Mid-Spatial Frequency Error (PSD-2) of optics induced during CCOS and full-aperture polishing

Transcription

1 J. Europ. Opt. Soc. Rp. Public. 8, Mid-Sptil Frequency Error PSD-2 of optics induced during CCOS nd full-perture polishing D. Lio Chengdu Fine Opticl Engineering Reserch Center, Chengdu, , Chin Z. Yun Chengdu Fine Opticl Engineering Reserch Center, Chengdu, , Chin C. Tng Chengdu Fine Opticl Engineering Reserch Center, Chengdu, , Chin R. Xie Chengdu Fine Opticl Engineering Reserch Center, Chengdu, , Chin X. Chen Chengdu Fine Opticl Engineering Reserch Center, Chengdu, , Chin Mid-Sptil Frequency MSF Wvefront Error of optics divided into the PSD-1 nd PSD-2 rnges plys n importnt role in the performnce of high power lser systems. The present work focuses on the PSD-2 rnge in terms of short ripples which hven t been well studied in the literture. Chrcteristics nd origins of these short ripples were detiled, wherefter smll tool computer controlled polishing CCP nd conventionl full perture polishing experiments were conducted on fused silic. It is reveled tht PSD2 error is independent of the min process prmeters including lp rotting rte nd polishing pressure in continuous polishing nd tool pth pitch nd crossfeed velocity in smll tool CCP processes. Wheres the type of polishing lp hs decisive effect on PSD2 error of the optics. The pitch lp shows superiority in restrint of short ripples over polyurethne pd. By introducing dimond conditioner for dressing polyurethne pd, the PSD2 error hs been gretly decresed. [DOI: Keywords: PSD2 error, pitch lp, polyurethne pd, smll tool CCP, continuous polishing 1 INTRODUCTION The wvefront qulity of the optics used in high power lsers is specified over continuous rnge of sptil frequencies from to mm 1 [1]. This rnge of frequencies is divided into the four seprte bnds: surfce figure sptil periods within 400~33 mm, wviness-1 i.e., PSD-1, 2.5~33 mm, wviness-2 PSD-2, 0.12~2.5 mm nd surfce roughness 0.01~0.12 mm. Figure determines most spects of the min focl spot, wviness-1 effects the tils of the focl spot, while wviness-2 determines pinhole loding. Both of the wviness regions influence ner-field modultion. Roughness contributes to pinhole loding nd hs been shown to ply role in filmenttion [2]. There re two types of specifiction for the Mid-Sptil Frequency MSF PSD-1 nd PSD-2 errors, specifiction for the Rms vlue <1.1 nm over tht rnge, nd the other not-to-exceed line for the power spectrl density PSD s function of sptil frequency [3]. Note tht the integrl of the mesured PSD over the ssocited sptil frequency bnd is equivlent to the Rms vlue [1]. The entire optic is tested using wvefront mesurement when evluting whether or not n optic meets PSD-1 spec. Much smller regions e.g., 10 x 10 mm of the surfces re smpled for evlution of the PSD-2 spec [3, 4]. Considering the trnsmitted or reflected wvefront qulity is specified respectively for optics tht re trnsmitting or reflecting the lser bemline, the reflection optics required of reflected wvefront qulity is considered nd evluted by the Rms vlue over the PSD-2 rnge in the present study. It is well known tht the PSD-1 error, in terms of long ripples i.e., wves, is intensively correltive to CCP processes chrcterized by smll tool [5]. This error is minly ffected by the initil surfce error distribution sptil nd frequency domin, the removl function chrcters profile, removl efficiency nd stbility nd the dopted pths [6, 7]. A lot of ttention hs been pid to the optimiztions of CCP process prmeters nd mchine configurtions [5],[7] [9]. There re lso some novel technologies nd processes developed to restrint the long ripples in the PSD-1 rnge, such s pseudo-rndom tool pths nd Vibe-finishing [10] [12]. Although there re mny reserches conducted on chrcteristics of the long ripples of the PSD-1 region in the literture, little hs been studied bout the PSD-2 region. PSD-2 error represents short ripples cross the opticl surfce with scle lengths between 0.12 nd 2.5 mm. These short ripples generted on optics my be ssocited with those fetures in the fbriction process tht with scle lengths lying in or close to 0.12~2.5 mm. The importnt fctors mybe include the nture or properties of the lp mteril, surfce chrcteristics of the lp/pd nd the kinemtics of the lp/pd nd optic which intensively correltive to the polishing process. Received Mrch 15, 2013; published My 02, 2013 ISSN

