Minimizing the Polarization Leakage of Geometric-phase Coronagraphs with Multiple Grating Pattern Combinations

• Published in: Publications of the Astronomical Society of the Pacific 2020-04, Vol.132 (1010), p.1-18
• Main Authors: Doelman, David S., Por, Emiel H., Ruane, Garreth, Escuti, Michael J., Snik, Frans
• Format: Article
• Language: English
• Subjects: Astronomical Instrumentation, Telescopes, Observatories, and Site Characterization
• ISSN: 0004-6280; 1538-3873

The design of liquid-crystal diffractive phase plate coronagraphs for ground-based and space-based high-contrast imaging systems is limited by the trade-off between spectral bandwidth and polarization leakage. We demonstrate that by combining phase patterns with a polarization grating (PG) pattern directly followed by one or several separate PGs, we can suppress the polarization leakage terms by additional orders of magnitude by diffracting them out of the beam. Using two PGs composed of a single-layer liquid crystal structure in the lab, we demonstrate a leakage suppression of more than an order of magnitude over a bandwidth of 133 nm centered around 532 nm. At this center wavelength we measure a leakage suppression of three orders of magnitude. Furthermore, simulations indicate that a combination of two multi-layered liquid-crystal PGs can suppress leakage to