New method to achieve the proper polarization state for a vector vortex coronagraph

Abstract

The vector vortex coronagraph (VVC) performance in the laboratory and in ground-based observatories has earned it a spot on the NASA mission concepts HabEx and LUVOIR. The VVC induces a phase ramp through the manipulation of the polarization state. Left-and right-circular polarizations get imprinted a phase ramp of opposite signs, which prevents model-based focal plane wavefront sensing and control strategies in natural light. We thus have to work with a polarization state than ensures circularly polarized light at the VVC mask. However, achieving this polarization state can be non trivial if there are optics that add phase retardance of any kind between the circular polarizer and the focal plane mask. Here we present the method currently used at the Caltech high contrast spectroscopy testbed (HCST) to achieve the proper circular polarization state for a VVC, which only uses the deformable mirror and appropriate rotation of the circular polarizer and analyzer optics. At HCST we achieve raw contrast levels of 10 −8 for broadband light with a VVC.

Laboratory demonstration of wavefront control through a single-mode fiber over a 20% bandwidth for the characterization of exoplanet atmospheres

“…More data correlating temperature changes, PSF drifts, and null degradation are needed to assess the effect of these temperature changes; HCST was recently equipped with a set of sensors to undertake this analysis. 18…”

“…An SMF (Thorlabs SM600) feeds the light into the testbed and is re-imaged onto a custom-made 4-μm pinhole. A circular polarizer is placed upstream of the pinhole and ensures the correct polarization state needed to conduct wavefront control with the HCST's vector vortex coronagraph (VVC) 18 is selected. An iris defines the pupil after the first collimating OAP.…”

“…The first is a known effect of the VVC given the way it imprints the vortex ramp through polarization changes. 26,27 Although there are ways to reduce the chromatic leakage with modifications to the polarization state that enters the testbed, 18 the leakage due to imperfect retardance in the VVC is inherent to the mask fabrication process, so it depends on the fabrication quality. We refer to Ref.…”