2 J. Europ. Opt. Soc. Rp. Public. 8, D. Lio, et.l The present work is n ttempt to understnding the genertion nd restrint of the short ripples of PSD-2 error. Experiments were crried out on fused silic flts with typicl full-perture polishing process continuous polishing, CP nd smll tool CCP processes. The extensively used pitch lp nd polyurethne pd were employed in the conventionl full-perture polishing to evlute the effect of lp mteril on PSD-2 error. For ssessing the effect of texture of polyurethne pd, fused silic smples were polished on those pds with or without in-situ dressing by dimond conditioners. Furthermore, series of polishing experiments were conducted by smll tool CCP process with pitch lps of vrible sizes nd differing rster too-pth pitches. PSD-2 error of these prepred fuse silic hve been mesured nd nlyzed. 2 PLISHING EXPERIMENTS AND PSD2 MEASUREMENT Conventionl full-perture polishing experiments were conducted on 36 continuous polisher. Round fused silic smples 180 mm dimeter 20 mm thick previously ground were polished using two typicl lps, i.e., pitch lp nd polyurethne pd LP66 by Universl Photonics Inc. to check the effect of the nture of lp mteril on PSD-2 error. All the smples were polished for 5 hours under the pressure of 0.4 psi. The plten speed ws set to 20 rpm, noticing tht the rotting rte of the optics ws slightly lower thn tht of lp/pd. Smll tool CCP process ws conducted on n OP1000 CNC polishing system developed for Plno optics. A series of experiments were crried out with pitch lp to ssess the effect of process prmeters on the PSD-2 error. The round fused silic smples were previously ground nd polished on the continuous polisher using pitch lp. The rster tool pth, one of the typicl tool-pths extensively used by sub-perture polishing processes ws dopted. The most importnt process prmeters including the lp size nd tool-pth pitch were considered, nd ll smples were removed uniform depth of 0.3 µm in polishing itertion. An opticl Surfce Profiler ws pplied to the mesurement of the reflected wvefront error over PSD2 rnge. The profiler uses filtered white light in Michelson interferometer configurtion. The mesurements were tken in phse mesuring mode with 0.5x objective nd 0.5x mgnifiction. This provided field of view of 14.1 mm by 10.6 mm. A set of 5 rdil-uniformly distributed smples were mesured for ech surfce nd the verge Rms vlue over PSD2 rnge ws obtined for evluting the PSD2 error. 3 RESULTS AND ANALYSIS 3.1 Effect of process prmeters on PSD2 error Continuous polishing hs been the mostly used full-perture polishing process for flt optics. Process prmeters determining the polishing performnce minly includes lp rotting rte, polishing pressure nd so forth s described by the Preston eqution [13]. Figure 1 shows PSD2 error of fused silic FIG. 1 PSD2 error of fused silic s function of polishing pressure nd rotting speed in continuous polishing. Polishing pressure is set to 0.4 psi nd Rotting speed is set to 20 rpm. Dimeter of Tool pth PSD2Rms/nm pitch lp/mm pitch/mm TABLE 1 PSD2 error of fused silic by smll tool CCP with vrible prmeters. polished with pitch lp under differing lp rotting rte nd polishing pressure. It is reveled tht the Rms vlues over the PSD-2 rnge keeps t ~0.72 nm for vrible polishing pressures. Also, t certin polishing pressure of 0.4 psi, differing rotting speeds result in stble PSD2 error of ~0.72 nm Rms. This my suggest tht PSD2 error is independent of these minly prmeters but some other crucil fctors. Tble 1 shows the experimentl configurtion nd results of the smll tool CCP process. The Rms