1064nm纳秒激光对熔石英元件后表面击穿的实验与数值研究

对1064 nm纳秒激光辐照下熔石英元件后表面损伤过程进行了时间分辨诊断.利用基于偏振原理的单发双帧阴影成像系统捕捉了脉冲上升沿开始到数百纳秒尺度内的瞬态材料响应,并结合剪切干涉成像系统分析了空气端等离子体微喷现象.结果表明,损伤在脉冲上升沿就己经发生,此时空气端等离子体的膨胀速度高达20 km/s,同时材料内部也存在高速扩张的不透明损伤区域,但其扩张过程在脉冲结束后迅速停止;损伤发生后数十纳秒后,空气端出现大量中性物质喷发基于激光支持的固态吸收波前模型与相爆炸理论对这些现象进行了讨论.对空气端等离子体扩张过程进行了数值模拟,结果表明空气端等离子体压强、温度与密度等参数值随延迟增加迅速下降,其瞬态压强最高达600 MPa;模拟结果还预测了向内扩张的内激波的形成.研究结果对理解熔石英元件的损伤机理有重要意义.     

熔石英光学材料紫外皮秒激光损伤的瞬态特性研究

光学元件激光损伤是限制高功率激光装置输出能力的关键因素,为了理解光学元件激光损伤过程,提高光学元件抗激光损伤性能,利用偏振阴影显微镜成像技术和光电探测技术研究了紫外皮秒激光诱使熔石英光学元件损伤的时间分辨动力学过程。结果显示了紫外皮秒激光作用过程中冲击应力波的传输特性、瞬态吸收的演变过程以及裂缝的发展过程。结果表明,冲击应力波的传输速度约为6.9μm/ns;532nm波长的激光瞬态吸收在激光作用之后2.5μs时激光吸收达到最大值,之后缓慢下降,整个持续时间可达50μs以上;损伤裂纹在7.5ns时刻就基本停止增长。研究结果对理解皮秒激光的损伤机制有重要意义。     

355 nm皮秒激光辐照下熔石英的损伤特性

利用光学元件激光损伤测试平台,测试了355 nm皮秒激光辐照下熔石英光学元件的初始损伤及损伤增长情况,并通过荧光检测分析了损伤区缺陷。研究结果表明:皮秒激光较高的峰值功率导致熔石英损伤阈值较低,前表面损伤阈值为3.98 J/cm2,后表面损伤阈值为2.91 J/cm2;前后表面损伤形貌存在较大差异,后表面比前表面损伤程度轻且伴随体内丝状损伤;随脉冲数的增加后表面损伤直径增长缓慢,损伤深度呈线性增长;皮秒激光的动态自聚焦和自散焦导致熔石英体内损伤存在细丝和炸裂点重复的现象;与纳秒激光损伤相比,损伤区缺陷发生明显改变。     

基于有限元法的激光等离子体诱导玻璃 损伤特性研究

研究了熔石英玻璃元件在纳秒激光等离子体冲击波作用下的表面和横截面损伤形貌,利用有限元法模拟了冲击波在熔石英玻璃内部的传播规律,并基于冲击波在玻璃内部的应力分布规律分析了损伤形成机理.研究发现:在冲击波作用下,石英玻璃会受到沿波面方向的压应力和沿波面切方向的拉应力,在这两种力的作用下,造成以激光辐照中心的弧状层状断裂和沿径向的断裂;冲击波的反射叠加还会使局部拉应力增大,造成靠近后表面的损伤.有限元法能够直观地分析等离子体冲击波对光学元件的作用,并分析光学元件在等离子体冲击波下的损伤机理.     

