Laser-induced damage in fused silica under multi-wavelength simultaneous laser irradiation
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摘要: 对比研究了3ω单独辐照、3ω+2ω和3ω+1ω双波长同时辐照下熔石英元件的初始损伤和损伤增长规律,重点研究3ω能量密度在其阈值附近时,低能量密度的2ω和1ω对初始损伤和损伤增长的影响,分析了波长间的能量耦合效应。结果表明:双波长同时辐照下,当2ω和1ω能量密度远低于其自身阈值时,它们对初始损伤几率和损伤增长阈值的影响可以忽略,但也会参与初始损伤和损伤增长过程,会增加初始损伤程度和损伤增长系数。基于飞秒双脉冲成像的冲击波速度测量表明,3ω和1ω同时辐照下,波长间的能量耦合效应会促进激光能量向材料沉积的效率。Abstract: The initial damage and damage growth of fused silica optical elements irradiated by 3ω alone, by two wavelengths (3ω+2ω and 3ω+1ω) at the same time were studied. When the energy density of 3ω is near its threshold, the influence of 2ω and 1ω of low energy density on the initial damage and damage growth is studied. The energy coupling mechanism between wavelengths is analyzed. The results show that: When the energy density of 2ω or 1ω is much lower than its threshold, irradiating at the same time, their effects on the initial damage probability and damage growth threshold can be ignored. But the initial damage degree and damage growth coefficient will increase. The measurement of shock wave velocity based on femtosecond dual pulse imaging shows that, when 3ω and 1ω irradiate at the same time, the energy coupling effect between wavelengths will promote the deposition efficiency of laser energy to materials.
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Key words:
- laser induced damage /
- fused silica /
- multiwavelength laser /
- initial damage /
- damage growth
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为满足物理实验需要,ICF激光驱动器装置的终端光学组件工作在以3ω为主的三波长激光同时辐照环境下,且3ω通量往往都接近元件的损伤阈值,终端光学组件是装置稳定、高效和安全运行的薄弱环节[1-5]。由于同时存在波长效应和波长间的能量耦合效应,多波长同时辐照下的初始损伤和损伤增长的物理机制比单波长要复杂很多[6-9]。研究多波长激光同时辐照下的初始损伤和损伤增长的实验现象,分析多波长激光同时辐照下损伤过程的能量耦合机制,有助于我们理解激光诱导光学元件损伤的物理机制,预测光学元件的使用寿命,对系统进行优化设计,提高系统的负载能力并降低运行成本[10-14]。本文研究了3ω单独辐照、3ω+2ω和3ω+1ω双波长同时辐照下熔石英的初始损伤和损伤增长实验规律,获得了在3ω阈值附近,低能量密度的2ω和1ω对初始损伤和损伤增长的影响规律,利用基于飞秒双脉冲的阴影成像方法获得了冲击波速度,分析了多波长激光同时辐照下光学元件损伤的能量耦合机制。
1. 实验系统
本文所采用损伤测试实验系统及实验方法见文献[15]。为了使研究结果对工程应用具有参考意义,选择各波长能量密度比与工程装置终端组件相近的情况作为实验条件。损伤坑面积采用辐照后离线显微成像的方式获得。采用图1所示的泵浦−探针阴影成像系统进行冲击波速度测量,Nd:YAG纳秒多波长激光作为泵浦脉冲,钛宝石飞秒激光作为探针脉冲。飞秒脉冲经1∶1分光镜分成两束,其中一束经延迟线产生延迟时间Δt。利用飞秒激光器外同步触发信号发生器(DG645),信号发生器控制纳秒激光与飞秒激光的延时ΔT。控制CCD曝光时间,采用单发双脉冲成像方法,单次成像可获得同一冲击波的两个波前,经长度定标可得到Δt内冲击波前的平均速度。利用光电探头和示波器获取ΔT,由于飞秒种子和纳秒激光器输出的时间抖动,ΔT存在±10 ns的抖动。实验样品厚度2 mm,透镜聚焦250 mm。
实验样品基材为Corning 7980,尺寸为100 mm×70 mm×5 mm和100 mm×10 mm×2 mm,表面精抛光,实验前用氢氟酸(HF)和氟化氨(NH4F )溶液在超声波环境下进行表面刻蚀,经去离子水清洗,并经高纯酒精脱水处理后擦净。实验样品在1ω,2ω和3ω单独辐照下的最大零几率损伤阈值分别约为64,45和19 J/cm2,损伤增长阈值分别约为15,10和6 J/cm2[15]。
