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The ESA-Darwin mission is devoted to direct detection and spectroscopic characterization of earthlike exoplanets. Starlight rejection is achieved by nulling interferometry from space so as to make detectable the faintly emitting planet in the neighborhood. In that context, Alcatel Alenia Space has developed a nulling breadboard for ESA in order to demonstrate in laboratory conditions the rejection of an on-axis source. This device, the Multi Aperture Imaging Interferometer (MAII) demonstrated high rejection capability at a relevant level for exoplanets, in singlepolarized and mono-chromatic conditions. In this paper we report on the new multi-axial configuration of MAII and we summarize our late nulling results.

Today, optical instruments with FOV of ±30 degrees or more can be achieved by combining several optical modules thus increasing the complexity at instrument level in the design, integration, validation and calibration phases. For instance the MERIS instrument on the ENVIronment SATellite (ENVISAT) has a 68.5 degree FOV shared between 5 identical modules. Each one of those modules needs to be calibrated independently and the modules require to be co-registrated, which leads to stringent constraints at system level in terms of stability. Reducing the number of modules to achieve large FOVs will have potentially beneficial impact on the mass and volume of the instrument and will ease the calibration and co-registration process.

Direct detection and spectroscopic characterization of earth-like exoplanets require to eliminate the starlight in the recorded signal at a rejection level (collected flux / residual flux) around 10= 80 nm).

ESA's Darwin mission is devoted to direct detection and spectroscopic characterisation of Earth-like planets in the thermal infrared domain by nulling interferometry in space. This technique requires deep and stable starlight rejection to an efficiency around 106 over the whole spectral band. Darwin is a major target for Thales Alenia Space, and is considered as a strategic part of its programme roadmap. In this paper we present the main outcomes of the Darwin mission study conducted by Thales Alenia Space from Oct. 2005 to Jul. 2007. Studying this mission in depth, our proposed most promising configuration features spacecraft in non planar arrangement (called Emma). It offers the best science return in terms of number of stars detected and sky accessibility while staying compliant with mass and volume constraints of a single Ariane 5 launch. Our solution dramatically alleviates engineering constraints thanks to a fully non deployable concept. As compared to the more conventional planar arrangement (called Charles), Emma suppresses Single Point Failures and spurious flexible modes, thus maximising both the system reliability and the stability of the dynamical environment. Emma is fully compatible with either 3 or 4 collectors.

The ESA-Darwin mission is devoted to direct detection and spectroscopic characterization of earth-like exoplanets in the thermal infrared domain by nulling interferometry in space. This technique yields the rejection of starlight so as to make detectable the faintly emitting planet in the neighborhood. In that context, Alcatel Alenia Space has developed a nulling breadboard for ESA in order to perform the rejection of an unresolved on-axis source. This device, the Multi Aperture Imaging Interferometer (MAII) demonstrated high rejection capability at a relevant level for exoplanets, in single-polarized and mono-chromatic conditions. In this paper we report on our late investigations using the MAII focussed on modal filtering. The dependence of the nulling ratio on the degeneracy of the guided modes in the modal filter is put into evidence.

This paper presents the development and tests in the thermal infrared of Integrated Optics (IO) technology in preparation of ESA's space mission Darwin . This mission aims to detect and characterize earth-like planets orbiting solar-type stars, using nulling interferometry in the spectral range 6 - 20 μm. Since typically 1:1e6 rejection of starlight is required, wavefront modal filtering is mandatory. Thus, mid-infrared single-mode IO is being developed in the framework of the ESA-funded "Integrated Optics for Darwin" project. Beyond its wavefront filtering capabilities, an IO component may support various optical functions, and is thus likely to ease instrumental design. This paper addresses the manufacturing process and the characterization tests results of newly developed IO devices. Investigated solutions are dielectric waveguides based on Chalcogenide glasses and Hollow Metallic Waveguides. In a first phase, the pre-selected technological solutions were validated and modal behavior of the manufactured devices was demonstrated, both through polarization and spectral analysis. Preliminary nulling ratios up to 5000 have been obtained with an IO modal filter in the 6 - 20 μm range. In a second phase of the project, the development of more complex IO functions was attempted. The methods used to validate the waveguide behavior and interferometric capabilities are also discussed. After achieving 1:1e5 polychromatic extinctions with similar solutions in the near IR, the presented results further underline the credibility of a mid-infrared IO concept for Darwin.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper reports the design, realisation, and characterisation of singlemode hollow conductive waveguides for stellar interferometry. These waveguides are developed in the frame of technological developments for the ESA DARWIN mission, which aims at direct detection of exoplanets and biomarkers on them (proof of life) using nulling interferometry in the 6-20 μm spectral range. The use of singlemode waveguides is mandatory in order to meet DARWIN required performance by achieving a modal filtering better than 10 -6 . While there is ongoing developments of infrared dielectric fibers or integrated waveguides, both using chalcogenide glasses or silver halide compounds, this paper presents the first realisation and characterisation of singlemode hollow conductive waveguides in the DARWIN spectral range, by means of standard microelectronic and wafer bonding technologies.

