Study Of The Laser Induced Shockwave Characteristic Under Water

This paper discusses photoacoustic effect, analyzing and comparing t he four mechanis ms of laser acoustic excitat ion in liq uid: ther ma l expans ion, vaporizat ion, optica l breakdown, radiation pressure. This art ic le a ims at th e ther ma l expans ion ma inly w hich is t he ma in mechanis m o f t he low- power laser exc itat ion. The features of laser- generated shock wave in water is derived fro m the theory in the art icle, and establis hes ideal model of the photoacoustic convers ion of underwat er ther mal expa ns ion exc ited by pulsed laser beam. We research and obtain the characterist ics of the sound fie ld excited in water, inc lud ing t he time- domain and freque ncy- domain characterist ics, as well as the imp ulse response,the sound pressure distr ibut io n, the direct iona l character ist ic, the ha lf- power width. We compare the theoretical datawith the experimental data to ensure the theoretical model.When laser power gets stronger, t he vaporizat io n is the ma in mecha nis m, which co nta ins two s it uat ions : s ingle bubble a nd bubbles. First ly, we simulate the energy t hresho ld for vaporizat ion and calc ulate the size of bubble as well as discuss ing how t he laser focus depth a nd laser wa vele ngth influence on the sound fie ld. Then we compare the sound character ist ics between the two situations.When laser power gets even higher, the water molecule ionizes. And optical breakdown excites the plas ma and shockwave in water. We calculate energy t hresho ld for breakdown which is far more the n vapor izat io n. Then we calculate the lengt h of plas ma and research the variatio n trend of sound in water.The t hresho ld pla ys the bond to for m four mecha nis ms into a who le, which is the basis to discuss how the laser characteristics influence on sound.