala temporale dei femtosecondi dove si arresta la risoluzione strumentale. Dalle simulazioni risulta che all'interno degli impulsi laser sono presenti delle substrutture della durata di alcune centinaia di femtosecondi. La presenza di tali strutture implica una potenza di picco maggiore. Abbiamo inoltre una conferma indiretta dei risultati numerici tramite le misure spettrali. Rimuovendo la cavità ottica e focalizzando un laser esterno nel primo ondulatore si può passare alla cosiddetta configurazione in ``singolo passaggio''. Nel primo ondulatore, l'interazione con il laser esterno (``seed'') produce una modulazione nell'energia degli elettroni, la quale viene trasformata in separazione spaziale (``bunching''). Un'analisi di Fourier del fascio di elettroni mostra componenti a tutte le armoniche (pari e dispari), per cui gli elettroni sono in grado di emettere a qualsiasi armonica nel secondo ondulatore. In questa configurazione la prima parte del lavoro di tesi è stata il design della linea e lo studio numerico dei risultati attesi. A questo studio preliminare è seguita l'installazione dell'esperimento, a partire dall'alloggiamento e la messa in funzione del laser esterno (Ti:Sapphire, lambda = 796 nm) fino alla realizzazione del sistema di sincronizzazione del seed con gli elettroni. Prima di ottenere la radiazione armonica coerente e poter confrontare le aspettative con i risultati sperimentali abbiamo dovuto dedicare molti turni di fisica di macchina al perfezionamento della sovrapposizione spaziale e temporale tra elettroni e laser esterno. La prima radiazione armonica coerente è stata ottenuta alla terza armonica (265 nm) del laser esterno. Dopo una prima caratterizzazione della sorgente, abbiamo introdotto un cristallo nonlineare per generare la seconda armonica del laser esterno e usare questa come seed. Attualmente il LEL di Elettra è in grado di produrre radiazione fino a 99.5 nm (la quarta armonica del seed) con la stessa stabilità della radiazione di sincrotrone (flutuazioni dell'ordine del %). Queste caratteristiche, insieme al numero di fotoni per impulso (~10^9) che supera di almeno due ordini di grandezza l'emissione di sincrotrone, permettono l'utilizzo della luce prodotta per esperimenti simili a quelli proposti per le sorgenti di quarta generazione. Riassumendo, la sorgente sviluppata durante la mia tesi è attualmente l'unica in grado di fornire luce coerente di durata variabile tra 100 fs e 1 ps con potenze dell'ordine del mega-Watt e polarizzazione variabile (lineare-circolare) in un ampia gamma spettrale nell'ultravioletto. Negli ultimi turni, questa radiazione è stata usata su due diversi tipi di esperimenti, uno in fase gassosa l'altro di stato solido. I risultati ottenuti dimostrano che la radiazione emessa può essere appetibile per gli utenti. Le prospettive sono estendere il range di lunghezze d'onda sotto i 100 nm e migliorare la tunabilità della sorgente.
XX Ciclo
1979

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
Neste trabalho descreve-se a geração de pulsos ópticos curtos estáveis, com durações na faixa de alguns psicossegundos, e diversas taxas de repetição. Para isto foi construído um laser a fibra dopada com érbio, na configuração em anel e operando nos regimes de mode- locking ativo harmônico e racional (MLEFL). As características temporal e espectral dos pulsos são estudadas. Os pulsos de saída apresentam potência de pico elevada e são limitados pela transformada, ou seja, na forma de sólitons. Esses pulsos são extremamente adequados para sistemas de comunicações solitônicos. Os lasers a fibra dopada com érbio operando no regime de mode-locking ativo (MLEFL), têm-se mostrado como um dos candidatos mais promissores para os sistemas solitônicos. Isto se deve ao fato desses dispositivos além de produzirem pulsos ultracurtos e com as características necessárias a esses sistemas, eles apresentam alta potência de saída e possibilitam a variação da taxa de repetição. Para a montagem desses lasers de forma razoavelmente compacta são utilizados componentes ópticos, tais como controladores de polarização, filtros e moduladores, em versões integradas (pigtailed) e já disponíveis comercialmente.
