Mapping Spatio-Temporal and Spectral Features of Underwater Acoustic Signals from Nanosecond Laser Filamentation

This study presents a comprehensive spatio-temporal and spectral mapping of acoustic pressure impulses generated by nanosecond laser-induced filamentation (ns-LIF) in water. As the incident optical-energy increases, the peak overpressure of these signals increases almost linearly. Meanwhile, the constant arrival time indicates that the propagation velocity of the nanosecond laser-induced underwater acoustic signal (ns-LIUAS) in the water remains unchanged. Spatio-temporal mapping of the ns-LIUAS enables estimation of the ns-LIF source length, which was observed to increase with increasing laser energy. Furthermore, mapping of reflected signals from the water–air interface demonstrates the potential for estimating the location, size, and shape of underwater objects. Spectral mapping revealed consistent peak frequencies at ~ 60 kHz and ~ 120 kHz were observed across all scanning positions and energy levels. The Δω/ω ratio shows the energy-dependent spectral broadening at peak frequency of ~ 60 kHz, but remains constant at ~ 120 kHz. Time-frequency analysis simultaneously visualizes the arrival times and the spectral content specifically the ~ 60 kHz and ~ 120 kHz instantaneous peak frequency components of each signal. Our results demonstrate the feasibility of laser-based sonar, offering a valuable tool for characterizing the properties and intricate dynamic processes of ns-LIF through acoustic diagnostics, thus benefiting its remote sensing applications.