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

Astrophotonics will be central to the next generation of astronomical instrumentation, enabling lightweight, compact, and environmentally stable photonic integrated circuits for both ground-based observatories and future space missions. One key application is beam combination for nulling interferometry, which suppresses starlight to reveal exoplanets and companions. Compact, broadband photonic beam combiners are essential for enabling complex circuitry on a single chip and for scalable solutions for single- and multi-telescope instruments, and are investigated herein. Two-waveguide photonic combiners rely on symmetric evanescent coupling to interfere light, which is inherently chromatic and requires modification for broadband operation. 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 two evanescent tri-couplers and a multimode interference coupler (MMI) across the 1.5-1.8 µm band, evaluating exoplanet throughput, starlight attenuation, sensing characteristics, and estimations on fabrication tolerance. All three tri-couplers achieved >40 dB attenuation over a ≥270 nm bandwidth. However, the standard tri-coupler was outperformed by both a bespoke tapered tri-coupler and the MMI, each of which achieved exoplanet throughput >85% across the band, excluding component losses. Including component loss, the tapered tri-coupler has the highest total throughput, averaging ∼96%. The standard tri-coupler began with an equivalent exoplanet throughput, falling 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 40 dB (10^-4) 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. This work provides a simulation suite for three tri-couplers. They can be selected based on robustness to common fabrication tolerances (the MMI), exoplanet throughput (the tapered tri-coupler), and/or the sensing performance (the tapered tri-coupler).