relevance of carbon dioxide laser to remove scratches on

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DOI: 10.1002/adem.201400383
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Relevance of Carbon Dioxide Laser to Remove Scratcheson Large Fused Silica Polished Optics

By Philippe Cormont,* Antoine Bourgeade, Sandy Cavaro, Thierry Donval,Thomas Doualle, Gael Gaborit, Laurent Gallais, Laurent Lamaignereand Jean-Luc Rullier

Scratches at the surface of fused silica optics can be detrimental for the performance of opticalsystems. A carbon dioxide (CO2) laser is an interesting tool to remove those scratches because it canmelt efficiently the silica in a rapid and localized way, without generating debris. In this article, wepropose a new process for optical fabrication, which uses a CO2 laser to remove scratches betweenpolishing and finishing steps. This is a linear process with no iterative polishing operations forscratch removal. This process is applied on an optic representative of laser megajoule facilityproduction. Indeed, we succeed in removing a 10mmdeep scratch and we demonstrate that this laseroperation increases the laser damage threshold by a factor of three in fluence.

1. Introduction be obtained by polishing for a long time, with frequent

Being able to produce a defect free glass interface hasalways been and is still a concern for an optical manufacturer;and this for applications such as lithography, astronomy, orhigh power lasers. These interfacial defects are likely to triggerdamage in the case of high power lasers (HPL) such as theFrench Megajoule Laser facility (LMJ).[1,2] LMJ will contain22 bundles, each consisting of eight laser beams, and so therewill be around 8000 large optics (40� 40 cm2). The beams willbe focused onto micron-sized targets containing deuteriumand tritium to initiate the thermonuclear fusion reaction. Thishuge device, like other HPLs, entails high exploitation costsprincipally linked with the lifetime of optical componentsunder intense laser irradiation. This lifetime depends onseveral factors.[3,4] One of them is the optical quality in term ofsurface defects after polishing. The manufacture of theseoptical components includes a double challenge associatedwith the polishing step. The first challenge is to minimizewavefront distortion because of its main impacts on beamalignment, focal spot at target, and energy loss.[5] The secondchallenge is to have no surface defects on the opticalcomponents. High surface quality in terms of flatness can

[*] P. Cormont, A. Bourgeade, S. Cavaro, T. Donval, G. Gaborit,L. Lamaignère, J.-L. RullierCEA CESTA, F-33114 Le Barp, FranceE-mail: [email protected]. Doualle, L. GallaisInstitut Fresnel, CNRS, Aix-Marseille Universit�e, EcoleCentrale Marseille 13013, Marseille, France

DOI: 10.1002/adem.201400383 © 2014 WILEY-VCH VerlaADVANCED ENGINEERING MATERIALS 2014,

controls during it. On the other hand, the safest way toprevent surface defects is to limit polishing time and opticshandling. The optics costs can be prohibitive if the propercompromise is not found between optical specification andsurface defects. Surface defects after polishing are mostlyscratches and pits. We essentially discuss scratches becausethey are the mainly visible defect and also the mostproblematic for LMJ application but the carbon dioxide(CO2) laser process that will be presented can also be appliedto various kinds of surface defects.

Conventional fabrication methods attempt to take offscratches by an iterative step of polishing, but with lowremoval rates that require a long polishing time. Moreover,any further treatment may also spoil the quality of surfacewaveform and create new scratches. The Fraunhofer Institutefor Laser Technology ILT has developed a laser-based processchain for manufacturing optics.[6] This process offers manyadvantages compared to conventional polishing specially forfree form optics but it is not yet operational for large optics.We propose to adapt the method for removal of scratches onfused silica optics by using a CO2 laser on a small area of theoptics.[7] So we add a new step using CO2 laser in aconventional optics fabrication process in order to make itmore predictable and less iterative. It is intended to validatetechnological choices made for LMJ and to prepare for itsexploitation.

Section 2 develops the key principles for scratch repair andits advantages. In Section 3, we describe the dedicated toolsthat we use to demonstrate this new process on a silica platerepresentative of LMJ optical fabrication. Then, the operations

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P. Cormont et al./Relevance of Carbon Dioxide Laser to Remove Scratches…

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conducted on our demonstrative silica plate are given in
Section 4. The Section 5 contains studies for our processacceptance including surface topology, calculation of beampropagation, and ultraviolet laser damage test.We present ourconclusions in Section 6.

