Self-seeded lasing in ionized air pumped by 800 nm femtosecond laser pulses

We report on the lasing in air and pure nitrogen gas pumped by a single 800 nm femtosecond laser pulse. Depending on gas pressure, incident laser power and beam convergence, different lasing lines are observed in the forward direction with rapid change of their relative intensities. The lines are attributed to transitions between vibrational and rotational levels of the first negative band of the singly charged nitrogen molecule-ion. We show that self-seeding plays an important role in the observed intensity changes.Comment: 9 pages, 4 figure

Superfilamentation in air

The interaction between a large number of laser filaments brought together using weak external focusing leads to the emergence of few filamentary structures reminiscent of standard filaments, but carrying a higher intensity. The resulting plasma is measured to be one order of magnitude denser than for short-scale filaments. This new propagation regime is dubbed superfilamentation. Numerical simulations of a nonlinear envelope equation provide good agreement with experiments.Comment: 5 pages, 4 figure

Lasing of ambient air with microjoule pulse energy pumped by a multi terawatt IR femtosecond laser

We report on the lasing action of atmospheric air pumped by an 800 nm femtosecond laser pulse with peak power up to 4 TW. Lasing emission at 428 nm increases rapidly over a small range of pump laser power, followed by saturation above ~ 1.5 TW. The maximum lasing pulse energy is measured to be 2.6 uJ corresponding to an emission power in the MW range, while a maximum conversion efficiency of is measured at moderate pump pulse energy. The optical gain inside the filament plasma is estimated to be excess of 0.7/cm. The lasing emission shows a doughnut profile, reflecting the spatial distribution of the pump-generated white-light continuum that acts as a seed for the lasing. We attribute the pronounced saturation to the defocusing of the seed in the plasma amplifying region and to the saturation of the seed intensity

Laser-induced periodic annular surface structures on fused silica surface

International audienceWe report on the formation of laser-induced periodic annular surface structures on fused silica irradiated with multiple femtosecond laser pulses. This surface morphology emerges after the disappearance of the conventional laser induced periodic surface structures, under successive laser pulse irradiation. It is independent of the laser polarization and universally observed for different focusing geometries. We interpret its formation in terms of the interference between the reflected laser field on the surface of the damage crater and the incident laser pulse

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

Acoustic signals generated by filamentation of ultrashort TW laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.Comment: 9 pages, 12 figure

Two-color interferometer for the study of laser filamentation triggered electric discharges in air

International audienceWe present a space and time resolved interferometric plasma diagnostic for use on plasmas where neutral-bound electron contribution to the refractive index cannot be neglected. By recording simultaneously the plasma optical index at 532 and 1064 nm, we are able to extract independently the neutral and free electron density profiles. We report a phase resolution of 30 mrad, corresponding to a maximum resolution on the order of 4 × 10 22 m −3 for the electron density, and of 10 24 m −3 for the neutral density. The interferometer is demonstrated on centimeter-scale sparks triggered by laser filamentation in air with typical currents of a few tens of A

Utilisation d'une source laser pulsée à haute energie comme source acoustique large bande en milieu liquide Source acoustique générée par un laser pulsé intense

