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干燥环境球形颗粒与平面基底间的吸附力以范德华力为主。然而,由于颗粒材料或多或少都具有顺从性,在吸附力的作用下,近似为球形的颗粒会产生形变,称之为吸附形变。
颗粒的尺寸与材料不同时,吸附形变模型也就不同。目前被广泛使用、通用的形变理论模型分别为1971年由Johnson等人提出的JKR接触模型和1975年由Derjaguin等人提出的DMT接触模型[18]。对于光学元件表面的典型颗粒污染物,通常采用DMT模型进行解释。DMT模型给出了形变产生的接触半径rc与颗粒半径R之间的关系式:
$$ {r_c} = \sqrt[{\mathop {\mathop {}\limits^{} }\limits^{\mathop {}\limits^{\mathop {}\limits^3 } } } ]{{\left( {\dfrac{{{A_{132}}{R^2}}}{{8E ^* {H^2}}}} \right)}} $$ (1) 其中,系统Hamaker常数A132的计算方法由公式(2)给出;E*计算方法由公式(3)给出:
$$ A_{132}=A_{12}+A_{33}+A_{13}+A_{32}=\left(\sqrt{A_{11}}-\sqrt{A_{33}}\right)\left(\sqrt{A_{22}}-\sqrt{A_{33}}\right)$$ (2) $$ \frac{1}{E^*}=\frac{1-\sigma_1^2}{E_1}+\frac{1-\sigma_2^2}{E_2}$$ (3) 式中:Aii 为物质 i 的真空Hamaker常数; Aij 为物质 i 与物质 j 之间相互作用的Hamaker常数;E1、E2 分别为颗粒与基底的杨氏模量; σ1 、σ2 分别为颗粒与基底的泊松系数。
在计算范德华力时,形变量的受力也需要考虑在内。此时的范德华力Fv′可表示为:
$$ {{F}_{v}}{{'}}=\frac{{A}_{132}R}{6{H}^{2}}\left(1+\frac{{{r}_{c}}^{2}}{RH}\right) $$ (4) 根据公式(4),可对光学元件表面典型颗粒污染物所受的吸附力数值进行计算。
计算熔石英基底与二氧化硅颗粒间的范德华力时,从结构上来讲,熔石英本身就是浓度高于99%的非晶态二氧化硅,熔石英与二氧化硅颗粒两者力学性能类似,因此基底与颗粒的杨氏模量、泊松系数可近似视为相同值,其杨氏模量为70 GPa,泊松系数为0.17。熔石英基底的Hamaker常数为6.6×10−20 J,空气的Hamaker常数为1.94×10−20 J,微米与亚微米级颗粒与基底间的距离通常取为0.4 nm。根据上述公式即可对形变引起的接触半径与范德华力进行计算。经计算,微米与亚微米级污染颗粒受到的吸附力大小为10−10~10−7 N量级。
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对于光学元件表面典型SiO2颗粒污染物的去除形式为颗粒对激光强吸收,基底弱吸收。当颗粒温度升高时,瞬态热效应产生向外的热膨胀加速度即相当于颗粒受到热膨胀带来的应力,当应力大于颗粒吸附力时,颗粒达到激光清洗的脱附条件,可以实现污染颗粒的去除。
位于高斯光束的光斑中心的污染颗粒,微米级的颗粒尺寸远小于光斑尺寸,因此,可利用颗粒的平均温升来近似估算热膨胀应力,计算一个脉宽时间内的热膨胀应力时,由于脉宽很短,仅为纳秒级别,因此颗粒比热容等随温度变化的参量也可近似地取一个定值进行定性估算。其升高的温度
ΔT可近似表示为: $$ \Delta T=\frac{\eta {F}_{l}}{\rho c\mu } $$ (5) 式中:η为颗粒对激光的吸收率;Fl为激光能量密度;ρ为颗粒密度;c为颗粒比热容;μ为热穿透深度。一个脉宽时间内热穿透深度μ可表示为:
$$ \mu =\sqrt{4\alpha \tau } $$ (6) 式中:α为颗粒的热扩散率;τ为激光脉宽。由温升ΔT引起的热膨胀位移H′可表示为:
$$ {H}{{'}}=2\beta R\Delta T $$ (7) 式中:β为颗粒的热膨胀系数。一个脉宽内作用于颗粒的热应力F’为:
$$ {F}{{'}}=\frac{2\pi \rho {R}^{3}{H}{{'}}}{3{\tau }^{2}} $$ (8) 通过以上的方法可以对颗粒受到的热膨胀应力进行定性估算。激光清洗SiO2颗粒时所需要用到的参数如表1所示。
表 1 计算所需的SiO2颗粒的热物性参量
Table 1. Thermal and physical parameters of SiO2 particles used in calculation
Parameter Value ρ/g·cm–3 2.5 c/J·(kg·K)–1 730 α/cm2·s–1 0.06 β/K–1 0.5×10−6 颗粒对波长为355 nm的激光吸收率
$ \eta $ 约为60%,激光脉宽$ \tau $ 为10 ns,颗粒半径R取典型值1 μm。对于半径为1 μm的颗粒以0.1~0.8 J/cm2的能量密度进行干式激光清洗时,颗粒的平均温升约为103 K量级,颗粒受到的热应力约为10−9~10−7 N量级。当激光能量密度为0.042 J/cm2时,光学表面半径1 μm的SiO2颗粒受到的吸附力与热应力相等,可判定为理论起始清洗阈值。当激光能量密度为2.9 J/cm2时基底温度达到熔点,即造成激光损伤,判定为理论基底损伤阈值。综上所述,激光清洗光学元件表面微米级SiO2污染颗粒的理论工艺区间为激光能量密度0.042~2.9 J/cm2。 -
针对高能量激光装置中受污染大口径真空隔离片的边缘污染区域与中心通光区域分别进行单发干式激光清洗。经过图像分析软件对清洗效果进行分析,获得不同激光能量密度下作用真空隔离片后残留颗粒数量曲线,如图5(a)和(b)所示。统计结果表明:污染区域与通光区域的清洗起始阈值均为0.57 J/cm2,均在激光能量密度为2.28 J/cm2时清洗效果最好,对半径为5 μm以上的污染颗粒去除效果最明显。当能量密度达到2.85 J/cm2时,颗粒统计数量增长并不显著,以上结论与镀膜熔石英样品的实验结论基本一致。在同样的激光工艺参数下,中心通光区域的清洗效果明显低于边缘污染区域,这是由于中心通光区域在激光装置实际运行时受到一定激光清洗的效果,残留的污染颗粒与基底的吸附效应更强,更难以去除,因此清洗效果较弱。
