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A linear variable filter is a multilayer band-pass coating deposited with a thickness gradient that gives a significant wavelength shift of the transmission peak along the corresponding direction. In the perpendicular direction the thickness should be as uniform as possible, corresponding to a stable centring wavelength. Such components, associated with matrix detectors are of great interest for the design of compact and light weight spectro-imagers. For such a filter, each of the rejection bands must be able to cover the whole spectral range that must be swept by the transmission peak. When this spectral range is rather large, extending for example from 400 to 1000 nm as in the case we studied, this rejection requirement rapidly becomes the most demanding one. In that case, the standard approach consists to use a metal dielectric band-pass structure, known as induced transmission filter [1]: A metallic layer is embedded in a symmetrical dielectric stacks to form a two-cavity Fabry-Perot filter, each cavity being closed with a metallic and a dielectric mirror. The major advantage of the metallic layer is to provide the wide rejection bands that are required, while the dielectric surrounding stacks are designed to minimize the absorption in the metallic layer at the peak wavelength. However, the reflectance of this metallic layer, obviously linked with its thickness, is not only responsible for the transmission level in the rejection bands, but has also a direct impact on the filter’s band-width and maximum peak transmission. As a result, these three main characteristics of the filter cannot be designed separately. The only way to overcome this major drawback is to avoid the use of any metallic layer. In that case, rejection bands must be formed with the help of broad-band dielectric mirrors which requires a high number of layers. Instead of a few tens layers for a metal-dielectric structure, several hundreds are necessary for an all-dielectric solution. This paper aims to describe the major steps of the work performed at Institut Fresnel to develop such a solution. We will first describe the design of the component and coatings, then the masking mechanism we developed to manufacture the coatings with the right thickness gradient. At last, we will give some partial results that prove the feasibility of this new concept.
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