Over the last decade, the general trend towards using freeform optics in illumination systems has been successfully extended to imaging optical systems suitable for practical applications. Customized design approaches and algorithms as well as significant progress in manufacturing, referencing and measurement technology are decisive factors in this regard. The application of aspheres and freeforms with a focus on shorter wavelengths require a significant improvement of shape deviation, waviness and roughness of the optical components to address diffraction-limited systems in the visual spectral range. Unlike conventional rotationally symmetric manufacturing, the production of asymmetrical, freeform optics requires the use of deterministically controlled machining tools that act on specific points or sub-apertures. This article highlights measurements and manufacturing strategies and methods for preventing and correcting high spatial, mid spatial and low spatial surface deviations on metal optical aspheres and freeform optics. The potential of freeform manufacturing, but also its current limitations, are discussed using examples.

Optical systems are constantly becoming smaller. This requires innovative approaches to integrate multiple optical functionalities into single components. A promising way forward is to combine a diffraction grating onto the curved surface of a lens. However, established diffraction grating inscription techniques face challenges when applied to curved surfaces. In this article, we present a proof-of-concept method to combine a diffraction grating, lithographically inscribed on a planar wafer, with a bi-convex cylindrical lens through hydrophilic direct bonding. We characterize the resulting novel hybrid lens by measuring the line width in the focus position. Our findings demonstrate the potential for enhancing the performance of optical systems in terms of improved aberration correction and new optical functionalities while minimizing their size and weight.

In high-precision refractive optical systems, e.g. telescopes and spectrometers for space applications, temperature differences influence the optical performance in the form of aberrations, especially thermal defocus. By temperature changes not only the radii of the optical elements itself, but also the distances between the optical elements are changing in dependency of the used mount materials. This causes a thermal defocus of the system and needs to be compensated. There are multiple approaches for compensation, generally divided into active and passive compensation. Active compensation techniques, e.g. piezo actuators, are highly accurate but require a control system, which is a cost and complexity driver. Passive compensation techniques often use combinations of several materials with different coefficients of thermal expansion. This increases the overall complexity and mass. The focus of this research is to develop a mechanical structure that passively compensates for thermal defocus over a wide temperature range. Auxetic unit cells are the basis of this structure. Auxetic structures exhibit a negative Poisson ' s ratio. By three-dimensional combination of auxetic unit cells, these can be arranged in a way that a transformation from radial deformation into longitudinal deformation is realized. The geometrical complexity requires additive manufacturing as manufacturing process. This concept is used to develop a mounting structure that passively compensates for thermal defocus by opposing longitudinal deformation, which is triggered by the thermally induced radial deformation of the structure itself under temperature changes. By analytical and numerical investigations, the mechanical behavior of the auxetic cells is studied in dependency of geometrical parameters. With these results, an auxetic mounting structure is developed. The aim is to compensate 5 mu mK axial spacing over a temperature range of 80 K. For proof of concept, a demonstrator system is numerically investigated under thermal loads and on its structural-dynamic properties.

We present what we believe to be a novel, geometrically scalable manufacturing method for creating compact, low-resonance frequency, and high quality factor fused silica resonators. These resonators are intended to be used in inertial sensors for measuring external disturbances of sensitive physics experiments. The novel method uses direct bonding and chemical-mechanical polishing (CMP) in order to overcome the limitations of current subtractive manufacturing methods, which face prohibitive cost and complexity as material removal increases, inherently restricting the design flexibility of the resonator. We demonstrate a prototype with a test mass of only 3 g that reaches a quality factor of Q = 118 000 ± 400 at a resonance frequency of below 20 Hz. This advancement is particularly significant for future gravitational wave observatories, such as the Einstein Telescope.

Quantum communication is considered to be a key feature for secure communication e.g. between government organisations or other institutions with high security requirements. Therefore, the QuNET initiative was founded. It focuses on developing a quantum-secure German governmental agency network based on quantum key distribution (QKD). Freespace optical (FSO) links are a valuable part of infrastructure because they can be deployed temporarily, such as at summits or to bridge the last miles where there is no fiber infrastructure. In particular, high-throughput telescopes are of great importance as optical antennas for terrestrial networks or links between mobile nodes. The paper describes the development and manufacturing of an unobscured, a focal four-mirror metal telescope which is already tested for ground-based quantum communication. The off-axis system, operating with a full telescope aperture of 200 mm, a magnification of 20x, and a FOV (field of view) of 3.5 mrad and is designed to yield diffraction-limited performance for an operational wavelength of 810 nm and 1550 nm. The addressed wavefront error-target of the whole system amounts to 66 nm RMS (root mean square). The use case of the telescope implied an operational temperature range of -40 degrees C up to +50 degrees C. Therefore, an athermal system is realized using an aluminum-silicon alloy substrate material combined with a nickel-phosphorus polishing layer that allows to reach the required surface quality of the mirrors. To simplify the alignment of the telescope, its mechanical concept relies on a snap-together approach using two substrates with two optical mirrors on a common substrate, each. The manufacturing chain of these two so called mirror substrates is described in detail. That includes the CNC pre-manufacturing, ultra precision diamond turning and subsequent polishing steps. The resulting quality of the mirror substrates as well as of the telescope system is demonstrated by optical measurements using interferometric setups.

