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arxiv: 2511.19790 · v1 · submitted 2025-11-24 · 🌌 astro-ph.IM

Asgard/NOTT: L-band nulling interferometry at the VLTI -- III. The mid-infrared integrated optics beam combiner for NOTT

A. Sanny (1 , 2 , 3) , L. Labadie (1) , S. Gross (2 , 5) , K. Barjot (1) , R. Laugier (4)
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G. Garreau (4) M.-A. Martinod (4) D. Defr\`ere (4) M. J. Withford (2) ((1) I. Physikalisches Institut Universit\"at zu K\"oln K\"oln Germany (2) MQ Photonics Research Centre School of Mathematical Physical Sciences Macquarie University Australia (3) Department of Physics Astronomy University of California Los Angeles USA (4) Institute of Astronomy KU Leuven Leuven Belgium (5) MQ Photonics Research Centre School of Engineering Australia)
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Pith reviewed 2026-05-17 04:17 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords nulling interferometryintegrated opticsbeam combinerVLTIL-bandexozodiacal dustultrafast laser inscriptionmid-infrared
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The pith

A four-telescope double-Bracewell integrated optics beam combiner achieves a self-calibrated null of 1.14 x 10^-3 in the L' band.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that a single-mode four-telescope beam combiner can be made in mid-infrared glass and delivers deep broadband nulling at room temperature. The device follows a double-Bracewell layout to combine light from four telescopes while suppressing on-axis starlight. Laboratory tests with a 200-nanometer bandwidth centered at 3.8 microns produced an average raw null of 8.13 x 10^-3 and a self-calibrated null of 1.14 x 10^-3, together with a theta^6 null shape. The combiner also shows achromatic splitting ratios and 37 percent throughput before anti-reflection coatings. These results mark the first reported broadband L' deep null from any four-telescope integrated optics combiner and set the stage for cryogenic validation.

Core claim

We manufactured a single-mode four-telescope double-Bracewell IOBC in GLS mid-infrared transparent glass using Ultrafast Laser Inscription. The directional couplers forming the 4T-nuller exhibit achromatic splitting ratios across 3.65-3.85 um, with 40/60 and 50/50 splitting for the side and central couplers. Operating at room temperature with 200 nm bandwidth centered at 3.8 um and without polarization control, we measure an average raw null of 8.13+/-0.03x10-3 and a self-calibrated null of 1.14+/-0.01x10-3 while reproducing a theta^6 broad null. This is the first measurement of a broadband L' deep null obtained with a four-telescope integrated optics beam combiner.

What carries the argument

The single-mode four-telescope double-Bracewell integrated optics beam combiner (4T-nuller) fabricated by ultrafast laser inscription in GLS glass, whose directional couplers provide the achromatic splitting and nulling.

If this is right

  • The measured achromatic splitting ratios allow broadband L' operation without additional polarization control.
  • A 37 percent throughput leaves room for improvement once Fresnel losses are reduced by anti-reflection coatings.
  • Reproduction of the theta^6 null shape demonstrates the combiner can handle the expected on-sky signal geometry.
  • The achieved self-calibrated null depth meets the requirement for detecting hot exozodiacal dust and young planets near the water snowline.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Successful cryogenic operation would allow the NOTT instrument to begin routine observations of exozodiacal dust at the VLTI.
  • The same ultrafast laser inscription approach could be adapted to produce combiners for other mid-infrared bands or for instruments on future facilities.
  • Integration with existing VLTI adaptive optics would test whether the demonstrated null depth holds under real atmospheric conditions.

Load-bearing premise

The room-temperature laboratory setup with four coherent beams from a double Michelson interferometer accurately predicts the null performance the combiner will deliver when cooled and fed by the actual VLTI telescopes.

What would settle it

A direct measurement of the null depth after the combiner is cooled to cryogenic temperature and illuminated by the four VLTI telescope beams would confirm or refute the laboratory results.

Figures

Figures reproduced from arXiv: 2511.19790 by ((1) I. Physikalisches Institut, 2, (2) MQ Photonics Research Centre, 3), (3) Department of Physics, (4) Institute of Astronomy, 5), (5) MQ Photonics Research Centre, A. Sanny (1, Astronomy, Australia, Australia), Belgium, D. Defr\`ere (4), Germany, G. Garreau (4), K. Barjot (1), K\"oln, KU Leuven, Leuven, L. Labadie (1), Los Angeles, Macquarie University, M.-A. Martinod (4), M. J. Withford (2), Physical Sciences, R. Laugier (4), School of Engineering, School of Mathematical, S. Gross (2, Universit\"at zu K\"oln, University of California, USA.