2. Key Principles of Our CO2 Laser Operation forDefects Removal

Localized CO2 laser heating of silica glass has demonstrat-ed its capacity to mitigate surface damage sites on optics usedin high power laser application.[8–10] In that way, severalworks have been carried out to optimize the process.[11,12] As,for example, thermal analysis of the laser silica interactionunder CO2 laser irradiation has beenwidely investigatedwithboth experiments[13,14] and simulations.[15,16] A schema of thegeneral succession of steps from laser irradiation to surfacecooling is represented in Figure 1. The first step is to localizethe defect that will be annihilated (a). Then during laserirradiation (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 isusually a crater with a surrounding raised rim and a laser-affected zone.

The application of two successive heatings by CO2

laser with adapted different parameters offers the possibilityto improve damage repairing sufficiently to extend thelifetime of the silica components.[17] Results of this studyare resumed in Figure 2. For this experiment, each damagedsite was heated by a first CO2 laser (1 s, 0.6mm at 1/e2 and5.5W), and then a second heating (1 s, 1.4mm at 1/e2 and12.5W) was applied. Thanks to the polariscope analysis,[18]

we observed unambiguously the area of major stress. In ourconcern, residual stress area is initially correlated with thedamage including all surrounding fractures (a). After the firstirradiation by CO2 laser, which eliminated completely thedamage, it forms a ring adjacent to the crater (b). The secondheating reduces these stresses sufficiently to make theminvisible to the polariscope but it creates a new stress area at a

Fig. 1. Successive steps of the defects mitigation method.

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greater distance from the crater (c). This new zone is largerthan the older one with equivalent intensity of relativeretardation. Although the second heating by CO2 laserremoves debris and smoothes crater edges, the polariscopemeasurement shows that it also causes the initially damagedarea to spread out over a much larger surface. Nevertheless,the damage test at 355 nm demonstrates that such amodification has a beneficial effect regarding UV laserirradiation (d).

3. Productions Tools

The experimental set-up shown in Figure 3 has beendeveloped 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 CO2

laser. For the detection, we use a damage mapping system(DMS) that includes a transversal sample illumination and ahigh-resolution camera.[19] Then we record the x – y coordi-nate for each defect. By means of an automatic x – ytranslation, the selected defect is placed in front of the CO2

laser beam. A long working distance microscope with a fieldof view of 3.6� 2.7mm2 observes this area of interest. As it canbe seen in Figure 3, the microscope is equipped from theopposite site compared to the camera system. The CO2 laserfrom Synrad (Firestar V20) operates at a 10.6mm wavelengthwith a 20W maximum power. The pulse length is adjustablein a large range of duration (milliseconds to several seconds).Mean power control is achieved by pulsed width modulationat a 5 kHz frequency: a duty cycle of 10% corresponds to apower of 2Wand 100% to 20W. The laser output power can beadjusted by a half wave plate and a polarizer. The beam isfocused with a ZnSe lens with a 254mm focal length. Thelatter is mounted on a z-translation stage to adjust the beamdiameter on the sample from400 to 2500mmmeasured at 1/e2.Different diagnostics in the laser path measure its power, andits temporal or spatial profile. Finally, adjusting the followingparameters: beam focus size, mean power, and pulse length,we can easily vary the laser energy deposition.

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Fig. 2. Polariscope images for a typical damage site (a), its transformation after the first heating by CO2 laser(b) and the impact of the second heating on the previous crater (c). The laser damage probability as a functionof the fluence of irradiation at 355 nm is given for the three configurations (d).


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To stabilize a defect longer than 100mm, we need to heat itwith numerous x – y positions. Thus, it is useful to computer-ize the procedure for complete automation. The objective is todetermine each shot position for running automatically thelaser and for moving the sample. Then, the operator has onlyto select the defect to mitigate and the CO2 laser parameters.The different steps for image processing in the case ofscratches on an optical component are summarized in

Fig. 3. Defect observation and laser operation setups.