高功率固体激光装置负载问题研究进展

高功率固体激光装置的负载问题是制约装置建设与运行的瓶颈问题。在高通量紫外纳秒激光辐照下，熔石英后表面的损伤不断产生和增长，严重限制了装置的负载能力。在提升熔石英抗损伤性能的基础上修复既有损伤，循环使用光学元件，是现阶段提升装置负载能力的主要手段。主要介绍了国内外近年来在熔石英损伤的规律与机制、光学元件循环处理的支撑技术以及提升负载能力的新材料与新技术方面所取得的重要进展。     

杂质诱导熔石英激光的损伤机理

发展了355 nm纳秒激光下亚波长杂质粒子引起熔石英损伤的基本模型。通过Mie散射理论和热传导方程,计算了粒子与熔石英边界处的温度随粒子尺寸的变化关系,并分析了达到临界温度时,不同粒子诱导损伤所需的关键能量密度,讨论了各粒子最易引起熔石英损伤的尺寸。实验采用355 nm纳秒激光脉冲作用熔石英及其HF刻蚀样品,测得两者的损伤概率。研究表明:粒子吸收激光能量,随着粒子半径的增加,其边缘温度先增大后减小,一定尺寸范围内的粒子才会引起熔石英的损伤;关键能量密度所对应的粒子半径为最易引起熔石英损伤的关键粒子半径;经刻蚀后,熔石英样品表面杂质数密度降低,损伤概率降低,损伤阈值提高。     

CF_4/Ar/O_2等离子体对熔石英元件的修饰工艺

采用感应耦合等离子体刻蚀技术,以CF4/Ar/O2为反应气体对熔石英元件表面进行修饰,研究并分析了CF4和Ar流量对刻蚀速率、熔石英表面粗糙度和微观形貌的影响。结果表明,CF4化学刻蚀与Ar的物理轰击对熔石英样品表面修饰效果存在一定竞争关系,当它们达到平衡时表面粗糙度最小。通过对不同流量气体刻蚀过后熔石英表面粗糙度和光学显微形貌分析获得了较为理想的气流量配比,该研究为反应等离子体修饰熔石英光学元件以获得较高光学性能提供工艺参考。     

表面Al膜污染物诱导熔石英表面损伤特性

在熔石英表面人工溅射一层Al膜污染物,分别测试污染前后熔石英基片在355 nm波长激光辐照下的损伤阈值,并采用透射式光热透镜技术、椭偏仪和光学显微镜研究了污染物Al膜的热吸收、厚度以及激光辐照前后熔石英的损伤形貌。用355 nm波长的脉冲激光分别辐照位于污染的熔石英和洁净的熔石英前后表面的损伤点,并用显微镜在线采集损伤增长图样,测试损伤点面积。实验表明:熔石英前表面的金属Al膜污染物导致基片损伤阈值的下降约30%,后表面的污染物导致基片下降约15%,位于熔石英样片后表面损伤点面积随激光辐照次数呈指数增长,而位于前表面的损伤点面积与激光脉冲辐照次数呈线性增长关系;带有污染的熔石英样片的增长因子比洁净的熔石英样片的增长因子高30%。     

高功率脉冲电子束辐照SiO$lt;sub$gt;2$lt;/sub$gt;的光学和激光损伤性能

研究了不同剂量的60 kW高功率脉冲电子束辐照对高纯熔石英玻璃的微观结构、光学性能和激光损伤特性的影响规律. 光学显微图像表明, 辐照后熔石英样品由于热效应导致表面破裂, 裂纹密度和尺寸随辐照剂量增加而增大, 采用原子力显微镜分析表面裂纹的微观形貌, 裂纹宽度约1 um, 同时样品表面分布着大量尺寸约0.1–1μm的碎片颗粒. 吸收光谱测试表明, 所有样品均在394 nm处出现微弱的吸收峰, 吸收强度随着电子束辐照剂量增大呈现先增加后减小的趋势. 荧光光谱测试发现辐照前后样品均有3个荧光带, 分别位于460, 494和520 nm, 荧光强度随辐照剂量的变化趋势与吸收光谱一致. 利用355 nm激光研究了不同剂量电子束辐照对熔石英激光损伤阈值的影响, 结果表明熔石英的损伤阈值随着辐照剂量的增加而降低. 在剂量较低时, 导致熔石英激光损伤阈值下降的原因主要是色心缺陷; 剂量较高时, 导致损伤阈值降低的原因主要是样品表面产生的大量微裂纹和碎片颗粒对激光的调制和吸收. 关键词： 熔石英 电子束辐照 色心 激光损伤阈值     

