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.
Nulling interferometry has emerged as a promising technique for imaging exoplanets, effectively overcoming the challenges of contrast and angular resolution faced by ground-based telescopes. The Guided Light Interferometric Nulling Technology (GLINT) instrument at the Subaru telescope in Hawaii utilises this technique, where a new integrated optics beam combiner will be deployed, capable of simultaneously performing nulling interferometry and fringe tracking. Fabricated using the femtosecond laser direct write technique in boro-aluminosilicate glass, the beam combiner integrates three single-mode waveguides as inputs to form 3 interferometric baselines. The design, fabrication and laboratory characterisation of the new beam combiner will be presented. The beam combiner comprises of 3D-printed micro lenses, a chrome mask, Y-Junctions, achromatic phase shifters and tricouplers to produce for each baseline an achromatically-nulled and two phase-sensitive bright outputs. This results in a total of 12 outputs that are directed onto a CRED2 camera.
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 π 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.
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