Manufacturing of freeform mirror by milling and altering its optical characteristics by ALD SiO₂ coating

In this study four aluminium and brass freeform mirrors used as fiber optic spectrometer probes were micro-milled and ALD SiO₂ coating was added. Freeform surfaces were designed by combining optical modeling with the mechanical structure. Moore 350 FG ultra precision machine tool was used for milling the freeform parts. To evaluate the surface roughness of the machining one aluminium and one brass freeform mirror also contained a 40 mm radius reference lens. The surface roughness's of parts containing reference lenses were analysed prior to coating them with SiO₂ on atomic layer deposition (ALD) device. The uncoated and coated reference lenses were measured with optical profiler. The Ra values of micro-milled reference lenses on uncoated aluminium and brass surfaces were in good correlation to theoretical values. On uncoated aluminium mirror surface roughness Ra was 6.8 nm and for uncoated brass mirror Ra was 5.7 nm. On coated aluminium mirror surface roughness Ra was 5.2 nm and on coated brass mirror Ra was 5.5 nm. For the testing of the spectral functionality of the system the mirrors were coupled to a fiber-optic UV-VIS spectrometer. Spectral measurements were done on coated and uncoated mirrors with several different coloured reflectance standards. Spectral measurements show that SiO₂ coating affects the reflectance characteristics of both mirror surfaces while maintaining high reflectivity characteristics.