vlues over the PSD-2 rnge re observed to keep stble t ~0.73 nm Rms for vrible pitch lp dimeters nd tool-pth pitches. It my be concluded tht effect of the min process prmeters is negligible. Notble is tht the PSD2 errors obtined in vlues re lmost identicl s obtined in the full-perture polishing using the pitch lps. Generlly, modern sub-perture deterministic opticl fbriction processes re more prone to ripple errors. The size of removl function nd the pitch of tool pth ply n importnt role in the sptil frequency of the ripples or wves. In most smll tool CCP processes, the polishing tools re usully mnufctured by pitch or polyurethne pd. These tools generlly hve dimeter of tens of millimeters. Regulr tool pths commonly spirl or rster with pitch lrger thn severl mm re dopt in considertion of efficiency. Fetures of these scles lie in the sptil frequency of long ripples, nmely the PSD-1 rnge of 2.5~33 mm, rther thn in the PSD2 rnge of 0.12~2.5 mm. Thus they don t hve much impct on the PSD-2 error in terms of short ripples. 3.2 Effect of lp type on PSD2 error Pitch lp nd polyurethne pd, the two most used polishing lps, were determined herein. Figure 2 shows PSD- 2 error of the opticl surfces polished with the pitch lp nd polyurethne pd. The Rms vlue over PSD-2 rnge is ~0.71 nm for pitch lp polished surfce, wheres the optic polished by pd hs much higher Rms vlue of ~1.62 nm. It cn

3 J. Europ. Opt. Soc. Rp. Public. 8, D. Lio, et.l FIG. 3 SEM imges showing surfces of pitch lp nd polyurethne pd. FIG. 2 PSD2 error of fused silic polished by pitch lp nd polyurethne pd. Rms=0.71 Rms=1.62. be seen tht the pitch polished surfce is much smooth while the pd polished surfce shows sub-mm & mm sized fetures, nmely protrudes nd conglomertions. SEM imge of the pitch lp shows uniformly smooth structure of the surfce see Figure 3. This smooth surfce is beneficil to restrint of ripples or mid-frequency wves. Furthermore, the viscoelstic properties of pitch ply n importnt role. Pitch cts s highly viscous Newtonin fluid for long time scles it undergoes sher motion tht is proportionl to the sher stress, so it flows to conform to the shpe of the optic [14]. This fluidity t long time scles ensures smoothness of the whole lp surfce when the lp strokes on the optic. Nevertheless, pitch will ct like solid for short time period under stress [15]. It does not flow fst enough to conform to locl short ripples tht my exist on the optic when the pitch lp trverses the optic with certin crossfeed velocity. As result, these short ripples of PSD-2 rnge would be smoothed. For the polyurethne pd, the texture nd micro-structure of pd surfce plys vitl rule in ripple genertion. Generlly, the pd surfce hs numerous independent pores seprted by sperities [16] [18]. Most of these isolted pores nd sperities hve size of severl tenths of mm i.e., sub-mm, s shown in the SEM imge of LP66 polyurethne pd produced by Universl Photonics Inc. Figure 3. As brsiveoccupied sperities yield mteril removl during polishing, it is prone to bringing fetures of their sub-mm scles on the optic. During pd polishing, the surfce of the workpiece is polished when the brsive prticles re pressed ginst the surfce by the sperity of the pd. Under the force of the sperity, the prticles re pushed ginst the surfce, drgged long by the sperity t the reltive velocity of the polishing pd with respect to the workpiece [19]. It is well known tht mteril removl rte is mrkedly dependent of the pd surfce roughness, which cn be chrcterized by the roughness of the pd sperities. Pd sperities were glzed s result of wer nd plstic deformtion by the workpiece during polishing, leding to roughness vrition [20]. Thus dimond conditioning is continuously performed to brek up the pd surfce nd restore the sperity roughness during polishing. The sub-mm sized sperity fetures especilly the surfce roughness my hve significnt impct on the roughness nd short wviness of the polished surfce. For these resons, we introduce dressing process in pd