Operating the National Ignition Facility (NIF) near its power and energy performance limits has revealed a new damage initiation mechanism in the final UV optics. The typical damage event involves the last three optics in the NIF beamline: the final focusing lens, the grating debris shield, and the target debris shield. It occurs on high power shots from intensifications from small phase defects (pits) on the exit surface of the focusing lens that travel through the grating debris shield before reflecting off the AR-coated target debris shield about 75 cm downstream, then propagate back upstream and damage the input surface of the grating debris shield optic which is 15 cm downstream of the focusing lens. Ray tracing has firmly established the direct relationship between the phase defects on the final focusing lens and the damage on grating debris via the reflection from the target debris shield. In some cases, bulk filamentary damage is also observed in the 1-cm thick fused silica grating debris shield. It is not fully understood at this point how there can be enough energy from the reflected beam to cause damage where the forward-going beam did not. It does not appear that interaction between the forward-going beam and the backward-going reflected beam is necessary for damage to occur. It does appear necessary that the target debris shield be previously exposed to laser shots and/or target debris. Furthermore, there is no evidence of damage imparted to the target debris shield or the final focusing lens. We will describe all the conditions under which we have (and have not) observed these relatively rare events, and the steps we have taken to mitigate their occurrence, including identification and elimination of the source phase defects.
The National Ignition Facility (NIF) uses an in-situ system called the Final Optics Damage Inspection (FODI) system to monitor the extent of damage on installed optical components. Among this system's uses is to alert operators when damage sites on a Grating Debris Shield (GDS) require repair (≈300 microns) and triggers the removal of the damaged optic. FODI, which can reliably detect damage sites larger than 50 microns, records the size and location of observed sub-critical damage observed on the optic, so each of these sites can be repaired before the optic is next installed. However, by only identifying, and hence repairing sites larger than ≈50 microns, optics are left with numerous smaller sites, some fraction of which resume growing when the host optic is reinstalled. This work presents a method of identifying and repairing damage sites below the FODI detection limit that have a significant probability of growth. High resolution images are collected of all likely damage candidates on each optic, and a machine learning based automated classification algorithm is used to determine if each candidate is a damage site or something benign (particle, previously repaired site, etc.). Any damage site greater than 20 microns is flagged for subsequent repair. By repairing these smaller sites, recycled optics had a 40% increased lifetime on the NIF.
The National Ignition Facility (NIF) regularly operates at fluences above the onset of laser-induced optics damage. To do so, it is necessary to routinely recycle the NIF final optics, which involves removing an optic from a beamline, inspecting and repairing the laser-induced damage sites, and re-installing the optic. The inspection and repair takes place in our Optics Mitigation Facility (OMF), consisting of four identical processing stations for performing the repair protocols. Until recently, OMF has been a labor-intensive facility, requiring 10 skilled operators over two shifts to meet the throughput requirements. Here we report on the implementation of an automated control system—informed by machine learning— that significantly improves the throughput capability for recycling of NIF optics while reducing staffing requirements. Performance metrics for mid-2018 show that approximately 85% of all damage sites can be automatically inspected and repaired without any required operator input. Computer keystrokes have been reduced from about 6000 per optic to under 300.
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.
Modeling of laser-induced optics damage has been introduced to benchmark existing optic usage at the National Ignition
Facility (NIF) which includes the number of optics exchanged for damage repair. NIF has pioneered an optics recycle
strategy to allow it to run the laser at capacity since fully commissioned in 2009 while keeping the cost of optics usage
manageable. We will show how the damage model is being used to evaluate strategies to streamline our optics loop
efficiency, as we strive to increase the laser shot rate without increasing operating costs.
Optics damage growth modeling and analysis at the National Ignition Facility (NIF) has been performed on fused silica. We will show the results of single shot growth comparisons, damage site lifetime comparisons as well as growth metrics for each individual NIF beamline. These results help validate the consistency of the damage growth models and allow us to have confidence in our strategic planning in regards to projected optic usage.
The National Ignition Facility has recently achieved the milestone of delivering over
1.8 MJ and 500 TW of 351 nm laser energy and power on target, which required average fluences up to 9 J/cm2 (3 ns equivalent) in the final optics system. Commercial fused silica laser-grade UV optics typically have a maximum
operating threshold of 5 J/cm2. We have developed an optics recycling process which enables NIF to operate above the laser damage initiation and growth thresholds. We previously reported a method to mitigate laser damage with laser
ablation of the damage site to leave benign cone shaped pits. We have since developed a production facility with four
mitigation systems capable of performing the mitigation protocols on full-sized (430 mm) optics in volume production.
