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Abstract
Laser-induced damage of fused silica with different parameters was studied by means of photothermal spectroscopy and optical microscope. The development of damage area with laser parameters was discussed, and the photothermal absorption signal and its stability of the damaged pits were studied. It was found that the signal is stronger when the fused silica material was molten or broken. The laser damage area visibility ratio is defined. The change of the laser damage area visibility ratio with laser parameters is studied, and it is suggested that it can be used as a parameter to indicate the degree of damage in engineering.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fused silica is often used as the window glass for high power laser systems [1–6] because of its superior properties. However, the fused silica glass window could be damaged due to the high energy UV laser output. This limits the maximum energy output from the laser. Recent studies have shown that the morphology of laser-induced damage is related to the pulse wavelength [7]. Laser-induced damage (LID) usually occurs on the exit surface of optical elements, although it sometimes occurs on the incident surface. Mary A. Norton et al. studied two mechanisms of damage growth on the incident surface [8]. The exit surface damage of fused silica glass under high power laser irradiation has been extensively studied. And then under subsequent irradiation an irreversible fatal injury grows exponentially in the lateral size of the damage site on the exit surface [9]. Therefore, the research on laser-induced damage of fused silica is a key problem to improve the maximum output energy of large laser [10–14].
The LID properties of fused silica remain an important issue in high-power/energy laser systems. There are many factors for laser induced damage, such as impurities introduced during polishing [15–17] and mechanical fracture. Most impurities can be eliminated by acid leaching [18,19]. After mitigation process, the performance of fused silica optics exhibits a considerable increase [20]. However, there are still a large number of surface damage precursors in the fused silica optics [21–22]. Recent studies have investigated the morphology of laser-induced damage and the defects induced by the manufacturing processing of the components (polishing, chemical etching, structuring, etc.) [23–24].
Photothermal techniques have been shown to be a useful tool for the nondestructive characterization of fused silica optics [25–30]. Photothermal technology is based on the phenomenon of photothermal effect. In this process, the sample irradiated by laser absorbs laser energy and produces various physical responses due to local heating. These physical responses are detected by another weak laser (probe laser). By analyzing the results of the probe laser, the absorption of the sample is measured and analyzed. Compared with other absorption measurement methods, photothermal technology has obvious advantages such as high sensitivity (< 10 ppb), high spatial resolution (< 1 micron), non-contact, non- destructive and so on. The instrument can detect the maximum damage depth of 10 mm. Transverse damage dimensions of 100mmX100 mm can be probe with reliable measurements. In previous studies, the dependence of photothermal absorption signal on pump laser parameters has been reported, while the dependence the damage degree on the damage laser parameters has not been reported. The development of LID has been studied by means of optical microscopy. But there is no comparison between the two methods to describe the development of LID.
In this paper, the influence of laser parameters on the photothermal absorption signal has been discussed, and the damage area development of LID of fused silica is further studied. At last we have defined the laser damage area visibility ratio (LDAVR), which describes the comparison the area obtained by photothermal spectroscopy and optical microscopy. The parameters LDAVR that may be used in engineering are suggested. And these investigations have not been reported.
2. Sample manufacturing and experimental details
Detailed descriptions of sample preparation have been discussed in our previous article [31]. UV-grade Heraeus Suprasil S312 high-quality optical fused silica samples were used in this experiment. It was divided into several block samples with a dimension of 20 mm×20 mm×10 mm. All six surfaces of the sample are polished by a conventional pitch-polishing process with CeO2. The Nd:YAG laser used in the experiment can operate at three wavelength. Dichroic mirrors are used to ensure that the laser irradiated on the sample is of a single wavelength. In the experiment, the incident laser is divided into two beams by a spectroscope, and measured by two energy meters. The ratio of the energy of the two beams is calculated. Then the energy value of the irradiated sample is calculated by using the value of one of the energy meters. The beam is focused on the sample through a convex lens with a focal length of 5 meters. The size of the laser spot is about 0.22 mm in diameter on the sample surface. The length of laser pulse measured by oscilloscope is about 6.8 ns. The fact that multiple longitudinal modes may exist in the laser cavity is known to influence by a lot the damage phenomena [32,33]. But the impact is not taken into account here. 355 nm, 6.8 ns UV pulsed laser was used in radiation damage test. The laser emission frequencies were 1, 2, 5, and 10 Hz, respectively. The laser irradiation fluences vary from 0.1 to 45 J/cm2, and the numbers of shots were from 1 to 100. In the experiment, there are 16 irradiation damage points on each sample.
