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Context. The Annular Groove Phase Mask (AGPM) is one possible implementationof the vector vortex coronagraph, where the helical phase ramp is produced by aconcentric subwavelength grating. For several years, we have been manufacturingAGPMs by etching gratings into synthetic diamond substrates using inductivelycoupled plasma etching. Aims. We aim to design, fabricate, optimize, andevaluate new L-band AGPMs that reach the highest possible coronagraphicperformance, for applications in current and forthcoming infrared high-contrastimagers. Methods. Rigorous coupled wave analysis (RCWA) is used for designingthe subwavelength grating of the phase mask. Coronagraphic performanceevaluation is performed on a dedicated optical test bench. The experimentalresults of the performance evaluation are then used to accurately determine theactual profile of the fabricated gratings, based on RCWA modeling. Results. TheAGPM coronagraphic performance is very sensitive to small errors in etch depthand grating profile. Most of the fabricated components therefore show moderateperformance 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 orderto optimize the parameters of the grating and hence significantly increasetheir coronagraphic performance. Starlight rejection up to 1000:1 isdemonstrated in a broadband L filter on the coronagraphic test bench, whichcorresponds to a raw contrast of about 1e-5 at two resolution elements from thestar for a perfect input wave front on a circular, unobstructed aperture.Conclusions. Thanks to their exquisite performance, our latest L-band AGPMs aregood candidates for installation in state-of-the-art and future high-contrastthermal infrared imagers, such as METIS for the E-ELT.

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