  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.
