Optimisation of laser-written tricouplers for nulling interferometry in the J- and H-band

The technique of nulling interferometry is helping to overcome the challenges of directly detecting exoplanets and is the foundation for the Guided Light Interferometric Nulling Technology (GLINT) instrument located at the Subaru telescope in Hawaii. A new integrated optics beam combiner will be deployed to simultaneously perform nulling interferometry and fringe tracking. This is enabled by using a photonic device called a tricoupler fabricated using the femtosecond direct-write technique. The tricoupler has 3 input single-mode waveguides, producing 3 outputs consisting of an interferometric output and two phase-sensitive bright outputs. We present a rigorous optimisation of the tricouplers which includes a numerical solution to coupled mode equations to obtain coupling coefficients and propagation constants, used to optimise the fabrication process for the J- and H- bands. The polarisation behaviour, the wavelength behaviour and interferometric performance has been investigated to create an accurate transfer matrix of the device.