International audienceIn this work the non-linear opto-acoustic problem which consists in generating an acoustic signal in water from an intense ultra short laser pulse has been studied. The acoustic source obtained could be related to the phenomenon of filamentation which produces a contraction of the initial beam accompanied by the formation of plasma. Relatively recent work has shown that lasers of this type could be used to produce remote acoustic sources with interesting applications to underwater acoustics. The spectrum of the sound source obtained was investigated and its directivity pattern in both planes (plane of the filament and plane 96 TS. Volume 33 – n° 1/2016 perpendicular to the filament) was measured. The sound level of the source as a function of energy, duration, and wavelength of the laser pulse was also measured.Ce travail est relatif à l'étude expérimentale d'un problème d'opto-acoustique non linéaire, consistant à générer un signal acoustique dans l'eau à partir d'un laser pulsé térawatt (TW). La source acoustique obtenue a pu être reliée au phénomène de filamentation qui produit une contraction du faisceau initial, accompagnée de la formation d'un plasma. Des travaux relativement récents ont montré que les lasers de ce type pouvaient être utilisés pour produire des sources acoustiques déportées, avec des applications intéressantes pour l'acoustique sous-marine. Le spectre de la source acoustique obtenue a été étudié à l'aide de plusieurs hydrophones couvrant une très large bande de fréquence, et le diagramme de directivité a été mesuré dans deux plans (plan du filament et plan perpendiculaire au filament). Le niveau acoustique de la source en fonction de l'énergie, de la durée, et de la longueur d'onde de l'impulsion laser, a également été étudi

Diffusion des ondes électromagnétiques par une surface rugueuse monodimensionnelle sous incidences modérées et rasantes (application au domaine maritime)

Le contexte de cette thèse est l'étude de la diffusion des ondes électromagnétiques par une surface rugueuse sous incidences modérées et rasantes. Par exemple, le radar côtier, objet de l'étude, observe la mer à des angles d'incidences proches de l'horizontale. Pour une telle configuration, il est intéressant de calculer la puissance rétrodiffusée par la mer afin de dimensionner des équipements futurs d une part, et d autre part, de détecter la présence d objets flottants. Pour cela, il existe des modèles asymptotiques basés sur la Méthode des Petites Perturbations, où les irrégularités de la surface la hauteur des vagues - sont supposées inférieures à la longueur d'onde. Ce modèle est référencé sous le sigle MPP dans la littérature. En utilisant une méthode rigoureuse, développée récemment au laboratoire, il est montré que la MPP dite classique, développée pour des incidences modérées, n est plus valide pour des incidences rasantes. Une méthode, inspirée de la méthode des diagrammes en théorie quantique des champs, est utilisée. La MPP est alors modifiée puisqu une impédance effective liée à la rugosité de la surface intervient dans la formulation. De plus, des travaux réalisés au siècle dernier pour une surface plane très conductrice mettent en évidence l existence d une onde de surface se propageant le long de l interface air-Terre. Cette solution est alors adaptée au cas d une surface de mer. Les notions d'impédance de surface rugueuse et onde de surface sont des facteurs déterminants dans l'évaluation de la puissance diffusée lorsque la mer est illuminée sous incidences rasantes. La MPP modifiée est alors comparée avec la méthode numérique de référenceThis thesis is devoted to the study of scattering of electromagnetic waves by a rough surface at moderate and grazing angles of incidence. As an example, the coastal radar observed the sea surface at angles of incidence close to horizontal. For such a set-up, it is interesting to work out the backscattered intensity from the sea surface in order to design future equipments, firstly, and secondly, to detect floating objects. Asymptotic methods based on the Small Perturbations Method can be used. Irregularities sea height waves- are then assumed to be smaller than the wavelength. This model can be found with the acronym SPM in the literature. The use of a rigorous method, recently implemented at the laboratory, shows that SPM is valid at moderate angles of incidence, but fails at low grazing angles. A method, based on the extension of the diagram method in quantum field theory, is then used. The SPM is then modified and an effective surface impedance is included into the solution. Moreover, many works done in the previous century proved the existence of a guided surface wave along the interface air-Earth. This solution is adapted to the case of a sea surface, goal of this study. Notions of effective surface impedance and surface wave are determining factors to estimate the scattered intensity from the sea at low grazing angles. Eventually, the modified SPM is compared with the rigorous methodNANTES-BU Sciences (441092104) / SudocSudocFranceF

Rigorous prediction of the ground wave above flat and rough highly-conducting one-dimensional sea surfaces in VHF band

International audienc

Runtime and jitter of a 1-cm scale atmospheric air spark gap triggered by a femtosecond-laser-filament

International audienceoral presentatio