A family of phase masks for broadband coronagraphy example of the wrapped vortex phase mask theory and laboratory demonstration

Abstract

Context. Future instruments need efficient coronagraphs over large spectral ranges to enable broadband imaging or spectral characterization of exoplanets that are 108 times fainter than their star. Several solutions have been proposed. Pupil apodizers can attenuate the star intensity by a factor of 1010 but they only transmit a few percent of the light of the planet. Cascades of phase and/or amplitude masks can both attenuate the starlight and transmit most of the planet light, but the number of optics that require alignment makes this solution impractical 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 in the previous solution, this leads to a complex instrument with numerous optics that require alignment and stabilization. Aims. We propose simple coronagraphs that only need one scalar phase mask and one binary Lyot stop providing high transmission for the planet light (> 50%) and high attenuation of the starlight over a large spectral bandpass (∼30%) and a 360° field-of-view. Methods. 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. Results. From numerical simulations, we predict the wrapped vortex can attenuate the peak of the star image by a factor of 104 over a 29% bandpass and 105 over a 18% bandpass with transmission of more than 50% of the planet flux at ∼4λ/D. We confirm these predictions in the laboratory in visible light between 550 and 870 nm. We also obtain laboratory dark hole images in which exoplanets with fluxes that are 3 × 10−8 times the host star flux could be detected at 3σ. Conclusions. Taking advantage of a new technology for etching continuous 2D functions, a new type of mask can be easily manufactured opening up new possibilities for broadband coronagraphy.