Development and exploitation of an infrared coronagraphic test bench for vortex phase mask performance assessment

Coronagraphy is a high-contrast imaging technique aiming to reduce the blinding
glare of a star in order to detect a potential companion in its close environment. Vortex
phase mask coronagraphy is widely recognized as one of the most promising
approaches.
This thesis is dedicated to the performance assessment of the vortex coronagraph.
For this purpose, it was crucial to have our own dedicated facility at the University
of Liège. In the first part of this thesis we describe how was built the Vortex
Optical Demonstrator for Coronagraphic Applications (VODCA) to optically characterize
infrared phase masks, in particular vortex masks. The layout and salient
features of VODCA are presented, as well as its operations, and its limitations in
terms of optical quality. The bench is then used to assess the performance of L-
(3575-4125 nm) and M-band (4600 to 5000 nm) annular groove phase masks (AGPM)
manufactured by our team. We discuss the results obtained with other facilities and
put them in perspective with the new measurements on VODCA. We demonstrate
the highest rejection ratio ever measured for an AGPM at L-band: 3.2x10^3 in a
narrow band filter (3425-3525 nm) and 2.4x10^3 in a broad L band filter. We also describe
the first results obtained at shorter wavelengths (H- and K-bands, from 1500
to 2400 nm). Vortex phase masks with higher topological charges VVC (Vector Vortex
Coronagraph) are theoretically more resilient to aberrations and resolved stars,
a key feature for next generation coronagraphs. We present here various designs for
the SGVC4 (charge 4 Subwavelength Grating Vortex Coronagraph) and the performance
of the first manufactured components. We finally investigate the influence of
optical aberrations on the AGPM and SGVC4, and confirm the simulations results.
The AGPM is sensitive to low order aberrations while the SGVC4 is significantly less
affected by tip/tilt and defocus.
By providing measurements close to the intrinsic limit of science-grade AGPMs
and accurately describing their behavior, VODCA proves to be a step forward in
terms of the evaluation of vortex phase masks performance.