But the intense heat creates debris and residual stress around this crater. .... conducive to its damage threshold, which is good for use on power laser chain.
Effect of CO2 laser annealing on residual stress and on laser damage resistance for fused silica optics P. Cormonta, L. Gallais b, L. Lamaignèrea, T. Donvala, J.L. Rulliera a
b
CEA CESTA, F-33114 Le Barp, France Institut Fresnel, CNRS, Aix-Marseille Université, Ecole Centrale Marseille, 13013 Marseille, France ABSTRACT
CO2 laser is used to prolong the lifetime of large optics for high power lasers such as the NIF and LMJ. Indeed, on silica optical components, damaged sites, whose diameter is in the order of tens of microns, appear at high UV laser fluence, and the size of such sites increases exponentially with each UV laser shot. An intense heat by CO2 laser ejects the material from the surface of the optical component and removes all fractures around the damaged site so that this site will not be damaged at fluences of operation of the UV laser. A crater is formed at the site of initial damage. But the intense heat creates debris and residual stress around this crater. Due to these debris and stress, the optical component is again weakened. We show here that a second heating process, done with different settings of the CO2, named here laser annealing, eliminates the debris and reduce stress. The results presented here establish that annealing significantly improves the resistance of laser optics. Key words: Fused silica, laser damage, laser mitigation, stress, CO2 laser
1. INTRODUCTION CO2 laser is tested for a long time, as a surface post-treatment, to manufacture optical components without surface defects, thanks to a superficial melting of the silica1. Since these first interests, several studies describe how to use the CO2 laser to improve glass cutting2, optical polishing3, micro-optics fabrication4 or damage repairing5. CO2 laser treatment is also used to prolong the lifetime of large optics for high power lasers such as the National Ignition Facility (NIF) and Laser Megajoule (LMJ)6,7. Indeed, on silica optical components, damaged sites appear at high laser fluence. The diameter of such sites is of the order of tens of microns and their size increases exponentially with each laser shot8. An intense heat by CO2 laser removes the material from the surface of the optical component and all fractures around the damage site9 so that this site will not be damaged at fluences of operation of the laser7,10. But this intense heat creates debris and stress around the crater formed at the site of initial damage. Consequently, the optical component is again weakened11-14. We show in this paper that a second heating process, using with different settings of the CO2 and operating as a laser annealing, eliminates the debris and reduce the effect of residual stress. The results presented establish that this annealing significantly improves the resistance of optics to laser beam. 2. HEATING PROCESS To accomplish this, the device developed, whose description is given in reference 12, uses a CW laser emitting at a wavelength of 10.6 μm with a maximum power of 20 W. The spatial profile of the beam is an axisymmetric Gaussian. The particularity of this system is to have a movable lens to adjust quickly and accurately the size of the laser beam on the surface to be treated. For the study described in this paper, a planar convex lens ZnSe, with a focal length of 20 inches, is used. After the first heating, the lens is moved backward for annealing with a greater beam size. The lens was then used in two configurations: the first one to obtain a beam size at the sample of 0.6 mm and the second one to obtain a beam size at the sample of 1.4 mm. For both configurations, an optical profiler was used to measure the dimensions of the crater formed by CO2 laser on fused silica samples.
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radius (µm) 100 0
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depth (µm)
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d=0,7 mm / P= 5,5 W d=1,4 mm / P= 12 W d=1,4 mm / P=10.5 W d=1,4 mm / P=9 W
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Figure 1: profiles of the crater formed on fused silica samples by CO2 laser. Each curve corresponds to different beam diameter and laser power. The different set of laser parameters that were used for this study, with the corresponding crater dimensions are listed in table 1. Table 1: Details of CO2 laser parameters and corresponding crater dimensions on silica samples. Irradiation parameters Crater Dimensions Diameter (1/e²) Pulse duration Power Diameter Depth 0.7 mm 1 sec 5.5 W 170 µm 49 µm 1.4 mm 1 sec 12 W 305 µm 7 µm 1.4 mm 1 sec 10.5 W 300 µm 0.7 µm 1.4 mm 1 sec 9W 300 µm < 0.5 µm
The results presented in figure 1 and table 1 were obtained on samples free of damage and with craters corresponded to just one CO2 laser heating. In the following paragraphs the study is based on real damage sites and two heating operations, corresponding to the two configurations characterized here, were applied on the same site.
