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Abstract
The absorption coefficient of fused silica for a mid-infrared (IR) laser is higher than that for a near-IR laser, but smaller than that for a far-IR laser. Therefore, the energy coupling efficiency of the mid-IR laser is higher than that for the near-IR laser, while the penetration depth is higher than that for the far-IR laser. Thus, the mid-IR laser is highly efficient in mitigating damage growth. In this study, a deuterium fluoride (DF) laser with a center wavelength of 3.8 µm was used to interact with fused silica. The temperature variation, changes in the reflected and transmitted intensities of the probe light incident on the laser irradiation area, and the vaporization and melting sputtering process were analyzed. The results demonstrate that when the laser intensity was low (<1.2 kW/cm2), no significant melting was observed, and the reflection and transmission properties gradually recovered after the end of the laser irradiation process. With a further increase in the laser intensity, the sample gradually melted and vaporized. At a laser intensity above 5.1 kW/cm2, the temperature of the sample increased rapidly and vapors in huge quantity evaporated from the surface of the sample. Moreover, when the laser intensity was increased to 9.5 kW/cm2, the sample melted and an intense melting sputtering process was observed, and the sample was melted through.
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1. Introduction
The transmittance of fused silica glass is high over a wide range of wavelengths, from near-ultraviolet to near-infrared (IR). Moreover, it has good chemical and thermal stability. Hence, fused silica is widely used in the manufacture of optical instruments (e.g., lenses, prisms, and windows), mechanical systems, and laser systems. In large laser systems that are set up at the National Ignition Facility (NIF) in the United States, the Laser Megajoule (LMJ) facility in France, and the Shenguang (SG) laser facility in China, the laser damage resistance of optical components restricts the development of lasers generating high energy/power. The threshold of laser damage is significantly lower than the intrinsic threshold of the material owing to the defects in the bulk and surface/subsurface of optical elements [1–3]. In particular, the grinding and polishing processes form defects such as dents, scratches, microcracks, and impurities on the surface/subsurface of the components. These defects can induce the following effects as described in [4,5] : 1) the modulation of the light field (local light amplification), 2) increased absorption of the laser energy, and 3) reduced mechanical strength of the material. Therefore, numerous studies have focused on examining the mechanism of the laser-induced damage in optical materials to improve the damage threshold and propose methods for damage repair.
The two major types of the laser-based mitigation approaches are the non-evaporative method [6] and the evaporative method [7]. The first method involves the usage of a laser to heat the material till it attains a state that lies between the melting and evaporation points. The damaged area is healed by the melting and reflowing processes. The second method is used to heat the material above the evaporation point to evaporate the damaged material. A CO2 laser with a wavelength of 10.6 µm is commonly used for laser mitigation. High absorption coefficients are obtained at the wavelengths that are approximately equal to the stretching vibration frequency of the Si-O bond of fused silica. Hence, high laser energy can be absorbed by the material causing it to melt and evaporate [8]. However, the maximum healing depth that can be attained with the non-evaporative method using the 10.6 µm laser is limited to 200 µm [9] . In contrast, a mid-IR laser with a lower absorption coefficient can be used to mitigate the damage at sites with deeper subsurface cracks. Guss et al. [9] used a mid-IR laser with a wavelength of 4.6 µm to mitigate the damage at sites measuring 500 µm in diameter and 200 µm in depth. By implementing a suitable mitigation strategy, the damage threshold in the mitigated region reached 25 J/cm2 with a shallower mitigation dent than that obtained with 10.6 µm laser mitigation. Steven et al. [10,11] compared site damage mitigation by using a 4.6 µm mid-IR laser and a 10.6 µm far-IR laser. The results demonstrated that the mitigation for the damage at sites with cracks up to a depth of 500 µm was more effective (rating is obtained by dividing the required laser power with the crack healing depth) with a 4.6 µm laser than that with a 10.6 µm laser. Matthews et al. [12] showed that by using the non-evaporative method, the mid-IR laser was more effective because a depth of approximately 1 mm could be reached for laser energy coupling. The absorption coefficient of the deuterium fluoride (DF) laser with a center wavelength of 3.8 µm is lower than that of the 4.6 µm laser. The corresponding penetration depth attained at room temperature is approximately five times that with the 4.6 µm laser. This indicates that the DF laser can be effectively used to heal deeper subsurface cracks. Hence, this research focused on studying the interaction of a 3.8 µm laser with fused silica owing to its great importance.
