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. Comprising of 3 input single-mode waveguides, the tricoupler produces 3 outputs, consisting of the interferometric nulled output and two phase-sensitive bright outputs.

These 3D waveguide tricouplers are fabricated using the femtosecond laser direct-write technique. This process involves a tightly focused laser to modify the refractive index of a boro-aluminosilicate glass sample, creating optical waveguides. We present a rigorous optimisation of the tricouplers which includes a numerical solution to coupled-mode equations to obtain coupling coefficients and propagation constants that are used to optimise the fabrication process for the J (1.1 μm - 1.4 μm) and H (1.5 μm - 1.8 μm) wavelength bands. Furthermore, the polarisation behaviour, the wavelength behaviour and interferometric performance has been investigated to create an accurate transfer matrix of the device.