FIRST, the pupil-remapping fiber interferometer at Subaru telescope: towards photonic beam-combination with phase control and on-sky commissioning results (Conference Presentation)

FIRST (Fibered Imager foR a Single Telescope) is a post-AO instrument module that enables high-contrast imaging and spectroscopy at sub-diffraction limited spatial scales. FIRST achieves this through a unique combination of sparse aperture masking, spatial filtering, pupil remapping Fizeau interferometry, and cross-dispersion in the visible. The telescope pupil is divided into sub-pupils using a honeycomb array of micro-electro-mechanical mirrors, and the light from each sub-pupil injected into a separate single mode fiber that provides spatial filtering. The fibers, which are pathlength-matched to within a few tens of micrometers, reformat the sub-apertures into a linear non-redundant array allowing for the extraction of fringes from each possible baseline as well as wavelength dispersion to create ~130 spectral channels for every baseline combination over the 600-900nm spectral range. In this presentation, we will first report on the latest on-sky results obtained with FIRST. In its current design, the instrument was successfully integrated on the 3-m telescope at Lick Observatory and is now a module of the SCExAO extreme adaptive optics instrument on the 8-m Subaru Telescope. The latest on-sky results obtained from commissioning data show the detection of the stellar companion of the Alpha Equu binary system at an angular separation of 0.6 λ/D (11mas). Even at such a separation, the FIRST data delivers information on the companion spectrum, providing valuable constraints on the stellar parameters of the system such as the effective temperatures. The second part of this presentation will focus on the ongoing instrument upgrades with the aim of increasing the instrument’s stability and sensitivity, thus improving the dynamic range. We initiated a comprehensive upgrade of FIRST’s interferometric components to a new series of photonic on-chip beam combiners and automated optoelectronic delay lines for rapid phasing of each sub-pupil. The new photonic beam combining chips split light from each sub-aperture and combines them to provide a simultaneous measurement of the fringes from every baseline. Another function of the new photonic chips is the inclusion of waveguides in crystalline electro-optic material (Lithium niobate) that enable on-chip active phase control of the light at high speeds (up to kHz). This type of photonic architecture has not been implemented previously for astronomical interferometry of this kind and could potentially provide FIRST with key advantages (see Martin et al., these proceedings). While the beam-combiner output no longer requires non-redundancy, the fiber array that feeds the chip input still requires accurate pathlength-matching to achieve high fringe contrasts. The existing fibers were individually manufactured to ensure identical length. However, while this method was successful, it was not very flexible particularly if any photonic components are added that change the overall fiber length. Thus, another key FIRST upgrade is the use of actively controlled fiber delay lines capable of compensating for up to 100 mm of differential pathlength in each fiber, with sub-micron accuracy. This type of active pathlength control allows FIRST to not only correct for unwanted environmental phase delays, but also makes it entirely reconfigurable regardless of the back-end photonics used.