Optimizing the subwavelength grating of L-band annular groove phase masks for high coronagraphic performance

¹ Department of Engineering Sciences, Ångström Laboratory, Uppsala University, PO Box 534, 751 21 Uppsala, Sweden
e-mail: mikael.karlsson@angstrom.uu.se
² Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19c Allée du Six Août, 4000 Liège, Belgium
³ LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 Pl. J. Janssen, 92195 Meudon, France
⁴ Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA
⁵ Department of Astronomy, California Institute of Technology, 1200 E. California Blvd, MC 249-17, Pasadena, CA 91125, USA
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 19 April 2016
Accepted: 4 August 2016

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 10^-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.