熔石英表面铜膜污染物诱导损伤实验研究

 在熔石英元件表面溅射一层厚度小于10 nm的金属铜膜污染物,并测试元件的透过率。测试355 nm熔石英元件的激光损伤阈值,并用光学显微镜观测损伤形态。实验结果表明：污染后的熔石英元件的损伤阈值降低20%左右,元件表面的金属污染物薄膜经强激光辐照,在熔石英表面形成很多坑状微损伤,分布不均的热应力导致表面起伏,并有明显的烧蚀现象,导致基底损伤阈值下降。建立的光吸收和热沉积传输模型初步解释污染物膜层导致熔石英元件损伤的机理。     

The damage mechanism of fused silica surface induced by 355 nm nanosecond laser

In order to understand the physical mechanism, time-resolved dynamics of 355 nm nanosecond laser induced entrance and exit surface damage on fused silica was investigated by using shadow graphic technique. The results show that the damage mechanism is different between the entrance and exit surface during nanosecond laser interaction with fused silica. The plasma and shock waves in air is relatively higher at the entrance surface. The entrance surface damage is reduced because plasma shielding limits energy deposition. Without plasma shielding, the exit surface damage is more serious for more laser energy deposition in material. And without the stress of plasma and shock waves, the material is ejected easily at rear surface. These are confirmed by damage micrograph at the entrance and exit surface.     

Laser-induced damage on large-aperture fused silica gratings

Two sets of laser-damage experiments on large-aperture fused silica optics have been carried out in a high-power laser facility. Severe damage has been found on the grating which presented dense craters on the front surface. This phenomenon is quite different from other fused silica optics, which are damaged on the rear surface. The damage possibility due to the redeposition layer was ruled out by acid-etching the grating’s front surface. The remarkable stimulated Brillouin scattering (SBS) effect was observed in grating and the reason for the front surface damage is thought to be the backward SBS.     

Damage characteristics of laser plasma shock wave on rear surface of fused silica glass

The damage to the rear surface of fused silica under the action of high power laser is more severe than that incurred by the front surface, which hinders the improvement in the energy of the high power laser device. For optical components,the ionization breakdown by laser is a main factor causing damage, particularly with laser plasma shock waves, which can cause large-scale fracture damage in fused silica. In this study, the damage morphology is experimentally investigated, and the characteristics of the damage point are obtained. In the theoretical study, the coupling and transmission of the shock wave in glass are investigated based on the finite element method. Thus, both the magnitude and the orientation of stress are obtained. The damage mechanism of the glass can be explained based on the fracture characteristics of glass under different stresses and also on the variation of the damage zone's Raman spectrum. In addition, the influence of the glass thickness on the damage morphology is investigated. The results obtained in this study can be used as a reference in understanding the characteristics and mechanism of damage characteristics induced by laser plasma shock waves.     

Numerical simulation of the modulation to incident laser by the repaired damage site in a fused silica subsurface

One of the main factors of laser induced damage is the modulation to incident laser which is caused by the defect in the subsurface of the fused silica.In this work,the repaired damage site irradiated by CO 2 laser is simplified to a Gaussian rotation according to the corresponding experimental results.Then,the three-dimensional finite-difference time-domain method is employed to simulate the electric field intensity distribution in the vicinity of this kind of defect in fused silica front subsurface.The simulated results show that the modulation is notable,the E max is about 2.6 times the irradiated electric field intensity in the fused silica with the damage site (the width is 1.5 μm and depth is 2.3 μm) though the damage site is repaired by CO 2 laser.The phenomenon and the theoretical result of the annular laser enhancement existed on the rear surface are first verified effectively,which agrees well with the corresponding experimental results.The relations between the maximal electric field intensity in fused silica with defect depth and width are given respectively.Meanwhile,the corresponding physical mechanism is analysed theoretically in detail.     