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Figure 4. The first step is to select the defectthat will be reappeared in the DMS image ofthe optical component. In that way, wedelimit a rectangular area including thewhole parts of the scratch even if notcontiguous (a). Then, the selected image isbinarized and the noise eliminated to isolatethe different constituent of the scratch (b) and(c). The presence of slick steps is frequent.From all these fragments, the defect isoutlined (d), and its skeleton is calculated(e). Finally, laser positions are generated onthis latter following the distance between twosuccessive irradiations defined by the opera-tor (f). We note that this work configurationincluding the image processing is well suitedtowork on large optics such as LMJ optics butthey can also be easily adapted to other opticsand applications.

4. Fabrication Process

In this study, we are interested in improv-ing an optics manufacture by introducing theCO2 laser heating after the step of polishing,and before the finishing step. To address thisissue, we investigate the scratches removal

by CO2 laser on a large optic. The fused silica sampleused is glass 7980 from Corning (NY, USA) polished byTHALES-SESO (http://www.seso.com). The sample is 15mmthick with a surface of 200� 200 mm2 that corresponds to thehalf-scale of LMJ components. As displayed by observationswith the DMS in Figure 5a, a scratch of about 50mm large and10 cm long was formed at one corner of the optic after

polishing. We split this later in three zones toevaluate the surface modification during ourrepair using successively two laser heatings,as shown in Figure 5. The zone 1 has not beenirradiated by CO2 laser in order to remain areference of the initial scratch. We know fromprevious works[7] that adjusting subtly theparameters of the CO2 laser can heal cracks. Afirst heating has been done on zones 2 and 3and then a second heating only on zone 3.Zone 2 in Figure 5b indicates that the scratchis well reduced but is still visible. Therefore,the second heating is useful not only toimprove laser damage resistance[7] but also toremove the whole defect in view of the DMScharacterization. In fact, this second heatingwas realized with a run of 20 shots beforemoving the optical component to the nextposition along the scratch. Compared to theshots of the first heating, shots for the secondheating have the same pulse duration but awider beam and a higher power to partially

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Fig. 4. Image processing in order to extract medial axis of scratches and to get the positions of the CO2 laser shots. The treatment progress is illustrated by following each imagesfrom (a) to (f).


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compensate this beam widening. After laser operation, wehave finished the fabrication process with slightly polishingthe optical component, resulting in the DMS image ofFigure 5c. During this final step of polishing, that we callfinishing, the uniform removal was 3.3mm.

5. Results of Studies for Our Process Acceptance

5.1. TopographySurface deformations of our fused silica sample were

characterized using a 3D optical profiler. This microscopefrom ZYGO (New View 7300) is based on coherencecorrelation interferometry. An objective with a magnificationof 10� and a numerical aperture of 0.4 was used, whichpermits us to attain a measurement area of 0.9� 0.9 mm2 withan optical resolution of 1.1mm in x–y. Step height standardsprovided by the manufacturer were used to calibrate theinstrument. The manufacturer specifies the vertical resolutionas about 1 nm. From our measurements, we obtain a 3D-mapcorresponding to the surface level in the irradiated area.

Fig. 5. DMS images of a component after each final step of fabrication: (a) polishing,operations during the step (b).


Figure 6 shows the impact of two successive heatings alonga scratch mitigated with different procedures. For the threezones detailed in the preceding part, 3D-map before (upperrow) and after (middle row) the final polishing are shown,followed by comparison of their profiles (lower row). Thecolor scales of these six images were automatically adjustedwith minimum and maximum values and so are different foreach picture.

The form of the cracks visible in zone 1, independently ofthe finishing, indicates that this is a typical trailing indentscratch if we refer to the categorization made by Suratwalaet al.[20] After the first heating by CO2 laser (zone 2), wedistinguish a central regionwith significantmatter removal[21]

and a surrounding area where silica has been distorted byviscous 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 presentalthough it was indiscernible with DMS characterization asmentioned in paragraph 4. Nevertheless, comparing profile ofzones 1 and 2, there is a moderate enhancement of the initial

(b) CO2 laser irradiation, (c) finishing. Zones 1–3 correspond to different CO2 laser


Fig. 6. The two first rows of images obtained by ZYGONew view 7300 are 3D-maps before and after the surface finishing, and the lower compares their profiles related with thedashed 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 otherscratch 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 noticethe scale difference between the different zones.


