【作者】 饶欢乐；

【导师】 付绍军；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2014， 博士

【摘要】 在惯性约束核聚变(Inertial Confined Fusion ICF)系统中,光束采样光栅(Beam Sampling Grating BSG)是其终端光学系统中重要的一种光学元件。作用是从三倍频主光路中取出极小部分作为采样光束,对入射光的能量、功率、空间分布及波长畸变等进行诊断。采样效率均匀性和激光损伤阈值(Laser-induced damage threshold LIDT)是评价BSG性能的两个至关重要的参数。采样效率均匀性直接关系到对入射光能量响应的特点。均匀性越好,测量相对误差越小,采样准确性越好。激光损伤阈值代表其抗激光损伤的能力,决定了其在强激光环境中的使用寿命。BSG指标要求是采样效率均匀性RMS≤5%,激光损伤阈值达到14J/cm2。目前国内采用全息离子束刻蚀方法制作的BSG,由于技术条件限制,均匀性不能满足要求,同时光栅制作过程中引入的污染物也导致了BSG抗激光损伤能力的下降。为提高大尺寸BSG的效率均匀性和激光损伤阈值的特性,本论文的主要研究工作包括：1、将熔石英的抛光技术用于大尺寸BSG,初步解决了提高BSG采样效率均匀性的关键技术。通过抛光的方法改变BSG局部衍射效率较高区域的光栅槽形结构,降低其衍射效率,使之达到与周围区域的衍射效率相近,最终使BSG衍射效率趋于均匀。研究了大尺寸BSG抛光的关键技术,光栅各结构参数及抛光工艺参数对光栅栅线高度去除速率的影响,掌握了抛光过程中BSG的槽形演化规律,获得了优化的大尺寸BSG抛光工艺参数。此外还将BSG效率模拟结果与实验结果进行比较分析,掌握了大尺寸BSG抛光过程中槽形轮廓演化情况；建立了大尺寸BSG的效率检测系统。利用建立的BSG抛光技术,可以将大尺寸BSG效率均匀性控制在RMS小于5%的水平,与美国NIF的水平相当。BSG抛光技术已经成为有效控制大尺寸BSG效率均匀性的关键工艺环节,显著降低了对全息光刻和离子束刻蚀过程工艺均匀性的要求。2、利用二次离子质谱技术,比较了离子束刻蚀、衍射效率均匀性修正及清洗等不同工艺条件下,碳、氟、金属及铈、镧等元素的浓度在BSG表层的深度分布变化,定量表征了BSG的关键工艺环节引入的化学污染及其清洗效果,获得了分别基于组合强酸和基于氢氟酸(HF)的两种BSG清洗方案。二次离子质谱实验结果显示,这两种清洗方案均可将BSG制备过程中引入的主要杂质污染分别降低到与初始熔石英基底相当或更低的水平,基于HF的清洗方案略优于基于组合强酸的清洗方案。基于稀释HF的清洗方案虽然提高了阈值,但在一定程度上降低了BSG衍射效率的均匀性。3、利用3ns脉宽,355nm波长激光对清洗前及不同清洗工艺下的BSG阈值进行了状态确认。初步BSG阈值实验结果显示,目前熔石英基底的损伤阈值为10-12J/cm2,采用基于组合强酸和基于HF两种清洗工艺后,BSG的损伤阈值分别为9J/cm2、12J/cm2；与熔石英基底的激光损伤阈值相比,BSG的激光损伤阈值低了10%-20%左右。说明使用基于氢氟酸清洗的效果非常的理想,消除了光栅制作和抛光工艺引入的污染物的影响,而基于组合强酸清洗方案未使BSG阈值提升到基底水平的原因待进一步分析。美国NIF对终端组件中熔石英基底和BSG的阈值要求分别是15J/cm2和14J/cm2(355nm,3ns).因此,下一步迫切需要提高熔石英基底和BSG光栅的激光损伤阈值。更多还原

【Abstract】 Beam sampling grating (BSG) is one important optical component of the final optical systems of the Inertial Confined Fusion (ICF) system. It is used to diagnose the energy, power, spatial distribution and wavelength distortion of the incident laser by diffracting and focusing a small fraction of the total laser energy to a calorimeter. The uniformity of diffraction efficiency and laser-induced damage threshold (LIDT) are two important parameters of BSG. The uniformity of diffraction efficiency is related directly to the response characteristics of incident laser energy. The analysis indicated that the better the uniformity of diffraction efficiency is, the less relative error of the measurement. And the LIDT represents its resistance to high power laser and also determine the lifetime of BSG. At present, the efficiency uniformity of BSG which was fabiricated by holographic ion beam etching (HIBE) can not meet the requirements due to the limit of technical conditions in China. Various surface contaminants introduced during the grating manufacture process lead laser-induced damages to grating. In order to improve the efficiency uniformity and LIDT of large-aperture BSG, the main works of this thesis are follows:1, Fused silica polishing technology was applied to large-aperture BSG, which become the key technology of improving the efficiency uniformity of BSG. After the HIBE of BSG, polishing process is utilized upon the local area of BSG, where exhibits higher efficiency. After polishing process, its groove profile can be modified and its efficiency can be reduced. The efficiency uniformity of whole-aperture BSG can be improved accordingly.The key polishing technology of large-apture BSG, including the influence of the grating structure and polishing process parameters on the polishing rate has been researched. The optimized polishing parameter for large-aperture BSG has been gotten.2、The BSG surface contaminations in the ion beam etching, polishing and cleaning processes have been analysed using the high-sensitivity secondary ion mass spectrometry. The results show two cleaning solution can remove the contaminants to the original level of initial fused silica substrate or ever lower. The cleaning process based on hydrofluoric acid (HF) cleaning solution is better than that based on multi-acid solution. It’s should be noted that the multi-acid cleaning method can preserve the efficiency uniformity of BSG.3、The LIDT of BSG with different cleaning processes has been measured at the wavelength of355nm and8ns. The results show that the LIDT of fused silica substrate is10-12J/cm2. And the LIDT of BSG cleaned based on the multi-acid cleaning process and the hydrofluoric acid (HF) cleaning solution is9J/cm2and12J/cm2, respectively. Comparing to the fused silica substrate, the LIDT of BSG has dropped by10%-20%. Therefore, the HF cleaning process is an effective way to improve the LIDT of BSG.Since the peak UV fluences required for the fused silica substrate and BSG at the National Ignition Facility (NIF) is15J/cm2and14J/cm2respectively, the next urgent step is to improving the LIDT of large-aperture BSG and its fused silica substrate.更多还原