2. 实验研究
2.1 双波长同时辐照下的初始损伤几率
表1为不同能量密度组合的3ω+2ω,3ω+1ω同时辐照下熔石英的初始损伤几率,虽然从3ω@17 J/cm2+2ω@9 J/cm2,3ω@17 J/cm2+1ω@10 J/cm2能量密度组合已经出现损伤,但随着能量密度逐渐增加,损伤几率没有明显变化,考虑样品差异和损伤的随机性、离散性,可以认为2ω和1ω对损伤几率没有明显贡献。在3ω@24 J/cm2+2ω@9 J/cm2,3ω@24 J/cm2+1ω@20 J/cm2时损伤几率分别增加到10%和14%,此时,可认为2ω和1ω对损伤几率有贡献。
表 1 双波长同时辐照下熔石英的初始损伤几率Table 1. Damage probability of fused silica irradiated by dual wavelength laserwavelengths energy density/(J·cm−2) damage probability/% wavelengths energy density/(J·cm−2) damage probability/% 3ω+2ω 17+3 0 3ω+1ω 17+5 0 17+6 0 17+10 3 17+9 2 17+15 4.5 17+12 5.5 17+20 6 20.5+3 7 20.5+5 4.5 20.5+6 5 20.5+10 4 20.5+9 1.5 20.5+15 3 20.5+12 3.5 20.5+20 8.5 24+3 3 24+5 5 24+6 8 24+10 7 24+9 10 24+15 8 24+12 12 24+20 14 27.5+3 31 - - 2.2 双波长同时辐照下的初始损伤面积
表2为不同能量密度(组合)的3ω单独、3ω+2ω和3ω+1ω双波长同时辐照下的损伤几率和损伤坑平均面积的实验结果。3ω单独辐照下,随着3ω能量密度增加,损伤几率和损伤坑平均面积都快速增加。在双波长同时辐照下,随着2ω和1ω的能量密度增加,虽然损伤几率没有明显变化,但损伤坑平均面积呈单调增加趋势。
表 2 2ω和1ω对损伤几率和损伤坑平均面积的影响Table 2. Influence of 2ω and 1ω wavelengths on damage probability and damage degreewavelengths energy density/(J·cm−2) damage probability/% average area/μm2 3ω 20.5 1.5 2 043 24 4 3 180 30.5 28 4 565 31 33.5 4 737 33 47 6 455 3ω+2ω 20.5+3 4 2 105 20.5+6 5 2 517 20.5+9 1.5 4 029 20.5+12 3.5 5 134 3ω+1ω 20.5+5 4.5 1 903 20.5+10 4 2 480 20.5+15 3 2 507 20.5+20 8.5 3 701 2.3 双波长同时辐照下的损伤增长
表3和表4是3ω+2ω,3ω+1ω双波长同时辐照下熔石英的损伤增长实验规律。可以看出,在3ω@5 J/cm2+2ω@2.4 J/cm2和3ω@6 J/cm2+1ω@2.4 J/cm2的情况下,2ω和1ω对损伤增长开始有明显贡献。
表 3 3ω+2ω同时辐照下熔石英的损伤增长几率Table 3. Damage growth probability of fused silica irradiated by 3ω and 2ω simultaneouslyNo. 3ω energy density/(J·cm−2) 2ω energy density/(J·cm−2) damage growth probability/% 1 4.4 2.4 0 2 5 0 0 3 5 1.2 0 4 5 2.4 5 5 6 0 0 6 6 1.2 11 7 6 2.4 17 8 6.5 0 13 9 6.5 1.2 18 10 6.5 2.4 26 表 4 3ω+1ω同时辐照下熔石英的损伤增长几率Table 4. Damage growth probability of fused silica irradiated by 3ω and 1ω simultaneouslyNo. 3ω energy density/(J·cm−2) 1ω energy density/(J·cm−2) damage growth probability/% 1 4.4 2.4 0 2 5 0 0 3 5 1.2 0 4 5 2.4 0 5 6 0 0 6 6 1.2 0 7 6 2.4 7 8 6 3.6 12 9 6.5 0 10 10 6.5 1.2 14 11 6.5 2.4 20 2.4 双波长同时辐照下的超快成像
图2是脉冲能量均为30 mJ的1ω,3ω单独辐照和同时辐照下,熔石英后表面损伤的典型阴影成像图,Δt=28 ns。其中1,2和3分别为体内一次冲击波前、体内二次冲击波前和空气冲击波前,1′,2′和3′为Δt时间间隔后的冲击波前。选择体内一次冲击波为研究对象,图2(a),(b)和(c)对应的冲击波前在Δt时间间隔内的平均速度分别为4.58×103,6.07×103和6.42×103 m/s。
3. 分析与讨论
双波长同时辐照下,在3ω单独辐照损伤阈值和损伤增长阈值附近,当能量密度比2ω∶3ω≥0.2∶1(损伤增长)和≥0.5∶1(初始损伤)时,2ω会对损伤增长和初始损伤产生明显贡献;当能量密度比1ω∶3ω≥0.4∶1(损伤增长)和≥0.8∶1(初始损伤)时,1ω会对损伤增长和初始损伤产生明显贡献;在初始损伤中,能量密度比2ω∶3ω<0.5∶1的2ω和1ω∶3ω<0.8∶1的1ω虽然对损伤几率影响较小甚至没有影响,但会引起损伤坑平均面积增加。
对于多波长同时辐照下熔石英的初始损伤和损伤增长实验现象一个可能合理的解释:
