This PDF file contains the front matter associated with SPIE Proceedings Volume 11852, including the Title Page, Copyright information, and Table of Contents.

Ongoing CNES space optics developments

IASI-NG: when improving our understanding of the Earth atmosphere requires to multiply innovations in an operational meteorological programme

LISA: Observing Universe with Gravitational Waves from space

The development of mechanical structures for space telescopes is mainly driven by reducing the mass and in the same step increasing the stiffness, thus to achieve a high margin of safety against failure during launch and low costs for the satellite mission. These aspects attract more and more attention for the NewSpace movement, e.g. for constellations of earth observation and communication systems. Considering the state of the art, design of mass-reduced housings is limited by conventional manufacturing processes like CNC-milling. The possibility of additive manufacturing of metallic materials opens the door for an advanced light-weighted design based on topology optimization and thereby an ideal use of the existing material, which is distributed to optimally sustain loads occurring during launch. This article deals with the development, manufacturing and test of a three-mirror-anastigmatic telescope. In more detail, different concepts for light-weighting (topology optimization and a more traditional shell concept) will be presented. After the additive manufacturing of the AlSi40-mirrors and AlSi40-housing, the parts can be further processed by the typical process chain (e.g. single-point-diamond-turning, plating with electroless nickel), as known from high-performance optics. After integration, the mechanical behaviour will be verified with dynamic tests using a shaker and the results (e.g. the eigenfrequencies) will be compared with simulations finite-element analysis and an optical characterization will be carried out.

Over more than four decades ago Safran Reosc was a pioneer in the development of lightweight glass ceramic space optics in Europe with FAUST, SPOT and METEOSAT projects for CNES. Since then we developed our best efforts to improve our skill and capabilities on the subject through many projects. We still hold a world record with the SOFIA primary mirror being the world largest machined lightweight glass ceramic mirror. Despite our strong orientation toward the highly demanded Silicon Carbide material for space optical instrumentation Safran Reosc is still conducting high performance optical activities with glass ceramic material. Therefore, after recalling our solid heritage on the subject, we want to share with the community our recent developments in glassy ultralightweight optics with MERLIN, Kompsat-7 and thin shells for AO. Other aspects of our progresses lie 1) in the improvements in term of schedule because the space industry is more and more schedule driven and 2) the evolution toward the use of fast and freeform optics enabling more capable and more compact, thus less heavy, and therefore lower total mission costs.

There is an urgent need for the imaging spectrometer with high spatial resolution and wide swathing range in the field of environment monitoring, agriculture evaluation and emergency disaster reduction. At the same time, higher request for the performance of spectrometers, such as signal-to-noise, is put forward to realize high-precision data inversion and quantitative detection. A telescope optical system with long focal length, large relative aperture and wide field-of-view is designed using for imaging spectrometer. The focal length is 1200 mm, the relative aperture is 1/3 and the field-of-view is 10°×1°. A new re-imaging off-axis six-mirror anastigmat (6MA) configuration is adopted. It is beneficial to control the stray light and achieve the telecentric design in the image space. A real entrance pupil is located in front of the system to minimize the aperture of scanning mirror. The whole system is divided into two modules and each module can image perfectly at the axial field of view. Therefore, each module can be aligned and tested individually to reduce the alignment difficulty of the whole system. The off-axis freeform surface is used to control the distortion lower than 0.5% and advance the imaging quality. Finally, the manufacture and alignment tolerance are simulated and analyzed. The design and analysis results show that the six-mirror anastigmat (6MA) off-axis freeform system can satisfy the demand of high-performance hyperspectral imaging for earth monitoring. Good imaging quality and loose tolerances indicate that this optical system is of strong engineering application value.

The PLAnetary Transits and Oscillations of stars mission (PLATO) is the M3 mission in ESA’s Cosmic Vision 2015-2025 Programme. The PLATO mission aims at detecting and characterizing extrasolar planetary systems, including terrestrial exoplanets around bright solar-type stars in the habitable zone. In order to achieve its scientific objectives, PLATO must perform uninterrupted high precision photometric monitoring of large samples of stars during long periods to detect and characterize planetary transits. The PLATO light curves will also contain information on the seismic activity of the stars, which will lead to the determination of radii and ages of parent stars. The scientific payload of PLATO, developed and provided by the PLATO Mission Consortium (PMC), is based on a multi-telescope configuration consisting of 24 “Normal” (N) cameras and 2 “Fast” (F) cameras, so as to provide simultaneously a large field of view and a large collecting aperture. The optical design is identical for all cameras and consists of a 6-lens dioptric design with a 120 mm entrance pupil and an effective field of view of more than 1000 deg2. The calibration and characterization of PLATO’s cameras is a real challenge, especially in terms of quantities: there are 24 FMs + 2 Flight Spares + 2 Qualification Models and finally 1 EM to calibrate. In this context, the on-ground calibration and characterization plan of the cameras was developed to the strict minimum needed. This means that all the measurements that can be performed in-orbit will not be calibrated on-ground. Our aim is to give an overview of the on-ground activities planned in the coming years to calibrate, characterize and verify the PLATO’s cameras, both in terms of organization and technical solutions. In particular, a detailed description of the geometric calibration used for the Fine Guidance System (FGS) and the focusing calibration will be given. PLATO’s Cameras have indeed the unusual but powerful feature to use a temperature control system to refine their focus. A description of the in-flight calibration plan (including for example the repetition of the focusing calibration, the micro-scanning procedure to determine high-resolution PSFs) will then be given to get the full overview of the calibration, characterization and verification of PLATO’s cameras. Calibrating and verify so many cameras in space, without any calibration targets/sources on board, only using pointing capabilities of the satellite, stellar targets and advanced data processing is a real challenge for this mission. A particular attention will be given to the micro-scanning procedure and inversion techniques required for precise PSF Modelling.

Meteosat Third Generation is the next ESA Program of Earth Observation dedicated to provide the International Community, with an operational satellite system able to support accurate prediction of meteorological phenomena, and climate monitoring. The satellites will be operating from the Geostationary orbit using a 3 axes stabilized platform. The main instrument is called the Flexible Combined Imager (FCI), currently under development by Thales Alenia Space France, the PFM being integrated and tested in our Cannes facilities. This instrument will provide full images of the Earth every 10 minutes in 16 spectral channels between 0.44 and 13.3 µm, with a ground resolution ranging from 0.5 km to 2 km. The FCI is composed of a TMA telescope developed by OHB, followed by an IR objective lenses (called SSA) which collimate the beam and split it into several spectral groups. The output beams then reach a cryostat, in which the cold optics (CO-I) focalize them onto the IR detectors. The assembly of the Spectral Separation Assembly (SSA), the CO-I and the detectors is called the SSDA and has been specifically tested as a sub-system. This article will first recall the optical design of the FCI. Then we will describe the principle of the main optical tests both at telescope level and SSDA level. The test results will be presented and compared to the success criteria and specification. In particular the telescope WFE, focal length and line of sight have been proved to be compliant to the success criteria. Concerning the SSDA, we will present the results of MTF measurements, co-registration between spectral channels, polarization and spectral transmittance, which were also all found compliant to their respective success criterion. Stray-light for both TA and SSDA has been characterised on-ground in order to provide the initial input necessary for the development of correction algorithm. At the beginning 2020, these two successfully tested sub-systems were integrated together to build the FCI PFM, which is expected to undergo optical vacuum testing at the timeframe of September 2020.

2020 has been a key year in the MeteoSat Third Generation (MTG), with the integration and tests of the Flexible Combined Imager (FCI) proto-flight model(PFM). The FCI is the imaging instrument of the MeteoSat Third Generation mission, whose first satellite MTG-I1 will be launched in the second half of 2022. Its large spectral coverage, its fast and flexible scanning, associated with demanding radiometric and optical performances will allow a step forward in Europe weather nowcasting. In 2018, three complementary development models were successfully integrated and tested. The Engineering Model validated the optical and radiometric performances of the detection chain. The Structural and Thermal Model qualified the robustness of the design against launch and in-orbit environments and validated the consistency with the thermal and microvibration mathematical model predictions. The Avionic Test Bench with the software which reached a very good level of maturity, validated the control, command and data handling of the instrument. The completion of these developments enabled to successfully hold the instrument Critical Design Review (CDR) end 2018. In 2019, the two main components of the instrument, namely the telescope assembly and the detection control electronics assembly (DCEA) successfully passed the acceptance tests and have been delivered. The article will present first an overview of the instrument design and the main outcomes of the development models. Then, it will discuss the up-to-date status of the FCI PFM development. Finally, it will introduce the overall planning for the four FCI models to be delivered to the MTG-I satellite series. This work has been performed under an ESA contract to Thales Alenia Space-France.

The new degrees of freedom offered by the new family of freeform surfaces helps compensating for off axis aberrations in optical designs. We define a freeform surface as a non-rotationally symmetric surface which cannot be described as an off-axis part of a conicoid. The use of freeform surfaces allows an increase in performance such as on the field of view, the F-number and compactness of unobscured all reflective telescopes. Reflective designs have the advantage of being light, achromatic, and potentially low-cost due to cheaper materials involved. It could result in industrial production of compact panchromatic systems for various applications ranging from automotive sensors to imaging nanosatellites. In this paper we address the challenges posed by the use of these surfaces in optical design optimization, specifically the minimization of the surface counting, in order to simplify the manufacturing, assembly and integration phases. We focus on three mirror designs and work on the compactness with various flavors in the 3D space. We will discuss our results on a fast, unobscured, non reimaging and well corrected three mirror anastigmat (TMA) design for a nanosatellite thermal imaging payload using uncooled bolometers. Optimization method of a TMA under volume and straylight constraints is described. A tolerance analysis is performed and minimal alignment is required even for large tolerances.

CarbonHIGS is a 2D imager designed to measure the enhancement of CO₂ concentration in emission plumes from power plants. The origin of HIGS, and the physics of the working principle will be explained. Two possible implementations of HIGS will be presented, i.e. one that uses power splitting while the other one uses polarization splitting. The characteristics of these implementations will be discussed. In the final part of this paper the preliminary design of the CarbonHIGS system will be shown.

Meteosat Third Generation (MTG) is the next challenging mission for accurate observation of the Earth dedicated to meteorology forecast. MTG encompasses two different missions: MTG-I (Imaging) and MTG-S (Sounding). MTG-I optical payload counts the FCI (Flexible Combined Imager), a multi-spectral imager in the continuity of Meteosat Second Generation (MSG) with improved performance and additional spectral bands. MTG-S transports the IRS instrument which is an Infrared Fourier Transform Spectrometer dedicated to the sounding of the atmosphere inspired by the previous IASI mission. Located in a geostationary orbit, the satellites are covering the full hemisphere for imaging (FCI) in less than 10 minutes in 16 different spectral bands from Visible to Very Long Wave IR at a resolution of 0.5-2 km and make a complete IR sounding of the atmosphere (IRS) in less than 1h at a spatial resolution of 4 km and a spectral resolution of 0.625 cm^-1. In this article, we describe the detection chains of FCI and IRS instruments which give accurate sight to the remote sensing and afford the capability to reach the objectives of MTG-I and S missions. Some particularities of designs and key performance are presented. MTG on ground tests have provided interesting results which are therefore presented. These results have been obtained after 10 years of intensive research and development at Thales Alenia Space (Prime FCI) and OHB(Prime IRS), with the support and supervision of ESA and EUMETSAT. This research and development work has been performed under an ESA contract.

HgCdTe Avalanche Photo Diodes (APDs) are developed at CEA/Leti to enable applications that require the detection of information contained in a low number of photons in each spatial and/or temporal bin, such as LiDAR and free space optical communications. The requirements for such detectors are strongly application dependent, which is why both the HgCdTe APD technology and the proximity electronics, used to extract the detected photocurrent, needs to be optimized for each application. The present communication reports results obtained from the development of detectors for high dynamic range LiDAR applications, made within the scope of the H2020 project HOLDON, and high data rate FSO, made in collaboration with Mynaric Lasercom AG. For FSO applications, we have measured 10 GHz bandwidth at unity gain for APDs with 10 μm diameter. At higher APD gain and diameter, the BW is presently limited by carrier transit and by resistance-capacitance product in small and large area APDs, respectively. For LiDAR we have developed APDs with an made of an array of diodes in parallel with a diameter up to 200 μm and large avalanche gain, M<100, that will be hybridized with a dedicated CMOS amplifier. This circuit was designed to enable photon shot noise limited linear detection over a dynamic range of 6 order of magnitude of signal for observation times ranging from ns up to μs. First characterizations made at unity APD gain shows that the HOLDON detector will meet most of the required performance parameters in terms of sensitivity and linear dynamic range.

Optical communication systems are picking up pace in the various industries for its many scenarios in which it can be used. This includes satellite, aircraft and ground systems for which the DLR Institute of Communications and Navigation develops laser terminals. Free Space Optics (FSO) based optical systems have different applications ranging from terrestrial to airborne to space, and thus also the associated requirements to these application domains vary even though the primary functional goals remain the same. Furthermore, quantum communication became of interest for its inherently secure key generation process that allows two nodes to securely communicate with each other and share secrets for long term security. Out of the different use cases of free-space optical laser terminal came the concept of having a modular FSO optical system design that could quickly adapt to the changing needs from mission to mission. The idea of a modularity appeals to many despite its challenges in the beginning, as in the long run the pros outweigh the cons and the gained advantages make the initial effort worth the while. The concept was realised by carefully studying different requirements from various on-going and past projects and thereafter segmenting different aspects of the optical system based on three main categories, namely; the received path, the transmitted path and the shared path. To elaborate modularity in system here; a fast steering mirror (FSM) can be replaced with a fold mirror for a simpler design should the requirements not require it, or an exchangeable CPA with integrated telescope mounted at the aperture of the optical bench to allow for different magnifications and ex-aperture beam diameter, all these without major impact on other subsystems. This greatly reduces the effort involved in re-evaluating system performance in the long run. In this study the design and development of such an optical system is presented that lays its ground in the Institute of Communications and Navigation’s OSIRIS project, but still continues to serve as the primary optical system design of choice for future projects.

Honeywell Aerospace is implementing on behalf of the Canadian Space Agency the Quantum Encryption and Science Satellite (QEYSSat), a Canadian-owned and operated scientific and technology demonstration mission aimed at developing the next-generation of secure communications infrastructure backed by quantum physics. The mission management is led by the Canadian Space Agency and the science is led by the Institute for Quantum Computing at the University of Waterloo. Quantum key distribution (QKD) is a method for issuing, via single-photon transmission, verifiably-confidential encryption keys between two parties. This capability is a powerful tool for the transfer of sensitive data (e.g. financial transactions, health records, etc.), however current terrestrial QKD networks are limited to a few hundred kilometres in geographic reach between nodes. The QEYSSat mission will use a satellite receiver as a trusted node to demonstrate the distribution of secure keys between ground stations separated by at least 400 km. In addition, Honeywell intends to fly an optical intersatellite link (OISL) terminal as a hosted payload on this mission. The QEYSSat mission will utilize both weak coherent pulse (WCP) sources and entangled photon sources in an uplink configuration to study the performance of QKD, and to perform Bell tests of long-range quantum entanglement. Honeywell is building the QKD receiver terminal consisting of a front-end telescope, a precision pointing and tracking system and single-photon detectors. Major technical challenges include polarization management throughout the optical chain, accurate pointing and tracking, and suppression of background and stray light sources. To address these challenges, Honeywell is leveraging its existing commercial optical communications solutions to meet the more stringent performance requirements for space-based QKD. The QKD terminal architecture consists of an afocal front-end telescope, a wide FOV high-precision pointing and tracking assembly, a polarization analyzer and single-photon photodetector system. A large-diameter on-axis telescope for geostationary optical communications forms the basis for the terminal’s front-end optics, and Honeywell’s commercial Optical Pointing and Tracking Relay Assembly for intersatellite Communications (OPTRAC) has been adapted as a quantum-ready pointing and tracking unit (QTRAC). For each element, substantial effort has been made to develop an optical system that preserves single-photon states with high fidelity despite the large number of optical surfaces in the chain. The optical assembly for the QKD terminal was developed and tested at the breadboard level in 2020; this paper will highlight the development and testing of these units as well as the overall architecture and concept of the QEYSSat mission.

Secure communication networks are the critical infrastructure of the information age. To ensure secure communication between governmental institutions and other high-security environments, the German Federal Ministry of Education and Research (BMBF) initiated an ambitious project { the QuNET initiative. In a joint effort, the Max Planck Institute for the Science of Light (MPL), the German Aerospace Center (DLR) and the Fraunhofer Society aim to develop the technological basis of a German quantum key distribution (QKD) infrastructure. This paper describes the infrastructure used in a first link demonstrator within this project and how we achieved the transition from initial quantum transceiver concepts to first link experiments.

Due to the distance limitation of quantum communication via ground-based fibre networks, space-based quantum key distribution (QKD) is a viable solution to extend such networks over continental and, ultimately, over global distances. Compared to LEO, QKD from GEO would offer substantial advantages, i.e. large coverage, continuous link to ground stations (cloud cover limited), 24/24h operation (background limited), no tracking required, however, coming with large link losses seen the space-ground distance. TNO, together with Eutelsat and CGI-NL, performed a detailed study on the feasibility of QKD from GEO, including a high level system design of payload and ground segment. We conclude that QKD from GEO is technically feasible, and a favourable solution if the satellite needs to act as an untrusted node (that is, no security assumptions required for the space segment). However, the optimal solution, generating a higher value-for-money, is to use a hybrid system, implementing an untrusted and trusted mode BBM92 QKD protocol. In order to arrive at a minimum required secure bit rate of ~1 bit/s in untrusted mode, a 2x ~0.5m diameter telescope in the space segment is required with <0.25µrad pointing accuracy, a <1GHz entangled photon source, in combination with ~2m diameter telescopes on ground. Details on our assumptions and results and drawings of the high level system design will be presented, as well as the development roadmap describing the required technology improvements and building blocks for the overall hybrid approach, which is applicable to non-GEO applications as well.

