During the last few years, one of our main research topics has been developing a new type of spectropolarimeter intended for space applications. Initially analyzed numerically, the instrument has a compact, stable design without rotating components. The entire Stokes vector can be determined in a single shot in a vast spectral range. The simulations proved that the modulation schemes that can be obtained for this instrument are close to the optimal form. The objective of the current research is the experimental validation of this instrument. Here, we present the first results for determining the instrumental matrix and the demodulation results for a s eries of polarization states. In conclusion, we present the possible further developments of that project. @en

The Ultraviolet Imager (UVI) is a challenging instrument developed in the frame of the SMILE mission, a collaboration between ESA and CAS. The UVI instrument is a CMOS-based ultraviolet camera developed to image Earth's northern auroral regions. It is centered on the 160-180 nm UV waveband, with a 10° × 10° field of view. At the core of the instrument, four thin film-coated mirrors guide light into its detector and ensure most of the signal filtering, crucial to achieve an out-of-band rejection ratio as low as possible to reject light from solar diffusion, dayglow and unwanted atomic lines. We developed an interferometric coating based on an MgF2/LaF3 multilayer stack deposited by ion-assisted electron-beam deposition. We gradually improved our evaporation setup to reach a high degree of homogeneity, precision and repeatability on the material thicknesses, over the entire mirrors surface. The reflectivity maximum is above 85% and the wavelength at which it occurs is adjustable within 1 nm, while the out-of-band reflectivity between 120 and 155 nm and between 200 nm and 1100 nm is kept below 6% on average never exceeding 8 %. The coating has been space qualified and shows stable performances in conditions representative of the instrument operation environment (thermal cycling under vacuum, radiations, UV exposure…). @en

The Large Interferometer For Exoplanets (LIFE) is a proposed space mission that enables the spectral characterization of the thermal emission of exoplanets in the solar neighborhood. The mission is designed to search for global atmospheric biosignatures on dozens of temperate terrestrial exoplanets and it will naturally investigate the diversity of other worlds. Here, we review the status of the mission concept, discuss the key mission parameters, and outline the trade-offs related to the mission’s architecture. In preparation for an upcoming concept study, we define a mission baseline based on a free-formation flying constellation of a double Bracewell nulling interferometer that consists of 4 collectors and a central beam-combiner spacecraft. The interferometric baselines are between 10–600 m, and the estimated diameters of the collectors are at least 2 m (but will depend on the total achievable instrument throughput). The spectral required wavelength range is 6–16 μm (with a goal of 4–18.5 μm), hence cryogenic temperatures are needed both for the collectors and the beam combiners. One of the key challenges is the required deep, stable, and broad-band nulling performance while maintaining a high system throughput for the planet signal. Among many ongoing or needed technology development activities, the demonstration of the measurement principle under cryogenic conditions is fundamentally important for LIFE. @en

Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L′-band (3.5 to 4.0) μm, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of 10-3 in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements of beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss <6.4% of the best efficiency for the injection and a null depth of 1/42.10-3, mainly from optical path delay errors outside the scope of this work.

@en

We describe the state of the development of a coherence scanning interferometer to measure local changes in topology and local induced vibrations of a mirror at cryogenic temperatures. The metrology instrument incorporates an optical phase mask and a microlenses array, enabling the acquisition of complete white light interferograms within a single-camera frame. This stands in contrast to traditional temporal phase-shifting interferometers. We design the optical phase mask as a combination of steps of different thicknesses, so each step introduces a different optical path difference to the rays. The local interferograms for each camera frame provide us with information on the local topology of the mirror. The interferogram displacement between camera frames allows us to monitor the mirror’s local induced vibrations. In this work, we report the metrology instrument’s working principle through numerical simulations and present the latest results of a proof of concept developed at the laboratory. The metrology instrument shown is of extensive usability in diverse applications related to real-time measurements of various fast physical processes and real-time characterization of the optical components topology.@en

