Multi-shot laser-induced damage experiments and mechanisms of optical coatings

光学薄膜元件是激光器中的抗激光薄弱元件，其破坏问题一直是激光系统向高功率、大能量方向发展的瓶颈之一。从实际应用出发，光学薄膜元件总要经受多次激光的作用。相比于单脉冲激光作用，在重频多次激光作用下，很多光学材料（包括光学薄膜）的多脉冲破坏阈值要比单脉冲低，这种现象称之为光学元件多脉冲激光损伤的累积效应。其关系到元件的长期工作稳定性，同时也是影响整个激光系统使用寿命的决定性因素之一。本文针对光学薄膜在重频多次纳秒脉冲激光作用下的损伤实验和物理过程进行了深入研究，具体工作如下： 研究了典型薄膜元件（1ω、2ω和3ω三个波长的反射薄膜、增透薄膜等）在低重频多次激光作用下的破坏统计特性和阈值变化规律，对单脉冲及重频多脉冲激光引起薄膜材料的破坏源头、过程和机制进行了深入分析。发现纳秒激光单脉冲及多脉冲损伤均由亚微米级缺陷引起，通过缺陷密度统计模型半定量研究了不同波长下多脉冲激光诱导材料的微观变化过程。结合损伤特性分析，发现基频HfO2/SiO2高反膜、减反膜和二倍频HfO2/SiO2高反膜均表现出明显地激光诱导材料产生再生缺陷的累积效应，但受薄膜与基底的耦合作用的影响，基频减反膜的初始损伤形貌等宏观特性并不受脉冲数目的影响。三倍频多脉冲激光辐照下，沉积在有较高抗激光损伤能力基底上的SiO2单层膜更易表现出累积效应，且无论是SiO2单层膜还是Al2O3/SiO2高反膜，当其单脉冲阈值相对较低即薄膜中含有对三倍频激光非常敏感的固有缺陷时，在100%几率损伤阈值附近由多脉冲激光诱导产生的再生缺陷的作用将更加明显。同时，基底对单层膜、减反膜的影响和高反膜的不均匀性都会对薄膜本身累积效应的观测产生一定影响。 通过数学统计模型和Max-of-N模型进一步证实了实验观察到的累积效应并不是简单由数学统计效应或者激光的波动引起的，主要是由多脉冲激光辐照诱导的再生缺陷导致的，但是在损伤阈值随脉冲数目的下降幅度较小的情况下，激光波动的影响需要较精确的定量分析。在现有的薄膜与激光相互作用的理论基础上，提出了适用于不同波长的纳秒低重复频率多脉冲激光作用下光学薄膜的破坏机制和理论模型，该模型对多脉冲下光学薄膜材料微观变化的累积损伤过程给出了较合理的解释。One of the weak points in laser application is the optical coatings, and its laser-induced damage has been a limiting factor in the development of high-power laser systems. For practical use, the optical coatings are always subjected to multiple pulse irradiations. In many materials (including optical coatings), the damage threshold subjected to multi-pulse exposure is lower than that generated under single-pulse illumination, which is referred to as fatigue effect. It affects the long-term working stability of the optics and even the lifetime of the laser systems. In this paper, the experiments and physical process under multiple-pulse irradiations of laser-material interaction were studied. The detail of our work is listed as followings: The statistical damage characteristics of the typical optical coatings (such as high-reflective and anti-reflective coatings of 1ω, 2ω and 3ω) were investigated to verify the damage initiators, process and mechanism under single- and multi-shot irradiations respectively. It is clarified that the damage sites are all induced by submicron absorbing defects for nanosecond laser irradiation no matter what the pulse number is. By using a model assuming Gaussian distribution of defect threshold, the information on the evolution of defects with shot numbers, namely the cumulative laser-induced material modifications process is studied semi-quantitatively. Combining the damage characteristic, 1ω and 2ω HfO2/SiO2 high-reflective, anti-reflective coatings and 2ω HfO2/SiO2 high-reflective coatings all exhibits apparent cumulative material modifications. Whereas, affected by the coupling effect of coatings and substrate, the 1ω anti-reflective coatings does not show accumulation of macroscopic characteristic such as damage morphology. For 3ω laser irradiations, the SiO2 coatings deposited on the substrate of higher laser resistance will manifest more evident fatigue effect. When the LIDT of the SiO2 or Al2O3/SiO2 high-reflective coatings is relatively low namely there are very sensitive native defects to the 355 nm laser wavelength, the effects of the reproductive defects induced by multi-pulse laser irradiations will be more distinct around the 100% probability damage threshold. Besides, the effects of the substrate to single layer or anti-reflective coatings and the nonuniformity of high-reflective coatings prevent the observation of fatigue effect to a certain extent. A statistical model and the Max-of-N model both further confirm the observed fatigue effect is not simply caused by statistical behavior or laser fluctuation, but mainly cumulative laser-induced material modifications. However, accurate quantitative analysis of laser fluctuation is needed when the LIDT decreases less. On the basis of the existing laser-induced damage mechanism of optical coatings, a theoretic model of low repetition frequency nanosecond laser-induced damage which can apply to different wavelengths is built to give a more reasonable explication of the cumulative laser-induced material modifications process of the optical coatings