A commercialized photothermal measuring system (PTS-2000) was used to characterize the photothermal absorption of fused silica before and after irradiation damage. Figure 1 shows the sketch of the experimental measurement of the photothermal absorption signal of fused silica damage by 355 nm, 6.8 ns UV pulsed laser. In the experiment, the samples were placed on the sample stage which can move in x, y and z direction. 2.5 W 355 nm laser was used as pump laser beam and 5 mW CW He-Ne laser was used as probe laser beam in our experiments. The probe laser is at an angle of 30° to the surface of the sample, as shown in the figure.
Fig. 1. Sketch of the photothermal spectroscopy measurement for fused silica irradiated by a UV laser.
Irradiated by a 355 nm UV laser with a given laser intensity, the absorption defect in the fused silica sample absorption energy, resulting in a nearby neighborhood temperature, thereby making refraction rate changes. Using the probe laser, we can measure the photothermal absorption signal. Therefore, we can characterize the material structure of the fused silica by using the measured photothermal absorption signal, and obtain the change information of the damaged fused silica samples by different conditions. The sensitivity is 10 ppb, the spatial resolution is 10 microns in this experiment.
3. Results
In order to understand the effect of laser parameters on the laser induced damage, we investigated laser induced damage of fused silica irradiated by different parameter laser pulses. In the LID experiment, fused silica samples were irradiated by a UV laser. The size of damage sites in the longitudinal and lateral direction measured by high quality transmission and reflection polarizing microscope (DYP-702) is influenced by several major factors. Figure 2 shows the dependence of the size of damage sites on the damage laser parameters. Figure 2(a) shows that with increasing laser fluence when fixed 1 Hz and 1shot, the size of the damage site increases gradually. The laser-induced damage threshold of the sample pumped by 355 nm laser pulses is 3.9J/cm2 as indicated by the arrow in the figure. However, owing to the small laser focal spot, the size of the damage site would reach a saturation value when the laser fluence increased to a certain value [34]. This is due to the aperture effect. Figure 2(b) shows that with the increasing of shot number when fixed 1 Hz and 4.5J/cm2, the size of damage sites gradually increases with the accumulation of laser energy absorption. The transverse size of the damage spot increases with the number of pulses in an accelerated way which is well approximated by an exponential function. There is little research on the change of laser damage size with laser frequency. Figure 2(c) shows that the size of the damage sites slightly decreases with increasing pulse repetition frequency (PRF) when fixed 50shots and 4.5J/cm2. This may be due to a slight decrease in the size of the laser spot emitted by the laser as the frequency increases. As can be seen from Fig. 2, the size of damage sites in the longitudinal and lateral direction changes similarly with the change of a single laser parameter.
Fig. 2. Dependence of the size of damage sites on the damage laser parameters by optical microscope: a) laser fluence (1 Hz & 1shot), b) number of shots (1 Hz & 4.5J/cm2), c) PRFs (50shots & 4.5J/cm2). The size of each damage point was measured four times to get the average value. The uncertainty of the results is less than 0.01.
3.1 Influence of laser fluence
In nanosecond regime, the development of the exit surface damage of fused silica optics induced by small spot 355 nm laser initially presents an exponential growth with laser fluence and then tends to saturation at high fluence as shown in Fig. 2(a). So far, there is no satisfactory explanation for these results. For this reason, we have studied the change of the photothermal absorption intensity of the damage site with the laser fluence by photothermal technique.
In the experimental measurement of the photothermal absorption intensity of the fused silica samples irradiated by a 355 nm laser with different fluence. The effect of the laser fluence on photothermal absorption intensity is shown in Fig. 3. When the laser fluence is less than 3.9 J/cm2, the fused silica sample is not damaged, and the photothermal absorption intensity is the same as before irradiation. When the laser fluence is greater than 3.9 J/cm2, the damage of fused silica samples occurs, and the photothermal absorption intensity increases with the increase of laser fluence. When the laser fluence reaches 4.5 J/cm2, a splash appears on the exit surface of fused silica sample. At this time, the photothermal absorption intensity decreases with the laser fluence because the pit surface is lower than before. When the laser fluence reaches 9.1 J/cm2, cracks appear around the pits on the exit surface of fused silica sample, and a strong photothermal absorption signal appears in the damage site. When the laser fluence is more than 22.7 J/cm2, the melting pits around the fracture region appear on the exit surface of fused silica sample, and the photothermal absorption intensity of the central melting region is lower than that of the undamaged one. There may be two reasons for this phenomenon, one is that the central region of the pit is lower than the periphery due to splashing, and the other is that the central melting region has better structure due to recrystallization after melting, which leads to the decrease of absorption. Previous literatures have also reported that subsurface microcracks and surface defects can lead to enhanced absorption [35,36].
