High-average-power solid-state lasers are often severely limited by thermally induced stress birefringence, leading to depolarization losses and damage issues. Depolarization losses can be mitigated by selecting appropriate polarization optics to compensate the space-variant stress birefringence distribution. While some small-aperture, low-fluence technologies are available, no solution so far can provide a combination of high fluence and large aperture with a high degree of control of the beam quality. In this work, we demonstrate the use of magnetorheological finishing (MRF) technology to carve a prescribed thickness profile in a quartz waveplate to achieve precise space-variant polarization control. An arbitrary distorted polarization distribution can be converted into a uniform linearly polarized beam using two freeform MRF crystal optics with their crystal axes offset by 45 degrees. This technology is readily scalable to high-fluence, large-aperture applications, potentially enabling new regimes of laser operation.
Loose abrasive grinding was performed on a wide range of optical workpiece materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2-Al2O3-P2O5-Li2O glass-ceramic (Zerodur); and glasses of SiO2 : TiO2 (ULE), SiO2 (fused silica), and P2O5-Al2O3-K2O-BaO (phosphate)]. Using the magneto rheological finishing (MRF) taper wedge technique (where a wedge was polished on each of the ground workpieces and the resulting samples were appropriately chemically etched), the subsurface mechanical damage (SSD) characteristics were measured. The SSD depth for most of the workpiece materials was found to scale as E11/2 / H1, where E1 is the elastic modulus and H1 is the hardness of the workpiece. This material scaling is the same as that for the growth of lateral cracks, suggesting that lateral cracks are a dominant source for SSD rather than radial/median cracks, as previously proposed. Utilizing the SSD depth data from both this study and others, semiempirical relationships have been formulated, which allows for estimating the SSD depth as a function of workpiece material and important grinding parameters (such as abrasive size and applied pressure).
C. Haefner, A. Bayramian, S. Betts, R. Bopp, S. Buck, J. Cupal, M. Drouin, A. Erlandson, J. Horáček, J. Horner, J. Jarboe, K. Kasl, D. Kim, E. Koh, L. Koubíková, W. Maranville, C. Marshall, D. Mason, J. Menapace, P. Miller, P. Mazurek, A. Naylon, J. Novák, D. Peceli, P. Rosso, K. Schaffers, E. Sistrunk, D. Smith, T. Spinka, J. Stanley, R. Steele, C. Stolz, T. Suratwala, S. Telford, J. Thoma, D. VanBlarcom, J. Weiss, P. Wegner
Large laser systems that deliver optical pulses with peak powers exceeding one Petawatt (PW) have been constructed at dozens of research facilities worldwide and have fostered research in High-Energy-Density (HED) Science, High-Field and nonlinear physics [1]. Furthermore, the high intensities exceeding 1018W/cm2 allow for efficiently driving secondary sources that inherit some of the properties of the laser pulse, e.g. pulse duration, spatial and/or divergence characteristics. In the intervening decades since that first PW laser, single-shot proof-of-principle experiments have been successful in demonstrating new high-intensity laser-matter interactions and subsequent secondary particle and photon sources. These secondary sources include generation and acceleration of charged-particle (electron, proton, ion) and neutron beams, and x-ray and gamma-ray sources, generation of radioisotopes for positron emission tomography (PET), targeted cancer therapy, medical imaging, and the transmutation of radioactive waste [2, 3]. Each of these promising applications requires lasers with peak power of hundreds of terawatt (TW) to petawatt (PW) and with average power of tens to hundreds of kW to achieve the required secondary source flux.
Over the last eight years we have been developing advanced MRF tools and techniques to manufacture meter-scale
optics for use in Megajoule class laser systems. These systems call for optics having unique characteristics that can
complicate their fabrication using conventional polishing methods. First, exposure to the high-power nanosecond and
sub-nanosecond pulsed laser environment in the infrared (>27 J/cm2 at 1053 nm), visible (>18 J/cm2 at 527 nm), and
ultraviolet (>10 J/cm2 at 351 nm) demands ultra-precise control of optical figure and finish to avoid intensity modulation
and scatter that can result in damage to the optics chain or system hardware. Second, the optics must be super-polished
and virtually free of surface and subsurface flaws that can limit optic lifetime through laser-induced damage initiation
and growth at the flaw sites, particularly at 351 nm. Lastly,
ultra-precise optics for beam conditioning are required to
control laser beam quality. These optics contain customized surface topographical structures that cannot be made using
traditional fabrication processes. In this review, we will present the development and implementation of large-aperture
MRF tools and techniques specifically designed to meet the demanding optical performance challenges required in large aperture
high-power laser systems. In particular, we will discuss the advances made by using MRF technology to expose
and remove surface and subsurface flaws in optics during final polishing to yield optics with improve laser damage
resistance, the novel application of MRF deterministic polishing to imprint complex topographical information and
wavefront correction patterns onto optical surfaces, and our efforts to advance the technology to manufacture largeaperture
damage resistant optics.
