Asgard/NOTT: the performance of the integrated optics 4-telescope beam combiner for double-Bracewell nulling interferometry

Le résumé fourni par la source

The Asgard/NOTT visitor instrument employs mid-infrared L′-band nulling interferometry to probe hot exozodiacal dust and young Jupiter-like planets near the water snowline by combining beams from the four telescopes of the VLTI. Its integrated optics beam combiner, the 4T-nuller, is based on the double-Bracewell architecture, allowing self-calibration through kernel nulling. This 4T-nuller is fabricated in gallium lanthanum sulfide (GLS) glass substrate using ultrafast laser inscription (ULI). To verify its on-sky capability, we conducted extensive laboratory testing with an interferometric test bench that produces four coherent beams simultaneously to simulate VLTI operations. The 4T-nuller features three cascaded achromatic directional couplers (DCs) in its interferometric section, including two DCs with 40/60 splitting ratios in the first stage and a central DC with a 50/50 splitting ratio in the second stage, across 3.65–3.85μm. Achromatic 40/60 Y-junctions are used for routing signals between the photometry and interferometry sections, respectively. The total throughput is 37% (including Fresnel losses), with further improvements expected once anti-reflection coatings are applied. The device exhibits low differential birefringence. Under ambient conditions without polarization control, we measure an average broadband raw null of 8.13 ± 0.03 × 10−3 and a self-calibrated null of 1.14 ± 0.01 × 10−3 over 20 seconds. To our knowledge, this is the first demonstration of a deep null in the L′-band using an integrated-optics beam combiner that also meets the requirements of the NOTT instrument.

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