等离子体镜是一种有效提升高功率超短脉冲激光信噪比的方法,但在一部分实验中使用等离子体镜后焦斑出现了退化现象。为了定量研究等离子体镜的焦斑退化问题,提出基于等离子体膨胀和衍射传输的时空聚焦多步传输算法,通过光束质量评价函数定量分析等离子体膨胀时间、波前误差幅值和空间频率对焦斑退化的影响。仿真结果表明,等离子体镜引起的远场焦斑退化主要是由等离子体膨胀时间和波前误差引起的等离子体膨胀不均匀所致,其中,等离子体膨胀时间是主导因素。并且波前误差的幅度越大、空间频率越低,对聚焦能力的影响相对越大。从高信噪比、高功率激光系统输出能力的角度考虑,对于使用等离子体镜的高功率超短脉冲激光系统提出时空上的光束质量要求,以避免远场焦斑的退化问题。

Plasma mirrors can be effectively used to improve the signal-to-noise ratio (SNR) of high-power ultrashort laser; however, some experiments conducted using plasma mirrors denote that plasma mirrors may cause focal spot deterioration. Herein, we establish a spatiotemporal focusing multistep propagation algorithm based on plasma expansion and diffraction propagation to quantitatively investigate the plasma-mirror-induced focal spot deterioration. Further, the influences of the plasma expansion time as well as the amplitude and spatial frequency of wavefront error on focal spot deterioration are quantitatively analyzed using a beam quality evaluation function. The simulation results reveal that the plasma-mirror-induced far-field focal spot deterioration can be mainly attributed to the non-uniform plasma expansion based on the plasma expansion time and wavefront error, with the plasma expansion time observed to be the dominant factor. Additionally, higher-amplitude and lower-spatial-frequency wavefront errors have a relatively greater influence on the focusing ability. From the perspective of high SNR ultraintense output capability, the spatiotemporal quality requirement of the pulse is introduced to avoid far-field focal spot deterioration when a plasma mirror is used in a high-power ultrashort laser system.