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Application of total internal reflection microscopy for laser damage studies on fused silica

Description: Damage studies show that the majority of damage on ultraviolet grade fused silica initiates at the front or rear surface. The grinding and polishing processes used to produce the optical surfaces of transparent optics play a key role in the development of defects which can ultimately initiate damage. These defects can be on or breaking through the surface or can be sub-surface damage. Total Internal Reflection Microscopy has been documented as a tool for revealing both sub-surface and surface defects in transparent materials. Images taken which compare both Total Internal Reflection Microscopy and Atomic Force Microscopy show that the observed defects can be less than one micron in size. Total Internal Reflection Microscopy has the added benefit of being able to observe large areas (1 square millimeter) with sub-micron detection. Both off-line and in-situ systems have been applied in the Lawrence Livermore National Laboratory`s damage laboratory in order to understand defects in the surface and subsurface of polished fused silica. There is a preliminary indication that TIRM quality can be related to the damage resistance. The in-situ microscope is coupled into a 355 run, 7.5 ns, 10 Hz Nd:YAG laser system in order to study damage occurring at localized scatter sites revealed with the Total Internal Reflection Microscopy method. The tests indicate damage initiating at observed artifacts which have many different morphologies and damage behaviors. Some of the scatter sites and damage morphologies revealed have been related back to the finishing process.
Date: December 1997
Creator: Sheehan, L. M.
Partner: UNT Libraries Government Documents Department

Large area damage testing of optics

Description: The damage threshold specifications for the National Ignition Facility will include a mixture of standard small-area tests and new large-area tests. During our studies of laser damage and conditioning processes of various materials we have found that some damage morphologies are fairly small and this damage does not grow with further illumination. This type of damage might not be detrimental to the laser performance. We should therefore assume that some damage can be allowed on the optics, but decide on a maximum damage allowance of damage. A new specification of damage threshold termed {open_quotes}functional damage threshold{close_quotes} was derived. Further correlation of damage size and type to system performance must be determined in order to use this measurement, but it is clear that it will be a large factor in the optics performance specifications. Large-area tests have verified that small-area testing is not always sufficient when the optic in question has defect-initiated damage. This was evident for example on sputtered polarizer and mirror coatings where the defect density was low enough that the features could be missed by standard small- area testing. For some materials, the scale-length at which damage non-uniformities occur will effect the comparison of small-area and large-area tests. An example of this was the sub-aperture tests on KD*P crystals on the Beamlet test station. The tests verified the large-area damage threshold to be similar to that found when testing a small-area. Implying that for this KD*P material, the dominate damage mechanism is of sufficiently small scale-length that small-area testing is capable of determining the threshold. The Beamlet test station experiments also demonstrated the use of on-line laser conditioning to increase the crystals damage threshold.
Date: April 26, 1996
Creator: Sheehan, L.; Kozlowski, M. & Stolz, C.
Partner: UNT Libraries Government Documents Department

Full aperture laser conditioning of multilayer mirrors and polarizers

Description: The Inertial Confinement Fusion (ICF) program at LLNL is beginning the design of a 1.8 Megajoule, 0.35-{mu}m, laser system called the National Ignition Facility (NIF). In order to reduce cost, and increase performance, high damage threshold optics are essential. As a result of damage initiating defects, only a small percentage of current as-deposited optical coatings can meet the required damage threshold specification. Work has been conducted in the area of understanding the causes of these nodular defects and how they are related to laser damage. While it is not yet possible to produce defect-free coatings, it has been found that the damage threshold of some coatings can be increased by as much as 2 or more times as a result of pre-illumination at incrementally increasing fluences. This process, termed laser conditioning, has been associated with the ejection of the damage-initiating defects. With current damage thresholds, mirrors and polarizers for the NIF will have to be laser conditioned in order to meet the laser requirements for fluence propagation. LLNL has constructed a system dedicated to laser conditioning of meter-sized HfO{sub 2}/SiO{sub 2} multilayer polarizers and mirrors. The optic is moved in a raster pattern through a stationary 10-Hz rep-rated, 1.064 {mu}m beam with 10-ns pulses. A scatter measurement diagnostic allows on-the-fly evaluation of laser-induced damage during a scan. This system has been used to laser condition optics as large as 73 cm {times} 37 cm. Such optics are now being used on the Beamlet laser system at LLNL. A description of the conditioning process, its effect on the optic, and an analysis of its application to large areas is presented.
Date: May 26, 1995
Creator: Sheehan, L.; Kozlowski, M. & Tench, B.
Partner: UNT Libraries Government Documents Department

