高能拍瓦激光的高精度脉宽测量对离轴抛物面镜焦斑功率密度的诊断以及光栅损伤阈值的分析都具有重要意义。分析了光束指向性和近场分布的周期性调制这两方面的误差影响。结果表明, 当反射镜稳定性在5 μrad时,光束指向性的误差最大为0.03%。当近场调制周期增加时, 误差降低; 而调制度增加时, 误差增大。另外, 采用镜像结构能降低近场缺陷导致的测量误差。当调制深度为1.5、调制周期大于10时, 综合误差小于20%, 最小可降至10%。镜像结构的误差均小于15%, 最小可降至0。在拍瓦级激光脉宽测量实验中, 证实了近场调制对于脉宽测量的影响及改善效果。

Precise pulsewidth measurement of petawatt laser with high energy is important to diagnosis of the focal intensity of the off-axis parabolic mirror and analysis of the gratings′ damage threshold. Two sources of error are discussed in this article, namely, the beam pointing and the periodic modulation of the near field distribution. The results show that when the stability of mirrors is within 5 μrad, the leam pointing error is 0.03%. The error decreases when the modulation period of near field increases. The error increases when the modulation depth increases. Besides, adopting mirror structure can reduce the measurement error caused by the near-field defects. When the period modulation depth of the near field is 1.5 and the modulation period is greater than 10, the comprehensive error is less than 20% and the minimum can be reduced to 10%. Total error of mirror structure is overall below 15% and the minimum can be reduced to 0. In the petawatt laser pulse width measurement experiment, the effect and improvement of the modulation of the near field on pulse width measurement is demonstrated.