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extrasolar planet; high contrast imaging; small IWA; vector vortex coronagraphy; phase mask; subwavelength grating
Abstract :
[en] 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 2pi 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.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Delacroix, Christian ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS)
Absil, Olivier ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS)
Carlomagno, Brunella ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS)
Piron, Pierre ; Université de Liège - ULiège > Département de physique > Optique - Hololab
Forsberg, Pontus
Karlsson, Mikael
Mawet, Dimitri
Habraken, Serge ; Université de Liège - ULiège > Département de physique > Optique - Hololab
Surdej, Jean ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Astroph. extragalactique et observations spatiales (AEOS)
Language :
English
Title :
Development of a subwavelength grating vortex coronagraph of topological charge 4 (SGVC4)
Publication date :
08 July 2014
Event name :
SPIE Astronomical Telescopes and Instrumentation 2014
Event organizer :
SPIE
Event place :
Montreal, Canada
Event date :
22/6/2014 - 28/6/2014
Audience :
International
Main work title :
Ground-based and Airborne Instrumentation for Astronomy V