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).
A comprehensive study of laser-induced damage associated with particulate damage on optical surfaces is presented. Contaminant-driven damage on silica windows and multilayer dielectrics is observed to range from shallow pitting to more classical fracture-type damage, depending on particle-substrate material combination, as well as laser pulse characteristics. Ejection dynamics is studied in terms of plasma emission spectroscopy and pump-probe shadowgraphy. Our data is used to assess the momentum coupling between incident energy and the ejected plasma, which dominates the laser-particle-substrate interaction. Beam propagation analysis is also presented to characterize the impact of contaminant-driven surface pitting on optical performance.
Surface particulate contamination on optics can lead to laser-induced damage hence limit the performance of high power laser system. In this work we focus on understanding the fundamental mechanisms that lead to damage initiation by metal contaminants. Using time resolved microscopy and plasma spectroscopy, we studied the dynamic process of ejecting ~30 μm stainless steel particles from the exit surface of fused silica substrate irradiated with 1064 nm, 10 ns and 355 nm, 8 ns laser pulses. Time-resolved plasma emission spectroscopy was used to characterize the energy coupling and temperature rise associated with single, 10-ns pulsed laser ablation of metallic particles bound to transparent substrates. Plasma associated with Fe(I) emission lines originating from steel microspheres was observe to cool from <24,000 K to ~15,000 K over ~220 ns as τ-0.22, consistent with radiative losses and adiabatic gas expansion of a relatively free plasma. Simultaneous emission lines from Si(II) associated with the plasma etching of the SiO2 substrate were observed yielding higher plasma temperatures, ~35,000 K, relative to the Fe(I) plasma. The difference in species temperatures is consistent with plasma confinement at the microsphere-substrate interface as the particle is ejected, and is directly visualized using pump-probe shadowgraphy as a function of pulsed laser energy.
We investigate the multipulse degradation of fused silica surfaces exposed at 351 nm for up to 109 pulses at pulse fluences greater than 10 J/cm2. In vacuum, the transmission loss increases as a function of the number of shots at low pulse intensity. However, as the pulse intensity increases, the transmission loss decreases and is not measureable above a certain intensity. Transmission loss is highest when measured at shorter wavelengths, and decreases towards the IR. Absorption is the primary mechanism that leads to transmission loss and is from photo-reduction of the silica surface.
Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will
review important optical damage precursors in silica up to UV fluences as high as 45J/cm2 (3ns) along with studies of
the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface
damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of
a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily
responsible for optic damage below about 10J/cm2; they can be mitigated by an optimized oxide etch processes. At
fluences above about 10J/cm2, precipitates of trace impurities are responsible for damage; they can be mitigated by
eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica
damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating
fluences these optics see.
Surface modification of fused silica windows caused by the laser ablation of surface-bound microparticles is
investigated. Using optical and electron microscopies between laser pulses, we detail the ablation, fragmentation
and dispersal of 2-150 μm diameter particles of various materials. Following complete ablation and ejection of all
debris material, surface pitting was found to be highly dependent on material type and particle size. Subsequent
light propagation modeling based on pit morphology indicates up to ~4x intensification. Understanding this class of
non-local, debris-generated damage is argued to be important for effective design of high-power optical windows
and debris-mitigation strategies.
Chemical vapor deposition (CVD) has been used for the production of fused silica optics in high power laser
applications. However, relatively little is known about the ultraviolet (UV) laser damage threshold of CVD films
and how they relate to intrinsic defects produced during deposition. We present a study relating structural and
electronic defects in CVD films to the 355 nm pulsed laser damage threshold as a function of post-deposition
annealing temperature (THT). Plasma-enhanced CVD, based on SiH4/N2O under oxygen-rich conditions, was used
to deposit 1.5, 3.1 and 6.4 μm thick films on etched SiO2 substrates. Rapid annealing was performed using a
scanned CO2 laser beam up to THT~2100 K. The films were then characterized using X-ray photoemission
spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and photoluminescence (PL). A gradual
transition in the damage threshold of annealed films was observed at THT up to 1600 K, correlating with a decrease
in NB silanol and broadband PL emission. An additional sharp transition in damage threshold also occurs at ~1850
K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is
proposed, and the potential of using high-THT CVD SiO2 to mitigate optical damage is also discussed.
