Achromatic nulling interferometry and fringe tracking with 3D-photonic tricouplers with GLINT

Nulling interferometry is one of the most promising techniques for imaging exoplanets at solar system scales as it simultaneously meets the stringent requirements for contrast and angular resolution. The GLINT instrument, operating at Subaru telescope behind the SCExAO extreme adaptive optics system, has delivered significant advances in performance, paving the way for a science-ready instrument. Results from previous commissioning runs have confirmed that integrated optics and self-calibration methods yield a robust pathway to detecting exoplanets below the diffraction limit of the telescope. Furthermore, limitations to the current performance have been identified. The most critical are the compensation of remaining phase fluctuations and delivering deep achromatic nulls. Specifically, one technology that offers enormous promise to resolve these issues is the use of tricouplers to perform simultaneous nulling and fringe tracking, or wavefront characterization and correction free of non-common path aberrations. By adding a broadband p radian phase-shifter, fringe tracking and path length stabilization is performed around a deep achromatic null, providing excellent high contrast performance at small angular scales. In this paper, we present the commissioning of GLINT, its current challenges and describe the modelling of devices to overcome them. We discuss the expected performance of a nuller based on these principles of phase control and null depth as implemented within the GLINT instrument. We further present laboratory characterization of 3D-written tricouplers made using ultrafast laser inscription.