光学元件改性处理对激光损伤阈值的影响

Effect of chemical modification technology laser damage threshold of fused silica optical elements

针对355nm激光作用于熔石英光学元件后其损伤阈值容易变差的问题,提出使用1.7%纯HF溶液和0.4%HF与1.2%NH4F混合的BOE溶液对样品进行处理来提高它们的激光诱导损伤阈值(LIDT)。在相同的条件下将熔石英光学元件浸没到上述两种不同的刻蚀溶液中进行处理,通过测量刻蚀过程中元件重量变化来计算刻蚀速率,利用Zygo轮廓仪测试元件表面粗糙度,然后对355nm激光照射下熔石英元件的损伤阈值情况进行研究。损伤测试表明,LIDT与元件的材料去除深度有关系,用两种刻蚀液刻蚀去除一定深度后,LIDT均有增加,但是进一步去除会显著地降低元件的LIDT。在处理过程中,这两种刻蚀液的去除速率都很稳定,分别为85.9nm/min和58.6nm/min左右。另外,元件表面的粗糙度会随着刻蚀时间的增加而变大。在刻蚀过程中还通过纳米技术测量了熔石英元件表面的硬度及杨氏系数,不过没有证据表明其与激光诱导损伤有明确的关系。

The Laser-induced Damage Thresholds （LIDTs） of fused silica optical elements would be deteriorate, when it suffers the 355 nm laser irradiation. This paper proposes a method to improve their LIDTs. A silica element was immersed into two types of HF-based etchants= 1.7%wt. HF acid and buffer oxide etchant （BOE. the mixture of 0.4%wt. HF and 12%wt. NH4F）to be treated in the same experimental condition , respectively. The etching rate was calculated by measuring the weight variation of the etching element during the etching process, and the surface roughness of the element was tested by a Zygo contourgraph. Then, the LIDT characteristics of the fused quartz element under the 355 nm laser irradiation were studied. The damage testing shows that the LIDT is relative to the depth of material removal. Both the etchants increase the LIDT at a certain depth of material removal, but further removal of material lowers the LIDT markedly. The etching rates of both etchants keep steady in our processing procedure, they are -85.9 nm/min and -58.6 nm/min, respectively. The micro-surface roughness （RMS＆PV） increases as etching time extended. Furthermore, the hardness and Young＇s modulus of the fused silica etched were measured by a nano-indenter, and there is no solid evidence that LIDT can be related to hardness or Young＇s modulus.