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The Radial Velocity Spectrometer (RVS) of Gaia measures the red shift of the stars in the spectral band between 847 nm and 874 nm. The spectrometer is a fully refractive optics consisting of 2 Fery prisms, 2 prisms, a pass band filter and a blazed transmission grating (instrument mass about 30 kg). It is located in the vicinity of the focal plane and illuminates 12 of the 106 Charge Coupled Devices (CCDs).
Gaia is in the implementation phase, the launch of the 2120 kg mass satellite is planned in Dec. 2012.
In this work, we demonstrate that self-organized, statistical Black Silicon structures, fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), can be used to effectively suppress interface reflection. More importantly, it is shown that antireflection can be achieved in an image-preserving, non-scattering way. This enables Black Silicon antireflection structures (ARS) for imaging applications in the MIR. It is demonstrated that specular transmittances of 97% can be easily achieved on both flat and curved substrates, e.g. lenses. Moreover, by a combined optical and morphological analysis of a multitude of different Black Silicon ARS, an effective medium criterion for the examined structures is derived that can also be used as a design rule for maximizing sample transmittance in a desired wavelength range. In addition, we show that the mechanical durability of the structures can be greatly enhanced by coating with hard dielectric materials like diamond-like carbon (DLC), hence enabling practical applications.
Finally, the distinct advantages of statistical Black Silicon ARS over conventional AR layer stacks are discussed: simple applicability to topological substrates, absence of thermal stress and cost-effectiveness.
A pinhole free embedding of the grating is essential, since even tiny air pockets will reduce the efficiency of the diffraction optic. This has been successfully realized. However, the ALD coating produces indentations on the surface of the embedded grating. The method to remove the indentations in the excess layer on the embedded grating is discussed. The planarization is done by ion beam etching and the oxygen depletion of the top TiO2 component is fixed by thermal treatment in O2 atmosphere.
Finally, we developed an embedded grating with transmission efficiency higher than 97.0 % at 1030 nm wavelength. The experimentally measured efficiency is in excellent agreement with the theoretical value obtained by rigorous coupled wave analysis. In contrast, a conventional, binary grating with the same period reaches only a maximum theoretical efficiency of 92.3 % at the same wavelength in Littrow-configuration.
Highly efficient polarization-independent transmission gratings for pulse stretching and compression
Analog proximity-photolithography with mask aligners for the manufacturing of micro-optical elements
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