Improving the knowledge of the spectral and temporal properties of plasma-based XUV lasers is an important issue for the ongoing development of these sources towards significantly higher peak power. The spectral properties of the XUV laser line actually control several physical quantities that are important for applications, such as the minimum duration that can be achieved (Fourier-transform limit). The shortest duration experimentally achieved to-date is ~1 picosecond. The demonstrated technique of seeding XUV laser plasmas with a coherent femtosecond pulse of high-order harmonic radiation opens new and promising prospects to reduce the duration to a few 100 fs, provided that the gain bandwidth can be kept large enough.XUV lasers pumped by collisional excitation of Ni-like and Ne-like ions have been developed worldwide in hot plasmas created either by fast electrical discharge, or by various types of high-power lasers. This leads to a variety of XUV laser sources with distinct output properties, but also markedly different plasma parameters (density, temperature) in the amplification zone. Hence different spectral properties are expected. The purpose of our work was then to investigate the spectral behaviour of the different types of existing collisional excitation XUV lasers, and to evaluate their potential to support amplification of pulses with duration below 1 ps in a seeded mode.The spectral characterization of plasma-based XUV lasers is challenging because the extremely narrow bandwidth (typically ∆λ/λ ~10-5) lies beyond the resolution limit of existing spectrometers in this spectral range. In our work the narrow linewidth was resolved using a wavefront-division interferometer specifically designed to measure temporal coherence, from which the spectral linewidth is inferred. We have characterized three types of collisional XUV lasers, developed in three different laboratories: transient pumping in Ni-like Mo, capillary discharge pumping in Ne-like Ar and quasi-steady state pumping in Ne-like Zn. Besides the accurate measurement of the temporal coherence of the laser in each case, we have studied the spectral behaviour when the laser is operated in the saturation regime and (in Ni-like Mo) when it is seeded with high-order harmonic radiation. We have also investigated the temporal behaviour of the Ni-like Mo transient XUV laser, using an ultrafast X-ray streak camera. Our linewidth measurements are compared with detailed numerical calculations including relevant broadening mechanisms as well as radiative transfer effects. The evolution of the spectral profile with amplification and saturation was studied for different plasma parameters, and corresponding Fourier-transform limit duration were evaluated.The shortest temporal coherence (ie the largest bandwidth) is measured for the quasi-steady state pumping XUV laser, which operates at the highest density and ionic temperature.

La caractérisation spectrale détaillée des lasers XUV générés dans des plasmas est un enjeu important des projets actuels de développement qui visent à augmenter la puissance crête de ces sources. En effet les propriétés spectrales de ces lasers conditionnent d'autres propriétés importantes, telle que la durée minimum accessible (limite de Fourier). La plus courte durée actuellement démontrée expérimentalement est de 1 picoseconde. La technique d'injection d'un plasma de laser XUV avec une impulsion femtoseconde de rayonnement harmonique d'ordre élevé offre des perspectives très prometteuses pour réduire la durée d'impulsion jusqu'à quelques 100 femtosecondes, pourvu que l'on sache maintenir une bande spectrale de gain suffisamment large.Les lasers XUV pompés par excitation collisionnelle dans des ions néonoïdes et nickeloïdes ont été développés dans des plasmas chauds créés aussi bien par décharge électrique rapide que par différents types de lasers de puissance. On a ainsi accès à une large variété de sources lasers XUV, qui diffèrent par les caractéristiques du faisceau émis, mais aussi par les paramètres du plasma (densité, température) dans la zone de gain. On peut donc s'attendre à des propriétés spectrales différentes. Le but du travail que nous présentons est d'étudier les propriétés spectrales des différents types de lasers XUV collisionnels existants, et d'évaluer leur capacité à amplifier des impulsions de durée inférieure à 1 picoseconde, dans un mode injecté.La caractérisation spectrale des lasers XUV est expérimentalement difficile parce que la résolution spectrale nécessaire (∆λ/λ ~10-5) n'est pas accessible avec les meilleurs spectromètres actuels. Dans notre étude, nous avons atteint cette résolution en mesurant la cohérence temporelle de la source à l'aide d'un interféromètre à division de front d'onde, spécifiquement conçu pour ces mesures, à partir desquelles largeur spectrale peut être déduite.Nous avons caractérisé trois types de lasers XUV collisionnels, développés dans trois laboratoires différents: pompage transitoire dans le molybdène nickeloïde, pompage par décharge électrique dans l'argon néonoïde et pompage quasi-stationnaire dans le zinc néonoïde. Dans chaque cas la cohérence temporelle a été mesurée précisément. De plus nous avons étudié l'effet de la saturation de l'amplification et (pour le Ni-like Mo) l'influence du mode injecté. Nous avons également étudié le comportement temporel du laser transitoire Ni-like Mo à l'aide d'une caméra streak X ultra-rapide. Nos mesures spectrales sont comparées à des résultats de simulations numériques prenant en compte les différents mécanismes d'élargissement ainsi que les effets de transfert radiatif. Nous avons étudié l'évolution du profil spectral avec l'amplification et la saturation, et nous avons évalué les limites de Fourier correspondantes.Le temps de cohérence le plus court (ie la largeur spectrale la plus grande) est mesuré pour le laser XUV quasi-stationnaire, qui correspond au plasma qui a la plus forte densité et la plus forte température ionique. / Improving the knowledge of the spectral and temporal properties of plasma-based XUV lasers is an important issue for the ongoing development of these sources towards significantly higher peak power. The spectral properties of the XUV laser line actually control several physical quantities that are important for applications, such as the minimum duration that can be achieved (Fourier-transform limit). The shortest duration experimentally achieved to-date is ~1 picosecond. The demonstrated technique of seeding XUV laser plasmas with a coherent femtosecond pulse of high-order harmonic radiation opens new and promising prospects to reduce the duration to a few 100 fs, provided that the gain bandwidth can be kept large enough.XUV lasers pumped by collisional excitation of Ni-like and Ne-like ions have been developed worldwide in hot plasmas created either by fast electrical discharge, or by various types of high-power lasers. This leads to a variety of XUV laser sources with distinct output properties, but also markedly different plasma parameters (density, temperature) in the amplification zone. Hence different spectral properties are expected. The purpose of our work was then to investigate the spectral behaviour of the different types of existing collisional excitation XUV lasers, and to evaluate their potential to support amplification of pulses with duration below 1 ps in a seeded mode.The spectral characterization of plasma-based XUV lasers is challenging because the extremely narrow bandwidth (typically ∆λ/λ ~10-5) lies beyond the resolution limit of existing spectrometers in this spectral range. In our work the narrow linewidth was resolved using a wavefront-division interferometer specifically designed to measure temporal coherence, from which the spectral linewidth is inferred. We have characterized three types of collisional XUV lasers, developed in three different laboratories: transient pumping in Ni-like Mo, capillary discharge pumping in Ne-like Ar and quasi-steady state pumping in Ne-like Zn. Besides the accurate measurement of the temporal coherence of the laser in each case, we have studied the spectral behaviour when the laser is operated in the saturation regime and (in Ni-like Mo) when it is seeded with high-order harmonic radiation. We have also investigated the temporal behaviour of the Ni-like Mo transient XUV laser, using an ultrafast X-ray streak camera. Our linewidth measurements are compared with detailed numerical calculations including relevant broadening mechanisms as well as radiative transfer effects. The evolution of the spectral profile with amplification and saturation was studied for different plasma parameters, and corresponding Fourier-transform limit duration were evaluated.The shortest temporal coherence (ie the largest bandwidth) is measured for the quasi-steady state pumping XUV laser, which operates at the highest density and ionic temperature.