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scratch depth (less than a factor two) due mainly to matterejection during the first heating. Then comparing profile ofzones 2 and 3, the scratch depth is reduced by a factor fivethanks to the second heating. We mention that after thepolishing removal of 3.3mm, the two blue profiles for zones 1and 2, which we have shifted by 3.3mm, are in goodagreement with the depth before the finishing step. In zone 3,the blue curve has also been positioned 3.3mm lower than theblack one to take into account the polishing removal.Surprisingly, the expected eradication of the 2mm deeptrench is not total. The shallow residual trench after polishingcan be caused by the neighboring silica affected by the laser. Inthe laser-affected zone, the temperature reached during thesecond heating is higher than the annealing point (1315K forCorning 7980 fused silica). The width of this zone isapproximately equal to the beam diameter at 1/e2 and ismuch deeper than 50mm.[7]

5.2. Beam PropagationDefects on optical components have an impact on beam

characteristics in the neighborhood of the defects and atlonger distances due to downstream propagation mecha-nisms.[2] This problem is even more critical for final optics ofLMJ beam baseline,[2] which are, respectively, the 3v focusing

DOI: 10.1002/adem.201400383 © 2014 WILEY-VCH Verlag GADVANCED ENGINEERING MATERIALS 2014,

grating and the vacuum window. The defects on LMJ 3vfocusing grating may trigger damage on vacuum window.This section presents the numerical approach we carry out toevaluate the effect of fused silica surface defects on the laserbeam propagation. Here, we consider the previously investi-gated scratch. A transverse profile of the scratch, that issufficiently long to be supposed infinite, has been introducedinto a code solving the plane 1D-transverse propagationequation:

@A@z

þ 12ik0

@2A@x2

¼ 0

where A is the laser beam envelope, k0 is the wave number atthe used wave frequency, z is the abscissa on the propagationaxis, and x is the coordinate in the transverse direction. Suchprofile impact corresponds to a phase perturbation that is asimple way to introduce a defect into the laser beampropagation modeling. Then, we calculate the evolution ofthe maximum of the ratio between the laser beam intensity ata distance z and the initial intensity at z¼ 0 (notedCMAX). Werealized this simulation for the two different profiles of thezone 3 of the Figure 6, i.e., after the complete cracks repairbefore and after the finishing process. For a 3v planar wave,

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the evolution of CMAX 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 theninduces a very rapid increase of the laser beam intensity,CMAX¼ 5.7 at z� 1mm. During the propagation this oscillat-ing value decreases, but is still around 2.5 at the vacuumwindow. By opposition, the smoother profile after finishing islinked with a low increase that reaches 60% after 0.65m ofpropagation (CMAX¼ 1.6). This is 2.5 less than before finishing.It is noticeable that improving the finishing process, usingRMS technical as, for example, could enhance such an effect.

5.3. UV-Laser Damage TestsLaser damage resistance tests were performedwith a table-

top laser, whose main characteristics are comparable with theLMJ laser beam. The laser used is a Nd:YAG, which deliversan equivalent pulse length of 2.5 ns at 355 nm with a 10Hzrepetition rate. The laser beam is focused by a 5m focal lengthto get a Gaussian spatial profile with a diameter of 0.9mm at1/e2. Damages are detected in situ with a mobile visualinspection system. In order to scan the whole area, the sampleto be tested is translated continuously along a first directionand stepped along a second direction. Repeating this test atseveral fluences on different zones allows us to determine thenumber of damage sites versus fluence, thus the damagedensity. More details of this method of characterization arefurnished elsewhere.[24]

We have applied this method on our demonstrationcomponent presented in Section 4. The whole surface wastested except the three zones. We have measured a density ofdamage equal to 0.2 damage sites per cm2 at 14 J cm�2. Thisresult is in good agreement with results obtained on similaroptics without visible defects. In parallel, we have tested zone1 in mode 1-on-1, which consists in having one laser shot forone position on the optical surface. At fluence of 5 J cm�2, 50%

Fig. 7. Evolution of CMAX as a function of the distance of propagation; z¼ 0corresponds to the 3v grating and z¼ 0.65m to the vacuum window. The black curveis the evolution of CMAX for the black profile of the zone 3 in Figure 6, the blue curvenamed “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 laserprocess.


