Many different environmental factors can have an effect on optical coating durability for space applications. This includes in-orbit effects such as vacuum exposure, UV radiation, particle radiation, atomic oxygen, thermal cycling, contamination and orbital debris, as well as ground based effects such as cleaning, contamination and humidity [1].
We investigate the capacity of magnetorheological finishing (MRF) process to remove surface and subsurface defects of fused silica optics. Polished samples with engineered surface and subsurface defects were manufactured and characterized. Uniform material removals were performed with a QED Q22-XE machine using different MRF process parameters in order to remove these defects. We provide evidence that whatever the MRF process parameters are, MRF is able to remove surface and subsurface defects. Moreover, we show that MRF induces a pollution of the glass interface similar to conventional polishing processes.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of
energy at 351 nm, on a millimetre scale target in the centre of an experiment chamber. The final optic assembly of this
system operating at a wavelength of 351 nm is made up of large fused silica optics, working in transmission, that are
used to convey and focus the laser beam. Under high fluences (i.e. more than 5 J/cm2 for 3 ns pulses), the limited lifetime of final optical assembly is a major concern for fusion scale laser facilities. Previous works have shown that surface finishing processes applied to manufacture these optical components can leave subsurface cracks (SSD), pollution or
similar defects that act as initiators of the laser damage. In this work, we used epi-fluorescent light scanning microscopy
(ELSM) and Stimulated Emission Depletion (STED) in confocal mode with fluorescent dye tagging to get a better
knowledge of size and depth of these subsurface cracks. Magnetorheological fluid finishing technique (MRF) was also
used as a tool to remove these cracks and thus assess depths measured by confocal microscopy. Subsurface cracks with a
width of about 120 nm are observed up to ten micrometers below the surface.
The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of energy of UV light, on a millimeter scale target in the centre of an experiment chamber. After amplification and transport at the wavelength of 1053 nm, frequency conversion at 351 nm is done with KH2PO4 crystals. The final optic assembly of this system is made up of large fused silica optics, working in transmission, that are used to convey, focus or shape the laser beam. When exposed to fluences of some joules per square centimeter at 351 nm within nanosecond pulse duration, fused silica optics can exhibit localized damage. Damage sites grow exponentially after further laser exposition and therefore dramatically limit the optic lifetime. The nature of the surface finishing process has been established to determine the lifetime of these components under high UV fluences (i.e. more than 5 J/cm2 for 3 ns pulses). Being able to reduce or eliminate the damage initiators such as subsurface cracks present in subsurface damage (SSD) layer of conventionally polished optical components aims this study. Magneto-rheological fluid finishing (MRF) is chosen as a final polishing tool to remove layers of material without inducing further damages. MRF enables to process optics with very small normal stresses applied to the surface during material removal and thus permits the elimination of the residual subsurface cracks. This study offers a better understanding of the efficiency of MRF polishing on the elimination of subsurface cracks in SSD layers.
The physical and chemical behaviour of materials is strongly correlated with their microstructure. Therefore, much effort
is invested in the advanced microstructural design of metallic thin films.
Laser Interference Metallurgy (LIMET) is used to locally tune the grain architecture of metallic thin films from the nanoto
the microscale. This means a defined size and orientation of the grains with lateral periodicity, by interfering on the
sample surface two or more laser beams of a high power nanosecond pulsed Nd:YAG laser. This technique enables the
local nucleation and crystallization of amorphous or nanocrystalline metallic thin films, thus combining nano- and
microcrystalline regions ordered in periodic line- or lattice-like arrangements in a composite architecture.
After having locally modified the microstructure of e-beam evaporated Pt and Au thin films by laser irradiation a wet
chemical etching procedure was induced in hot aqua regia. Doing so, a selective etching is achieved without using
conventional lithography. Due to the laser-induced recrystallization in periodic structures, these microcrystalline zones
of specific oriented grains show a higher resistance against the wet chemical etchant than the as-deposited,
nanocrystalline areas, which are completely removed down to the substrate. Therefore, this procedure may have the
potential to be an alternative, low cost approach to conventional lithographic techniques and provides a novel method for
a straight-forward patterning of metallic thin films.
Innovative surfaces are successful, if we succeed to put in the correct place the correct property with technological
efficiency. Until now, material surfaces can be systematically structured in different ways in order to fulfil chemical or
mechanical requirements such as corrosion protection or wear resistance for example. Moreover, the properties of
materials are strongly related to their microstructure as well as to their spatial distribution. For that reason, the design of
materials with tailored microstructures is a key for the functionalization of surfaces. This is possible by an artificial
fabrication technique called Laser Interference Metallurgy.
In this context, textured or functionalized surfaces are beneficial in overcoming stiction and adhesion in MEMS devices.
With regard to tribological applications, a systematic study of the effect of geometrically differing laser interference
patterns on the wetting behaviour of metallic gold thin films with a thickness of about 300 nm and 125 μm thick
polyimide foils should be presented. It could be shown that in case of gold films, a laser interference patterning
reinforces the hydrophilic sample behavior whereas the polyimide foils reveal a significant increase in hydrophobicity
after the laser patterning process. Both wetting regimes are advantageous under dry or lubricated friction conditions. The
corresponding geometrical limits of the abovementioned method concerning the structure depth, periodicity and pattern
form has been determined. All the samples have been characterized by scanning electron and focused ion beam
microscopy and white light interferometry. Additionally, IR spectroscopy has been applied to the polyimide samples in
order to separate topographic and chemical influences.
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