There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility
of optical components and both the surface quality of the optics, and the presence of near surface
fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal
time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation
at indentation sites. The combination of localized polishing and variations in indentation loads
allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed
and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are
strongly correlated with fracture surfaces, while densified and plastically flowed material is
relatively benign, and 2) fracture events result in the formation of an electronically defect rich
surface layer which promotes energy transfer from the optical beam to the glass matrix.
Over the past year we have been working on specialized MR fluids for polishing KDP crystals. KDP is an extremely
difficult material to conventionally polish due to its water solubility, low hardness, and temperature sensitivity. Today,
KDP crystals are finished using single-point diamond turning (SPDT) tools and nonaqueous lubricants/coolants. KDP
optics fabricated using SPDT, however, are limited to surface corrections due to tool/method characteristics with surface
quality driven by microroughness from machine pitch, speed, force, and diamond tool character. MRF polishing offers a
means to circumvent many of these issues since it is deterministic which makes the technique practical for surface and
transmitted wavefront correction, is low force, and is temperature independent. What is lacking is a usable nonaqueous
MR fluid that is chemically and physically compatible with KDP which can be used for polishing and subsequently
cleaned from the optical surface. In this study, we will present the fluid parameters important in the design and
development of nonaqueous MR fluid formulations capable of polishing KDP and how these parameters affect MRF
polishing. We will also discuss requirements peculiar to successful KDP polishing and how they affect optical
figure/finish and laser damage performance at 1064 nm and 532 nm.
Magnetorheological Finishing (MRF) techniques and tools have been developed to imprint complex,
continuously varying topographical structures onto 430 x 430 millimeter optical surfaces. These optics,
known as continuous phase plates (CPPs) are important for
kilojoule- and megajoule-class laser systems requiring precise control of beam-shape, energy distribution and wavefront profile. MRF's sub-aperture polishing characteristics make it possible to imprint complex computer generated topographical information at spatial scale-lengths approaching 1 millimeter and surface
peak-to-valleys as high as 22 micrometers to within 30 nanometers of design specifications. This paper presents the evolution of MRF imprinting technology for manufacturing large-aperture CPPs.
The Mercury laser uses ytterbium-doped strontium fluorapatite (Yb:S-FAP) crystals as the gain medium with a nominal
clear aperture of 4 x 6 cm. Recent damage test data have indicated the existence of bulk precursors in Yb:S-FAP that
initiate damage starting at approximately 10 J/cm2 at 9 ns under 1064 nm irradiation. In this paper, we report on
preliminary results on bulk damage studies on Yb:S-FAP crystals.
Managing subsurface damage during the shaping process and removing subsurface damage during the polishing process is essential in the production of low damage density optical components, such as those required for use on high peak power lasers. Removal of subsurface damage, during the polishing process, requires polishing to a depth which is greater than the depth of the residual cracks present following the shaping process. To successfully manage, and ultimately remove subsurface damage, understanding the distribution and character of fractures in the subsurface region introduced during fabrication process is important. We have characterized the depth and morphology of subsurface fractures present following fixed abrasive and loose abrasive grinding processes. At shallow depths lateral cracks and an overlapping series of trailing indentation fractures were found to be present. At greater depths, subsurface damage consists of a series of trailing indentation fractures. The area density of trailing fractures changes as a function of depth, however the length and shape of individual cracks remain nearly constant for a given grinding process. We have developed and applied a model to interpret the depth and crack length distributions of subsurface surface damage in terms of key variables including abrasive size and load.
Understanding the behavior of fractures and subsurface damage in the processes used during optic fabrication plays a key role in determining the final quality of the optical surface finish. During the early stages of surface preparation, brittle grinding processes induce fractures at or near an optical surface whose range can extend from depths of a few μm to hundreds of μm depending upon the process and tooling being employed. Controlling the occurrence, structure, and propagation of these sites during subsequent grinding and polishing operations is highly desirable if one wishes to obtain high-quality surfaces that are free of such artifacts. Over the past year, our team has made significant strides in developing a diagnostic technique that combines magnetorheological finishing (MRF) and scanning optical microscopy to measure and characterize subsurface damage in optical materials. The technique takes advantage of the unique nature of MRF to polish a prescribed large-area wedge into the optical surface without propagating existing damage or introducing new damage. The polished wedge is then analyzed to quantify subsurface damage as a function of depth from the original surface. Large-area measurement using scanning optical microscopy provides for improved accuracy and reliability over methods such as the COM ball-dimple technique. Examples of the technique's use will be presented that illustrate the behavior of subsurface damage in fused silica that arises during a variety of intermediate optical fabrication process steps.