In this study, a DF laser was used to interact with fused silica. This study focused on the temperature rise, reflection, and transmission characteristics. Additionally, the vaporization and melt sputtering processes at different laser intensities were investigated.
2. Experimental setup
The experimental setup is demonstrated in Fig. 1. A DF laser was focused on the sample after reflection using a mirror. A small portion of the incident laser transmitted through the mirror and was received at the diffuse reflection screen. The PbSe detector (sensitive to the wavelength ranging from 1.5 µm to 4.8 µm) was used to detect the temporal distribution of the DF laser. A probe laser with a wavelength of 532 nm was focused at the DF laser spot on the sample surface. The probe beam was reflected and refracted onto the sample surface. Both the reflected and transmitted beams were detected using the Si detectors. A narrow-band filter with a pass wavelength of 532 nm was used to eliminate the influence of the light emission induced due to laser heating. The light intensities received by the two Si detectors were affected by thermal deformation, melting, vaporization etc., caused by DF laser irradiation. The IR radiation pyrometer KMGA 740-USB system (emissivity 0.1–1 adjustable, response time of 6 µs, minimal sampling interval time of 10 µs, spectral range of 2–2.5 µm, and temperature measuring range of 350–3500 °C) was used to measure the temperature evolution of the sample surface. The spot center on the sample surface measured 150 µm in radius and was comparatively smaller than the spot size of the DF laser. Hence, the measured result can be considered as the temperature evolution related to the spot center of the DF laser. A high-speed camera (GigaView, Southern Vision Systems, Inc.) was placed adjacent to the sample for capturing the vaporization and melt sputtering processes. The experiment was recorded with a frame rate of 200 fps. Different exposure times were set based on the interaction of the laser at different intensities with fused silica.
The temporal distribution of the DF laser obtained using the PbSe detector is demonstrated in Fig. 2. The blue vertical lines in the figure denote the beginning of irradiation and the stable irradiation stage, and the end of the stable irradiation stage and laser irradiation, which were recorded at 0.55, 0.61, 1.85, and 2.01 s, respectively. Therefore, the duration of the stable laser output was determined to be 1.23 s, obtained using the start and end time of the stable irradiation stage.
The laser spot on the front surface of the sample was 3.8 mm wide and 11.5 mm high. The samples used in the experiment were fused silica (JGS1) with 25.4 mm diameter and 3 mm height. This study investigated the irradiation processes and morphologies induced by a DF laser at various intensities of 0.47 kW/cm2, 1.2 kW/cm2, 2.3 kW/cm2, 5.1 kW/cm2 and 9.5 kW/cm2.
3. Results and discussion
3.1 Surface morphology
Micrographs of the surface morphologies of the sample surfaces were obtained using an Optical Microscope, Metallurgical Microscope and Scanning Electron Microscopy (SEM). No changes were observed when the laser intensity was 0.47 kW/cm2. Figure 3 demonstrates the melting morphologies caused by the DF laser at three different intensities of 1.2 kW/cm2, 2.3 kW/cm2, and 9.5 kW/cm2. As observed from Fig. 3(a), a large number of discrete damage sites with sizes up to 40 µm can be observed in the laser-irradiated area of the sample surface (highlighted with blue ellipses). It is noticed from Fig. 3(b) that a laser at an intensity of 2.3 kW/cm2 resulted in the formation of a molten dent on the surface of the sample. The dent shape is similar to the spatial distribution of the laser beam, and the dent measured 3.9 × 10 mm2. Five magnified images (highlighted with red rectangles) of different areas are shown around the dent. The image marked 1 on the top left corner of Fig. 3(b) is 50× magnified picture of the dent-edge area obtained by Metallurgical Microscope. A small amount of molten material had flowed out of the pool against the surface tension, viscous force, and gravity under the influence of the recoil pressure generated during evaporation [13]. The other four images are magnified SEM pictures of the dent edge, dent center and the area above the dent edge. The image marked 2 shows a large number of granular structures distributed on the outer edge of the upper half of the dent. Increased surface roughness at the lower edge of the dent is shown in the image marked 3, where a lot of microcracks distributed on the surface. No micro-scale structures can be observed inside the dent as the image marked 4 shown. The uneven area in the image marked 5 represents the condensed vapor that adhered to the sample surface. Figure 3(c) shows that the sample melted through under laser irradiation at an intensity of 9.5 kW/cm2. A partial enlarged view (30×) of the left edge of the molten dent is demonstrated, where the molten material had flowed out of the edge. The dominant driving force was the recoil pressure. Few bubbles were observed in the solidified molten material. The SEM image on the bottom show a rough surface of the melts flowing out of the edge. Additionally, owing to the high laser intensity, large amount of vapor was generated during laser irradiation. This vapor condensed around the ablated area and adhered to the sample surface.
