[論文レビュー] Tri-coupler geometries for achromatic nulling interferometry in the near infrared

Harry-Dean Kenchington Goldsmith, Nemanja Jovanovic+5 more|arXiv (Cornell University)|Feb 27, 2026

Adaptive optics and wavefront sensing被引用数 0

ひとこと要約

The paper compares three tri-coupler designs (standard evanescent, tapered evanescent, and MMI) for broadband achromatic nulling interferometry in the 1.5–1.8 μm band, assessing exoplanet throughput, starlight attenuation, sensing, and fabrication tolerance. The tapered tri-coupler and MMI outperform the standard design in throughput, with the MMI showing greatest fabrication tolerance, while achieving deep starlight suppression across bandwidth.

ABSTRACT

Astrophotonics will be central to astronomical instrumentation, enabling lightweight, compact, and environmentally stable systems for both ground-based observatories and space missions. One key application is beam combination for nulling integrated with a photonic lantern, and long baseline nulling interferometry, which suppresses starlight to reveal exoplanets and companions. Compact, broadband photonic beam combiners are essential for providing a pathway to complex circuitry on a single chip and scalable solutions for single and multi-telescope instruments, and are investigated herein. Two-waveguide photonic combiners rely on symmetric coupling to interfere light, which is chromatic and requires modification for broadband operation. A three-waveguide configuration (tri-coupler) offers the potential for deeper, broader, and stable achromatic nulls. This work compares simulations of two evanescent tri-couplers and a multimode interference coupler (MMI) across the 1.5 - 1.8 $μ$m band, evaluating exoplanet throughput, starlight attenuation, sensing characteristics, and estimations on fabrication tolerance. The standard tri-coupler was outperformed by both a tapered tri-coupler and the MMI, each of which achieved exoplanet throughput >85% throughout the band. The standard design recorded a minimum exoplanet throughput of 50% at the waveband's extremes. The tapered tri-coupler was further redesigned to achieve a non-degenerate sensing state. The MMI, while limited to a starlight attenuation of 40 dB $\left(10^{-4} ight)$ by uncoupled light, showed the greatest tolerance to fabrication errors, offering strong practical potential. Future designs aim to combine high exoplanet throughput, deep starlight attenuation, and non-degenerate sensing within a single integrated architecture.

研究の動機と目的

• Motivate the use of photonic integrated circuits for nulling interferometry in exoplanet detection.
• Assess three tri-coupler geometries (standard evanescent, tapered evanescent, and MMI) for broadband achromatic nulling.
• Quantify exoplanet throughput, starlight attenuation, and fabrication tolerance across 1.5–1.8 μm.

提案手法

• Model three tri-coupler geometries (standard evanescent, tapered, and MMI) in lithographically fabricated silica-based waveguides.
• Simulate TE-mode light propagation and compute exoplanet throughput, starlight attenuation, and losses across 1.5–1.8 μm.
• Define attenuation via Destructive output relative to total output and relate to exoplanet S/N.
• Use phase-triggered two-input-one-output nulling with 180° phase difference between outer inputs.
• Evaluate sensitivity to fabrication tolerances and bend losses.

(a) Standard tri-coupler

実験結果

リサーチクエスチョン

• RQ1 Which tri-coupler geometry provides the best compromise among exoplanet throughput, deep starlight attenuation, and bandwidth achromaticity?
• RQ2 How do bend losses and material losses affect overall nulling performance across the 1.5–1.8 μm band?
• RQ3 Can a design be found that enables non-degenerate sensing while maintaining high exoplanet throughput?
• RQ4 How robust are the nulling and fringe-tracking capabilities to fabrication tolerances across designs?

主な発見

• The bespoke tapered tri-coupler and the MMI achieve exoplanet throughput >85% across the 1.5–1.8 μm band, versus ~50% for the standard design at band edges.
• The tapered tri-coupler provides an approximately achromatic exoplanet throughput, maintaining near-100% transmission across the band.
• The MMI shows the greatest tolerance to fabrication errors, suggesting strong practical potential despite higher intrinsic losses.
• The standard tri-coupler exhibits chromatic exoplanet throughput with peak around 1.55 μm and declines at longer wavelengths.
• Bend losses contribute to overall device loss, with MMI incurring higher intrinsic losses but more stable performance across wavelengths.
• A non-degenerate sensing state was demonstrated for the tapered tri-coupler, enabling potential continuous fringe tracking.

(b) Tapered tri-coupler