We have developed an experimental technique that combines magnetorheological finishing (MRF) and microscopy to examine fractures and/or artifacts in optical materials. The technique can be readily used to provide access to, and interrogation of, a selected segment of a fracture or object that extends beneath the surface. Depth slicing, or cross-sectioning at selected intervals, further allows the observation and measurement of the three-dimensional nature of the sites and the generation of volumetric representations that can be used to quantify shape and depth, and to understand how they were created, how they interact with surrounding material, and how they may be eliminated or mitigated.
Antireflection (AR) coatings typically damage at the interface between the substrate and coating. Therefore the substrate finishing technology can have an impact on the laser resistance of the coating. For this study, AR coatings were deposited on Yb:S-FAP [Yb3+:Sr5(PO4)3F] crystals that received a final polish by both conventional pitch lap finishing as well as magnetorheological finishing (MRF). SEM images of the damage morphology reveals laser damage originates at scratches and at substrate coating interfacial absorbing defects. Previous damage stability tests on multilayer mirror coatings and bare surfaces revealed damage growth can occur at fluences below the initiation fluence. The results from this study suggest the opposite trend for AR coatings. Investigation of unstable HR and uncoated surface damage morphologies reveals significant radial cracking that is not apparent with AR damage due to AR delamination from the coated surface with few apparent cracks at the damage boundary. Damage stability tests show that coated Yb:S-FAP crystals can operate at 1057 nm at fluences around 20 J/cm2 at 10 ns; almost twice the initiation damage threshold.
We report initial operation of the Mercury laser with seven 4 x 6 cm S-FAP amplifier slabs pumped by four 80 kW diode arrays. The system produced up to 33.5 J single shot, 23.5 J at 5 Hz, and 10 J at 10 Hz for 20 minute runs at 1047 nm. During the initial campaign, more than 2.8 x 104 shots were accumulated on the system. The beam quality of the system was measured to be 2.8 x 6.3 times diffraction limited at 110 W of output, with 96% of the energy in a 5X diffraction limited spot. Static wavefront glass plates were used to correct for the low order distortions in the slabs due to fabrication and thermal loading. Scaling of crystal grown has begun with the first full size slab produced from large diameter growth. Using an energetics optimization code we find the beam aperture is scalable up to 20 x 30 cm and 4.2 kJ.
Magnetorheological finishing (MRF) techniques have been developed to manufacture continuous phase plates (CPPs) and custom phase corrective structures on polished fused silica surfaces. These phase structures are important for laser applications requiring precise manipulation and control of beam-shape, energy distribution, and wavefront profile. The MRF’s unique deterministic-sub-aperture polishing characteristics make it possible to imprint complex topographical information onto optical surfaces at spatial scale-lengths approaching 1 mm. In this study, we present the results of experiments and model calculations that explore imprinting two-dimensional sinusoidal structures. Results show how the MRF removal function impacts and limits imprint fidelity and what must be done to arrive at a high quality surface. We also present several examples of this imprinting technology for fabrication of phase correction plates and CPPs for use at high fluences.
Installation and commissioning of the first of forty-eight Final Optics Assemblies on the National Ignition Facility was completed this past year. This activity culminated in the delivery of first light to a target. The final optics design is described and selected results from first-article commissioning and performance tests are presented.
The large-aperture (up to 40 cm × 80 cm) mirrors required for the National Ignition Facility have very stringent specifications. The specifications include requirements for transmitted and reflected wavefront over a wide spectral frequency, surface quality, laser resistance, spectral characteristics, etc. In order to validate optic performance, metrology tools were fielded at optic fabrication vendors to assure production control. These tools include interferometers, large-area conditioning stations, and photometers. Of the 1800 large-aperture mirrors required for the NIF, approximately 35% have been completed. This presentation will review the types of large-aperture mirrors used on NIF along with the performance of NIF optics as measured and received from our vendors.