One of the most ambitious goals of modern astronomy is to uncover signs of extraterrestrial biological activity, primarily achieved through spectroscopic analysis of light emitted by exoplanets to identify specific atmospheric molecules. Most exoplanets are indirectly identified through techniques like transit or Doppler shift of the host star's flux. Long-term surveys have yielded statistical insights into the occurrence rates of different planet types based on factors such as radius/mass, orbital period, and the spectral type of the host star. Initial estimates of terrestrial planets within the habitable zone have also emerged. However, the difficulty of detecting light from these exoplanets leaves much unknown about their nature, formation, and evolution. As the number of rocky exoplanets around nearby stars rises, questions about their atmospheric composition, evolutionary trajectory, and habitability increase. Direct measurement of an exoplanet's atmospheric composition through its spectral signature in the infrared can provide answers. Measuring the infrared spectrum of these planets poses significant challenges due to the star/planet contrast and very small angular separation from their host stars. Previous research showed that space-based telescopes are mandatory, and unless large primary mirrors (>30m in diameter) can be sent into space, interferometric techniques become essential. Combining light from distant telescopes with interferometric techniques allows access to information at minimal angular separation, operating within the diffraction limit of individual telescopes. Successful demonstrations of on-ground nulling interferometry open a new era for such space-based missions. They are vital to sidestep and tackle these scientific questions. We recently initiated a new study with the European Space Agency to explore the design parameters and the performances related to an interferometric concept based on a single spacecraft and sparse multiple sub-apertures. Launch constraints are linked to the use of an Ariane 6 launch vehicle. Our parametric study covers a range of 1-4 m for the diameter of the telescope and a 10-60 m baseline. The most promising concept working in the infrared range (3-20μm) will be highlighted. This study is conducted by TUDelft in cooperation with KULeuven, CSL/ULiège, and Amos with the support of the European Space Agency. @en

This paper proposes a novel approach to improve the performance of free-space optical communication intersatellite links by combining fundamental Gaussian and higher-order Laguerre-Gaussian beams. We present a comprehensive mathematical model to analyze the system’s performance, including received power statistics, average bit error probability, and outage probability. To generate the desired beam profiles, we propose an optical system capable of creating a superposition of orthogonally polarized Laguerre-Gaussian beams that yield the far-field irradiance distributions that optimize the communication performance. Our theoretical analysis demonstrates that the combination of fundamental Gaussian and higher-order modes can significantly enhance system performance compared to conventional fundamental Gaussian beams. In some scenarios, the proposed approach offers savings on the order of 20% to 40% of the required transmitted power.@en

Reflectance spectroscopy is a technique widely used to investigate the composition and physical properties of a surface. The spectro-polarimetry adds the investigation of the polarimetric state of the light, while keeping the spectroscopy dependency. This technique is currently limited for the characterization of the surface, but can bring another clue on the composition and physical properties of the studied surface. We present here the design of a novel ellipsometer, optimized for the investigation of the polarization state of the light reflected by a granular surface. This instrument is able to measure the linear and circular components of the polarization over a wide spectral range from the ultraviolet to near-infrared and at a wide choice of geometrical configuration. The wide spectral range is achieved with the use of a photoelastic modulator acting like a retardance waveplate over the whole working range. Spectro-polarimetric investigations of terrestrial and extra-terrestrial samples will have application to mineralogical investigations, planetary surface explorations, and improve our understanding of the Solar System. @en

Intersatellite optical communication links will be crucial for the development of future global optical and quantum communication networks. Under the harsh space environment satellite optical terminals will suffer pointing jitter and wavefront errors. In this paper, the impact of the combination of these errors on the transmitter side is modeled. Combining the far-field diffraction patterns obtained through computational Fourier optics and the statistics of the pointing jitter, the received power statistics are derived numerically for different scenarios. The computational model is first used to evaluate the optimum nominal parameters of the transmitted beam. Then, several optical aberrations are added to the transmitted beam and their impact on the communication performance is evaluated through the average bit error rate.@en

NOTT (formerly Hi-5) is the L’-band (3.5-4.0μm) nulling interferometer of Asgard, an instrument suite in preparation for the VLTI visitor focus. The primary scientific objectives of NOTT include characterizing (i) young planetary systems near the snow line, a critical region for giant planet formation, and (ii) nearby mainsequence stars close to the habitable zone, with a focus on detecting exozodiacal dust that could obscure Earthlike planets. In 2023-2024, the final warm optics have been procured and assembled in a new laboratory at KU Leuven. First fringes and null measurements were obtained using a Gallium Lanthanum Sulfide (GLS) photonic chip that was also tested at cryogenic temperatures. In this paper, we present an overall update of the NOTT project with a particular focus on the cold mechanical design, the first results in the laboratory with the final NOTT warm optics, and the ongoing Asgard integration activities. We also report on other ongoing activities such as the characterization of the photonic chip (GLS, LiNbO3, SiO), the development of the exoplanet science case, the design of the dispersion control module, and the progress with the self-calibration data reduction software.@en

The light emitted or reflected by a medium can exhibit a certain degree of polarization. Most of the time, this feature brings valuable information about the environment. However, instruments able to accurately measure any type of polarization are hard to build and adapt to inauspicious environments, such as space. To overcome this problem, we presented recently a design for a compact and steady polarimeter, able to measure the entire Stokes vector in a single shot. The first simulations revealed a very high modulation efficiency of the instrumental matrix for this concept. However, the shape and the content of this matrix can change with the characteristics of the optical system, such as the pixel size, the wavelength or the number of pixels. To assess the quality of the instrumental matrices for different optical characteristics, we analyze here the propagation of errors, together with the impact of different types of noise. The results show that the instrumental matrices are converging towards an optimal shape. On this basis, the theoretical limits of sensitivity on the Stokes parameters are inferred. @en