polishing to decrese pd surfce roughness nd my thus the SPD2 error of optics. To ssess the effect of pd surfce roughness on the PSD-2 error, fused silic flt ws polished using continuous polisher. The polyurethne pd fixed on the plten ws in-situ conditioned by dimonds of vrible sizes. The surfce roughness of pd sperity ws mesured fter ech polishing itertion using surfce roughness tester Mitutoyo SJ-400. Considering the sub-mm sized pores nd sperities, smpling length of 0.08 mm ws dopted s shown in Figure 4. The smpling loction for roughness mesurement should be crefully selected for ensuring tht the stylus trverses on the sperity voiding flling into deep pores during the mesuring. A set of 5 rndomly selected loctions were mesured nd the verge Rq vlue ws obtined for evlution of the surfce roughness. Figure 5 indictes tht dimond dressing of polyurethne pd hs significnt effect on pd roughness nd PSD2 error of fused silic. The pd without being dressed yields high PSD2 error ~1.68 nm Rms, but fter dressed with 100 µm nd 70 µm dimonds the Rms vlue decreses to ~1.55 nd ~1.28 nm, respectively. Furthermore, dressing by 40 µm leds

4 J. Europ. Opt. Soc. Rp. Public. 8, D. Lio, et.l FIG. 4 Schemtic of micro-structure of pd nd roughness mesuring. FIG. 5 Effect of dimond size on the pd roughness nd the resultnt PSD2 error of fused silic. Roughness of polyurethne pd, PSD-2 error Rms of the opticl surfces. to fvorble Rms vlue of ~1.02 nm, which stisfies the specified eligible vlue 1.1 nm. Smpling surfces polished by nondressing pd, 100 µm, 70 µm nd 40 µm dimond dressed pds re shown in Figure 6. It is obvious tht the topogrphy of the surfce polished by non-dressing pd is much nonuniform with submicron & micron sized protrudes nd conglomertions. These protrudes nd conglomertions re lightly smoothed by using 100 µm nd 70 µm dimond dressed pd, nd further lmost eliminted for 40 µm dimond dressed pd leding to n eligible Rms vlue of ~1.0 2 nm. Although dressing process wouldn t lter the size of pores nd sperities on the pd surfce, the effect of pd dressing on the pd surfce roughness hs been confirmed see Figure 5. Some other reserch hs lso reveled pd dressing hs n impct on pd surfce roughness nd hence the mteril removl [21]. Conditioner with lrger dimonds is deemed to increse the surfce roughness of the sperities, which is most likely to promote the scrping process of the optic surfce by the sperities. Tht is to sy, smll ripples on optic induced by these sperities re intensified. As result, pd dressing using smll dimonds would decrese the pd roughness nd hence the PSD2 error of optics in terms of smll ripples. FIG. 6 Typicl unfiltered rw dt of surfces polished by polyurethne pd with no dressing, 100 um dimond dressing, c 70 µm dimond dressing nd d 40 µm dimond dressing. 4 CONCLUSION Short ripples in the higher frequency region of MSF error ws studied in terms of reflected wvefront of fused silic. The conventionl full perture polishing experiments reveled tht the pitch lp shows significnt dvntge of restrining short ripples over polyurethne pd due to its fluidity. The sub-mm & mm sized pores nd sperities on the polyurethne pd surfce seem to be the origin of short ripples on the pd-polished optics. Pd dressing hs significnt effect on PSD-2 error. Rms vlue over PSD-2 rnge hs been decresed from 1.68 nd 1.55nm with the newly-used nd 100µm dimond dressed pd, respectively, to n eligible vlue of 1.02nm with the 40µm dimond dressed pd. This my be scribed tht fined dimond conditioner reduces surfce roughness of the pd nd thus diminishes scrpe effect on optic surfce by the pores nd sperities. In the typicl subperture polishing processes, smll tool CCP process exhibited superiority in restrint of short ripples s the lp size nd tool-pth pitch go completely beyond the sptil wvelengths of PSD-2 rnge. The study hs provided some insights into the genertion nd restrint of short ripples on optics induced during polishing

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