Fig. 3. Influence of the damage laser fluence on photothermal signals (@1 Hz and 1 shot): a) 3.9 J/cm2, b) 4.1 J/cm2, c) 4.5J/cm2, d) 6.8J/cm2, e) 9.1 J/cm2, f) 13.6 J/cm2, g) 22.7 J/cm2, h) 45.4 J/cm2.
The increasing laser fluence of single shot leads to the instantaneous absorption of laser energy in fused silica. With an increase in laser fluence, the laser energy absorbed by fused silica increases, which results in an increase in the internal broken bonds in fused silica. Therefore, the photothermal absorption signal is gradually enhanced.
With an increase in laser fluence, the absorbed energy increases, and the damage area will be enlarged. Considering that the spot of the Gauss pulse laser used in our experiment is circular, and energy transfer in fused silica is also isotropic for fused silica is an isotropic amorphous material, the LID site shows an isotropic pattern in the horizontal and vertical dimensions.
3.2 Influence of the number of pulses
High quality optics have low density of defect precursors. When irradiated by a low fluence laser, the absorption will remain constant with the increase of the number of shots. Therefore, the effect of some micro-cracks on the performance of optical elements is not obvious. However, when the laser fluence is up to a certain value, the damaged area of the material rapidly increases with irradiation by subsequent continuous pulse and finally exceeds a critical value; as a result, this leads to the breakdown of the material.
Figure 4 shows the effect of the pulse number on photothermal absorption signals. When the number of laser pulses is from 1 shot to 10 shots, the area of the damaged area is small. As the energy is absorbed by the material, laser-induced defects in materials increase gradually., the size of the damage area becomes large, and the photothermal absorption signals intensity of the damage pit gradually increase. When the number of laser pulses is greater than 20 shots, the increased thermal pressure enables internal damage of the material that results in breakage. The high temperature and pressure inside the fracture area enable the internal material to break when the deposition energy in the internal material increases to a certain value. As fracture increases, the exit surface of the sample appears splash, thereby leading to dishing and rough. Therefore, the photothermal absorption signal in central regions is not received, as the result shows, the phenomenon of photothermal absorption signal intensity is lower than the undamaged areas.
Fig. 4. Influence of the number of shots on photothermal signals (@1 Hz and 4.5J/cm2): a) 1 shot, b) 10 shots, c) 20 shots, d) 40 shots, e) 50 shots, f) 100 shots
3.3 Influence of PRFs
Figure 5 depicts the PRF dependence of the 2D image of photothermal absorption signal. With increasing laser PRF, the size of damage pit decreases slightly as shown in Fig. 2(c). The possible reason is that the area of laser spot decreases slightly due to the increase of PRF. Because the laser needs to work at a stable frequency to achieve thermal balance, especially for the third-harmonic laser wavelength, the thermal balance state is inconsistent at different operating frequencies, and the beam quality will change accordingly. The corresponding laser beam is no longer the same 0.22 mm in diameter, but slightly smaller with the increasing PRF. As a result, the damage area decreasing trend with the increase of working frequency is resulted from the decreasing spot size. In Fig. 5, the intensity of the photothermal absorption signal decreases with the increase of PRF.
Fig. 5. Influence of the PRFs on photothermal signals (@50 shots and 4.5 J/cm2): a) 1 Hz, b) 2 Hz, c) 5 Hz, d) 10 Hz
For single laser irradiation, the effect of a slight decrease in laser spot can be neglected, but with the increase of the number of laser spots, the effect of a slight decrease in laser spot will gradually increase. When 50 laser beams were irradiated in this experiment, the results have been reduced a little. Consequently, the damaged area slightly decreases with the frequency increasing, as shown in Fig. 2(c). When the sample is irradiated by 50 shots laser, the central melting region around the surrounding fracture region appears in the damage pit. Considerable fracture defect also leads to a strong photothermal absorption signal. These findings are consistent with the experimental results.
