reflectivity of the boundary surface between the optical waveguide 8 and the liquid is clearly increased and thus the opto receiver 4 continues to receive light of high intensity until the cavitation bubbles 22 at the exposed end 10 collapse by measuring the reflectivity of this boundary surface not only can the formation of cavitation bubbles 22 be established but their lifetimes can be determined as well the free cavitation bubbles 22 found in the liquid essentially do not contribute to the signal that is received at the opto receiver 4 since the fraction of light which is reflected into the optical waveguide 8 by diffuse reflection is insignificant the light source 2 and the opto receiver 4 are connected to a control and evaluation unit 5 this unit 5 performs an analysis to determine if the signal measured by the opto receiver 4 contains a cavitation signal indicating the existence of cavitation bubbles 22 because of the characteristics of the cavitation signal that occurs in conjunction with the cavitation bubbles 22 the cavitation signal can be easily detected using suitable means the control and evaluation unit 5 may include such means for example unit 5 may include means by which the shock wave signal measured at the opto receiver 4 can be differentiated from the cavitation signal this can be accomplished for example by a comparator circuit arrangement which measures the amplitude of the signal at the opto receiver 4 and compares it to a specified threshold value the shock wave signal which occurs before the cavitation signal can be used effectively as a reference signal for time and amplitude values for example only those measured signals which occur within a specified period.