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In this paper, we show how thin film multilayer coatings can be a solution to answer this problematic as it is possible to design accurate spectral response that presents a very low level of reflectance with a zero value of transmittance.
After a description of the design steps, we will focus on the realization of such sophisticated metal-dielectric multilayer stacks using dense coating techniques; in particular, we will show that master of refractive index of very thin metallic layers is an asset to achieve accurate performances and how in situ optical broadband monitoring allows excellent reproducibility of production processes even for few nanometers-thick layers required in metal-dielectric absorbers.
Spectral and angular measurements of different coatings solutions will be given on various types of substrates (glass or metallic). Environmental qualification tests and spectral characterizations are also presented showing the stability of the performances in severe conditions compatible with severe environment. In particular, coatings developed for various projects will illustrate this study.
Geostationary satellite COMS-1 (Communication, Ocean, Meteorological Satellite-1) of Astrium has the role of ensuring meteorological observation as well as monitoring of the oceans. It is equipped with a colour imager to observe the marine ecosystem through 8 bands in the visible spectrum with a ground resolution of 500m. For that, this very high technology instrument is constituted with a filters wheel in front of the oceanic colour imager with 8 narrow band filters carried out and qualified by Cilas.
For the project, two specific coatings have been implemented and qualified by CILAS: first, an antireflection coating deposited on the entrance and exit facets of the immersed grating prism, which reaches a very low value of reflectivity in the infrared [2305nm; 2385nm] spectral range and for a wide angular range [0° to 47°] of incidence of the transmitted light, and second, a metal-dielectric absorbing coating for the third facet of the prism to eliminate parasitic light inside the silicon prism.
All these components were manufactured using Plasma Assisted Reactive Magnetron Sputtering (PARMS) technics offered by the HELIOS machine and monitored in real time with an OMS5000 in-situ optical monitoring, both developed by BUHLER Optics.
Compressive mechanical stress of 364 MPa and 55 MPa respectively for SiO2 and Nb2O5 are measured, final sag of 326 nm and 13 nm, and uniformity from -0.05% to 0.10% and from -0.10% to 0.20% are obtained respectively for the two manufactured filters.
We will see how its wide variety of configurations based on seven 2.5 meters-long planar magnetrons, that may be powered in various modes (RF, MF, DC, ..) allows us producing complex optical functions such as multilayer dielectric mirrors, antireflective coatings and protected or enhanced metallic mirrors from ultraviolet to infrared applications. Moreover, an all-fibered broadband optical monitoring system developed in collaboration by Institut Fresnel is installed on this industrial machine and allows extremely accurate monitoring carried out on samples and fine characterization directly on components.
Many spectral measurements and cosmetic results will be given that show that this equipment is well adapted for producing large optical components with high quality for various applications in the fields of industry, space or astronomy. In particular, for space application, PACA2M coating machine can be used to produce in a same batch all the required models as it is possible to simultaneously coat flight models, qualification witnesses and spare models.
The stability of the performances in severe environmental conditions compatible with space environment will be illustrated with qualifications tests and characterizations.
After a description of the design steps, we will present the manufacturing of such multilayer stacks using magnetron sputtering technique and we will see how such coating technique is very well suited for production due to its high process reproducibility even for very thin layers required in metal-dielectric absorbers.
Monitoring of such coatings is also presented with the help of a powerful in situ optical system developed in collaboration with Institut Fresnel that allows characterization of in-situ refractive indices of deposited materials and broadband monitoring of the multilayer stack.
Many results will be given on qualification samples, such as environmental tests and spectral characterizations that show the stability of the performances in severe environmental conditions.
At last, we will focus on the spectral and angular scattering behavior of such absorbing coatings and we will present several measurements performed on glass or metallic substrates with different roughnesses.
The use of an in-situ optical monitoring system in visible and near infrared range (up to 2500nm) permits to reach severe spectral specifications and to have a good agreement with the theoretical designs.
In this paper, we will focus on some complex optical functions that have been manufactured with DIBS technique: narrow band-pass filters and wide band filters including large blocking range with more than 100 layers and 25 μm total thickness, antireflection coatings which reach very low values of reflectivity in the near infrared spectral domain for wide angular ranges, and also metal-dielectric absorbing coatings.
We will also give an analysis of the cosmetics performances of the deposited layers, which shows the high quality of the coatings even with functions with large total thickness.
Many experimental results of qualification tests in temperature, humidity, thermal vacuum, radiations,… are presented and show the reliability of these multidielectric components in space or cryo-vacuum environment.
Luminescence, absorption and morphology studies of laser-damage sites in silica glasses and coatings
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