of the site irradiated showed damage growth, and at 6 J cm�2

the totality of sites (100%). These results on scratches confirmusual threshold for damage growth. We have also tested zone3 in mode 1-on-1 at higher fluences in order to evaluate ourrepair process. The results of these UV-laser damage tests arepresented in Figure 8. We can notice that after our repairprocess, the laser resistance is even better than in Figure 2. Themain difference between the process tested on Figure 2 andthe one tested on Figure 8 is the slight polishing after laserprocess. So we assume that the finishing after laser process isan explanation of this excellent result. These results validatethe relevance of this new process for LMJ optics fabricationbecause themaximum fluence expected on LMJ is 14 J cm�2, asindicated by the dashed line in Figure 8.

6. Summary and Conclusion

We have presented a new fabrication process formanufacturing optics. In this fabrication process, the conven-tional polishing loop for scratch removal has been replaced bya CO2 laser operation including two main steps. The first stepuses a smaller beam and a higher power density than thesecond step. The first step replaces scratch fractures by asmooth trench and the second step improves UV laserresistance and fills in the trench so that the surface is smoothenough to avoid beam propagation problems.

To validate our process, we have applied our method to anoptical component of large dimensions. This 20 cm� 20 cmwindow was polished with conventional tooling in order toobtain specified flatness. After polishing, a several millimeterslong scratch was visible on the surface. This 10mm deepscratch was then removed by CO2 laser operation. Finally, aconventional finishing step was done in order to obtain thelow roughness necessary for LMJ optics. We have observedthat a shallow residual trench has replaced the scratch and wehave evaluated the impact on beam propagation. The mostimportant result is that the laser resistance of the window hasbeen greatly improved. The laser damage thresholdmeasured

Fig. 8. Laser damage probability as a function of the fluence of irradiation at 355 nmbefore and after repairing the scratch. The green dashed line indicates the LMJmaximum fluence.



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at 355 nm with a 3 ns pulse duration has been increased from5 to 15 J cm�2, which is higher than the maximum fluence onLMJ.

In this study, we have demonstrated that CO2 laser is anappropriate tool to remove scratches on fused silica opticsbecause it is rapid, localized to the scratch, and it creates nodebris. Nonetheless, we are still looking for finishingtechniques to reduce the impact on beam propagation afterthe laser process.

Received: August 15, 2014Final Version: September 23, 2014

[1] J. Ebrardt, J. M. Chaput, J. Phys.: Conf. Ser. 2010, 244,032017.

[2] S. Mainguy, B. Le Garrec, M. Josse, Proc. SPIE 2005, 5991,599105.

[3] N. Bloembergen, Appl. Opt. 1973, 12, 661.[4] H. Bercegol, P. Bouchut, L. Lamaignère, B. Le Garrec,

G. Raz�e, 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, 28272.

[8] E. Mendez, K. M. Nowak, H. J. Baker, F. J. Villarreal,D. R. Hall, Appl. Opt. 2006, 45, 5358.

[9] S. Palmier, L. Gallais, M. Commandr�e, P. Cormont,R. Courchinoux, L. Lamaignère, J.-L. Rullier, P. Legros,Appl. Surf. Sci. 2009, 255, 5532.

DOI: 10.1002/adem.201400383 © 2014 WILEY-VCH Verlag GADVANCED ENGINEERING MATERIALS 2014,

[10] I. L. Bass, G. M. Guss, M. J. Nostrand, P. J. Wegner, Proc.SPIE 2010, 7842, 784220.

[11] S. T. Yang, M. J. Matthews, S. Elhadj, D. Cooke,G. M. Guss, V. G. Draggoo, P. J. Wegner, Appl. Opt.2010, 49, 2606.

[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, 063106.

[14] S. T. Yang, M. J. Matthews, S. Elhadj, V. G. Draggoo,S. E. Bisson, J. Appl. Phys. 2009, 106, 103106.

[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, 26068.

[18] L. Gallais, P. Cormont, J.-L. Rullier,Opt. Express 2009, 17,23488.

[19] R. Prasad, M. Bernacil, J. Halpin, J. Peterson, S. Mills, RHackel. 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, 3888.[24] L. Lamaignère, G. Dupuy, T. Donval, P. Grua,

H. Bercegol, Appl. Opt. 2011, 50, 441.

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