In this work we showcase the formulation of an anti-reflective (AR), UV/ozone-curable polysilsesquioxane or glass resin (GR) system with a high laser damage threshold suitable for Third-Harmonic Generator (THG) optics. Potassium Dideuterium Phosphate (DKDP) single crystals are used for these non-linear optics at the National Ignition Facility (NIF). However, DKDP is thermally sensitive making it crucial to avoid elevated thermal conditions, therefore UV/ozone-curing glass resins are the optimal choice. The commercial GR, utilized in this study was specifically engineered to undergo complete curing during UV processing, while also offering the flexibility of a low thermal cure at temperatures below 100 °C. UV/ozone exposure resulted in a significant loss of methyl silane peaks and a thermally cured network formed a total reduction of R-OH & Si-OH peaks. Laser damage testing confirmed equivalent damage-resistance for each network based on their curing condition. The newly formulated GR is a highly shelf-stable solution that allows peak optical performance on large-high-power laser systems where unique optics and operating conditions exist.
Lawrence Livermore National Security’s (LLNS) National Ignition Facility (NIF) requires over 600 large crystals of KH2PO4 (KDP) and KH0.6D1.4PO4 (DKDP) for use in polarization rotation and frequency conversion assemblies. There are near term plans to increase the energy on NIF to 2.2 MJ and up to 3 MJ in future years. Managing optics’ damage is one critical aspect of achieving these aggressive goals. Frequency conversion crystals are being examined for readiness to meet the goals for higher energy operations. All aspects of the crystal fabrication process are being studied including growth, processing, laser conditioning, and mitigation to strengthen the crystals against damage. THG crystals have an approximately 10× lower exchange rate than fused silica optics, so unlike NIF fused silica optics which are micromachined to mitigate surface laser damage between recycle loops, THG crystals are refinished to remove surface laser damage. Given the long time to grow NIF size DKDP crystals (3 years) and the impact of THG thickness on frequency conversion, a good understanding of the evolution of the inventory is necessary. The number of damage sites that require laser blocking dictates the exchange rate of THG crystals. Understanding the difference between bulk and surface damage, is important to limit the number of blocked sites. Also, in preparation for higher power operations on NIF, the current KDP polarization rotators on half of the NIF beams are being exchanged with DKDP due to an ~5% reduction in absorption at 1053 nm thus reducing beam contrast in the NIF amplifiers.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to 351nm light or 3w. It was built to access the extreme high energy density conditions needed to support the nation’s nuclear stockpile in the absence of further underground nuclear tests, including studying Inertial Confinement Fusion (ICF) and ignition in the laboratory.
Over the last year, important results have been obtained demonstrated a fusion yield of 1.35MJ with 1.9MJ of laser energy (and 440 TW power) injected in the target, bringing the NIF to the threshold of ignition [2-3]. As the yield curve near ignition is steep, the laser performance team has focused on providing improved power accuracy and precision (better shot-to-shot reproducibility) with a high-fidelity pulse shaping system (HiFiPS), and also on extending the NIF operating power and energy space by 15% to 2.2MJ and 500TW.
The final optics in the National Ignition Facility (NIF) are protected from target debris by sacrificial (disposable) debris shields (DDS) comprised of 3-mm thick Borofloat. While relatively inexpensive, Borofloat has been found to have bulk inclusions which, under UV illumination, damage, grow, and occasional erupt though the surface of the DDS. We have shown previously that debris generated from Input Surface Bulk Eruptions (ISBE) are a significant source of damage on NIF. Inclusion-free fused silica debris shield (FSDS) have been installed in between the DDS and the final optics on some NIF beam lines to test their efficacy in mitigating damage initiation. We will show results of the damage performance of the FSDS and its role in protecting the final optics. These results will help in our economic analysis of the potential benefits of using FSDS to protect NIF final optics.
Contaminants can severely limit the efficiency, laser damage threshold, and strength of photonic crystal fiber-based lasers. Such contamination can occur due to environmental exposure during the pulling or stacking of rods and tubes or improper handling and storage of these glass components. A preform made by the “stack and draw” process is susceptible to incorporating surface contaminants into the bulk laser glass.