UBK7玻璃后表面缺陷诱导体内激光损伤

 对UBK7玻璃在波长为1.064靘的短脉冲激光作用下产生的损伤进行了研究。通过对体损伤的形貌的相衬显微镜观测,发现体内和后表面炸裂点间的丝状损伤,从而提出后表面缺陷导致体损伤的理论解释。同时分析了表面损伤和体损伤的机理,对后表面损伤阈值低于前表面的原因作了讨论。     

预应力对熔石英损伤的影响

采用ANSYS进行形变-应力模拟。对不同应力状态下的熔石英表面进行三倍频激光损伤测试,结果发现,预加压应力为0~50 MPa时损伤阈值有明显提高的趋势,用应力耦合作用对此给出了解释:0~50 MPa的预加压应力可以降低和抵消激光辐照产生的张应力破坏,大于50 MPa预应力的耦合作用会使得该处机械性能下降,另外,损伤增长在预应力存在时更容易发生。因此,0~50 MPa预加压应力时的表面预应力可以提高熔石英的抗激光辐照能力。     

激光诱导光学材料后表面损伤的数值模拟

 采用3维时域有限差分方法和完全匹配吸收层,模拟了长方体缺陷在熔石英前后表面时对入射激光为TM波的调制作用,绘出了截面上的电场强度分布及最大电场强度随熔石英深度变化的曲线,并进行了比较和分析。结果表明：缺陷在前表面上时,后表面附近的最大电场强度2.522 41 V/m大于缺陷附近的1958 83 V/m;缺陷在后表面上时,材料中的最大电场强度为2.799 38 V/m,且出现在后表面附近。无论该缺陷在前表面还是在后表面,最大电场强度都是出现在后表面附近,表明光学材料的后表面在一定程度上更容易被损伤。     

超声波辅助酸蚀提高熔石英损伤阈值

为了提高熔石英元件表面抗激光损伤阈值,利用超声波辅助HF酸研究平滑光学元件表面缺陷形貌和去除刻蚀后残留物效果,通过扫描电子显微镜电镜和原子力显微镜记录表面形貌结构,以及单脉冲激光辐照测试抗损伤阈值确定实验参数。研究表明,超声波场的引入能催化HF酸的刻蚀速率、提高钝化效果并且更易剥离嵌入的亚μm级杂质粒子。经过实验测试,获得了熔石英类元件相匹配的超声辅助HF酸刻蚀实验参数,研究结果对应用超声波辅助HF酸研究熔石英表面抗激光损伤有重要意义。     

纳秒激光诱导熔石英光学玻璃的损伤增长

利用Nd:YAG激光器研究了纳秒激光诱导熔石英光学玻璃的初始损伤及损伤增长,对比研究了损伤程度和损伤形貌随激光波长、能量密度、脉冲数及位置的变化规律,并对损伤机制进行了分析和讨论。研究结果表明:初始损伤受损伤先驱的物理化学性质和激光参数的影响,而损伤增长规律与初始损伤程度、激光参数和位置有关;后表面的损伤随脉冲数的增加呈指数关系增长,前表面则呈线性关系;裂纹的产生及其在后续脉冲辐照下的发展是后表面损伤增长的主要原因,高温等离子体表面刻蚀是前表面损伤增长的主要原因     

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse. Drilling rates in excess of 100 μm per pulse are achieved by taking advantage of the propagation characteristics of the plasma created at the drilling front. The plasma during rear-side drilling generates a laser-supported detonation wave into the bulk material. The geometry also seems to increase the efficiency of the laser-induced plasma combustion and shock wave during the pulse by confining it in front of the channel tip. Received: 1 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000