FIRST 5T 3D: interferometric performances of a laser written bream combiner for FIRST/SUBARU

FIRST (Fibered Imager foR a Single Telescope instrument) is an on-sky instrument at SUBARU Telescope that enables high-contrast imaging and spectroscopy, thanks to a unique combination of pairwise combination of sub-apertures using a Photonic Integrated Circuit that allows spatial filtering by single-mode waveguides and cross-dispersion in the visible (~ 600-800 nm), a high-efficiency alternative to sparse aperture masking. To reduce crosstalk observed in planar PICs, and therefore increase the instrument’s stability and sensitivity, we have designed and fabricated a series of 3D laserwritten optical PICs with 5T beam combiners. Different laser powers were used to address different single-mode spectral ranges. The multi-aperture beam combiner consists of five input waveguides spaced by 250 μm. Each input is split into four waveguides and the twenty channels undergo pairwise recombination, using Y-junctions, to produce ten outputs. In this work, we present the interferometric performances of these 3D PICs when exposed to a point-like light source (single star) or a two-points-like light source (binary) on the FIRST/SUBARU instrument replica built at LESIA – Observatoire de Paris.

In previous work, we identified the optimal 5T 3D device, as being single-mode between 550-800 nm and showing good internal transmission in all input channels, above 45% at 635nm. The internal transmission (sum of the output values obtained for the four waveguides of the 1x4 splitter as normalized to the output signal obtained from the straight waveguide used as a reference) was measured. Two inputs achieved 80% transmission. The PIC was installed in the FIRST/SUBARU optical bench simulator at LESIA, to inject light into five inputs simultaneously and scan the fringes using independent MEMS segments, inducing a relative OPD modulation. The results of this study, comparing the signature obtained for a single source (star) as compared to a binary, will be presented in this work. We will show that both polarizations are guided, with no crosstalk, and analyze the interferometric performances as a function of the source type, showing that the binary companion can be detected.