Generation of Underwater Sound by a Moving High-Power Laser Source

摘要

The generation of underwater sound by high-power laser pulse is analyzed both theoretically and experimentally. The mechanism of sound generation is the thermal expansion of the water caused by heat imparted by the laser. The basic physical properties of such a laser-induced thermoacoustic source are investigated with a time domain approach which is valid in the nearfield of the source. Results are then extended to the case of a thermoacoustic source moving at subsonic, transonic, or supersonic velocities on the surface of the water. Special attention is paid to the case of a source moving at a velocity close to the speed of the sound in the water, because, at such a velocity, high pressure transients can be generated, making the thermoacoustic mechanism of sound generation more attractive for practical applications. Experimental results were obtained with a laser system providing up to 5 joules of energy over a pulse duration of approximately 1 ms, during which the intensity was modulated at a single frequency between 5 and 80 kHz. The laser emitted either in the infrared region of the spectrum or in the visible region of the spectrum. Most of the experimental results presented in this study were obtained with the infrared Neodymium:Glass laser. Four kinds of results were obtained: pressure waveforms, directivity patterns, sound level dependence on source velocity, and spreading curves. In general, the experimental results are in good agreement with the theoretical predictions.