Ultrafast laser inscription of achromatic phase shifters for the GLINT integrated nulling interferometer

The characterization of exoplanets is critical to understanding planet formation, and the search for life. To directly characterize Earth-like exoplanets, a contrast ratio of 10−10 between the star and planet must be overcome. The GLINT instrument is an integrated photonic nulling interferometer fabricated using ultrafast laser inscription, where all routing, beam splitting, phase shifting and recombination is done within a single photonic chip. The instrument uses pupil sub-aperture's to create different baselines. An achromatic π phase shift is then applied to baseline pairs and recombined in a tricoupler. This suppresses the on-axis starlight while off-axis planet light is observed in the "null" output. The Achromatic phase shifters are designed using differential waveguide dispersion where lengths of three different widths combinations are used to create a differential achromatic phase shift. Adiabatic tapers are then used to transition between regions of different widths to reduce losses. Using this method, a 3.6 mm long 180° achromatic phase shifter was fabricated, and measured to have a phase shift of 171.5 ± 4° from 1440-1640 nm. This phase shift is comparable to GRAVITYs 90° phase shifters (85 ± 3°).