The high precision Slit Assembly is a key component of the FLEX instrument. Two different input beams for the Low- and the High-resolution spectrometer, respectively, will be generated by the Slit assembly. The paper presents design and implementation of its optical key components, which are a highly precise double slit and two mirrors achieving spatial channel separation and spectral filtering. High alignment and stability requirements, as well as stringent envelope restrictions are driving the mechanical design of the Slit Assembly. We demonstrate solutions in design and manufacturing techniques as well as achieved performance under thermal and mechanical loads. The main focus of the paper is on the development and realization of the double slit device. This device is designed to mount a silicon double slit chip into a mechanical holder providing mechanical and gauge interfaces to the slit assembly. The slit position is aligned to mechanical interfaces to meet the tight positioning requirements. A dedicated lithographic structuring process chain was developed for the manufacturing of the double slit to fulfill a number of challenging requirements; i.e. the absolute slit width accuracy of less than 2 μm peak to valley, and slit planarity of less than 10 μm peak to valley. It is based on the adaptation of lithographic structuring techniques for etching of Silicon wafers. The overall manufacturing process is the result of an extended technology development phase. The manufactured slits are coated with a black coating layer to reach the specified optical reflectivity and the optical density. The results of environmental tests of the Double slit device Breadboard, i.e. the thermal vacuum test, the shock test, and the vibration test, are discussed.

The analysis of recent Earth observation spectrometer missions revealed the impact of spatially heterogeneous Earth radiance scenes on the spectral accuracy of the instruments. One of the most critical observations is the distortion of the instrument spectral response function (ISRF) induced by radiometric contrast in the Earth radiance scene. In order to meet the high precision and accuracy of quantifying the spatial distribution of the atmospheric composition, stringent requirements on the ISRF knowledge are defined such as shape stability, centroid position of the spectral channel centre and the Full Width at Half Maximum (FWHM). In the framework of the CO2M A/B1 study, Airbus investigated a new slit concept called 2D-slit homogenizer (2DSH) for the mitigation of spatially non-uniform scenes. This is done by replacing the classical spectrometer slit with non-circular core multimode fibres, which scramble the light in along-track (ALT) and across-track (ACT) direction and hence average the contrast of the Earth scene in both dimensions. The final 2DSH will be made of several adjoined fibres, assembled in a bundle. A single fibre core dimension defines the spectral extent of the slit (ALT) and the minimum achievable spatial sample (ACT). Consequently, the full swath width covered by the instrument is given by the total size of the fibre bundle in ACT. Here, we present an experimental validation of the 2DSH in terms of scrambling efficiency and radiometry. In order to probe the fibre characteristics for non-uniform scenes, we designed and constructed a setup which allows us to track and tune multiple high contrast scene cases as a fibre input facet illumination. The scrambling efficiency performance of the fibre is assessed by measuring the near- and far-field intensity distribution of light transmitted through the fibre for different scene cases. With the acquired data, we propagate the fibre output through the subsequent spectrograph by simulation and thereby translate the measured near- and far-field distributions in ISRF errors. In particular we quantify the ISRF shape, centroid and FWHM for different Earth scenes. Furthermore, we analyze the impact of focal ratio degradation in terms of radiometric losses and compare the results in the NIR and SWIR wavelength for different stress cases on the fibre.

Polarized light is generally considered as an added value and is often used to improve the viewing of scenes and samples through various optimization processes. However, there are a number of situations where the polarization of the light is penalizing, and for which it is important that the polarized light is transformed into an unpolarized light. For illustration, many space applications require on-board detectors to analyze optical flows from the Earth or the environment. These flows are collected after being scattered and reflected by the different elements encountered, which can partially polarize the light studied. This depolarization is not predictable because it strongly depends on the environments crossed (clouds of water or dust, atmosphere ...) whereas it strongly influences the calibration of the instruments. Although the polarization state of light can be easily transformed into another arbitrary state, or from unpolarized light to polarized light, the reverse situation of depolarizing light is less common and often accompanies optical losses and a reduction in spatial or temporal coherence. As a result, different types of devices and systems have been designed and constructed to achieve this depolarization function. Nevertheless, the systems currently used lead to substantial losses of energy or to a division of the beam considered. In addition, these systems generally operate in transmission and are not integrable. Finally, they do not offer a spectral control of the degree of polarization. We proposed an original alternative technique based on the principle of spatial depolarization (with regard to temporal depolarization). It requires an optical multilayer type component having a transverse gradient of optical properties. In this paper, we will show under which conditions this gradient satisfies a spatial depolarization condition, without creating energy loss. This synthesis step simultaneously takes into account the spatial and spectral variations of the optical properties of the filter. These conditions are then refined to minimize the diffraction effects of the depolarized beam. A compromise is sought between the value of the spatial gradient of the filter, and the spectral variation speed of its polarimetric phase shift. The results obtained are remarkable and the technique is extended to provide specular depolarizers operating in broadband or narrowband . The components are finally manufactured by vacuum deposition technologies. A specific method is used to reach the desired spatial gradient. The metrology that we have implemented reveals an excellent agreement with the theoretical predictions, both for depolarization and diffraction over a wide spectral range.

In next generation space instrumentation for Earth and Universe Observation, new instrument concepts include often non planar gratings. Their realization is complex and costly. We propose a new technology for designing and realizing convex blazed gratings for high throughput spectrographs. For this purpose, our requirements are driven by a Digital-Micromirror-Device-based (DMD) MOS instrument to be mounted on the Telescopio Nazionale Galileo (TNG) and called BATMAN. The two-arm instrument is providing in parallel imaging and spectroscopic capabilities. The objects/field selector is a 2048 x 1080 micromirrors DMD, placed at the focal plane of the telescope; it is used as a programmable multi-slit mask at the entrance of the spectrograph. The compact Offner-type spectrograph design contains a low density convex grating to disperse light. For optimization of the spectrograph efficiency, this convex grating must be blazed. A blazed reflective grating has been designed with a period of 3300 nm and a blaze angle of 5.04°, and fabricated into convex substrates with 225 mm radius of curvature and a footprint diameter of 63.5 mm. The blaze is optimized for the center wavelength of 580 nm within the spectral range of 400 – 800 nm. Such gratings have been fabricated and coated with a silver-based layer, with a final 7° blaze angle over the whole surface. Efficiency close to 90% on the 1st diffraction order at 700nm has been obtained, measured on BATMAN spectroscopic arm. Detailed mapping of the blazed grating showed a very good period uniformity with up to 0.5% deviation. Grating depth and blaze angle have higher deviation, up to 7%. An optimized device with the exact required blaze angle would reach the same efficiency and be centered on the mid of 400-800nm wavelength band: its realization is on-going. The grating brings a significant contribution in the total amount of straylight at instrument level. Their straylight level remains a critical issue, and its reduction by specific and controlled implementation of improvements in manufacturing process is a challenge to tackle. Preliminary straylight measurement has been done and shows a lowest straylight level below 10^-2 sr^-1 between the diffraction orders. This new type of non-planar reflective gratings will be the key component for future high throughput spectrographs in space missions.

Climate on Earth is determined by the Earth Radiation Budget (ERB), which quantifies the incoming and outgoing radiative energy fluxes at the top-of-atmosphere. The ERB can be monitored from space by non-scanning wide field-of-view radiometers, or by scanning narrow field-of-view radiometers. In this paper, we investigate the use of 2 wide field-of-view radiometers geometries, each of which composed of a baffle and precision aperture featuring a field-of-view of a minimum of 127°, allowing to monitor the Earth’s radiative fluxes from limb-to-limb, from an altitude of about 700 km. For both designs, the cavity geometry is analyzed and an evaluation and comparison of different interior cavity coatings is made. The key feature of our first radiometer design proposes is a near-hemispherical cavity enabling a uniform angular sensitivity, in combination with a conical part and baffle to ensure a wide field-of-view. For the second radiometer design, we consider an integrating sphere with a traditional active-cavity radiometer oriented along an axis at right angles with the precision aperture. The throughput of an ideal integrating sphere does not depend on the spatial distribution of the incident radiation. Both designs are equipped with practical coating materials, i.e. Black Velvet for the spherical radiometer and Spectralon for the integrating sphere. To compare both designs, we evaluate the optical absorbance using radiation view factor calculations, and we rely on scattering and stray-light analyses conducted with ASAP® optical design software from Breault Research. We optimize the cavity shape using the outgoing cavity flux as a merit function. We conclude that optimal radiometer designs and coating configurations can be obtained, with great potential for fabricating next-generation Earth Radiation Budget radiometers.

METimage is a cross-purpose multi-spectral optical imaging instrument dedicated to operational meteorology, oceanography, and climate applications with medium resolution in 20 spectral bands with wavelengths ranging from 0.4 to 13.5 μm. The instrument operates onboard MetOp Second Generation satellite A, which is placed in a Sun-synchronous polar orbit with an average altitude of 830km. Firstly, we will provide an overview of the optical architecture of the METimage instrument, detailing its specificities: - In-beam scanner mirror with a cylindrical curvature, - TMA telescope with very low distortion, - 5-mirror all SiC derotator, - Dichroic beamsplitters working from blue to far infrared, - Cryogenic filters and relay optics for infrared imaging. Secondly, the key optical design drivers will be described and explained, together with the solutions that have been put in place to address these demands: - Stringent need on co-registration leading to a very low distortion on the telescope design, and the implementation of masks at the infrared focal planes, - Pupil imaging and calculation of pupil diffraction generated by the infrared field masks, - Straylight requirements leading to specific baffling, tight roughness and particulate contamination requirements, careful handling of ghosts and associated post-processing, - Polarisation sensitivity leading to the specific scanner and derotator combination, and development of optical coatings. Finally, our current development state will be briefly summarized: - Successful instrument CDR confirming the maturity of the technical status, - Design and ongoing manufacturing of the subsystems, focusing on the optical components.

The TRISHNA program marks a step further in the fruitful cooperation built between CNES and ISRO since many years, through a new Earth observation mission dedicated to the improvement of water cycle understanding and water resource management. Thanks to its unprecedented high spatial resolution in the thermal infrared domain, together with a high revisit frequency, TRISHNA mission will significantly contribute to the detection of ecosystem stress and to the optimization of water use in agriculture in a context of global climate change. This paper presents the TRISHNA mission context and main requirements, gives an overview of TRISHNA system architecture and describes TIR (Thermal InfraRed) instrument design with a highlight on its specific innovative features and the current development status.

The CO2M mission, part of the Copernicus Expansion programme, will measure the amount of CO2 released into the atmosphere through human activity. Due to the high accuracy requirements of this measurement, it is important to measure and characterise the aerosol population within the CO2 instrument footprint. For this purpose, ESA have requested that a Multi Angle Polarimeter (MAP) be part of the instrument suite. As part of the overall mission consortium led by OHB (DE), Thales Alenia Space (UK) have been selected to provide this instrument. The instrument itself is a multi angle (<40 angles) multi spectral (7 bands, 400-900nm) polarimeter, based on well known approaches (POLDER, 3MI). However, MAP achieves the demanding level of polarimetric accuracy in a compact volume by using a focal plane assembly that performs both polarization and spectral separation. Use of simple telescopes also allows for wide field-of-view (300km swath) while minimising issues associated with straylight. This paper outlines the main MAP instrument requirements, and details the instrument operating concept and design. Results from the early technology predevelopment activity on the focal plane will be presented, as well as a brief look at other missions for which MAP is suited.

Teledyne e2v has developed the Capella CIS120 which is a general purpose CMOS image sensor with high quantum efficiency that has been specifically designed for space applications and is planned to be used for different Copernicus programs. We will describe the considerations that led to this CMOS sensor concept and then outline the specifications and design details of this back-illuminated detector. Key features include 2048 × 2048 pixels, 10µm pixel pitch, on-chip ADC giving digital outputs in LVDS, both rolling shutter and global shutter modes are available, 45 ke- or 80 ke- full well capacity, peak QE of 90% at 600 nm and latch-up immune and high SEU threshold by design. We will show how the CIS120 can be used for different modes including multi-linear operation, multi-windowing with independent left and right addressing for startracker applications and full frame operation in global or rolling shutter mode for array imaging applications including hyperspectral imaging. The CIS120 is designed to be highly flexible in format allowing; larger variants for astronomical applications and multi die focal plane arrangement, a range of backthinning process options and larger pixel sizes for x-ray applications. Currently the high pixel full capacity version of the CIS120 with 80 ke- per pixel in being characterised. We will present detailed characterisation results and validation data qualifies CIS120 to TRL6 level. We will also present the three different package types have been designed so far for CIS120; standard ceramic package, three side buttable package and sealed Peltier package. Finally, we will summarise plans for future activities including the design and production of a HiRho variant to give significantly improved NIR sensitivity by applying voltage to back surface to be able to operate in full depletion.

We have studied how high-energy electron irradiation (12 MeV, total dose 66 krad(Si)) and long term humidity exposure (75%, 75 ˚C, 500 hours) influence the induced junction black silicon or planar photodiode characteristics. In our case, the induced junction is formed using n-type silicon and atomic-layer deposited aluminum oxide (Al₂O₃), which contains a large negative fixed charge. We compare the results with corresponding planar pn-junction detectors passivated with either with silicon dioxide (SiO₂) or Al₂O₃. The results show that the induced junction detectors remain stable as their responsivity remains nearly unaffected during the electron beam irradiation. On the other hand, the SiO2 passivated counterparts that included conventional pn-junction degrade heavily, which is seen as strongly reduced UV response. Similarly, after humidity test the response of the induced junction detector remains unaffected, while the pn-junction detectors passivated with SiO₂ degrade significantly, for instance, the response at 200 nm reduces to 50% from the original value. Interestingly, the pn-junction detectors passivated with Al₂O₃ exhibit no degradation of UV response, indicating that the surface passivation properties of Al₂O₃ are more stable than SiO₂ under the studied conditions. This phenomenon is further confirmed with PC1D simulations suggesting that the UV degradation results from increased surface recombination velocity. To conclude, the results presented here suggest that black silicon photodiodes containing Al₂O₃-based induced junction are highly promising alternatives for applications that require the best performance and long-term stability under ionizing and humid conditions.

Curved sensors enable a substantial simplification of optical systems and significant improvement of the image quality. Both lead to the reduction of the mass-volume budget of the instruments, simplify the manufacturing and shrink the integration time, resulting in a drop of the consolidated cost. The field curvature is an optical aberration describing the light going through an imager and producing an image on a portion of a sphere. This aberration is present in every single optical system and particularly in wide-angle imaging. Current solutions applied to correct the images imply the complexification of the instruments by using field flatteners, ie. more optics of complex shape, thus increasing the mass-volume and cost budget. The use of curved sensors helps overcoming these issues. It corrects the field curvature directly at the focal surface. For 10 years, the CNRS-LAM demonstrated, through theoretical studies and prototyping, that the use of curved sensors allows to remove 30 to 50 percent of the lenses count and drastically releases constraints in optical systems. I am currently leading a parametric study regarding the contribution of curved sensors for UV imaging system. This study is done through an imager aiming at observing aurora with a wide field of view (more than 60 degrees) in 130-138nm bandwidth with a signal to noise ratio higher than 10. The first main constraint in this project is to get a high transmissive system in FUV bandwidth. To address this problematic, the number of lenses must be reduced. However, reducing the number of lenses means reducing the number of degrees of freedom to correct the aberrations of such a wide field system. To maintain high optical performance, I have considered the use of curved and freeform sensors to correct the field curvature and astigmatism directly in focal surface. In this talk I will present the parametric study I have been leading on the design of this FUV imager. I will quantify the benefits of using curved and freeform sensors compared to flat sensor to keep high performance whilst reducing the number of lenses from 6 to 3 to increase the throughput by a factor 3.

3D PLUS has developed in the framework of RD activities a new high-performance and high-resolution CMOS camera head for space applications. With the collaboration of the Centre National d’Etudes Spatiales (CNES), also called French Space Agency, which is leading this development, the 12 Mpixels 3DCM800 Space Camera Head is integrated using 3D PLUS technology and expertise in designing high performances and high reliability electronics modules. This development intends to offer the best compromise for a highly miniaturized, highly reliable, all-in-one optoelectronic module solution for space applications. Particular attention has been paid to ensure radiation tolerance to cover a wide range of application such as planetology, equipment monitoring, in-flight spacecraft navigation and rendezvous application, and earth observation applications. Technological developments were necessary to overcome challenges such as heat dissipation, and mechanical assembly of complex components, as will be presented in this paper.

Several experiments need to acquire images with different linear polarisation. Acquisition of successive images through a rotating polarising elements give the possibility to achieve this goal. However, the subsequent acquisition of these images implies a temporal modulation and a rotation mechanism (with a consequent consumption of space, power, etc. . . ). The PolarCam©, a camera where an array of linear micropolarizers with orientation are matching the sensor’s pixels, tries to solve these problems. In the present manuscript the features of this camera and the main aspects of its calibration are shown. Finally, an example of its application is presented. In particular, our application of this camera is for the Antarctica solar Coronagraph -AntarctiCor- for the “Extreme Solar Coronagraphy Antarctic Program Experiment” -ESCAPE- that used this technology for a ground-based polarimetric study of the solar K-corona from the Italian-French Concordia Base (Dome C - Antarctica).

Quantum key distribution (QKD) is a promising field within telecommunication and cryptography. QKD allows two parties to generate a shared secret key, which only the two communicating parties know, and can later be used to encrypt and decrypt messages. This leads to unbreakable data transmission. QKD was experimentally already tested on a Chinese mission called MICIUS. Future QKD protocols may also benefit from coupling into an optical single-mode fiber. Adaptive optics is, from a quantum communication perspective, very interesting. In this paper, we present the overall system design, the optical and mechanical design, and the factory performance test results, of an adaptive optics module for post-compensation of the quantum signal in a QKD ground station. The set-up is integrated in a compact and transportable “AO-box”. The AO-box has been designed to be directly mounted on the flange of an 80 cm fork mount telescope and delivers the fiber-coupled quantum signal at its output interface.

