Optimizing the subwavelength grating of L-band Annular Groove Phase Masks for high coronagraphic performance

TL;DR

This paper presents the design, fabrication, and optimization of L-band Annular Groove Phase Masks (AGPMs) using subwavelength gratings to enhance coronagraphic performance for infrared high-contrast imaging, achieving up to 1000:1 starlight rejection.

Contribution

The study introduces new re-etching processes to optimize grating profiles, significantly improving AGPM coronagraphic performance in the L-band.

Findings

Achieved up to 1000:1 starlight rejection in the L-band.

Demonstrated the effectiveness of re-etching for performance enhancement.

Validated performance improvements through optical testing and RCWA modeling.

Abstract

Context. The Annular Groove Phase Mask (AGPM) is one possible implementation of the vector vortex coronagraph, where the helical phase ramp is produced by a concentric subwavelength grating. For several years, we have been manufacturing AGPMs by etching gratings into synthetic diamond substrates using inductively coupled plasma etching. Aims. We aim to design, fabricate, optimize, and evaluate new L-band AGPMs that reach the highest possible coronagraphic performance, for applications in current and forthcoming infrared high-contrast imagers. Methods. Rigorous coupled wave analysis (RCWA) is used for designing the subwavelength grating of the phase mask. Coronagraphic performance evaluation is performed on a dedicated optical test bench. The experimental results of the performance evaluation are then used to accurately determine the actual profile of the fabricated gratings, based on RCWA modeling. Results. The AGPM coronagraphic performance is very sensitive to small errors in etch depth and grating profile. Most of the fabricated components therefore show moderate performance in terms of starlight rejection (a few 100:1 in the best cases). Here we present new processes for re-etching the fabricated components in order to optimize the parameters of the grating and hence significantly increase their coronagraphic performance. Starlight rejection up to 1000:1 is demonstrated in a broadband L filter on the coronagraphic test bench, which corresponds to a raw contrast of about 1e-5 at two resolution elements from the star for a perfect input wave front on a circular, unobstructed aperture. Conclusions. Thanks to their exquisite performance, our latest L-band AGPMs are good candidates for installation in state-of-the-art and future high-contrast thermal infrared imagers, such as METIS for the E-ELT.