(1)初始损伤及损伤增长过程均可分为两个阶段。第一阶段,缺陷(损伤坑)吸收激光产生自由电子并形成低温等离子体(其膨胀不足以产生损伤(增长));第二阶段,低温等离子体高效吸收后续激光,内能增加,形成高温等离子体,其剧烈膨胀产生损伤(增长)。
(2)双波长同时辐照下,当3ω能量密度在其单独辐照的阈值附近,能量密度远低于其自身损伤阈值和损伤增长阈值的2ω和1ω被3ω产生的自由电子、低温等离子体和高温等离子体吸收,从而引起损伤几率(程度)增加。即,如果2ω和1ω被3ω产生的自由电子或低温等离子体大量吸收,形成高温等离子体,则主要引起损伤几率增加;如果2ω和1ω主要被3ω产生的高温等离子体吸收,导致等离子体内能进一步增加,则主要引起损伤程度增加。从这个角度来看,多波长同时辐照下,只要其中一个波长对元件产生了损伤或损伤增长现象,其产生的自由电子和等离子体就会对其他波长激光产生吸收,从而对初始损伤和损伤增长造成影响。
根据Taylor等人[16]提出的等离子体点爆炸球面波模型
(1) 式中:v是冲击波前速度;E是等离子体能量;γ为无量纲常数;ρ是介质密度。根据2.4的实验测试结果,
,即双波长同时辐照下等离子体积累的能量大于两个波长单独辐照下等离子体积累的能量之和,表明波长间的耦合效应促进了激光能量向材料(等离子体)沉积的效率。当然,这需要忽略缺陷、激光及相互作用过程的差异。这种现象可以解释为,较高能量密度的3ω被缺陷吸收产生自由电子和等离子体,自由电子和等离子体吸收1ω,使得原本对1ω不吸收(或吸收很小)的缺陷产生了吸收;同时,吸收1ω后,电子密度和等离子体温度增加,又会增加其对3ω脉冲后续能量的吸收效率;使得原本单独吸收3ω不会损伤的产生了损伤,原本会产生损伤的增加了损伤程度。4. 结 论
多波长激光同时辐照下熔石英元件的初始损伤和损伤增长过程中,同时存在激光与材料相互作用的波长效应和波长间的能量耦合效应,自由电子和等离子体是波长间能量耦合效应的吸收媒介,会促进激光能量向材料沉积的效率。当3ω能量密度在其损伤阈值和损伤增长阈值附近,能量密度远低于其自身阈值的2ω和1ω会被3ω产生的自由电子和等离子体吸收,虽然可能不会对损伤几率和损伤增长阈值产生影响,但会增加初始损伤程度和损伤增长系数。
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表 1 双波长同时辐照下熔石英的初始损伤几率
Table 1. Damage probability of fused silica irradiated by dual wavelength laser
wavelengths energy density/(J·cm−2) damage probability/% wavelengths energy density/(J·cm−2) damage probability/% 3ω+2ω 17+3 0 3ω+1ω 17+5 0 17+6 0 17+10 3 17+9 2 17+15 4.5 17+12 5.5 17+20 6 20.5+3 7 20.5+5 4.5 20.5+6 5 20.5+10 4 20.5+9 1.5 20.5+15 3 20.5+12 3.5 20.5+20 8.5 24+3 3 24+5 5 24+6 8 24+10 7 24+9 10 24+15 8 24+12 12 24+20 14 27.5+3 31 - - 表 2 2ω和1ω对损伤几率和损伤坑平均面积的影响
Table 2. Influence of 2ω and 1ω wavelengths on damage probability and damage degree
wavelengths energy density/(J·cm−2) damage probability/% average area/μm2 3ω 20.5 1.5 2 043 24 4 3 180 30.5 28 4 565 31 33.5 4 737 33 47 6 455 3ω+2ω 20.5+3 4 2 105 20.5+6 5 2 517 20.5+9 1.5 4 029 20.5+12 3.5 5 134 3ω+1ω 20.5+5 4.5 1 903 20.5+10 4 2 480 20.5+15 3 2 507 20.5+20 8.5 3 701 表 3 3ω+2ω同时辐照下熔石英的损伤增长几率
Table 3. Damage growth probability of fused silica irradiated by 3ω and 2ω simultaneously
No. 3ω energy density/(J·cm−2) 2ω energy density/(J·cm−2) damage growth probability/% 1 4.4 2.4 0 2 5 0 0 3 5 1.2 0 4 5 2.4 5 5 6 0 0 6 6 1.2 11 7 6 2.4 17 8 6.5 0 13 9 6.5 1.2 18 10 6.5 2.4 26 表 4 3ω+1ω同时辐照下熔石英的损伤增长几率
Table 4. Damage growth probability of fused silica irradiated by 3ω and 1ω simultaneously
No. 3ω energy density/(J·cm−2) 1ω energy density/(J·cm−2) damage growth probability/% 1 4.4 2.4 0 2 5 0 0 3 5 1.2 0 4 5 2.4 0 5 6 0 0 6 6 1.2 0 7 6 2.4 7 8 6 3.6 12 9 6.5 0 10 10 6.5 1.2 14 11 6.5 2.4 20 -
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