Soutenance de thèse de Kévin BARJOT le jeudi 20 avril 2023

Kévin BARJOT

Title

"Characterization and deployement of Photonic Integrated Circuits for the FIRST fibered interferometer at the Subaru Telescope in the context of accreting protoplanets studies"

Abstract

During my thesis, I worked in the field of high contrast and high angular resolution (HRA) imaging for the study of protoplanets. This scientific case is interesting because it allows to study the mechanisms of planetary formation and are characterized by the presence of emission lines in their spectrum. The one we are interested in is the Ha line and the contrast of the system is then weaker at these wavelengths and thus more accessible.

The Fibered Interferometer foR a Single Telescope (FIRST) instrument uses the concept of fibered pupil remapping in the visible, integrated on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) telescope. Its principle is to sub-divide the entrance pupil into sub-pupils whose beams are injected into single mode optical fibers. The latest have the double advantage of applying a spatial filtering on the wavefront thus suppressing the optical aberrations at the scale of the sub-pupils, at the same time as the remapping of the sub-pupils. The interferograms resulting from the interference of the sub-pupil beams are then measured. FIRST has demonstrated that it can detect stellar binaries at a resolution below the diffraction limit of the telescope with this concept. The replica of a new version of this instrument (FIRSTv2) has been built in laboratory to allow the development of the integrated optics technology.

My thesis work is to evaluate its performances and its feasibility for HRA imaging, in order to improve the contrast performances of the instrument. The recombination of each pair of sub-pupils is encoded on an output of the photonic chip then dispersed and imaged on few pixels of the camera, allowing to increase the sensitivity. The fringe sampling is performed by temporal modulation of the sub-pupil optical paths by the segments of the deformable mirror. In addition, I continued the development of the testbed, both in terms of the optical assembly and the control software, to develop a specific procedure for data acquisition and to develop a program for data reduction and analysis. The goal of this data analysis is to calculate the spectral differential phase, which is a self-calibrated observable of the atmospheric and instrumental perturbations. In that purpose, the phase measurements of the complex visibilities of each baseline are calibrated by the continuum signal highlighting a possible signal in the line of interest. Thus, from a protoplanetary source simulator that I integrated on the testbed, I was able to demonstrate that FIRSTv2 could detect a protoplanetary companion at a separation equivalent to 0.7 l / B from the central source, with a contrast of about 0.5.

Finally, I participated in the integration and the first light of FIRSTv2 on the SCExAO bench for which the software developed in laboratory was deployed. The tests and data acquired during several observation nights show that an isolation of the optical fibers on the bench is necessary and that the method of acquisition of the interferograms by temporal modulation may need to be changed for an on-chip modulation by changing the recombination technology using an ABCD chip.