Activities of the cavitation bubbles in the wake of a shock pressure pulse recorded by an optical fiber hydrophone

Abstract

The shock pulse used in an extracorporeal shock wave treatment (ESWT or ESWL) has a large negative pressure (< -5MPa) which can always produce acoustic cavitation. The resulting cavitation bubbles are known to play an important role in therapeutic effects, however, the bubble activities are not readily measurable yet. The present study considered a weird tail after the negative peak in the time history of pressure sensed by an optical fiber hydrophone which was usually abandoned in typical pressure field measurements. A shock pressure pulse in water causes change of mass density which modulates the optical refractive index. The change of the refractive index can be measured by the light reflection at the tip of the glass fiber submerged in water. The loss of water contact by cavitation bubbles at the fiber tip leads to an abnormal increase of high reflection which is clearly identified. This suggests that the weird tail of the hydrophone signal beyond the negative cycle of a shock pulse is closely related...