This work describes a simple and stable harmonically mode- locked erbium-doped fiber ring laser, that produces high power, ~ 1 ps transform-limited sech optical pulses. Pulse trains with different high repetition rates were obtained using harmonic mode-locking and rational harmonic mode- locking techniques. The temporal and spectral characteristics of the pulses are studied. The pulses are extremely appropriate for soliton based systems communications. Actively mode-locked erbium doped fiber ring lasers (MLEFL), have attracted much attention and are one of most promising candidates for soliton systems. This is due to some characteristics which are very convenient for high capacity optical systems: they can produce very short transform limited optical pulses at gigahertz rates. Such lasers also present high output powers, long term stability and can be easily tuned to operate in a wide region of wavelengths.

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses. By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated. High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam. Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration. The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.
QC 20100831

Dans cette thèse, nous étudions expérimentalement la génération d'impulsions carrées très énergétiques et accordable à l’échelle nanosecondes et d'impulsions ultracourtes à haute puissance moyenne de sortie dans les lasers à fibre utilisant les nanomatériaux comme absorbant saturable. Tout d'abord, puisque la dynamique des impulsions est dominée par l'interaction de la non linéarité et de la dispersion chromatique cubique de la fibre avec un mécanisme de discrimination d'intensité appelé absorbant saturable, la stabilité d'une distribution harmonique en mode verrouillé est étudiée par injection externe d'une onde continue.Enfin, nous avons utilisés des absorbant saturable à base de nanomatériaux déposés sur des tapers optiques dans les lasers à fibre pour générer des impulsions ultracourtes avec une puissance de sortie moyenne élevée
In this thesis, we investigate experimentally the generation of high energy nanosecond tunable square pulses and high output power ultrashort pulses in fiber lasers. First, since pulse dynamics are dominated by the interaction of the fiber's cubic Kerr nonlinearity and chromatic dispersion with an intensity-discriminating mechanism referred to as a saturable absorber, the stability of a harmonic mode-locked distribution is studied by external injection of a continuous wave. Finally, we implemented nanomaterial based saturable absorbers in fiber laser configuration to generate ultrashort pulses with high average output power. Different techniques of achieving such components are explicitly detailed: ultrashort pulse generation in ring cavities where graphene and topological insulators are deposited on optical tapers to form a saturable absorber

Un des problèmes à résoudre lors de la réalisation d'un endoscope non linéaire pour des applications biomédicales concerne la propagation d'impulsions ultra courtes dans une fibre optique. Les processus non linéaires concernés nécessitent de grandes puissances d'excitation, réalisables seulement pour des impulsions de très courte durée qui sont déformés et allongés par la dispersion et les non linéarités des fibres. La plupart des techniques d'illumination fibrées pour la microscopie non linéaire emploient des systèmes de pré-compensation pour neutraliser les effets de ces phénomènes. Dans ce travail, nous explorons les possibilités offertes par la formation de solitons de grande énergie dans une fibre à bandes interdites photoniques à coeur solide. Les solitons optiques ont la propriété de conserver leur forme lors de leur propagation, et leur durée reste proche de la valeur minimum définie par la limite physique imposée par leur largeur spectrale, sans avoir besoin de recourir à un système de pré-compensation. De plus, la longueur d'onde et le retard relatif des solitons peuvent être accordés en changeant la puissance lumineuse en entrée de fibre. Plusieurs sources de lumière ont été conçues et réalisées, pour générer de nombreux contrastes non linéaires. Des images d'échantillons biologiques ont d'abord été réalisées en tirant profit de la courte durée des solitons. Puis, des mesures d'absorption transitoire ont été menées dans une configuration pompe-sonde en contrôlant le retard des solitons dans la fibre. Enfin, un montage de CRS basé sur le principe de focalisation spectrale a été réalisé, et son utilité a été démontrée en suivant un équilibre chimique