图 5 不同激光能量密度下作用真空隔离片后残留颗粒数量曲线。(a)边缘污染区域;(b)中心通光区域
Figure 5. Number of residual particles on the contaminated vacuum separator after laser cleaning process under different laser energy density. (a) Border zone of vacuum separator; (b) Central zone of vacuum separator
由图5 (a)和(b)可知,当以激光清洗后颗粒的总残留数量为判据时,激光能量在2.28 J/cm2时颗粒去除效果最佳。但此时基底已产生了激光损伤的痕迹,而1.71 J/cm2时基底表面状态良好,无膜层损伤现象,这表明对于边缘污染区域而言,单发激光清洗的最佳能量密度为1.71 J/cm2。图6为真空隔离片非通光区域的激光清洗前后暗场成像图。使用类似的方法对中心通光区域的损伤状况进行观察可知,2.28 J/cm2时未对通光区域基底造成损伤,这也同样表明,污染物密度越高,基底的损伤阈值越低。图7为真空隔离片中心通光区域的激光清洗前后暗场成像图。对最佳能量密度下的颗粒去除率进行计算,边缘污染区域的颗粒去除率为57.67%,这与镀膜熔石英样品的颗粒去除率规律保持一致。
图 6 真空隔离片非通光区域的激光清洗前后暗场成像图。(a)未激光清洗;(b)激光清洗后(激光能量密度为1.71 J/cm2)
Figure 6. Dark field image of border zone on vacuum separator before and after laser cleaning. (a) Unprocessed; (b) Post processed with laser cleaning method (1.71 J/cm2 laser density)
图 7 真空隔离片中心通光区域的激光清洗前后暗场成像图。(a)未激光清洗;(b)激光清洗后(激光能量密度为1.71 J/cm2)
Figure 7. Dark field image of central zone on vacuum separator before and after laser cleaning. (a) Unprocessed; (b) Post processed with laser cleaning method (1.71 J/cm2 laser density)
将大口径真空隔离片与镀膜熔石英样品的单发激光清洗结论进行对照,对照结果如表2所示。两种样品由于污染状态略有差距造成基底损伤阈值与清洗工艺区间不同,去除率略有不同,但规律性结论基本吻合。
表 2 大口径真空隔离片与小尺寸镀膜熔石英样品的单发激光清洗参数对比
Table 2. Single-shot laser cleaning parameters comparison of small size coated fused silica and large-aperture vacuum separator
Samples Range of laser cleaning parameters/J·cm–2 Optimal laser energy density/J·cm–2 Removal rate Small size coated fused silica 0.57-2.45 1.72 54.61% Large-aperture vacuum separator 0.57-2.28 1.71 57.67% -
激光清洗实验过程中,大量的脱附颗粒可能会对激光清洗的效果造成影响。尤其是针对大口径光学元件的激光清洗场景,需要考虑清洗二次产物的空间弥散及再吸附问题,
自研的气流置换系统的原理是通过高速伺服电机驱动叶轮风机高速旋转,系统内部出现瞬时真空,和外界的大气环境形成负压差,高速吸入激光清洗过程中产生的大量脱附颗粒,提升清洗效果,避免二次污染。与常用的侧面吹扫气流方案不同,侧面吹扫方案使用为正压气流吹扫,虽然可将脱附颗粒吹离表面,但会造成颗粒在空间的弥散,特别是大口径光学元件安装于密闭的光传输管道中,会带来密封环境污染和二次附着问题。而气流置换方法采用负压原理,系统本身集成了多级HEPA过滤循环单元,可及时回收激光清洗过程的脱附颗粒避免二次污染,对可在线的工程应用具有重要意义。
与2.2节的实验设置相同,使用波长为355 nm、脉宽为10 ns的Nd: YAG脉冲激光器,于样片下侧平行夹持放置真空度为18 kPa、流速为15 m/s的气流置换系统,与光斑位置的直线距离约为4 cm,所使用的气体种类为洁净的压缩空气。选取大口径真空隔离片表面的边缘污染区域进行激光清洗实验,实验方案如图8所示。
图 8 气流置换系统辅助的激光清洗大口径光学元件实验装置示意图
Figure 8. Schematic diagram of the large aperture optics laser cleaning setup with the airflow displacement system
选取单点作用模式,取激光能量为40~200 mJ,光斑面积为7 mm2 (椭圆形,长、短轴为3.4 mm×2.6 mm @1/e2),计算得实验所用激光能量密度约为0.57~2.86 J/cm2。对两种样品进行气流置换系统辅助的单发干式激光清洗实验,使用光学显微镜、暗场成像法、图像分析软件对清洗样品上残留的污染颗粒以1~5 μm、5~15 μm、15~25 μm、25 μm以上四种颗粒半径范围分类统计。实验参数条件保持与单发干式激光清洗时相同,据此可通过控制变量法对气流置换系统的辅助效果进行分析,并与单发干式激光清洗的清洗效果进行对比。
