Programmable beam spatial shaping for the National Ignition Facility
Megajoule-class lasers for inertial-confinement fusion require large-aperture optics capable of withstanding high-fluence (energy per unit area) operation at UV wavelengths.1 In the past decade, work associated with the construction and commissioning of the National Ignition Facility (NIF) has led to major advances in the manufacturing and processing of high-damage-threshold UV optics.2 While the number of damage-relevant flaws is very low, it is not yet zero, and the few flaws that remain on the optics can still act as small initiation sites for damage that have the potential to grow in size in response to subsequent high-fluence laser shots. Our ability to temporarily shadow these isolated sites from high-fluence laser pulses enhances operational flexibility by enabling uninterrupted use of the facility at near-peak laser performance until a suitable time when the optics can be removed, repaired offline, and replaced.
The obscurations that produce the shadows, referred to as ‘blockers,’ are introduced in the laser's low-fluence, IR region, upstream of the main amplifier chain. By imaging and aligning the shadows onto flaws in the output UV optics, we can ensure that these flaws are protected at the expense of a modest loss of beam area. Initially, we introduced static chrome masks, each containing a single blocker obscuration, into the pre-amplifier modules feeding the NIF's beamlines. While these solved an immediate need, we preferred a programmable means for introducing this type of blocker to provide enhanced flexibility and reliability of NIF operations. Projection-display technology has improved the performance and driven down the cost of pixelated, incoherent spatial-light modulators. Unfortunately, their direct application to coherent laser beams presents multiple challenges, including diffraction artifacts resulting from the pixelated matrix, poor throughput, wavefront distortions, and spectral distortions resulting from multilayer reflections, converting imposed frequency modulation into undesirable temporal modulation.
To address these challenges, we adopted optically addressable light-valve (OALV) technology.3 These systems leverage incoherent pixelated-display technology, but solve the difficult issues through use of a custom, single-pixel analog modulator. The resulting hardware system consists of two stages. In the first, bitmapped patterns are written onto an auxiliary, incoherent address beam derived from a blue LED source. The address image is written using a commercially available liquid-crystal-on-silicon modulator (as can be found on rear-projection TV displays). The address beam is then projected onto a large, single-pixel liquid-crystal light modulator containing a layer of photoconductive bismuth silicon oxide. The photoconductive layer enables bright and dark regions of the address beam to control the valve's transmission for the coherent beam at 1053nm. As a result, this second stage imprints the incoherent pattern onto the coherent laser beam without introducing spurious artifacts. Figure 1 shows a pre-amplifier module upgraded with a programmable spatial-shaper package, an OALV, and a beam profile imprinted with an array of nine blockers.
The clear aperture of the OALV accommodates an 18×18mm2 square beam profile. At its installed upstream location, the beam fluence is less than 5mJ/cm2. Downstream, the beam is magnified and further amplified until it grows to 372×372mm2, operating at 1ω (1053nm) fluences up to 16J/cm2 and 3ω (351nm) fluences up to 8J/cm2. Using this technique, we can realize arbitrarily defined masks with smooth shapes, high transmission (>90%), low wavefront distortion (<0.5 wave), and without spectral distortions. Figure 2 exhibits the resultant image fidelity. We carefully chose the blocker diameters as a tradeoff between maintaining usable beam area and minimizing diffraction ripples at the edges (that can intensify and lead to damage). Propagation studies suggest that the blocker holes can be as small as 1mm at the OALV (2cm on the magnified beam downstream) before edge diffraction, when coupled with Kerr nonlinear effects, this introduces significant beam modulation.4 Figure 3 demonstrates that we have achieved a programmable means of introducing our desired blocker shape.
The new generation of high-energy laser systems for fusion research will rely on continuing advances in high-damage-resistant optics. Active equipment protection in the form of programmable beam shaping will nevertheless offer significant advantages for flexible and reliable operations. A retrofit of each of the 48 NIF pre-amplifier modules with this new upgrade package was completed in February 2010. These devices, in combination with a flaw-inspection system and optics-registration strategy, represent a new approach for extending the operational lifetime of high-fluence laser optics. Future work will involve optimizing the blocker shapes and adapting the system to extract the maximum energy while operating safely and reliably.
This work was performed under the auspices of the US Department of Energy under contract DE-AC52-07NA27344.
John Heebner completed his PhD in 2003 at the Institute of Optics of the University of Rochester (New York), where he conducted research in nonlinear optics. He is currently a lead engineer working on front-end laser sources and ultrafast diagnostics for megajoule- and petawatt-class fusion-laser systems.
