Full-aperture laser conditioning of multilayer mirrors and polarizers

摘要

Abstract: The inertial confinement fusion (ICF) program at LLNL is beginning the design of a 1.8 megajoule, 0.35-micrometer, laser system called the National Ignition Facility (NIF). In order to reduce cost and increase performance, high damage threshold optics are essential. It has been found that the damage threshold of some coatings can be increased by as much as 2 or more times as a result of pre-illumination at incrementally increasing fluences. This process, termed laser conditioning, has been associated with the ejection of damage-initiating defects present in some optical coatings. With current damage thresholds, mirrors and polarizers for the NIF will have to be laser conditioned in order to meet the laser requirements for fluence propagation. LLNL has constructed a system dedicated to laser conditioning of meter-sized HfO$-2$//SiO$-2$/ multilayer polarizers and mirrors. The optic is moved in a raster pattern through a stationary 10-Hz rep-rated, 1.064 micrometer beam with 10-ns pulses. A scatter measurement diagnostic allows on-the-fly evaluation of laser-induced damage during a scan. This system has been used to laser condition optics as large as 73 cm by 37 cm. Such optics are now being used on the Beamlet laser system at LLNL. With this large area conditioning system, it has been observed that the scattered light from an optical coating increases during the conditioning process. This increase in scattered light has been related to the removal of nodular defects. !3