SUPPPPRESS: Prototyping and testing liquid-crystal vector vortex coronagraphs with reduced polarization leakage
The vortex coronagraph is one of the most promising candidates for the Habitable Worlds Observatory (HWO) due to its excellent theoretical performance for an off-axis telescope. A practical realization can be achieved using liquid-crystal polymers to form a vector vortex coronagraph (VVC). Reaching the $10^{-10}$ contrast required for Earth-like planet detection is, however, limited by polarization leakage caused by wavelength-dependent deviations from half-wave retardance. This effect can be mitigated using multi-layer twisted retarders to minimize leakage, and by combining the VVC with multiple polarization gratings (mgVVC) to diffract the polarization leakage out of the science path. We present recent progress within the ESA-funded SUPPPPRESS project, which aims to advance the manufacturing, assembly, and testing of high-performance VVCs. Central singularities of 2 and 6 $μ$m have been achieved for charge 2 and charge 6 VVCs, respectively, with patterning accuracies better than 1 degree root-mean-square error. Fabrication procedures have been developed to produce individual components with a polarization leakage of $3\times10^{-4}$ over a 10% bandwidth and $8\times10^{-4}$ over a 20% bandwidth. We also report on the development of assembly and alignment procedures for mgVVCs and their metrology. Furthermore, we present initial high-contrast tests at the THD2 bench for both regular VVCs and a double-grating VVC. The double-grating VVC reaches an average contrast between 3 and 10 $λ$/D of $2 \times 10^{-8}$ over a small bandwidth and $6\times 10^{-8}$ over a 10% bandwidth. Finally, we report on successful space-environment tests of the assembled liquid-crystal masks.