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22 July 2016 A three-layer eight-octant phase mask towards broadband high-contrast observations
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We designed and fabricated an achromatic eight-octant phase mask (8OPM) for broadband coronagraphic observations of exoplanets. The fabricated 8OPM is composed of three-layer eight-octant half-wave plates based on photonic crystals. By using Jones calculus, it is shown that the three-layer 8OPM achieves much higher contrast over broad wavelength range than that of the previous single-layer design. We carry out preliminary laboratory experiments of the coronagraph using the fabricated three-layer 8OPM. As a model star, we use several visible laser light sources for characterizing the coronagraphic performance. As a result, we obtain higher contrasts than theoretical ones of the single-layer 8OPM. However, the achieved contrasts are lower than the theoretical values of the three-layer one. At present we suspect that manufacturing errors of the half-wave plates in the 8OPM limit the achieved contrasts.
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Naoshi Murakami, Jun Nishikawa, Natsumi Akaiwa, Yusuke Komuro, Naoshi Baba, and Motohide Tamura "A three-layer eight-octant phase mask towards broadband high-contrast observations", Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99126G (22 July 2016);


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