Characteristics of Nanosecond Laser-Induced Underwater Acoustic Signals Across the Water–Air Interface

The temporal and spectral characteristics of nanosecond laser-induced underwater acoustic shockwave for varying input laser energies are obtained. The underwater acoustic shockwave results in reflection and transmission at the water-air interface due to enormous acoustic impedance mismatching between the water and air. With increasing input laser energies, the peak-to-peak overpressures of the reflected and transmitted signals increase linearly. The relationship between the reflected and transmitted acoustic energy coefficients across the water-air interface and the incident laser energy is studied. The advantage of power spectral density in classifying reflected and transmitted signals is based on their peak frequency and full width at half maximum (FWHM). With increasing incident laser energy, the FWHM decreases, and the area under the curve increases for reflected and transmitted signals. In addition to the experimental investigation, finite element analysis (FEA) was used to visualize underwater acoustic signal reflection and transmission at the interface. Experimental data is used as input for FEA in visualizing the acoustic signal in the spatio-temporal domain at the water-air interface. These studies could also be useful for understanding the blast wave evolution of underwater explosions and their interactions at the interfaces.