Damage thresholds for single layers of common coating materials such as MgF/sub 2/, SiO/sub 2/, ThF/sub 4/, Al/sub 2/O/sub 3/, ZrO/sub 2/, and TiO/sub 2/ are given. Laser-induced damage of coated and uncoated optically polished surfaces has been studied at LLL for laser pulsewidths between 0.17 ns and 3 ns. Two 1064 nm Nd lasers generated this range of pulsewidths. This report contains a review of the results. (MOW

Lowdermilk, W.H.

Milam, D.

Wirtenson, G.R.

UNT (University of North Texas) Digital Library

7S Cent-7269 71--Z, PREPRINT UCRL- 81630 Lawrence Livermore Laboratory LASER DAMAGE TESTING AT LLL - AN OVERVIEW AND AN UPDATE* D. Milam, W. H. Lowdermilk and G. R. Wirtenson fEF-r September 1, 1978 This paper was prepared for submission to the Electro Optics/Laser '78, Industrial and Scientific Conference Management, Inc. September 19, 1973 - Boston, Massachusetts This is a preprint of a paper intended for publication in a journal or proceedings. Since chdnges may be made before publication, this preprint is made available with the understanding ;hat it will not be cited or reproduced without the permission of the author. oisntmcnow or THIS DOCUMENT IS tiwujuiTEn Ar; OVERVIEW AND UPDATE A brief review of the experimenta1 technique and apparatus used to obtain optical damage threshold values with iiriearly polarized l.Ou' nm pulses is presented. Short pulse damge threshold for six commonlv use.- iptical coating materials for a ranne of refractive irdices is given. Data comparing the damage of hare surfaces, anti-reflection coatings with and without barrier layers and multilayer high reflectors is presented for 125 p sec pulse durations. Results from recent experiments are shown describing the pulse duration dependence of damage threshold (125 p sec to 3.2 n sec) for bare surfaces, both bowl feed and conventional polish and for various optical coatings inluriing multilayer reflectors, polarizers and anti-reflection designs with and without layers. Ztg&BA*^ :'i- "i.' LASER DAMAGE TESTING AT LLL AN OVERVIEW AND AN UPDATE* by D. Milam, W. H. Lowdermilk and G. R. Wirtenson I. INTRODUCTION The threshold for laser-induced damage of optical surfaces limits laser power output and is therefore a central problem in the development of high power laser fusion systems. Laser-induced damage of coated and uncoated optically polished surfaces has been studied at LLL for laser pulsewidths between 0.17 ns and 3 ns. Two 1064 nm Nd lasers generated this range of pulsewidths. The oscillator for one laser system was passively modelocked and produced the 0.17 ns pulses. The second laser oscillator was actively O-switched to produce 30 ns single-axial-mode pulses. A portion of that pulse, with a continuously variable width greater than 1 ns, was switched-out by a Pockels cell and then amplified. II. SUBNANOSECOND PULSEWIDTH DAMAGE The following observations summarize the results of numerous measurements at LLL of damage thresholds for subnanosecond pulses. 1. The oxide and fluoride films commonly used in the visible and near infrared spectral region have damage thresholds less than one-half the damage thresholds of polished glass or fused silica surfaces. * Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under contract No. W-7405-Eng-48. -2-2. Damage thresholds for multilayer high-reflectivity (HR) coatings exceed the thresholds for anti-reflection (AR) coatings by 20 to 30 percent. This result was unexpected because the standingwave electric field strength is greater in the HR coating's outer high-index layers than in high-index layers of the AR coating. Penetration of the electric field to the interface between substrate and coating for AR but not for HR coatings suggests that surface chemical or physical interactions in the interface region plays a fundamental role in determining the damage threshold. L 3. The interface region's importance is further suggested by the statistical improvement in AR coating damage threshold when a low-[21 index barrier layer is first applied to the substrate. Fig. 1 illustrates this improvement (30%) in the mean damage threshold for four 7ayer Si02/Ti02 AR coatings on BK-7 glass. 