Detection of inherent and laser-induced scatter in optical materials

Description: As Lawrence Livermore National Laboratory moves forward with the design of the National Ignition Facility (NIF) in the Inertial Confinement Fusion (ICF) program, issues relating to the detection and measurement of laser-induced damage on large optics must be addressed. Currently, microscopy is used to evaluate surface quality and measure damage thresholds on small witness samples. In order to evaluate large areas, an automated system was constructed which can scan optics with dimensions as large as 1 meter and weighing as much as 400 pounds. The use of microscopy as the main test diagnostic has been replaced with an optical scatter detection system. Now large areas can be rastered, and maps can be generated, reflecting inherent and laser-induced scatter in multilayer optical coatings and bulk materials. The integrated scattered light from a test piece is measured in transmission using a HeNe laser as the probe source. When the probe beam is overlapped on a pulsed, high power, ND:YAG laser beam, damage related scatter may be measured. This technique has been used for: (1) mapping of inherent scatter in an optic, (2) on-the-fly damage detection during a high fluence raster scan of an optic, and (3) single site damage evaluation for the determination of a laser damage threshold. Damage thresholds measured with the scatter diagnostic compare within measurement error to those attained using 100 {times} microscopy.
Date: July 5, 1995
Creator: Sheehan, L. & Kozlowski, M.
Partner: UNT Libraries Government Documents Department

Diagnostics for the detection and evaluation of laser induced damage

Description: The Laser Damage and Conditioning Group at LLNL is evaluating diagnostics which will help make damage testing more efficient and reduce the risk of damage during laser conditioning. The work to date has focused on photoacoustic and scattered light measurements on 1064-nm wavelength HfO{sub 2}/SiO{sub 2} multilayer mirror and polarizer coatings. Both the acoustic and scatter diagnostics have resolved 10 {mu}m diameter damage points in these coatings. Using a scanning stage, the scatter diagnostic can map both intrinsic and laser-induced scatter. Damage threshold measurements obtained using scatter diagnostics compare within experimental error with those measured using 100x Nomarski microscopy. Scatter signals measured during laser conditioning can be used to detect damage related to nodular defects.
Date: January 3, 1995
Creator: Sheehan, L.; Kozlowski, M. & Rainer, F.
Partner: UNT Libraries Government Documents Department

NIF small optics laser damage test specifications

Description: The Laser Damage Group is currently conducting tests on small optics samples supplied for initial evaluation of potential NIF suppliers. This document is meant to define the specification of laser-induced damage for small optics and the test methods used to collect the data. A rating system which will be applied for vendor selection is presented. Presented here is the plan for qualification of NIF small optics vendors based on laser damage performance. The raster scan test method was chosen in order to provide for testing of significant areas of the samples. The vendor performance at a given fluence is categorized as either ''qualified'', ''probable'', or ''fail'', depending on the level of damage observed. This binning system allows a conservative stance for qualification, while providing some insight into lever of risk associated with a lowering on the specifications. The R: 1 mapping technique is reserved in cases where comparisons, not qualifications are needed. Once suppliers are selected, an less intensive pass/fail test will be instituted for production optics.
Date: April 1, 1999
Creator: Sheehan, L.
Partner: UNT Libraries Government Documents Department

National Ignition Facility small optics laser-induced damage and photometry measurements program

Description: The National Ignition Facility will require upwards of 25,000 small optical components in its various beam conditioning and diagnostic packages. A quality control program designed to ensure that the elements meet the required specifications will test these optical elements. For many of the components, damage performance is one of the critical specifications, which will require state-of-the-art performance from the industry participants. A program was initiated to understand the current performance level of such optics. The results of this study as it pertains to laser-induced damage is shown. The use of ratio reflectometry is also addressed as the method of choice for photometry measurements on these industry supplied optics. Key words: Optical coatings, qualification, specifications, laser-induced damage, photometry, ratio reflectometry
Date: July 1, 1999
Creator: Battersby, C; Hendrix, J; Oberhelman, S & Sheehan, L
Partner: UNT Libraries Government Documents Department

NIF small optics laser damage test specifications

Description: The Laser Damage Group is currently conducting tests on small optics samples supplied for initial evaluation of potential NIF suppliers. This document is meant to define the specification of laser-induced damage for small optics and the test methods used to collect the data. A rating system which will be applied for vendor selection is presented.
Date: April 20, 1999
Creator: Sheehan, L
Partner: UNT Libraries Government Documents Department

Depth profiling of polishing-induced contamination on fused silica surfaces

Description: Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. Here secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finished process contamination on fused silica surfaces. Contaminants detected include the major polishing compound components (Ce or Zr from CeO2 or ZrO2), Al presently largely because of the use of Al2O3 in the final cleaning process (Fe, Cu,Cr) incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-100 ppm range, except for Al with exceeds 1000 ppm. A strong correlation has been shown between the presence of a gray haze damage morphology and the use of CeO2 polishing compound. No strong correlation was found however between high levels of Ce, or any other contaminant and the low damage threshold was observed. In fact one of the strongest indications of a correlation is between increased damage thresholds and increased Zr contamination. This suggests that the correlation between redeposition layer and laser damage threshold is not simple an absorbing contaminant issue.
Date: December 20, 1997
Creator: Kozlowski, M.R.; Carr, J.; Hutcheon, I,; Torres, R.; Sheehan, L. Camp, D. & Yan, M.
Partner: UNT Libraries Government Documents Department