Super-polished optics are crucial components for extremely short wavelength applications. One of the main applications are illumination optics. Short operational wavelengths of similar to 13,5 nm lead to high demands on substrate's surface quality, especially for microroughness features in the high spatial frequency range (HSFR). The excellent surface quality is essential for high-performant reflective mirrors. Typically, mirror substrates for EUV applications are made of glass, glass-ceramics and silicon. This paper describes an alternative approach by using metallic substrates made from electroless Nickel-Phosphorous (NiP) plated Aluminum alloys. The X-Ray amorphous NiP alloy enables several polishing and correction techniques to reduce the surface roughness down to the addressed sub-nm range. The process chain of metal optics includes ultra-precise diamond turning (DT) and chemical mechanical polishing (CMP) steps. Microroughness (HSFR) of < 0.2 nm RMS (AFM, 1 x 1 mu m(2)) by CMP could be generated on flat and curved surfaces as well as on freeform optics. The process chain will be verified by surface characterization techniques, e.g. atomic force microscopy and white light interferometry. The data is evaluated using the established PSD (power spectral density) analysis method. Conclusions about the surface errors concerning the different processing technologies can be verified. This paper shows that super-polished metallic freeform optics are suitable for curved EUV illumination optics.

In this article, the phenomenon of water stress corrosion (WSC) at borofloat glass interfaces joined by hydrophilic direct bonding is explored. In particular, the impact of the surface waviness and surrounding atmosphere is studied through time-resolved measurements of the bonding energy during surface separation. We present a model for sub-critical crack growth and discuss the underlying WSC reaction. Key findings are that, firstly, the presence of humid air, and secondly, mechanical stress stored at the interface due to elastic contact point deformation increase the number of water molecules with sufficient kinetic energy to participate in the WSC reaction, that is, increase the reaction rate. This study provides crucial insights into the conditions aggravating WSC and gives implications for improving the durability and performance of mechanically stressed glass interfaces in various applications, such as micro-electro-mechanical systems (MEMS) and advanced optics.

This paper reports on a novel concept of thermally stable lattice structures for next-generation refractive optical systems. The aim is to develop a metallic mount for optical lenses based on auxetic structures fabricated by additive manufacturing. Auxetic structures exhibit an effective negative Poisson ratio at the macroscopic level. This property is used to develop a mounting structure that compensates for thermal defocus between refractive optical elements. Numerical and experimental studies of the mechanical behavior of additively manufactured body-centered cubic cells provide the basis for the development of this application-specific mounting structure. Several static load cases are analyzed and deviations between numerical and experimental data are examined to quantify differences in dependence of element type, element size, and manufacturing-related geometrical inaccuracies. These results are used to compare the numerical analysis of the Poisson ratio of an auxetic unit cell with the analytical descriptions. This comparison is then applied to two-dimensional auxetic unit cell compounds. These studies provide the basis for the further development process of an auxetic mounting structure.

Reflective optical systems typically combine precise aligned mirrors, housings, interface structures et cetera. For MICADO (Multi-AO Imaging Camera for Deep Observations), a first-light instrument for ELT, Fraunhofer IOF realize several optical sub-systems, including ten freeform shaped metal optics for the Collimator, the high-resolution Imager, the low-resolution Imager and the Camera. In this paper, the challenges of freeform manufacturing and metrology will be described. For manufacturing of those mirror substrates suitable technologies, as slow tool servo (STS) and fast tool servo (FTS) diamond turning and for further correction (e.g., magnetorheological finishing) and smoothing steps (e.g., chemical-mechanical polishing), sub-aperture tools are required. For interferometry of freeform shaped optical surfaces, computer generated holograms including reference fiducials are realized. After manufacturing the mirror substrates, the optical surface will be coated with a high-reflective gold coating.

Joining technologies are of great importance for space-based applications.Given the very low environmental temperatures, high temperature differences, vacuum, and high acceleration loads during rocket launch, the (thermo-)mechanical requirements on the joining technologies are demanding.Plasma-activated bonding (PAB) and silicate bonding (SB) meet all these requirements.We developed PAB and SB to assemble an all-glass four-channel beam splitter.This development was initiated by a satellite mission concept, devoted to transient astronomy.Central part of this satellite mission is a novel beam splitter that divides the incoming telescope beam into four near infrared channels (λ = 800 -1700 nm), by using a Kösters prism type design.As a final result, we built a demonstrator for the validation of the developed technological concept.