We have successfully repaired over 700 NIF optics (unique serial numbers), some of which have been recycled as many
as 11 times. We describe the mitigation systems, the optics recycle loop process, and optics recycle production data.
We present results from a study to determine an acceptable CO2 laser-based non-evaporative mitigation protocol for use
on surface damage sites in fused-silica optics. A promising protocol is identified and evaluated on a set of surface
damage sites created under ICF-type laser conditions. Mitigation protocol acceptability criteria for damage re-initiation
and growth, downstream intensification, and residual stress are discussed. In previous work, we found that a power
ramp at the end of the protocol effectively minimizes the residual stress (⪅25 MPa) left in the substrate. However, the
biggest difficulty in determining an acceptable protocol was balancing between low re-initiation and problematic
downstream intensification. Typical growing surface damage sites mitigated with a candidate CO2 laser-based
mitigation protocol all survived 351 nm, 5 ns damage testing to fluences ⪆12.5 J/cm2. The downstream intensification
arising from the mitigated sites is evaluated, and all but one of the sites has 100% passing downstream damage
expectation values. We demonstrate, for the first time, a successful non-evaporative 10.6 m CO2 laser mitigation
protocol applicable to fused-silica optics used on fusion-class lasers like the National Ignition Facility (NIF).
A new method of mitigating (arresting) the growth of large (>200 m diameter and depth) laser induced surface damage
on fused silica has been developed that successfully addresses several issues encountered with our previously-reported5,6large site mitigation technique. As in the previous work, a
tightly-focused 10.6 m CO2 laser spot is scanned over the
damage site by galvanometer steering mirrors. In contrast to the previous work, the laser is pulsed instead of CW, with
the pulse length and repetition frequency chosen to allow substantial cooling between pulses. This cooling has the
important effect of reducing the heat-affected zone capable of supporting thermo-capillary flow from scale lengths on
the order of the overall scan pattern to scale lengths on the order of the focused laser spot, thus preventing the formation
of a raised rim around the final mitigation site and its consequent down-stream intensification. Other advantages of the
new method include lower residual stresses, and improved damage threshold associated with reduced amounts of redeposited
material. The raster patterns can be designed to produce specific shapes of the mitigation pit including cones
and pyramids. Details of the new technique and its comparison with the previous technique will be presented.
Phase-defects on optics used in high-power lasers can cause light intensification leading to laser-induced damage of
downstream optics. We introduce Linescan Phase Differential Imaging (LPDI), a large-area dark-field imaging
technique able to identify phase-defects in the bulk or surface of large-aperture optics with a 67 second scan-time.
Potential phase-defects in the LPDI images are indentified by an image analysis code and measured with a Phase
Shifting Diffraction Interferometer (PSDI). The PSDI data is used to calculate the defects potential for downstream
damage using an empirical laser-damage model that incorporates a laser propagation code. A ray tracing model of LPDI
was developed to enhance our understanding of its phase-defect detection mechanism and reveal limitations.
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.
The Optical Sciences Laser (OSL) Upgrade facility, described in last year's proceedings, is a kJ-class, large aperture (100cm2) laser system that can accommodate prototype optical components for large-scale inertial confinement fusion lasers. High-energy operation of such lasers is often limited by damage to the optical components. Recent experiments on the OSL Upgrade facility using fused silica components at 4 J/cm2 (351-nm, 3-ns) have created output surface and bulk damage sites that have been correlated to phase objects in the bulk of the material. Optical Path Difference (OPD) measurements of the phase defects indicate the probability of laser-induced damage is strongly dependent on OPD.
A large aperture, kJ-class, multi-wavelength Nd-glass laser system has been constructed at Lawrence Livermore National Lab which has unique capabilities for studying a wide variety of optical phenomena. The master-oscillator, power-amplifier (MOPA) configuration of this "Optical Sciences Laser" (OSL) produces 1053 nm radiation with shaped pulse lengths which are variable from 0.1 - 100 ns. The output can be frequency doubled or tripled with high conversion efficiency with a resultant 100 cm2 high quality output beam. This facility can accommodate prototype hardware for large-scale inertial confinement fusion lasers allowing for investigation of integrated system issues such as optical lifetime at high fluence, optics contamination, compatibility of non-optical materials, and laser diagnostics.