Darwin is one of the most challenging space projects ever considered by the European Space Agency (ESA). Its principal objectives are to detect Earth-like planets around nearby stars, to analyze the composition of their atmospheres and to assess their ability to sustain life as we know it. Darwin is conceived as a space “nulling interferometer” which makes use of on-axis destructive interferences to extinguish the stellar light while keeping the off-axis signal of the orbiting planet. Within the frame of the Darwin program, definition studies of a Ground based European Nulling Interferometry Experiment, called GENIE, were completed in 2005. This instrument built around the Very Large Telescope Interferometer (VLTI) in Paranal will test some of the key technologies required for the Darwin Infrared Space Interferometer. GENIE will operate in the L’ band around 3.8 microns as a single Bracewell nulling interferometer using either two Auxiliary Telescopes (ATs) or two 8m Unit Telescopes (UTs). Its science objectives include the detection and characterization of dust disks and low-mass companions around nearby stars.

The coupling between a turbulence-distorted optical beam and a single-mode waveguide is addressed. The coupling efficiency and the coupled phase are derived, both without aberrations and with small aberrations. These analytical expressions are validated by numerical simulations. Correction with adaptive optics is investigated. In the general case, the Strehl ratio is a pessimistic estimator, and the coupled phase is different and has a smaller variance than the classical phase averaged over the pupil. Application fields are heterodyne detection and stellar interferometry, for which spatial and modal filtering are distinguished.

We report on first scientific observations of a few bright late type stars by direct long baseline interferometry in the thermal infrared (3.4 to 4.1 microns) obtained with the TISIS (Thermal Infrared Stellar Interferometric Set-up) experiment of the IOTA (Infrared and Optical Telescope Array) interferometer. Beam combination is provided by a single-mode fluoride glass coupler optimized for operation in that wavelength domain and yielding visibility measurements with 2% typical relative accuracy. First precise estimations of uniform disk diameters for α Orionis, a Herculis, o Ceti and R Leonis are presented in the L band. Very large increases (50 to 70%) in apparent angular diameters have been found for the 2 Mira stars o Ceti and R Leonis with respect to previous measurements obtained at shorter infrared wavelengths and same luminosity phase. Extended optically thin close-by dust shells characterized by Infrared Spatial Interferometer (ISI) measurements are not found to play a significant role in the observed L band intensity distribution. Gas properties are likely to have a greater impact at these wavelengths. Our o Ceti interferometric observations look indeed in good agreement with the presence of very extended circumstellar gas layers (mostly H 2 O and SiO) derived from recent Infrared Space Observatory (ISO) thermal infrared spectral data.

The FLUOR project started in 1991 with a prototype fiber recombination unit that transformed a pair of independent 80 cm telescopes into a stellar interferometer. An improved version of this unit is now used as part of the instrumentation at the IOTA interferometer on Mt Hopkins (Arizona). The system is based on fluoride glass single-mode waveguides (non polarization-preserving) for observations at infrared wavelengths between 2 and 2.4 μm. A triple coupler performs the coherent recombination of the beams and extracts two calibration signals. A passive polarization control is sufficient to maintain the interferornetric efficiency above 80 %, with variations of the order of a few percents from one night to the next. The combination FLUOR/JOTA now routinely provides stellar interferograms on baselines ranging between 5 and 38 m, with an accuracy of 1 % or better in the fringe visibility measurements.

Because they have demonstrated very high visibility accuracies and have greatly simplified conventional interferometric recombination devices, single-mode fibers are being seriously considered in several optical interferometry projects. This paper deals with light coupling into single-mode fibers. An analytical expression of the coupling efficiency is derived for the monochromatic case. Then, the effect of purely static aberrations is considered. Finally, coupling in the presence of atmospheric turbulence is investigated for long exposure times. Using temporal sequencies of turbulent wavefronts, simulations are performed for a wide range of seeing conditions with both uncorrected turbulence and various levels of correction by an adaptive optics system.

In the area of long-baseline nulling interferometry, high rejection ratios are needed to cancel out the light of a bright central source and look for nearby faint structures or companions. These rejection requirements directly translate into drastic optical constraints. We discuss the possibility of using single-mode waveguides for such applications. Conversely to simple pinholes, single-mode waveguides efficiently correct wave-front defects of both high- and low-order spatial frequencies, ensure a perfect matching of the amplitude profiles coming from the various beams, and can be used with almost optimum coupling efficiency over a broad optical bandpass (typically an octave). They then appear to greatly enhance the feasibility of high-dynamic interferometric coronagraphs.