Relevance of Carbon Dioxide Laser to Remove Scratches on Large Fused Silica Polished Optics

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1 DOI: /adem Relevance of Carbon Dioxide Laser to Remove Scratches on Large Fused Silica Polished Optics By Philippe Cormont,* Antoine Bourgeade, Sandy Cavaro, Thierry Donval, Thomas Doualle, Gael Gaborit, Laurent Gallais, Laurent Lamaignère and Jean-Luc Rullier Scratches at the surface of fused silica optics can be detrimental for the performance of optical systems. A carbon dioxide (CO 2 ) laser is an interesting tool to remove those scratches because it can melt efficiently the silica in a rapid and localized way, without generating debris. In this article, we propose a new process for optical fabrication, which uses a CO 2 laser to remove scratches between polishing and finishing steps. This is a linear process with no iterative polishing operations for scratch removal. This process is applied on an optic representative of laser megajoule facility production. Indeed, we succeed in removing a 10 mm deep scratch and we demonstrate that this laser operation increases the laser damage threshold by a factor of three in fluence. 1. Introduction Being able to produce a defect free glass interface has always been and is still a concern for an optical manufacturer; and this for applications such as lithography, astronomy, or high power lasers. These interfacial defects are likely to trigger damage in the case of high power lasers (HPL) such as the French Megajoule Laser facility (LMJ). [1,2] LMJ will contain 22 bundles, each consisting of eight laser beams, and so there will be around 8000 large optics (40 40 cm 2 ). The beams will be focused onto micron-sized targets containing deuterium and tritium to initiate the thermonuclear fusion reaction. This huge device, like other HPLs, entails high exploitation costs principally linked with the lifetime of optical components under intense laser irradiation. This lifetime depends on several factors. [3,4] One of them is the optical quality in term of surface defects after polishing. The manufacture of these optical components includes a double challenge associated with the polishing step. The first challenge is to minimize wavefront distortion because of its main impacts on beam alignment, focal spot at target, and energy loss. [5] The second challenge is to have no surface defects on the optical components. High surface quality in terms of flatness can [*] P. Cormont, A. Bourgeade, S. Cavaro, T. Donval, G. Gaborit, L. Lamaignère, J.-L. Rullier CEA CESTA, F Le Barp, France philippe.cormont@cea.fr T. Doualle, L. Gallais Institut Fresnel, CNRS, Aix-Marseille Universite, Ecole Centrale Marseille 13013, Marseille, France be obtained by polishing for a long time, with frequent controls during it. On the other hand, the safest way to prevent surface defects is to limit polishing time and optics handling. The optics costs can be prohibitive if the proper compromise is not found between optical specification and surface defects. Surface defects after polishing are mostly scratches and pits. We essentially discuss scratches because they are the mainly visible defect and also the most problematic for LMJ application but the carbon dioxide (CO 2 ) laser process that will be presented can also be applied to various kinds of surface defects. Conventional fabrication methods attempt to take off scratches by an iterative step of polishing, but with low removal rates that require a long polishing time. Moreover, any further treatment may also spoil the quality of surface waveform and create new scratches. The Fraunhofer Institute for Laser Technology ILT has developed a laser-based process chain for manufacturing optics. [6] This process offers many advantages compared to conventional polishing specially for free form optics but it is not yet operational for large optics. We propose to adapt the method for removal of scratches on fused silica optics by using a CO 2 laser on a small area of the optics. [7] So we add a new step using CO 2 laser in a conventional optics fabrication process in order to make it more predictable and less iterative. It is intended to validate technological choices made for LMJ and to prepare for its exploitation. Section 2 develops the key principles for scratch repair and its advantages. In Section 3, we describe the dedicated tools that we use to demonstrate this new process on a silica plate representative of LMJ optical fabrication. Then, the operations DOI: /adem WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1

