플라즈마 거울을 이용한 페타와트 레이저 펄스 대조비 향상 연구

Abstract

The contrast ratio by preceding pulse and Amplified Spontaneous Emission (ASE) has to be taken account to develope high-power laser with chirped-pulse amplification (CPA) technique. Preceding pulse and ASE are amplified together with main pulse and arrive on the target before main pulse, which makes unintentional effect on the laser-plasma interaction. Therefore, high contrast ratio between the main pulse and preceding is required to measure accurate laser plasma dynamics. In order to obtain a high contrast ratio, various techniques such as cross-polarized wave (XPW) generation, double CPA, saturable absorber and plasma mirror can be applied. XPW, double CPA and saturable absorber can be used for the laser with few mJ scale energies whereas plasma mirror technique can be applied to the laser pulses with ten of Joule energy. In this study, increase in contrast ratio up to is investigated by using saturable absorber which suppresses preceding pulse at low energy stage, and the contrast ratio was enhanced further by plasma mirrors after amplifying laser pulse to 36 J. The principle of achieving high contrast ratio using plasma mirror is as below. In order to enhance the contrast ratio by plasma mirrors, the maximum energy fluence is set to be 70 J/cm and 127 J/cm for single plasma mirror and double plasma mirror, respectively, using off-axis parabolic mirror (OAP). Plasma mirrors are anti-reflection (AR) coated with the center wavelength of 800 nm, and focused laser beam generates plasma on the mirror surface. The preceding pulse and ASE are not reflected because they do not have enough energy to generate plasma, and the main pulse is only reflected by plasma, which results in the contrast ratio increment. In conclusion, single plasma mirror and double plasma mirror improve the contrast ratio up to 10 and 10, respectively. We also measured and presented the correlation between reflectivity of plasma mirror and fluence of incident laser pulse. At the end of the study, modifications of the laser spectrum and beam profile by the plasma mirror are evaluated.