Analysis of nulling phase functions suitable to image plane coronagraphy

TL;DR

This paper investigates various phase functions for space-based coronagraphs, analyzing their effectiveness in starlight suppression and imaging exoplanets, with a focus on azimuthal cosine functions as promising alternatives.

Contribution

It introduces and compares new phase functions, including azimuthally cosine-modulated profiles, for improved coronagraph performance in exoplanet imaging.

Findings

Azimuthal cosine functions show potential as alternatives to optical vortices.

Numerical simulations evaluate null depth, inner working angle, and sensitivity to errors.

Central obscuration effects and throughput are analyzed for different phase functions.

Abstract

Coronagraphy is a very efficient technique for identifying and characterizing extra-solar planets orbiting in the habitable zone of their parent star, especially when used in a space environment. An important family of coronagraphs is based on phase plates located at an intermediate image plane of the optical system, that spread the starlight outside the "Lyot" exit pupil plane of the instrument. In this communication we present a set of candidate phase functions generating a central null at the Lyot plane, and study how it propagates to the image plane of the coronagraph. These functions include linear azimuthal phase ramps (the well-known optical vortex), azimuthally cosine-modulated phase profiles, and circular phase gratings. Numerical simulations of the expected null depth, inner working angle, sensitivity to pointing errors, effect of central obscuration located at the pupil or image planes, and effective throughput including image mask and Lyot stop transmissions are presented and discussed. The preliminary conclusion is that azimuthal cosine functions appear as an interesting alternative to the classical optical vortex of integer topological charge.