Fig. 3. Ablated morphologies generated at different laser intensities. Fm = (a) 1.2 kW/cm2, (b) 2.3 kW/cm2, (c) 9.5 kW/cm2.
3.2 Analysis of the interaction process
The accuracy in temperature measurements using a pyrometer is highly affected by the emissivity, which in turn is wavelength and temperature dependent. Barnes et al. [14] measured the emissivity of fused quartz. It decreased from 0.775 to 0.67 when the temperature increased from 100 to 500 °C. Sully et al. [15] obtained the emissivity of fused silica in the temperature range of 400–800 °C. They observed that the emissivity decreased from 0.68 to 0.5. The emissivity of fused silica at 1300 K in the frequency range of 800 to 5000 cm-1 was measured by Sova et al. [16]. The emissivity was recorded to be less than 0.2 in the spectral range of 2–2.5 µm. The discussed results demonstrate that the emissivity decreases with an increase in temperature. Considering the temperature rise of the material at different stages of laser irradiation, the emissivity was uniformly set to 0.35 during the entire experiment. It should be noted that the transmission range of fused silica used in the present study and the spectral range of the pyrometer are 0.17–2.2 µm and 2–2.5 µm, respectively. The pyrometer can receive the thermal radiation from the bulk sample, especially when the sample temperature is far from the melting temperature, because the transmission of fused silica is temperature dependent [10,17]. Hence, in most cases, the measured temperature was higher than the actual temperature. Nevertheless, the time dependent temperature curves obtained at different laser intensities can reflect the temperature rise of the laser-irradiated area to a certain extent. Comparatively better measurements can be obtained by using a pyrometer or thermal imager that is sensitive to the thermal radiation with a spectrum located in the far-IR range.
Figure 4 demonstrates the temperature variations obtained using the pyrometer due to laser irradiation at different intensities. The black curves correspond to the black colored y-axis coordinates on the left, and the red curves correspond to the red colored y-axis coordinates on the right, as indicated by the arrows in Fig. 4. A maximum temperature of 410 °C was recorded at the laser intensity of 0.47 kW/cm2. The actual emissivity at temperatures lower than 400 °C was greater than 0.35. This indicates that the actual maximum temperature was less than 400 °C. A maximum temperature of 760 °C was recorded at the laser intensity of 1.2 kW/cm2, which was lower than the melting point of fused silica at 2000 °C [18]. Subsequently, when the laser intensity was increased above 2.3 kW/cm2, the temperature exceeded the melting point of the material. The temperature rise curve corresponding to 2.3 kW/cm2 increased at a lower rate after reaching the melting point, which can be attributed to the evaporation process. Toward the end, a drastic temperature drop was recorded after laser irradiation. As observed in Fig. 4, at the laser intensity of 5.1 kW/cm2, the temperature increased faster before reaching the upper limit 3500 °C at 4.85 s. Shortly after 0.020 s, a sudden decrease in the temperature was recorded. This can be attributed to the faster retreat of the interface of the air in front of the sample and the molten material (air-liquid interface) and the stronger shielding effect of the vapor. A different temperature evolution is observed at a laser intensity of 9.5 kW/cm2, where the temperature oscillated in the range of 2250 to 3500 °C for 0.22 s. The oscillation is associated to the heavy evaporation process owing to the high laser intensity.
