光学元件的损伤是高功率激光技术发展的一个瓶颈,为此以连续CO2激光辐照K9玻璃为例,研究了连续激光辐照光学材料的热力损伤机理。通过积分变换方法,求解热传导和热弹性力学方程组,由此得到激光辐照引起的温度场和热应力场的瞬态分布。研究中发现在高斯光束作用下,热扩散长度的概念不再适用,因此通过曲线拟合方法,推导出最大热应力的位置与辐照时间的关系,并由此计算出材料的损伤阈值。由于K9玻璃的应力损伤阈值小于熔融损伤阈值,因此当激光作用引起的环向热应力大于材料的抗拉强度时,材料发生永久性损伤,损伤形态为拉伸解理。将理论模型的相关结论与实验结果相对比,两者吻合得很好。

Laser induced damage in optical components is a limiting factor to the improvement of high-power laser. Using K9 glass irradiated with the CO2 continuous-wave laser as an example, the thermal and mechanical damage mechanism of optical material induced by continuous-wave laser is investigated. The equations of heat conduction and thermoelastic dynamics are solved through integral-transform method. Transient distribution of temperature field and thermal stress field is described. It is found that the definition of the thermal diffusion length is not applicable when incident laser is Gaussian beam; therefore, the dependence of the site of the maximal thermal stress on irradiation time is derived by curve fitting method, then the analytical solution of the damage threshold intensity is calculated. Because the stress damage threshold intensity is smaller than the melting damage threshold intensity, permanent damage in optical material is produced when circumference stress exceeds the tensile strength. The damage morphology of K9 glass induced by laser is proved to be cleavage burst. The model exhibits good agreement with the experiment result.