Figure 6 depicts the stability of photothermal absorption signal in different types of regions with 2.5 W 355 nm laser as a pump laser. Figure 6(a) shows the undamaged area; the internal structure of the fused silica material herein is intact, and thus the photothermal absorption signal stability will not change over time. Figure 6(b) shows the sample with contaminants on the surface. The contaminants may be organic matter, which absorbs energy and evaporates under laser irradiation. Consequently, the absorption signal suddenly increases, then decreases, and stabilizes at a certain value that is equivalent to the photothermal absorption signal intensity at the undamaged area. The internal structure of the sample material is not destroyed. Figure 6(c) shows the pre-damaged area. The laser-induced defects appear inside the fused silica material, which rapidly absorb large amounts of energy. Hence, the photothermal absorption signal suddenly increases and the value gradually stabilizes because of the relaxation effect. This value is about three times of the photothermal absorption signal strength in the undamaged area. Figure 6(d) shows the photothermal absorption signal in the area of physical damage. In the radiation damage experiment, the splash occurs on the material surfaces. The center of the surface damaged area is lower than the surrounding area, and the surface becomes rough. The photothermal absorption signals cannot be obtained in this area.
Fig. 6. Stability of the photothermal absorption signal dependence on 2.5 W 355 nm laser in different damage sites: a) undamaged, b) contaminant existed, c) pre-damaged, d) damaged
Figure 7 shows the relationship between the peak of photothermal absorption signals and the laser parameters. Unirradiated fused silica sample has low absorption signal strength and good stability. When the laser fluence irradiated on the sample is far below the damage threshold (3.9J/cm2), the photothermal absorption signal of the sample remains unchanged. With an increase in laser fluence, the laser-material interaction is enhanced. Accordingly, the laser energy absorbed by the material will increase the temperature of the material, which will lead to the fusion phenomenon. In this case, the stability of the photothermal absorption signals of the material decreases, and the pre-damage phenomenon occurs. After the relaxation effect, the molten material is recrystallized. Owing to the multiphoton absorption, the photothermal absorption signal peak of the material present a sharp peak, as shown by the first peak in Fig. 7a). As the laser fluence increases, the temperature increases to a certain extent. Consequently, the thermal stress of the material increases, and the material undergoes thermal explosion or dissociation. With the fracture of materials, the photothermal absorption signals of the material increases, which lead to a physical damage phenomenon to the material, as shown by the second peak in Fig. 7(a) and the peak in Fig. 7(b). Figure 7(b) shows the photothermal absorption signal peak of the material changes with the laser pulse number. With the accumulation of laser energy absorbing, the material physical damage occurs when the irradiated material reaches 50 times. Figure 7(c) shows the photothermal absorption signal peak change with PRFs. With increasing frequency, the peak of the signal decreases and reaches a constant value.
Fig. 7. Photothermal absorption signal intensity dependence on the laser parameters: a) laser fluence, b) number of pulses, c) PRFs. The intensity of the photothermal absorption signal is the peak value of the experimental data obtained by the photothermal testing instrument.
The sizes of laser damage pits were studied with the variation of laser parameters as in Fig. 2. Therefore, the photothermal spectroscopy method is adopted for comparative study. Figure 8 shows the comparison of the morphology and area of damage pits with different damage degrees measured by an optical microscope (OM) and photothermal (PT) spectroscopy. Figure 8(a) is the morphology of the damage pits irradiated by 1shot UV laser with a given laser intensity of 4.2 J/cm2 obtained by the optical microscope test, Fig. 8b) is the result of the damage pits measured by photothermal spectroscopy method. By comparing Figs. 8(a) and 8(b), we can clearly observe from the test results that the photothermal spectroscopy technology has more obvious advantages than the optical microscope test. Optical microscopy results can only measure visible damage of materials, it can’t measure the damage below the surface, the area of the subsurface damage. Whereas photothermal spectroscopy method can also measure the subsurface damage of the material. Figure 8(c) is the morphology of the damage pits irradiated by 50shots 355 nm 6.8 ns UV laser with a given laser intensity of 4.2 J/cm2 obtained by the optical microscope test, Fig. 8(d) is the corresponding result of the damage pits in Fig. 8(c) using the photothermal spectroscopy method. Compared to optical microscope method, we found that the area of damage craters can be more precise described by photothermal spectroscopy before the fused silica material was molten.