Recent work has shown that the damage resistance of both ICF-class (1600 cm2) DKDP tripler crystals and SiO2 components (lenses, gratings and debris shields) benefits from laser raster scanning using pulsed lasers in the 350 nm range. For laser raster scanning to be a viable optical improvement tool for these large optics, damage improvement must be optimized while maintaining scan times of less than 8 hours/optic. In this paper we examine raster scanning with small beams from tabletop laser systems. We show that 120 Watts of average power is required for a tabletop scanning system at one optic/day. Next, we develop equations for total scan time for square and round top heat beams and round and rectangular Gaussian beams. We also consider the effect of packing geometry (square vs. hexagonal), examine the deviations from uniform coverage with each scan geometry and show that hexagonal packing yields lower scan times but is less efficient in coverage than square geometry. We also show that multiple passes at low packing densities are temporally equivalent to a single pass with higher packing density, and discuss the advantages of each method. In addition, we show that the differences between hexagonal and square scan geometries are negated when pointing errors and fluence fluctuations from the laser are considered.
Fluorescing surface defects that led to damage upon 351-nm laser exposure below 7 J/cm2 (3-ns) in DKDP optics were reported in these proceedings by this group a year ago. Subsequent laser damage experiments have correlated the density of these damage precursors to single-pont diamond finishing conditions. Every diamond-finishing schedule contains brittle-mode cutting and ductile-mode cutting in a taper-down sequence. Finishing experiments have traced the occurrence of these defects to insufficient ductile-mode removal of subsurface damage incurred during piror brittle-mode cutting. Additionally, a correlation between defect fluorescence, laser-induced damage, and defect meorphology has been established. Laser-induced damage tests also suggest a correlation between growth method and damage probability. Current experiments indicate that damage-prone defects can be minimized with the proper choice of diamond finishing conditions.
We report an experimental investigation of mitigating surface damage growth at 351 nm for machine-finished DKDP optics. The objective was to determine which methods could be applied to pre-initiated or retrieved-from-service optics, in order to stop further damage growth for large aperture DKDP optics used in high-peak-power laser applications. The test results, and the evaluation thereof, are presented for several mitigation methods applied to DKDP surface damage. The mitigation methods tested were CW-CO2 laser processing, aqueous wet-etching, short-pulse laser ablation, and micro-machining. We found that micro-machining, using a single crystal diamond tool to completely remove the damage pit, produces the most consistent results to halt the growth of surface damage on DKDP. We obtained the successful mitigation of laser-initiated surface damage sites as large as 0.14 mm diameter, for up to 1000 shots at 351 nm and fluences in the range of 2 to 13 J/cm2, ≈ 11 ns pulse length. Data obtained to-date indicates that micro-machining is the preferred method to process large-aperture optics.
In this paper we present the results of bulk damage experiments done on Type-II DKDP triple harmonic generator crystals that were raster conditioned with 351 - 355 nm wavelengths and pulse durations of 4 and 23.2 ns. In the first phase of experiments 20 different scan protocols were rastered into a sample of rapid growth DKDP. The sample was then rastered at damage-causing fluences to determine the three most effective protocols. These three protocols were scanned into a 15-cm sample of conventional-growth DKDP and then exposed to single shots of a 1-cm beam from LLNL's Optical Sciences Laser at fluences ranging from 0.5 - 1.5X of the 10% damage probability fluence and nominal pulse durations of 0.1, 0.3, 0.8, 3.2, 7.0 and 20 ns. The experiment showed that pulse durations in the 1 - 3 ns range were much more effective at conditioning than pulses in the 16.3 ns range and that the multiple pass "peak fluence" scan was more effective than the single pass "leading edge" scan for 23.2 ns XeF scans.
Energy (frequency) of phonons is the main parameter which determines ratio between probabilities of radiative and radiationless transitions in luminescence. Single crystals of double chlorides KPb2C15 and bromides KPb2Br5 , which are formed by heavy ions and have a low energy phonon spectrum (h? <200 and 150 cm-1, respectively), of optical quality were obtained using the Bridgmen-Stockbarger technique. We studied the optical spectra and luminescence kinetics of RE —doped crystals (RE= Pr3+, Nd3+, Tb3+, Ho3+, Er3+ etc). Intensity parameters were determined by the Judd-Ofelt method, radiative and non-radiative transition probabilities were calculated. It was shown that low multiphonon relaxation rate in these crystals together with high values of radiative probabilities leads to the evidence of high intensity luminescence in spectral domain from 360 to 9000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers with laser diode pumping.
