Description
Nulling interferometry provides a method for detecting faint companions close to bright stars by suppressing the on-axis stellar signal while transmitting light from an off-axis source. This approach is implemented in the Guided Light Interferometric Nulling Technology (GLINT) instrument at the Subaru Telescope, where an integrated photonic beam combiner is used to perform simultaneous nulling interferometry and fringe tracking. This work presents the design, fabrication, and laboratory characterisation of an upgraded three-input beam combiner developed to improve optical path-length control and interferometric stability.
The device is fabricated in borosilicate glass using the femtosecond laser direct write technique, in which a tightly focused laser produces a localised refractive index modification to form single-mode waveguides. Light from three selected regions of the telescope’s pupil is focused by a custom microlens array and coupled into the three input waveguides of the beam combiner. Each input is divided using a Y-junction, splitting light for photometry and interferometry. The waveguides are then remapped using a revised geometry that improves optical path-length matching between the interferometric arms. For each baseline, the interferometric channels are routed to an achromatic phase shifter, which introduces the required (\pi) phase delay across the H band, before entering a tricoupler resulting in one nulled output and two phase-sensitive bright outputs for fringe tracking. Laboratory characterisation showed that the upgraded Y-junctions produced splitting ratios close to the target values of (50/50) and (33/67), while the beam combiner achieved a monochromatic null depth of ($2\times10^{-3}$) at 1550~nm and a broadband null depth of ($6\times10^{-3}$). Thermal phase shifters are integrated above selected waveguides to provide active post-fabrication control of the optical pathlength. Together, these upgrades improve the stability and phase control required for multi-baseline nulling interferometry.
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