Mitigated Sites Research Articles

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

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%.

Effect of the repaired damage morphology of fused silica on the modulation of incident laser

Local CO2 laser treatment has proved to be the most promising method to extend the life-time of fused silica. However, previous experimental data show that some raised rims are observed around the mitigated sites left from the mitigation process, which will result in hazardous light modulation to the downstream optics. In this work, the morphology features of mitigated sites on the surface of fused silica optics were analyzed in detail. According to measured morphology features, a 3D analytical model for simulating the modulation value induced by mitigated site has been developed based on the scalar diffraction theory. The diffraction patterns at a discrete distance downstream from each mitigated site are measured. The influences of geometry, laser wavelength and refractive index of substrates on the modulation by repaired damage morphology at different distances are discussed, respectively. The analytical model is usable and representative to evaluate the hazardous modulation induced by repaired damage morphology to downstream optics. Results on this research suggest that the downstream intensification can be suppressed by controlling the morphology features of mitigated sites, which provides a direction for the development and improvement of the mitigated techniques of damage optics.

Influence of secondary treatment with CO2 laser irradiation for mitigation site on fused silica surface**Project supported by the National Natural Science Foundation of China (Grant Nos. 61505170, 61505171, and 51535003), the Joint Fund of the National Natural Science Foundation of China, the Chinese Academy of Engineering Physics (Grant No. U1530109), and the China Postdoctoral Science Foundation (Grant No. 2016M592709).

The ablation debris and raised rim, as well as residual stress and deep crater will be formed during the mitigation of damage site with a CO2 laser irradiation on fused silica surface, which greatly affects the laser damage resistance of optics. In this study, the experimental study combined with numerical simulation is utilized to investigate the effect of the secondary treatment on a mitigated site by CO2 laser irradiation. The results indicate that the ablation debris and the raised rim can be completely eliminated and the depth of crater can be reduced. Notable results show that the residual stress of the mitigation site after treatment will reduce two-thirds of the original stress. Finally, the elimination and the controlling mechanism of secondary treatment on the debris and raised rim, as well as the reasons for changing the profile and stress are analyzed. The results can provide a reference for the optimization treatment of mitigation sites by CO2 laser secondary treatment.

Effect of thermal annealing on damage growth of mitigated site on fused silica

Residual stresses will be formed around the mitigated site after the damaged site is irradiated by 10.6 m CO2 laser. Using those mitigated sites can improve the damage resistance ability in optics, and once the reinitiating damage occurs, the damaged site will grow under the subsequence irradiation and large fracture may form around the mitigated site. In this study, the annealing temperatures 650, 750 and 850 ℃, and time durations 6, 8, 10 and 12 h are used to anneal the samples. The sample annealed at 750 ℃ is the main research object of this study, while the sample annealed at 650 ℃ or 850 ℃ is only treated for 10 h. The differences of damage growth morphology and velocity of mitigated site on fused silica treated under those annealing conditions are investigated when it is damaged once again. Results are also compared with the damage growth behaviors of the unannealed substrate and mitigated site. It is indicated that the damage growth data still fit to an exponential curve even for the unannealed mitigated site. However, for the unannealed mitigated site, a more serious and larger size of damage site will be formed when the reinitiating damage occurs. It is mainly attributed to the fast propagation of crack under the effect of residual stress around the mitigated site. This behavior can be effectively controlled by the annealing treatment. Results show that the crack propagation behavior can be avoided when the retardation of mitigated sites is controlled in the range of 25 nm; moreover, the damage growth velocity and coefficient will gradually decrease with the increase of the annealing duration and annealing temperatures. A notable result indicates that there is no difference between the mitigated site and substrate when the retardation of mitigated sites is controlled below 10 nm, especially for the samples treated at 750 ℃ for 12 h and 850 ℃ for 10 h. Moreover, the reported investigation indicates that the stresses can still improve the damage resistance ability in optics. This is the most desirable outcome of the annealing treatment. Thus, the investigation results can provide a reference on how to analyze the effect of stress on damage growth of mitigated site and optimize the annealing parameters.

