【作者】 高慧；

【导师】 刘伟伟；

【作者基本信息】 南开大学 ， 光学工程， 2013， 博士

【摘要】 高功率超快激光脉冲在光学介质中传输引起的成丝现象是当前科学研究领域的前沿问题。在成丝过程中,超快激光脉冲可以保持高强度长距离(长至千米量级)传输。一般认为,激光成丝现象的物理机制主要是光学克尔效应引起的自聚焦与等离子体散焦效应间的动态平衡。激光成丝过程包含丰富的非线性过程,如自聚焦、光致电离、强度钳制、自相位调制、自陡峭与时空聚焦等,它在远程遥感、激光引雷、脉冲压缩等领域具有光明的应用价值。当入射激光脉冲峰值功率远大于自聚焦闽值功率时,由于出射激光脉冲本身强度分布的不均匀性或传输介质的折射率扰动,实验中通常会观察到分布无序不均匀的多丝现象。多丝间会随间距、交叉角度、相位的不同而发生相互排斥、吸引、融合与能量的交换等互作用过程,最终导致多丝数量与空间分布不规律性。在实际应用中,如白光阵列、微波通道、精密微加工中,则需要多丝空间分布规律,因此近年来对超快激光多丝控制已成为相关领域的研究热点。面向这一学科发展的前沿方向,本论文重点研究了光丝阵列的产生机理。本文首先研究了利用轴锥镜聚焦超快激光脉冲在甲醇溶液中产生光丝阵列的动力学过程。实验结果表明,光丝阵列中光丝分布于轴锥镜聚焦产生的贝塞尔光束的中心零级与次级圆环上。理论研究结果表明轴锥镜聚焦条件下,激光强度空间分布的非柱对称性是光丝阵列产生的主要原因。本文又以半圆形相位板、四分之一圆形相位板与八分之一圆形相位板为例,研究了利用相邻单元相位差为π的相位板产生光丝阵列的机理。研究结果揭示光丝阵列的空间分布图案由相位板的几何结构决定,而光丝间间距、长度等参数则可通过使用不同焦距的透镜来调控。研究结果进一步证明,如果利用轴锥镜代替透镜聚焦光束后产生的光丝阵列其间距几乎不随传输距离发生变化,且光丝长度被延长,研究成果为产生具有光子晶体结构和特性的光丝阵列提供了新的技术途径。本文最后研究了无电离超快激光多光通道自引导传输的机理。研究结果显示激光成丝结束后,光束被分成多个显著的毫米量级的光斑,光斑以很小的发散角进行传输,并被低强度的厘米量级的背景能量池所环绕,其传输的发散角甚至小于一个拥有相同直径(FWHM)与功率大小的光斑非线性传输的发散角。理论揭示背景能量池仍然是维系这一非线性传输过程的主要物理机制。更多还原

【Abstract】 Ultrafast laser filamentation occurs when high power femtosecond laser pulses propagate in optical medium. During fialmentation, ultrafast laser pulses can propagate long distances without significantly losing peak intensity. The major physical mechanism of filamentation is a dynamic balance between the optical Kerr effect induced self-focusing and the de-focusing effect caused by either plasma diffraction or high-order-Kerr-effect. Fruitful nonlinear processes are involved during filamentation, inculding self-focusing, photonization, intensity clamping, self-phase modulation, self-steepening, and space-time focusing, etc. Due to the bright prospects in the wide range of applications, such as remote sensing, lightning control and pulse compression, ultrafast laser filamenatation has attracted considerable interest.When the laser power is higher than the critical power for self-focusing, multi-filamentation can be frequently observed in practice due to the perturbation in the intensity distribution of the initial beam pattern or the refractive index perturbation of the optical media. Depending on the phase differences, crossing angles or distances among them, multiple filaments will interact with each other, manifesting as repelling, attraction, fusion or energy exchange etc. As a consequence, multiple filaments are normally distributed disorderly in space. However, in some specific applications, such as white light array, filament assisted microwave guiding and massive micro-fabrication using filament arrays, spatial regularization of multiple filaments are demanded. Therefore control of multiple filamentation has become a hot research in related field recently. In this dissertation, we focus on the filament array generation mechanism.Firstly, this dissertation studied the filament array generation dynamics by focusing ultrafast laser pulses with axicon in methanol. The experimental results demonstrate multiple filaments are located on the central spot and ring structures of the quasi-Bessel beam created by the axicon. The outcome of simulation suggests the cylindrical symmetry breaking in the initial beam profile is the main reason for the filament array generation when focusing ultrafast laser pulses with the axicon.Secondly, we study the filament arrays generation mechanism in air by using three kinds of step phase plates with π phase lag, namely, the semicircular phase plate, the quarter-circle phase plate, and eight-octant phase plate. Experimental results and simulations show that the spatial arrangement of the filament array is determined by the geometrical shapes of the phase plates. The separation distances and the length of the filmanet array can be controlled by different focal lenses. The study results further indicate that by using an axicon, filament array in the form of ring shape could be realized. The separation distances between filaments are almost independent of the propagation distance, while the lengths of the filaments could be significantly elongated at the same time. Our research has provided a new technical approach to produce a filament array potentially possessing photonic crystal structure and characteristics.At the end, we study the self-guided propagation mechanism of multiple light channels without ionization at the post-filamentation stage. The experimental results show that after the filament was ended, the laser beam was divided into multiple distinguished millimeter-scale spots with larger low intensity energy background surrounded. These spots propagated with low divergence which is even significantly lower than that given by a nonlinear propagation of a laser beam with similar diameter (FWHM) and power. The corresponding numerical simulation reveals that the low intensity energy background is the main mechanism to support this nonlinear propagation process.更多还原