2 conducted on our demonstrative silica plate are given in Section 4. The Section 5 contains studies for our process acceptance including surface topology, calculation of beam propagation, and ultraviolet laser damage test. We present our conclusions in Section Key Principles of Our CO 2 Laser Operation for Defects Removal Localized CO 2 laser heating of silica glass has demonstrated its capacity to mitigate surface damage sites on optics used in high power laser application. [8 10] In that way, several works have been carried out to optimize the process. [11,12] As, for example, thermal analysis of the laser silica interaction under CO 2 laser irradiation has been widely investigated with both experiments [13,14] and simulations. [15,16] A schema of the general succession of steps from laser irradiation to surface cooling is represented in Figure 1. The first step is to localize the defect that will be annihilated (a). Then during laser irradiation (b), local heating increases the surface temperature (c) so that there are various transformations of the silica (d) including crack healing. Finally after treatment (e), there is usually a crater with a surrounding raised rim and a laseraffected zone. The application of two successive heatings by CO 2 laser with adapted different parameters offers the possibility to improve damage repairing sufficiently to extend the lifetime of the silica components. [17] Results of this study are resumed in Figure 2. For this experiment, each damaged site was heated by a first CO 2 laser (1 s, 0.6 mm at 1/e 2 and 5.5 W), and then a second heating (1 s, 1.4 mm at 1/e 2 and 12.5 W) was applied. Thanks to the polariscope analysis, [18] we observed unambiguously the area of major stress. In our concern, residual stress area is initially correlated with the damage including all surrounding fractures (a). After the first irradiation by CO 2 laser, which eliminated completely the damage, it forms a ring adjacent to the crater (b). The second heating reduces these stresses sufficiently to make them invisible to the polariscope but it creates a new stress area at a greater distance from the crater (c). This new zone is larger than the older one with equivalent intensity of relative retardation. Although the second heating by CO 2 laser removes debris and smoothes crater edges, the polariscope measurement shows that it also causes the initially damaged area to spread out over a much larger surface. Nevertheless, the damage test at 355 nm demonstrates that such a modification has a beneficial effect regarding UV laser irradiation (d). 3. Productions Tools The experimental set-up shown in Figure 3 has been developed to stabilize large fused silica optical components. This facility detects and localizes defects of the component, and then allows us to repair a selected surface defect with CO 2 laser. For the detection, we use a damage mapping system (DMS) that includes a transversal sample illumination and a high-resolution camera. [19] Then we record the x y coordinate for each defect. By means of an automatic x y translation, the selected defect is placed in front of the CO 2 laser beam. A long working distance microscope with a field of view of mm 2 observes this area of interest. As it can be seen in Figure 3, the microscope is equipped from the opposite site compared to the camera system. The CO 2 laser from Synrad (Firestar V20) operates at a 10.6 mm wavelength with a 20 W maximum power. The pulse length is adjustable in a large range of duration (milliseconds to several seconds). Mean power control is achieved by pulsed width modulation at a 5 khz frequency: a duty cycle of 10% corresponds to a power of 2 Wand 100% to 20 W. The laser output power can be adjusted by a half wave plate and a polarizer. The beam is focused with a ZnSe lens with a 254 mm focal length. The latter is mounted on a z-translation stage to adjust the beam diameter on the sample from 400 to 2500 mm measured at 1/e 2. Different diagnostics in the laser path measure its power, and its temporal or spatial profile. Finally, adjusting the following parameters: beam focus size, mean power, and pulse length, we can easily vary the laser energy deposition. Fig. 1. Successive steps of the defects mitigation method WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /adem

3 Figure 4. The first step is to select the defect that will be reappeared in the DMS image of the optical component. In that way, we delimit a rectangular area including the whole parts of the scratch even if not contiguous (a). Then, the selected image is binarized and the noise eliminated to isolate the different constituent of the scratch (b) and (c). The presence of slick steps is frequent. From all these fragments, the defect is outlined (d), and its skeleton is calculated (e). Finally, laser positions are generated on this latter following the distance between two successive irradiations defined by the operator (f). We note that this work configuration including the image processing is well suited to work on large optics such as LMJ optics but they can also be easily adapted to other optics and applications. Fig. 2. Polariscope images for a typical damage site (a), its transformation after the first heating by CO 2 laser (b) and the impact of the second heating on the previous crater (c). The laser damage probability as a function of the fluence of irradiation at 355 nm is given for the three configurations (d). To stabilize a defect longer than 100 mm, we need to heat it with numerous x y positions. Thus, it is useful to computerize the procedure for complete automation. The objective is to determine each shot position for running automatically the laser and for moving the sample. Then, the operator has only to select the defect to mitigate and the CO 2 laser parameters. The different steps for image processing in the case of scratches on an optical component are summarized in Fig. 3. Defect observation and laser operation setups. 