Bulk damage and absorption in fused silica due to high-power laser applications

F. Nürnberg, B. Kühn, A. Langner, M. Altwein, G. Schötz, R. Takke, S. Thomas, J. Vydra
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引用次数: 20

Abstract

Laser fusion projects are heading for IR optics with high broadband transmission, high shock and temperature resistance, long laser durability, and best purity. For this application, fused silica is an excellent choice. The energy density threshold on IR laser optics is mainly influenced by the purity and homogeneity of the fused silica. The absorption behavior regarding the hydroxyl content was studied for various synthetic fused silica grades. The main absorption influenced by OH vibrational excitation leads to different IR attenuations for OH-rich and low-OH fused silica. Industrial laser systems aim for the maximum energy extraction possible. Heraeus Quarzglas developed an Yb-doped fused silica fiber to support this growing market. But the performance of laser welding and cutting systems is fundamentally limited by beam quality and stability of focus. Since absorption in the optical components of optical systems has a detrimental effect on the laser focus shift, the beam energy loss and the resulting heating has to be minimized both in the bulk materials and at the coated surfaces. In collaboration with a laser research institute, an optical finisher and end users, photo thermal absorption measurements on coated samples of different fused silica grades were performed to investigate the influence of basic material properties on the absorption level. High purity, synthetic fused silica is as well the material of choice for optical components designed for DUV applications (wavelength range 160 nm - 260 nm). For higher light intensities, e.g. provided by Excimer lasers, UV photons may generate defect centers that effect the optical properties during usage, resulting in an aging of the optical components (UV radiation damage). Powerful Excimer lasers require optical materials that can withstand photon energy close to the band gap and the high intensity of the short pulse length. The UV transmission loss is restricted to the DUV wavelength range below 300 nm and consists of three different absorption bands centered at 165 nm (peroxy radicals), 215 nm (E’-center), and 265 nm (non-bridging oxygen hole center (NBOH)), which change the transmission behavior of material.
高功率激光应用对熔融二氧化硅体损伤和吸收的影响
激光聚变项目正朝着具有高宽带传输、高抗冲击和耐高温、长激光耐久性和最佳纯度的红外光学方向发展。对于这种应用,熔融二氧化硅是一个很好的选择。红外激光光学的能量密度阈值主要受熔融石英的纯度和均匀性的影响。研究了不同合成熔融硅等级对羟基含量的吸收行为。受OH振动激发影响的主吸收导致富OH和低OH熔融二氧化硅的红外衰减不同。工业激光系统的目标是最大限度地提取能量。贺利氏石英开发了一种掺镱熔融石英光纤来支持这一不断增长的市场。但是激光焊接和切割系统的性能从根本上受到光束质量和聚焦稳定性的限制。由于光学系统中光学元件的吸收对激光聚焦位移有不利的影响,因此必须在块状材料和涂层表面上尽量减少光束能量损失和由此产生的加热。与一家激光研究所、一家光学精加工商和最终用户合作,对不同熔融二氧化硅等级的涂层样品进行了光热吸收测量,以研究基本材料特性对吸收水平的影响。高纯度的合成熔融二氧化硅也是用于DUV应用(波长范围160 nm - 260 nm)的光学元件的首选材料。对于更高的光强度,例如准分子激光器,紫外线光子可能会产生缺陷中心,影响使用过程中的光学性能,导致光学元件老化(紫外线辐射损伤)。强大的准分子激光器要求光学材料能够承受接近带隙的光子能量和短脉冲长度的高强度。紫外透射损失限制在300 nm以下的DUV波长范围内,由以165 nm(过氧自由基)、215 nm (E ' -中心)和265 nm(非桥接氧空穴中心(NBOH))为中心的三个不同的吸收带组成,改变了材料的透射行为。
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