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First fringes with an integrated-optics beam combiner at 10 μm. A new step towards instrument miniaturization for mid-infrared interferometry

Abstract : Context. Observations of milliarcsecond-resolution scales and high dynamic range hold a central place in the exploration of distant planetary systems in order to achieve, for instance, the spectroscopic characterization of exo-Earths or the detailed mapping of their protoplanetary disc birthplace. Multi-aperture infrared interferometry, either from the ground or from space, is a very powerful technique to tackle these goals. However, significant technical efforts still need to be undertaken to achieve a simplification of these instruments if we wish to recombine the light from a large number of telescopes. Integrated-optics concepts appear to be a suitable alternative to the current conventional designs, especially if their use can be extended to a higher number of astronomical bands.
Aims: This article reports, for the first time to our knowledge, the experimental demonstration of the feasibility of an integrated-optics approach to mid-infrared beam combination for single-mode stellar interferometry.
Methods: We fabricated a two-telescope beam combiner prototype integrated on a substrate of chalcogenide glass, a material transparent from ~1 μm to ~14 μm. We developed laboratory tools to characterize in the mid-infrared the modal properties and the interferometric capabilities of our device.
Results: We obtain interferometric fringes at 10 μm and measure a mean contrast V = 0.981 ± 0.001 with high repeatability over one week and high stability over a time-period of ~5 h. We show experimentally - as well as on the basis of modeling considerations - that the component has a single-mode behavior at this wavelength, which is essential to achieve high-accuracy interferometry. From previous studies, the propagation losses are estimated to be 0.5 dB/cm for this type of component. We also discuss possible issues that may impact the interferometric contrast.
Conclusions: The IO beam combiner performs well at the tested wavelength. We also anticipate the requirement of a closer matching between the numerical apertures of the component and the (de)coupling optics to optimize the total throughput. The next step foreseen is the achievement of wide-band interferograms.
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L. Labadie, G. Martín, N. C. Anheier, B. Arezki, H. A. Qiao, et al.. First fringes with an integrated-optics beam combiner at 10 μm. A new step towards instrument miniaturization for mid-infrared interferometry. Astronomy and Astrophysics - A&A, EDP Sciences, 2011, 531, ⟨10.1051/0004-6361/201116727⟩. ⟨insu-03624667⟩

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