基于约化电子数密度增长速率方程，建立了熔石英导带电子数密度随脉冲持续时间变化的模型。利用电子数临界密度这一概念，得到了150 fs~10 ps脉宽下，熔石英激光损伤阈值范围。分析表明，5~10 ps，雪崩电离仍然起主要作用，而光致电离提供的初始电子使雪崩电离不再依赖材料原有的初始电子; 当脉宽减小到约为4 ps时，光致电离与雪崩电离作用相等; 之后，光致电离起主要作用。通过仿真出的损伤阈值拟合，得到了该脉宽区间下新的脉宽定律: 熔石英的损伤阈值正比于脉宽的0.38次方; 考虑温度对熔石英损伤阈值的影响，熔石英的损伤阈值正比于脉宽的0.34次方。

Based on the reduced growth rate equation of electron density, a theoretical model is established to describe the change of the conduction band electron density of fused silica with the laser pulse duration. Using the concept of the critical electron density, a scope of the laser-induced damage of fused silica is calculated with the pulse width from 150 fs to 10 ps. The analysis shows that the avalanche ionization still takes a leading role from 5 ps to 10 ps. Instead of electrons provided by the fused silica itself, the initial electrons produced by the photoionization make a contribution to the impact ionization. It reaches a balance between avalanche ionization and photoionization when the pulse width shrinks to 4 ps. After that, the photoionization plays a main role in the growth of the conduction band electron density. Through the fitting curve, a new pulse width law is obtained to satisfy the laser-induced damage threshold changing from 150 fs to 10 ps. The results demonstrate that the threshold is proportional to tp0.38(tp is the pulse width), and changes to tp0.34 when the temperature dependence is concerned.