We have adapted cleaning and handling protocols originally developed for processing large fused silica optics for the National Ignition Facility. The etch cleaning process reported here mimics the “AMP” or “Advanced Mitigation Process” developed for NIF optics that see high fluence 351nm light. In addition, all cleaning, fixturing and assembly processes used to prep a stack for pulling into a fiber are done in a Class 100 cleanroom. Glass rods (1-3mm in diameter and 10” long) are assembled into a Teflon fixture that only contacts the rods at each end. The loaded fixture receives 120kHz ultrasonic cleaning in 10% sodium hydroxide at 45C and 3% Brulin 1696 detergent at 55C. Parts are thoroughly rinsed using ultrasonicated ultrapure water and spray rinses. A 200nm etch in buffered hydrofluoric acid (6:1 BOE diluted 2:1 in DI water) is followed by additional ultasonicated (120kHz-270kHz) ultrapure water and spray rinse. Finally, the components are allowed to fully dry inside the Teflon frame. The rods are cleaned, stacked, and assembled into a fused silica tube.
The preform stack is then returned to a non-cleanroom area to be pulled into fiber using standard telecom fiber-based draw tower equipment and without clean air filters around the draw area. Four fibers were made to test independently the damage threshold and the background loss, two Yb core active fibers and two silica core (F clad) fibers. One of each was cleaned with the AMP process, and one of each with a methanol wipe cleaning process. The active fiber was coated with a dual acrylate coating, first with a low-index inner coating to provide a pump cladding, and then with a relatively hard coating to protect the relatively soft primary coating. The active fibers were pumped at 980nm in a double Fresnel cavity configuration and the power increased until the fiber was damaged up to 1kW. The passive fiber background loss was measured using a standard cut-back technique. Replacing the former methanol wipe clean process with this aqueous cleaning process improved the 1060nm damage threshold of a fiber laser by >30x to above the kW level in the laboratory and reduced the background attenuation by >18x. Early indications are that the acid etching also makes the tensile strength of the fiber consistently high.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence
Livermore National Laboratory under Contract DE-AC52-07NA27344.
The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to
two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a
disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on
absorbing glass armor at higher than expected fluences. We show that the composition of the particles is
secondary to the energetics of their delivery, such that particles from either source are essentially benign if they
arrive at the GDS with low temperatures and velocities.
We describe the cleaning processes, treatment methods, facilities, and cleanliness verification techniques developed to
achieve and maintain the demanding cleanliness requirements for both hardware and optics used in the National
Ignition Facility (NIF).
Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will
review important optical damage precursors in silica up to UV fluences as high as 45J/cm2 (3ns) along with studies of
the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface
damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of
a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily
responsible for optic damage below about 10J/cm2; they can be mitigated by an optimized oxide etch processes. At
fluences above about 10J/cm2, precipitates of trace impurities are responsible for damage; they can be mitigated by
eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica
damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating
fluences these optics see.
A system of customized spatial light modulators has been installed onto the front end of the laser system at the National
Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the
amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam
profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might
otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48
existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was
accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide
photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper
packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach
for extending the operational lifetime of high fluence laser optics on NIF.
Current methods for the manufacture of optical components inevitably leaves a variety of sub-surface imperfections
including scratches of varying lengths and widths on even the finest finishes. It has recently been determined that these
finishing imperfections are responsible for the majority of laser-induced damage for fluences typically used in ICF class
lasers. We have developed methods of engineering subscale parts with a distribution of scratches mimicking those found
on full scale fused silica parts. This much higher density of scratches provides a platform to measure low damage
initiation probabilities sufficient to describe damage on large scale optics. In this work, damage probability per unit
scratch length was characterized as a function of initial scratch width and post fabrication processing including acidbased
etch mitigation processes. The susceptibility of damage initiation density along scratches was found to be strongly
affected by the post etching material removal and initial scratch width. We have developed an automated processing
procedure to document the damage initiations per width and per length of theses scratches. We show here how these
tools can be employed to provide predictions of the performance of full size optics in laser systems operating at 351 nm.
In addition we use these tools to measure the growth rate of a damage site initiated along a scratch and compare this to
the growth measured on an isolated damage site.
Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in
fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain,
"blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical
components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap
images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS)
modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address
beam images are projected onto custom fabricated
optically-addressable light valves. Each valve consists of a large,
single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables
bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in
turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been
fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and
etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and
optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.
In many high energy laser systems, optics with HMDS sol gel antireflective coatings are placed in close proximity to
each other making them particularly susceptible to certain types of strong optical interactions. During the coating
process, halo shaped coating flaws develop around surface digs and particles. Depending on the shape and size of the
flaw, the extent of laser light intensity modulation and consequent probability of damaging downstream optics may
increase significantly. To prevent these defects from causing damage, a coating flaw removal tool was developed that
deploys a spot of decane with a syringe and dissolves away the coating flaw. The residual liquid is evacuated leaving an
uncoated circular spot approximately 1mm in diameter. The resulting uncoated region causes little light intensity
modulation and thus has a low probability of causing damage in optics downstream from the mitigated flaw site.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.