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When laser optics are exposed to continuous radiation or to pulse lengths long enough so that the damage level is determined simply by thermal heating, the mechanisms causing catastrophic damage are fairly clear. In particular, the role of surface defects should be predictable from an analysis of the standing wave pattern at the surface, the optical properties and shape of the defects, and simple heat transfer calculations. The conclusion drawn from such an analysis is that although dust and other surface defects may cause small isolated damage sites to occur, these sites will not grow under continued irradiation and produce catastrophic damage unless the defects exceed a critical diameter or are so closely spaced that they significantly affect the average absorption of the surface. Experimental evidence supports this hypothesis. Dust and other small surface defects are thus acceptable in reasonable concentrations on high power continuous wave laser optics provided that they do not exceed the critical size.
Coating imperfections, cw laser irradiation, digs, dust, laser damage, mirrors, optical components, scratches, surface defects
Michelson Laboratory, Physics Division, Naval Weapons Center, China Lake, California