Quantum communication has seen rapid progress towards practical large-scale networks, with quantum key distribution (QKD) spearheading this development. While fibre-based systems have been shown to be well suited for metropolitan scales, suitable fibre infrastructure may not always be in place. Here, we make the case for an entanglement-based free-space quantum network as a practical and efficient alternative for metropolitan applications. We developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. For a representative 1.7-km free-space link, we showcase its ad hoc deployability and achieve secure key rates of up to 5.7 kbps, with 2.5 kbps in direct noon sunlight. By extrapolating experimental data, we show that kbps key rates are achievable even for 10-km distances and multi-user scenarios. We anticipate that our work will establish free-space networks as a viable solution for metropolitan applications and an indispensable complementary building block in the future global quantum internet.

An accurate measurement of the bonding energy of an interface is important in many areas of applied research. We present a novel method for measuring the bonding energy, which is based on the principle of non-planar direct bonding, i.e., direct bonding of originally planar wafers onto non-planar substrates. We discuss in detail the advantages and disadvantages compared to the commonly used double cantilever beam method. To demonstrate the practical relevance, by using the example of glass wafers, the evolution of the bonding energy during different de-bonding steps is investigated, focusing on how the surface shape variations and the surface roughness affects water stress corrosion. We find that the bonding energy in the corroded state is not affected by the original surface shape variations and mid-spatial frequency range roughness, anymore. A molecular mechanism to explain this phenomenon is proposed.

A novel, to the best of our knowledge, dual-state reflective optical relay system based on the Alvarez system is proposed, which can be used for remote sensing applications. By keeping the image and pupil positions constant, it can be combined with a telescope to achieve two different magnifications. As a compact structure with only two moving parts, freeform optical mirrors and a nearly diffraction limited performance for the infrared wavelength 8 µm make it an attractive subsystem for space applications. Different design tradeoffs and the preferred layout properties are discussed in detail.

The GALA (Ganymede Laser Altimeter) is one of eleven scientific instruments of the ESA mission JUICE (Jupiter Icy Moons Explorer) with the goal of exploring the icy moons of Jupiter, with a special interest in Ganymede. By its atmosphere, magnetic field, and water abundance, Ganymede is similar to Earth [1]. GALA is a laser altimeter that generates a surface profile with a resolution of < 15 cm based on an emitted laser pulse that is reflected by the surface of the moon 500 km away [2]. The mechanical development of the receiver telescope with an extremely thin-walled primary mirror (thickness 4-8 mm; diameter ~ 300 mm) was driven by tough boundary conditions. These are a small envelope and mass budget with high mechanical loads, such as a quasi-static acceleration of 120 g during rocket launch and a temperature range from -50 °C up to 150 °C, at the same time. The athermal design is based on the use of a silicon particle reinforced aluminum compound (AlSi40) and an amorphous nickel-phosphorous plating to allow various shape correction and polishing processes. Another challenge was the high radiation load of 1012 protons/cm2 @ 10 MeV. Fraunhofer IOF developed and qualified a gold HR coating based on nanolaminate with R < 98% @ 1064 nm and high resistance. Thus, almost all process steps from development through manufacturing to integration and characterization could be carried out at Fraunhofer IOF. With a shape deviation of 27 nm RMS of the primary mirror and 8 nm RMS of the secondary mirror, a system performance of 90% encircled energy could be achieved with a pupil radius of 38 µm. The telescope was handed over to HENSOLDT in spring 2020 and will start its eight-year journey to Jupiter in 2023.

We present new dispersive optical components, namely TIR-GRISMs, that exhibit three advantages when compared to standard diffraction gratings. First, high diffraction efficiency, second, enormous angular dispersion and, third, a nearly constant angular resolution profile.

Sub-aperture fabrication techniques such as diamond turning, ion beam figuring, and bonnet polishing are indispensable tools in today's optical fabrication chain. Each of these tools addresses different figure and roughness imperfections corresponding to a broad spatial frequency range. Their individual effects, however, cannot be regarded as completely independent from each other due to the concurrent formation of form and finish errors, particularly in the mid-spatial frequency (MSF) region. Deterministic Zernike polynomials and statistical power spectral density (PSD) functions are often used to represent form and finish errors, respectively. Typically, both types of surface errors are treated separately when their impact on optical performance is considered: (i) wave aberrations caused by figure errors and (ii) stray light resulting from surface roughness. To fill the gap between deterministic and statistical descriptions, a generalized surface description is of great importance for bringing versatility to the entire optical fabrication chain by enabling easy and quick exchange of surface topography data between three disciplines: optical design, manufacturing, and characterization. In this work, we present a surface description by stitching the amplitude and unwrapped phase spectra of several surface topography measurements at different magnifications. An alternative representation of surface errors at different regimes is proposed, allowing us to bridge the gap between figure and finish as well as to describe the well-known MSF errors.