Photonic beam-combiner for visible interferometry with SCExAO/FIRST: laboratory characterization and design optimization

TL;DR

This paper reports on the development and laboratory testing of a photonic integrated circuit for visible interferometry, aiming to improve stability and sensitivity in astronomical measurements, specifically for the FIRST project.

Contribution

It introduces optimized designs of a visible PIC for interferometry, including experimental characterization and theoretical modeling of different combiner architectures.

Findings

Successful laboratory characterization of waveguides.

Optimized designs for symmetric and asymmetric combiners.

Potential for improved stability and sensitivity in astronomical interferometry.

Abstract

Integrated optics are used to achieve astronomical interferometry inside robust and compact materials, improving the instrument's stability and sensitivity. In order to perform differential phase measurements at the H$\alpha$ line (656.3 nm) with the 600-800 nm spectro-interferometer 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 sub-apertures 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, in order 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.