Underwater acoustic signals induced by intense ultrashort laser pulse

Abstract

International audienceAcoustic signals generated in water by terawatt (TW) laser pulses undergoing filamentation are studied. The acoustic signal has a very broad spectrum, spanning from 0.1 to 10 MHz, and is confined in the plane perpendicular to the laser direction. Such source appears to be promising for the development of remote laser based acoustic applications

Underwater acoustic wave generation by filamentation of terawatt ultrashort laser pulses

“…In figure 12 a comparison with experimental results is shown for the angular dependence of the amplitude for selected frequencies. Most of the sound wave energy is distributed perpendicularly to the beam propagation axis, as measured and shown by Y. Brelet et al [14]. We note that the directivity is sharply peaked for higher frequencies.…”

“…The conversion efficiencies from light to the acoustic signal was however reported to be enhanced with nanosecond laser pulses, leading to optical breakdown, rapid heating of the focal volume producing pressures in the gigapascal range and explosive expansion followed by the emission of a shock wave [11][12][13]. Femtosecond laser pulses open up new possibilities in this field as they were recently shown to lead to ultrabroad acoustic signals [14]. The nonlinear propagation of femtosecond laser pulses in gases or liquids leads to light-plasma filaments, where the laser beam shrinks upon itself due to the Kerr nonlinearity, to reach intensity levels that exceed the threshold for optical field ionization [15].…”

Energy deposition from focused terawatt laser pulses in air undergoing multifilamentation

“…One could think that as some energy transfer channels, like avalanche ionization, are favored by longer pulses, it could positively affect energy deposition. Such behavior has indeed been observed during filamentation in fused silica [17] and water [18]. Conversely, it seems that maximum intensity gives the best results in air, because it promotes a more efficient nonlinear ionization and Raman absorption, in the limit of pulse durations that can be achieved with our laser system.…”

Utilisation d’une source laser pulsée à haute energie comme source acoustique large bande en milieu liquide

“…On a également mesuré les diagrammes de directivité dans un plan du filament et dans le plan perpendiculaire au filament. Ces mesures ont été faites avec différentes lentilles de focalisation, ce qui a permis non seulement de focaliser le faisceau laser à différentes profondeurs, mais aussi de mettre en évidence l'influence de la focalisation sur la directivité de la source acoustique (Brelet et al, 2015). Les signaux temporels présentés sur les figures 2a et 3a sont exprimés en Pa et représentent donc une pression acoustique.…”