Free space optical (FSO) satellite communications has very attractive properties for Quantum Key Distribution (QKD). The quantum channel loss and noise are major factors in setting the maximum achievable secure key rate of QKD systems. Primarily for a LEO satellite node and the BB84 protocol a number of technologies are proposed to reduce the quantum channel loss and noise. These technologies apply to the receiver side of the QKD link, the Optical Ground Terminal. It is shown that an optical beam shaper to optimize fiber mode matching, dedicated thermo-mechanical design to reduce misalignment induced by temperature gradients and an Adaptive Optics (AO) system to counteract optical turbulence effects can result in a significant reduction of loss and background noise. A breadboard verification experiment shows that using an medium-size AO system can maintain a fiber coupling efficiency up to 40%. The developed technologies have a general applicability with respect to satellite orbit and QKD protocol used.

Quantum cryptography promises unconditionally secure communication. Today, there exists a vast array of different QKD protocols that claim to offer security. However, looking in more detail, many subtleties lead to different security levels, or in the worst-case to no security at all. Here, we introduce the most crucial QKD security properties, ranging from different attack schemes to actual implementation security considerations. We present three different QKD use-cases with different network topologies: one trusted-node-based scenario (BB84 decoy) and two trusted-node-free (entanglement-based BBM92 and twin-field QKD). Using our, in cooperation with the European Space Agency (ESA), in-house developed simulations package, we simulate all relevant performance parameters, including the expected finite secret-key-rate. Furthermore, we assess all simulated QKD protocols’ applicability to satellite-based QKD networks and identify essential technologies. Interestingly, not the single-photon detection modules are the key drivers in terms of secure-key-rate performance, but the optical sending and receiving telescopes.

Metis is a multi-wavelength coronagraph onboard the European Space Agency (ESA) Solar Orbiter mission. The instrument features an innovative instrument design conceived for simultaneously imaging the Sun's corona in the visible and ultraviolet range. The Metis visible channel employs broad-band, polarized imaging of the visible K-corona, while the UV one uses narrow-band imaging at the HI Ly 􀄮, i.e. 121.6 nm. During the commissioning different acquisitions and activities, performed with both the Metis channels, have been carried out with the aim to check the functioning and the performance of the instrument. In particular, specific observations of stars have been devised to assess the optical alignment of the telescope and to derive the instrument optical parameters such as focal length, PSF and possibly check the optical distortion and the vignetting function. In this paper, the preliminary results obtained for the PSF of both channels and the determination of the scale for the visible channel will be described and discussed. The in-flight obtained data will be compared to those obtained on-ground during the calibration campaign.

Metis coronagraph is one of the remote-sensing instruments of the Solar Orbiter mission launched in February 2020. The mission profile will allow for the first time the remote-sensing observation of the Sun from as close as 0.28 AU and from ecliptic latitudes as high as 30?. Metis, in particular, is aimed at the study and the overall characterization of the solar corona and solar wind. This instrument is an innovative inverted-occultation coronagraph that will image the solar corona for the first time simultaneously in two different wavelength band-passes: in the linearly-polarized visible-light (VL), between 580 and 640 nm, and in the ultraviolet (UV) Lyman-a line of hydrogen, HI at 121.6 nm by combining in the same telescope UV interference mirror coatings (Al/MgF2) and spectral bandpass filters. The visible channel includes a broad-band polarimeter to observe the linearly polarized component of the K corona. These measurements will allow a complete characterization of the physical parameters, such as density and outflow speed, of the two major plasma components of the corona and the solar wind: electrons (protons) and hydrogen. After a period of commissioning, by the summer of 2020, Metis will have performed the First-light Science Observations during the “Remote-Sensing Check-out Window” (RSCW) that is a telemetry contact period, specifically allocated before entering the operational phase at the end of 2021. This presentation will report the first-light science observations of Metis represented by the UV and polarized VL images of the corona. The calibration results from the commissioning will be used for the correction of the instrumental effects. The resulting first-light maps of the coronal electron and hydrogen distributions will be presented.

The Flexible Combined Imager (FCI), main instrument of the Meteosat Third Generation Imager (MTG-I) satellite, will image the on-Earth instrument swath with high image quality and highly stable line of sight. It is composed by a Three Mirror Anastigmatic (TMA) telescope assembly, followed by a Spectral Separation and Detection Assembly (SSDA) which has the function to separate the different spectral channels and focus them onto the detector. The Telescope assembly, developed by OHB System AG in Oberpfaffenhofen, will be the subject of this article, presenting first its optical design concept and its main characteristics. Thereafter, the FCI Telescope Assembly (FCI-TA) alignment will be presented, followed by the full test campaign accomplished to characterize the full optical performance of the FCI-TA. The telescope optical design is based on an off-axis field TMA telescope. It is composed by three aspheric off-axis mirrors, having a focal length of 1650 mm, an F/# of 5.5 and an entrance pupil diameter of 302 mm. A fourth, folding, mirror deflects the beam onto the TA focal plane, while in front of the first TMA mirror there is a Scan mirror, in order to let the telescope scan the whole Earth disc. In mid-2019 OHB ended the alignment of this first flight model of FCI-TA. We will show in this article the good results in terms of optical performance obtained from that alignment with a worst WFE of 63 nm and a LOS error of 7.8 arcsec.

This contribution presents the on-ground characterization and video chain development of the CMOS detector implemented in SPEXone, the five-angle space spectro-polarimeter for the NASA PACE observatory scheduled for launch in 2023. SPEXone is a Dutch compact payload contribution developed in a partnership between SRON and ADSN, and supported by TNO. Making use of spectral modulation, this polarimeter will enable in-depth and global characterization of the microphysical properties of fine particulate matter or aerosols in the atmosphere from low Earth orbit. In SPEXone, the spectrally modulated images are captured by means of a commercial-off-the-shelf detector module (DEM) from 3Dplus, which is equipped with a CMOS image sensor with integrated front-end-electronics. Video chain developments, including DEM firmware, read-out, flexible binning and DEM interfacing through SpaceWire have been carried out in-house. Making use of the firmware, the optimal detector parameters with associated random noise, full-well capacity, and photo response non-uniformity (PRNU) of the DEM were determined by placing the DEM in front of an integrating sphere fiber-fed with a stable white light source with accurately adjustable intensity and a highly linear reference detector, providing highly uniform illumination of the whole detector area at well-known relative light intensities. The rationale behind the measurement sequences is explained, and the full-well and read noise performance under different gain settings is described. The full-well capacity of the DEM is found to be not constant, but increasing significantly with illumination intensity.

The Gamma-Ray Module (GMOD) is a novel gamma-ray detector developed for the study of high energy astrophysical transients called Gamma-Ray Bursts. GMOD has been designed in-house and will be flown on board EIRSAT-1, intended to be Ireland’s first satellite, a 2U CubeSat developed as part of the European Space Agency’s Fly Your Satellite! programme. The detector comprises a 25×25×40mm CeBr₃ scintillator, coupled to a tiled array of 16 OnSemiconductor Silicon Photomultipliers with front-end readout provided by the IDE3380 SIPHRA. The readout is received by the GMOD Motherboard which provides temporary storage and support functionality for the instrument operation, including the transfer of Time-Tagged Event data to the EIRSAT-1 On Board Computer. The Engineering Qualification Model was environmentally tested following an approach tailored from the ECSS standards in early February 2020 at the CubeSat Support Facility in Transinne, Belgium. This campaign was conducted to qualify the hardware for low Earth orbit, including multi-axis vibration testing and thermal-vacuum cycling under qualification test levels and durations. GMOD was mounted on a 20kN electrodynamic shaker in which it underwent predefined sine and random vibration test profiles, demonstrating its ability to withstand the launch environment. The instrument was then thermally cycled under vacuum over a range spanning -31°C to +75°C, simulating the low Earth orbit environment. The results of the successful environmental qualification campaign of the Gamma-Ray Module from EIRSAT-1 are presented.

The ELFIS2 image sensor has a unique combination of properties, being true high dynamic range (HDR), charge domain global shutter (GS), backside illumination (BSI) and a TID and SE hard design. It is stitchable to a resolution of n*1k by m*1k pixels on a 15μm pitch.

Future space-borne synthetic aperture radar, hyper-spectral imaging, and high spatial and temporal resolution imaging will place increasing demands on space network data throughput. It is expected that space data relay systems will require capacities of multiple Terabits per second (Tbps). Free-space optical communication is likely the only technology capable of supporting these data demands. A previous study1 has shown that implementing these high-capacity optical feeder links through a traditional geostationary constellation will pose significant development challenges. Meshed satellite constellations in low earth orbit (LEO), operating at significantly shorter ranges, were shown to have the potential to support Tbps feeder links using the technical capabilities of current free-space optical communication systems. Meshed LEO constellations, however, provide unique challenges, including complex constellation maintenance, dynamic meshing and data routing, and short contact periods with ground stations. We review the design requirements for high capacity LEO relay optical terminals and satellites. The global coverage and ground station downlink opportunities of constellation options are evaluated to develop constellation designs utilizing the minimum number of satellites while providing required feeder link performance. The constellations are designed to provide mesh links with Tbps feeder link capabilities using optical terminals comprised of components within current state-of-the-art and with demonstrated capabilities. These meshed feeder link constellations were then modelled to demonstrate the ability of a LEO constellation to gather Tbps of data over a global network, relay this information around the constellation, and downlink this data to a limited number of ground stations.

To concurrently cope with the scarcity of RF frequency bands, the growing capacity demand and the required lower cost of the ground segment, Very High Throughput Satellites systems must rely on new technical solutions. Optical feeder links are considered as a promising alternative to surpass classical RF technology, offering assets inherent to optical technologies (large bandwidth, no frequency regulation, low beam divergence, components availability). Nevertheless the potential of this technology shall not conceal the remaining challenges to be overcome to make it relevant for operational missions : clouds, turbulence, power generation and high efficiency modulations. VERTIGO (Very High Throughput Satellite Ground Optical Link) is a 3-year H2020 project funded by the European commission and started mid-2019 focusing on the optical link itself regardless of site diversity aspect and aiming at demonstrating in a ground demonstration required technologies to implement very high capacity optical feeder links. In particular, VERTIGO is built on 3 pillars each addressing a key issue for the implementation of optical feerder links: 1) Throughput increase through the use of advanced schemes with high spectral and power efficiency compared to current modulations used in space, as well as RF-over-Fiber approach. 2) High optical power generation to close the demanding link budgets by developing on-board and ground means to raise the transmitted optical power, not only based on amplifier power increase, but also on incoherent/coherent power combining. 3) Opto-mechanical and digital techniques for the mitigation of atmospheric propagation impairments, to make full use of throughput and power increases. Several demonstrations in-flight or on-ground already demonstrated separately key aspects (atmospheric propagation and impairments mitigation techniques, modulation format, high power…), for the implementation of optical (feeder) links. These aspects are closely linked since the solutions to each of them are necessary but not sufficient to allow for high throughput transmissions. VERTIGO concept is to address each key issue with at least one solution and to combine them in an unprecedented manner. To reach these objectives, VERTIGO will lean on a highly skilled consortium composed of : CREONIC, ETH Zürich, Fraunhofer HHI, Gooch and Housego, Leo Space Photonics RD, ONERA, Thales Research and Technology, Thales Alenia Space in France and Switzerland. This paper will present the VERTIGO project and its status.

This paper describes the technical challenges and the experimental set-up for an optical feeder-link demonstration between ESA’s optical ground station in Tenerife, Spain, and the Alphasat satellite in geostationary orbit (25º East). In the absence of clouds, reliable optical feeder-uplinks require pre-distortion of the transmit beam to minimize the influence of the continuously changing atmospheric turbulence effect. Without beam pre-distortion, the quality of service (minimum impact of scintillations and outages) required for commercially viable feeder-links is hardly achievable. Power efficient feeder-links require the wave-front distortions of an optical downlink beam from a satellite to be inversely applied onto the optical feeder-uplink beam to the satellite with certain adaptations due to the difference in the uplink and downlink paths. The underlying assumption is that pre-distortion and atmospheric turbulence cancel each other out and the main goal of the planned experiment is to determine to which extent this is true. The closed-loop bandwidth of pre-distortions has to be higher than the atmospheric turbulence fluctuations and optical beam reception and transmission should use the same telescope aperture. The former requires a fast controller and the later poses stringent requirement on the stray-light performance of the set-up such that the powerful transmit beam does not blind the receiver. Avoidance of receiver blinding is a particularly difficult task when doing feeder-uplink tests with the coherent laser communication terminal on board the Alphasat satellite as will be explained. The paper will describe the design of the feeder-uplink system that will be installed in the OGS as well as initial measurements that have been performed to evaluate feasibility.

Space data highway networks are currently being deployed to provide high data rate connectivity to non-fibre-connected areas around the world and to prepare the incoming growth of Internet Of Things market. Among other solutions, GEO optical feeder links are seen as the backbone of the future globalized very high data rate satellite-based telecommunication networks. The FEEDELIO experiment developed for ESA in April 2019 demonstrated the relevance of angular decorrelation models to assess the statistical characteristics of the pre-compensated uplink irradiance that is deeply affected by anisoplanatism. Based on the conclusion drawn from this experiment, we present the results of a preliminary design study of an Adaptive Optics (AO) pre-compensated optical feeder links ground station. Exploiting end-to-end simulations under relevant turbulence conditions, time-correlated fading statistics are investigated to provide typical fading durations for AO pre-compensated optical channel, taking into account the impact of point-ahead mispointing errors.

We report on the visible LGS-AO experimental facility which we are building to be installed at the 1m ESA Optical Ground Station at Observatorio del Teide, Tenerife, Canary Islands. We focus on the system aspects related to optics. The instrument will be a novel facility to perform strategic LGS-AO technology R&D in future years, demonstrating a 50+ W CW 589nm laser, uplink laser beam pre-compensation on sodium LGS in pulsed laser operation. The rationale for the LGS-AO facility is to have a synergy between astronomical and space awareness adaptive optics, to technologies for astrophysics and optical communication with satellites (Optical Feeder Links), including daytime LGS-AO, for future OGS systems.

Variable filters, which central wavelength of the bandpass shifts along one dimension of the component, is a promising way to simplify and to miniaturize imaging spectrophotometer systems for spatial observation. In fact, by combining a dispersive element and a variable filter, it is possible to realize a spectral and a spatial filtering based on pushbroom technique. While there were numerous methods developed in order to produce variable filters, at this date, there is no simple, reliable and repeatable method. In this paper, we present a new method for the fabrication of variable filters based on plasma assisted reactive magnetron sputtering (PARMS). Variable filters are obtained upon the variation of the thickness of some or all the layers deposited on the substrate. The layers are deposited using a Bühler HELIOS machine in order to benefit from the high stability of the deposition process. To insure a non-uniformity of the deposited layers, adapted masks are placed in front of the targets for the low and high refractive index materials. The uniformity of these layers is measured via a custom set-up allowing a local measurement of the transmission to perform a mapping of the transmission over the whole component aperture. In this paper, we present the fabrication of layers and filters with thickness. Various types of thickness gradients are presented.

Due to the diversity and complexity of optical functions they address, optical coatings are widely used in space optics but the level of both their specifications and performances is now so high that the characterization of their optical properties requires the development of extreme metrology. In this context we present in this paper the last state-of-the-art instrument for thin-films characterization. This apparatus is unique by the performances and the levels of detectivity achieved. It operates spectrally and angularly resolved characterization of Angle Resolved Scattering (ARS) over a wide and continuous spectral range from 400 nm to 1.7 μm. The spectral resolution is lower than 1 nm, while the angular resolution is lower than 1° with a precision better than 1%. The lowest levels of scattering measured are close to 10^-8 sr^-1, near to the limit of air particles scattering. The large angular range available offers the opportunity to evaluate scattering functions both in the forward and backward half-spaces. In specific configurations, this same setup records specular properties, achieving again unreached levels of detection and precision over the same spectral range and with the same resolution. All measurements have shown great agreement with theoretical designs and numerical predictions, for both scattering and transmittance records. This instrument is an upgraded version of an apparatus developed in the Scattering group for more than five years formerly operating in the visible wavelength spectrum and now extended to near-infrared with the same level of performances.

We manufactured and tested ion beam sputtered antireflection coatings for 355 nm using several defect mitigation strategies to improve the damage resistance of laser optics for LIDAR satellite missions. The first approach to avoid the damage precursors in the film is based on a secondary ion source. The goal is to remove the particles that settle on the surface of the growing film. A second strategy is to avoid the damage precursors in the ultraviolet wavelength range by laser conditioning of the thin films during the coating process. A laser beam at the application wavelength scans the deposited layer on the sample during deposition still in vacuum. Optical coatings produced with both technologies show a significantly increased damage threshold when tested in ramped raster scan tests.

ELOIS VISWIR is a science-grade, smallsat compatible, pushbroom hyperspectral payload covering the entire VISWIR spectral range (400-2450 nm) with over 200 contiguous 10 nm wide spectral bands. Its unique all reflective optical design enables covering a 70 km swath with 35 m ground sampling distance (GSD), while guaranteeing high imaging performances across the full field of view. At the core of the instrument, the spectrometer formula expands upon AMOS’ state of the art Offner-based ELOIS VNIR long-slit spectrometer [1], including key features such as built-in slit de-magnification, and the use of a fine-tuned freeform diffraction grating enabling the appropriate compensation of the optical aberrations affecting the fast, F/2.1 focal plane. Besides its excellent imaging performances and advanced control of distortions, the ELOIS VISWIR delivers unparalleled throughput in a single optical chain thanks to the inclusion of leading-edge technical solutions including a broadband multi-blazed freeform diffraction grating, a variable dichroic beam-splitter plate, and high efficiency order sorting filters. In this paper, we discuss these key ingredients in the optical design of the ELOIS VISWIR spectrometer and report the optical performances of the manufactured optical components.