4. Damage thresholds are greater for films of lower-index materials, m continuing a trend observed in the bulk. The measurements at LLL on quarter and half-wave films are summarized in Table 1. TABLE 1: Damage Thresholds for Single Layers of Common Coating Materials 2 Damage Threshold (J/cm ) Material Index Mean Best MgF 2 1.35 5 7 Si0 2 1.45 5 8 ThF 4 1.50 5 7 A1 2 0 3 1.60 5 6 Zr0 2 2.00 4 5 Ti0 2 2.30 4 5 -3-5. Linear absorption in coatings may alone be large enough to account for observed thresholds. III. PULSE-DURATION DEPENDENCE OF DAMAGE THRESHOLD Because solid state lasers for fusion research will operate with pulsewidths in the range of 0.1 ns to 3 ns,'- •* measurements were made to determine the damage thresholds and scaling laws in this pulsewidth range. Thresholds of bare polished glass surfaces and optical thin films have been measured using linearly polarized 1064-nm pulses with durations of 0.17 ns, 1.0 ns, 1.6 ns, and 3.2 ns. The beam profile in the sample plane was elliptical, but approximately Gaussian. The beam diameter was greater than 2 mm at the e" intensity level. Details of the experimental procedure have T5l been presented elsewhere.1 J Polished Glass Surfaces BK-7 Sample A: Sample A was a 2-inch diameter, 3 -wedge window whose surfaces were produced by conventional polishing. Details of grinding and polishing were unknown, except for the usual LLL requirement that polishing be done with "non-absorbing" abrasives. In practice, this requirement simply bans the use of iron oxide. The sample had been used for several months as a beam splitter and was cleaned many times with alcohol and acetone. Damage thresholds measured for sample A are shown in Fig. 2. Guenther ejt al. suggested that damage thresholds should increase in proportion to the square root of the pulsewidth. The curves drawn in Fig. 2 are best-fit square root functions. Entrance surface thresholds appear to obey the square-root scaling law, and the deviation of the exit surface thresholds is slight. -4-For a sample with linear refractive index n = 1.50, irradiated at normal incidence, the intensity at the exit surface exceeds the intensity at the ? ? T7l entrance surface by the ratio R = 4n /(n+1) = 1.44L If the surfaces of a sample sre identical their damage thresholds should have this ratio. The measured ratios of entrance-surface to exit-surface thresholds for all samples are presented in Table 2. Figure 2 presents two sets of measurements. The first measurements, denoted by squares were made after the sample was cleaned by pulling a wetted tissue across the surface. After such cleaning, examination by back-lighting with an intense light revealed a high level of scattering from both surfaces. This scattering was initially thought to be due to a marginal polish. However, the thresholds measured at 1.6 and 3.2 ns were lower than expected, and the entrance and exit thresholds at 3.2 ns were nearly equal, suggesting that the thresholds were not intrinsic properties of the surfaces. After these measurements, the sample was scrubbed vigorously with soap and water and then witn acetone and alcohol. The surfaces then exhibited much less scatter. This cleaning procedure significantly increased the damage thresholds as the data denoted by circles shows. This experiment illustrates that particulate matter, airborne or in solvents, adheres so well that it is not easily removed by simple cleaning, and significantly reduces damage thresholds at the longer pulse lengths. Our earlier measurements showed, however, that careful cleaning does not affect the damage threshold for 0.17 ns pulses. -5-BK-7 Sample 081 Sample 081 was PH-3 grade BK-7, two inches in diameter, 0.40 inches thick. Grinding was done with alumina; at each abrasive size the surfaces were ground to a depth equal to three times the mean diameter of the previous grit size. Final polishing was done with commercially available Ce0~. Surface etching at LLL uncovered no subsurface fracture and the rms surface j-Q"| roughness was measured by the Naval Weapons Center to be 11 A\ Typical, commercial optical polished BK-7 surfaces have rms roughness between 15 A and 40 A. Damage thresholds for sample 081 were 25% le^s at all pulse durations than those measurements for sample A. Thresholds for Loth surfaces agreed with the square-root scaling rule, although use of the full error range was necessary to obtain the fit. Because thresholds were below those of sample A, this sample was vigorously cleaned to determine whether its thresholds could be raised. Soap and water, alcohol, acetone, and a commercial calcium carbonate compound were used. This additional