Acoustic transient generation by holmium-laser-induced cavitation bubbles

Abstract

The acoustic effects of free-running 2.12 μm Cr:Tm:Ho:YAG laser pulses delivered in water are studied. Laser pulses of 10 to 1200 mJ energy and 230 μs duration (full width at half-maximum) are used. Delivery fiber diameters of 200–600 μm are investigated. Combined fast flash video imaging and needle probe hydrophone pressure sensing are used. The experimental results show that the laser-induced water vapor bubbles can generate strong acoustic transients at the bubble collapse several hundreds of μs after the start of the laser pulse. Pressures of up to 3600 bar are measured. Above a laser fluence threshold of 40 J/cm2 the pressure amplitude increases sharply, reaching a maximum value between 100 and 200 J/cm2. At higher fluences up to more than 1000 J/cm2, the pressure amplitude is found to decrease again. A two-phase mechanism is proposed to describe the complex bubble dynamics generated by the free-running pulses: The isotropic expansion of an initially superheated water volume is followed by a continuous ablation phase. The results suggest a mechanism of possible unwanted acoustic damage during Holmium laser medical applications in a liquid environment.