Focusing property for ultra-short pulse laser reflected by off-axis parabolic mirrors

In order to study the dependence of the focus on the quality of the femtosecond laser beam,the tighten focusing of the femtosecond laser beam reflected by a 90° off-axis parabolic mirror was studied experimentally and theoretically trough the classical geometrical optical model.The obtained theoretical and experimental results indicate that the peak power density of the laser spot will reduce by half when the declination reaches 3mrad.Simultaneously the optical path difference resulted from incident angle causes time broadening of the spot on the target surface and impairs the focal spot intensity to some degrees.A 3TW Ti∶ sapphire laser system was adopted in the present work.The best focal spot obtained in the present work with an f/1 off-axis parabolic mirror is 5.6μm×5.4μm and the peak power density is 3.83×1017W/cm2 and 1.23×1019W/cm2 for 3mJ and 96mJ ultra-short pulse laser separately.The study of the tighten focusing for ultra-short laser pulse reflected by 90° off-axis parabolic mirror.This research provides important information for the study on the interactions between the femtosecond laser pulses and solid,gas,clusters and so on.