Image Processing And Control Of A Programmable Spatial Light Modulator For Spatial Beam Shaping Page: 3 of 14
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Image processing and control of a programmable spatial light
modulator for spatial beam shaping
Abdul A. S. Awwal, Charles Orth, Eddy Tse, JoAnn Matone, Mitanu Paul, Carla Hardy, Gordon
Brunton, Mark Hermann, Steve Yang, J.M. DiNicola, Matt Rever, Sham Dixit, Paul Wegner and
John Heebner
Integrated Computer Control System, National Ignition Facility
Computer Engineering division
Lawrence Livermore National Laboratory, Livermore, CA. 94551
E-mail: awwall @llnl.gov
ABSTRACT
Programmable spatial shapers using liquid-crystal-based spatial-light-modulators in the National Ignition Facility lasers
enable spatial shaping of the beam profile so that power delivered to the target can be maximized while maintaining
system longevity. Programmable spatial shapers achieve three objectives: Introduce obscurations shadowing isolated
flaws on downstream optical elements that could otherwise be affected by high fluence laser illumination; Spatial
shaping to reduce beam peak-to-mean fluence variations to allow the laser to operate at higher powers so that maximum
power can be delivered to the target; And finally gradually exposing the optical regions that have never seen laser light
because they have always had shadowing from a blocker that is no longer needed. In this paper, we describe the control
and image processing algorithms that determine beam shaping and verification of the beam profile. Calibration and
transmittance mapping essential elements of controlling the PSS are described along with spatially nonlinear response
of the device such as scale and rotation.
Key word: control systems, laser alignment, beam shaping, liquid crystal device, spatial light modulator
1. INTRODUCTION
Inertial confinement fusion research at the National Ignition Facility (NIF) of the Lawrence Livermore National
Laboratory, is performed on a 192-beam, 1.8 megajoule, 500-terawatt, ultraviolet, laser system [1-3]. The key functions
of the NIF laser such as specifying pulse shape, alignment, amplification and beam control are managed by the
integrated computer control system (ICCS) in order to produce proper timing, high-energy density and pressure leading
to a controlled fusion reaction [4-7]. ICCS divides alignment, optics inspection and shot planning, etc. into various
subsystems. While the automatic alignment (AA) system analyzes beam images to determine beam centering to
perform optical alignment, the optics inspection (OI) system, is used to identify flaws of different sizes on various
optical elements in the beam path and track them over time.
One of the challenges of operating NIF is to maintain the high quality of the optical elements that are subjected to
repetitive laser shots. Exposure to high-fluence laser light can cause some optical flaws to grow over time and impact
performance. The programmable spatial shaper (PSS) based on liquid-crystal spatial light modulators (SLM) was
designed and implemented within the NIF laser to introduce arbitrarily shaped obscurations that act to reduce laser
irradiation around selected flaw sites [8]. The OI system and Laser Performance Operations Model (LPOM) [9] supply
the flaw site information, while the PSS system provides "blockers" that cast a shadow in the beam effectively reducing
flaw irradiation exposure. A second objective of this spatial shaper, in addition to deploying blockers, is to reduce beam1
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Awwal, A. S.; Orth, C.; Tse, E.; Matone, J.; Paul, M.; Hardy, C. et al. Image Processing And Control Of A Programmable Spatial Light Modulator For Spatial Beam Shaping, article, January 22, 2013; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc828291/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.