To achieve the expected level of sensitivity of third-generation gravitational-wave (GW) observatories, more accurate and sensitive instruments than those of the second generation must be used to reduce all sources of noises. Amongst them, one of the most relevant is seismic noise, which will require the development of a better isolation system, especially at low frequencies (below 10 Hz), the operation of large cryogenic silicon mirrors, and the improvement of optical wavelength readouts. In this framework, this article presents the activities of the E-TEST (Einstein Telescope Euregio Meuse-Rhine Site & Technology) to develop and test new key technologies for the next generation of GW observatories. A compact isolator system for a large silicon mirror (100 kg) at low frequency ( < 10 Hz) is proposed. The design of the isolator allows the overall height of the isolation system to be significantly compact and also suppresses seismic noise at low frequencies. To minimize the effect of thermal noise, the isolation system is provided with a 100 kg silicon mirror which is suspended in a vacuum chamber at cryogenic temperature (25-40 K). To achieve this temperature without inducing vibrations to the mirror, a radiation-based cooling strategy is employed. In addition, cryogenic sensors and electronics are being developed as part of the E-TEST to detect vibrational motion in the penultimate cryogenic stage. Since the commonly used silicon material is not transparent below the wavelengths typically used in the 1 µm range for GW detectors, new optical components and lasers must be developed in the range above 1500 nm to reduce absorption and scattering losses. Therefore, solid-state and fiber lasers with a wavelength of 2090 nm, matching high-efficiency photodiodes, and low-noise crystalline coatings are being developed. Accordingly, the key technologies provided by E-TEST serve crucially to reduce the limitations of the current generation of GW observatories and to determine the technical design for the next generation.

@en

We present the experimental results of the proof of concept of a metrology instrument developed to characterize the cryogenic mirror of the Einstein Telescope (ET) prototype. ET is a proposed gravitational-wave observatory. The metrology instrument uses the principle of low-coherence interferometry to measure the local change in topology and local induced vibrations of the mirror resulting from the cooling down process. We implement an innovative optical phase mask and a microlens array to obtain a depth map of the mirror on a single camera frame. With our instrument prototype, we can obtain 25 interference patterns of the same mirror spot for each camera frame. Each interference pattern corresponds to a difference Optical Path Difference (OPD). Then by reconstructing the interference patterns, we can measure the mirror’s local topology change and local induced vibration. Moreover, in this proceeding, we describe the analysis of the white-light interference patterns through numerical simulations and depict the metrology instrument’s optical design. Finally, we discuss how we can use the metrology instrument for real-time characterization of other optical components with all the advantages of white light interferometry. @en

Context. NOTT (formerly Hi-5) is a new high-contrast L′ band (3.5–4.0 µm) beam combiner for the VLTI designed with an ambitious aim to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and contributions of instrumental noise. This motivates the development of end-to-end simulations to optimize these instruments.

Aims. The aim of this study is to enable a performance evaluation of NOTT and inform the design of such instruments with current and future infrastructures in mind, taking into account the different sources of noise and their correlation.

Methods. SCIFYsim is an end-to-end simulator for single-mode-filtered beam combiners, with an emphasis on nulling interferometers. We use it to compute a covariance matrix of the errors. We then use statistical detection tests based on likelihood ratios to compute compound detection limits for the instrument.

Results. With the current assumptions as to the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6–7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant.

Conclusions. SCIFYsim is suited to anticipating some of the challenges of design, tuning, operation, and signal processing for integrated-optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to 105 in the L band at separations of 5–80 mas around bright stars.@en

The diffractive zone thicknesses of conventional diffractive optical elements (DOEs) are generally obtained using the thin element approximation (TEA). However, the TEA yields inaccurate results in the case of thick multilayer DOEs (MLDOEs). The extended scalar theory (EST) is an alternative thickness optimization method that depends on the diffractive order and the optimization wavelength. We developed an algorithm to research suitable EST input parameters. It combines ray-tracing and Fourier optics to provide a performance estimate for each EST parameter pair. The resulting “best”MLDOEdesigns for three different material combinations are analyzed using rigorous finite-difference time-domain. Compared to the TEA, the proposed algorithm can provide performing zone thicknesses. @en