We studied the optical spectra and luminescence kinetics of double chloride Kpb2Cl5:TR3+ crystals as a new luminescent material promising for UV, VIS and mid-IR lasers, pumped with laser diodes. Intensity parameters were determined by the Judd-Ofelt method, radiative and non- radiative transition probabilities were calculated. It is shown that low multiphoton relaxation rate in the se crystal together with high values of radiative probabilities leads to the evidence of high intensive luminescence in spectral domain from 360 to 5000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers.
We have investigated the surface degradation of bare and sol-gel coated deuterated potassium dihydrogen phosphate (DKDP) crystals when exposed to 351-nm laser pulses in atmospheric air and nitrogen and at pressures ranging from atmospheric down to 10-5 Torr vacuum. Optical microscopy, surface topography, surface chemical analyses, 351-nm pumped photoemission maps, and photometry results have been used to characterize these samples. We report the occurrence of two potentially linked surface degradation phenomena: the development of increased photoemission and the development of unacceptable surface roughening in the region exposed to the beam in vacuum. We note no degradation for surfaces exposed in air or nitrogen at pressures exceeding 1 Torr. Diamond-turned DKDP surfaces show a ubiquitous, low-intensity photoemission signature before exposure to any laser fluence. The observed reduction of this emission signal as a function of operating pressure and accumulated laser energy when crystals are exposed to 351-nm laser pulses in air can be correlated with the removal of surface carbon.
Embedded gold and mechanical deformation in silica were used to investigate initiation of laser-induced damage at 355 nm (7.6 ns). The nanoparticle-covered surfaces were coated with between 0 and 500 nm of SiO2 by e-beam deposition. The threshold for observable damage and initiation site morphology for these engineered surfaces was determined. The gold nanoparticle coated surfaces with 500 nm SiO2 coating exhibited pinpoint damage threshold of <0.7 J/cm2 determined by light scattering and Nomarski microscopy. The gold nanoparticle coated surfaces with the 100 nm SiO2 coatings exhibited what nominally appeared to be film exfoliation damage threshold of 19 J/cm2 via light scattering and Nomarski microscopy. With atomic force microscopy pinholes could be detected at fluences greater than 7 J/cm2 and blisters at fluences greater than 3 J/cm2 on the 100-nm-coated surfaces. A series of mechanical indents and scratches were made in the fused silica substrates using a non-indentor. Plastic deformation without cracking led to damage thresholds of approximately 25 J/cm2, whereas indents and scratches with cracking led to damage thresholds of only approximately 5 J/cm2. Particularly illuminating was the deterministic damage of scratches at the deepest end of the scratch, as if the scratch acted as a waveguide.
This work is an experimental investigation to evaluate the potential of fluorescence microscopy as a tool to detect surface contamination as well as reveal surface damage precursors on DKDP and SiO2 optics. To achieve these technical objectives, microscopic imaging systems were built that also incorporate in-situ damage testing capabilities. Fluorescence imaging experiments were performed using 351-nm laser excitation while damage testing was performed at relatively high laser fluences. The experimental results demonstrated the potential of this technique to address the aforementioned technical issues.
In this work we studied the optical spectra of dysprosium and ytterbium doped double chloride MePb2Cl5:RE3+ (Me equals K, Rb), and double fluoride LiYF4:RE3+ (RE3+ equals Dy, Yb) crystals. RE3+ doped double chloride and double fluoride crystals have been grown, by using Bridgeman technique. Optical spectra were studied, intensity parameters are determined using Judd-Ofelt method and radiative probabilities and branching ratio were calculated. Yb3+ -Dy3+ energy transfer processes in KPb2Cl5:Dy3+,Yb3+ and LiYF4:Dy3+,Yb3+ were considered.
In single crystals of AgCl, BaCl2, PbCl2, SrCl2, KPb2Cl5 with an unextended phonon spectrum (E less than 200 cm-1), doped by Dy, the luminescence spectrum was found to cover a wide range from visible region to IR (4.3 mkm for KPb2Cl5), including well pronounced 1.31 mkm band, which is important for using in telecommunication amplifiers. The Rare Earth impurity was shown to enter the host matrice as RE3+ in chlorides, the only type of impurity centers being formed in the case of rhombic MeCl2 crystals with Me equals Ba, P, S.
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