Fig. 8. Comparison of the morphology of the damage pits irradiated by a 355 nm 6.8 ns UV laser with a given laser intensity of 4.2 J/cm2 measured by OM and PT: a) OM-1 shot, b) PT-1 shot, c) OM-50 shots, d) PT-50 shots.
From the above results, the photothermal absorption signal on the damage area is related to the number of Si-O bond broken inside the silica sample. In our other work, it has been proved that the Si-H bond and interstitial O2 generated in the material after the Si-O bond broke in the region where the photothermal absorption signal was strong [31]. With increasing laser fluence the material absorbing the laser energy increases when the laser fluence is below the LIDT. The number of broken Si-O bond also increases, and therefore the intensity of light and heat absorption signal increases. When the laser fluence is higher than the LIDT, as laser fluence increases, the material of the central damaged region is melted and recrystallization occurs after relaxation. In the process of the material absorbing significant heat, in-situ measurement should obtain high photothermal absorption signals. However, owing to the sputtering generated during irradiation, the surface of the central region is lower than the peripheral region, and the surface becomes rough, resulting in low photothermal absorption signals. The large amount of material damage near the peripheral area occurs with a large number of Si-O covalent bonds broken. Therefore, the photothermal absorption signal around the peripheral area presents higher values than the melted center of the area.
With an increase in the laser fluence, the damage to the pit size becomes large, and the photothermal absorption signals become strong. However, when the laser fluence reaches a certain value, the damage pit tends to a stable value that the damaged laser focal spot is small. With increasing laser frequency and the gradual accumulation of absorbed energy in the material, the photothermal absorption signal intensity in the damaged increases and pit size in damaged area also increases. The irradiation process produces a low damage surface in the central region, and high surface roughness. When the material melts and recrystallization occur, the photothermal absorption signal strength in the center area is lower than in the surrounding fracture area and even lower than in the undamaged area. The photothermal signal intensity from the damaged area and the size of the damaged area both decrease with increasing laser emission frequency.
The comparison results of the area of multiple sets of damage pits measured using optical microscopy and photothermal microscope dependence on the laser parameters are shown in Fig. 9. By comparing the results in Fig. 9, it illustrates that before the central melting damage area surrounded by the fracture region occurs, the visual damage area accounts for less than 50% of the total damage area; after a central melting damage area surrounded by the fracture region occurs, the visual damage area accounts for about 80% of the total damage area. Therefore, the ratio of visible damage area to total damage area is significant for damaged fused silica optical elements. As far as we know, this result has not been reported in the previous work.
Fig. 9. Comparison of the area of the damage pits measured by OM and PT: a) laser fluence (1 shot-1 Hz), b) the number of shots (4.2 J /cm2-1 Hz), c) PRF (4.2 J /cm2-50 shots), d) PRF (18 J/cm2-10 shots). The areas of each damage point were measured four times to get the average value. The uncertainty of the results is less than 0.03.
4. Discussion
Firstly, the variation of laser damage size with laser parameters was obtained by means of optical microscope. Similar results can be obtained by using photothermal spectroscopy. The difference of the morphology and photothermal image of the edge of the damage crater is caused by subsurface microcracks and the laser induced defect absorption in the damage crater. The different types and quantity distribution of the defects lead to the difference of the photothermal absorption intensity. As for the reproducibility and stability of the result, the same results were obtained using another photothermal spectrometer. The difference of signal variation under different laser parameters in Fig. 3, Fig. 4 and Fig. 5 is due to the different number and distribution of subsurface microcracks and laser-induced defects under different laser parameters. Since we are discussing the area visibility ratio, which is an available macroscopic parameter in engineering, only refers to area itself, not to the number and distribution of subsurface microcracks and laser-induced defects. We consider a crater as a whole, regardless of the difference of signal changes caused by the number and distribution of defects in the crater. We find that the ratio of the area obtained by the two methods is regular. Therefore, we define such a ratio to study the law of damage development. We defined the LDAVR, which refers to the ratio of area measured by optical microscope and photothermal microscope. The LDAVR of fused silica irradiated by 355 nm 6.8 ns laser varies with laser parameters are shown in Fig. 10.
