Photonic beam-combiner for visible interferometry with Subaru coronagraphic extreme adaptive optics/fibered imager for a single telescope: laboratory characterization and design optimization

Integrated optics are used to achieve astronomical interferometry inside robust and compact materials, improving the instrument's stability and sensitivity. To perform differential phase measurements at the Hα line (656.3 nm) with the 600- to 800-nm spectro-interferometer fibered imager for a single telescope (FIRST), a photonic integrated circuit (PIC) is being developed in collaboration with TEEM Photonics. This PIC performs the interferometric combination of the beams coming from subapertures selected in the telescope pupil, thus implementing the pupil remapping technique to restore the diffraction limit of the telescope. In this work, we report on the latest developments carried out within the FIRST project to produce a high-performance visible PIC. The PICs are manufactured by TEEM Photonics, using their technology based on K + : Na + ion exchange in glass. The first part of the study consists in the experimental characterization of the fundamental properties of the waveguides, to build an accurate model, which is the basis for the design of more complex functions. In the second part, theoretical designs and their optimization for three types of combiner architectures are presented: symmetric directional coupler, asymmetric directional couplers, and ABCD cells, including achromatic phase shifters.