A new photonic integrated circuit for the FIRST instrument: towards high throughput with a compact photonic chip

A photonic integrated circuit (PIC) is a lightweight, compact alternative to bulk optics. The Fibered Imager foR a Single Telescope (FIRST) instrument, is a spectro-interferometer performing pupil remapping and designed to operate in the 600 to 800 nm visible wavelength range. It is installed on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument's platform at the Subaru telescope. In the integrated optical version of FIRST (FIRST PIC), the interferometric combination of the beams occurs by the pairwise combination of five sub-apertures to achieve 20 baselines. This paper introduces a PIC design with novel components for splitting and pairwise coupling the light for FIRST PIC, fabricated and packaged by LioniX International. A high index difference between the waveguide fundamental mode and cladding material was selected to allow compact circuits with prospects of increasing the number of combined sub-apertures with designs of much greater complexity. The high confinement waveguides were simulated to produce approximately 50% injection loss, a tapering system in both height and width to a low confinement waveguide at the PIC interface ameliorated the loss. The optimised throughput prediction is estimated at 80%. Standard couplers and splitters are expected to have high losses due to fabrication tolerances, and due to the high confinement, standard couplers' performance is highly dependent on the wavelength. Presented here are novel component designs to replace them. Tapered directional couplers, which maintain an acceptable coupling ratio over the entire bandwidth, and tri-couplers, with higher throughput than conventional splitters but high intolerance to fabrication issues, were designed and fabricated for this purpose. The designs and individual experimental verification will be discussed, focusing on polarization and wavelength response. The full combination of components was used to create a five sub-aperture combiner for FIRST. Cross-talk between crossed waveguides was measured independently for the two polarisations. The cross-coupling over the full bandwidth was measured as low as 2% in one polarization, without active subtraction for scattered light to confirm the light in the ports was guided from the cross-coupling, and as low as 8% in the other. Additional work, and potential refinement of the photonic chip components, are required to differentiate the scattered light from the cross-talk to ensure it is reduced in both polarisations.