Tri-coupler geometries for achromatic nulling interferometry in the near-infrared

TL;DR

This paper compares different tri-coupler designs for achromatic nulling interferometry in the near-infrared, demonstrating high starlight suppression and throughput with potential for improved exoplanet detection.

Contribution

It introduces and evaluates a simulation suite for tri-coupler geometries, highlighting their advantages over traditional two-waveguide approaches in nulling interferometry.

Findings

All tri-couplers achieved >40dB attenuation over 270nm bandwidth.

The tapered tri-coupler achieved an average throughput of 97%.

The MMI design showed the greatest tolerance to fabrication errors.

Abstract

Astrophotonics is central to the next generation of astronomical instrumentation, enabling compact photonic integrated circuits for both ground-based observatories and future space missions. Beam combination for nulling interferometry suppresses starlight, revealing exoplanets and companions. Two-waveguide photonic combiners rely on symmetric evanescent, inherently chromatic, coupling to interfere light. A three-waveguide configuration, or tri-coupler, offers the potential for deeper, broader, and more stable achromatic nulls compared with two-waveguide approaches. This work compares the simulated performance of evanescent tri-couplers and a multimode interference coupler across the 1.5-1.8 micron band, evaluating exoplanet throughput, starlight attenuation, sensing characteristics, and estimations on fabrication tolerance. All three tri-couplers achieved >40dB attenuation over a 270nm bandwidth. Including component loss, the tapered tri-coupler has the highest total throughput, averaging 97%, whereas the standard tri-coupler began with an equivalent exoplanet throughput and fell to 50% at the band edges. The tapered tri-coupler was further redesigned to achieve a non-degenerate sensing state. The MMI, while limited to a starlight attenuation of 40dB by uncoupled light, showed the greatest tolerance to fabrication errors. Future designs aim to combine high exoplanet throughput, deep starlight attenuation, and non-degenerate sensing within a single integrated architecture. This work provides a simulation suite for three tri-couplers.