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When a laser irradiates into the liquid medium, the medium absorbs the laser energy and induces sound source. As a new method to generate underwater sound wave, laser-acoustic has a variety of commercial and oceanographic applications on the information transmission between aerial and underwater platform, underwater target detection, marine environment measurement etc. due to its merits such as high acoustic intensity, spike pulse and wide frequency spectrum. According to different energy intensity of the laser pulse and the spatial and temporal distribution of energy interaction region, the mechanism of the laser interacting with water that generating sound are classified as thermoelastic, vaporization and optical breakdown mainly. Thermoelastic is an important mechanism of laser-acoustics. The characteristics of photoacoustic signal that induced by thermoelastic mechanism was summarized and analyzed comprehensively. According to different induce conditions, theoretical models of the photoacoustic signal induced by a δ pulse and a long pulse laser are summarized respectively, and its nature characteristic in the time domain and frequency domain were analyzed. Through simulation, the theoretical curve of the sound directivity was drawn. These studies will provide a reference for the practical application of laser-acoustics technology.
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