One of the issues that has to be overcome to realize a nonlinear endoscope for biomedical applications is the propagation of ultra-short pulses in an optical fiber. Nonlinear processes require high peak powers in the focal volume in order to generate observable signals, so the pulses should be as short as possible. This makes them sensitive to the dispersion and nonlinearities of the fibers. Most of the existing techniques of ultra-short pulses fiber-delivery rely on complex pre-compensation systems to counteract these effects. In this work, we explore the possibilities offered by the generation of high-energy solitons in a custom-built solid-core photonic bandgap fiber, for nonlinear microscopy and spectroscopy. Optical solitons preserve their shape when they propagate in a fiber, and their duration remains close to the minimum value physically allowed by their bandwidth, without the need of any pre-compensation. Moreover, the wavelength and delay of the soliton can be tuned by changing the power at the input of the fiber. Several soliton-based light sources were designed and realized, generating contrast in the most prevalent nonlinear microscopy modalities. TPEF and SHG images of biological samples were first realized by taking advantage of the short duration of the solitons. By controlling the delay of the soliton, transient absorption measurements were then realized in a pump-probe configuration. Finally, the wavelength tunability of the soliton was used to generate the Stokes beam in a CRS setup based on the spectral focusing technique. The capabilities of this scheme were demonstrated by performing CRS microspectroscopy to monitor a chemical equilibrium

L’amplification par dérive de fréquence démontrer en 1985 a permis la création d’installations laser en impulsions courtes tels que Petal (Petawatt Aquitaine Laser). La montée en puissance de ces lasers est limitée par la résistance au flux laser des composants placés après la compression. L’objectif de cette thèse est d’améliorer la résistance au flux laser de ces composants qui sont des miroirs qui consiste en un empilement multicouches. Trois approches sont envisagées le changement de designs des empilements couches minces (nombre de couche, épaisseurs), de matériaux et/ou de procédés de fabrication. Une étude numérique a permis d’envisager théoriquement le changement de matériaux et/ou de design et de quantifier les améliorations possibles. Cette étude a mené au développement d’un algorithme d’optimisation des designs qui nécessite la caractérisation préalable des matériaux. Par conséquent, une variété de matériaux déposés en monocouches a été testée au flux laser et caractérisé optiquement pour évaluer l’adéquation matériaux et technique de dépôt. Les résultats obtenus montrent une forte dispersion qui ne peut être expliqué par des lois préalablement établi dans la littérature. Cependant, une bonne corrélation entre le seuil de tenue au flux laser intrinsèque dans l’infrarouge et l’absorption dans l’ultraviolet a été observé ce qui confirme l’influence de l’absorption multi-photonique sur l’endommagement laser en impulsions courtes. Pour finir, l’ensemble de ces résultats expérimentaux et de l’algorithme d’optimisation ont permis la fabrication d’échantillons de miroirs qui montrent une amélioration du seuil de tenue au flux laser de 73% par rapport à des miroirs quart d’onde classiques
The chirped pulse amplification demonstrated in 1985 allowed the development of petawatt class laser such as Petal (Petawatt Aquitaine Laser). The increase of power of those facilities is limited by the resistance to laser-induced damage of the optical components placed after the compression stage. The aim of this thesis is to improve the laser-induced damage threshold of those components which are multilayer dielectric mirrors. Three paths of improvement are considered the change of design (number of layer, thicknesses), of materials and/or deposition process. A numerical study allows evaluating the potential improvement brought by two of those paths. This led to the development of a design optimization algorithm that required the prior characterization materials. Consequently, various materials deposited as single layers were laser damage tested and optically characterized to evaluate the adequacy of the materials with the deposition process. The results show a wide discrepancy that cannot be explained by the laws exposed in the literature. However, a good correlation was found between the intrinsic laser-induced damage thresholds in the infrared with the absorption in the ultraviolet confirming the influence of the multiphoton absorption in the laser-induced damage mechanisms. Finally, those experimental results combined with the optimization algorithm allowed the development of mirror samples that exhibit laser-induced damage threshold 73% higher than one of classical mirrors

The PhD project, entitled "Second harmonic generation in disordered nonlinear crystals: application to ultra-short laser pulse characterization", is devoted to the study of second harmonic generation in nonlinear ferroelectric crystals formed by a random distribution of domains with inverted quadratic nonlinear susceptibility (such as the Strontium Barium Niobate and Calcium Barium Niobate crystals) and its application to the single-shot characterization of ultrashort laser pulses. The basic principle of operation is related to the unique type of emission associated to those kinds of crystals where the second harmonic signal is emitted transversally to the beam propagation direction. Using the transverse second harmonic generation from these crystals we measure the pulse duration, the chirp parameter and the temporal profile in a single-shot configuration. This method has been implemented