基于全时域等效吸收强度演化的纳秒脉冲串激光作用机理研究

In order to attain the quick and accurate capture of distant, ultrasonic and hypersonic flying target, the pulse train laser in lidar is urgently needed to develop to higher single pulse energy and smaller pulse spacing. Nevertheless, the laser induced damage of transmission components in the system has become the limitations. Even the absorptive defects in nanoscale and inner cracks hiding in submicron to dozens of micron area beneath the top surface are likely to become the damage inducements. Unlike the single pulse with low pulse repetition frequency or continuous laser, the interaction mechanism of pulse train laser and the influence of different types of defects on damage properties are unknown. Thus, this project focuses on the damage events of transmission components at 1064 nm wavelength. After obtaining the initial and final states of absorption characteristics of defects by photo-thermal absorption method, the pump-probe technique is used to capture the equivalent transient absorption and relaxation process by the same defect after the first pulse irradiation, and the multiplier characteristics of the secondary pulse irradiation to establish the full-time domain equivalent absorption intensity evolution. Then, the relationship among the different initial defect characteristics, double-pulse delay time and laser-induced damage threshold are explored. The similarities and differences of the full-time domain equivalent absorption intensity graph and damage behaviors between the absorptive and structural defects are investigated. In addition, the project is trying to be technique support for breakthrough on research and development of the transmission elements used in pulse train laser by clarifying the defect-induced damage mechanism under double-pulse laser irradiation, developing a limiting factor discrimination and criterion based on the initial photo-thermal absorption features.

为实现远距离、高超音速飞行目标的快速和准确识别，迫切要求激光雷达中的脉冲串激光不断向更高单脉冲能量和更小脉冲间距方向发展。而系统中透射元件的激光损伤问题已成为限制性因素，即使纳米尺度吸收型缺陷和内部裂纹都可能成为损伤诱因。不同于低重频单脉冲和连续激光，脉冲串激光的作用机理、不同类型缺陷对损伤性能的影响机制尚不明确。本项目以基频透射元件为研究对象，利用光热吸收方法研究缺陷的初始和终态光热吸收特性，通过泵浦探测技术获得同一缺陷在一次脉冲辐照后的等效瞬态吸收和弛豫过程、二次脉冲辐照的倍增特性，建立全时域等效吸收强度演化规律；探索不同缺陷初始特征、双脉冲延迟时间和损伤阈值间的对应关系；明确不同类型缺陷全时域等效吸收强度图谱和损伤行为的共性和差异性，阐明双脉冲激光作用下缺陷诱导损伤的作用机理，发展基于初始光热吸收特征的损伤诱因甄别和判定准则，为我国在脉冲串激光用透射元件的研制上实现突破提供技术支撑。

不同激光频率对激光诱导损伤样品的结果有着重要的影响，随着激光频率的增加，激光损伤的累积效应、前次脉冲对后续脉冲的影响、产生的等离子体特性均有所不同。随着高重频激光应用范围的增加，纳秒脉冲激光诱导光学元件，从低重频、中重频、高重频再到准连续的频率范围内，对同一样品的损伤物理机制的研究仍比较少。研究工作通过可控频率的双脉冲激光系统，对不同类型样品的脉冲串激光作用下损伤机理进行了相关物理机制和弛豫过程研究。研究工作的开展基本上按照项目的研究计划和实施方案执行，完成了项目的研究内容和研究目标。主要取得了以下四方面的研究成果：1.首先对可调间隔的双脉冲系统的搭建、难点及注意点进行了阐述，并给出了两种通过图像灰度值来进行损伤弛豫过程中强度变化的计算方式，为等效瞬态吸收强度的表征做支撑。2.采用了三种不同的光学元件，对两种不同形式的激光诱导产生的等效瞬态吸收强度随激光工作频率的变化进行了实验研究,最终得出激光与这三类样品的作用过程和持续时间。3.分别对单脉冲和双脉冲诱导下激光能量密度与等效吸收强度关系进行了阐述，并与光热吸收结果进行对比。4.通过模拟，对体材料损伤的弛豫过程进行了模拟，将模拟结果与体材料损伤的实验结果进行比较，给出了材料在激光诱导后发生的弛豫过程的形式及时间跨度，这对理解激光与物质相互作用的过程有着十分重要的意义。