4. Fabrication Process In this study, we are interested in improving an optics manufacture by introducing the CO 2 laser heating after the step of polishing, and before the finishing step. To address this issue, we investigate the scratches removal by CO 2 laser on a large optic. The fused silica sample used is glass 7980 from Corning (NY, USA) polished by THALES-SESO ( The sample is 15 mm thick with a surface of mm 2 that corresponds to the half-scale of LMJ components. As displayed by observations with the DMS in Figure 5a, a scratch of about 50 mm large and 10 cm long was formed at one corner of the optic after polishing. We split this later in three zones to evaluate the surface modification during our repair using successively two laser heatings, as shown in Figure 5. The zone 1 has not been irradiated by CO 2 laser in order to remain a reference of the initial scratch. We know from previous works [7] that adjusting subtly the parameters of the CO 2 laser can heal cracks. A first heating has been done on zones 2 and 3 and then a second heating only on zone 3. Zone 2 in Figure 5b indicates that the scratch is well reduced but is still visible. Therefore, the second heating is useful not only to improve laser damage resistance [7] but also to remove the whole defect in view of the DMS characterization. In fact, this second heating was realized with a run of 20 shots before moving the optical component to the next position along the scratch. Compared to the shots of the first heating, shots for the second heating have the same pulse duration but a wider beam and a higher power to partially DOI: /adem WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3

4 Fig. 4. Image processing in order to extract medial axis of scratches and to get the positions of the CO 2 laser shots. The treatment progress is illustrated by following each images from (a) to (f). compensate this beam widening. After laser operation, we have finished the fabrication process with slightly polishing the optical component, resulting in the DMS image of Figure 5c. During this final step of polishing, that we call finishing, the uniform removal was 3.3 mm. 5. Results of Studies for Our Process Acceptance 5.1. Topography Surface deformations of our fused silica sample were characterized using a 3D optical profiler. This microscope from ZYGO (New View 7300) is based on coherence correlation interferometry. An objective with a magnification of 10 and a numerical aperture of 0.4 was used, which permits us to attain a measurement area of mm 2 with an optical resolution of 1.1 mm in x y. Step height standards provided by the manufacturer were used to calibrate the instrument. The manufacturer specifies the vertical resolution as about 1 nm. From our measurements, we obtain a 3D-map corresponding to the surface level in the irradiated area. Figure 6 shows the impact of two successive heatings along a scratch mitigated with different procedures. For the three zones detailed in the preceding part, 3D-map before (upper row) and after (middle row) the final polishing are shown, followed by comparison of their profiles (lower row). The color scales of these six images were automatically adjusted with minimum and maximum values and so are different for each picture. The form of the cracks visible in zone 1, independently of the finishing, indicates that this is a typical trailing indent scratch if we refer to the categorization made by Suratwala et al. [20] After the first heating by CO 2 laser (zone 2), we distinguish a central region with significant matter removal [21] and a surrounding area where silica has been distorted by viscous flow, [13] densification, [22] or tensile surface forces. [23] Then, we see obviously that even after the second heating (zone 3), the imprint of the initial scratch is still present although it was indiscernible with DMS characterization as mentioned in paragraph 4. Nevertheless, comparing profile of zones 1 and 2, there is a moderate enhancement of the initial Fig. 5. DMS images of a component after each final step of fabrication: (a) polishing, (b) CO 2 laser irradiation, (c) finishing. Zones 1 3 correspond to different CO 2 laser operations during the step (b) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /adem

