This article describes a new athermal approach for high performance metal optics, particularly with regard to extreme environmental conditions as they usually may occur in terrestrial as well as in space applications. Whereas for mid infrared applications diamond turned aluminium is the preferred mirror substrate, it is insufficient for the visual range. For applications at near infrared wavelengths (0.8 µm – 2.4 µm) as well as at on cryogenic temperatures (-200°C) requirements exist, which are only partially met for diamond turned substrates. In this context athermal concepts such as optical surfaces with high shape accuracy and small surface micro-roughness without diffraction effect and marginal loss of stray light, are of enormous interest.
The novel, patented material combination matches the Coefficient of Thermal Expansion (CTE) of an aluminium alloy with high silicon content (AlSi, Si ≥ 40 %) as mirror substrate with the CTE of the electroless nickel plating (NiP). Besides the harmonization of the CTE (~ 13*10-6 K-1), considerable advantages are achieved due to the high specific stiffness of these materials. Hence, this alloy also fulfils an additional requirement: it is ideal for the manufacturing of very stable light weight metal mirrors.
Metal optics made of Aluminium 6061 have been widely used to fulfill the demands of an athermal instrument design. Diamond turned metal mirrors are standard optical components in mid infrared astronomical instrumentations working at cryogenic temperatures. Structures and optics can be made from the same material (aluminium) to avoid thermal stress due to different CTEs. However, surface roughness, scattering behavior and form accuracy of aluminium mirrors are limited due to the crystallographic and mechanical properties of the substrate material. Mirrors made of zero expansion glass ceramic or silicon carbide (SiC) can be used for cryogenic applications. However, this requires enormous efforts concerning manufacturing and mounting. Therefore the designer tries to avoid the use of glass or ceramics at these working conditions. The use of the same material for optics and structures even for near infrared applications would be a big step forward.
The usage of aluminium substrates with an NiP layer is possible to overcome the performance limitation of aluminium mirrors.
Various polishing techniques may be applied. Nevertheless the significant mismatch in the CTE has to be reduced for the cryogenic use. Looking at the scaling behavior of the deformation due to the CTE mismatch of a simple bi-metallic plate the determining factors becomes obvious:
Deformation CTE * ThicknessNiP * ENiPlayer / (Esubstrate * Thicknesssubstrate2)
For an athermal approach an expansion controlled AlSi alloy is a promising substrate material. Both the higher Youngs Modulus of AlSi compared to standard aluminium and the small CTE mismatch between AlSi and NiP have a positive impact on reducing the bimetallic bending. Very thin NiP layers, which are necessary for standard aluminium, are not longer mandatory.
Source: Fraunhofer-Publica - Publication Database of the Fraunhofer-Gesellschaft