A three-layer eight-octant phase mask towards broadband high-contrast observations

摘要

We designed and fabricated an achromatic eight-octant phase mask (80PM) for broadband coronagraphic observations of exoplanets. The 80PM 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 80PM 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 80PM. However, the achieved contrasts are lower than the theoretical values of the three-layer one. At present we suspect that fabrication errors of the half-wave plates in the 80PM limit the achieved contrasts.