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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.
The main achievements reached during these activities have been:
- to evaluate by design, manufacturing and test of LVF samples the achievable performances compared with target requirements;
- to evaluate the reliability of the projects by analyzing their repeatability;
- to define suitable measurement methodologies
We will give first a description of the structure of these filters completed by a presentation of their manufacturing procedure, then a detailed description of our experimental set-up, and at the end a presentation of the results of our measurements on these two specific narrow bandpass filters.
The Engineering Tool is indeed designed to achieve in vacuum reflectance and transmittance measurements between 600 and 1700 nm. It is also designed to evaluate during the vacuum cycle partially the nature of the outgassing elements, using mass spectrometry.
We will present in our paper the concept of this equipment and the associated test method. The preliminary characterizations will be done in June 2006 on reflective coatings, one anti reflective coating and dichroic filters.
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.
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.
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 Course all in English aims to provide high-level theoretical and technical competence in the fields of optics, photonics and optoelectronics. Students on the programme have a unique opportunity to study within a truly multicultural environment, by spending time at each participating institution in turn: Marseille; Berlin and Milan. The organization and the pedagogy will be presented.
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