Since its introduction at the Very Large Telescope Interferometer (VLTI), the GRAVITY instrument has emerged as a key player in interferometry. Recognizing its substantial contributions, the European Southern Observatory and the GRAVITY consortium have embarked on an initiative to enhance the instrument's functions. This initiative, named GRAVITY+, aims to broaden the instrument's utility for the international astronomical community and foster new areas of research. GRAVITY+ incorporate advanced features such as a novel laser Adaptive Optics system and an improved fringe tracker to augment interferometric observation sky coverage. This paper details the development of a new Germanium (Ge) prism. This new technical capability of GRAVITY+ will serve to substantially increase the spectral resolution of the instrument to approximately R~15000. The design process, along with the scientific rationale underpinning this advancement, are thoroughly examined in this study.
The knowledge of the bi-directional scattering distribution function (BSDF) of an optical component is an import requirement for the design and assessment of high-performance optical instruments. However, precise BSDF measurement with high resolution close to the specular beam can be very challenging and require sophisticated instrumentation. In this paper, we present a newly developed scatterometer, the “Enhanced Resolution Light Scattering Analyzer for Curved Gratings (single detector axis)” – ELSA/CG-S which is designed specifically to measure the BSDF of curved optical components with a very high resolution not only close to the specular direction, but throughout the whole angular measurement range with an instrument signature that can compete with the top of the class of current commercially available instruments. The distinguishing feature of the instrument is the use of a high-resolution silicon sCMOS imaging detector which enables fast acquisition times and provides access to a two-dimensional section of the BSDF around the main detection plane of the instrument with an out-of-plane FoV of about ±0.6°. In the following, we will describe the general design of the instrument and explain the measures that have been taken to enable a very low stray light signature with the chosen detection scheme. After this, we will assess the instruments capabilities and present measurements of the instrument signature and BRDF measurements of plane and curved diffraction gratings with high groove densities. These measurements will also demonstrate the additional value that is provided by using an imaging detector. All measurements will be compared to results obtained with ESTEC’s commercial CASI scatterometer from The Scatter Works, that represent the current state of the art.
Holography represents a very elegant and versatile method for the manufacturing of high-performance diffraction gratings. This is particularly true within the frame of space applications, e.g., for utilization in customized, high end optical spectrometers. Holography enables both symmetrical (sinusoidal or binary) and blazed groove profiles on arbitrarily shaped substrates, ranging from plane over spherical to freeform surfaces. In addition, holographic recording of grating lines is not restricted to straight lines of constant density (either on a plane surface or on a plane projection plane above a curved substrate figure) but can rather be deterministically controlled to yield defined groove distortions and locally varying groove densities. Being able to control not only the groove density but also grooves local curvature represents a major advantage for the optical design of spectrometers, since it adds another degree of freedom to the design – enabling improved focusing and / or aberration correction features within the grating surface. In contrast to frequently encountered misbeliefs, it is shown that holography can also address very low groove density gratings with periods well above 10 µm (i.e. g < 100 lines/mm). Furthermore, two strategies are presented that allow for flattening a grating’s spectral diffraction efficiency (which is often of particular importance for gratings with low groove density): (1) multi-zone gratings with zone-wise varying blaze wavelength and (2) “kinked” groove profiles with more than just one effective blaze facet within the grating period.
In addition to applications based on particle accelerator sources the XUV wavelength range is interesting for several further fields of research – for example in astronomy. On one hand, due to the short wavelengths a diffraction grating for these applications must provide a high-quality surface figure of its polished grating substrate or blank. On the other hand, the grating profile must be very smooth with no relevant phase errors to provide high diffraction efficiency as well as minimum stray light levels. Due to these challenging specifications it is advantageous if a manufacturer has access to lithographic technologies and sophisticated polishing processes at once. For instance, it is sometimes crucial to adapt the polishing to the employed technology chain in order to reach optimum grating performance. Furthermore, a capable grating manufacturing technology should enable flexible line distributions ranging from equally spaced lines on plane or curved substrates up to variable-line-spacing gratings (VLS) as well as even curved lines for imaging grating types in general. Grating surface figures ranging from plane, spherical and cylindrical up to freeform and – in case of beamline optics for grazing incidence operation mode – comparable massive grating blanks must be manageable as well. By employing holographic exposure in combination with new and state-of-the-art etching techniques it is feasible to address all the mentioned features. We will address the degrees of freedom in grating design arising due to holographic pattern definition and present latest improvements that go beyond the so far reported status of XUV grating manufacturing. Beside the flexibility of holography to achieve excellent roughness and best peak efficiency on silicon the option for local blaze adaption can be extremely beneficial. Thus, it will be in the focus of this text.
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