Development of a subwavelength grating vortex coronagraph of topological charge 4 (SGVC4)

TL;DR

This paper presents a new design for a subwavelength grating vortex coronagraph with topological charge 4, aiming to improve high contrast imaging for future large telescopes by overcoming manufacturing challenges.

Contribution

The authors develop a realistic design for a charge-4 vortex coronagraph using subwavelength gratings, with minimized discontinuities and optimized phase ramp, advancing previous charge-2 designs.

Findings

Preliminary RCWA simulations show promising phase ramp improvements.

Design minimizes discontinuities compared to previous models.

Further validation with 3D FDTD simulations and lab tests is planned.

Abstract

One possible solution to achieve high contrast direct imaging at a small inner working angle (IWA) is to use a vector vortex coronagraph (VVC), which provides a continuous helical phase ramp in the focal plane of the telescope with a phase singularity in its center. Such an optical vortex is characterized by its topological charge, i.e., the number of times the phase accumulates 2{\pi} radians along a closed path surrounding the singularity. Over the past few years, we have been developing a charge-2 VVC induced by rotationally symmetric subwavelength gratings (SGVC2), also known as the Annular Groove Phase Mask (AGPM). Since 2013, several SGVC2s (or AGPMs) were manufactured using synthetic diamond substrate, then validated on dedicated optical benches, and installed on 10-m class telescopes. Increasing the topological charge seems however mandatory for cancelling the light of bright stars which will be partially resolved by future Extremely Large Telescopes in the near-infrared. In this paper, we first detail our motivations for developing an SGVC4 (charge 4) dedicated to the near-infrared domain. The challenge lies in the design of the pattern which is unrealistic in the theoretically perfect case, due to state-of-the-art manufacturing limitations. Hence, we propose a new realistic design of SGVC4 with minimized discontinuities and optimized phase ramp, showing conclusive improvements over previous works in this field. A preliminary validation of our concept is given based on RCWA simulations, while full 3D finite-difference time-domain simulations (and eventually laboratory tests) will be required for a final validation.