Influence of secondary treatment with CO
                    <sub>2</sub>
                    laser irradiation for mitigation site on fused silica surface

Jiang, Yong; Zhou, Qiang; Qiu, Rong; Gao, Xiang; Wang, Huili; Yao, Caizhen; Wang, Junbo; Zhao, Xin; Liu, Chunming; Xi, Xiang; Zu, Xihong; Yuan, Xiaodong; Miao, Xinxiang

doi:10.1088/1674-1056/25/10/108104

Cited by 3 publications

(1 citation statement)

References 16 publications

(22 reference statements)

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“…They found that these surface phased defects result in strong intensity modulation after some propagation distance. Downstream light-field modulation for mitigated glass has been the subject of research, and it was found that some mitigation structure would be harmful to the downstream optics [18,19] . Fused silica elements are traditionally placed after frequency converters made from KDP materials, which impose a lower relative laser damage threshold at these positions versus other components [5] .…”

Section: Introductionmentioning

confidence: 99%

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Yang

Cheng

Liu

et al. 2020

High Pow Laser Sci Eng

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To determine whether a potassium dihydrogen phosphate (KDP) surface mitigated by micro-milling would potentially threaten downstream optics, we calculated the light-field modulation based on angular spectrum diffraction theory, and performed a laser damage test on downstream fused silica. The results showed that the downstream light intensification caused by a Gaussian mitigation pit of 800 μm width and 10 μm depth reached a peak value near the KDP rear surface, decreased sharply afterward, and eventually kept stable with the increase in downstream distance. The solved peak value of light intensification exceeded 6 in a range 8–19 mm downstream from the KDP rear surface, which is the most dangerous for downstream optics. Laser damage sites were then induced on the fused silica surface in subsequent laser damage tests. When the distance downstream was greater than 44 mm with a downstream light intensification of less than 3, there were no potential damage threats to downstream optics. The study proves that a mitigated KDP surface can cause laser damage to downstream optical components, to which attention should be paid in an actual application. Through this work, we find that the current manufacturing process and the mitigation index still need to be improved. The research methods and calculation models are also of great reference significance for related studies like optics mitigation and laser damage.

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Section: Introductionmentioning

confidence: 99%

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Yang

Cheng

Liu

et al. 2020

High Pow Laser Sci Eng

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Secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface

et al. 2020

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Micro-milling has been proved to be the most effective method to mitigate the growth of laser-induced surface damage on potassium dihydrogen phosphate (KDP) crystals used in high power laser systems. However, the secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface would cause potential risk to damage downstream optics. In order to explore the effect of the mitigation pits on the secondary peak, we numerically calculated the downstream light field modulations caused by Gaussian mitigation pits on the rear KDP surface based on the angular spectrum diffraction theory. The results suggest that the secondary peaks are dependent on the parameters of the width, depth, depth error and title error. Among them, the tilt error and depth have greater influence on the mitigation effect. To reduce the laser damage risk caused by the secondary peak, the depth of the pre-designed mitigated contour should be optimized according to the actual operating conditions. The tilt error and depth error are proposed to be controlled within 1′ and 2 μm, respectively, during the micro-milling. Also, the experiments verified the calculation results of downstream modulations and the effects of these parameters on the secondary peak. This work can not only provide available models for evaluating the laser damage risk of secondary peak caused by mitigation pits on the KDP surface but also contribute to the development of optimal micro-milling parameters for laser damage mitigation as well as the installation strategy of optical components employed in the high power laser systems.

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Formation and control of bubbles during the mitigation of laser-induced damage on fused silica surface

Zhang¹,

Zhang²,

Chen³

et al. 2018

Acta Phys. Sin.

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Surface damage on fused silica optics initiated by high fluence 351 nm laser is one of the major bottlenecks for the high power laser systems, such as, Shenguang Ⅲ (SG-Ⅲ) laser facility. Generally, the CO2 laser, which is strongly absorbed by fused silica and thus can effectively heat fused silica above melting temperature, is used to locally mitigate the damages, called the non-evaporative mitigation method. However, subsurface bubbles may be introduced in the damage mitigation process by CO2 laser melting. Unfortunately, the mitigated damage sites with subsurface bubbles can be easily re-initiated upon subsequent laser shots. In this article, in order to eliminate the subsurface bubbles, we systematically investigate the influences of mitigation protocols in different ways of laser irradiation preheating on the formation and control of subsurface bubbles. Based on the simulated results of the temperature distribution and structural changes under CO2 laser irradiation, two CO2 laser-based non-evaporative mitigation methods are proposed, which are adopted for the mitigation of surface damage sites ranging in size from 150 m to 250 m, and systematically investigated to assess the effect of eliminating subsurface bubbles. The process of mitigation method I is that multiple laser irradiations with short time and increasing power are initially used to preheat the damage site and then a higher power laser irradiation is adopted to mitigate the damage site. The process of mitigation method Ⅱ is that a long time, low power laser irradiation is first used to preheat the damage site and then a high power laser irradiation is adopted to mitigate the damage site. The detailed morphologies of the mitigation sites and subsurface bubbles produced by the two mitigation methods are measured by optical microscope with high magnification. A large number of small subsurface bubbles are observed in mitigation method I. While, less subsurface bubbles are observed in mitigation method Ⅱ. The statistical results indicate that among the thirty-four mitigated sites, only eight have no surface bubbles in method I. In contrast, among the fifty-four mitigated sites, forty-nine have no surface bubbles in mitigation method Ⅱ. The experimental results suggest that the formation probability of subsurface bubbles is effectively suppressed by the mitigation method Ⅱ. The mechanism of eliminating subsurface bubbles in the mitigation method Ⅱ is discussed based on the structural changes of the fused silica in the mitigation process. It is found that the fused silica is not melted by the long time, low power laser irradiation, which means that a long time preheating without melting could provide enough time to effectively reject air and impurities enwrapping in cracks, and thus reducing the formation probability of subsurface bubbles in the form of the crack closing due to rapid melting. With the mitigation method Ⅱ, the probability of mitigated sites without subsurface bubbles can reach 98%.

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Influence of secondary treatment with CO ₂ laser irradiation for mitigation site on fused silica surface

Cited by 3 publications

References 16 publications

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface

Formation and control of bubbles during the mitigation of laser-induced damage on fused silica surface

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Influence of secondary treatment with CO 2 laser irradiation for mitigation site on fused silica surface

Cited by 3 publications

References 16 publications

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Potential damage threats to downstream optics caused by Gaussian mitigation pits on rear KDP surface

Secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface

Formation and control of bubbles during the mitigation of laser-induced damage on fused silica surface

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Influence of secondary treatment with CO ₂ laser irradiation for mitigation site on fused silica surface