Electric field distribution, in the wavelength range 1053 nm and 0◦ high reflection coatings, with different truncated conical pits has been estimated by using the finite difference time domain method (FDTD). Results of simulations indicate that the smaller the angle between the pit’s edge and the normal line, the higher the damage threshold of the mitigation pit. In the experimental process, the dimension of this angle mainly depends on two factors, i.e. the influencing area of the focal spot and the depth of mitigation pits. Because the ratio between them is the angle’s tangent, decreasing the influencing area of the focal spot and increasing the depth of the machined area could yield a mitigation pit with a smaller angle. By optimizing the focal spot size, pulse energy, step size and the number of machining passes of femtosecond laser micromachining, a pit with an angle of 25◦ and a depth of 14 µm is obtained. The typical damage threshold of the mitigation pit is about 21 J/cm2, which is 2.3 times greater than the fluence-limited defect. Moreover, the laser damage testing results of 50 mitigation pits show that the mitigation process has a good repeatability. The correlation between the cone angle and the damage threshold is also examined, the simulations are in agreement with the experimental results. The ratio of the maximum intensification between 45◦ and 25◦ cone angles is∼2.5 and that of the damage threshold between the two angles is ∼0.5. At the same time, the relationship between the micromachining pulse width and the damage threshold is also estimated: if other process parameters are kept constant, a longer pulse length tends to produce lower laser-resistant mitigation pits. Compared to the result of 260 fs laser pulse, the truncated conical pit created by 6 ps laser pulse has a smaller depth, which implies that more thermal effect occurs during the miromachining process. However, cracks are not found around the pit. Thus, thermal damage is not the major reason for the decrease of damage threshold. Meanwhile, smaller depth also indicates that the pit has a large cone angle. According to the result of former FDTD simulation, the decrease of damage threshold is mainly caused by electric field enhancement in a pit with a large cone angle.%针对波长1053 nm,0◦高反介质膜元件,采用有限时域差分法,模拟分析了损伤修复点边缘与法线的夹角对膜层内电场强度分布的影响,该角度越小,修复点的损伤阈值越高.通过优化飞秒激光微加工过程中的焦斑尺寸、脉冲能量、扫描步长和扫描次数等参数,获得了夹角为25◦、深度为14µm的修复点.该修复点典型的损伤阈值为21 J/cm2,是修复前的2.3倍,50个修复点的测试结果表明该修复参数具有非常好的可重复性.修复点的测试结果还验证了修复点边缘与法线的夹角大小与其损伤阈值的关系,45◦的电场强度最大值约为25◦的2.5倍,而45◦的损伤阈值约为25◦的1/2,模拟和实验结果一致性较好.同时,实验验证了微加工的激光脉宽对修复点损伤阈值的影响,在只改变脉宽的情况下,脉宽越长,损伤阈值越低.
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