Lynn Matthew Sheehan

Depth profiling of polishing-induced contamination on fused silica surfaces

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. In the present study, secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminating detected include the major polishing compound components, Al present largely because of the use of Al2O3 in the final cleaning process, and other metals 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 10-10 pm range, except for Al which often exceeds 1000 ppm.

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

For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large 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 test are typically carried out with Gaussian beams on relatively small test ares. The test 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. We 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. The defect density is found to vary as a power of the fluence.

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

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 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 result show that a wet etch exposes sub-surface damage while maintaining or improving the laser damage performance. The benefits of a wet etch must be evaluated for each polishing process.

The advantages of evaporation of Hafnium in a reactive environment to manufacture high damage threshold multilayer coatings by electron-beam deposition

Electron-beam deposition is the current method to produce large-aperture high laser-induced damage threshold coatings for the National Ignition Facility, a 1.8 MJ fusion laser. The e-beam process is scalable to large optics up to 0.25 m2 and with laser conditioning has relatively benign coating defect ejections resulting in high damage threshold thin films. The latest technological breakthrough in manufacturing high damage threshold coatings is e-beam deposition of hafnia by evaporation from a metallic instead of an oxide source in a reactive environment. Although the damage threshold is not significantly increased, a 3-10x defect reduction occurs resulting in significantly less coating modification during laser conditioning. Additional benefits of this technology include improved interfaces for the elimination of flat-bottom pits and up to 3x reduction in plume instability for improved layer thickness control and spectral performance.

Large-area conditioning of optics for high-power laser systems

This paper presents the procedure and apparatus used to laser condition meter-scale HfO2/SiO2 multilayer polarizers which will be used in the Beamlet laser system. A study of different conditioning techniques, the effects of conditioning, and the determination of a practical process for conditioning large-area optics is presented.

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

A damage morphology study was performed with a 355 nm, 8-ns 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 water-crack interactions. The samples were polished, then chemically etched in a buffered HF solution to remove 45, 90, 135, and 180 nm of surface material. Another set of sample was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluences and characterized by Normarski optical microscopy and scanning electron microscope. Damage was initiated at micro-pits on both input and output surface of the polished fused silica sample. At higher fluences, the micro-pits generated cracks on the surface. Laser damage of the etched fused silica surface shoed that the real density of micro-pits decreased with etched thickness. SIMS analysis of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pits formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving cracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.

Historical perspective on fifteen years of laser damage thresholds at LLNL

We have completed a fifteen-year, referenced and documented compilation of more than 15,000 measurements of laser-induced damage thresholds (LIDT) conducted at the Lawrence Livermore National Laboratory. These measurements cover the spectrum from 248 to 1064 nm with pulse durations ranging from < 1 ns to 65 nm and at pulse-repetition frequencies from single shots to 6.3 kHz. We emphasize the changes in LIDTs during the past two years since we last summarized our database. We relate these results to earlier data concentrating on improvements in processing methods, materials, and conditioning techniques. In particular, we highlight the current status of anti-reflective coatings, high reflectors, polarizers, and frequency-conversion crystals used primarily at 355 nm and 1064 nm.

Laser modulated scattering as a nondestructive evaluation tool for defect inspection in optical materials for high power laser applications.

Laser modulated scattering (LMS) is introduced as a tool for defect inspection and characterization of optical materials for high power laser applications. LMS is a scatter sensitive version of the well-known photothermal microscopy techniques. Because only the defects of a super-polished optic generate a scattering signal, the technique is essentially a method for dark-field photothermal microscopy. Experimental results show that the technique (1) measures the local absorption properties of defects, contamination, and laser damage sites; (2) when used in conjunction with DC scattering, can differentiate between absorbing and non-absorbing defects; and (3) detects thermal transport inhomogeneities.

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

Damage studies show that the majority of damage on UV 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 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 with sub-micron detection. Both off-line and in- situ systems have been applied in the Lawrence Livermore National Laboratorys 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 nm, 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.

Spectroscopic investigation of SiO2 surfaces of optical materials for high-power lasers

High quality surfaces of fused silica optical materials were studied using microscopic fluorescence imaging as well as Raman and emission micro-spectroscopy. For as-polished surfaces optically active defect formations were detected on the surface of the material which vary in geometry, relative intensity and concentration depending on the polishing process. A partial correlation of these defects with subsequent laser damage sites was indicated. Following laser-induced damage the Raman and photoluminescence spectra indicated extensive materials modification within the damage sites. Emission spectra show at least three characteristic luminescence bands centered at 1.9 eV, 2.2 eV and approximately 4.7 eV. Raman scattering indicates that laser irradiation leads to compaction.