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8 December 1995 Low-efficiency gratings for third-harmonic diagnostics applications
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Proceedings Volume 2633, Solid State Lasers for Application to Inertial Confinement Fusion (ICF); (1995)
Event: Solid State Lasers for Application to Inertial Confinement Fusion (ICF), 1995, Monterey, CA, United States
The baseline design of the National Ignition Facility (NIF) calls for sampling gratings to provide third-harmonic energy diagnostics in the highly constrained area of the target chamber. These 40 multiplied by 40 cm transmission gratings are to diffract at (order plus 1) nominally 0.3% of the incident 351 nm light at a small angle onto a focusing mirror and into a calorimeter. The design calls for a plane grating of 500 lines/mm, and approximately 30 nm deep, etched into a fused silica focusing lens and subsequently overcoated with a sol-gel antireflective coating. Gratings of similar aperture and feature size have been produced for other applications by ion etching processes, but, in an effort to reduce substantially the cost of such optics, we are studying the feasibility of making these gratings by wet chemical etching techniques. Experimentation with high-quality fused silica substrates on 5 and 15 cm scale has led to a wet etching process which can meet the design goals and which offers no significant scaleup barriers to full sized optics. The grating is produced by holographic exposure and a series of processing steps using only a photoresist mask and a final hydrofluoric acid etch. Gratings on 15 cm diameter test substrates exhibit absolute diffraction efficiencies from 0.2 - 0.4% with a standard deviation of about 15% of the mean over the full aperture. The efficiency variation is due to variation in linewidth caused by spatial nonuniformities in exposure energy. Uniformity improvements can be realized by using a smaller, more uniform portion of the exposure beam and exposing for longer times. The laser damage threshold for these gratings has been measured at LLNL and found to be identical to that of the fused silica substrate. Scaleup to full-sized substrates will use techniques such as meniscus coating for photoresist, large-aperture holography and other processes already established at LLNL for optics of this size. A prototype sampling grating to be installed on the Beamlet laser will be produced in early 1996.
© (1995) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Jerald A. Britten, Robert D. Boyd, Michael D. Perry, Bruce W. Shore, and Ian M. Thomas "Low-efficiency gratings for third-harmonic diagnostics applications", Proc. SPIE 2633, Solid State Lasers for Application to Inertial Confinement Fusion (ICF), (8 December 1995);

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