A novel method for space object identification is proposed, based on full Stokes spectropolarimetry in the visible and near-infrared wavelength range. Space objects that have been previously detected and are illuminated by the sun can be observed with a telescope to simultaneously obtain intensity, spectra, and polarimetry, and compose light curves of these parameters as function of time. The intention is to thus assign a unique identification, or at least a classification to these objects. Single, double, and multiple reflections of sunlight off the space object (natural or artificial objects, including debris) will introduce spectrally dependent polarisation into the scattered light, the spectral signature of which is affected by the complex refractive index of the scattering materials and the geometry. The simultaneous measurement of the full Stokes vector allows separation of the light source unpolarised spectral signatures on the one hand from the polarisation spectral features on the other hand. To illustrate the concept, we have performed a number of simulations for double scattering off a small selection of materials, for a large range of scattering geometries. Examples of individual scattering geometries and statistical summaries of all geometries are presented. A demonstrator spectropolarimeter is being built, we present an overview of the design and the high level planning, as well as some predicted performance parameters.@en

Growing interest in free-space optical communication, due to the high bandwidth and security provided by these links, has generated the necessity of designing high-performance satellite terminals. In order to develop these terminals, the opto-thermo-mechanical phenomena that appear in the space environment and their effect on optical communication links have to be understood in detail. A review of the opto-thermo-mechanical phenomena occurring in spaceborne terminals is presented, describing the relevance of each of them. The methods found to compute the impact on the communication performance due to opto-thermo-mechanical phenomena are collected by building the bridge between the optical and communication performance parameters. Finally, techniques available to mitigate the detrimental effects of these phenomena are classified, and the relevant research challenges are identified.@en

Light management strategies using photonic crystals have been proven to efficiently improve light harvesting and subsequently conversion efficiency of various optoelectronic devices. This study focuses on 3D inverse opal CH3NH3PbI3 photoanodes in perovskite solar cells from a combined numerical and experimental approach. Varying the pore size and the layer thickness in numerical simulations, we first determined theoretical optimum from a purely optical point of view. Corresponding 3D inverse opal photonic nanostructures were then fabricated through spin-coating protocols using polystyrene nanospheres of various diameters as hard templating sacrificial agents. It demonstrates how the photonic nanostructuration of the perovskite layer impacts both optical and electronic properties of experimental samples. Regarding the individual 3D inverse opal perovskite layers, an optimum of light absorption is reached for an ∼500 nm diameter pore photonic nanostructure, with a photonic absorption enhancement as high as 16.1% compared to an unstructured compact benchmark. However, in addition to electronic-related countereffects, local light absorption in the hole transporting material is observed in assembled solar cells, weakening the light management benefits of the perovskite layer nanostructuration to only ∼3% photonic enhancement.@en

The Hi-5 instrument, a proposed high-contrast L' band (3.5-4.0 μm) nulling interferometer for the visitor focus of the Very Large Telescope Interferometer (VLTI), will characterize young extra-solar planetary systems and exozodiacal dust around nearby main-sequence stars. Thanks to VLTI's angular resolution (λ=B = 5 mas for the longest UT baseline), it will fill the gap between young giant exoplanets discovered by ongoing single-aperture direct imaging surveys and exoplanet populations discovered by radial velocity surveys. In this paper, we investigate the exoplanet detection yield of Hi-5. First, we present the latest catalog of stars identified as members of young stellar associations within 150 pc of the Sun thanks to the BANYAN algorithm and other searches for young moving group members. Realistic exoplanet populations are then generated around these stars and processed with the SCIFYsim tool, the end-to-end simulator for the Hi-5 instrument. Then, two formation models are used to estimate the giant planet's luminosity. The first is the New Generation Planetary Population Synthesis (NGPPS), also known as the Bern model, and the second is a statistical model based on gravitational instability (hot-start model - AMES-Dusty model). We show that Hi-5 is insensitive to cold-start planets but can detect giant hot-start planets. With ATs, more than 40 planets could be detected assuming 20 nights of observations. With its unique capabilities, Hi-5 is also able to constrain in mass the observed systems. Hi-5 is sensitive to planets with a mass > 2 Mjup around the snow line.@en

Hi-5 is the L’-band (3.5-4.0 μm) high-contrast imager of Asgard, an instrument suite in preparation for the visitor focus of the VLTI. The system is optimized for high-contrast and high-sensitivity imaging within the diffraction limit of a single UT/AT telescope. It is designed as a double-Bracewell nulling instrument producing spectrally-dispersed (R=20, 400, or 2000) complementary nulling outputs and simultaneous photometric outputs for self-calibration purposes. In this paper, we present an update of the project with a particular focus on the overall architecture, opto-mechanical design of the warm and cold optics, injection system, and development of the photonic beam combiner. The key science projects are to survey (i) nearby young planetary systems near the snow line, where most giant planets are expected to be formed, and (2) nearby main sequence stars near the habitable zone where exozodiacal dust that may hinder the detection of Earth-like planets. We present an update of the expected instrumental performance based on full end-to-end simulations using the new GRAVITY+ specifications of the VLTI and the latest planet formation models.@en