both in transverse auto-correlation and transverse cross-correlation schemes for the measurement of pulses with durations in the range from several tens up to several hundreds of femtoseconds. The main advantages gained with the developed techniques against other traditional methods include the removal of the requirement of thin nonlinear crystals for harmonic generation, the possibility to get automatic phase matching without angular alignment or temperature control over a very wide spectrum and a simplified operation process. Different types of pulses have been measured in different conditions and the limits of validity of the technique have been explored. Since this work relies strongly upon the characteristics of emission of the second harmonic signal by these random crystals, an important part of this work has been focused on the characterization of the distribution of domains of the random nonlinear ferroelectric crystals and its relation with the angular emission of the second harmonic signal. The domain distribution of the nonlinear polarization implies an associated distribution of reciprocal lattice vectors, which can compensate the phase mismatch in the nonlinear interaction. Any change in the domain distribution would have a direct impact in the second harmonic generated and in its intensity angular distribution. Based on these fundamental concepts we demonstrate an indirect non-destructive optical method for the characterization of nonlinear domain statistics based on the analysis of the second harmonic generation intensity angular distribution. This method has been implemented experimentally and tested in crystals with different types of distributions. To gain a deeper insight on these processes, numerical simulations have been performed using a split-step fast-Fourier transform beam propagation method. It has been demonstrated that the analysis of the dependence of the second harmonic generation angular emission with the fundamental beam wavelength can be used to obtain relevant information about complicated domain structures. This method could be used for real time monitoring of the unknown domain distribution during the poling or crystal growing process
Esta tesis doctoral es un estudio de la generación de segundo armónico en cristales no lineales compuestos por dominios ferroeléctricos que alternan el signo de la non linealidad de segundo orden y distribuidos de una forma aleatoria (como por ejemplo niobato de estroncio y bario o niobato de calcio y bario). Como primera aplicación proponemos una técnica de caracterización de pulsos laser ultracortos, cuyo principio de operación está relacionado con la manera singular en la que este tipo de cristal emite la señal de segundo armónico en una dirección transversal a la dirección de propagación del pulso a medir. Utilizando esta señal no lineal podemos determinar la duración del pulso, el parámetro de chirp y el perfil temporal en una configuración de single-shot. Hemos implementado este método en dos configuraciones distintas -auto correlación y correlación cruzada- para la medida de pulsos con duraciones entre 10 fs y 1 ps. Este método, en comparación con otros métodos tradicionales para la caracterización de pulsos ultracortos, permite obtener el ajuste de fase (phase matching) de forma automática sobre un rango espectral muy amplio, sin necesidad de aliñamiento crítico ni ajuste de temperatura, elimina el requisito de utilizar cristales delgados y tiene un proceso de operación más sencillo. Se han medido diferentes tipos de pulsos y se han explorado las limitaciones de la técnica. Como este trabajo se basa en las propiedades específicas de la emisión de segundo armónico en los cristales no lineales con distribución aleatoria de dominios, un objetivo importante ha sido la caracterización del tamaño y la distribución de los dominios ferroelectricos y su relación con la distribución angular específica del segundo armónico generado. La distribución espacial de los dominios implica una distribución correspondiente de vectores en la red recíproca que puede compensar el ajuste de fase en la interacción no lineal. Cualquier cambio en la distribución de dominios tendrá pues un impacto directo en la intensidad y distribución angular de la señal de segundo armónico generado. Basándolos en estos conceptos, demostramos un método óptico non destructivo indirecto para la caracterización estadística de los dominios no lineales basado en el análisis de la intensidad y la distribución angular del segundo armónico generado. Implementamos este método experimental en la caracterización de cristales con diferentes tipos de dominios. Para un estudio más detallado hemos desarrollado un modelo numérico basado en el método de "split-step fast-Fourier transform beam propagation" que simula el proceso no lineal observado experimentalmente. Demostramos que el análisis de la dependencia angular del segundo armónico puede aportar información relevante sobre estructuras con distribuciones complejas de dominios. Este método se puede utilizar para la monitorización en tiempo real de distribuciones desconocidas en el mismo proceso de crecimiento o del poling del cristal ferroelectrico.
