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10 June 2004 Study of surface contamination layer in silica thanks to laser damage investigation
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During the first steps of optical silica substrate preparation, cracks due to mechanical stress appear on the surface in a thickness of a few microns. The following stage of polishing is useful to reduce those cracks and to decrease the roughness thanks to specific abrasive liquids. The consequence of this process is a contamination of the silica on the considered layer. These different contaminants could be suspected to be precursor centers of laser damage. An estimation of this critical thickness can be done by studying the morphology of the laser-induced damages. Results obtained by observation after irradiation with an atomic force microscope confirms that a thickness of a few microns seems to be involved. The study proposed here, consists in an investigation of the layer which is responsible of surface damages. We use a statistical model previously developed to determine the precursors density from laser damage probability curves. At first, we will present results based on the study of liquids used for the polishing of bare silica glasses. Subsequently, we will correlate these results with the laser-induced damage threshold of the substrates. Finally, we will describe a method allowing us to measure the thickness of this surface contamination layer. To illustrate our purpose, we will present results achieved on bare silica.
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Bertrand Bertussi, Jean-Yves Natoli, Mireille Commandre, and Claude Amra "Study of surface contamination layer in silica thanks to laser damage investigation", Proc. SPIE 5273, Laser-Induced Damage in Optical Materials: 2003, (10 June 2004);

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