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

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 and achromatic null together with phase measurements for fringe tracking. We present a derivation of the transfer matrix of the tricoupler, including its chromatic behavior, and our 3D design of a fully symmetric tricoupler, built upon a previous design proposed for the guided-light interferometric nulling technology instrument. It enables a broadband null with symmetric, baseline-phase-dependent splitting into a pair of bright channels when inputs are in antiphase. Within some design trade space, either the science signal or the fringe tracking ability can be prioritized. We also present a tapered-waveguide 180-deg-phase shifter with a phase variation of 0.6 deg in the 14-to-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.