During the last decade, meteorology domain has lived many breakthroughs. In the field of remote sensing from space, one of the most significant advancement is in the use of Thermal Infrared sounders. Thales Alenia Space delivered sounders flagship METOP/IASI, resulting in the enhancement of meteorological events forecast and unprecedented 3D atmospheric knowledge. The next generation of European geostationary meteorological satellites, Meteosat Third Generation is currently being realized through the well-established and successful Cooperation between EUMETSAT and ESA with Thales Alenia Space being overall MTG prime contractor. MTG encompasses two kinds of satellites: an imaging mission MTG-I with the FCI imaging payload derived from the MSG-SEVIRI with a payload complement of a lightning detection system (LI as the Lightning Imager) and a sounding mission MTG-S with the payload complement including the thermal infrared sounding payload IRS (full disc coverage) and the Copernicus payload UVN/S4, a spectrometer in UV/VIS/NIR bands to retrieve air quality in Europe. Stemming from IASI, Thales Alenia Space is now developing the IRS interferometer as well as the detection and onboard processing chain. These key elements will be integrated by OHB which is the IRS instrument as well as MTG-S satellite Prime Contractor. Reaching performance from a geostationary orbit is a two-order of magnitude more stringent challenge compared to a LEO instrument. Thales Alenia Space did elaborate new and innovative technology to ensure sub nanometric interferometer metrology and thus to achieve real time on-board corrections. Two essential validation milestones have been achieved in 2019. • The interferometer engineering model was thoroughly tested successfully, especially in terms of spectral performance vs. microvibrations. • Thales Alenia Space has also tested the IRS “core spectrometer” engineering model, allowing to test the full spectrometer chain of the instrument by synchronizing the interferometer itself with the two 160x160 pixel detector arrays of the detection chain and the on-board data processing unit that is able to process the 2Gbit/s raw data flow generated by the instrument. The paper will focus on the two major outcomes of this test campaign: • The demonstration of the functionality of IRS new 3D metrology system, its associated processing and its benefits. • An overview of performances achieved during those two intensive test campaigns. In particular Spectral Response Function (SRF) knowledge/prediction, which is essential to support an unprecedented interpretation of the spectra and final products retrieval accuracy.

In the context of the creation of a Distributed Space Weather Sensor System (D3S), the European Space Agency (ESA) is planning to fly a small payload for the monitoring of the auroral emissions with the project Wide Field Auroral Imager (WFAI). In order to meet ESA's requirements for an Auroral Optical Spectral Imager (AOSI), CSUG will build a breadboard as a proof of concept based on Imager Spectrometer on Chip (ImSPOC) technology. Imaging Spectrometer on Chip (ImSPOC) technology consists of a matrix composed by multiple, low finesse, Fabry-Perot modules mounted in one unique structure. Each of these Fabry-Perot has a fixed optical path difference distributed between optical contact up to maximum elevation in order to have the desired spectral resolution and the spectral coverage. So, in one snapshot it is possible to ""scan"" several optical path differences and reconstruct the spectrum for each point of the field of view. The advantage of this technology is to reduce the mass and dimensions but also that it collects sufficient number of photons allowing the observation of the faintest aurora (Non visible with naked eye) with short integration time. This makes it a particularly well-suited technology for small satellite payload design in the framework of polar lights monitoring. We present here the optimization of the ImSPOC technology in the context of WFAI mission. This optimization allows us to recorded polar lights spectra in the wavelength range of 350 to 950 nm within an exposure between 10 and 60 s. The field of view of the instruments is 56.7° × 56.7°, obtained by combination of 9 individual modules with small dimension (one module fits in a 60 × 45 × 55 mm3 box). The simulated performances of the instrument are really encouraging. The spectrometer must have a spectral resolution of 1 nm at 350 nm.

On the 30th of July 2020, NASA launched the Mars2020 mission. This mission, very similar to Mars Science Laboratory, consists in landing the Perseverance rover on the Martian surface in order to characterize the geology and history of Jezero Crater landing site, investigate Mars habitability, seek potential bio-signatures, cache samples for a future return to Earth, and demonstrate in-situ production of oxygen needed for human exploration. The SuperCam instrument, an improved version of the ChemCam instrument on Curiosity rover, implements a remote micro-scale characterization of the mineralogy and elemental chemistry of the Mars surface, along with the search for extant organic materials. In addition to the elemental characterization offered by Laser Induced Breakdown Spectroscopy (LIBS), Time-Resolved Raman and Luminescence Spectroscopy (TRR/L) and visible-infrared spectroscopy (VISIR) have been added for a complete mineralogical characterization of the samples. LIBS and TRR/L techniques will be exercised from the Rover calibration targets (1.5 m range) up to 7 m, whereas VISIR spectroscopy can be used on targets up to the horizon. A context color imagery capability is also implemented to place the analyzed samples in their geological context. A microphone allows recording aeolian phenomena, rover noises and the shock waves produced by the laser blasts on target up to 4 meters. SuperCam consists of three units: the Body Unit built by the Los Alamos National Laboratory in the US, the Mast Unit built by a French consortium of 6 laboratories and CNES, and the Calibration Target Unit led by the University of Valladolid in Spain.

In this paper, we report on the on-going development of a compact analytical instrument for future missions to the Moon, using the LIBS technique. The instrument, christened VOILA (Volatiles Identification by Laser Ablation), is part of the “LUVMI-X” scenario funded by the European Union’s Horizon 2020 programme which envisions the concept of a rover carrying a suite of instruments for detecting and characterizing volatiles as a component of lunar soil (“regolith”) at high latitudes on the Moon. Behaviour of the plasma created by LIBS target ablation depends on the prevalent atmospheric pressure. Mars atmospheric pressure is close to ideal for the LIBS technique. On bodies without atmosphere such as the Moon, the lack of confining pressure leads to the plasma dissipating quickly causing weaker signals. India has developed the first, modest LIBS instrument for operation on the Moon with however a number of limitations in its optical design. Moreover, the lander of the corresponding mission Chandrayaan-2 crashed during landing in 2019. Our VOILA is designed to be between the quite capable but large and massive Mars LIBS instruments of NASA (ChemCam on the CURIOSITY ANCE rover), and the very modest Indian LIBS for Chandrayaan- 2. The VOILA working range will vary between 0.3 and 0.5 m, with a laser pulse energy of ~15 mJ.

The global representation of aerosol, clouds, and their impact on the energy budget remains a major source of uncertainties in global models for climate predictions, but also strong weather events in numerical weather prediction. Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) mission, the sixth Earth Explorer Mission of the European Space Agency (ESA) Living Planet Programme, shall improve the understanding by three dimensional observations of global profiles of cloud, aerosol, precipitation and associated radiative properties. A major contribution will be provided by the ATmospheric LIDAR (Light Detection and Ranging), ATLID, being developed and tested by Airbus Defence and Space S.A.S in Toulouse. ATLID is a High Spectral Resolution-Polarization LIDAR and will provide vertical profiles of optically thin cloud and aerosol layers, the aerosol type, as well as the altitude of cloud boundaries. After the development, qualification and delivery of all units, ATLID integration has finally been completed and the self-standing instrument was successfully tested to demonstrate its performance under ambient and space environment conditions, in Toulouse and CSL/Liège respectively. All preliminary test results obtained up to now are very promising and indicate ATLID’s capability and compliance to all performance goals and specifications. The instrument has been now delivered, to Airbus Defence and Space GmbH Immenstaad, and integrated into the EarthCARE platform. Results of ATLID ambient test campaign, EMC, mechanical test campaign, and thermal balance test campaign, along with final performance and calibration test results shall be presented in this paper.

We present recent progress in developing miniaturized optical transmitters and receiver amplifiers for space communications. Three C-band high-speed optical transmitter designs are presented: a bespoke 300 mW version as part of TNO’s “SmallCat” terminal to fly on-board NordSat and two variants that provide 300 mW and 3 W of optical power complying to standard cubesat form factors. In addition to these transmitters, an ultra-small form factor, high gain, low noise amplifier, for boosting received signals is presented.

H2020-SPACE-ORIONAS is a 3-year Research and Innovation Action program funded by the European Commission focusing on the development of compact optical transceiver and amplifier modules applicable to new generation optical inter-satellite links. ORIONAS explores photonic integrated circuits and small form factor fiber optics leveraging their success in datacenter interconnect and hi-rel aerospace applications to deliver miniaturized modules and devices that can shrink considerably the SWaP of lasercom terminals. This paper presents the most recent project achievements.

An adaptive optical satellite network (AOSN) for next-generation optical satellite network systems is proposed. To decrease operational cost of complicated network systems, the AOSN can monitor status of satellite optical links and modify the link parameters of laser-communication terminals (LCTs) automatically. To realize the AOSN, we utilize adaptive optical transmitters, digital coherent optical receivers and software defined network (SDN) and incorporate them into a conventional concept of adaptive optical networks. The AOSN can realize the adaptive control of link parameters for a variety of satellite optical links with the utilization of artificial intelligence (AI), machine learning and operation policy.

Molecular adhesion is a well-known process used on terrestrial optics and other fields. This process consists in joining two surfaces without the use of any adhesive or additional material. Molecular adhesion is a high-precision production process, and assemblies obtained present a dimensional stability due to the absence of mechanical part or glue. In addition, since no adhesive materials are used in this process, the risks of contamination associated with degassing are avoided. Based on this benefits, this process is of particular interest for optical system manufacturing for spatial applications. For over the past 20 years, Thales SESO develop its own process of molecular adhesion. For example among others, it has been applied on the manufacturing of Fabry-Perot interferometers. Such as fused silica Fabry-Perot for ALADIN program in the frame of AEOLUS mission (launched in August 2018) or fused silica/Zerodur® Fabry-Perot for ATLID program. Both interferometer have under vacuum cavity. On the ATLID Fabry-Perot the sealing is also made using molecular adhesion. Back then, the molecular adhesion mechanical strength admissible was around 1 MPa. Then, Thales SESO improve its molecular adhesion process - through lots of collaborations with CNES and other partners – in order to be able to bond more complex designs and increase the mechanical resistance. Today the molecular adhesion mechanical strength admissible has been multiplied by a factor five. Those improvement have been applied in the frame of the Third Generation Meteorological satellite (MTG-IRS) for the manufacturing of corner cubes in accordance with the mechanical and thermal environments of a geostationary orbit coupled with very strict optical performances. Today, several corner cubes have been manufactured, and the strength of adhered bond have been demonstrated in terms of thermal, shock and random solicitations through mechanical qualifications. The major advantage of molecular adhesion for the corner cube application is the stability of the WFE under vacuum – which have been also demonstrated. Here, an overview of the Thales SESO realizations starting from Fabry-Perot first results to the latest results on corner cubes performances as well as the future of molecular adhesion for space applications imagined by Thales SESO.

The Multi-viewing, Multi-channel, Multi-polarisation Imager (3MI) is one of the instruments of the “Satellite A” payload of MetOp-SG, developed to provide information on atmospheric aerosols. 3MI is a space based, wide-field-ofview spectroradiometer that is designed to acquire sequential images of the same ground target which are combined with multiple spectral views in both un-polarized and polarized channels. The spectral coverage in the multiple bands from 410 nm to 910 nm and from 910 nm to 2130 nm shall be done using a Filter Wheel Assembly which included the Filter Wheel Disk (FWD). OBJ was selected for development and production of these optical elements. This contribution addresses the manufacturing and characterization of the demanding dielectric optical coatings for the sophisticated BPFs developed by Optics Balzers.

In the present article, complex optical UV bandpass filters with tantalum pentoxide and zirconium dioxide as high refractive index materials, respectively, have been manufactured on the EOSS® sputtering system. Four different bandpass filters with central wavelengths between 316 nm and 399 nm and a blocking up to 1100 nm were realized. Transmittance of the filters was higher than 80% although the band pass was located close to the absorption edge of the high index material. For two of the filters, ZrO₂ was used as the high index material. It is demonstrated that the ZrO₂ containing filters have very good optical properties and ZrO₂ is a good alternative to the use of the costly HfO₂. The bandpass filters will be used as order-sorting filters for the grating spectrograph of the Sunrise UV Spectropolarimeter and Imager (SUSI) onboard the balloon-borne solar observatory Sunrise III. The filters are designed to filter out unwanted light that hits the detectors. The observatory will be launched in June 2022. Among other topics, the mission is dedicated to the investigation of magnetic fields and convective plasma flows in the lower solar atmosphere.

Gold shows a very high reflectivity in the IR range. In addition, Au (and protected Au) is more robust than Ag (and protected Ag). Therefore, Au based coatings are of high interest. Common techniques for the deposition of optical Au coatings are sputtering and evaporation. In this contribution, both techniques, sputtering and evaporation, as well as unprotected and protected Au will be considered. A comparison of reflectivity between sputtered and evaporated Au-layers shows a slightly higher reflectivity for evaporated Au. Beside reflectivity after coating, decrease of reflectivity due to interdiffusion at increased temperatures (250°C) between adhesion layer and the reflective Au-layer is considered. In case of space-based applications, interdiffusion in thin film coatings could be activated due to particles of lower energies. This phenomenon is not necessary tested by radiation tests, performed by applying particles of higher energy. By a sputterd TiOX adhesion layer underneath a protected Au-coating (protected by an Al2O3-Si3N4-laminate), resistance against interdiffusion, and the successful passing of radiation-, cleanability- and abrasion tests could be achieved. This high reflective coating (reflectivity of 98 % at 1064 nm) was applied to the different mirrors of the GALA-instrument.

Able to monitor very precisely CO2 gases concentration in the atmosphere (better than 1 ppm), MicroCarb is a CNES mission which instrument is a passive Short Wave InfraRed spectrometer that will help scientists to better understand the planet’s major ecosystems and gain a clearer picture of its carbon budget at regional scales. Reaching such a monitoring precision means that the instrument has very stringent requirements especially in terms of radiometry, spectral filtering, spectral bands co-registration and polarization sensitivity. The MicroCarb instrument is currently under integration at Airbus Defence and Space facilities in Toulouse (France). It is based on an innovative concept permitting the acquisition of the spectral bands using a single split-pupil telescope, spectrometer and detector. Spectral bands multiplexing is obtained by the grating, each band corresponding to a different diffraction order, between 15 and 40. The split-pupil concept is characterized by 3 sets of 4 to 5 assembled prisms at entrance telescope pupil, spectrometer slit and in front of the detector, all participating to the complex spectral filtering. All the components considered as critical for the performances of the instrument have been manufactured and are under integration: the echelle grating replicated in a SiC substrate, the assembled optical elements with narrow spectral filtering coatings and the Dual Babinet polarization scrambler. In this paper we will focus on these optical components, detailing for each one how the stringent requirements of the instrument have been transposed to their specification, close to the limits of manufacturability. We will be able to present their development and achievement with respect to these specifications and how they impact the global performances of the instrument.

Off-axis systems with freeform mirrors is a design approach of increasing importance for space instruments. While the off-axis reflective approach allows simple, versatile and obstruction free designs, the use of freeform surfaces allows to achieve better optical performance and/or compactness. In recent years, many such instruments have been designed and manufactured at TNO. Some important examples are the Tropomi (S5/precursor) and TSBOA (Sentinel-5) telescopes and several pushbroom spectrometers of the Spectrolite family. Despite the recognized potential of these systems, there is to our knowledge no available theory that allows to describe and predict the aberrations of plane symmetrical (off-axis) systems with freeform mirrors. In this context, an effort has been started 4 years ago at TNO to develop an approach that can describe and explore systematically off-axis freeform mirror systems with an arbitrary geometry. Since then the developed theory has been proven to be very useful in scanning the solution space for better starting designs, folding geometries and correction of lower aberrations in an early design phase. To circumvent difficulties linked to a wavefront formalism, generalized ray-tracing equations were derived, that include aberration terms up to 3rd order in X/Y object and pupil coordinates. These equations were recently published in two papers for the case of pure mirror systems [1,2]. The theory was also expanded to describe flat reflective gratings, opening a way to a complete description of reflective freeform spectrometers. In the present paper, after providing a high-level description and introduction to the aberration theory for freeform mirror systems, we will report some of its outcomes that have some practical relevance for space instruments. In particular: - two-mirror telescope designs for slit spectrometers (thus having a 1D field along the slit) that are inherently corrected for spatial smile will be presented. For these designs the slit projection in object space is exactly straight. - a new family of mirror spectrometers will be introduced that uses flat gratings and no collimator. In a collimator-less spectrometer, the aberrations induced by the grating under diverging light are corrected with freeform mirrors. The presented designs are entirely calculated from theory, with only optimization of the higher order freeform terms. A simpler architecture than traditional designs is obtained, with less optical surfaces. - finally we present a systematic classification of distortions in imaging slit spectrometers. The difference between distortions originating in the collimator and at the grating or in the imager is clarified and described mathematically. We discuss aberration-induced non-linear dispersion, as well as distortions from the keystone and smile families. The proposed classification also applies to catadioptric and refractive systems, the only requirement being to have a plane of symmetry.

The Back Telescope Assembly (BTA) is one of the optical sub-systems composing the IRS instrument. It consists in an afocal off-axis four-mirror telescope whose primary function is to transfer the optical beam between the interferometer and cold detector units. The design, manufacturing and AIT activities are performed under the responsibility of AMOS (Liege, Belgium) with OHB System (Wessling, Germany) as the prime contractor of the IRS instrument. In order to meet the challenging performance requirements, an innovative telescope design with four-mirror anastigmat optical configuration has been developed, using a full aluminum solution, i.e. mirrors and structure. It is manufactured and aligned under ambient conditions while it is operated close to 0°C in a non-uniform thermal environment. The aluminum structure of the BTA supports the 4 mirrors and is interfaced via 6 Titanium needles to the CFRP optical bench of the IRS instrument. The aluminum mirrors and the BTA structure are realized by the single-point diamond turning technique in AMOS manufacturing facilities. In view of its qualification, the BTA models underwent an extensive testing campaign including: thermal-vacuum cycles, mechanical loads testing and model correlation, thermal balance and model correlation, measurement of the optical performances (wavefront error, magnification, optical axis orientation including its stability and pupil position) under ambient and operational conditions. The results of the tests allow to demonstrate the compliance of the BTA to the requirements.

