The effect of distance between lens and water surface on the acoustic characteristics induced by a 532 nm nanosecond laser under water

Abstract

Experimental and simulation investigations on laser-induced acoustic wave propagation and the optical refraction effects by varying the focusing lens distance from the water-surface have been presented. The pulses from Nd:YAG laser (532 nm, 10 ns pulse width) were directed onto a water medium to generate the underwater plasma acoustic waves. Initially, the effect of varying focus lens positions in water on the acoustic wave signal was analyzed using the spectroscopy approach. Secondly, the charge-coupled device (CCD) photography technique was employed to capture the acoustic wave propagation underwater at different focusing lens positions (60–10 mm). We found that strength of underwater plasma signal was reduced gradually as the distance between focusing lens and the horizontal plane decreased. The CCD photography was confirmed that radiation intensity of plasma images became weaker with decrease in focusing lens distance underwater due to the greater optical refraction effect on it. The research efforts indicated that the simulation results could also address the underwater acoustic wave characteristics and their optical refraction effects, which were found in consistence with the experimental evidences.