Effects of wet etch processing on laser-induced damage of fused silica surfaces

Description: Laser-induced damage of transparent fused silica optical components by 355 nm illumination occurs primarily at surface defects produced during the grinding and polishing processes. These defects can either be surface defects or sub-surface damage.Wet etch processing in a buffered hydrogen fluoride (HF) solution has been examined as a tool for characterizing such defects. A study was conducted to understand the effects of etch depth on the damage threshold of fused silica substrates. The study used a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser to damage test fused silica optics through various wet etch processing steps. Inspection of the surface quality was performed with Nomarski microscopy and Total Internal Reflection Microscopy. The damage test data and inspection results were correlated with polishing process specifics. The results show that a wet etch exposes subsurface damage while maintaining or improving the laser damage performance. The benefits of a wet etch must be evaluated for each polishing process.
Date: December 22, 1998
Creator: Battersby, C.L.; Kozlowski, M.R. & Sheehan, L.M.
Partner: UNT Libraries Government Documents Department

Laser conditioning methods fo hafnia silica multiplayer mirrors

Description: Large aperture multilayer hafnia silica high reflector coatings at 1064 nm, deposited by reactive electron-beam deposition, were prepared to examine different laser conditioning methods for manufacturing high fluence optics in the National Ignition Facility. Laser conditioning is a process where the damage threshold of the coating is increased or the damage that is created is minimized so that it does not grow upon further irradiation. Two laser conditioning methods were examined for coatings deposited from only oxide starting materials. Off-line laser conditioning consists of raster scanning a mirror past a 1 mm diameter Gaussian beam over the entire clear aperture; a process that takes approximately 24 hours per scan. On-line laser conditioning consisted of a large aperture 300 mm x 300 mm beam from the Beamlet laser that irradiated the entire full clear aperture of a series of mirrors; a process that was limited by a 2-4 hour shot rate. In both cases a six-step process was used with the mirror first irradiated at a low fluence, then successively higher fluences increased in equal increments up to the peak laser operating fluence. Mirrors that were only partially laser conditioned damaged catastrophically while fully conditioned mirrors survived fluences exceeding the safe operating Beamlet fluence. An alternative off-line laser conditioning method was examined for coatings deposited from hafnia or metallic hafnium sources. Single-step laser conditioning consists of off-line raster scanning an optic at the peak operating fluence, thus decreasing the laser conditioning cost by reducing the number of scans and required laser conditioning stations to process all the mirrors for the National Ignition Facility. Between pulses the optic is stepped approximately one fourth of the l/e* Gaussian beam diameter so each area of the coating is irradiated by different segments of the beam starting at a low fluence at the outer edge ...
Date: January 6, 1998
Creator: Stolz, C.J.; Sheehan, L.M.; Maricle, S.M. Schwartz, S.; Kozlowski, M.R.; Jennings, R.T. & Hue, J.
Partner: UNT Libraries Government Documents Department

Study of the effects of polishing, etching, cleaving, and water leaching on the UV laser damage of fused silica

Description: A damage morphology study was performed with a 355 nm Nd:YAG laser on synthetic UV-grade fused silica to determine the effects of post- polish chemical etching on laser-induced damage, compare damage morphologies of cleaved and polished surfaces, and understand the effects of the hydrolyzed surface layer and waste-crack interactions. The samples were polished , then chemically etched in buffered HF solution to remove 45,90,135, and 180 nm of surface material. Another set of samples was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluencies and characterized by Normarski optical microscopy and scanning electron microscopy. Damage was initiated as micro-pits on both input and output surfaces of the polished fused silica sample. At higher fluencies, the micro-pits generated cracks on the surface. Laser damage of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pit formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving tracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.
Date: December 23, 1997
Creator: Yoshiyama, J.; Genin, F.Y.; Salleo, A.; Thomas, I.; Kozlowski, M.R.; Sheehan, L.M. et al.
Partner: UNT Libraries Government Documents Department

Laser modulated scattering as a nondestructive evaluation tool for optical surfaces and thin film coatings