The 532 nm probe laser was reflected and transmitted at the center of the DF laser spot on the sample surface. The variations of the reflected and transmitted intensity with time are demonstrated in the Fig. 5 and Fig. 6, respectively. The blue vertical line to the left in both the figures corresponds to the start of steady light emission from the laser, while the blue line to the right denotes the time after which the laser intensity begins to decrease. For example, in Fig. 5(a), steady light emission started at 0.61 s while the laser intensity decreased after 1.85 s. The pink arrow represents important time instances when sharp change in the signal relative to the time of beginning of stable irradiation (0.61 s) is observed.
Fig. 5. Variation of the reflected signal intensity with time during laser irradiation at different intensities. Fm = (a) 0.47 kW/cm2; (b) 1.2 kW/cm2; (c) 2.3 kW/cm2; (d) 9.5 kW/cm2.
Fig. 6. Variation of the transmitted signal intensity with time during laser irradiation at different intensities. Fm = (a) 0.47 kW/cm2; (b) 1.2 kW/cm2; (c) 2.3 kW/cm2; (d) 9.5 kW/cm2.
The reflected and transmitted signals at a laser intensity of 0.47 kW/cm2 are demonstrated in the Fig. 5(a) and Fig. 6(a), respectively. The reflected signal was steady for the entire duration of laser irradiation, whereas the transmitted signal decreased from 0.90 to 1.23 s, when the sample was irradiated with stable light. Later, the transmitted signal gradually increased and attained its initial intensity.
Figure 5(b) and Fig. 6(b) show the results at a laser intensity of 1.2 kW/cm2. In Fig. 5(b), the reflected signal decreased after the sample was stably irradiated for 1.18 s and reached a minimum value at 1.23 s. Subsequently, a gradual increase in the signal was recorded and it finally recovered the initial intensity. Figure 6(b) illustrates that the transmitted intensity decreased after 0.42 s of stable irradiation. Further, it was observed that the transmitted signal increased after 1.23 s whereas the laser power decreases.
The results at the laser intensity of 2.3 kW/cm2 are shown in the Fig. 5(c) and Fig. 6(c). Figure 5(c) shows that the reflected signal decreased rapidly after the sample was stably irradiated for 0.51 s. Whereas, the transmitted signal began to decrease significantly after a period of 0.19 s of stable irradiation, as demonstrated in Fig. 6(c). Both the reflected and transmitted signals were heavily attenuated after the DF laser interaction. In particular, the transmitted part of the probe laser was completely obscured by the laser-irradiated area.
Figure 5(d) and Fig. 6(d) demonstrate the results at the laser intensity of 9.5 kW/cm2. As demonstrated in Fig. 5(d), a sharp decrease in the reflected intensity was observed after the sample was stably irradiated for 0.022 s. Subsequently, a sharp rise in the intensity was observed after a period of 0.12 s of stable irradiation. Subsequently, the reflected intensity increased beyond its initial value and reached its peak between 0.12 s and 0.55 s. Later, the signal rapidly decreased at 0.55 s. The variation in the signal after 0.55 s can be divided in two separate time ranges. The fluctuation between 0.55 s and 1.18 s was higher than that of other periods, and the rate of decline was lower than that after 1.18 s. Finally, the reflected light intensity tended to zero after the end of the laser irradiation process. In Fig. 6(d), the transmitted signal rapidly decreased to near zero after the sample was stably irradiated for 0.034 s. Besides, the intensity increased to 0.6 V after the end of the laser irradiation process.