cleaning failed to increase the damage threshold. Sample 081 was one of a set of 200 BK-7 substrates prepared by a single vendor. Two additional substrates from this set were tested at 1.0 ns only. Their thresholds were 15-17 J/cm , the same as 081. It was unexpected that thresholds for this carefully prepared sample set were 25% less than for sample A which had an uncertain history. A possible explanation for these lower thresholds is that surface damage may be governed by absorbing impurities. Iron-oxide compounds for example have been shown to reduce damage threhsolds for 30 ns pulses. [9] Recent chemical analysis found large metal impurity concentrations in commercial Ce0„ polishing compounds.HO] Further studies of the polishing compound impurities and efforts to produce pure CeO ? for polishing are in progress. -6-BK-7 Sample C18 This sample was 1.5 inches in diameter and 6 ram thick, and was not large enough to allow measurements at all four pulse durations. Both surfaces were conventionally polished, and one surface was later bowl-feed polished. 8y comparing it with a standard sample, rms surface roughness of —10 A was estimated for the bowl-feed surfaces. The thresholds for sample C18, shown in Fig. 3, were the lowest observed for BK-7 glass in this study, and they do not follow the square-root scaling rule. Fused Silica A single fused silica sample, 2 inches in diameter and ,4 inches in thickness, was studied. This sample was a production part "polished with non-absorbing abrasives." Damage threshold data are shown in Figure 4. By using the complete error bar, a square root function can almost be fit to the front and rear surface data. The ratios of entrance to exit damage thresholds are given in Table 2. The expected ratio for fused silica is 1.40. TABLE 2: Ratios of Entrance to Exit Surface Damage Thresholds Pulsewidth (ns) Entrance/Exit Threshold Ratio A 081 C18 Fused Silica 0.17 1.20 - 1.43 1.16 1.0 1.27 1.18 1.24 1.21 1.6 1.28 - 1.09 3.2 1.47 1.46 1.40 0.95 Conclusions: Polished Glass Surfaces From this study, the following general conclusions are drawn concerning the damage thresholds of polished surfaces: 1. Exit surfaces have lower damage thresholds than entrance surfaces, in agreement with theory!-'-! and earlier observations. Entrance surface/ exit surface threshold ratios varied from 1 to 1.45 whereas the ratio 1.44 is expected for identical surfaces on a sample with a linear index of 1.5. 2. Thresholds for most samples increase approximately as the square root of pulse duration. Departure from this scaling law was greatest for sample C18 which had the lowest threshold. 3. Short-pulse (<3 ns) thresholds on surfaces with good optical finish o (rms roughness <40 A) were not directly related to surface smoothness. 2 4. The mean exit surface threshold at 1.0 ns was 17 J/cm . The range observed in this study was 15-22 J/ci/, including both the BK-7 and SiOp samples. 5. Careful cleaning of optical surfaces was required to obtain re­producible results, particularly at 3.2 ns. The presence of ordinary-laboratory dust on focal sites caused threshold reductions of up to 50%. Dielectric Thin Films Six films deposited in large production coating machines for the Shiva laser were chosen for testing. Four of the films were TiCu/SiCL AR films with V 2 undercoats on BK-7 substrates, one was a 45° turning mirror, and one was a maximum reflector at 0 incidence. Thresholds for one of the AR films and one reflector are shown in Figs. 5 and 6 respectively. Results for the other films were similar. Conclusions: Dielectric Thin Films 1. Thresholds increased more slowly than the square root cf pulse duration. 2. The mean threshold for AR films was 6.5 J/cm at 1.0 ns, the range o was 5.5 - 8.5 J/cm . 2 3. The mean threshold for reflectors was 8.5 J/cm . 4. Threshold damage in AR films was not accompanied by emission of light, and the damage morphology did not vary significantly with pulse duration, suggesting that damage was due to absorption. Had damage been due to a mechanism such as avalanche breakdown, associated with an absorbing plasma, light emission would be expected, and the damage at each microsite should expand when driven by longer pulses. 