MRF Applications: On the Road to Making Large-Aperture Ultraviolet Laser Resistant Continuous Phase Plates for High-Power Lasers

Over the past two years we have developed MRF tools and procedures to manufacture large-aperture (430 X 430 mm) continuous phase plates (CPPs) that are capable of operating in the infrared portion (1053 nm) of high-power laser systems. This is accomplished by polishing prescribed patterns of continuously varying topographical features onto finished plano optics using MRF imprinting techniques. We have been successful in making, testing, and using large-aperture CPPs whose topography possesses spatial periods as low as 4 mm and surface peak-to-valleys as high as 8.6 {micro}m. Combining this application of MRF technology with advanced MRF finishing techniques that focus on ultraviolet laser damage resistance makes it potentially feasible to manufacture large-aperture CPPs that can operate in the ultraviolet (351 nm) without sustaining laser-induced damage. In this paper, we will discuss the CPP manufacturing process and the results of 351-nm/3-nsec equivalent laser performance experiments conducted on large-aperture CPPs manufactured using advanced MRF protocols

Imprinting continuously varying topographical structure onto large-aperture optical surfaces using magnetorheological finishing

Over the past four years we have advanced Magnetorheological Finishing (MRF) techniques and tools to imprint complex continuously varying topographical structures onto large-aperture (430 x 430 mm) optical surfaces. These optics, known as continuous phase plates (CPPs), are important for high-power laser applications requiring precise manipulation and control of beam-shape, energy distribution, and wavefront profile. 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 and surface peak-to-valleys as high as 22 {micro}m. During this discussion, we will present the evolution of the MRF imprinting technology and the MRF tools designed to manufacture large-aperture 430 x 430 mm CPPs. Our results will 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 in high-power laser applications

Effect of rogue particles on the sub-surface damage of fused silica during grinding/polishing

The distribution and characteristics of surface cracks (i.e., sub-surface damage or scratching) on fused silica formed during grinding/polishing resulting from the addition of rogue particles in the base slurry has been investigated. Fused silica samples (10 cm diameter x 1 cm thick) were: (1) ground by loose abrasive grinding (alumina particles 9-30 {micro}m) on a glass lap with the addition of larger alumina particles at various concentrations with mean sizes ranging from 15-30 {micro}m, or (2) polished (using 0.5 {micro}m cerium oxide slurry) on various laps (polyurethanes pads or pitch) with the addition of larger rogue particles (diamond (4-45 {micro}m), pitch, dust, or dried Ceria slurry agglomerates) at various concentrations. For the resulting ground samples, the crack distributions of the as-prepared surfaces were determined using a polished taper technique. The crack depth was observed to: (1) increase at small concentrations (>10{sup -4} fraction) of rogue particles; and (2) increase with rogue particle concentration to crack depths consistent with that observed when grinding with particles the size of the rogue particles alone. For the polished samples, which were subsequently etched in HF:NH{sub 4}F to expose the surface damage, the resulting scratch properties (type, number density, width, and length) were characterized. The number density of scratches increased exponentially with the size of the rogue diamond at a fixed rogue diamond concentration suggesting that larger particles are more likely to lead to scratching. The length of the scratch was found to increase with rogue particle size, increase with lap viscosity, and decrease with applied load. At high diamond concentrations, the type of scratch transitioned from brittle to ductile and the length of the scratches dramatically increased and extended to the edge of the optic. The observed trends can explained semi-quantitatively in terms of the time needed for a rogue particle to penetrate into a viscoelastic lap. The results of this study provide useful insights and 'rules-of-thumb' relating scratch characteristics observed on surfaces during optical glass fabrication to the characteristics rogue particles causing them and their possible source

The distribution of subsurface damage in fused silica

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

Sub-surface mechanical damage distributions during grinding of fused silica

The distribution and characteristics of surface cracking (i.e. sub-surface damage or SSD) formed during standard grinding processes has been investigated on fused silica glass. The SSD distributions of the ground surfaces were determined by: (1) creating a shallow (18-108 {micro}m) wedge/taper on the surface by magneto-rheological finishing; (2) exposing the SSD by HF acid etching; and (3) performing image analysis of the observed cracks from optical micrographs taken along the surface taper. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The SSD depth distributions are typically described by a single exponential distribution followed by an asymptotic cutoff in depth (c{sub max}). The length of the trailing indent is strongly correlated with a given process. Using established fracture indentation relationships, it is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and it is likely the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth was observed to increase with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related to the SSD length distribution to gain insight into ''effective'' size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth (c{sub max}). These relationships can serve as useful rules-of-thumb for nondestructively estimating SSD depth and to identify the process that caused the SSD. In certain applications such as high intensity lasers, SSD on the glass optics can serve as a reservoir for minute amounts of impurities that absorb the high intensity laser light and lead to subsequent laser-induced surface damage. Hence a more scientific understanding of SSD formation can provide a means to establish recipes to fabricate SSD-free, laser damage resistant optical surfaces

Using ultra-high resolution methodologies to imaging active submarine faults: The STRENGTH 2023 cruise in the Alboran sea

8th International Colloquium on Historical Earthquakes, Palaeo- Macroseismology and Seismotectonics, Past earthquakes and advances in seismology for informed risk decision-making, 17-20 September 2023, Lixouri, Kefalonia Island, Greece.-- 4 pages, 2 figuresGreat earthquakes and the possibility to generate destructive tsunamis are geohazards of key societal concern, as they may impact world economies, disturb submarine structures and affect coastal areas with the associated risk for local populations (Bilham, 2010). We still have in our mind catastrophic episodes, such as the giant events of the Sumatra earthquake and tsunami in 2004 in the Indian Ocean of magnitude (Mw) 8.7, and the Tohoku-Oki earthquake and tsunami in 2011 in the northwest of Japan, of magnitude (Mw) 9.0-9.1. Nevertheless, seismic events of moderate to large magnitude (Mw 6-7.3) in areas of low to moderate tectonic deformation, and with long recurrence intervals, such as the Alboran Sea in the Western Mediterranean, might also have a significant effect. Accordingly, during the last decades there has been an expansion of paleoseismology to marine areas, both on-fault and off-fault investigations (Pantosti et al., 2011; Perea et al., 2021a). [...]This research was supported by the grant STRENGTH (PID2019-104668RB-I00) funded by MCIN/AEI/10.13039/501100011033. This project acknowledges the „Severo Ochoa Centre of Excellence‟ accreditation (CEX2019-000928-S), the grant UNrIDDLE (2018-T1/AMB-11039) “Atracción de Talento Investigador” call 2018 funded by Comunidad de Madrid and the Ocean Facilities Exchange Group (OFEG) for allowing us to use the AUV “AsterX” from IFREMERPeer reviewe

Full System Operations of Mercury; A Diode-Pumped Solid-State Laser

Operation of the Mercury laser with two amplifiers activated has yielded 30 Joules at 1 Hz and 12 Joules at 10 Hz and over 8 x 10{sup 4} shots on the system. Static distortions in the Yb:S-FAP amplifiers were corrected by magneto rheological finishing technique