A high-speed camera was used to study the surface vaporization and melt sputtering process during DF laser irradiation. The results at different laser intensities are demonstrated in Fig. 7. The time instance at which the laser irradiates at a stable intensity is indicated at the upper-right corner. Figure 7(a) and Fig. 7(b) demonstrate the results obtained during laser irradiation at intensities of 1.2 kW/cm2 and 2.3 kW/cm2, respectively. Bright spots were observed at 0.86 s and 0.42 s at the two laser intensities, at the centers of the laser-irradiated areas, as highlighted with the pink rectangle. Subsequently, the brightness around the laser spot gradually increased until the laser intensity decreased. Figure 7(c) and Fig. 7(d) demonstrate the vapors spread toward the incident laser at 0.22 s and 0.12 s under laser irradiation at intensities of 5.1 kW/cm2 and 9.5 kW/cm2, respectively. At a laser intensity of 9.5 kW/cm2, the time at which the vapor spreads into the ambient atmosphere coincided with the sudden increase in the reflected signal (see Fig. 5(d) at 0.12 s). In addition, Fig. 7(d) demonstrates that the laser-heated area ejected molten material at 0.56 s, which is consistent with the fast decreasing moment of the reflected signal, as demonstrated in Fig. 5(d), at 0.55 s. The above analysis indicates that the high reflectivity between 0.12 s and 0.55 s, demonstrated in the Fig. 5(d), was specifically related to the rapid evaporation process of the sample. Further from Fig. 7(d), it is observed that the molten material ejected from the back surface of the sample at 0.71 s, indicating that the sample was melted through at this moment.
Fig. 7. Sequence images of the laser irradiation process conducted at different intensities captured using the high-speed camera. (a) Fm = 1.2 kW/cm2, exposure time texp was set as 100 µs (b) Fm = 2.3 kW/cm2, texp = 50 µs (c) Fm = 5.1 kW/cm2, texp = 20 µs (d) Fm = 9.5 kW/cm2, texp = 20 µs.
3.3 Discussion
At a laser intensity of 0.47 kW/cm2, the peak temperature recorded by the pyrometer was approximately 400 °C, which was very small as compared to the transformation temperature (∼1080 °C [19]). The decrease in the transmitted intensity demonstrated in Fig. 6(a) is related to the variation in the refractive index of the material. This change in the refractive index is due to the high temperature attained by the sample caused due to DF laser irradiation. The refractive index variation can be expressed as δn = (δn)temp +(δn)stress, where (δn)temp and (δn)stress denote the refractive index variation induced by the temperature rise and the stress-induced birefringence, respectively [20]. The temperature and the stress were not sufficiently high to generate irreversible damage as the laser intensity was low. When the laser intensity decreased, the refractive index of the sample gradually recovered with the decreasing temperature. This resulted to an increase in the transmitted intensity of the probe laser. In addition, the behavior of the reflected and the transmitted signals as demonstrated in Fig. 5(a) and Fig. 6(a), respectively, are consistent with the observation wherein no irreversible modification is observed on the sample surface.
When the laser intensity increased to 1.2 kW/cm2, discrete melting sites were observed at the sample surface. This observation can be attributed to the following two factors. First, the discrete melting sites could have formed due to the presence of the inclusions, grooves, and cracks at the surface and subsurface of the sample [4]. This easily damaged the surrounding materials because of the higher absorption coefficient or local enhancement of the light intensity. Thus, forming small damage sites centered at these defects. Second, the uneven spatial distribution of the laser caused the local laser intensity to be higher than that of the surrounding areas. Thus, the temperature at the locations with relatively high laser intensity is higher than that in other areas and is more prone to melting. However, the maximum temperature demonstrated in Fig. 4 was lower than the melting point, which is inconsistent with the morphology as demonstrated in Fig. 3(a). The reason for this difference is that the detection area had melted partially. Hence, the flux of the thermal radiation received by the pyrometer was lower than that in the case where the entire detection area had melted. Owing to the discrete distribution of the molten sites as well as their small size, the reflected signal recovered shortly after the end of the laser irradiation process, as shown in Fig. 3(a).