Fig. 10. The LDAVR varies with laser parameters: a) laser fluence, b) the number of shots, c) PRF. The ratio is obtained by the ratio of the average area measured by two different methods.
There is no doubt that the higher the laser fluence, the more severely the material is damaged shown in Fig. 10(a). After reaching a certain extent, the ratio no longer increases, which is the saturation effect. The damage degree of the material becomes more serious with the increase of the laser shot number shown in Fig. 10(b). But after a certain amount of shots, the laser energy can no longer be transmitted outward, when it is saturated. When the laser fluence is 4.2 J/cm2 and the number of laser shots is bigger than 40, the ratio reaches saturation. Similar experiments have been carried out with another instrument of the same type from National University of Defense Technology. The laser fluence used in laser irradiation damage is 18 J/cm2. Other laser conditions are the same as those of previous experiments. The experimental results on the red curves in Fig. 10(b) and 10(c) are shown. While the laser fluence is 18 J/cm2 and the number of laser shots is bigger than 25, the ratio reaches saturation. Therefore, the larger the laser fluence, the easier the LDAVR can reach saturation. Because the spots of the laser are given. If the spots are larger, the size of the damage pit would be large. But the ratio doesn’t get any bigger because it has been already saturated. In fact, the effect of aperture effect is excluded in this case. When the ratio reaches 0.8, the optical element has failed. The LDAVR slightly decreases with the increase of laser frequency shown in Fig. 10(c), and the ratio is about 0.8 when the LDAVR reaches saturation. When the laser fluence is 4.2 J/cm2 and the number of laser shots is 50, the ratio is still around 0.8 because the LDAVR reaches saturation when the number of laser shots is 40. But there is little difference between different laser frequencies. While the laser fluence is 18 J/cm2 and the number of laser shots is 15, the ratio reaches to about 0.5. Results reveal that before the fused silica material molten, the ratio is less than 0.5; after the fused silica material molten, the ratio is larger than 0.5. Therefore, we use 0.5 as the criterion of laser damage degree. The sample we selected is that used of the laser in our laboratory, i.e. UV-grade Heraeus Suprasil S312 high-quality optical fused silica samples mentioned earlier. Sample preparation before irradiation damage experiment is also polished with the same standard, which makes the surface roughness stable at 0.3 nanometer. Other manufacturing processes can be further studied. In a word, the LDAVR increases with the increase of laser fluence. The most significant effect on the LDAVR is the number of laser shots. The effect of laser frequency on it can be negligible.
5. Conclusion
In summary, using the photothermal spectroscopy technique and optical microscopy method, the photothermal absorption signal and its stability of the damaged pits were studied and the size of the damage sites are successfully characterized and analyzed. It was found that the signal is stronger when the fused silica material was molten or broken. This shows that the change has taken place when the fused silica material was molten or broken. It was investigated that the laser irradiation damage in fused silica under 355 nm 6.8 ns laser with different laser parameters, the relationship between damage pits’ area and different laser parameters, such as the laser fluence, the number of shots and PRFs. We defined the LDAVR, and found that there would be a melting pit around the fracture region on the surface of irradiated fused silica when the ratio reached to 0.8. In our study, we believe that before the ratio reaching 0.5, it is valuable to take the optical element off the high-power laser systems for repair. To prolong the service life of bulk fused silica optical element and to reduce repair costs, we should repair the optical element before the ratio reaching 0.5. If the ratio exceeds 0.5, the laser damage size will increase exponentially and a central melting damage area surrounded by the fracture region will appear on the exit surface of the material. The optical element will fail when the ratio reached to 0.8. If the optical element is to be repaired it must be before the LDAVR reaches this value. So, we suggest that the LDAVR defined by the combined photothermal spectroscopy technique and optical microscope method can be used as the parameter in engineering for repairing fused silica optical elements.
Funding
National Natural Science Foundation of China (NSFC) (51402173); Fundamental Research Funds for the Central Universities (FRF-TP-15-099A1).
Acknowledgments
We acknowledge that Dr. Wenyong Cheng of Shandong University for their help during the preparation of samples, Professor Zhouling Wu of ZC Optoelectronic Technologies, Ltd. and Dr. Feng Shi of National University of Defense Technology for the photothermal absorption characteristic measurement of the damaged fused silica.
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