A Family of Phase Masks For Broadband Coronagraphy Example of the Wrapped Vortex Phase Mask Theory and Laboratory demonstration

Abstract

Future instruments need efficient coronagraphs over large spectral ranges to enable broadband imaging or spectral characterization of exoplanets 1e8 fainter than their star. Several solutions were proposed. Pupil apodizers can attenuate the star intensity by a 1e10 factor but they transmit a few percents of the planet light only. Cascades of phase/amplitude masks can both attenuate the starlight and transmit most of the planet light but the number of optics to align is not a practical solution for an instrument. Finally, vector phase masks can be used to detect faint sources close to bright stars but they require the use of high quality circular polarizers and as for the previous solution, this leads to a complex instrument with numerous optics to align and stabilize. We propose simple coronagraphs that need one scalar phase mask and one binary Lyot stop only providing high transmission for the planet light (>50%) and high attenuation of the starlight over a large spectral bandpass (~30%) and a 360 degree field-of-view. From mathematical considerations, we find a family of 2D-phase masks optimized for an unobscured pupil. One mask is an azimuthal wrapped vortex phase ramp. We probe its coronagraphic performance using numerical simulations and laboratory tests. From numerical simulations, we predict the wrapped vortex can attenuate the peak of the star image by a factor of 1e4 over a 29% bandpass and 1e5 over a 18% bandpass with transmission of more than 50% of the planet flux at ~4 lambda/D. We confirm these predictions in laboratory in visible light between 550nm and 870nm. We also obtain laboratory dark hole images in which exoplanets with fluxes that are 3e-8 times the host star flux could be detected at 3sigma. Taking advantage of a new technology for etching continuous 2D-functions, new type of masks can be easily manufactured opening new possibilities for broadband coronagraphy.

11 pages, 17 figures, accepted in Astronomy & Astrophysics

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Cited by in corpus (11)

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• Laboratory demonstration of the wrapped staircase scalar vortex coronagraph
• Topological Designs for Scalar Vortex Coronagraphs
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• Spectroscopic fourth-order coronagraph for the characterization of terrestrial planets at small angular separations from host stars
• Benefits of adding radial phase dimples on scalar coronagraph phase masks
• Active focal-plane coronagraphy with liquid-crystal spatial-light modulators: Broadband contrast performance in the visible
• Comparative laboratory study of electric field conjugation algorithms
• Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging
• Future exoplanet direct imaging instruments: Simulating spatial light modulator-based pixelated focal-plane coronagraphy
• Extended Linearity in the High-Order Wavefront Sensor for the Roman Coronagraph