5 P. Cormont et al./relevance of Carbon Dioxide Laser to Remove Scratches Fig. 6. The two first rows of images obtained by ZYGO New view 7300 are 3D-maps before and after the surface finishing, and the lower compares their profiles related with the dashed lines, respectively, dark and blue. The first column (zone 1) is the characterization of a part of the initial scratch, the second (zone 2) is the transformation of the other scratch area after the first heating by CO2 laser, and the third (zone 3) shows the impact of the second heating on a section equivalent to the previous zone. It is important to notice the scale difference between the different zones. scratch depth (less than a factor two) due mainly to matter ejection during the first heating. Then comparing profile of zones 2 and 3, the scratch depth is reduced by a factor five thanks to the second heating. We mention that after the polishing removal of 3.3 mm, the two blue profiles for zones 1 and 2, which we have shifted by 3.3 mm, are in good agreement with the depth before the finishing step. In zone 3, the blue curve has also been positioned 3.3 mm lower than the black one to take into account the polishing removal. Surprisingly, the expected eradication of the 2 mm deep trench is not total. The shallow residual trench after polishing can be caused by the neighboring silica affected by the laser. In the laser-affected zone, the temperature reached during the second heating is higher than the annealing point (1315 K for Corning 7980 fused silica). The width of this zone is approximately equal to the beam diameter at 1/e2 and is much deeper than 50 mm.[7] 5.2. Beam Propagation Defects on optical components have an impact on beam characteristics in the neighborhood of the defects and at longer distances due to downstream propagation mechanisms.[2] This problem is even more critical for final optics of LMJ beam baseline,[2] which are, respectively, the 3v focusing DOI: /adem grating and the vacuum window. The defects on LMJ 3v focusing grating may trigger damage on vacuum window. This section presents the numerical approach we carry out to evaluate the effect of fused silica surface defects on the laser beam propagation. Here, we consider the previously investigated scratch. A transverse profile of the scratch, that is sufficiently long to be supposed infinite, has been introduced into a code solving the plane 1D-transverse propagation þ where A is the laser beam envelope, k0 is the wave number at the used wave frequency, z is the abscissa on the propagation axis, and x is the coordinate in the transverse direction. Such profile impact corresponds to a phase perturbation that is a simple way to introduce a defect into the laser beam propagation modeling. Then, we calculate the evolution of the maximum of the ratio between the laser beam intensity at a distance z and the initial intensity at z ¼ 0 (noted CMAX). We realized this simulation for the two different profiles of the zone 3 of the Figure 6, i.e., after the complete cracks repair before and after the finishing process. For a 3v planar wave, 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 5

6 the evolution of C MAX as a function of the propagation from the 3v grating to the vacuum window is shown in Figure 7. Without optic finishing, the profile is corrugated and then induces a very rapid increase of the laser beam intensity, C MAX ¼ 5.7 at z 1 mm. During the propagation this oscillating value decreases, but is still around 2.5 at the vacuum window. By opposition, the smoother profile after finishing is linked with a low increase that reaches 60% after 0.65 m of propagation (C MAX ¼ 1.6). This is 2.5 less than before finishing. It is noticeable that improving the finishing process, using RMS technical as, for example, could enhance such an effect UV-Laser Damage Tests Laser damage resistance tests were performed with a tabletop laser, whose main characteristics are comparable with the LMJ laser beam. The laser used is a Nd:YAG, which delivers an equivalent pulse length of 2.5 ns at 355 nm with a 10 Hz repetition rate. The laser beam is focused by a 5 m focal length to get a Gaussian spatial profile with a diameter of 0.9 mm at 1/e 2. Damages are detected in situ with a mobile visual inspection system. In order to scan the whole area, the sample to be tested is translated continuously along a first direction and stepped along a second direction. Repeating this test at several fluences on different zones allows us to determine the number of damage sites versus fluence, thus the damage density. More details of this method of characterization are furnished elsewhere. [24] We have applied this method on our demonstration component presented in Section 4. The whole surface was tested except the three zones. We have measured a density of damage equal to 0.2 damage sites per cm 2 at 14 J cm 2. This result is in good agreement with results obtained on similar optics without visible defects. In parallel, we have tested zone 1 in mode 1-on-1, which consists in having one laser shot for one position on the optical surface. At fluence of 5 J cm 2, 50% of the site irradiated showed damage growth, and at 6 J cm 2 the totality of sites (100%). These results on scratches confirm usual threshold for damage growth. We have also tested zone 3 in mode 1-on-1 at higher fluences in order to evaluate our repair process. The results of these UV-laser damage tests are presented in Figure 8. We can notice that after our repair process, the laser resistance is even better than in Figure 2. The main difference between the process tested on