As the laser intensity increased above 2.3 kW/cm2, the temperature exceeded the melting point of 2000 °C and the evaporation point 2700 °C [6], as demonstrated in Fig. 3(b) and Fig. 4. The evaporation process occurred during the second half of laser-heating period. Due to the relative low laser intensity, the vapor spread out near the surface after being evaporated. Hence, plenty of vapor condensed and adhered around the dent, especially the upper half of the dent. The thickness of the deposited layer decreased from top to bottom, as the two SEM images marked 5 and 2 in Fig. 3(b) shown. After the end of laser irradiation, the molten pool began to cool. The resolidification of the melt led to the shrinkage of the material rapidly, which further created cracks on the surface. Hence, the surface roughness increased in the image marked 3 in Fig. 3(b). The cooling speed at the edge of the molten pool was faster than the inner area. Therefore, the microcracks were more likely formed at the edge of the molten pool. The air-liquid interface retreated during the evaporation process, resulting in the deviation from the detection focal plane of the pyrometer. In addition, the vapor distributed in front of the sample can cause a shielding effect, which further reduces the detected flux of the thermal radiation. Hence, the rate of increase of the temperature rise curve corresponding to the laser intensity of 2.3 kW/cm2 decreased after exceeding the melting point, as demonstrated in Fig. 4. When the laser intensity further increased to 5.1 kW/cm2, the evaporation process intensified and a large amount of the vapor diffused in the ambient atmosphere (see Fig. 7(c)). In addition, the recoil pressure generated during the rapid evaporation process excludes the molten material to the surroundings. Hence, the air-liquid interface retreated faster, and the thermal radiation was shielded more heavily, decreasing the thermal radiation flux shortly after reaching the evaporation point. The loss of the detected flux of the thermal radiation gave rise to the behavior of the decrease of the temperature demonstrated in Fig. 4. The evaporation further intensified when the laser intensity was increased to 9.5 kW/cm2. The vapors evaporated at a faster rate, and travelled farther distance, as demonstrated in Fig. 7(d). The thermal radiation radiating from these dense vapors reduces the loss due to the interface retreat and the shielding effects. Therefore, the temperature curve was maintained for a longer time span of 0.22 s after reaching the saturation point, as demonstrated in Fig. 4. The oscillation behavior during the period of 0.22 s was associated with the instability of the evaporation process. The laser intensities applied in out text were far from the plasma ignition threshold reported in Refs. [21–23], where the ignition thresholds for metal targets at 3.8 µm were in the order of 10 MW/cm2. Hence, plasma was absent in our experiments and only melting and vaporizing processes should be considered. On the other hand, the increased intensity of reflected signal between 0.12 s and 0.55 s shown in Fig. 5(d) was also occurred for the laser intensity of 5.1 kW/cm2. The start and end times for 5.1 kW/cm2 were 0.24 s and 1.23 s, respectively. During this time interval, numerous vapors was evaporated as shown in Fig. 7(c). Hence, the increase in the reflected signal at 0.12 s in Fig. 5 (d) can be attributed to the scattering of the probe laser by dense vapor. More signals can be detected by the Si detector arranged in front of the sample. The increase in the transmitted signal after the end of the laser irradiation process was caused by the enlargement of the hole after the sample melted through.
4. Conclusion
The ablation morphology and the process induced by the interaction between a 3.8 µm DF laser and fused silica at different laser intensities were experimentally studied. The variations in the temperature, and changes in the reflected and transmitted signal features were investigated. In addition, the vaporization and the melt sputtering processes were analyzed using a high-speed camera. The results demonstrated that the reversible heating process gradually converted into an irreversible ablation process with the increase in laser intensity. At a laser intensity lower than 1.2 kW/cm2, the reflected and the transmitted signals recovered after laser irradiation. However, when the laser intensity increased, the surface morphology, and the transmitted and reflected signal features permanently changed owing to the melting process. When the laser intensity exceeded 5.1 kW/cm2, the temperature increased sharply, and the evaporation process became significant. In particular, the sputtering of the molten material driven by the recoil pressure (generated during the intense vaporizing process) along with the phenomenon was observed when the laser intensity increased to 9.5 kW/cm2.
Funding
National Natural Science Foundation of China (61805120); Scientific Research Foundation of Education Department of Shanxi Province (21JK0945); Natural Science Foundation of Science and Technology Department of Shanxi Province (2022JQ-664).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting the findings of this study are available from the first and corresponding authors upon reasonable request.
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