We report on the current design and preliminary developments of the airborne Lidar Emitter and Multi-species greenhouse gases Observation iNstrument (LEMON), which is aiming at probing H₂O and its isotope HDO at 1982 nm, CO₂ at 2051 nm, and potentially CH₄ at 2290 nm, with the Differential Absorption Lidar method (DIAL). The infrared emitter is based on the combination of two Nested Cavity OPOs (NesCOPOs) with a single optical parametric amplifier (OPA) line for high-energy pulse generation. This configuration is enabled by the use of high-aperture periodically poled KTP crystals (PPKTP), which provide efficient amplification in the spectral range of interest around 2 μm with slight temperature adjustments. The parametric stages are pumped with a Nd:YAG laser providing 200 mJ nanosecond double pulses at 75 Hz. According to parametric conversion simulations supported by current laboratory experiments, output energies in the 40 - 50 mJ range are expected in the extracted signal beam whilst maintaining a good beam quality (M² < 2). The ruler for all the optical frequencies involved in the system is planned to be provided by a GPS referenced frequency comb with large mode spacing (1 GHz) against which the emitter output pulses can be heterodyned. The frequency precision measurement is expected to be better than 200 kHz for the optical frequencies of interest. The presentation will give an overview of the key elements of design and of preliminary experimental characterizations of sub-systems building blocks.

We report on recent advances at ONERA on fiber-based laser and lidar systems, in the perspective of future space-borne monitoring of greenhouse gas, especially CO₂ and CH₄. We first present an advancement status of research activities on high-peak-power single-frequency fiber lasers at 2.05 μm and 1.58 μm for CO₂ monitoring, and at 1.64 μm for CH₄, as well as perspectives for future amplification and frequency-stabilization of these lasers. The second parts deals with activities led on electro-optic dual comb spectroscopy (EO-DCS), which could be a potential future alternative technique to conventional space-borne IPDA lidars (Integrated Path Differential Absorption). We report on an EO-DCS experiment performed in pulsed regime, using an Er-Yb-doped amplifier delivering up to 92 μJ, 2μs pulses at 10 kHz repetition rate. An absorption line of HCN is sampled simultaneously by a 13-teeth frequency comb, and its transmission is measured with a good agreement with respect to the expectation.

We are developing a DIfferential Absorption Lidar (DIAL) for CO₂ remote sensing using coherent detection, based on a pulsed hybrid laser emitter at 2.05 μm. In the 2 μm region, the R30 CO₂ absorption line has been identified as one of the most promising for space-borne DIAL instrument to provide precise sounding of the atmospheric boundary layer where CO₂ sources and sinks are located. In this paper, we present the Tm-doped all-fiber part of the emitter and we demonstrate its ability to measure the CO₂ Volume Mixing Ratio (VMR) in the atmosphere. The lidar setup is presented, including the telescope, the heterodyne detection system and a spectral reference system used to infer the absolute frequency of the laser and the ON-line/OFF-line energy difference. We investigate an Integrated-Path-DIAL measurement on a range of 426 m. Results compare well with an in-situ reference sensor (Picarro probe), and the estimate statistics show good agreement with theoretical error calculations. This lidar demonstrator has been called HELENA (Hybrid Emitter Lidar for ENvironmetal Applications). Once completed, it will feature an hybrid emitter combining a semiconductor laser, a high-power Tm-doped fiber amplifier stage, and a final Ho:YLF crystal single-pass amplification stage. This combination aims at benefitting of the versatility, compactness and alignment easiness of a fiber amplifier on one hand, and of the high-peak power attainable with a solid-state amplifier on the other hand. It could allow overcoming the traditional Brillouin limit met in fiber amplifiers, without sacrificing much in compactness and robustness.

A new era of manned lunar exploration is dawning. Optical communication technology is expected to become indispensable for high-speed data transmission from lunar explorers to the Earth. This paper describes the expectations for contents from the Moon and the estimated amount of information. Then, two types of Lunar-Earth optical communication systems are discussed, including a comparison between direct communication with an optical ground station (OGS) and communication through a data relay satellite in geostationary orbit (GEO) around the Earth.

MPB has developed a 10W Polarization Maintaining Optical Fiber amplifier (1550 nm) for space applications. The prototype is based on three stages of optical amplification with photodiodes at each stage, monitoring the output power. It includes the control electronics and software with feedback loops to dynamically control and monitor the amplifier. The design had to overcome many challenges to comply with the mechanical, thermal, radiation, and vacuum requirements for the LEO satellite space environment, while at the same time meeting the price targets for LEO constellations by maximizing the use of commercial off the shelf (COTS) components. The following were the main challenges: a) to effectively dissipate the heat generated (75-90 W); b) to select radiationtolerant electronics to drive the needed electrical current; c) to source and effectively implement components, such as the combiners and isolators, in the high power optical path compatible with vacuum at 10W output. The major challenge with regard to heat management was to find an optimal method to dissipate the heat from the third stage (high power) Erbium Ytterbium Doped Fiber. Commonly, this fiber is spooled on an Aluminium spool. The difference in the Constant Temperature Extension (CTE) between the fiber (low) and Aluminium (high) leads to a detachment of the fiber at low temperature with a high risk of breaking the fiber when passing from OFF to ON. At high temperatures, the Aluminium extends much more than the fiber, leading to an over tension on the fiber with a high risk of mechanical breakage. Different designs of the spool, supports inside the box, selection of materials, and process implementations were tried. An innovative, proprietary method was developed to satisfy this requirement. The unit successfully passed performance testing between -20°C and +40°C in vacuum with 10W output, with a wall plug efficiency of 11%. The lower temperature limitation was due to the specification of the high-power laser diodes. The higher temperature was limited by the local heating and risk of mechanical breaking of the third-stage COTS combiner and isolator. Vibration and mechanical shock are not foreseen to be an issue. The simulation demonstrated the prototype is complying with these requirements. Moreover, MPB has built similar instruments at lower power levels that have successfully passed these qualification tests. The components used were available as COTS products, including the radiation-tolerant electronics. All the components were qualified individually for >30 krad, in vacuum, and for the temperature range -35°C to +65°C except for the highpower laser diodes which were limited to -25°C. MPBC is continuing the qualification, implementing minor design changes, in order to satisfy the complete temperature range (-35°C to +65°C).

Athena, the largest space-based x-ray telescope to be flown by the European Space Agency, uses a revolutionary new modular technology to assemble its 2.6 m diameter lens. The lens will consist of several hundreds of smaller x-ray lenslets, called mirror modules, which each consist of about 70 mirror pairs. Those mirror modules are arranged in circles in a large optics structure and will focus x-ray photons with an energy of 0.5 to 10 keV at a distance of 12 m onto the detectors of Athena. The point-spread function (PSF) of the optic shall achieve a half-energy width (HEW) of 5” at an energy of 1 keV, with an effective area of about 1.4 m2, corresponding to several hundred m2 of super-polished mirrors with a roughness of about 0.3 nm and a thickness of only 150 µm. Silicon Pore Optics (SPO), using the highest grade double-side polished 300 mm wafers commercially available, have been invented to enable such telescopes. SPO allows the cost-effective production of high-resolution, large area, x-ray optics, by using all the advantages that mono-crystalline silicon and the mass production processes of the semi-conductor industry provide. SPO has also shown to be a versatile technology that can be further developed for gamma-ray optics, medical applications and for material research. This paper will present the status of the technology and of the mass production capabilities, show latest performance results and discuss the next steps in the development.

The Athena mission, under study and preparation by ESA as its second Large-class science mission, requires the largest X-ray optics ever flown, building on a novel optics technology based on mono crystalline silicon. Referred to as Silicon Pore Optics technology (SPO), the optics is highly modular and benefits from technology spin-in from the semiconductor industry. The telescope aperture of about 2.5 meters is populated by around 700 mirror modules, accurately co-aligned to produce a common focus. The development of the SPO technology is a joint effort by European industrial and research entities, working together to address the challenges to demonstrate the imaging performance, robustness and efficient series production of the Athena optics. A technology development plan was established and is being regularly updated to reflect the latest developments, and is fully funded by the ESA technology development programmes. An industrial consortium was formed to ensure coherence of the individual technology development activities. The SPO technology uses precision machined mirror plates produced using the latest generation top quality 12 inch silicon wafers, which are assembled into rugged stacks. The surfaces of the mirror plates and the integral support structure is such, that no glue is required to join the individual mirror plates. Once accurately aligned with respect to each other, the surfaces of the mirror plates merge in a physical bonding process. The resultant SPO mirror modules are therefore very accurate and stable and can sustain the harsh conditions encountered during launch and are able to tolerate the space environment expected during operations. The accommodation of the Athena telescope is also innovative, relying on a hexapod mechanism to align the optics to the selected detector instruments located in the focal plane. System studies are complemented by dedicated technology development activities to demonstrate the capabilities before the adoption of the Athena mission.

The optics of ATHENA (Advanced Telescope for High-ENergy Astrophysics) consists of several hundreds of Silicon Pore Optics mirror modules integrated and co-aligned onto a Mirror Assembly Module (MAM). The selected integration process exploits an optical bench to capture the focal plane image of each mirror module when illuminated by an UV plane wave at 218 nm. Each mirror module focuses the collimated beam onto a CCD camera placed at the 12 m focal position of the ATHENA telescope and the acquired point spread function is processed in real time to calculate the centroid position and intensity parameters. This information is used to guide the robot-assisted alignment sequence of the mirror modules. The ATHENA Assembly Integration and Testing (AIT) facility has been designed and is now under construction. It consists of a vertical tower, in which clean room conditions are maintained. Inside the tower, the MAM is supported at ground level on a gravity release system and a robot device above the MAM is used for alignment of the SPO Mirror modules. A paraboloid mirror that collects the light from an ultraviolet point source and generates a single reference plane wave large enough to illuminate the 2.6 m aperture of the X-ray telescope is placed 6 m below the MAM, whereas a CCD camera for the detection of the focused beam is placed at the top of the tower, 12 m above the MAM.

The ALBA synchrotron (Barcelona, Spain) is building MINERVA a new X-ray beamline designed to support the development of the ATHENA mission (Advanced Telescope for High Energy Astrophysics). The beamline design is originally based on the monochromatic pencil beam XPBF 2.0 at the Physikalisch-Technische Bundesanstalt (PTB), at BESSY II. MINERVA will provide metrology capabilities to integrate stacks produced by cosine company into a mirror module (MM) and characterize them. It will provide photons with a fixed energy of 1.0 keV with a residual divergence below 1 × 1 arcsec² rms. The beam dimensions at the mirror module is adjustable from 10 × 10 μm² up to 8 × 8 mm². Interoperability between MINERVA and XPBF 2.0 will be preserved in order to reinforce and boost the production and characterization of the mirror modules. MINERVA is funded by the European Space Agency (ESA) and the Spanish Ministry of Science and Innovation. Still in the detailed design phase, MINERVA will take 2 years to be completed for operation in 2022.

The European Space Agency ATHENA mission is an x-ray observatory that will study the formation of galaxy clusters and the growth of black holes. Due for launch in early 2030s, with adoption planned for 2021, ATHENA will use silicon pore optic mirror modules to create a 2.6 m diameter x-ray mirror. The PANTER x-ray test facility has over 40 years of experience in testing and calibrating x-ray optics. PANTER plays a key part in testing developments of silicon pore optics, manufactured by Cosine Measurement Systems, for the ATHENA mission. This year, as part of these developments, PANTER has measured an x-ray optic unit comprising two mandrels, manufactured by Zeiss. This optic was produced as a standard against which the performance of both silicon pore optics and the PANTER facility setup for silicon pore optics was measured. The first confocal mirror module measured at PANTER was entered into an environmental program, to determine whether particulate contamination causes loss of effective area. The contamination and the pre- and post-contamination effective area measurements were carried out at PANTER. The paper will provide an overview of the most recent testing carried out at PANTER, and the performance of the ATHENA optics tested to date.

Significant progress has been made over the past years in the understanding of Earth’s climate; however, there are still uncertainties in the modelling of atmospheric radiative processes. The main radiative processes relevant for climatological models are related to atmospheric water vapour, clouds, and surface emissivity across the full thermal infrared spectrum. Whilst the mid-infrared outgoing radiation is currently measured from space (e.g. IASI), there is a lack of spectrally resolved measurements over a significant portion of Earth’s thermal emission in the Far-InfraRed (FIR) i.e. between 100 and 667 cm-1 (15-100 micron). FORUM (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected as the ESA’s ninth Earth Explorer mission in September 2019. FORUM aims at measuring the Earth’s Top-Of-Atmosphere emission spectrum from 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). Two independent instrument concepts, based on a Fourier Transform Spectrometer, have been thoroughly studied in Phase A preparatory activities, including the development of a breadboard of the interferometer. In addition, critical technologies have been identified and subjected to comprehensive breadboard studies in order to mitigate development risks and to raise the Technology Readiness Level (TRL). This has led to the development and characterisation of several beamsplitter candidates, the measurement of the emissivity of the black coating for the on-board blackbody, the verification of noise and responsivity performance of pyroelectric detectors and the characterisation of the interferometer mechanism.

In 2019, the FORUM mission (Far-infrared Outgoing Radiation Understanding and Monitoring) has been selected by ESA as Earth Explorer 9 mission. FORUM aims to record the emission spectrum of the Earth’s stratosphere and troposphere in the spectral range of 100 to 1600 cm^-1 (i.e. 6.25μm to 100 μm). It will measure the spectral features of the far-infrared contribution to the Earth radiation budget with the focus on the water vapour contribution, the cirrus cloud properties, and the ice/snow surface emissivity. FORUM’s primary optical instrument is a non-imaging Fourier Transform Spectrometer (FTS) scanning the Earth atmosphere in a step-and-stare acquisition mode sounding a 15 km ground sampling with a ground sampling distance of 100 km. A consortium led by TAS-UK as mission and platform prime and OHB as instrument prime are currently developing the satellite and payload together with Leonardo Italy and MICOS during an ESA phase A / B1 study. This consortium is one of the 2 independent consortia selected for FORUM’s phase A/B1. The paper will describe the design concept of the FORUM FTS instrument with main focus on the optical design. The key optical subsystems are the pointing unit and a double pendulum interferometer with an entrance three-mirror-anastigmatic telescope. The prediction of the performance with respect to threshold and goal requirements will be presented. Furthermore, the status of the beam splitter pre-development will be reported, in particular the development of an optical element based on a CVD-diamond substrate and a technology to manufacture broad band anti-reflective microstructures on a diamond surface. The microstructures are designed to suppress parasitic reflections in the spectral range between 6.25μm and 100μm. The highest efficiency occurs between 13 and 30μm, where the Fresnel reflection is reduced from 17% down to below 3.5%.

Four compact planetary ultraviolet spectrographs have been built by Southwest Research Institute and successfully operated on different planetary missions: Rosetta-Alice, New Horizons-Alice, LRO-LAMP, and Juno-UVS. A fifth, JUICE-UVS, has been delivered to Airbus in preparation for ESA’s JUICE mission to Ganymede and the Jovian system. This spectrograph features advancements that improve spatial resolution, maximum instantaneous count rates, and radiation background rejection when compared to previous spectrographs. A high-resolution port (HP) is added to improve the spatial resolution of the base UVS design by a factor of approximately 4, at a cost of 16 times worse signal-to-noise. The new design also allows solar occultations at Jupiter and its moons by reworking the SOCC that flew on New Horizons- Alice. JUICE-UVS will explore the Galilean satellites, investigate the dynamics and energetics of Io’s atmosphere and torus, and characterize the dynamics and vertical structure of Jupiter’s upper atmosphere.

Developed by Airbus Defense and Space under CNES overall responsibility in partnership with Eumetsat, the Infrared Atmospheric Sounding Interferometer New Generation (IASI-NG) is a key payload element of the second generation of European meteorological polar-orbit satellites (METOP-SG). It will continue and improve the IASI mission in the next decades (2020-2040) in the field of operational meteorology, climate monitoring, and characterization of atmospheric composition related to climate, atmospheric chemistry and environment. The main challenge is the performance improvement by a factor two compared to IASI with regard to spectral resolution and radiometric error. The measurement technique is based on wide field Fourier Transform Spectrometer (operating in the 3.5 - 15.5 μm spectral range) with an innovative Mertz compensated interferometer to manage the so-called self-apodisation effect and the associated spectral resolution degradation. End 2019, IASI-NG instrument Critical Design Review has been successful and since nearly all the sub-units of the PFM instrument have been fully characterized and delivered to ADS. Interferometer principle has been partially validated on a Bread Board Model and Interferometer EQM and PFM have completed qualification and performance test. The aim of this paper is first to present the performance budget of the instrument after CDR, based on sub-units consolidation (sub-unit CDR) or measurement (sub-assemblies PFM measurement). We will then focus on the overall instrument development status and the different upcoming milestones until the delivery.

