|
Anti-reflecting (AR) surface relief nano-textures have been integrated with fused silica diffraction gratings to
demonstrate the potential of stable diffractive 3ω beam samplers with increased energy to target at the National
Ignition Facility (NIF). TelAztec’s AR texturing process was used to etch Random-type AR (RAR) microstructures
in sub-scale NIF Grating Debris Shields consisting of large pitch, shallow line gratings. This superposition yielded
the desired ~3.5% increase in zero-order transmission uniformly over the full aperture without compromising the
grating function. Another fused silica window fabricated with RAR nano-textures in both faces for a 3ω (351nm)
transmission of 99.5%, was subjected to capillary condensation tests to evaluate the resistance of the RAR texture to
the adsorption of organic compounds. It was found that for a one day exposure time to a surrogate suite of organic
contaminants, the RAR textured fused silica surfaces adsorbed less than one fourth the amount of organic
contaminants found on a NIF baseline hardened sol-gel AR coated optic. In two additional exposure cycles, further
RAR process refinement reduced the amount of adsorbed organics to a level nearly 200 times below the current NIF
baseline. Significantly, the 3ω transmission of the RAR textured window remained unchanged after all three
exposure cycles, whereas the sol-gel coated windows showed losses up to 4.9% for the highest contaminant
concentration. Large beam pulsed laser damage testing of RAR textured fused silica windows was conducted with
the Optical Sciences Laser (OSL) at NIF. The RAR sample damage resistance was found to be equivalent to the
current NIF baseline - even after multiple aggressive chemical cleaning cycles. Lastly, a series of RAR textured
and sol-gel AR coated windows were subjected to commercial 3ω pulsed laser damage testing at Quantel. The
results indicate an average RAR damage threshold of 26 J/cm2, a level about 80% of the two NIF fused silica
samples tested, two times higher than the best performing ion beam sputtered thin-film AR coatings reported in the
literature, and six times higher than catalog high power laser window coating specifications.
|
