基于电子密度演化模型，借助数值方法，研究了飞秒激光作用下光学薄膜内的电子密度演化过程，讨论了初始电子密度Ni和激光脉冲宽度τ对光学薄膜激光损伤阈值Fth的影响，分析了激光诱导薄膜损伤过程中MPI和AI的性质和作用.研究结果表明，对应于一定的脉宽，存在一个临界初始电子密度，当Ni低于这一临界密度时，Fth不受Ni影响；当Ni高于临界密度时，Fth随Ni增加而降低.临界初始电子密度随着脉宽的减小而增加。对于FS和BBS介质薄膜，Fth随脉宽的增加而升高。初始电子密度Ni对BBS中的MPI和AI基本没有影响；同样Ni对FS中的AI基本不产生影响，但当Ni>1011 cm－3时，FS中MPI电子密度随Ni增加而降低.在所研究的脉宽范围τ∈［0.01,5］ps，AI是FS介质激光诱导损伤的主要机制.而对于BBS，当脉宽τ∈［0.03,5］ps，AI是激光诱导损伤的主要机制；当脉宽τ∈［0.01,0.03］ps，MPI在激光诱导损伤中占主导地位.

Both the nature of avalanche ionization (AI) and the role of multi-photon ionization (MPI) in the studies of laser-induced damage have remained controversial up to now. According to the model proposed by Stuart et al., we study the role of MPI and AI in laser-induced damage in two dielectric films, fused silica (FS) and barium aluminum borosilicate (BBS), irradiated by 780-nm laser pulse with the pulse width range of 0.01-5 ps. The effects of MPI and initial electron density on seed electron generation are numerically analyzed. For FS, laser-induced damage is dominated by AI for the entire pulse width regime due to the wider band-gap. While for BBS, MPI becomes the leading power in damage for the pulse width \tau less than about 0.03 ps. MPI may result in a sharp rise of threshold fluence Fth on \tau, and AI may lead to a mild increase or even a constant value of Fth on \tau. MPI serves the production of seed electrons for AI when the electron density for AI is approached or exceeded before the end of MPI. This also means that the effect of initial electron can be neglected when MPI dominates the seed electron generation. The threshold fluence Fth decreases with the increasing initial electron density when the latter exceeds a certain critical value.