We present the initial tests of a novel correlation wind lidar (CWL). The instrument scans the atmosphere and detects inhomogeneities in the aerosol content. Depending on whether it is being exploited from a ground-based, airborne or space platform, the instrument scans the atmosphere and by cross-correlation determines the viable structures for analysis and then determines wind from the motions of these viable structures. The current instrument exploits recent advances in photon-counting array detectors, advanced high-resolution time-tagging and image processing techniques. To improve the SNR of the system, a Rayleigh / Mie Spectral Separator is employed so that only the Mie signal from Aerosols is measured. In situations that are frequently encountered with low to modest aerosol content, this enhances the visibility of low-contrast aerosol structures, thus increasing the frequency and accuracy of wind determination. We are investigating the possibility of exploiting this on future Space Missions aimed at global wind measurement. While generally limited to the troposphere and tropopause regions, the technique does not require complex or sophisticated transmitter or receiver systems. Extension beyond Tropopause is proposed by complementing the CWL lidar with a Wind Doppler Interferometer operating in Limb Sounder on O2 lines. The CWL lidar concept can be extended to different applications domains.

We present the mission proposal for a spaceborne multi-frequency lidar that is able to perform simultaneous Doppler Rayleigh, Mie and resonance measurements to gain temperature and wind data from ground to thermosphere. The suitability of the technology is demonstrated with a novel ground-based lidar with a diode-pumped alexandrite laser that acts as a unit for a lidar network able to cover the atmosphere up to 100 km over a large area at day- and nighttime in polar regions. The performance of a spaceborne lidar at the iron resonance line at 386 nm is derived from simulations with parameters similar to Aeolus. The multi-frequency lidar achieves the same resolution and uncertainties in wind measurements as Aeolus but additionally provides the wind and temperature up to 120 km. The reduction of the solar background by three to five orders of magnitude by means of the usage of narrow bandwidth filters and laser allows for significantly lower laser pulse energies (~ 10 mJ) and higher repetition rates (~500 Hz). Thereby the laser as a key-component and with the highest risk of failure is reduced in complexity, as no amplification stages are necessary to yield the high pulse energy. The roadmap to space is discussed in light of the heritage from Aeolus and with the diode-pumped alexandrite laser that is in line with the FULAS (Future Laser System) platform that is also the basis for the emitter of Merlin. The necessary development steps are identified and the current activities to address them are described.

Over the past years, Synopta have successfully applied adaptive optics (AO) in optical ground stations to enhance the capabilities in satellite-to-ground laser communication links. In this paper, we present the outcome of an upgrade activity to the ESA Optical Ground Station (ESA OGS). The activities comprise the design, fabrication and site acceptance test results of the Cassegrain Adaptive Receiver Optics (“CARO”) add-on instrument to the 1 m telescope of the ESA OGS. Additionally, we present a baseline design concept for the future use of a bi-directional adaptive optics system in the Coudé path of the ESA OGS, expected performance characteristics for a selected uplink application.

The Grasse Satellite Laser Ranging (SLR) station (ID7845), part of the International Laser Ranging Service (ILRS) network, participates to various laser applications including SLR, Lunar Laser Ranging (LLR), Time Transfer by Laser Link (T2L2), high-resolution imagery, and debris detection. Since 2014, in collaboration with the France Space Agency (CNES), the Grasse station has been upgrading in order to be able to take part in laser communication (lasercom) experiments. The main challenges of these experiments are the implementation of a complete laser communication chain from space to ground and the characterization of the turbulent atmosphere during the optical data transfer. Several campaigns have been successfully performed and planned since 2015 between Grasse station and lasercom terminal at low Earth orbit, SOTA onboard SOCRATES [5], OPALS integrated on ISS [6], and recently DLR's OSIRIS mission [7]. To prepare for possible higher data-rate lasercom campaigns in the near future (OSIRISv1 at 40 Mbps, OSIRISv2 and Optelµ at 1 Gbps, OSIRISv3 at 10 Gbps) Grasse SLR station is upgrading on lasercom optical bench, with a fine auto-tracking system and a high sensitivity telecom detector. In the proceeding, we will describe the architecture of our optical bench for high data-rate lasercom link and some primary results of the lasercom link between OSIRISv1 [14] (onboard FlyingLaptop satellite) and the station.

In recent years, spatial optical communication has been attracting attention as a large-capacity communication method that connects satellites and the ground. Radiofrequency communications are used as a method for transmitting data acquired by satellites to the ground. However, the radio frequencies used make it difficult to improve the communication speed, owing to interference problems and the carrier frequency. Space optical communication is expected to be a solution to this issue. A beacon method is often used to emit light from an optical ground station toward a satellite for initial acquisition. It is desirable that the divergence angle of the irradiating beacon covers the range of satellite orbit prediction. Therefore, it is important to know the orbit prediction accuracy of a satellite. In the experiment, satellite flare was observed from a ground station and verified by comparing the telescope tracking data with the GPS data and NORAD-TLE information mounted on the satellite.

The optical turbulence measurement and characterization has become an essential information for optical link (telecommunication, telemetry, time transfert, ...) and for high-angular resolution imagining in astron- omy. For example, it has an impact on the quality and on the bit error rate of optical communication signals and it decrease the resolution of astronomical images. The J.L. Lagrange Laboratory of the Observatoire de la Côte d’Azur has a long and recognized expertise in atmospheric optics and turbulence for astronomical purposes. We have developed recently a new generation of autonomous instruments for daytime and night- time atmospheric optical turbulence measurement. In 2015, a complete instrumental platform, the Calern Atmospheric Turbulence Station (CATS) has been installed at the Calern observatory. CATS is an auto- matic station equipped with a set of complementary instruments using original techniques for monitoring optical turbulence from the first meters above the ground up to the top of the atmosphere including the dome seeing. The station is based on 4 instruments. PML measures continuously the vertical profiles of the refractive index structure constant Cn2 with a high vertical resolution (~ 100m at ground level). GDIMM monitors the wavefront coherence parameters (seeing, isoplanatic angle, coherence time, scintillation, outer scale). A weather station provides the ground meteorological conditions (pressure, temperature, relative humidity, wind speed and direction). And the nighttime cloud fraction is given by an all-sky camera. Another instrument called INTENSE (INdoor TurbulENce Sensor) is occasionally associated with CATS station to measure the seeing inside the dome of the 1.5m MeO telescope to evaluate its contribution to the whole turbulence. A new tool has been integrated to CATS station consisting in a forcasting model using the Weather Research and Forecasting (WRF) system coupled to a turbulence model to predict daytime and nighttime meteorological and optical turbulence conditions for the next 48h.

Infrared (IR) remote sensing offers a huge range of applications, mostly addressing make-or-break issues of our century (crops water content monitoring, forest fires and volcanic eruption detection and imaging, etc.). These applications fall under different spectral bands, known as mid and long-wave infrared, which are very hard to combine in a single compact instrument. In this article we propose to explore the infrared (IR) behaviour of a dual-band diffractive component: the multilayer diffractive optical element (MLDOE). We use and discuss the thin element approximation as a valid phase model. Using Fourier optics, we are able to simulate the resulting image of the MLDOE. Thereby, ray-tracing software are not accurate to model a complex diffractive component. The Strehl ratio is used to determine the focalization efficiency for the working order, which is above 95% in the mid and long-wave infrared bands. This result, along with the very low energy content of the other orders, proves the strong imaging potential of MLDOEs for dual-band applications. It is also demonstrated that the MLDOE has the same chromatic behaviour as standard DOEs, making it a very useful component for infrared achromatization.

For space Astronomy as well as Earth Observation from space, more and more focal plane instruments are operating in the near or mid infrared and require optics operating at cryogenic temperature. The challenge here is to design an optical system reaching all optics, mechanics and thermal requirements. Thales SESO is presently under manufacturing of different opto-mechanical sub-assemblies for Meteosat Third Generation program. The imager missions MTG-I will produce images of the Earth simultaneously in different spectral channels, ranging from the visible spectrum to thermal infra-red, in order to fulfill the scientific, governmental & population needs. The sounder mission MTG-S addresses some interferometric measurements in the MWIR and LWIR ranges on the same instrument. The four CO-I objectives and the four one of CO-S work in different infrared ranges, thus, materials and coating have been chosen and designed especially for each spectral group. Each objectives is a stack of lenses bonded into barrels, themselves bonded inside a main barrel. Each objective is optimized based on the opto-mechanical values measured on each component. The objectives need to resist to mechanical and thermal environments in terms of opto-mechanical stability and work under vacuum, thus, with cleanliness specifications. All these requirements need to be taken into account on the design of different part of the cold optics. Through this on-going realization, the main technical issues and corresponding solutions in terms of design, manufacturing and testing of lens assemblies, mirrors and relevant optical coatings will be pointed out.

The imager that observes the ground surface with high spatial resolution from geostationary orbit has been studied. To obtain image data with high ground sampling distance (GSD) from a geostationary orbit, an extremely large-diameter and long-focal-length telescope is required. In order to realize this imager, we plan to adopt a synthetic aperture type reflection optical telescope (Korsch type) in which the primary mirror is composed of six segmented mirrors. The diameter of the synthesized aperture telescope is approximately 3.6m, and the focal length is approximately 30 m. Prior to the development of the actual imager, we have been producing a full-scale prototype of the segmented mirror that constitutes the primary mirror and its supporting structure. The aperture shape of the prototype segmented mirror is hexagonal, and its diagonal length is approximately 1.35 m. The supporting structure of the prototype segmented mirror incorporates a mechanism (6-axes adjustment mechanism) for adjusting the alignment of the segmented mirror with six degrees of freedom. A mechanism for adjusting the curvature of the segmented mirror (curvature adjustment mechanism) was also incorporated. In this paper, the design, the manufacturing, and the testing status of this prototype are described.

This paper describes the outcomes of a study funded by the European Space Agency aimed at identifying the technical challenges and trade-offs at the system level, performing preliminary designs of an active correction loop for large deployable telescopes, and defining technological roadmaps for the development of the active correction loop for the selected designs. This study has targeted two very different application cases, one for High Resolution Earth Observation from Geostationary orbit (called GeoHR, with a 4m diameter entrance pupil) and one for Science missions requiring very large telescopes (with a up to 18 m diameter entrance pupil) with high-contrast imaging capabilities for exo-Earth observations and characterization. For both application cases, this paper first summarizes the mission objectives and constraints that have influence on the telescope designs. It then presents the high-level trade-offs that have been led and the optical and mechanical design that have been developed, including the deployable aspects. Finally, the performance assessment is presented, and is the basis for the justification of an active optics correction chain, with a preliminary set of requirements for typical components of the system. The presentation is concluded with proposed technological roadmaps that aim to allow the development of the building blocks on which the next generation instruments will be able to rely on.

ZERODUR surfaces resulting from machining to spherical or aspherical forms have been investigated by visible light microscopy to identify subsurface damage (SSD). The rotation table peripheral grinding method has been used. Surface roughness (SR) of the different finished surfaces has been measured by a profilometer. We then examine the correlation between subsurface damage and roughness. This correlation allows for a precise determination and reduction of the SSD, enabling subsequent process steps such as polishing to continue with finer tool sizes resulting in a more efficient overall process.

This paper presents the firsts steps of the development of a new spectropolarimeter with a high potential for space applications. The instrument, designed on birefringent elements, avoids the need for rotating elements and may cover the entire Stokes vector on a large wavelength band. In addition, the architecture of the modulator enables a very high polarimetric efficiency, placing this concept above classical spectropolarimeters. A new proof for the solution uniqueness is presented, and base on that, a thorough analysis of the polarimetric efficiency.

CHIME, the Copernicus Hyperspectral Imaging Mission for the Environment, is one of the six High Priority Candidate Missions (HPCM) of the evolution in the Copernicus Space Component (CSC) foreseen in the mid-2020s that is proposed for further analysis. In this paper we summarize the results as retrieved by OHB (D) as part of the Phase A/B1. The contract was kicked off in 2018 and concluded in 2020 after finalisation of the Pre-development activities. The proposed instrument is a hyperspectral imager instrument with reflective telescope and grating-based spectrometer. The selected orbit is in the range of 625 ± 30 km, LTDN 10:45 – 11:15 am with a repeat cycle of 20 to 25 days for a single satellite and 10-12.5 days revisit for 2 satellites. The payload of each satellite records at a Spatial Sampling Distance (SSD) of 30m the full spectral range from 400 to 2500nm at a Spectral Sampling interval < 10nm with Low Keystone/Smile. On the front end a high performance TMA with wide-band coated optics collects the light from ground and feeds it to a highly linear almost distortion free spectrometer assembly attaining very good spectral stability. All units are integrated in an optical bench structure that offers excellent AIT access and provides a highly stable LOS. The electro-optical backend contains low-noise cold MCT detectors creating margin in the predicted NEDL performance. The instrument can be calibrated via on-board devices or using reference targets outside the spacecraft. We present the functional decomposition and the physical instrument architecture: the optical design and opto-mechanical layout, the electro-optical imaging chain ant thermal control system.

The Space CARBon Observatory (SCARBO) European program aims at assessing the monitoring of anthropogenic emissions of Green House Gas (GHG) with the uncovered goal of a sub-daily revisit of the Earth at an affordable cost. One of the main project scopes is the feasibility study of a hybrid constellation including both high accuracy reference missions (Copernicus CO2M or CNES MicroCarb mission) and 24 small-satellites onboarding innovative miniaturized payloads. The key GHG-sensor of the smallsat constellation is the NanoCarb concept, an unprecedently kilogram-class Fourier Transform imaging spectrometer. We are reporting here some preliminary experimental results from a demonstration airborne campaign. A low cost, 2-bands prototype designed for CO2 and CH4 measurement has been developed then integrated over a Falcon-20 from SAFIRE, jointly to SPEX aerosol sensor from SRON. During October 2020, we flew over Spain, Italy, and towards Poland from Francazal airport in Toulouse, France. Even if we did not have the opportunity to flight over a powerplant, a lot of data has been acquired and are being processed. After presented the instrument, mission and data products, we assess the data quality and the reliability of the model. We derive finally an expected sensitivity over CO2 and CH4 columns according to the background respectively about 1.5-2.5% and 5%. We finally demonstrate the operability of this first TRL5 prototype of NanoCarb.

Sentinel-5 is an Earth atmospheric monitoring mission developed within the European Union’s Copernicus program. The mission objective is to monitor the composition of the Earth’s atmosphere on a daily basis. Airbus DS GmbH acts as the prime contractor under a European Space Agency contract. The instrument design, development, and the instrument verification was and is in many aspects a fruitful co-operation between Airbus and TNO. The first part of this paper illustrates the optical design of the five optical channels of the Sentinel 5 instrument. The innovative compact optical design sets the basis for the acquisition of the variety of trace gas spectra. The design includes a two free-form mirror wide field telescope, a pointing insensitive Dual Babinet Pseudo Depolariser type polarisation scrambler, three one-dimensional waveguide type homogenizers, five reflective and refractive anamorphotic spectrometers including different disperser types as an a-spherical reflective diffraction grating for the ultra-violet, a grism for mainly the visible, a transmissive grating for the near-infrared, and immersed reflective gratings for the short wave infrared spectral ranges. The second part of this paper focusses on the design, qualification and verification of the UV1 spectrometer subsystem. Its optical design is based on an Offner-type spectrometer which has been adapted to employ freeform optics and an aspheric off-axis grating. A monolithic housing structure has been developed for optimum stability, accessibility and easy integration of the optical components. The qualification and performance verification of the first Proto Flight Model of the UV1 spectrometer was successfully completed in July 2020.

Kongsberg Defence & Aerospace (KDA) has developed and qualified the Laser Diode Module (LDM) for the Laser Source Electronics (LASE) for the IASI-NG instrument. The LDMs were Lot Acceptance Tested (LAT), and successfully qualified for use in space. The LASE unit provides a wavelength-stabilized optical source to be injected into the IASI-NG interferometer with a wavelength stability within ± 0.00015 nm throughout the instrument lifetime.

A side-pumped actively Q-switched Nd:YAG laser with pulse energy of >170 mJ, pulse duration of 6~7 ns and M²≈2.5 has been miniaturized, light-weighted and dimensioned for high electrical to optical efficiency. The key performance parameters and space qualification of the laser will be presented. Its performance and space environment reliability is suitable for space-borne altimeter.

For planetology applications, a SuperCam type laser emitting in the UV region (266 nm or 355 nm) would be of great interest. We investigated Third Harmonic Generation (THG) and Fourth Harmonic Generation (FHG) in LBO crystal and BBO crystal respectively. For potential lunar mission on vehicle smaller than a Curiosity class rover, we designed a laser based on SuperCam heritage focusing on size and mass reduction for a compact LIBS instrument.

For the satellite-based methane lidar instrument MERLIN a reliable laser source is needed that emits laser pulses at two wavelengths of around 1645 nm to measure the methane concentration of earth’s atmosphere with an Integrated Path Differential Absorption LIDAR (IPDA). To generate those pulses, the laser (LASO) consists of a seeded, actively qswitched, diode pumped Nd:YAG master oscillator power amplifier (MOPA) and a subsequent seeded and frequencycontrolled optical parametric oscillator (OPO). Due to the passive thermal control of the instrument the laser has to withstand a large non-operational and operational temperature range and also high mechanical loads while at the same time a compact envelope is required. Together with the demanding requirements on the laser performance a very robust optical design is needed. To meet those requirements, Fraunhofer Institute for Laser Technology (ILT) uses optomechanical mounts that were developed in a previous project and show very low tilting over a large operational temperature range, even after nonoperational temperature cycling and applying mechanical loads. The mounts are soldered and free of organic substances as the LASO is enclosed in a pressurized housing (LASH). Any outgassing could lead to a decay or damage of the optics and thus a failure of the laser. During the development of the optomechanical mounts many tests were performed to quantify the statistical behavior under mechanical and thermal loads. Based on those results and additional mechanical simulations, Monte-Carlo-Analyses have been performed to analyze the performance of the laser and to verify the fulfilment of the requirements.

