ACOUSTIC TRANSIENT GENERATION BY HOLMIUM-LASER-INDUCED CAVITATION BUBBLES

The acoustic effects of free-running 2.12 mu m Cr:Tm:Ho:YAG laser puls es delivered in water are studied. Laser pulses of 10 to 1200 mJ energ y and 230 mu s duration (full width at half-maximum) are used. Deliver y fiber diameters of 200-600 mu m are investigated. Combined fast flas h video imaging and needle probe hydrophone pressure sensing are used. The experimental results show that the laser-induced water vapor bubb les can generate strong acoustic transients at the bubble collapse sev eral hundreds of mu s after the start of the laser pulse. Pressures of up to 3600 bar are measured. Above a laser fluence threshold of 40 J/ cm(2) the pressure amplitude increases sharply, reaching a maximum val ue between 100 and 200 J/cm(2). At higher fluences up to more than 100 0 J/cm(2), the pressure amplitude is found to decrease again. A two-ph ase mechanism is proposed to describe the complex bubble dynamics gene rated by the free-running pulses: The isotropic expansion of an initia lly superheated water volume is followed by a continuous ablation phas e. The results suggest a mechanism of possible unwanted acoustic damag e during Holmium laser medical applications in a liquid environment.