Previous studies have identified two significant precursors of laser damage on fused silica surfaces at fluences <35 J/cm2: photoactive impurities from polishing and surface fractures. We evaluate isothermal heating as a means of remediating the defect structure associated with surface fractures. Vickers indentations are applied to silica surfaces at loads between 0.5 and 10 N, creating fracture networks. The indentations are characterized before and following thermal annealing under various time and temperature conditions using confocal time-resolved photo-luminescence (CTP) imaging, and R/1 damage testing with 3-ns, 355-nm laser pulses. Improvements in the damage thresholds with reductions in CTP intensity are observed at temperatures well below the glass transition temperature (Tg). The damage threshold on 0.5-N indentations improves from <8 to >35 J/cm2 after annealing at approximately 750°C. Larger fracture networks require longer or higher temperature treatment to achieve similar results. At an annealing temperature >1100°C, optical microscopy indicates morphological changes in some of the fractures surrounding the indentations, although remnants of the original fractures are still observed. We demonstrate the potential of using isothermal annealing to improve the laser damage resistance of silica optics, and provide a means of further understanding the physics of optical damage and mitigation.
Previously we have shown that the size of laser induced damage sites in both KDP and
SiO2 is largely governed by the duration of the laser pulse which creates them. Here we
present a model based on experiment and simulation that accounts for this behavior.
Specifically, we show that solid-state laser-supported absorption fronts are generated
during a damage event and that these fronts propagate at constant velocities for laser
intensities up to 4 GW/cm2. It is the constant absorption front velocity that leads to the
dependence of laser damage site size on pulse duration.
We show that these absorption fronts are driven principally by the temperatureactivated
deep sub band-gap optical absorptivity, free electron transport, and thermal
diffusion in defect-free silica for temperatures up to 15,000K and pressures < 15GPa. In
addition to the practical application of selecting an optimal laser for pre-initiation of
large aperture optics, this work serves as a platform for understanding general lasermatter
interactions in dielectrics under a variety of conditions.
Small micrometer-sized roughness on optical surfaces, caused by laser damage and/or redeposition of laser ablated
material, can cause local electric field intensification which may lead to damage initiation both on the optics and/or
downstream. We examined the smoothing of etched periodic surface structures on SiO2 substrate with 10.6μm CO2
laser using atomic force microscopy. The characteristic surface tension driven mass flow of the glass under
different laser parameters were simulated using computational fluid dynamics and correlated with experimental
results. We found that during CO2 laser polishing the estimate viscosity of the silica glass appears to be higher than
typical literature values measured at a temperature similar to the laser heating conditions. This discrepancy can be
explained by the observation that at high temperature, a significant portion of the hydroxyl content in the layer of
heated silica glass can diffuse out resulting in a much stiffer glass.
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.
Using high-sensitivity confocal time-resolved photoluminescence (CTP) techniques, we report an ultra-fast
photoluminescence (40ps-5ns) from impurity-free surface flaws on fused silica, including polished, indented or
fractured surfaces of fused silica, and from laser-heated evaporation pits. This fast photoluminescence (PL) is not
associated with slower point defect PL in silica which has characteristic decay times longer than 5ns. Fast PL is
excited by the single photon absorption of sub-band gap light, and is especially bright in fractures. Regions which
exhibit fast PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5eV
ns-scale laser pulses, making CTP a powerful non-destructive diagnostic for laser damage in silica. The use of CTP
to provide insights into the nature of damage precursors and to help develop and evaluate new damage mitigation
strategies will be presented.
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