Input pulse duration effect on laser-induced underwater acoustic signals.

The characteristics of laser-induced underwater acoustic signals (UASs) generated by focused 10 ns and 30 ps laser pulses of different energies under similar experimental conditions are compared. Time domain signals and time-frequency analysis of the UASs were used to understand the role of input pulse duration and energy on the evolution of UAS. In the time domain, the peak-to-peak (${{\rm P}_k} {-} {{\rm P}_k}$) overpressure of the UAS decreases, and the arrival time (${{\rm A}_t}$) increases as a function of propagation distance for both ns and ps laser-induced breakdown (LIB) of water. With increasing incident energy of both ns and ps laser pulses, the ${{\rm P}_k} {-} {{\rm P}_k}$ overpressures of acoustic signals increase almost linearly. In the time-frequency domain, the spectrogram obtained via short-time Fourier transform provides spectral information and ${{\rm A}_t}$ of both direct and reflected signals simultaneously. The spectrogram revealed that the transient UAS has broad acoustic spectra spanning from 10 to 800 kHz, perpendicular to the laser propagation direction. The initial acoustic impulse resulted in two major frequencies centered around 105 and 690 kHz with a standard error of 30 kHz. Upon reflection from the water-air interface, only the peak frequency corresponding to ${\sim}{105};{\rm kHz}$ was reflected, while the longer frequency was observed to dissipate. Our results demonstrate that the ns-LIB is more suitable for applications compared to the ps-LIB owing to stronger acoustic impulse of both direct and reflected signals.