Achromatic design of a photonic tricoupler and phase shifter for broadband nulling interferometry

TL;DR

This paper introduces an achromatic photonic tricoupler and phase shifter designed for broadband nulling interferometry, improving exoplanet imaging by addressing phase and chromaticity issues in nullers.

Contribution

It presents a new symmetric tricoupler design with broadband nulling capabilities and a near-achromatic phase shifter, enhancing interferometric performance for exoplanet detection.

Findings

Achieved a broadband null with symmetric, baseline-phase-dependent splitting.

Developed a tapered-waveguide phase shifter with 0.6° variation in 1.4-1.7 μm band.

Devices enable deep nulling and real-time fringe phase measurement.

Abstract

Nulling interferometry is one of the most promising technologies for imaging exoplanets within stellar habitable zones. The use of photonics for carrying out nulling interferometry enables the contrast and separation required for exoplanet detection. So far, two key issues limiting current-generation photonic nullers have been identified: phase variations and chromaticity within the beam combiner. The use of tricouplers addresses both limitations, delivering a broadband, achromatic null together with phase measurements for fringe tracking. Here, we present a derivation of the transfer matrix of the tricoupler, including its chromatic behaviour, and our 3D design of a fully symmetric tricoupler, built upon a previous design proposed for the GLINT instrument. It enables a broadband null with symmetric, baseline-phase-dependent splitting into a pair of bright channels when inputs are in anti-phase. Within some design trade space, either the science signal or the fringe tracking ability can be prioritised. We also present a tapered-waveguide $180^\circ$-phase shifter with a phase variation of $0.6^\circ$ in the $1.4-1.7~\mu$m band, producing a near-achromatic differential phase between beams{ for optimal operation of the tricoupler nulling stage}. Both devices can be integrated to deliver a deep, broadband null together with a real-time fringe phase metrology signal.