The flight units of the laser module for the Raman Laser Spectrometer (RLS) instrument of the ExoMars mission were manufactured, tested and finally delivered in March 2018. Based on the data collected during flight batch acceptance testing, FM and FS lasers were picked from that batch, and the former was subsequently tested at the next integration levels, i.e. RLS instrument, Analytical Laboratory Drawer (ALD) and ExoMars rover itself. As the system complexity increased, the data available for laser performance assessment became sparse due to shorter testing slots and to the basic information available through instrument telemetries and products. In order to assess the status of the laser module during these years of on-ground testing we have developed tools to extract as much information as possible from test sessions. Our starting inputs were some relevant house-keepings, such as laser optical power and temperature, and Raman spectra, from where laser spectral signature was obtained. Our analysis of the available on-ground test sessions at ALD and rover level show that the laser unit is working properly, but a fine tune of the working temperature setpoint may be required to provide optimal scientific return. The procedures developed will be also highly valuable for the definition of the laser performance assessment engineering tools during standard scientific operation in Mars.

Two types of multimode fibers (50μm and 400μm) are going to be used for the scalar sensor of the JUICE (Jupiter Icy Moon Explorer program) magnetometer. This optical sensor will be placed at the end of a boom and these multimode fibers will be used to transmit modulated laser light through the sensor, the light source and the detector electronics being located within the spaceship. The fibers will be exposed to very low temperatures while the optical connectors will be more protected. The simulation of the optical budget did not allow to assure that the optical power transmitted through the fibers would be OK within the entire thermal range. A 3-chambers thermal cycling test was proposed to thermally cycle in a synchronized way to different temperature plateaus, both high and low, while monitoring the transmitted optical power. Some pieces of fibers and mating connectors were stabilized to one temperature while others were at other values. The temperature ranges for each chamber were: -190ºC to 120ºC for one chamber, -160ºC to 100ºC for the second one and -80ºC to +45ºC for the third one. The light source used for the test was developed by the institute of experimental physics at TUG Graz using exactly the same lasers that will be used for JUICE. The stability of the setup was assured using a reference path. This paper presents the main challenges for this test and the results obtained.

ESA’s Soil Moisture and Ocean Salinity (SMOS) mission was launched 2 Nov 2009 being the first ESA satellite relying on a complete optical harness (OHA), which was initially selected for the mechanical properties of optical fiber, what facilitated the deployment of the 3 arms of the instrument. In addition, other interesting advantages of the optical harness played an important role in the instrument performance such as lower propagation losses, immunity to electromagnetic interference, high bandwidth, SWaP reduction, etc. The two main functions of optical harness in SMOS were the distribution of reference clock and the transmission of the IQ data signals. Based on the good results obtained by SMOS-OPS mission, under the ESA ITI contracts No 4000120740/17/NL/AI and 4000122980/18/NL/IA, DAS Photonics, along with Airbus DS, and SENER Aeroespacial have developed the required OHA and the advanced L-band receivers (ALR), respectively, for a future advanced L-band radiometer mission with a target requirement of 10 km spatial resolution. In particular, two OHA configurations have been manufactured and tested. The first configuration aims at solving some identified issues as well as at improving performance of SMOS thanks to lessons learnt from the in-orbit operation, but without attempting novel techniques of calibration or signal distribution. The main goals of the second configuration, based on WDM techniques, are the improvement of the electrical performance and the optimization of the optical harness in terms of layout, i.e, to reduce number of cables/fibres, size, weight, as well as power consumption.

Within this work, we present a novel approach for Structural Health Monitoring (SHM) of composite materials during the autoclave curing process. For this research, samples of Carbon Fiber Reinforced Polymer (CFRP) have been manufactured with optical fibers embedded, both in the middle of the stack and on its surface. The strain development of the CFRP during the curing process has been monitored employing the Optical Frequency Domain Reflectometry (OFDR) technique. This technique allows the conditions of the element to be tracked not only during the operational lifetime of the composite, as in most of the research in this field, but also throughout the manufacturing process. The reported measurements demonstrate the potential of distributed optical fiber sensors for monitoring CFRP structures while their manufacturing, providing a unique opportunity for early detection of damages that may result in critical failures (e.g. delamination and debonding) and for improving the quality of the manufacturing processes and the final strength of the product.

Photonic generation and transmission of Linearly Chirped Microwave Waveform (LCMW) with a high Time-BandWidth Product (TBWP), of the order of 102 or more, is a widely used approach in Synthetic Aperture Radar (SAR) payloads to realize high range resolution, improving the distinction between two or more targets on the same bearing. In standard payload systems, an electronic approach based on voltage-controlled oscillator or digital signal processing, is used as LCMW generator, with both limited TBWP and operating frequency. In this context, a photonic approach plays a crucial role, ensuring low phase noise, high TBWP and operation frequency of the order of GHz, with a significant reduction of mass and size with respect to the electronic counterparts. Here, we propose the design of a Ka-band photonic LCMW generator, based on a frequency-tuneable Opto-Electronic Oscillator (OEO) and a Recirculating Phase Modulation Loop (RPML), with high spectral purity and high chirp rate in a small footprint. The OEO heart is a one-dimensional photonic crystal ring resonator with an ultra-high Q factor value and an optical delay < 10 μs. A low-frequency optical signal, with a high chirp rate and TBWP values of the order of 102-103, is generated in the RPML section.

Synthetic Aperture Radar (SAR) is a well-known technique for remote sensing applications with great advantages like uninterrupted imaging capabilities even at night or in presence of cloud cover. However, spaceborne SAR sensors face major challenges like cost and size, which are among the great barriers against their applicability for future constellations of low-Earth observation applications. SAR sensors are not compact and require large or medium-sized satellites weighting hundred kilograms or more, which cost hundreds million dollars. To solve these challenges, the recently started SPACEBEAM project, funded by the European Commission, aims at developing a novel SAR receiver approach, i.e., the Scan-on-Receive (SCORE), exploiting a hybrid integrated optical beamforming network (iOBFN) that also realizes the electro-photonic down-conversion of RF signals. The compactness and frequency flexibility of the proposed photonic solution complies with the requirements of future constellations of low-Earth orbit satellites in terms of size, weight, power consumption, and cost. A high-level representation of the SCORE SAR receiver module based on the multi-functional hybrid photonic integrated circuit (PIC), with 12 input RF channels and 3 output beam-formed IF channels, is shown in the submitted PDF document. For this design, we target the development of an X-band SCORE-SAR receiver having a swath width of 50 km (5 times wider than state-of-art spaceborne SAR systems), and enabling 1.5 m spatial resolution in both along-track and across-track directions. During the conference, we will present the design and specifications of the SCORE-SAR receiver at equipment level, where we aim at a hermetically packaged PIC that is also designed for space compliance. We target a flight-design for the RF front-end and control electronics, enabling the electro-photonic frequency down-conversion of the RF signals and the fast control of the PZT-driven iOBFN with <300 ns switching time.

Centering and positioning of the single optical elements within a lens system are essential for the quality of high precision optics. The positioning of the lenses have to be long term stable and shall withstand space environment to keep the performance of the optical system over years. This applies in particular to the optical modules for the Sentinel-4/Ultraviolet/Visible/Near-Infrared (UVN) sounder instrument /1/. In order to fulfill these requirements an appropriate opto-mechanical design and a deterministic assembly technique based on well-defined assembly interfaces are essential. For this reason, the application of the alignment turning technology is particularly suitable /2/. The basic approach is to reach the best possible coincidence between the optical axis of the lens and the mechanical axis of the lens cell and drop the lens stack in a barrel with a tight fitting. By machining the lens housing in respect to the optical axis the resulting ultra-precise references planes significantly simplify the alignment and integration of the objective. The assembly becomes deterministic – extensive alignments are not required. The mechanical and environmental specifications are achieved by the consistent compliance with: ultra-precise machined mounting interfaces, avoiding adjustment mechanisms, the application of a defined pre load during lens mounting. The S4 UVN optical modules requires a resulting decenter shift of each optical surface to the best fitting optical system axis less than 4 µm for the telescope, camera and collimator optics and less than 6 µm for the NIR optics. The total tilt of each surface to the best fitting optical axis has to be within 6 arcsec. The individual lens position (air distances) as well as the position of the lens stack in direction of the optical axis have to be within 5 µm, respectively. Aiming to the specified values the alignment turning technology was carried out on the proto flight models of the UVN instrument optics. Using a customized alignment turning machine the decentration of the housed lens were detected and adjusted in respect to the turning axis, reaching a minimum. After machining of the housing the vertex height as well as the geometry of the lens cell were measured in sub micron precision. For the passive joining of the alignment turned lens elements in to the lens tube without further adjustment steps a specific mounting device was realized. A newly developed clamping process that ensures a dedicated pre-load completes the assembly procedure. The alignment turned and assembled flight models were characterized using a centration measurement of the optical subsystems (telescope, collimator, camera, NIR spectrometer). The alignment status of the optical axis of all individual lens systems w.r.t. the measurement reference axis was determined within the required specifications. Further optical verification measurements confirm the achieved optical performance of the optical modules to the full extent. The paper deals with the alignment turning, the integration and verification of the lens assemblies for the S4 UVN Instrument.

Thales Alenia Space has designed and developed space high performance observation instruments for more than 40 years. The future missions will have to deal with better performance, better optical quality while from affordability point of view, the total mass, the development schedule and the final cost have to be reduced. These constraints induce an innovative generation of solutions based on larger entrance optics associated to high lightweight ratio. In these conditions, the enhancement of the final performance can only be guaranteed by using active optics in flight. In this context, Thales Alenia Space has been selected by French National Space Agency (CNES) to develop an active optics full scale telescope to path the way for the new class of Extremely high resolution telescopes. The whole building blocks have been already tested and qualified over the last 4 years. So, a deformable mirror is implemented in the future large telescopes in order to correct manufacturing residues, ground/flight evolution including gravity. Moreover, low mass and high agility satellites require more compact designs which implies telescope solutions more sensitive to misalignments. An active positioning mechanism is then also needed in order to correct the telescope alignment during flight conditions. A wavefront sensor is implemented in the mission focal plane and the global active loop validated. This paper presents the validation of active optic telescope on a full scale Engineering Model of 1500mm of entrance pupil diameter. All the active optic building blocks already qualified are mounted on this demonstrator. This Engineering model has followed a complete integration and alignment process that confirms the breakthrough simplification expected. The test plan validates on a full scale model the wave-front correction algorithms on-ground and in flight conditions, ensuring the best performance achievable with the telescope all along the lifetime.

Euclid is a part of the European Space Agency Cosmic Vision Medium Class program. This mission’s goal is to investigate the nature of dark energy, dark matter and gravity by observing the geometry of the Universe and the formation of structures over cosmological timescales. Euclid Payload Module (PLM) includes a large TMA Korsch telescope in Silicon Carbide (SiC), feeding a visible imager (VIS) and a near-infrared spectrometer and photometer (NISP). Both instruments are mounted at their respective focal plane position.

The field of earth observation requires increasingly complex optical instruments to meet the final requirements . The anticipation of instrument integration and alignment activities on the subsystem side is essential. Thales SESO manufactures opto-mechanical subsystems assembled in such different space instruments. The evolution of instruments, as TMA type, concerning the reduction of the space allocated requires Thales SESO to offer opto-mechanical components and associated measurements that are increasingly precise and reliable. The challenge here for Thales SESO is to manufacture, integrate and measure off-axis mirrors while ensuring accurate apex positioning. We will share here the results on the instrument of the MTG program for Telescope Optics subsystems of the two instruments FCI and IRS. Through a specific metrology scheme, including accurate scanning of the optical surface Thales SESO delivers to the customer a reliable and accurate location of the optical reference frame of each sub - assembly toward its mechanical reference frame. From these relative location, the customer is able, in its assembly process, to “plug” the sub-assembly directly in its nominal position to start the alignment process with interferometric system. The data transmitted by Thales SESO made it possible to anticipate each adjustment of the optical subsystems and to make a very accurate prediction of the alignment requirement. With the data measures by Thales SESO, our customers realize a very quick final alignment procedure, with minimum displacements, to meet the final goal. In this process, the alignment budget is also minimized, leading to a final WFE largely under the predictions made by the customer before receiving the assemblies.

This paper presents the results of the technology development project “Enabling Technologies for Piezo-Based Deformable Mirrors in Active Optics Correction Chains” conducted by OHB System AG together with its partner Münster University of Applied Sciences (MUAS). The project was funded by ESA within their General Support Technology Programme (GSTP). We address in this paper mainly the definition, flow-down and verification of the requirements for the Deformable Mirror (DM). The requirements were derived from a set of real space mission applications. The deformation of the mirror is performed by piezo-ceramic actuators in an unimorph configuration. The finally developed DM is able produce Zernike modes with a stroke of several tens of μm over a clear optical aperture of 50 mm in diameter. It underwent successfully a full environmental qualification campaign including thermal cycling, shock- and vibration testing, as well as exposure to proton and γ–ray radiation. Thermal and performance tests were performed in the temperature range from 100 K to 300 K. Furthermore, the DM sustained all vibration (random 17.8 g RMS and sinus) and shock (300 g) testing. Thereby all criticalities which were identified a previous study have been overcome successfully. A Technology Readiness Level (TRL) of 5 is reached, as the component has been validated in relevant environment. Based on the high level of maturity, this deformable mirror is now ready for the incorporation in future flight instruments. The achieved TRL of 5 is sufficient for the status of a PDR at payload level and gives thus a very good basis for all kinds of potential B2, C/D payload developments.

High accuracy multi-angle polarimetry is of crucial importance for remote sensing of aerosol properties with accuracies demanded by climate and air quality studies. In this contribution, we discuss the polarimetric calibration of the multi-angle polarimeter "SPEX airborne". SPEX airborne is a multi-angle viewing instrument providing snapshot measurements of spectral radiance and degree of linear polarization at fixed viewing angles. Radiance and polarization are measured as a continuous function of wavelength in the 400-760nm range, at nine viewing angles equally distributed over an angular range of -56° to +56°. Each viewing-angle aperture has a swath of 7° with an instantaneous field of view of 0.5°l° (cross- times along-track). SPEX airborne measures the degree and angle of linear polarization of scattered sunlight by means of spectral modulation. For each field of view, the instrument records two modulated spectra. Ideally, these are perfectly in anti-phase, such that the sum of the modulated spectra is modulation free and gives the spectral radiance. The state of linear polarization is derived from the scaled difference of the modulated spectra. As a result of finite image quality at the focal plane, any spectropolarimeter using spectral modulation will show different polarimetric responses for the two modulated spectra, which breaks the anti-phase symmetry. For SPEX airborne this is indeed the case, and special care is taken both in the calibration and in the data processing. Ignoring this can lead to errors both in polarimetric and radiometric measurements. It is shown however that these errors are quite small. In this contribution, we will outline the spectral modulation technique employed by SPEX airborne to measure the state of linear polarization, explain which instrumental parameters are to be determined by calibration and how they feature in the data processing chain. We discuss the polarization calibration setup and how polarization calibration measurements are processed into useful calibration data. Taking a Mueller matrix approach, we show how differences in polarimetric responses can be mitigated, while also other imperfections like telescope polarization are accounted for without extra calibration effort. We also present a scheme to correct for sharp features in the solar spectrum, which would otherwise mix into the modulation spectra via the finite slit-width of the spectrometer and result in polarimetric errors. The effect of telescope polarization is also discussed Polarimetric calibration of SPEX airborne is relevant for SPEXone, a compact multi-angle polarimeter that builds on SPEX airborne heritage and space-borne spectrometer heritage within the Netherlands. SPEXone has been developed for NASA's PACE mission, which has a planned launch date in 2023.

The ESA Sentinel-4 Mission is part of the European Commission Copernicus Programme for monitoring the Earth. It is going to provide free data on trace gas concentrations and aerosols in the atmosphere to support real time operational services of air-quality for Europe. The Sentinel-4/UVN Instrument will be placed in a geo-stationary orbit on board of the Meteosat Third Generation Sounder (MTG-S) satellite. Jena-Optronik has been contracted to develop and build the core main optics of the Sentinel-4/Ultra-violet/Visible/Near- Infrared (UVN) instrument, which comprises a telescope, a UV-VIS spectrometer and a NIR spectrometer. Due to its geo-stationary orbit and its compactness the photon flux seen by the instrument is low. Simultaneously the lens based optics of the instrument set new standards concerning performance. In consequence straylight levels within the instrument need to be reduced to the bare minimum including high quality polishing of the optics as well as manufacturing facilities as well as processes, which reduce the overall allowed contamination of the optical surfaces to be <50ppm for particular and <50ng/cm² molecular contamination. Furthermore, the very demanding performance requirements led to an instrument design, which sets the demands on manufacturing and alignment quality to a new level. To realize the instruments performance over the complete specified thermal range Jena-Optronik developed a unique passive thermal focus compensation for the lens optics, which acts like a passive auto-focus system w.r.t. thermal changes. Moreover, we used precision lens mount manufacturing techniques to achieve the required lens positioning accuracy of less than 5μm. The design of the instrument will be presented as well as the highlights from the assembly, integration and test campaign of the three core optics. All three PFMs, the telescope as well as the UV-VIS and the NIR spectrometers have been successfully delivered to Airbus Defence & Space after ending their environmental test campaign successfully.

SPEXone is a multi-angle channeled spectropolarimeter that is developed by a Dutch consortium consisting of SRON and Airbus Defence and Space Netherlands with support from TNO. SPEXone will fly together with the Ocean Color Instrument (OCI) and the Hyper-Angular Rainbow Polarimeter-2 (HARP-2) on the NASA Plankton, Aerosol, Clouds and ocean Ecosystem (PACE) mission, which has a notional launch in 2023. SPEXone will deliver high quality hyperspectral multi-angle radiance and polarization products that, together with products from OCI and HARP2, enable unprecedented aerosol and cloud characterization from space. SPEXone employs dual beam spectral polarization modulation, in which the state of linear polarization is encoded in a spectrum as a periodic variation of the intensity. This technique enables high polarimetric accuracies in operational environments, since it provides snapshot acquisition of both radiance and polarization without moving parts. SPEXone has five viewing angles that are realized using a novel three-mirror segmented telescope assembly. The telescope focuses light captured by the five viewing angles onto a single image plane consisting of five stacked sub-slits. This multi-slit forms the entrance slit of a reflective grating spectrometer that consists of freeform mirrors and an order-sorting filter close to the focal plane, yielding an intrinsic spectral resolution of 2 nm and 5.4 km spatial resolution across the 100 km swath. The spectrometer re-images two spectral images per viewing angle following a dual beam spectral polarization modulation implementation. In this contribution, the optical performance of the telescope and spectrometer will be presented by means of star stimulus measurements at the slit plane and at the spectrometer focal plane. Measurements of the optical spot quality and preliminary measurements of stray light are compared with the optical design and with stray light simulations. We find that the measured optical performance of the telescope and spectrometer is better than modelled, showing higher resolution and lower slit keystone, thereby meeting all spatial and spectral resolution requirements. Also, preliminary stray light results indicate a higher diffuse but lower ghost contribution to the total stray light, which is in general beneficial for implementing stray light correction, which will enhance the polarimetric accuracy in inhomogeneous scenes.