Ce projet de thèse de doctorat est intitulé « Génération du second harmonique dans des cristaux non-linéaires désordonnés: application pour la caractérisation d'impulsions laser ultra-courtes ». Il est consacré à l'étude de la génération de deuxième harmonique dans des cristaux ferroélectriques non linéaires formés par une distribution aléatoire de domaines. Ceci conduit à une distribution aléatoire de la susceptibilité non linéaire quadratique (Tels que le nitrate de baryum de strontium –SBN- et les cristaux de nitrate de calcium et de calcium) et son application à la caractérisation unique des impulsions laser ultra-courtes. Le principe de base de l'opération est lié au type unique d'émission associé à ces types de cristaux où le second signal harmonique est émis transversalement à la direction de propagation du faisceau. En utilisant la génération transversale de deuxième harmonique à partir de ces cristaux, nous mesurons la durée de l'impulsion, le paramètre chirp et le profil temporel dans une configuration à un seul pulse laser. Cette méthode a été mise en oeuvre à la fois dans l'autocorrélation transversale et les schémas transversaux de corrélation croisée pour la mesure des impulsions avec des durées allant de plusieurs dizaines à plusieurs centaines de femtosecondes. Les principaux avantages obtenus avec les techniques développées par rapport à d'autres méthodes traditionnelles comprennent l'élimination de l'exigence de cristaux minces non linéaires pour la génération harmonique, la possibilité d'obtenir une correspondance automatique de phase sans alignement angulaire ou contrôle de la température sur un spectre très large et un processus d'opération simplifié. Différents types d'impulsions ont été mesurés dans différentes conditions et les limites de validité de la technique ont été explorées. Étant donné que ce travail repose fortement sur les caractéristiques de l'émission du second signal harmonique par ces cristaux ferroélectriques à distribution aléatoire des domaines, une partie importante de ce travail a été axée sur la caractérisation de la distribution des domaines des cristaux ferroélectriques non linéaires aléatoires et sa relation avec l'émission angulaire du signal de la deuxième harmonique. La distribution de la polarisation non linéaire implique une distribution associée de vecteurs de réseau réciproque, ce qui peut compenser le décalage de phase dans l'interaction non linéaire. Toute modification de la répartition des domaines aurait un impact direct dans la distribution angulaire de la deuxième harmonique et de sa distribution angulaire d'intensité. Sur la base de ces concepts fondamentaux, nous démontrons une méthode optique non destructive indirecte pour la caractérisation de statistiques des domaines non linéaire basées sur l'analyse de la distribution angulaire d'intensité de génération de la deuxième harmonique. Cette méthode a été mise en oeuvre expérimentalement et testée dans des cristaux avec différents types de distributions. Pour obtenir une meilleure compréhension de ces processus, des simulations numériques ont été effectuées en utilisant une méthode de propagation de faisceau adaptée aux matériaux non linéaires. Il a été démontré que l'analyse de la dépendance de l'émission angulaire de la deuxième génération harmonique avec la longueur d'onde fondamentale du faisceau peut être utilisée pour obtenir des informations pertinentes sur les structures de domaines compliquées. Cette méthode pourrait être utilisée pour la surveillance en temps réel de la distribution de domaines inconnue pendant le processus de polling ou de croissance des cristaux.

