The second type of damage observed can be classified as optical damage. However, an attempt to measure the damage threshold for Nd:YAG and Nd: Cr: GSGG samples having the coated surface in contact with the laser coolant was not successful. The coating was specified to meet a general requirement of 500 MW/cm2 at 1.06 micron with pulses of 20 ns duration. However, the estimated damage threshold was observed at significantly lower power. In figure 14, photographs of an optical damage site are shown. The damage sites generally appear to be free of craters, and it appears that the coating was simply blown away by the laser pulse. In absence of hard data for the damage threshold of the coating under its operating conditions, the damage threshold is estimated based on the laser performance. The goal of the 5 mm x 8 mm aperture laser output energy was 300 mJ, in a 20 ns pulse at 100 Hz repetition rate. The damage threshold was estimated at 190 mJ/p output energy. The damage threshold of the single layer A1203 coating was measured, using the conventional technique for 1.061 micron wavelength radiation, at 20 Hz repetition rate and was found to exceed the specification. The damage threshold measurement for an optically polished surface of Nd:YAG and Nd: Cr: GSGG material was also measured, separately, (see ref. 7) and was not the limiting factor. Thus, it was concluded that the poor damage threshold observed for the coating in its operating environment may be due to surface contaminants on the TIR surface itself. A process to clean slab surfaces before coating deposition was added. The slab coated using this process was tested recently with an output energy of more than 200 mJ/p without damage. 5. Conclusions Based on the data collected for parasitic oscillation suppression coatings on TIR surfaces of a slab laser, the following conclusions can be drawn: o A slab with TIR coated surfaces can be immersed in liquid coolant for efficient cooling. o A 5 mm x 8 mm x 93 mm size slab of Nd: Cr:GSGG material with the parasitic suppression coating provided 5.85 percent efficiency at a high repetition rate. The passive and active interferometry conducted show the pump cavity design provides a one dimensional thermal gradient in the slab, independent of the laser crystal quality. 0 The beam divergence and beam quality of the slab was measured. The data show that even at 50 Hz repetition rate the beam divergence and quality do not degrade. o A1203 single layer coating used for parasitic suppression provides almost negligible reflectivity from the normal incidence angle to grazing incidence angle. o Two types of damage in the parasitic suppression coating were observed and classified as i) non-optical and ii) optical. Non-optical damage shows that the coating peels off the surface when the coolant is boiled. This type of damage can be avoided by depositing an aluminum oxide material on the TIR surfaces with an E-beam. o Optical type damage of the parasitic suppression coating was not measured but was A test set-up capable of measuring the damage threshold of a sample with the 7. Acknowledgements This work was conducted under contract No. N60530-86-C-0187 and No. N60530-87-M-0349 from Naval Weapons Center, China Lake, CA. The authors also acknowledge help from Jackie Gibson, Phyllis Heatley, Mark Gall and Lauryn Erndl, in preparation of the manuscript. References [1] Z.J. Kiss and R.C. Duncan, Phys. Lett. 5, 200 (1964). [2] D. Pruss, V.V. Laptev, et.al., Appl. Phys. B28, 355 (1982). [3] E.V. Zharikov, G.I. D'yakonov, et.al., Sov. J. Quantum Electron. 18, 43 (1988). [4] M.A. Acharekar, "Comparison of Nd:YAG and Nd: Cr:GSGG Slab Lasers," Technical Report, Contract #N60530-86-C-0187 (1986). [5] W.S. Martin and J.P. Chemoch, US Patent #3,633,126 (1972). [6] D.C. Brown and K.K. Lee, CLEO's 1985 paper WM-37 (1985). [7] M.A. Acharekar, D.P. McCarthy, et.al., "Laser Damage in Optical Materials: 1984," NBS Special Publication 727, 39 (1986). [8] M.A. Acharekar, "Laser Damage in Optical Materials: 1985," NBS Special Publication 746, 170 (1988). [9] R.L. Byer, "High Energy Efficient Solid State Laser," NASA Grant 1-182, G.L. Report #3634 (1983). [10] M.A. Acharekar, "Comparison of Face and Edge Pumped Slab Laser," Technical Report Contract #N60530-86-C-9187 (1986). [11] A. Vasicek, "Optics of Thin Film," North-Holland Publication Co., (1959). MANUSCRIPT NOT RECEIVED ABSORPTION AND DAMAGE THRESHOLD OF DIELECTRIC T. Izawa, Y. Ishiwata, I. Hashimoto, and H. Shikakura 804 Hakusancho, Midori-ku, Yokohama, Kanagawa, 226 Japan Y. Owadano, Y. Matsumoto, and M. Yano Electrotechnical Laboratory 1-1-4 Tsukuba, Ibaraki, 305 Japan ABSTRACT Laser damage threshold of dielectric reflectors for UV lasers depends on absorption in the high index materials. High resistance reflector for ArF laser (193 nm) is developed by using LaF, as high index materials. In this paper, we present the results of damage threshold, overcoating effects, and degradation in ageing and in post-heating for LaF3/Na,AlF6 reflectors. Damage threshold of the reflectors was measured using 10pps 17 ns pulsewidth ArF laser. Damage threshold of the reflectors without overcoating was -0.45 J/cm2 just after the coating, and it showed the decrease of ~10%, 420 days after the coating. Absorption of reflectors without overcoating was -3.6% and it increased to -5.8%, 420 days after the coating. Postheating at 80°C showed the decrease of -50% in damage threshold. Na,AlF6 overcoating on the reflector improved about twice higher damage threshold of -0.8J/cm2. Degradation in ageing was also improved compared with the nonovercoating reflectors. No degradation after post-heating was observed. Damage threshold of MgF2 overcoating reflectors showed worse results than that of non overcoating reflectors. In the experiments of LaFl, single layer films, absorption of the film showed more than 10 times increase, 420 days after the coating. From this result, the cause of ageing effect is considered to be mainly due to the increase of absorption in LaF3 films. |