Effect of residual stress on laser-induced damage characterization of mitigated damage sites in fused silica

The influences of residual stress on damage morphology and laser-induced damage threshold (LIDT) of mitigated sites in fused silica optical components before and after annealing are investigated. The results indicate that the cracks occur either at the inner location or at the maximum retardation location of mitigated sites once the residual stress (retardation) exceeds 26.49±1.79MPa (22.25±1.5nm). Meanwhile, the discrepancy of stress at different locations results in the fluctuation of LIDTs. The results of isochronal and isothermal annealing indicate that annealing temperature is the most important influence factor and the residual stress can be effectively controlled if the residual stress induced by the maximum CO2 laser beam is eliminated. Furthermore, the LIDT of mitigated sites can be enhanced without dramatical fluctuation and the damage growth coefficient that is sharply increasing can be avoided if the retardation of mitigated site is equal to or less than 5nm. The study demonstrates that annealing significantly improves the resistance of fused silica optics.

Characterization of 355 nm laser-induced damage of mitigated damage sites in fused silica

The laser-induced damage threshold (LIDT) is one of the important methods to determine the mitigation efficiency of mitigated damage sites in fused silica optics. In this work, two sizes of laser beam with a wavelength of 355 nm and a pulse width of 6.3 ns are used to test the LIDTs and properties of mitigated damage sites. Meanwhile, the R-on-1 average threshold and threshold distribution are used to analyze damage behavior of mitigated sites. It is found that the R-on-1 average threshold shows an S-shaped curve and the threshold distribution obeys a normal law for the mitigated damage sites, which is similar to the law of pristine substrates reported before. The LIDT discrepancy of mitigated sites tested with different sizes of laser beam is attributed to the spot-size effect of the test laser beam, i.e., the LIDT of the mitigated site increases with decreasing test laser spot size. It is shown that most damage occurs at the edge of the laser affected zone, which could be ascribed to the weak location between laser affected and unaffected zones. Finally, the correlations between damage size and the laser fluence which causes damage initiation or damage growth are investigated for the pristine substrate and mitigated sites, and can be fitted by an exponential law.

Two localized CO2 laser treatment methods for mitigation of UV damage growth in fused silica

Two methods: high-power, short-time, single-shot irradiation (Method A) and low-power, long-time, multi-shot irradiation (Method B) are investigated to mitigate the UV damage growth in fused silica by using a 10.6-μm CO2 laser. To verify the mitigation effect of the two methods, the laser induced damage thresholds (LIDTs) of the mitigated sites are tested with a 355-nm, 6.4-ns Nd:YAG laser, and the light modulation of the mitigation sites are tested with a 351-nm continuous Nd:YLF laser. The mitigated damaged sites treated with the two methods have almost the same LIDTs, which can recover to the level of pristine material. Compared with Method A, Method B produces mitigated sites with low crater depth and weak light modulation. In addition, there is no raised rim or re-deposited debris formed around the crater edge for Method B. Theoretical calculation is utilized to evaluate the central temperature of the CO2 laser beam irradiated zone and the radius of the crater. It is indicated that the calculated results are consistent with the experimental results.

Mitigation of laser damage growth in fused silica by using a non-evaporative technique

A non-evaporative technique is used to mitigate damage sites with lateral sizes in a range from 50 μm to 400 μm and depths smaller than 100 μm. The influence of the pulse frequency of a CO2 laser on the mitigation effect is studied. It is found that a more symmetrical and smooth mitigation crater can be obtained by increasing the laser pulse frequency form 0.1 to 20 kHz. Furthermore, the sizes of laser-affected and distorted zones decrease with the increase of the laser pulse frequency, leading to less degradation of the wave-front quality of the conditioned sample. The energy density of the CO2 laser beam is introduced for selecting the mitigation parameters. The damage sites can be successfully mitigated by increasing the energy density in a ramped way. Finally, the laser-induced damage threshold (LIDT) of the mitigated site is tested using 355 nm laser beam with a small spot (0.23 mm2) and a large spot (3.14 mm2), separately. It is shown that the non-evaporative mitigation technique is a successful method to stop damage re-initiation since the average LIDTs of mitigated sites tested with small or large laser spots are higher than that of pristine material.

The Structure Evolution of Fused Silica Induced by CO2 Laser Irradiation

The structure evolution of fused silica induced by CO2 laser irradiation (with a wavelength of 10.6 μm) is studied in detail. In the non-evaporation mitigation process, the irradiation time should be long enough to completely eliminate damage. However, there is a raised rim around the mitigated site. The rim height is enhanced when the irradiation time increases, and the mitigated site can lead to off-axis and on-axis downstream light intensification. Volume shrinkage occurs during the irradiation and rapid cooling processes, and this may be due to a decrease in the Si—O—Si bond angle. The distribution of debris overlaps with the maximum phase retardance induced by stress. The debris arouses an enhanced light absorption in the region from 220 nm to 800 nm.