Description: Laser modulated scattering (LMS) is introduced as a non-destructive evaluation tool for defect inspection and characterization of optical surfaces and thin film coatings. This technique is a scatter sensitive version of the well-known photothermal microscopy (PTM) technique. It allows simultaneous measurement of the DC and AC scattering signals of a probe laser beam from an optical surface. By comparison between the DC and AC scattering signals, one can differentiate absorptive defects from non-absorptive ones. This paper describes the principle of the LMS technique and the experimental setup, and illustrates examples on using LMS as a tool for nondestructive evaluation of high quality optics.
Date: December 22, 1999
Creator: Feit, M D; Kozlowski, M R; Rubenchik, A M; Sheehan, L & Wu, Z L
Partner: UNT Libraries Government Documents Department

Large-aperture, high-damage-threshold optics for beamlet

Description: Beamlet serves as a test bed for the proposed NIF laser design and components. Therefore, its optics are similar in size and quality to those proposed for the NIF. In general, the optics in the main laser cavity and transport section of Beamlet are larger and have higher damage thresholds than the optics manufactured for any of our previous laser systems. In addition, the quality of the Beamlet optical materials is higher, leading to better wavefront quality, higher optical transmission, and lower-intensity modulation of the output laser beam than, for example, that typically achieved on Nova. In this article, we discuss the properties and characteristics of the large-aperture optics used on Beamlet.
Date: February 23, 1995
Creator: Campbell, J.H.; Atherton, L.J.; DeYoreo, J.J.; Kozlowski, M.R.; Maney, R.T.; Montesanti, R.C. et al.
Partner: UNT Libraries Government Documents Department

Extrapolation of damage test data to predict performance of large-area NIF optics at 355 nm

Description: For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large (40 x 40 cm) 351 nm final optics is inevitable. Planning and utilization of NIF therefore requires reliable prediction of the functional degradation of the final optics. Laser damage tests are typically carried out with Gaussian beams on relatively small test areas. The tests yield a damage probability vs energy fluence relation. These damage probabilities are shown to depend on both the beam fluence distribution and the size of area tested. Thus, some analysis is necessary in order to use these test results to determine expected damage levels for large aperture optics. The authors present a statistical approach which interprets the damage probability in terms of an underlying intrinsic surface density of damaging defects. This allows extrapolation of test results to different sized areas and different beam shapes (NIF has a flattop beam). The defect density is found to vary as a power of the fluence (Weibull distribution).
Date: December 22, 1998
Creator: Schwartz, S; Feit, M D; Genin, F Y; Kozlowski, M R; Rubenchik, A M & Sheehan, L M
Partner: UNT Libraries Government Documents Department

Single-beam photothermal microscopy - a new diagnostic tool for optical materials

Description: A novel photothermal microscopy (PTM) is developed which uses only one laser beam, working as both the pump and the probe. The principle of this single-beam PTM is based on the detection of the second harmonic component of the laser modulated scattering (LMS) signal. This component has a linear dependence on the optical absorptance of the tested area and a quadratic dependence on the pump laser power. Using a pump laser at the wavelengths of 514.5- and 532-nm high-resolution photothermal scans are performed for polished fused silica surfaces and a HfO{sub 2}/SiO{sub 2} multilayer coatings. The results are compared with those from the traditional two-beam PTM mapping. It is demonstrated that the single-beam PTM is more user-friendly (i.e. no alignment is needed) than conventional two-beam PTM and, offers a higher spatial resolution for defect detection.
Date: December 22, 1998
Creator: Feit, M. D.; Kozlowski, M.; Natoli, J. Y.; Rubenchik, A. M.; Sheehan, L.; Wu, Z. L. et al.
Partner: UNT Libraries Government Documents Department

Statistical description of laser damage initiation in NIF and LMJ optics at 355 nm

Description: Understanding the �extreme statistics� of failure at a weak link allows extrapolation of the results of small area laser damage tests to predict damage levels for the large areas pertinent to NIF/LMJ. Conceptually, it is important to focus on the fluence dependence of the surface density of damage sites. Results of different types of damage tests can be reported in terms of this sample characteristic property.
Date: July 30, 1998
Creator: Dijon, J.; Feit, M. D.; Garrec, P.; Genin, F. Y.; Hue, J.; Kozlowski, M. R. et al.
Partner: UNT Libraries Government Documents Department

Automated damage test facilities for materials development and production optic quality assurance at Lawrence Livermore National Laboratory

Description: The Laser Program at LLNL has developed automated facilities for damage testing optics up to 1 meter in diameter. The systems were developed to characterize the statistical distribution of localized damage performance across large-aperture National Ignition Facility optics. Full aperture testing is a key component of the quality assurance program for several of the optical components. The primary damage testing methods used are R:1 mapping and raster scanning. Automation of these test methods was required to meet the optics manufacturing schedule. The automated activities include control and diagnosis of the damage-test laser beam as well as detection and characterization of damage events.
Date: December 22, 1998
Creator: Battersby, C.; Dickson, R.; Jennings, R.; Kimmons, J.; Kozlowski, M. R.; Maricle, S. et al.
Partner: UNT Libraries Government Documents Department