5. Threshold damage in reflectors was accompanied by light, and the damage morphology did vary with pulse duration. Threshold damage at 0.17 ns produced micropits with a mean spacing of about 10 MTTI, each a few microns in diameter. Threshold damage at 1.0, 1.6, and 3.2 ns produced much larger microsites, 30 - 100 Mm in diameter, with a mean spacing of «100 «m. IV. SUMMARY LLL maintains an intense and continuing interest in laser-induced damage. Expriments are in progress to determine the effects of surface roughness, film materials, designs and deposition conditions, the use cf purified polishing compounds and controlled removal of surface layers. These and other experiments will aid our understanding of damage mechanisms and lead to development of materials and processes to improve optical component damage thresholds and the performance of lasers for fusion research. -9-REFERENCES J. H. Apfel, "Further Studies of the Role of Electric Field Strength in Laser Damage of D-'^lectric Layers," Laser Induced Damage in Optical Materials: 1977, NBS Special Publication 509, pp. 251-254. J. H. Apfel, E. A. Enema,-k, D. Milam, W. L. Smith, and K. J. Weber, "The Effects of Barrier Layers and Surface Smoothness on 150-ps, 1.064-MITI Laser Damage of AR Coatings on Glass," Laser Induced Damage in Optical Materials: 1977, NBS Special Publication 509, pp. 255-259. 0. R. Bettis, R. A. House, A. H. Guenther, and R. Austin, "Tne Importance of Refractive Index, Number Density, and Surface Roughness in the Laser-Induced Damage of Thin Films and Bare Serf aces," Laser Induced Damage in Optical Materials: 1975, NBS Special Publication 435, pp. 289-295. Laser Program Annual Report, 1976 Lawrence Livermore Laboratory, UCRL-50C21-7G, pp.2-184 D. Milam, "Laser-Induced Damage at 1064 nm, 125 psec," Appl. Opt. 16, 1204 (1977). R. A. House, J. R. Bettis, and A. H. Guenther, "Preparation Techniques and Hydroxyl Concentration ys. Surface Damage Threshold," Laser Induced Damage in Optical Materials: 1973, NBS Special Publications 462, pp. 310-314. M. D. Crisp, "Some Aspects of Surface Damge That Can be Explained with Linear Optics," Laser Induced Damage in Optical Materials: 1973, NBS Special Publication 387, pp. 80-85. N. J. Brown, Lawrence Livermore Laboratory and J. M. Bennett, K. W. C , China Lake, private communications. -10-9. N. L. Boling, G. Dube? and M. D. Crisp, "Laser Surface Damage Studies on Several Glasses," Laser Induced Damage in Optical Materials: 1973, NBS Special Publication 387, pp. 69-70. 10. R. L. Landingham, Lawrence Livermore Laboratory, private communiction. NOTICK "This report was prepared as an account of work sponsored by Che United Slates Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or respon­sibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately-owned rights." Reference lo a company or product name docs m i l imply approval or rceommciu la l ion of l i l t ; product by the I 'n ivers i i ) ol ' ( al i l 'ornia or the I S . Du'parmicnl of I ncrj>y lo the exclusion o l (H IKTN thai may *•; su iuh le . UNDERCOATS IMPROVE DAMAGE THRESHOLD OF AR FILMS AS WELL [OS T i0 2 /S iO, OCLI - Shiva four-layer ARC on BK-7 O Without undercoat T 0 D With undercoat - A l 2 0 3 or S i 0 2 X/4 or thicker Threshold Predicted (J/cm2) at 1 nsec 10 Mean • 0 B DP Mean v 15 • A ooo o o D D 2 3 4 5 Threshold (J/cm 2) at 150 psec 50-01-0378-0727 Figure 1 BARE BK-7 SAMPLE A .13 60 50 J 40 o •S 30 0) <2 20 E CO Q ____ I o Entrance surface • Exit surface After cleaning 10 0 • Entrance surface • Exit surface Prior to cleaning 1 2 Pulse duration, ns 50-00-0678-2362 Figure 2 BARE BK-7 SAMPLE C18 m 50 40 CM E o 2 o </> CD I -<u CO E (0 Q O Front Surface Bowl-feed Polish • Exit Surface Conventional Polish 50-00-0678-2359 1 2 Pulse Duration, ns Figure 3 FUSED SILICA m E O J= to 0) i_ J= \-QJ O) E ro Q 50 40 30 20 10 0 o Entrance Surface Conventionai Polish • Exit Surface Conventional Polish T 1 2 Pulse Duration, ns 50-00 0678-2358 Figure 4 FILM DAMAGE VERSUS PULSE DURATION IH3 14 12 CM E 10 u —> old, 8 x: V ) 03 i _ £. *-» 6 CD a> co E CO 4 50-03-0778-2434 0 I I Sample B T i0 2 /S i0 2 Anti-reflective film on BK-7 X/2Si0 2 Undercoat 1 2 Pulse duration, ns Figure 5 FILM DAMAGE VERSUS PULSE DURATION us 14 12 " E 1 0 o —i •n o £ 8 03 03 E Q 6 T i0 2 /S i0 2 Reflector 99% at 45° incidence 50-03-0778-2438 1 2 Pulse duration, ns Figure 6 

Laser damage testing at LLL: an overview and an update

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Laser damage testing at LLL: an overview and an update

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