KEYWORDS: Galactic astronomy, Space telescopes, Stars, Hydrogen, Telescopes, Solar system, Astronomy, Infrared telescopes, Spectroscopy, James Webb Space Telescope
This paper describes the beginning of the Far-Infrared Surveyor mission study for NASA’s Astrophysics Decadal 2020.
We describe the scope of the study, and the open process approach of the Science and Technology Definition Team. We
are currently developing the science cases and provide some preliminary highlights here. We note key areas for
technological innovation and improvements necessary to make a Far-Infrared Surveyor mission a reality.
This work is an experimental investigation to evaluate the potential of fluorescence microscopy as a tool to detect surface contamination as well as reveal surface damage precursors on DKDP and SiO2 optics. To achieve these technical objectives, microscopic imaging systems were built that also incorporate in-situ damage testing capabilities. Fluorescence imaging experiments were performed using 351-nm laser excitation while damage testing was performed at relatively high laser fluences. The experimental results demonstrated the potential of this technique to address the aforementioned technical issues.
Following initiation at absorbing surface flaws, UV laser- induced damage to polished fused-silica surfaces continues to grow upon subsequent illumination. In this study photoluminescence spectroscopy was used to detect the formation of a modified, absorbing layers of silica that could be responsible for the continued growth of the damage site. For damage sites created with pulsed 355 nm illumination, three characteristic photoluminescence peaks are detected within the damage sites when excited with a 351 nm CW beam. Two of the peaks are likely due to the well- known E' and NBOHC defects associated with oxygen vacancies and broken Si-O bonds, respectively. The third, and dominant, peak at 560 nm has not been clearly identified, but is likely associated with a change in stoichiometry of the silica. The relative intensities of the peaks are non- uniform across individual damage sties. The photoluminescence data is being combined with insights from various optical and optical and electron microscopies to develop an understanding of laser-induced damage sties. The objective is to develop strategies to slow or stop the growth of the damage sites.
The National Ignition Facility will require upwards of 25,000 small optical components in its various beam conditioning and diagnostic packages. A quality control program designed to ensure that the elements meet the required specifications will test these optical elements. For many of the components, damage performance is one of the critical specifications, which will require state-of-the-art performance from the industry participants. A program was initiated to understand the current performance level of such optics. The results of this study as it pertains to laser-induced damage is shown. The use of ratio reflectometry is also addressed as the method of choice for photometry measurements on these industry supplied optics.
Lynn Sheehan, Sheldon Schwartz, Colin Battersby, Richard Dickson, Richard Jennings, James Kimmons, Mark Kozlowski, Stephen Maricle, Ron Mouser, Michael Runkel, Carolyn Weinzapfel
The Laser Program at LLNL has developed automated facilities for damage testing optics up to 1 meter in diameter. The system were developed to characterize the statistical distribution of localized damage performance across large- aperture National Ignition Facility optics. Full aperture testing is a key component of the quality assurance program for several of the optical components. The primary damage testing methods used are R:1 mapping and raster scanning. Automation of these test methods was required to meet the optics manufacturing schedule. The automated activities include control and diagnosis of the damage-test laser beam as well as detection and characterization of damage events.
Laser-induced damage of transparent fused silica optical components by 355 nm illumination occurs primarily at surface defects produced during the grinding and polishing processes. These defects can either be surface defects or sub-surface damage. Wet etch processing in a buffered hydrogen fluoride solution has been examined as a tool for characterizing such defects. A study was conducted to understand the effects of etch depth on the damage threshold of fused silica substrates. The study used a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser to damage test fused silica optics through various wet etch processing steps. Inspection of the surface quality was performed with Nomarski microscopy and Total Internal Reflection Microscopy. The damage test data and inspection results were correlated with polishing process specifics. The result show that a wet etch exposes sub-surface damage while maintaining or improving the laser damage performance. The benefits of a wet etch must be evaluated for each polishing process.
Jerald Britten, Steve Herman, Leslie Summers, Michael Rushford, Lun Auyang, I. Barton, Bruce Shore, Sham Dixit, Thomas Parham, Curly Hoaglan, Charles Thompson, Colin Battersby, J. Yoshiyama, Ron Mouser
We have fabricated demonstration diffractive optic plates at full scale for the NIF laser. These include an off-axis focusing beam sampling grating, a color separation grating, and a kinoform phase plate for spatial beam smoothing. Fabrication methods and optical performance of these DOPs are discussed. IT was discovered that the sol-gel antireflective coating normally applied to high-power transmissive optics partially planarizes the diffractive structures, particularly on the color separation grating used for color management at target, to the extent that optical performance and laser damage threshold are negatively impacted. The effect of sol-gel coatings on grating performance, the feasibility of placing all diffractive structures on a single surface, and future work in this area are discussed.
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