根据前期研究表明,单独使用气流置换系统进行实验时,无法对此类污染状况产生清洗效果。气流置换辅助单发激光清洗的结果如图9所示。
图 9 加入气流置换前后不同样品颗粒激光清洗去除率对比
Figure 9. Comparison of surface particles removal rate of different samples laser cleaning with or without the airflow displacement method
由图9可知,从激光去除颗粒比例趋势上来看,加入气流置换系统前后颗粒去除率规律性基本一致,激光通量峰位位置相同;从数值上看,颗粒去除率提升了5%~30%。结合光学显微镜可知,加入气流置换系统对基底损伤阈值不会造成影响。此时大口径真空隔离片的最佳工艺参数为1.71 J/cm2,颗粒去除率为82.21%,相较于单纯的单发干式激光清洗,去除率提高了24.54%。这表明气流置换系统辅助单发激光清洗能有效提高其清洗效果。
以大口径真空隔离片为例,图10 (a)~(d)给出了在最佳激光清洗能量密度1.71 J/cm2下,加入气流置换系统前后的明场与暗场光学显微镜图片。结果表明,气流置换系统辅助的单发干式激光清洗的效果良好,有效减少了颗粒的重附着现象。图10 (a)为仅有单发激光清洗时的暗场图,图10(b)为气流置换系统辅助的单发激光清洗的暗场图。结果表明,气流置换系统辅助带来的清洗效果提升明显,并能够在单发次激光清洗的基础上有效减少粒径为1~5 μm的污染颗粒的残留数量。
图 10 气流置换辅助激光清洗的效果对比显微镜成像图。(a)无气流置换-暗场;(b)有气流置换-暗场;(c)无气流置换-明场;(d)有气流置换-明场
Figure 10. Microscopy comparison of surface particles with or without the airflow displacement method. (a) Dark field microscopy without the airflow displacement; (b) Dark field microscopy with the airflow displacement; (c) Bright field microscopy without the airflow displacement; (b) Bright field microscopy with the airflow displacement
In-situ laser cleaning of large-aperture optical components with sol-gel film (invited)
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摘要: 在高功率激光装置中,光学元件表面的污染物会降低光束质量甚诱导光学元件损伤。针对装置中受污染的镀有SiO2溶胶-凝胶增透膜的大口径真空隔离片(430 mm×430 mm),使用波长为355 nm的Nd:YAG脉冲激光器模拟在线激光清洗实验。实验中采用了单发次激光干式清洗与气流置换系统辅助的激光清洗系统,研究了关键特征参数对激光在线清洗效果的影响规律,获得了可用于激光在线清洗的工艺参数。光学元件的处理采用光学显微镜、暗场成像法表征以及图像处理软件分析。实验结果表明,激光在线清洗光学元件存在最佳工艺窗口。通过气流置换辅助的激光清洗方法后,相较于单纯的单发干式激光清洗,激光清洗能力有大幅提升。因此,气流置换系统辅助单发激光清洗能有效提高其清洗能力,为高功率激光装置中大口径光学元件表面污染物在线去除提供了一种有效手段。Abstract: In high-power laser facilities, contaminants on the surface of optical components can reduce beam quality and even induce damage to optical components. For the contaminated large-aperture vacuum separator (430 mm×430 mm) coated with SiO2 sol-gel antireflection film in the facility, a Nd:YAG pulsed laser with a wavelength of 355 nm was performed in laser cleaning experiment. The experiment adopted a single-shot laser dry cleaning and a laser cleaning system assisted by an airflow replacement system. The influence of key characteristic parameters on in-situ laser cleaning was studied, and the process parameters that could be used for laser in-situ laser cleaning were obtained. The processing of optical elements was characterized by microscope, dark field imaging and image processing software analysis. The experimental results suggest that there is an optimal process window for laser in-situ cleaning of optical components. After the laser cleaning method assisted by airflow replacement, the laser cleaning ability is greatly improved compared with the simple single-shot dry laser cleaning. Therefore, the single-shot laser cleaning assisted by the airflow displacement system can effectively improve its cleaning ability and provide an effective means for the in-situ removal of contaminants on the surface of large-aperture optical components in high-power laser facilities.
