Optical Vortex Phase Mask Development and Testing

The top priority of the 2020 Decadal Survey for Astronomy & Astrophysics is the imaging and spectroscopy of terrestrial exoplanets around nearby stars. However, bright diffracted and scattered starlight makes it very difficult to discern much dimmer, close-in exoplanets. High-precision wavefront correction and starlight suppression via stellar coronagraphy are therefore essential to be able to unveil faint exoplanets. One of the most promising coronagraphs is the optical vortex coronagraph, and in this proposal we have two aims in that regard: to broaden the technologies used for making high-performance optical vortex phase masks, and to bring one or more of those technologies up to the full performance level (a contrast of ~ 1e-10 for 20% bandwidth) needed by coronagraphs on the Habitable Worlds Observatory (HWO). There are two main types of vortex phase mask, the scalar vortex and the vector vortex, and the best demonstrated contrast performance to date (~ 1e-9 for 10% bandwidth) has been achieved with the vector vortex coronagraph. While good progress has thus been made, there are further limitations that need to be overcome. The first is that starlight suppression need to be deepened by another order of magnitude, and the suppression bandwidth needs to be doubled. This will be addressed with improved mask modeling and manufacturing techniques. Beyond that, it would be highly desirable to eliminate the need to separate the two polarization states in a vector vortex coronagraph, without doubling the number of optics or dropping one polarization state. Finally, vector vortex phase masks currently rely on a single technological approach (liquid crystal polymers) that are supplied by a single vendor, making the supply chain fragile. The latter two issues can both be dealt with by developing alternate technologies, such as the scalar vortex coronagraph, which we have been developing under an APRA award (reaching ~ 2e-7 contrast for 10% bandwidth). We now have two promising theoretical solutions for scalar vortex masks that go much deeper, based on a pair of modified azimuthal dielectric phase ramps, and the new technology of microstructure masks, respectively. Here we therefore plan to combine efforts on all three of these mask types (vector liquid crystal polymer, scalar dielectric modified azimuthal phase ramps, and scalar microstructure), in order to make the final push to develop and compare all three types of vortex phase mask for testing in the HCIT, aiming both to reach final HWO-like performance levels, and to broaden the number of technologies and vendors that HWO will be able to rely on. Our main goal is to show that at least one of these technologies is able to reach the requisite contrast levels for HWO in the unobscured aperture case. The integration time per exoplanet, and hence the number of terrestrial exoplanets to be characterized spectroscopically by HWO, depends on coronagraph throughput and bandwidth, as well as the stellar rejection level. Therefore, comparing these three vortex technologies one-to-one on the same testbed will be very valuable in several ways: the more advanced vector vortex coronagraph will lead the way to demonstrating HWO contrast levels, while the less developed but more efficient scalar vortex coronagraph will demonstrate higher throughput by at least a factor of 2, while decreasing instrument complexity and cost. Finally, establishing different technological approaches will serve to mitigate risk regarding long term mask availability. High-quality scalar vortex phase masks will also enable simpler wavefront sensing and control, as different polarization states will no longer interact differently with the vortex. Thus, as a result of this proposal, NASA will benefit from having demonstrated a successful coronagraphic approach with deeper rejection, broader bandwidth, higher throughput, shorter integration times, simpler wavefront control, and a simpler optical system.

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