The B3 prism segment was tested at two different angles of incidence on the TIR face as shown by the two entries in table 7. This was done to specifically test the effect the electric field would have on the damage threshold. The peak field data in table 7 indicates that the 73° test should have had~ 3 times the threshold compared to the 60° test. In fact the ratio of the 73° to 60° thresholds is ~2.2-2.4. But one must also consider that the threshold values are reported in a plane perpendicular to the test beam axis. The actual fluence on a tilted surface is the reported threshold multiplied by the cosine of the angle of incidence. Thus there should be a difference in the 73° and 60° thresholds which is the ratio of the cosine of the two angles. This cosine ratio equals 1.71. Most of the observed threshold ratio of 2.2-2.4 is therefore accounted for by the cosine ratio. The extra difference that the field analysis predicts is not observed. The TIR data supports each of the observations made with respect to the normal and non-normal incidence coatings in the preceding sections. The coated segments slightly outperformed the uncoated ones; the one segment with a high index material in the coating was the worst performer; there was little correlation between the electric field profile and damage threshold; and the IAD processing doesn't appear to have significantly degraded the damage threshold. 5.0 Summary Uncoated polished YAG surfaces have damage thresholds in the 9 -14 J/cm2 range. The ultimate potential for YAG surfaces is indicated by clean sites on fractured surfaces which damaged in the 100 - 110 J/cm2 range. Normal incidence AR coatings on YAG damaged in the 12-18 J/cm2 range, if they avoided the use of high index materials such as TiO2 and ZnSe. The non-normal incidence AR coatings, that excluded high index materials, also damaged in the 12-18 J/cm2 range for both polarizations. The TIR surfaces had the following damage thresholds: 4-9 J/cm2 uncoated, 7-10 J/cm2 coated. I wish to acknowledge and thank the following persons and groups: Alan Boxell, who fabricated most of the coatings; Art Braundmeier (of Southern Illinois University, at Edwardsville) who designed and fabricated some of the coatings; Steve Chelli and his coating group, who also designed and fabricated some of the coatings; Gordon Burkhart and the polishing group, who fabricated some of the substrates; and finally Carolyn Krebs and George Dube' for supporting this entire effort. References [1] Seitel, S. C. Laser Damage Test Handbook and Database of Nd:YAG Laser Optics, Montana Laser Optics, Inc., Bozeman (1988), custom report. [2] Lowdermilk, W.H.; Milam, D.; Rainer, F. "Damage to Coatings and Surfaces by 1.06 μm Pulses", Natl. Bur. Stand. U.S. Spec. Publ. 568, 391 (1981). [3] Seitel, S. C. Laser Damage Test Handbook and Database of Nd:YAG Laser Optics, Montana Laser Optics, Inc., Bozeman (1988), Tables C.14, C.15. [4] Bettis, J.R.; House, R.A.; Guenther, A.H. "The Importance of Refractive Index, Number Density, and Surface Roughness in the Laser-Induced Damage of Thin Films and Bare Surfaces", Natl. Bur. Stand. U.S. Spec. Publ. 435, 289 (1975). [5] Turner, A.F. "Ruby Laser Damage Thresholds in Evaporated Thin Films and Multilayer Coatings", Natl. Bur. Stand. U.S. Spec. Publ. 356, 119 (1971). [6] Thomas, I.; Wilder, J.; Gonzales, R.; George, D. "1064 nm and 350 nm Laser Damage Thresholds of High-Index Oxide Films Deposited from Organic Solutions and Sols", in Proceedings, Eighteenth Annual Symposium on Optical Materials for High Power Lasers, Boulder Colo., Nov. 1986, in process. [7] Thomas, I.; Wilder, J. "Al2O3-SiO2 HR Coatings Prepared from Colloidal Suspensions", in Proceedings, Nineteenth Annual Symposium on Optical Materials for High Power Lasers, Boulder Colo., Nov. 1987, in process. [8] Apfel, J.H. "Further Studies of the Role of Electric Field Strength in Laser Damage of Dielectric Layers", Natl. Bur. Stand. U.S. Spec. Publ. 509, 251 (1977). [9] Apfel, J.H.; Enemark, E.A.; Milam, D.; Smith, W.L.; Weber, M.J. "The Effects of Barrier Layers and Surface Smoothness on 150-ps, 1.064-μm Laser Damage of AR Coatings on Glass", Natl. Bur. Stand. U.S. Spec. Publ. 509, 255 (1977). [10] Deaton, T.F.; Rainer, F.; Milam, D.; Smith, W.L. "Survey of Damage Thresholds at 532 nm for Production-Run Optical Components", Natl. Bur. Stand. U.S. Spec. Publ. 620, 297 (1983). [11] Hart, T.T.; Lichtenstein, T.L.; Carniglia, C.K.; Rainer, F. "Effects of Undercoats and Overcoats on Damage Thresholds of 248 nm Coatings", Natl. Bur. Stand. U.S. Spec. Publ. 638, 344 (1983). [12] Newnam, B.E.; Foltyn, S.R.; Jolin, L.J.; Carniglia, C.K. "Multiple-Shot Ultraviolet Laser Damage Resistance of Nonquarterwave Reflector Designs for 248 nm", Natl. Bur. Stand. U.S. Spec. Publ. 638, 363 (1983). MANUSCRIPT NOT RECEIVED DAMAGE MECHANISM ON METAL MIRRORS INDUCED BY CO2 LASER K. Yoshida, M. Yamanaka, S. Nakai Institute of Laser Engineering, Osaka University Y. Tsunawaki Osaka Industrial University 3 Naka-Gaito, Daito, Osaka, 574 Japan H. Okamoto, K. Motoba, S. Aramaki, and K. Ohta Central Research Laboratory, Nippon Mining Co., LTD ABSTRACT It is very important to identify the mechanism of surface damage in a metal mirror, and consequently to increase the laser damage threshold. We studied the laser damage mechanism due to impurities and grain boundaries for a molybdenum mirror. Detailed microscopic observations are made of three different damage morphologies for a diamond-turned copper mirror. MANUSCRIPT NOT RECEIVED OPTICAL CHARACTERISTICS OF ZnSe COATED COPPER MIRROR K. Yoshida, M. Yamanaka, S. Nakai Institute of Laser Engineering, Osaka University Y. Tsunawaki Osaka Industrial University 3 Naka-Gaito, Daito, Osaka 574 Japan H. Okamoto, K. Motoba, S. Aramaki, and K. Ohta Central Research Laboratory, Nippon Mining Co., LTD 3-17-35 Nizominami, Toda, Saitama, 335 Japan ABSTRACT We have developed a ZnSe coated copper (Cu) mirror for high power CO2 laser. As the Vickers hardness of ZnSe coated Cu mirror is two times harder than a normal Cu mirror, it is rather easy to clean the coated mirror surface without scratching. For the high-power TEA CO2 laser, the ZnSe coated Cu mirror has a surface damage threshold two times higher than that of Au-coated Cu mirror. |