Feasibility study of a forked grating scalar vortex coronagraph for the Habitable Worlds Observatory

NASA’s ambitious plan of imaging 25 Earth-like planets with its future Habitable Worlds Observatory requires a coronagraph reaching contrast on the order of 10⁻¹⁰ across a broad band. One of the most promising architectures to achieve this goal is a vortex coronagraph, providing a good tradeoff between small inner working angle, and high throughput, as well as low sensitivity to low order aberrations. Current vortex coronagraphs are of vectorial nature and have shown promising results so far, however they imprint opposite phase ramps on the two circular polarizations making wavefront control in both polarizations simultaneously challenging. An alternative to the vector vortex is the polarization-independent scalar vortex coronagraph, which imprints the same phase ramp regardless of polarization. Current scalar vortex phase mask implementations are however limited by their chromaticity. Here, we present a design study for a scalar vortex phase mask combined with a phase grating. We show that this architecture is capable of spatially separating the chromatic leakage from the vortex beam, and present simulation results of this concept. We then discuss different implementations based on shaped dielectric masks, or metasurfaces. Finally, we discuss potential manufacturing approaches to fabricate the forked grating scalar vortex phase mask, and their feasibility in light of HWO.