Résumé : La génération d’impulsions de quelques cycles optiques stabilisées en CEP dans le moyen infrarouge utilisant la technique d’amplification paramétrique optique à fort taux de répétition est d’un grand intérêt pour diverses études de dynamiques ultra-brèves. Les travaux de cette thèse sont directement inscrits dans ce cadre. Nous décrivons un système émettant des impulsions dont le spectre est centré à 2.1 µm avec une durée de 19.5 fs et une énergie de 31 µJ opérant à 10 kHz avec une stabilité RMS de 0.54 %. Ce système se distingue de l’état de l’art par la mise en œuvre d’une technique de différence de fréquence en ligne permettant d’obtenir une stabilité de la CEP tir-à-tir de 107 mrad pendant quatre heures. De plus cette thèse à permis le dévelopement d’un dispositf émettant des impulsions de quelques cycles optiques à 1.55 µm opérant à haut taux de répétition (125 kHz). Ce système est le résultat de l’assemblage d’un amplificateur paramétrique optique et d’un systéme de compression non-lineaire dans une cellule multi-passage. La propagation non linéaire périodique dans la cellule en régime de dispersion anormale permet une compression solitonique, tout en moyennant les effets spatiaux de la nonlinéarité sur le faisceau. Nous démontrons ainsi l’autocompression d’impulsions initiales de 19 µJ 63 fs vers des impulsions en sortie de 14 µJ 22 fs
Abstract : The generation of carrier envelope phase (CEP) stable few-cycle pulses in the SWIR/Mid-IR using optical parametric chirped pulse amplification (OPCPA) at high repetition rate is of great interest for several applications in ultrafast dynamics. During this thesis the work was primarily focused on the development of OPCPA sources for attosecond science. We present an OPCPA operating at 2.1 µm delivering 19.5 fs pules duration with an energy of 31 µJ at 10 kHz with a RMS energy stability of 0.54 %. An original architecture using an all-inline difference frequency generation stage allows performances beyond the state of the art in terms of CEP stability. We report RMS CEP fluctuations of 107 mrad RMS measured shot-to-shot over four hours. On another hand, the development of a high repetition rate (125 kHz) OPCPA coupled with an innovative nonlinear compression scheme is described. The OPCPA provides pulses centered at 1.55 µm, with a pulse duration of 63 fs with an energy of 19 µJ after compression. Then a nonlinear compression stage based on a soliton dynamics in a multipass cell is implemented. The periodic propagation inside the cell allows to retain the temporal nonlinear effects, while the spatial nonlinear effects are washed out by the distributed nature of the nonlinearity over a large number of passes. We report the self-compression of 63 fs pulse at 1.5 µm down to 22 fs with an energy of 14 µJ

Master of Science
Department of Physics
Brian R. Washburn
This thesis reports our attempt towards achieving a phase stabilized free-space nonlinear polarization rotation (NPR) mode locked erbium doped fiber laser frequency comb system. Optical frequency combs generated by mode-locked femtosecond fiber lasers are vital tools for ultra-precision frequency metrology and molecular spectroscopy. However, the comb bandwidth and average output power become the two main limiting elements in the application of femtosecond optical frequency combs. We have specifically investigated the free-space mode locking dynamics of erbium-doped fiber (EDF) mode-locked ultrafast lasers via nonlinear polarization rotation (NPR) in the normal dispersion regime. To do so, we built a passively mode-locked fiber laser based on NPR with a repetition rate of 89 MHz producing an octave-spanning spectrum due to supercontinuum (SC) generation in highly nonlinear fiber (HNLF). Most significantly, we have achieved highly stable self-starting NPR mode-locked femtosecond fiber laser based frequency comb which has been running mode locked for the past one year without any need to redo the mode locking. By using the free-space NPR comb scheme, we have not only shortened the cavity length, but also have obtained 5 to 10 times higher output power (more than 30 mW at central wavelength of 1570 nm) and much broader spectral comb bandwidth (about 54 nm) compared to conventional all-fiber cavity structure with less than 1 mW average output power and only 10 nm spectral bandwidth. The pulse output from the NPR comb is amplified through a 1 m long EDF, then compressed by a length of anomalous dispersion fiber to a near transform limited pulse duration. The amplified transform limited pulse, with an average power of 180 mW and pulse duration of 70 fs, is used to generate a supercontinuum of 140 mW. SC generation via propagation in HNLF is optimized for specific polling period and heating temperature of PPLN crystal for SHG around 1030 nm. At last, we will also discuss the attempt of second harmonic generation (SHG) by quasi phase matching in the periodically polled lithium niobate (PPLN) crystal due to nonlinear effect corresponding to different polling period and heating temperature.