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Key words:
- laser cleaning /
- in-situ /
- large-aperture /
- optical components /
- airflow assist
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图 10 气流置换辅助激光清洗的效果对比显微镜成像图。(a)无气流置换-暗场;(b)有气流置换-暗场;(c)无气流置换-明场;(d)有气流置换-明场
Figure 10. Microscopy comparison of surface particles with or without the airflow displacement method. (a) Dark field microscopy without the airflow displacement; (b) Dark field microscopy with the airflow displacement; (c) Bright field microscopy without the airflow displacement; (b) Bright field microscopy with the airflow displacement
表 1 计算所需的SiO2颗粒的热物性参量
Table 1. Thermal and physical parameters of SiO2 particles used in calculation
Parameter Value ρ/g·cm–3 2.5 c/J·(kg·K)–1 730 α/cm2·s–1 0.06 β/K–1 0.5×10−6 表 2 大口径真空隔离片与小尺寸镀膜熔石英样品的单发激光清洗参数对比
Table 2. Single-shot laser cleaning parameters comparison of small size coated fused silica and large-aperture vacuum separator
Samples Range of laser cleaning parameters/J·cm–2 Optimal laser energy density/J·cm–2 Removal rate Small size coated fused silica 0.57-2.45 1.72 54.61% Large-aperture vacuum separator 0.57-2.28 1.71 57.67% -
[1] Huang L, Yan H, Yan L, et al. Improvement of the environmental stability of sol-gel silica anti-reflection coatings [J]. Journal of Sol-Gel Science and Technology, 2022, 101(3): 630-636. doi: 10.1007/s10971-022-05725-z [2] Tian H, Zhang L, Xu Y, et al. Comparision of silica anti-reflective films obtained via a sol-gel process in the presence of PEG or PVP [J]. Acta Physico-Chimica Sinica, 2012, 28(5): 1197. doi: 10.3866/PKU.WHXB201202231 [3] Li Y, Bai Q, Guan Y, et al. In situ plasma cleaning of large-aperture optical components in ICF [J]. Nuclear Fusion, 2022, 62(7): 076023. doi: 10.1088/1741-4326/ac555c [4] Wang S Y, Yan H W, Li D J, et al. TEM and STEM studies on the cross-sectional morphologies of dual-/tri-layer broadband SiO2 antireflective films [J]. Nanoscale Research Letters, 2018, 13: 49. doi: 10.1186/s11671-018-2442-4 [5] Yin J, Cao Y. Research of laser-induced damage of aluminum alloy 5083 on micro-arc oxidation and composite coatings treatment [J]. Optics Express, 2019, 27(13): 18232-18245. doi: 10.1364/OE.27.018232 [6] Li Y, Bai Q, Sun H, et al. Research