Asgard/NOTT: L-band nulling interferometry at the VLTI

III. The mid-infrared integrated optics beam combiner for NOTT

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
² MQ Photonics Research Centre, School of Mathematical and Physical Sciences, Macquarie University, New South Wales 2109, Australia
³ Department of Physics & Astronomy, University of California, Los Angeles, CA 90095, USA
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ MQ Photonics Research Centre, School of Engineering, Macquarie University, New South Wales 2109, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 June 2025
Accepted: 17 November 2025

Abstract

Context. The NOTT visitor instrument at the VLTI is designed to characterize hot exozodiacal dust and young Jupiter-like planets at the water snowline via L′ band nulling interferometry. The beam combination will be achieved by a four-telescope integrated optics beam combiner, which should fulfill specific requirements.

Aims. Our goal was to manufacture the mid-infrared integrated optics beam combiner for NOTT based on the double-Bracewell architecture and to run a detailed laboratory characterization in the L′ band. In particular, our focus was on the achievable raw and self-calibrated nulling ratios.

Methods. We used a setup based on a double Michelson interferometer to produce four broadband-coherent beams simulating the four telescopes of the VLTI and perform broadband nulling at room temperature. We also analyzed the modal, chromatic, and polarization behavior of the integrated optics beam combiner, and we measured its total throughput.

Results. We were able to manufacture a single-mode four-telescope double-Bracewell beam combiner in gallium lanthanum sulfide mid-infrared transparent chalcogenide glass using ultrafast laser inscription. We show that the directional couplers forming the four-telescope beam combiner (4T-nuller) have an achromatic splitting ratio across the band 3.65–3.85 μm with a 40/60 and 50/50 splitting for the side couplers and the central coupler, respectively. We report a total throughput of 37%, including the Fresnel losses that will be mitigated with antireflection coatings, and we quantified the differential birefringence. Operating at room temperature with a 200 nm bandwidth centered at 3.8 μm and without polarization control, we measured an average raw null of 8.13 ± 0.03 × 10⁻³ and a self-calibrated null of 1.14 ± 0.01 × 10⁻³. Finally, we show that a θ⁶ broad null can be experimentally reproduced in these conditions. This is, to our knowledge, the first measurement of a broadband L′ deep null obtained with a four-telescope integrated optics beam combiner.

Conclusions. Following these promising results, the next step would involve testing the performance of the 4T-nuller in cryogenic conditions.