La génération d'harmoniques d'ordre élevé en milieu gazeux est un phénomène habituellement décrit par un modèle à trois étapes : sous l'e et d'un champ laser intense, un atome (ou une molécule) est ionisé par e et tunnel. L'électron éjecté est accéléré dans le champ laser, puis il se recombine sur son ion parent en émettant un photon XUV. Ce rayonnement XUV, émis sous la forme d'impulsions attosecondes (1 as = 10^-18 s), est un outil idéal pour sonder la structure électronique des atomes ou des molécules, avec une résolution temporelle de l'ordre de l'attoseconde. Néanmoins, l'intensité de ce rayonnement n'est en général pas su sante pour induire des e ets non-linéaires (transitions à deux photons). Au cours des travaux réalisés pendant cette thèse, nous avons développé une source harmonique capable de produire un rayonnement XUV intense qui doit permettre d'accéder à la physique non-linéaire dans cette gamme de longueur d'onde. Pour parvenir à ces résultats, un travail important sur les impulsions infrarouges génératrices a été nécessaire, aussi bien dans le domaine spatial que dans le domaine temporel. Une technique de mise en forme spatiale de faisceaux laser intenses a donc été développée, ainsi qu'une technique de post compression adaptée aux impulsions laser intenses. Ce travail de thèse se divise donc en trois étapes : - Le développement de la source harmonique haute énergie et des diagnostics associés. Cette source est basée sur l'utilisation d'une chaîne laser Titane-Saphir qui délivre des impulsions de 150 mJ pour des durées de 40 fs à une cadence de 10 Hz. De bonnes conditions d'optimisation ont été obtenues, donnant lieu à des impulsions XUV dont l'énergie est de l'ordre du J lors de la génération dans l'argon. - Le développement d'une technique de mise en forme spatiale adaptée aux faisceaux laser intenses et à la génération d'harmoniques. Le dispositif est basé sur une optique en ré exion, et sur les interférences à deux faisceaux. Il permet de produire, dans la région focale, des faisceaux dont le pro l d'intensité est radialement constant (faisceaux at top) et ainsi d'apporter un contrôle supplémentaire sur la génération d'harmoniques d'ordre élevé. - Le développement d'une technique de post compression en propagation guidée basée sur l'élargissement spectral induit par ionisation. Cette technique est adaptée pour des impulsions intenses (3.5 TW) et permet de produire des impulsions de puissance crête supérieure au Térawatt dans le domaine sub-10 fs. Cette technique fournit donc une source unique pour la génération d'harmoniques d'ordre élevé. Ces deux approches ont été testées et validées pour la génération d'harmoniques d'ordre élevé, et les résultats obtenus ouvrent d'intéressantes perspectives telles que la génération d'impulsions attosecondes isolées de haute énergie (> 100 nJ).

The work described in this thesis was performed at the Laboratory for Intense Lasers (L2I) of Instituto Superior Técnico, University of Lisbon (IST-UL). Its main contribution consists in the feasibility study of the broadband dispersive stages for an optical parametric chirped pulse amplifier based on the nonlinear crystal yttrium calcium oxi-borate (YCOB). In particular, the main goal of this work consisted in the characterization and implementation of the several optical devices involved in pulse expansion and compression of the amplified pulses to durations of the order of a few optical cycles (20 fs). This type of laser systems find application in fields such as medicine, telecommunications and machining, which require high energy, ultrashort (sub-100 fs) pulses. The main challenges consisted in the preliminary study of the performance of the broadband amplifier, which is essential for successfully handling pulses with bandwidths exceeding 100 nm when amplified from the μJ to 20 mJ per pulse. In general, the control, manipulation and characterization of optical phenomena on the scale of a few tens of fs and powers that can reach the PW level are extremely difficult and challenging due to the complexity of the phenomena of radiation-matter interaction and their nonlinearities, observed at this time scale and power level. For this purpose the main dispersive components were characterized in detail, specifically addressing the demonstration of pulse expansion and compression. The tested bandwidths are narrower than the final ones, in order to confirm the parameters of these elements and predict the performance for the broadband pulses. The work performed led to additional tasks such as a detailed characterization of laser oscillator seeding the laser chain and the detection and cancelling of additional sources of dispersion.