Characterization of nanosecond laser-induced underwater filamentation using acoustic measurements

This investigation aims to understand the temporal and spectral characteristics of underwater acoustic impulses due to ∼10n s laser-induced filamentation at various incident laser energies. In addition to the experimental study, finite element analysis (FEA) has been used in conjunction with experimentally acquired data to simulate and visualize acoustic impulse propagation and interaction across the water-rigid boundary. In the time domain, the increase in the underwater filament length is measured by the peak-to-peak (P _k-P _k) overpressures as a function of input laser energies. The P _k-P _k overpressures are maximum around the focal plane and are symmetrical on either side. This variation in the P _k-P _k overpressure is traced to characterize the spatial extent of the underwater filament. The arrival time of underwater acoustic impulse varies within the 8-9 µs for input laser energies. It was observed that with increasing laser beam power, the conversion efficiency of the acoustic impulse decreases, and the underwater filament length increases. In the frequency domain, with increasing input laser energy, the peak frequency of the underwater acoustic impulse shifts toward the lower frequencies. Using FEA, the short-time Fourier transform spectrogram simultaneously visualizes the arrival time and instantaneous frequency of the simulated acoustic impulse. The simulated acoustic impulse produced a single instantaneous peak frequency of around 76 kHz. Upon reflection from the water-rigid boundary, we observed that the reflected signal also has a single instantaneous peak frequency of around 76 kHz, while higher-frequency components are completely dissipated. Our results are promising for the development of remote laser-based acoustic generation and sensing applications.