Figure 1
Figure 1. Figure 1: Functional phasor diagram of the double-Bracewell nuller with complex amplitudes of inputs 1 (green), 2 (yellow), 3 (red), and 4 (blue). DC1, DC2, and DC3 are three directional couplers with 50/50 splitting ratio in intensity. DC1 and DC2 produce constructive outputs at #1 and #4, while the two nulled outputs are feeding DC3. The nulled outputs #2 and #3 feature two transmission maps, which are centro-symm… view at source ↗
Figure 2
Figure 2. Figure 2: Top left: NOTT’s 4T-nuller photonic chip with the network of waveguides manufactured by ULI (see Sect. 3.2 for details). Top right: cross-section of the input/output facet of a triplet waveguide; Bottom left: schematic of the 4T-nuller showing the four inputs, the side-step S-bends, and the three cascaded directional couplers; Bottom right: vi￾sualization of the different outputs of the 4T-nuller (photomet… view at source ↗
Figure 3
Figure 3. Figure 3: Layout of the mid-infrared characterization testbench and its components. SCS: supercontinuum source; BBS: blackbody source; AC: achromat; AP: aperture; L′ : broadband filter; PH: pinhole; BS: beam splitter; M1: fixed mirror; M2, M3, M4: movable mirrors with delay lines; PL: mid-infrared linear polarizer; HeNe: metrology laser to calibrate the delay lines; Photonic Chip contains the 4T-nullers. Top inset: … view at source ↗
Figure 4
Figure 4. Figure 4: Measurement of the splitting ratio of the three directional couplers with 7.5 mm interaction length forming the 4T-nuller (three plots from the left) and for the Y-junction photometric tap. The error bars correspond to three times the standard deviation obtained from four independent measurements. In the last plot, the gray curves depict one of the four Y-junctions in the characterized 4T-nuller, while the… view at source ↗
Figure 5
Figure 5. Figure 5: Relative throughput in percent for the reference straight waveg￾uide and S-bends of 1.1 mm amplitude, along with the 4T-nuller. Error bars denote standard deviations from multiple identical straight and S￾bend waveguides on the same photonic chip, while the standard devia￾tion for the 4T-nuller is based on the throughputs of its four individual inputs. between 3.65 µm and 3.85 µm ( [PITH_FULL_IMAGE:figure… view at source ↗
Figure 6
Figure 6. Figure 6: Polarization contrast at the 4T-nuller output I2 as a function of the input polarization angle at inputs 2 and 3 in steps of 20◦ . The error bars show the standard deviation over 200 points for each input polar￾ization orientation. The arrows indicate the orientation of the polarized beam relative to the vertical position of a triple-track waveguide’s cross￾section in the inset [PITH_FULL_IMAGE:figures/fu… view at source ↗
Figure 7
Figure 7. Figure 7: Orientation of the output elliptical polarization measured at I2 as a function of the angle of the linear input polarization at inputs 2 (blue) and 3 (red). The dashed line corresponds to the case where the input polarization angle is maintained at the output. Benoît et al. 2021; Siliprandi et al. 2024), suggesting possible shape birefringence induced by the laser writing process. 4.5. High-contrast nullin… view at source ↗
Figure 9
Figure 9. Figure 9: Self-calibrated (scn) and raw (rn) nulling ratio measured with the photonic 4T-nuller. The yellow dashed line shows the limit in the attainable dynamic range at room temperature set by the thermal back￾ground. Each point gives the mean value and the error on the mean estimated over 50 frames. raw (Eq. 2) and self-calibrated (Eq. 3) nulls derived from the data of [PITH_FULL_IMAGE:figures/full_fig_p008_9.png] view at source ↗
read the original abstract

