Integrated Optics for Astronomical Interferometry. III. Optical Validation of a Planar Optics Two-Telescope Beam Combiner.

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Abstract
The optical characterization of a fiber-connected planar optics beam combiner dedicated to astronomical interferometry for two telescopes is presented. The beam combiner, fully integrated on a single 5 mm × 40 mm glass chip, is tested as the central part of an astronomical instrument. The single-mode waveguides are made by silver-ion-exchange technology upon glass substrates and provide spatial filtering, which improves the visibility measurement accuracy by selecting only the fundamental mode of the beams at the telescope focal plane. A global optical throughput of 43% is measured, and the sources of losses are identified and examined in detail. Solutions for improving this throughput are proposed. High and stable contrasts are obtained with a 1.55-µm laser diode (≳96%) and with a white-light source (∼92%) in the astronomical H filter (1.43 µm; 1.77 µm). The need for accurate control of differential instrumental polarization is demonstrated. In this context the intrinsic polarization-maintaining property of the planar optics component is characterized. This validation of the important potential uses of integrated planar optics should be valuable for future design of optical telescope arrays.

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