The application of fiber-coupling zig-zag beam deflection method on the investigation of plasma shock waves

Abstract

A novel method, namely, fiber-coupling zig-zag beam deflection method has been proposed to investigate the attenuation of plasma shock waves in air in this paper. The main innovation of this method is to use a zig-zag laser beam as the probing beam, instead of a straight beam in traditional beam deflection method. The zig-zag beam is formed by eight times successive reflections on a pair of parallel mirrors. Shock waves propagate through the space between two mirrors which contains nine zigzags in the horizontal plane. This space can be designated as the testing field. After the probing beam leaves the testing field, it is coupled into a single mode optical fiber which guides the beam into a photomultiplier to complete the process of photoelectrical conversion. Plasma shock waves are generated during laser ablation of Fe target in air. The laser used in our experiment is a Qswitched Nd: YAG laser operating at wavelength of 1064nm and pulse width (FWHM) of 7ns. The output of 160mJ/pulse of this laser is focused on the surface of the target which is far exceeds the ablation threshold of Fe. When a shock wave propagates in the testing field, it will meet the zig-zag probing beam nine times one after another. Correspondingly, nine deflection signals will be induced by the perturbations of the shock wave, which can be utilized to illuminate the propagating behavior of the shock wave. The whole attenuation process of the shock wave can be demonstrated intuitively only through one experimental curve with the nine deflection signals. From the curve, the average velocity of the shock wave can be calculated out with high and reliable precision. It is found that shock waves attenuate into acoustic waves within 10mm in air because of the inhalement of environmental media. The development of this novel optical technology provides a powerful tool for the detection of shock wave propagation and riches diagnostic methods of shock waves.