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

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 $\left(10^{-4}\right)$ 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.

Figures (10)

• Figure 1: The RSoft CAD of the three tri-couplers (not to scale).
• Figure 2: Normalized intensity in the three tri-coupler configurations (Standard, MMI, and Tapered) for the splitting of a single input (Split), the constructive interference in the coupler for two equal intensity inputs with a 0° phase offset (Constructive), and the destructive interference for two equal intensity inputs with a 180° phase offset (Destructive).
• Figure 3: Normalized intensity in the three tri-coupler configurations (Standard, MMI, and Tapered) for the left, center, and right output waveguides. Equal-intensity launch fields are injected into the two outer input ports across the full wavelength range as a function of phase difference.
• Figure 4: Comparison of constructive interference in the central output when injecting equal-intensity electric fields with a 0° phase difference into the outer waveguides, for the three tri-coupler designs, normalized to total transmission.
• Figure 5: Simulated loss of the MMI (orange) compared with the standard tri-coupler (green), tapered tri-coupler (blue), and a standard tri-coupler without bends (red).