Fundamental Physical Mechanisms of Mitigation Approaches to Laser-Induced Damage on Ultraviolet Optics (FY08 LDRD Final Report)

Technical Report · Thu Jul 12 04:00:00 EDT 2012

DOI:https://doi.org/10.2172/1129971· OSTI ID:1129971

Draggoo, Vaughn ^[1]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)

The objective was to explore and further develop the concept that laser–induced damage on surfaces of an optic can be mitigated to extend its lifetime in high-fluence laser systems. The project scope explored mitigation processes utilizing laser, micro-machining, and chemical techniques including laser-induced damage testing of the mitigated site, and characterization of the size and shapes of the mitigation site with respect to downstream intensification of the high energy laser beam. Advanced materials processing and characterization tools, and computational tools were developed to investigate physical properties of the mitigated regions. The material focus of the mitigation effort was fused silica and potassium di-hydrogen phosphate (KDP) crystal optics. and to develop a fundamental understanding of the physical mechanisms of the mitigation processes is sought in order to extend their applicability to larger and deeper damage sites and to larger laser fluences. When successful, these mitigation processes will significantly extend the useful lifetime of the optic and drastically reduce the associated operating costs. For silica optics, mitigation processes utilizing mid-infrared (4.6 μm) and far infrared (10.6 μm) laser radiation from carbon dioxide lasers was used to ablate, evaporate, melt and anneal the damaged silica surface. By varying the laser parameters (power, pulse rate, spot size, etc.) these investigations evaluated the almost 100x difference in the optical absorption depth between these wavelengths in the efficacy in arresting damage growth, and controlling the end-state of the material in terms of shape morphology, residual stress and process by-products (vapor and re-condensed material). For crystal optics, micro-diamond tool machining and micro re-crystallization of KDP were investigated. A precision three-axis micro machine tool was developed, tested and shown capable of treating damage sites to 1 mm in size that were robust against re-initiation of damage and exhibited acceptable downstream intensification. Micro re-crystallization of KDP was accomplished through Gibbs-Thompson condensation at the tip of an Atomic Force Microscope stylus. By controlling the humidity at the tip and raster scanning of the stylus over damaged crystal surfaces, filling of scratched surfaces with re-crystallized material was demonstrated.

• Research Organization: Livermore, CA; Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48; AC52-07NA27344
• OSTI ID: 1129971
• Report Number(s): LLNL--TR-564151
• Country of Publication: United States
• Language: English

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