【作者】 杨芳芳；

【导师】 薛亦渝； 夏志林；

【作者基本信息】 武汉理工大学 ， 材料加工工程， 2008， 硕士

【摘要】 光学薄膜几乎是所有光学系统中不可缺少的基本元件,并且也是激光系统中最薄弱的环节之一。前人研究发现,在长脉冲激光作用下,激光薄膜中的杂质吸收是导致薄膜的激光损伤阈值下降的主要因素之一。在杂质诱导激光损伤机制和过程的分析中,薄膜中温度分布的计算非常重要,温度分布计算的合理性影响到后续力学过程分析的可信度。因此,研究激光辐照下,薄膜的杂质吸收问题具有重要的理论价值和实际意义。现有理论在计算杂质以及其附近薄膜基体的温度分布时,忽略了杂质吸收的非线性效应,本文在这方面做了改进。本文根据经典热传导基本方程,建立了激光辐照下,杂质吸收导致周围薄膜和基底中瞬态温度场变化的物理模型,模型假设介质薄膜和基底为各向同性且热力参数不随温度变化,而杂质的热力学参数随温度变化。研究了网格大小和时间步长参数与有限元求解的稳定性关系,建立了优化的有限元模型。在理论分析基础上,模拟了各种形状、取向、尺寸、深度的体内杂质和体表杂质吸收激光能量导致的周围介质薄膜的瞬态温度场。数值模拟结果表明,薄膜在激光单脉冲作用时间内,最高温升不会出现在激光功率密度达到峰值的时刻,而是稍微滞后;对于体内杂质,任何取向的球状、柱状和楔形杂质吸收都会造成薄膜激光损伤阈值的降低,且大而浅的杂质吸收造成的周围薄膜温升较大,从而导致薄膜的激光损伤阈值更低;体内球状和轴向平行于膜厚方向的柱状杂质所导致的周围薄膜的最高温升同其尺寸分别存在一个近似的二次函数关系;对于体表杂质,存在一个尺寸,当杂质处于此尺寸范围之内时会造成薄膜的激光损伤,且此范围可以通过精确的计算机仿真模拟来确定;本文的仿真模拟结果表明,在波长1064nm,脉宽12ns,功率密度2.22×10~6W/cm~2的YAG激光器辐照下,体表球状杂质造成薄膜损伤的半径范围约为50～120nm之间。本文研究的结果可为杂质吸收导致激光薄膜热学损伤的理论研究和数值模拟提供参考,为后续力学过程分析的可信性提供保证,同时也对实际光学薄膜的生产中采取有效措施控制光学薄膜中杂质位置和尺寸有一定的指导意义。更多还原

【Abstract】 Optical coatings are almost the indispensable components of all the optical systems and are also one of the weakest links in the laser system.Previous studies have found that for long pulse laser inclusions absorption in the optical coatings is the main reason for the decline of laser induced damage threshold.In the analysis of the mechanism and process of damage induced by inclusions,it is very important to evaluate the temperature distribution.The rationality of temperature distribution will affect the following mechanics analysis.So the study of inclusions absorption has an important theoretical value and practical significance.The existing models have ignored the nonlinear absorption effect of inclusions.In this paper, we have made some improvements in this aspect.In accordance with classic thermal conduction equation,transient temperature field in coatings under laser radiation are analyzed in this paper.It is provided that dielectric film and substrate are isotropic,and the thermal physical properties of inclusion are variable,while the coating’s are not.An optimized finite element is developed to simulate transient temperature filed in coating/substrate systems based on a well understanding of the influences of element size and integration time step on the solution stability.Based on this theoretical model,transient distributions of temperature field around kinds of inclusions with different shapes,sizes and depths are simulated by using the finite element method(FEM).Numerical results indicate:In the time of single impulse irradiation,the highest temperature rise doesn’t synchronized with the peak of laser power density,but delayed more or less.For the body inclusion,every orientation of globosity,cylindrical and sphenoid inclusions will cause a decline of damage threshold.But the absorptions of inclusions with larger sizes and lighter positions will cause a higher temperature rise and result in a lower damage threshold.There is a relationship similar to a quadratic function between the highest temperature rise and the sizes of globosity inclusions and cylindrical inclusions whose axis is parallel to the direction of the coatings’ thickness.For the surface inclusions,there is a size range,in which the inclusions will cause a laser damage and the range can be calculated by sophisticated simulation.The result in this paper shows that under the irradiation of YAG laser generator with the the wavelength of 1064nm,pulse-width of 12ns,power density of 2.22×10~6 W/cm~2,the globosity inclusions in the radius of about 50～120nm will cuase a damage of coatings.These conclusions may provide reference for the theory research and numerical simulation of optical coatings’ thermal damage induced by the inclusions absorption under laser radiation.And they can guarantee the credibility of the following mechanical analysis.At the same time,they can also be a guide to take effective measures to control the inclusions in a certain position and size.更多还原