Sentinel-4/UVN is a high resolution UVN (UV-VIS-NIR) spectrometer system, being developed within the framework of the Copernicus programme, to support operational monitoring of air quality and climate over Europe and adjacent regions, through accurate atmospheric measurements of trace gas concentrations and aerosols. Stray light is one of the main drivers of the instrument performance. We have developed a stray light correction analysis framework, applicable to a wide range of spectrometer systems, to provide input for selecting the right correction method(s) and specifying calibration measurements, well before the on-ground calibration campaign. This was accomplished by using the instrument design parameters and simulated instrument response to evaluate the S4-UVN stray light performance, before and after stray light correction. This paper presents the developed framework and a stray light correction sensitivity analysis performed for Sentinel-4/UVN. The analysis results indicate that an iterative convolution correction algorithm can reduce stray light levels in case of smooth stray light, such as diffraction and scattering. The observed fast convergence of the algorithm suggests that a single iteration may suffice in many cases. The study shows that calibration uncertainties should be carefully considered during the preparation for the on-ground calibration campaign, as they can significantly degrade the correction quality.

The OGSE GESTA (Geometrical and Spectral Test Assembly) operates in vacuum chamber to perform the optical characterization of the IRS instrument. GESTA integrates both sources, gas cell and laser sources, in order to perform a spectral characterization and calibration. The MTF measurements are performed by measuring the signal coming from a hot pattern plate, illuminated by a stable and homogeneous optical beam. To measure the background signal, the pattern plate is removed and a cryogenic blackbody is used instead of. This paper describes this illumination sub-system used in GESTA, developed by Bertin Technologies, and using the infrared laser sources. The principle of the used solution is to apply a dynamic variation to the interference status responsible for the speckle at the output of the integrating sphere, in order to eliminate the impact of the spatial coherence of the output beam and to get a uniform radiance through temporal averaging over the integration time of the optical detector. For that, the laser beams hit a rotating diffusing disc located inside the sphere. For GESTA, the requirement is challenging as speckle contrasts need to be as low as 0.1% for a 400μs integration time. The solution developed shows good enhancements, in terms of radiance level, stability, uniformity, and is easily translatable to other spectral ranges. Final achieved performances and the related influencing parameters are reported.

The Sentinel-5/UVNS instrument is a spectrometer developed within the Copernicus program for monitoring trace gas concentrations and aerosols in Earth’s atmosphere with unprecedented accuracy and resolution. The performance of the satellite spectrometers in orbit relies upon accurate characterisation and radiometric calibration of the optical ground support equipment (OGSE). M Squared Lasers has developed a series of advanced tuneable, fully-automated, remotely controlled, narrow-linewidth laser systems, providing rapid scanning across the extensive wavelength operating range from 260-2700 nm, with a number of unique features that ensure stringent source requirements are met throughout the mission lifetime. To the best of our knowledge, these laser systems are the first of their kind, demonstrating a significant leap in laser technology with regards to remote controlled automated operation, gapless wavelength coverage from UV to SWIR, functionality and performance. This paper will detail each of the S5 laser sources, describe the engineering achievements in their development.

Laser launch telescopes are optical systems forming a reference beacon in the sky, to be used as a reference for ground telescope adaptive optics. One of the requirement of the system is that the laser beam should have a radius as small as possible at the target distance where the telescope detects the beacon. This is not trivial given the Gaussian nature of laser beams. Traditionally, laser launch telescopes consists of a simple afocal. Its magnification sets the output beam waist to an optimal radius that limits the laser beam divergence. We propose a configuration consisting of a combination of two afocal systems, each with a controlled defocus. Our system provides enough degrees of freedom to displace the waist at any distance in the sky and to achieve a smaller beam radius at the target distance. After presenting the theoretical concept of the combination, this paper describes the practical implementation of the system. We start by describing the design process and the expected performances. Then, the alignment and experimental verification process are emphasized. In particular, the verification of the controlled defocus is challenging as it cannot be done simply by putting a detector at several km from the optical system. Instead, the verification in laboratory is performed by performing interferometric measurement for different defocus and comparing them to theoretical simulations. The laser launch telescope will be used in various meteorological conditions. We describe the impact of thermal variations and how it can be compensated.

With this work, we present a new class of diode pumped solid-state based optical frequency comb (OFC) technology based on a Kerr-lens modelocked oscillator, which possesses robust self-starting capabilities. By combining this novel technology together with Airbus’ capabilities of building highly stable, bonded optical space instruments, a preliminary optics module is demonstrated that produces sub-150-fs pulses with a repetition rate of 1.51 GHz, an output power of 12 mW and a central wavelength of 1078 nm with a bandwidth of 9.1 nm. This novel technology is presented for the first time to provide some preliminary results and to show the capability of the technology. Operation has so far been demonstrated for tens of hours without performance degradation and it is expected to be significantly less sensitive against radiation than fibre based OFCs. All these features are packed in a mechanically robust and small micro-optical package, with the clear aim to deliver a compact solution fully suitable for space applications.

In this paper, we will describe the work carried out within the framework of the evaluation of the reliability of pump combiners under space environment. This work includes the definition of the optical and environmental specifications, the selection of the components and their evaluation. The evaluation of these pump combiner under spatial conditions will show which reference is the most resistant to such conditions but it will also highlight what kind of environment can be destructive.

Compared to conventional large satellites in the past, small satellite classes (less than 150 kg) show their advantages for mass production, such as short time and low cost for development and launch, to cope with the demand for emerging missions that require a sufficient number of satellites in orbit. However, the traditional communication method, in which a low earth orbit (LEO) small satellite sends data to a ground station using radio frequency, has several disadvantages. Firstly, the limitation of radio-frequency bandwidth leads to a low data rate and difficulty in getting a frequency license. Secondly, there is a significant delay during which data cannot be sent to the ground due to lacking a line of sight between the LEO satellite and the ground station. Additionally, the duration time for the small satellite to communicate with the ground station is just less than 10 minutes approximately. To resolve the above issues, we investigate the case that a less-than-150-kg satellite carries out a laser communication link from LEO to a satellite in geostationary orbit (GEO). Due to the constraints of size, weight, and power (SWaP), traditional bulky LEO-GEO relay systems cannot be applied for the small satellite. However, using the combination of the satellite body pointing and a piezo Fast-Steering Mirror (FSM), which reduces the SWaP considerably, makes it feasible that the LEO-to-GEO communication can be implemented in a small satellite for the first time. While utilizing laser communication can increase the data rate, the relay communication via the GEO satellite helps the small satellite to extend the communication duration significantly. Moreover, since there is a line of sight between the two terminals in any of about 15 orbits per day of the LEO satellite, data taken by the small satellite can be downloaded to the ground via the GEO one in almost real time. This research aims at investigating and proving the feasibility of a small satellite to transmit a laser communication link to its GEO counterpart. In this paper, we describe the LEO-to-GEO laser communication of the small satellite with a study of pointing-budget and link-budget analysis. Furthermore, a hardware-based simulation of the fine control mechanism is conducted. The hysteresis that affects severely to the piezo mechanism, and hence, the final control accuracy, is modeled accurately and its effect is shown.

Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.

Constellations of CubeSats will benefit from high data rate communications links and precision time transfer and ranging. The CubeSat Laser Infrared CrosslinK (CLICK) mission intends to demonstrate low size, weight, and power (SWaP) laser communication terminals, capable of conducting full-duplex high data rate downlinks and crosslinks, as well as high precision ranging and time transfer. A joint project between the Massachusetts Institute of Technology (MIT), the University of Florida (UF), and NASA Ames Research Center, CLICK consists of two separate demonstration flights: the initial CLICK-A, which will demonstrate a space-to-ground downlink and serve as a risk-reduction mission, and CLICK-B/C, a crosslink demonstration mission. The CLICK payloads each consist of laser transceivers and pointing, acquisition, and tracking (PAT) systems, and will fly on 3U CubeSat buses from Blue Canyon Technologies to perform their optical downlink and crosslink experiments in low Earth orbit (LEO). We present an update on the status of both the CLICK-A and CLICK-B/C payloads. At the time of writing, the final assembly and testing of the CLICK-A payload has been completed and the payload has been delivered for integration with the spacecraft bus. The final testing included the validation of the transmitter and the PAT system, the performance of both of which was characterized under various environmental test conditions and shown to meet their requirements for operation on orbit. On CLICK-B/C, the payload electronics have been designed and are under test. The optical bench of the payload has been assembled, the characterization of which is ongoing.

This paper applies non-reciprocal phase error analysis approach to the fiber optic gyroscope coil made of multi-core fiber and fan-in/fan-out devices to estimate the thermal sensitivity analytically for the first time. In the case of MCF symmetric coils, the relationship between the coil parameters, which can take advantage of MC-FOG in terms of sensitivity to temperature change, is quantitatively clarified in comparison with SMF symmetric coils. In addition, the non-reciprocal phase error of the FIFO element under temperature disturbance is quantitatively clarified. The design policy of the coils proposed from these results is also discussed. And method for connecting cores that minimizes the phase error is proposed. As analytical results, besides lowering the manufacturing cost of the coil due to the shorter fiber length, the multi-core fiber coil was found to further reduce the phase error sensitivity compared to a single-mode coil of equivalent length.

Lens R&D and SystematIC Design B.V. are currently involved, in frame of an Artes AT program, in designing a small true digital Sunsensor, intended to have an as large as possible Field Of View (FOV). Adding another 10 degrees to the common 60° on axis field of view will even provide a redundant full spherical coverage with only 8 sensors, (instead of the required 10 sensors when a sensor with 60° FOV is used). This conclusion prompted an investigation into the possibilities to increase the FOV to 69° on axis. During the following design considerations however, several optical issues were encountered, actually leading to a proposal to reduce the FOV to 62° in diagonal (54° on axis). This paper focusses on these issues and effects, like polarisation dependent reflection, Sun spot spreading and shadowing effects of on-chip interconnects.

Reliable temperature measurements of hot structures of re-entry vehicles is one of the main challenges associated with atmospheric heating. During re-entry, several minutes at hypersonic velocities results in severe aerothermal loads and a resultant temperature increase of more than 1000°C. In contrast to single-point measurements provided by thermocouples, optical fiber sensors allow temperature measurement at multiple positions along the fiber line. Key challenges of this technique include packaging, integration and extraction of the temperature contribution from a signal that is also influenced by strain effects. MPBC developed optical fiber sensors for such temperatures with special packaging optimizing between protective capability and fast thermal conductivity. The fiber sensors were initially calibrated with thermocouples using a standard oven, then by means of a test in the DLR arc-heated wind tunnel L3K at a material temperature of 1000°C. To well monitor the fast heat fluxes in reentry two special ruggedized interrogation modules (Interrogator) was developed with a data acquisition at 100 Hz and 3.5 kHz. The Interrogators have large memory capacity to save data during 1 hour, and a USB memory stick as back up. The Interrogators was validated for vacuum, thermal cycling and vibrations, being completely functional during the tests. The vibrations tests were successful even an accurate sweeping part the piezo of the tunable Fabry-Perot Interferometer was sweeping during the vibrations in X, Y and Z-axis. DLR integrated both 100 Hz and 3.5 kHz into the hypersonic flight experiment ATEK for measuring temperature distribution of the motor case of the second stage motor and hybrid module structure, respectively. The 3.5 KHz Interrogator was integrated in the hybrid module, which was part of the launcher block equipped with a parachute. The second stage performed the return flight without parachute and allowed testing the impact resistance of the new DLR’s data acquisition system and some measurement techniques. The ATEK flight experiment was successfully launched on 13th July 2019 from the launch site Esrange in Kiruna. The second stage and the payload reached an apogee of approx. 240 km and continued the descent without any thrust and landed approx. 500 seconds after the take-off at a distance of approx. 67 km from the launch site. The Health Monitoring System allowed the measurement of aerothermal and mechanical loads on the hybrid payload structure and the motor case along the complete flight. Part of the data has been transmitted during flight to ground via telemetry at a low sampling rate of several Hertz. In addition, several impact-resistant data acquisition units could acquire the data at a high sampling rate of several Kilohertz and stored it onboard. The housing of interrogator and memory stick box was nearly completely undamaged. All four fiber optic connectors were still attached. MPB will recuperate and evaluate its functionality. The 100Hz Interrogator, without protection, impacted the ground with a velocity of about 95 m/s and was damaged due to the impact

PLATO - PLAnetary Transits and Oscillation of stars - is a medium-class mission in the European Space Agency (ESA) Cosmic Vision programme, whose launch is foreseen by 2026. The objective is the detection and characterization of terrestrial exoplanets up to the habitable zone of solar-type stars by means of their transit signature in front of a very large sample of bright stars. The seismic oscillations of the parent stars orbited by these planets is measured in order to understand the properties of the exoplanatory systems. The PLATO payload consists of an instrument with 26 cameras for star observation; 24 normal cameras grouped in four subsets with six cameras each and two fast cameras. Besides providing scientific data for very bright stars, the fast cameras also serve as two redundant Fine Guidance System (FGS) and will be an integral part of the Attitude and Orbit Control System (AOCS). This ensures a very high pointing precision which is needed to achieve a high photometric precision. Working as a star-tracker, the attitude calculation is based on guide star positions on the focal plane and their reference directions given by a star catalogue. Compared to predecessor missions like CoRoT, Kepler, or TESS, the precision of the fine guidance algorithm needs to be increased significantly. This is especially challenging as the optical design is identical for all cameras and optimized to meet the science objectives rather than to serve as a star-tracker. Therefore, a novel approach based on a Gaussian fit is proposed. The shown algorithm provides a noise optimal estimation of the guide star positions which propagates to an optimal attitude estimation. Although, computational more expensive than conventional methods, its suitability for a real-time on-board application is proven with an implementation on the target hardware. Furthermore, its robustness and precision is assessed theoretically and with simulated star images sequences.

This paper presents the design and breadboarding of the proof of concept demonstrator for the so called retro-reflector interferometer scheme in off-axis configuration for the ‘Next Generation Gravity Mission’ (NGGM) studied at the European Space Agency (ESA). This configuration can offer benefits in terms of overall satellite configuration compared to the transponder scheme, which is currently flying on board of GRACE-FO. However, it relies on very low received laser signal levels due to the fact that the laser light is travelling about 100 km from the master satellite to the remote satellite and is reflected back to the master satellite by a retro-reflector. In comparison to the transponder scheme, where the signal is amplified on the remote satellite using a laser, which is optically phase locked to the laser signal of the master spacecraft, this reflection does not amplify the signal. Thus, even with higher emitted laser power, instead of some nanowatt, only a few picowatt are available on the according science detector. Therefore, less than a femtowatt of straylight within the detectable heterodyne frequency and angular range is allowed on the detector to fulfil the ranging noise requirement. The paper gives insights into the main opto-mechanical design topics of the Optical Bench Assembly (OBA). It includes the optical analysis results as well as mechanical design to suppress straylight below the required limit. The optomechanical design of the OBA is complemented by the opto-mechanical design of the test setup and by the electro-optical design of the phase read-out chain. Finally, preliminary results from the test campaign are presented.

This presented study is initiated in the frame of CNES advanced studies. It aims at providing a better understanding of driving parameters for this type of instruments, and exploring suitable, very compact hyperspectral instruments based on grating spectrometers. First a scalar model for multiblazed gratings is developed, and confronted with more detailed diffraction models. The spectral band is in the range of 400-2500 nm, with a spectral resolution of about 10 nm width. The Ground Sampling Distance (GSD) shall be between 10 and 15m, the Field of View (FOV) shall be larger than 8km, the orbit Height is 550km. Three types of instruments are proposed, a common spectrometer with a unique detector, a common spectrometer working in diffraction order 1 for the SWIR, and 2 for the VIS spectral band, and 2 spectrometers within field separa tion. For each instrument the grating blazed profile is optimized, the key instruments parameters computed, and an optical configuration is proposed. The study shows compactness optimization with these three instrument concepts, thanks to an entrance pupil diameter reduction along with better grating diffraction efficiency

The EUV High-Throughput Spectroscopic Telescope (EUVST) of Solar-C mission is a revolutionary spectrometer that is designed to provide high-quality and high cadence spectroscopic data covering a wide temperature range of the chromosphere to flaring corona to investigate the energetics and dynamics of the solar atmosphere. The EUVST consists of only two imaging optical components; a 28-cm clear aperture off-axis parabolic primary mirror and a two-split ellipsoidal grating without a blocking filter for visible light before the primary mirror to achieve unprecedented high spatial and temporal resolution in EUV-UV imaging spectroscopic observations. For this reason, about 53 W of sunlight is absorbed by the multilayer coating on the mirror. We present an instrumental design of the telescope, particularly, primary mirror assembly which enables slit-scan observations for imaging spectroscopy, an image stabilizing tip-tilt control, and a focus adjustment on orbit, together with an optomechanical design of the primary mirror and its supporting system which gives optically tolerant wavefront error against a large temperature increase due to an absorption of visible and IR lights.