基于飞秒激光微加工的介质膜损伤修复研究
文献类型：学位论文
| 作者 | 林圆圆 |
| 学位类别 | 硕士 |
| 答辩日期 | 2015 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 李学春 |
| 关键词 | 介质膜元件 损伤修复 飞秒激光微加工 损伤阈值 |
| 其他题名 | Study on damage mitigation in dielectric mirrors by using femtosecond laser micromachining |
| 中文摘要 | 大口径光学元件易损伤是制约高功率激光驱动装置提高传输能量的因素之一。本文主要围绕基于飞秒激光微加工的介质膜损伤修复技术研究展开的，目的就是探索出最佳的修复形貌以及相对应的修复工艺参数，提高光学元件的利用率。本文主要内容分为： 1、完成了基于有限时域差分方法(FDTD)的模拟计算。针对波长1053nm，0°高反介质膜元件，采用FDTD方法进行了膜层内电场强度最大值的模拟计算，着重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。损伤阈值测试结果验证了修复点边缘和法线夹角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。 2、首次提出了修复形貌边缘倾斜角度的控制方法。FDTD模拟和损伤测试结果都表明，微加工区域边缘与法线的夹角是决定修复点损伤阈值的关键因素。对于0°高反膜，模拟结果表明，修复点边缘与法线的夹角越小越好。影响修复点边缘角度有两大因素，即焦斑的作用范围和修复点切除深度，两者的比值即边缘与法线夹角的正切值。可以通过减小焦斑的作用范围，同时增加修复点的切除深度来减小夹角值。通过优化飞秒激光微加工过程中的参数，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21J/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。 3、完成了修复激光脉宽对修复点损伤阈值的影响的研究。在不改变扫描步长和次数的情况下，脉宽增加，降低了焦斑的峰值功率密度，导致每次加工深度下降，并且底部的平整度变差。坑底的微结构会对光产生调制，但由于底部的反射率较低，相比于边缘引入的硬边衍射，该部分贡献量较小。脉宽6ps激光的加工深度下降后，边缘与法线的夹角变大。根据FDTD的模拟结果，边缘角度变大致使膜层局部电场增强，是损伤阈值下降的主要原因。260fs和6ps两种激光脉冲条件下累计的总能量相同，但是脉宽变宽后，加工的总深度下降了，推断有部分激光能量转化为热效应。在扫描电镜下观测时，6ps的加工结果也没有发现微裂纹，所以该加工条件下的热效应还不是影响阈值的主要因素。 4、完成了基于象散法的对焦模拟计算。从软件模拟的角度说明了采用不同焦距的柱透镜组合所产生的灵敏度，在f1=f2=d=D的情况下，当柱透镜之间的距离d与柱透镜焦距f同时增大时，所得曲线斜率增大，即可以提高系统的灵敏度；当d相等时，f越大，系统的灵敏度越高；f相同时，d越大，系统的灵敏度越高。为选择能够满足实验需求灵敏度的光学对焦系统提供了理论依据。 |
| 英文摘要 | The easy damage for the large-aperture optical components is one of the most important factors to limit the high power laser system development.The research is mainly focous on damage mitigation in dielectric mirrors by using femtosecond laser micromachining and get the surper mitigation morphology and the process parameters. So that utilization is improved for the optical elements. The main content of the article are summarized as follows: 1、Electric field distribution within the 1053nm, 0° high reflection coatings with different truncated conical pits has been estimated by using the finite difference time domain method (FDTD). The result is the smaller the angle between the pit’s edge and the normal line, the higher the damage threshold of the mitigation pit. The correlation between the cone angle and the damage threshold is also examined and there is agreement between the experiment results and simulations, the ratio of the maximum intensification between 45° and 25° cone angles is ~2.5 and the ratio of damage threshold between 25° and 45° cone angles is ~0.5. 2、A method of control the angle between the pit’s edge and the normal line is successfully developed. The results of simulation and experiment also show that: the angle between the pit’s edge and the normal line is the main facor for the damage threshold of the mitigation pit. The smaller the angle between the pit’s edge and the normal line, the higher the damage threshold of the mitigation pit for the 1053nm, 0° high reflection coatings. The affect scope of the focal spot and the depth of the pit are two important factors that decide the edge angle. Reducing the angle can be improved by reducing the affect scope of the spot size and depth of machining. By optimizing the focal spot size, pulse energy, step size and number of machining passes of femtosecond laser micromachining, a pit with the angle of 25° and the depth of 14μm is obtained. The typical damage threshold of the mitigation pit referred above is about 21J/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. 3、The relationship between the micromachining pulse width and the damage threshold is verified. The pulse width is the important factor for the mitigation pit. If keeping the scan step and scan times constant, the peak power density of the focal spot will be reduced when the pulse width increased. The depth of the every machining will drop and the flatness of the bottom will become worse. The light will be modulated by the micro-structure on the bottom, which produces a smaller contribution than the hard edge diffraction induced by the edge for the reflectivity of the bottom is lower. The angle between the pit’s edge and the normal line will become larger when the depth of machining drops. The results of the simulation show that: the angle between the pit’s edge and the normal line is the main facor to the damage threshold of the mitigation pit. The cumulative total energy are same of the 260fs and 6ps. Maybe some laser energy is converted to thermal effects for the depth of the machining is smaller of the 6ps. There is no micro-crack of the pit is found when observed under the scanning electron microscope, so the thermal effect is not the main factor under this conditon. 4、The simulation based on the astigmatism method is verified. Autofocusing is an important step for the micromachining process. The level of the sensitivity for the different cylindrical lens combination is estimated by simulation. Under the condition of the f1=f2=d=D, the sensitivity can be improved when the D is larger; Under the condition of the d keeps constant, the sensitivity is higher when the f is larger; Under the condition of the f keeps constant, the sensitivity is higher when the d is larger. According to the conclusion, the autofocusing systerm that satisfy the requirement could be selected. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/16925]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 林圆圆. 基于飞秒激光微加工的介质膜损伤修复研究[D]. 中国科学院上海光学精密机械研究所. 2015. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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