Master thesis and internship[BR]- Master's thesis : Towards a laboratory validation of a dual-polarization wavefront sensing with the vortex coronagraph[BR]- Internship

Abstract

In the context of high-contrast imaging, dual-polarization wavefront sensing represents a very promising solution for non-common path aberrations and aberrations control using the vortex coronagraph. The interest of this method lies in the efficient phase retrieval using the unique polarization behaviour of the vortex coronagraph and the simplicity of its implementation. This method has been progressively implemented on Vortex Optical Demsontrator for Coronagraphic Application (VODCA) test bench using the vortex coronagraph called the Annular Groove Phase Mask (AGPM). The dual-polarization setup requires the most accurate selection of both orthogonal circular polarizations before and after the focal plane. An accurate polarization setup is optimized to ensure the purest circular polarization for both orthogonal states at more than 97% at the focal plane on VODCA. In order to achieve this level of precision, two polarization measurement methods have been presented. The classical method and the rotating quarter wave plate (QWP) method. Both are used and compared to characterize how the polarization state of the light was impacted by the optical elements of VODCA. The rotating QWP method is used to optimize the accuracy of the circular polarizer being more precise and consistent than the classical method. With the optimized circular polarizer, the dual polarization setup has been evaluated in terms of two particular performance values, the extinction ratio and the rejection ratio. The setup has achieved a non-expected high performance comparable to particularly efficient results obtained in previous studies. Considering the polarization setup without the AGPM, an extinction ratio of 1924 has been achieved. Due to intrinsic performance of the AGPM, the setup with the vortex coronagraph has achieved an extinction ratio of 1521. Moreover, a rejection ratio of 2010 has been obtained for the AGPM. This result has been obtained using a aberration minimization routine to avoid limiting performance due to aberrations in the focal plane. The total contrast achieved by the dual-polarization setup is up to 21e4. These performance ensured a sufficiently high accuracy of the setup to produce a required diversity for phase retrieval. This produced diversity has been proven by using the deformable mirror (DM) of VODCA to inject specific aberrations in the focal plane and verify the lifting of the sign ambiguity which was confirmed. Following this, some CNN training on aberration identification have led to relative successful wavefront reconstruction tests performed using machine learning. The dual-polarization setup developed in this work combined with the polarization behaviour of the phase mask allows a sufficient diversity to ensure a performing phase retrieval. However, these results are still preliminary in sight of a laboratory validation of the dual-polarization wavefront sensing using the vortex coronagraph.