Optical Measurement of a Laser Induced Micro Shock Wave on a Metal Surface

Abstract

Laser induced streaming is one of the interesting phenomena. When we provide a highly concentrated energy in a small region with laser beam, an induced streaming, such as micro shock wave or laser induced thermal acoustics is observed in the region. In the present paper, a micro shock wave in air which was induced by laser irradiation was observed by the optical techniques. The shock wave was generated on the metal surface due to the laser ablation. In the present experiment, we focused on a relatively low irradiation intensity region. As the laser power decreased, the plasma plume was not observed clearly on the metal target, but a strong micro shock was still induced. The trajectory of shock front was recorded with the shadowgraphic measurement and the Mach number was calculated experimentally. Alternatively, the induced density behind the shock wave was measured by speckle photographic technique. The shock Mach number and density distribution was analyzed and discussed with self-similar theory.