Acoustic transient generation in pulsed holmium laser ablation under water

Abstract

In this study the role of acoustical transients during pulsed holmium laser ablation is addressed. For this the collapse of cavitation bubbles generated by 2.12 micrometers Cr:Tm:Ho:YAG laser pulses delivered via a fiber in water is investigated. Multiple consecutive collapses of a single bubble generating acoustic transients are documented. Pulse durations are varied from 130 - 230 microsecond(s) and pulse energies from 20 - 800 mJ. Fiber diameters of 400 and 600 micrometers are used. The bubble collapse behavior is observed by time resolved fast flash photography with 1 microsecond(s) strobe lamp or 5 ns 1064 nm Nd:YAG laser illumination. A PVDF needle probe transducer is used to observe acoustic transients and measure their pressure amplitudes. Under certain conditions, at the end of the collapse phase the bubbles emit spherical acoustic transients of up to several hundred bars amplitude. After the first collapse up to two rebounds leading to further acoustic transient emissions are observed. Bubbles generated near a solid surface under water are attracted towards the surface during their development. The final phase of the collapse generating the acoustic transients takes place directly on the surface, exposing it to maximum pressure amplitudes. Our results indicate a possible mechanism of unwanted tissue damage during holmium laser application in a liquid environment as in arthroscopy or angioplasty that may set limits to the choice of laser pulse duration and energies.