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For example, for complex optical coatings (typically with more than 100 interfaces), the scattering response of the component is not directly correlated to the substrate spectrum, as it is the case for simple coatings (less than 40 layers). For this reason, an accurate modelling of light scattered by complex optical coatings requires a global analysis taking into account different parameters from the design and manufacturing to the conditions of use of the component.
We will present in this paper the last developments performed by the Scattering Group of Institut Fresnel for the modelling and the metrology of light scattered by complex optical components for space applications.
A specific care will be given to recent studies performed on narrow band filters for earth observation and high performances mirrors. For each of these applications, the specific problematics and corresponding challenges will be presented and experimental and numerical results will be detailed. The agreement between metrology and modelling will be highlighted.
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.
For these reasons, it is essential to develop a metrological tool which provides an accurate quantification of the spectral and angular scattering losses behavior with sufficient angular and spectral resolutions.
In order to face this issue, new investigations were performed by our group at Institut Fresnel and led to the development of the new scatterometer SALSA (Spectral and Angular Light Scattering characterization Apparatus). The use of both a broad-band light source and a tunable filter allows to accurately select the illumination wavelength and the spectral bandwidth on the whole spectral range of CCD detectivity. In this paper we will present the performances of the setup and some experimental results.
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