Optical reflector coatings for astronomical applications from EUV to IR

Abstract

Optical coatings are an integral part of superior optical components. Astronomical applications (ground- and space-based) place especially high demands on these coatings, not only with regard to their optical performance but also to their mechanical and environmental stability, their thermal properties, and their radiation resistance. This article presents a short overview of several coating solutions developed in recent years at Fraunhofer IOF in order to meet the challenging demands of astronomical applications. The focus is placed on high reflective coatings for different wavelength regions including coatings for the VUV range below 100nm, coatings for the DUV wavelength range above 100nm and VIS/NIR/IR coatings. Further, amorphous silicon layers will be introduced which can be polished to very low roughness values and therefore can act as polishing layer for the manufacture of ultraprecise optical components from metal substrates

Au coatings for spaced based reflectors

“…For the "reflectivity and durability of protected Au" (see chapter 2.6) the deposition were carried out in an inline coating system 11 that has seven targets of 100 x 750 mm² and a high-frequency etching station (located in two deposition chambers separated by the gate valves). Ar was used as process gas.…”

Development of reflective co-sputtered nanostructured metallic films

“…Aluminium (Al)-and silver (Ag)-based high reflective metal films/coatings are often used for the applications such as in spacecraft as thermal control reflective mirror surface, transparent heat mirrors (THM) devices, telescopes, high energy laser application, high reflective window, sun-glasses and so on [1][2][3][4][5]. However, the aforesaid metal films/coatings are prone to oxidation/ tarnish at high temperature and while exposing under harsh environment which alters their functional properties in particular thermo-optical characteristics [2][3][4]. To mitigate this issue, a single or multilayer dielectric thin films are deposited on top of the reflective metal coatings to protect the reflective layer from oxidation and environmental degradation.…”

“…To mitigate this issue, a single or multilayer dielectric thin films are deposited on top of the reflective metal coatings to protect the reflective layer from oxidation and environmental degradation. However, often it causes trade-off with the functional properties and thus a judicious selection of dielectric protective materials is required to be applied [3,4]. Further, sometimes technology becomes tricky to protect reflective layer.…”

Roughness reduction of large-area high-quality thick Al films for echelle gratings by multi-step deposition method

“…Although the history of Al thermal evaporation technology can be traced more than a century [12,13], the investigations of large-area Al films mainly concentrate on Al reflector coatings, which are widely utilized on a multitude of optical systems [14]. In addition, the fabrication technique of large-size Al mirror coating mostly used in the telescope is quite well-rounded and its thickness is generally fixed at a few hundred nanometers [15][16][17]. However, when the designed ruling depth of an echelle grating is around 5 µm, the thickness of Al film should be 2-2.5 times of the ruling depth, 10-15 µm, according to grating ruling experience.…”