The NOTT visitor instrument at the VLTI will 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 (IOBC) that fulfills specific requirements. Our goal was to manufacture the mid-infrared IOBC for NOTT based on the double-Bracewell architecture and run a detailed laboratory characterization in the L' band. We focus on the achievable raw and self-calibrated nulling ratios. We use 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 analyze the modal, chromatic, and polarization behavior of the IOBC, and measure its total throughput. We were able to manufacture a single-mode four-telescope double-Bracewell IOBC in GLS mid-infrared transparent glass using Ultrafast Laser Inscription. We show that the directional couplers forming the four-telescope IOBC (4T-nuller) have an achromatic splitting ratio across the band 3.65-3.85 um 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 anti-reflection coatings, and quantify differential birefringence. Operating at room temperature, with 200 nm bandwidth centered at 3.8 um and without polarization control, we measure an average raw null of 8.13+/-0.03x10-3 and a self-calibrated null of 1.14+/-0.01x10-3. Finally, we show that a theta^6 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. The next step foresees testing the 4T-nuller in cryogenic conditions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 2 minor

Summary. The manuscript reports the fabrication via ultrafast laser inscription of a single-mode four-telescope double-Bracewell integrated optics beam combiner (IOBC) in GLS glass for the NOTT L-band nulling instrument at the VLTI. Laboratory characterization at room temperature with a double-Michelson four-beam setup over a 200 nm bandwidth centered at 3.8 µm yields achromatic 40/60 and 50/50 splitting ratios in the directional couplers, 37% total throughput (including Fresnel losses), quantified differential birefringence, an average raw null depth of 8.13±0.03×10^{-3}, a self-calibrated null of 1.14±0.01×10^{-3}, and experimental reproduction of the expected θ^6 null shape. Cryogenic testing is identified as future work.

Significance. If the reported laboratory performance holds, this constitutes the first experimental demonstration of broadband L' nulling with a four-telescope IOBC, directly supporting the technical readiness of the ULI fabrication route for mid-IR single-mode beam combination. The direct measurements of splitting ratios, throughput, modal behavior, and null depths provide concrete, reproducible evidence for the device's room-temperature functionality without reliance on fitted parameters or circular derivations.

minor comments (2)
  1. [Results] Results section: expand the uncertainty budget to show explicit propagation from measured splitting ratios, throughput, and modal content into the quoted null-depth uncertainties; the current ±0.03×10^{-3} and ±0.01×10^{-3} values are given but the full chain is not visible.
  2. [Discussion] Discussion: clarify whether the measured differential birefringence will require active polarization control on-sky or whether the self-calibration procedure already mitigates it to the reported level.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their positive assessment of our manuscript and for recommending minor revision. The referee's summary accurately captures the key results and future plans. We respond to the summary below.

read point-by-point responses

  1. Referee: The manuscript reports the fabrication via ultrafast laser inscription of a single-mode four-telescope double-Bracewell integrated optics beam combiner (IOBC) in GLS glass for the NOTT L-band nulling instrument at the VLTI. Laboratory characterization at room temperature with a double-Michelson four-beam setup over a 200 nm bandwidth centered at 3.8 µm yields achromatic 40/60 and 50/50 splitting ratios in the directional couplers, 37% total throughput (including Fresnel losses), quantified differential birefringence, an average raw null depth of 8.13±0.03×10^{-3}, a self-calibrated null of 1.14±0.01×10^{-3}, and experimental reproduction of the expected θ^6 null shape. Cryogenic testing is identified as future work.

    Authors: We thank the referee for this accurate summary. All measurements were performed at room temperature as stated, and cryogenic testing is indeed identified as the next development step for the NOTT instrument. revision: no

Circularity Check

0 steps flagged

No significant circularity

full rationale

The manuscript reports direct experimental fabrication of a 4T double-Bracewell IOBC via ultrafast laser inscription in GLS glass followed by laboratory measurements of splitting ratios, throughput, birefringence, and null depths in a double-Michelson setup. No derivations, predictions, or fitted parameters are invoked; all reported values (raw null 8.13e-3, self-calibrated null 1.14e-3, 37% throughput, theta^6 shape) are obtained from physical characterization at room temperature. The text explicitly defers cryogenic testing to future work and makes no claim that laboratory results equal on-sky performance. No self-citations serve as load-bearing premises for the central results, and no equations or ansatzes reduce the measured quantities to prior inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Experimental instrumentation paper relying on established optical fabrication and interferometry principles without introducing new free parameters, axioms beyond standard optics, or invented entities.

axioms (1)
  • standard math Standard principles of single-mode propagation, directional coupling, and interferometric nulling apply to the GLS glass and ULI-fabricated waveguides in the L' band.
    Invoked implicitly in design of couplers and interpretation of null measurements.

pith-pipeline@v0.9.0 · 5851 in / 1227 out tokens · 27578 ms · 2026-05-17T04:17:50.384802+00:00 · methodology

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