There are 830 transport mirrors with a combined surface area of approximately 255 m<sup>2</sup> of precision multilayer coatings deposited on 50 metric tons of BK7 glass in the high fluence transport section of the National Ignition Facility (NIF). With peak fluences over 20 J/cm<sup>2</sup> at 1053 nm, less than five percent of these mirrors are exchanged annually due to laser damage since full system operations began in 2009. Multiple technologies have been implemented to achieve these low exchange rates. The coatings are complex dichroics designed to reflect the fundamental wavelength (1053 nm) and an alignment beam (374 nm) while suppressing target backscatter wavelengths (351 nm and 400-700 nm) from backward propagation up the beamlines. Each optic is off-line laser conditioned to nominally 50% over the average fluence and nominally 90% of the peak fluence allowing the final laser conditioning to occur on-line during NIF operations. Although the transport section of NIF is sealed in a clean argon environment, air knives were installed on upward facing transport mirrors to blow off particulates that could accumulate and initiate laser damage. Beam dumps were installed in between the final optics assembly and the final transport mirrors to capture ghost reflections from the anti-reflection coated surfaces on the transmissive optics used for polarization rotation, frequency conversion, and focusing the 192 laser beams on target. Spot blockers, normally used for the final optics, are sometimes used to project a shadow over transport mirror laser damage in an effort to arrest laser damage growth and extend transport mirror lifetime. Post analysis of laser-damaged mirrors indicates that the dominant causes of laser damage are from surface particulates and the 351-nm wavelength target backscatter.
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.
We report an investigation on the response to laser exposure of a protective capping layer of 1ω (1053 nm) high-reflector
mirror coatings, in the presence of differently shaped Ti particles. We consider two candidate capping layer materials,
namely SiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub>. They are coated over multiple silica-hafnia multilayer coatings. Each sample is exposed to a
single oblique (45°) shot of a 1053 nm laser beam (p polarization, fluence ~ 10 J/cm<sup>2</sup>, pulse length 14 ns), in the
presence of spherically or irregularly shaped Ti particles on the surface. We observe that the two capping layers show
markedly different responses. For spherically shaped particles, the Al<sub>2</sub>O<sub>3</sub> cap layer exhibits severe damage, with the
capping layer becoming completely delaminated at the particle locations. In contrast, the SiO<sub>2</sub> capping layer is only
mildly modified by a shallow depression, likely due to plasma erosion. For irregularly shaped Ti filings, the Al<sub>2</sub>O<sub>3</sub>
capping layer displays minimal to no damage while the SiO<sub>2</sub> capping layer is significantly damaged. In the case of the
spherical particles, we attribute the different response of the capping layer to the large difference in thermal expansion
coefficient of the materials, with that of the Al<sub>2</sub>O<sub>3</sub> about 15 times greater than that of the SiO<sub>2</sub> layer. For the irregularly
shaped filings, we attribute the difference in damage response to the large difference in mechanical toughness between
the two materials, with that of the Al<sub>2</sub>O<sub>3</sub> being about 10 times stronger than that of the SiO<sub>2</sub>.
Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To
date, the mechanism by which substrate surface defects affect the performance of coating layers under high power
laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations
to delineate the correlation between laser damage resistance of coating layers and the physical properties of the
substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological
evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the
trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize
the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference timedomain
(FDTD) method show that field intensification exists mostly near the top surface region of the coating near
convex focusing structures. The light intensification could be responsible for the reduced damage threshold.
Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show
that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our
preliminary results show that damage occurs on the region of the coating where substrate scratches reside and
etching of the substrate before coating does not seem to improve the laser damage resistance.
Replacing growing damage sites with benign, laser damage resistant features in multilayer dielectric films may enable
large mirrors to be operated at significantly higher fluences. Laser damage resistant features have been created in high
reflecting coatings on glass substrates using femtosecond laser machining. These prototype features have been damage
tested to over 40 J/cm<sup>2</sup> (1064nm, 3ns pulselength) and have been shown not to damage upon repeated irradiation at
40J/cm<sup>2</sup>. Further work to optimize feature shape and laser machining parameters is ongoing.
Removal of laser-induced damage sites provides a possible mitigation pathway to improve damage resistance of coated
multilayer dielectric mirrors. In an effort to determine the optimal mitigation geometry which will not generate
secondary damage precursors, the electric field distribution within the coating layers for a variety of mitigation shapes
under different irradiation angles has been estimated using the finite difference time domain (FDTD) method. The
coating consists of twenty-four alternating layers of hafnia and silica with a quarter-wave reflector design. A conical
geometrical shape with different cone angles is investigated in the present study. Beam incident angles range from 0° to
60° at 5° increments. We find that light intensification (square of electric field, |E|<sup>2</sup>) within the multilayers depends
strongly on the beam incident direction and the cone angle. By comparing the field intensification for each cone angle
under all angles of incidence, we find that a 30° conical pit generates the least field intensification within the multilayer
film. Our results suggest that conical pits with shallow cone angles (≤ 30°) can be used as potential optimal mitigation