The high-energy/high-power section of the NIF laser system contains 7360 meter-scale optics. Advanced optical
materials and fabrication technologies needed to manufacture the NIF optics have been developed and put into
production at key vendor sites. Production rates are up to 20 times faster and per-optic costs 5 times lower than could be
achieved prior to the NIF. In addition, the optics manufactured for NIF are better than specification giving laser
performance better than the design. A suite of custom metrology tools have been designed, built and installed at the
vendor sites to verify compliance with NIF optical specifications. A brief description of the NIF optical wavefront
specifications for the glass and crystal optics is presented. The wavefront specifications span a continuous range of
spatial scale-lengths from 10 μm to 0.5 m (full aperture). We have continued our multi-year research effort to improve
the lifetime (i.e. damage resistance) of bulk optical materials, finished optical surfaces and multi-layer dielectric
coatings. New methods for post-processing the completed optic to improve the damage resistance have been developed
and made operational. This includes laser conditioning of coatings, glass surfaces and bulk KDP and DKDP and well as
raster and full aperture defect mapping systems. Research on damage mechanisms continues to drive the development
of even better optical materials.
Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm, will be epxosed to backscattered light from plasmas created from fusion targets and backlighters. This backscattered light covers the UV and visible spectrum from 351 - 600 nm. The transport mirror BK7 substrates will be intentionally solarized to absorb >95% of the backscattered light to prevent damage to the metallic mechanical support hardware. Solarization has minimal impact on the 351- 1053-nm laser-induced damage threshold or the reflected wavefront of the multilayer hafnia silica coating. Radiation sources of various energies were examined for BK7 darkening efficiency within the UV and visible region with 1.1 MeV gamma rays from a Cobalt 60 source ultimately being selected. Finally, bleaching rates were measured at elevated temperatures to generate a model for predicting the lifetime at ambient conditions (20°C), before solarized BK7 substrates exceed 5% transmission in the UV and visible region. Over a 30-mm thickness, BK7 glass will bleach in 10 years to 5% transmission at 600 nm, the most transmissive wavelengths over the 351 - 600 nm regions.
This paper describes the effect of 355-nm laser conditioning on the concentration of UV-laser-induced surface damage sites on large-aperture fused silica optics. We will show the effect of various 355-nm laser conditioning methodologies on the reduction of surface-damage initiation in fused silica samples that have varying qualities of polishing. With the best, generally available fused silica optic, we have demonstrated that 355-nm laser conditioning can achieve up to 10x reduction in surface damage initiation concentration in the fluence range of 10-14 J/cm2 (355- nm at 3 ns).
Laser-induced damage initiation on fused silica optics can limit the lifetime of the components when used in high power UV laser environments. For example in inertial confinement fusion research applications, the optics can be exposed to temporal laser pulses of about 3 nsec with average fluences of 8 J/cm2 and peak fluences between 12 and 15 J/cm2. During the past year, we have focused on optimizing the damage performance at a wavelength of 355-nm (3(omega) ), 3-nsec pulse length, for optics in this category by examining a variety of finishing technologies with a challenge to improve the laser damage initiation density by at least two orders of magnitude. In this paper, we describe recent advances in improving the 3(omega) damage initiation performance of laboratory-scale zirconium oxide and cerium oxide conventionally finished fused silica optics via application of processes incorporating magnetorheological finishing (MRF), wet chemical etching, and UV laser conditioning. Details of the advanced finishing procedures are described and comparisons are made between the procedures based upon large area 3(omega) damage performance, polishing layer contamination, and optical subsurface damage.
A program to identify and eliminate the causes of UV laser- induced damage and growth in fused silica and DKDP has developed methods to extend optics lifetimes for large- aperture, high-peak-power, UV lasers such as the National Ignition Facility (NIF). Issues included polish-related surface damage initiation and growth on fused silica and DKDP, bulk inclusions in fused silica, pinpoint bulk damage in DKDP, and UV-induced surface degradation in fused silica and DKDP in a vacuum. Approaches included an understanding of the mechanism of the damage, incremental improvements to existing fabrication technology, and feasibility studies of non-traditional fabrication technologies. Status and success of these various approaches are reviewed. Improvements were made in reducing surface damage initiation and eliminating growth for fused silica by improved polishing and post- processing steps, and improved analytical techniques are providing insights into mechanisms of DKDP damage. The NIF final optics hardware has been designed to enable easy retrieval, surface-damage mitigation, and recycling of optics.
Cleanliness measurements made on AMPLAB prototype NIF laser amplifiers during assembly, cassette transfer, and amplifier operation are summarized. These measurements include particle counts from surface cleanliness assessments using filter swipe technique and from airborne particle monitoring. Results are compared with similar measurements made on the Beamlet and Nova lasers and in flashlamp test fixtures. Observations of Class 100,000 aerosols after flashlamp firings are discussed. Comparisons are made between typical damage densities on laser amplifier optics from Novette, NOVA, Beamlet, and AMPLAB.
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