Pulse shielding in Laser-Induced Breakdown of saline water on hydrodynamic time scales is experimentally characterized. Pairs of pulses from a Nd:YAG laser are focused into saline water with a controlled time delay between them. The Laser-Induced Breakdown produced by the first pulse creates a cavitation bubble that later collapses generating a plume of bubbles that evolves on hydrodynamic time scales. When the second pulse arrives, the light is scattered by this plume with a consequent reduction in the intensity at the focal spot resulting in a lower breakdown efficiency of this pulse. By means of acoustic measurements, we determine the breakdown energy threshold for the first pulse and characterize the shielding of the second pulse as a function of the salinity of the solution, the energy of the pulse, and the inter-pulse interval. A model for the blocking process that takes into account both linear and nonlinear absorption along the path is developed which satisfactorily explains the observations.
Note1, in cases where material débris are also present—and subject also to advection by the fluid flow—both bubbles and débris will contribute additively to the scattering and absorption of the laser light.