Figure 2 and the one tested on Figure 8 is the slight polishing after laser process. So we assume that the finishing after laser process is an explanation of this excellent result. These results validate the relevance of this new process for LMJ optics fabrication because the maximum fluence expected on LMJ is 14 J cm 2,as indicated by the dashed line in Figure Summary and Conclusion We have presented a new fabrication process for manufacturing optics. In this fabrication process, the conventional polishing loop for scratch removal has been replaced by aco 2 laser operation including two main steps. The first step uses a smaller beam and a higher power density than the second step. The first step replaces scratch fractures by a smooth trench and the second step improves UV laser resistance and fills in the trench so that the surface is smooth enough to avoid beam propagation problems. To validate our process, we have applied our method to an optical component of large dimensions. This 20 cm 20 cm window was polished with conventional tooling in order to obtain specified flatness. After polishing, a several millimeters long scratch was visible on the surface. This 10 mm deep scratch was then removed by CO 2 laser operation. Finally, a conventional finishing step was done in order to obtain the low roughness necessary for LMJ optics. We have observed that a shallow residual trench has replaced the scratch and we have evaluated the impact on beam propagation. The most important result is that the laser resistance of the window has been greatly improved. The laser damage threshold measured Fig. 7. Evolution of C MAX as a function of the distance of propagation; z ¼ 0 corresponds to the 3v grating and z ¼ 0.65 m to the vacuum window. The black curve is the evolution of C MAX for the black profile of the zone 3 in Figure 6, the blue curve named after finishing corresponds to the calculation for the blue profile in Figure 6 zone 3. The difference between the two curves is due to the polishing after laser process. Fig. 8. Laser damage probability as a function of the fluence of irradiation at 355 nm before and after repairing the scratch. The green dashed line indicates the LMJ maximum fluence WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: /adem

7 at 355 nm with a 3 ns pulse duration has been increased from 5to15Jcm 2, which is higher than the maximum fluence on LMJ. In this study, we have demonstrated that CO 2 laser is an appropriate tool to remove scratches on fused silica optics because it is rapid, localized to the scratch, and it creates no debris. Nonetheless, we are still looking for finishing techniques to reduce the impact on beam propagation after the laser process. Received: August 15, 2014 Final Version: September 23, 2014 [1] J. Ebrardt, J. M. Chaput, J. Phys.: Conf. Ser. 2010, 244, [2] S. Mainguy, B. Le Garrec, M. Josse, Proc. SPIE 2005, 5991, [3] N. Bloembergen, Appl. Opt. 1973, 12, 661. [4] H. Bercegol, P. Bouchut, L. Lamaignère, B. Le Garrec, G. Raze, Proc. SPIE 2004, 5273, 302. [5] S. Mainguy, Proc. SPIE 2013, 8602, 86020G. [6] S. Heidrich, A. Richmann, P. Schmitz, E. Willenborg, K. Wissenbach, P. Loosen, R. Poprawe, Opt. Lasers Eng. 2014, 59, 34. [7] P. Cormont, P. Combis, L. Gallais, C. Hecquet, L. Lamaignere, J. L. Rullier, Opt. Express 2013, 21, [8] E. Mendez, K. M. Nowak, H. J. Baker, F. J. Villarreal, D. R. Hall, Appl. Opt. 2006, 45, [9] S. Palmier, L. Gallais, M. Commandre, P. Cormont, R. Courchinoux, L. Lamaignère, J.-L. Rullier, P. Legros, Appl. Surf. Sci. 2009, 255, [10] I. L. Bass, G. M. Guss, M. J. Nostrand, P. J. Wegner, Proc. SPIE 2010, 7842, [11] S. T. Yang, M. J. Matthews, S. Elhadj, D. Cooke, G. M. Guss, V. G. Draggoo, P. J. Wegner, Appl. Opt. 2010, 49, [12] W. Dai, X. Xiang, Y. Jiang, H. J. Wang, X. B. Li, X. D. Yuan, W. G. Zheng, H. B. Lv, X. T. Zu, Opt. Lasers Eng. 2011, 49, 273. [13] L. Robin, P. Combis, P. Cormont, L. Gallais, D. Hebert, C. Mainfray, J.-L. Rullier, J. Appl. Phys. 2012, 111, [14] S. T. Yang, M. J. Matthews, S. Elhadj, V. G. Draggoo, S. E. Bisson, J. Appl. Phys. 2009, 106, [15] P. Combis, P. Cormont, L. Gallais, D. Hebert, L. Robin, J.- L. Rullier, Appl. Phys. Lett. 2012, 101, 21. [16] R. M. Vignes, T. F. Soules, J. S. Stolken, R. R. Settgast, S. Elhadj, M. J. Matthews, J. Am. Ceram. Soc. 2013, 96, 137. [17] P. Cormont, L. Gallais, L. Lamaignere, J. L. Rullier, P. Combis, D. Hebert, Opt. Express 2010, 18, [18] L. Gallais, P. Cormont, J.-L. Rullier, Opt. Express 2009, 17, [19] R. Prasad, M. Bernacil, J. Halpin, J. Peterson, S. Mills, R Hackel. Proc. SPIE 2004, 5647, 421. [20] T. Suratwala, R. Steele, M. D. Feit, L. Wong, P. Miller, J. Menapace, P. Davis, J. Non-Crystal. Solids 2008, 354. [21] M. D. Feit, A. M. Rubenchick, Proc. SPIE 2002, 4932, 91. [22] M. D. Feit, M. J. Matthews, T. F. Soules, J. S. Stolken, R. M. Vignes, S. T. Yang, J. D. Cooke, Proc. SPIE 2010, 7842, 78420O. [23] T. R. Anthony, H. E. Cline, J. Appl. Phys. 1977, 48, [24] L. Lamaignère, G. Dupuy, T. Donval, P. Grua, H. Bercegol, Appl. Opt. 2011, 50, 441. DOI: /adem WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 7