Abstract:During recent years, the filamentation of femtosecond laser in the atmosphere has contributed considerable interest to researchers. However, the actual atmosphere can result in different scattering medium, which are adverse to the application of filamentation in the atmosphere. In order to study the propagation of femtosecond laser in real scattering medium, the propagation of 800 nm femtosecond laser in ice cloud, water cloud, fog, aerosol and rainfall is simulated numerically. Combined with the theory of stratified medium model and Mie scattering theory, we constructed a scattering model with a changeable size distribution function in the nonlinear laser model. The results indicated that the different size distribution and phase state of particles have different influence on the propagation properties of the filaments. As the rainfall was dominated by large raindrops, the scattering on filament was the strongest, resulting in the lowest peak intensity and energy. In the case, the distribution of filament energy was extremely inhomogeneous, causing the shortest length of filament and generation of multi-filament. In the image of fluence distribution, a diffraction ring can be observed clearly in the rainfall but was blurred in other medium. The propagation properties of filaments in water cloud and fog were similar because of the same size distribution. However, due to the size of particle in fog was smaller than that in water cloud, the filaments had more higher energy and more concentrated distribution in fog. In addition, the scattering of ice particles was stronger than that of liquid droplets, so the energy of filament in ice cloud was lower than that in water cloud, resulting a reducing of the length and number of filaments in ice cloud. The size of aerosols was the smallest, which had the weakest influence on the filament. Accordingly, in the early of propagation, there had little perturbance on the filament and the beam was transmitting with a stable single filament, and results in the highest peak intensity and energy. With the propagation increasing, the accumulation of scattering attenuation produced the perturbation on filament at a position after the onset of filamentation. Keywords:femtosecond laser/ filamentation/ size distribution/ scattering medium
为了分析连续散射介质中的光丝辐射传输, 采用分层传输模式[13], 将激光传输路径划分为具有间距Δz的连续层(图1). 层与层之间为激光自由传输的部分, 飞秒激光与空气分子发生的电离反应、衍射以及自诱导聚焦等非线性光学效应都发生在该部分, 而粒子半径和数密度对光丝空间传输的散射影响则发生在屏上. 当激光光束打在粒子屏上时, 粒子群通过散射光束, 使光场信息重新分布. 随后经过自由传输部分影响传递到下一层, 下一层上的粒子群又会对光场产生新的扰动, 最终通过一系列的粒子散射屏来描述整个散射介质的散射特性. 已有的研究表明[20], 粒子表面等离子体的分布对大气中成丝过程的贡献基本可忽略不计, 因此文中模型并未考虑光丝与粒子的电离反应. 图 1 分层传输模式概念图 Δz为屏间距, L为散射屏的宽度 Figure1. Stratified-medium model. Δz is distance between screens, L is the width of the screen.
沿着光束传播方向, 粒子对激光场的散射扰动只考虑前向散射方向范围(θ < 10°), 而对后向散射方向的扰动不进行计算[21]. 同时, 大粒子的散射扰动强度强于小粒子对光场的散射扰动(图2(a)). 根据粒子的散射相函数分布(图2(b)), 利用相函数曲线的第一个波谷来选取前向散射角, 从而用这部分前向散射能量来计算对光场的扰动. 粒子尺度越大前向散射角度的取值范围越小(θ100 < θ15 < θ10 < θ5). 图 2 (a)粒子对激光光场的散射扰动图; (b)利用散射相函数获得不同粒径粒子(100, 15, 10, 5 μm)的前向散射角 Figure2. (a) Scattering on light field by particles; (b) Use the scattering phase function to obtain forward scattering angle of particles with different sizes.