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Interferometric spatio-temporal characterisation of ultrashort light pulsesMang, Matthias M. January 2014 (has links)
The main topic of this thesis is the development of novel diagnostics for the characterisation of infrared femtosecond and extreme-ultraviolet (XUV) attosecond pulses. High-resolution interferometric methods are applied to high harmonic radiation, both to measure the properties of the XUV light and to relate this information to the physics of the fundamental generation process. To do so, a complete high harmonic beamline has been built and optimised to enable the observation of strong signatures of the macroscopic response of the medium. The distinct spatial characteristics of long and short trajectories are studied, as well as the interference between them. An interferometric measurement allows the extraction of the atomic dipole phase, which gives direct access to the sub-cycle electron dynamics. A major focus of this thesis is on the development of a novel method which simultaneously characterises two independent electric fields as a function of any degree of freedom in which it is possible to shear one of the beams. Since each field alternately takes the role of the reference to retrieve the other field, this technique is referred to as mutual interferometric characterisation of electric-fields (MICE). One of the key features of MICE is that no sheared but otherwise identical replica of the test pulse needs to be generated, which is a typical requirement of self-referencing techniques. Furthermore, no a priori information is needed for the reconstruction. The strength and the wide applicability of MICE are demonstrated using two fundamentally different examples. First, the temporal pulse profiles of two infrared femtosecond pulses are simultaneously reconstructed in a single laser shot. In the second demonstration, the MICE approach is used to simultaneously reconstruct the wavefronts of two high harmonic beams. Having this new technique at hand, the phase properties of the different quantum trajectories are compared. All pulse characterisation techniques implicitly assume full coherence of the beam. This, however, is often not the case in practice, in particular when dealing with complex XUV light sources. Here the standard characterisation techniques fail to provide an accurate description of the electric field. Instead, the electric field must be seen as a statistical mixture of different contributions to the overall field. Here an interferometric experiment is first proposed and then performed involving multiple lateral shears to measure the two-point correlation function of high harmonic radiation. This directly provides information about the existence and the magnitude of partial coherence of high harmonics.
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Dispositifs expérimentaux pour la caractérisation spatio-temporelle de chaines laser femtosecondes haute-puissance / Experimental devices for the spatiotemporal characterization of femtosecond high-power laser chainsGallet, Valentin 26 September 2014 (has links)
Un des avantages des lasers femtosecondes de haute puissance (TW-PW) est de pouvoir obtenir, au foyer d'une optique focalisante, des intensités très élevées atteignant jusqu'à 10^22W.cm^-2 soit un champ électrique de 2.7 PV.m^-1. Pour cela, ces chaînes lasers délivrent nécessairement des faisceaux de grands diamètres (jusqu'à 40 cm) et des impulsions très courtes (de l'ordre de la dizaine de femtosecondes). En conséquence, les propriétés spatiales et temporelles de l'impulsion ne sont généralement pas indépendantes. Ce type de dépendance, appelée couplage spatio-temporel, a pour conséquence d'augmenter la durée d'impulsion et la taille de la tache focale, ce qui peut conduire à une diminution notable de l'intensité maximale au foyer. Les dispositifs de métrologie couramment utilisés sur ces chaînes lasers femtosecondes de haute puissance ne permettent de mesurer les profils spatial et temporel de l'impulsion que de façon indépendante.L'objectif de cette thèse était de développer des techniques permettant de mesurer les couplages spatio-temporels afin de pouvoir quantifier leur effet et de les corriger dans l'optique d'obtenir l'intensité maximale au foyer. Ainsi, nous avons tout d'abord adapté une technique de caractérisation spatio-temporelle existante à la mesure de lasers TW. Afin d'éviter les contraintes induites au foyer, comme celles liées aux fluctuations de pointé, les mesures ont été réalisées sur le faisceau collimaté. Ajouter une source de référence en parallèle du dispositif initial, nous a aussi permis de prendre en compte les artéfacts de mesure dus aux variations thermiques et mécaniques affectant l'interféromètre. Grâce à cette amélioration, il est possible de reconstruire le profil spatio-temporel complet du faisceau, en particulier son front d'onde.Cependant, les limitations induites par cette technique, nous ont conduit à développer un nouveau dispositif de mesure. Basé sur une corrélation croisée, cette technique consiste à faire interférer le faisceau laser à caractériser avec une partie de ce dernier, suffisamment petite pour ne pas être distordue spatio-temporellement. Nous avons également mis en œuvre une variante de ce dispositif permettant une mesure mono-coup selon une dimension transverse de l'impulsion.A l'aide de ces différentes techniques, nous avons pu caractériser, pour la première fois, plusieurs chaînes lasers TW. Les mesures réalisées ont mis en lumière l'existence de couplages spatio-temporels résiduels conduisant à une baisse significative de l'intensité pic au foyer. Ces résultats montrent qu'il est indispensable de caractériser spatio-temporellement des chaînes lasers femtosecondes de haute puissance dans l'optique d'obtenir l'intensité maximale au foyer. / One of the advantages of high-power femtosecond lasers (TW-PW) is to obtain, at the focus of a focusing optic, very high intensities up to 10^22W.cm^-2 (i.e. an electric field of 2.7 PV.m^-1. Therefore, these lasers chains necessarily deliver beams with large diameter (up to 40 cm) and very short pulses (of the order of tens of femtoseconds). As a consequence, the spatial and temporal properties of the pulse are generally not independent. Such dependence, called spatial-temporal coupling has the effect of increasing the pulse duration and the size of the focal spot, which can lead to a significant reduction of the maximum intensity at the focus. Metrology devices commonly used on these high-power femtosecond lasers allow retriving the spatial and temporal profiles of the pulse only in an independent manner. The aim of this thesis was to develop techniques for measuring spatiotemporal couplings in order to quantify their effect and correct them in order to obtain the maximum intensity at focus. First of all, we adapted an existing technique of spatio-temporal characterization to the measurement of TW lasers. To avoid the issues induced at the focus, such as those related to jittering, measurements were performed on the collimated beam. By adding a reference source to the original device, we managed to take into account the measurement artifacts due to thermal and mechanical variations affecting the interferometer. With this improvement, it was possible to reconstruct the complete spatio-temporal profile of the beam, particularly its wavefront. However, the limitations imposed by this technique led to the development of a new measurement device. Based on a cross-correlation, this technique consists of making the laser beam to interfere with a part of itself, small enough not to be spatiotemporally distorted. We have also implemented a variant of this device for a single-shot measurement along one transverse dimension of the pulse. Using these techniques, we performed the very first characterization of several TW laser chains. The measurements have revealed the existence of residual space-time couplings leading to a significant decrease of the peak intensity at focus. These results show that it is essential to spatio-temporally characterize high power femtosecond laser chains to obtain the maximum intensity at focus.
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