progress on contamination damage and cleaning technology of large-aperture diffraction grating [J]. Journal of Mechanical Engineering, 2022, 58(9): 270-282. (in Chinese) doi: 10.3901/JME.2022.09.270 [7] Yang L, Xiang X, Miao X X, et al. Influence of outgassing organic contamination on the transmittance and laser-induced damage of SiO2 sol-gel antireflection film [J]. Optical Engineering, 2015, 54(12): 126101. doi: 10.1117/1.OE.54.12.126101 [8] Li Y, Bai Q, Yao C, et al. Long-lasting antifogging mechanism for large-aperture optical surface in low-pressure air plasma in-situ treated [J]. Applied Surface Science, 2022, 581: 152358. doi: 10.1016/j.apsusc.2021.152358 [9] Wu G M, Shen J, Zou L P. Laser-induced damage on ordered and amorphous sol-gel silica coatings [J]. Optical Materials Express, 2014, 4(12): 2478. [10] Yang L, Xiang X, Yuan X D, et al. Bulk damage and stress behavior of fused silica irradiated by nanosecond laser [J]. Optical Engineering, 2014, 53(4): 047103. doi: 10.1117/1.OE.53.4.047103 [11] Yu J X, Xiang X, He S B, et al. Laser-induced damage initiation and growth of optical materials [J]. Advances in Condensed Matter Physics, 2014, 1(1): 364627. [12] Guo Y J, Zu X T, Jiang X D, et al. Laser-induced damage mechanism of the sol–gel single-layer SiO2 acid and base thin films [J]. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2008, 266(12-13): 3190. doi: 10.1016/j.nimb.2008.03.187 [13] Spaeth M L, Wegner P J, Suratwala T I, et al. Optics recycle loop strategy for NIF operations above UV laser-induced damage threshold [J]. Fusion Science and Technology, 2016, 69(1): 265. doi: 10.13182/FST15-119 [14] Yu H B, Wang C M, Zhang W, et al. Present status and outlook of laser cleaning application development [J]. Electric Welding Machine, 2014, 44(10): 80-84. (in Chinese) [15] Chen J F, Zhang Y K, Kong D J, et al. Research progress of cleaning tiny particles by short pulsed laser [J]. Laser Technology, 2007, 31(3): 301-305. (in Chinese) [16] Liu H, Miao X X, Yang K, et al. Atmosphere pressure plasma cleaning of grease contamination on sol-gel SiO2 coating [J]. High Power Laser and Particle Beams, 2015, 27(11): 112008. (in Chinese) [17] Li Y, Ye Y, Liu H, et al. Time-resolved imaging for investigating laser-material interactions during laser irradiation cleaning on murals [J]. Optics & Laser Technology, 2023, 157: 108679. [18] Ramakrishna S N, Nalam P C, Clasohm L Y, et al. Study of adhesion and friction properties on a nanoparticle gradient surface: Transition from JKR to DMT contact mechanics [J]. Langmuir the Acs Journal of Surfaces & Colloids, 2013, 29(1): 175-182.