3. SAMPLES PREPARATION AND CHARACTERIZATION Samples of Corning 7980 fused silica polished by SESO are used. Their diameter and thickness are respectively 50 mm and 5 mm. This study has been divided in four stages. First is the damage initiation step. A Nd:YAG laser delivering a pulse length of 2.5 ns at 355 nm has been used. The beam is focused by a 5 m focal length lens to get a Gaussian spatial profile with a diameter of 0.9 mm at 1/e². One shot is then applied on several sites at a fluence around 20 J/cm² in order to create damage sites of few tens of micometers. Secondly, the first CO2 laser heating is done always with a beam size at the sample of 0.6 mm at 1/e ² and a power of 5.5 W during 1 second. Then annealing is processed during 1 second with a beam size of 1.4 mm. For comparison of the process efficiency, three different powers were used for annealing in this study: 9 W, 10.5 W and 12 W. For a power of 13 W and more, we observe a significant evolution of the crater shaped by the first heating, which is not wanted. The last stage concerns the study of the laser resistance of the annealing sites, described in the next paragraph. At each step, the surface modifications were characterized by Nomarski microscopy, followed by confocal microscopy to measure the cracks depth9 and finally by polariscope to identify areas of stress12. Figure 2 shows these three types of observation after the first three steps.
After first heating
After annealing
Polariscope
Confocal
Nomarski
Damage site
Figure 2: the first two rows of images L1 and L2 were obtained respectively with Nomarski and confocal microscopy. The third row L3 of the image was made with the polariscope. The first column C1 corresponds to the damage site created by a Nd:YAG laser operating at 355 nm, the second column C2 is the transformation of the damage after the first heating by CO2. The third column C3 was obtained after annealing the previous crater.
The observations with microscopes show that, in this study, the typical damage site created by UV laser has a diameter of 60 µm and a depth of 20 µm. Similar observations of CO2 laser crater (diameter 180 µm and 40 µm depth) indicate that the chosen parameters permit to remove all surface fractures. The annealing has negligible effect on the crater dimensions (variation of 80% of Fmax
1000 µm (a)
1000 µm (b)
Figure 3: Constraints observations on sites which have been: (a) heated by the CO2 laser and then irradiated with the Nd: YAG and with a fluence of 12 J/cm² at the beam center , (b) heated and annealed by the CO2 laser and then irradiated with the Nd: YAG and with a fluence of 15 J/cm² at the beam center . The discs represent the clearest area illuminated by the beam at 355 nm with a fluence equal to at least 80% of the maximum fluence, which is at the center of the disc.
The polariscope observation in figure 3.(a), shows that the damage, created at 12 J/cm², is situated exactly on the area of maximal retardation. Although residual debris are not visible on the photo 2.(a), they are localized at the same area and could be answerable for damage initiation. On the other hand, during annealing, the debris was removed and the areas of maximal retardation were displaced from the crater, as shown in figure 3.(b). In that case, damage sites initiate at a fluence of the same order (about 13 J/cm² at their location), but they are formed on the new zone of high stress, in the absence of any debris. This results indicates clearly the much more important contribution played by stress compared with the debris ejected during crater formation. Irradiations, with the Nd:YAG laser beam pointed to areas of stress, are repeated on series of reproducible sites to measure the laser damage probability. Figure 4 reports the results of a statistical study of damage threshold as a function of CO2 laser power (around 20 sites were tested for each case).
without repair First heating Second heating with P=9W Second heating with P=10.5W Second heating with P=12W
Damage probability (%)
100 80 60 40 20 0 0
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10
Fluence (J/cm²)
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Figure 4: Laser damage probability as function of the fluence of irradiation at 355 nm. Before the damage test, each site has been heated by CO2 laser on a first time during 1 s with a beam diameter of 0.6 mm at 1/e² and a power of 5.5 W. Then, after annealing during 1 s, the beam diameter was 1.4 mm at 1/e². Each curve corresponds to a value of power used for annealing.
These results highlight the contribution of annealing to increase the laser resistance. Without annealing, all sites get damaged with a fluence of 10 J/cm². On the contrary, with the annealing realized at a power of 12 W, a fluence greater than 12 J/cm² is needed to initiate damage.
5. CONCLUSION By applying suitable laser annealing, we managed to increase laser damage resistance to a level of necessary for LMJ. This study has mainly demonstrated that the temperature changes induced by CO2 laser create constraints that are conducive to the development of damage. We have thus shown that it was appropriate to use the CO2 laser in two different ways in order to take advantage of properties of silica: a mode of mass ejection with temperatures reached above the melting temperature of silica and a mode annealing with a temperature range between the glass transition temperature and melting temperature. The latter mode allows a reorganization of the material more conducive to its damage threshold, which is good for use on power laser chain. [1] [2] [3] [4] [5] [6] [7]
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