An achromatic eight - octant phase - mask coronagraph using photonic crystal

For direct detection of faint exoplanets, high-contrast imagers would be indispensable for strongly suppressing central bright stars. In particular, extremely high contrast of 10−10 will be needed for detecting Earth-like planets. An eight-octant phase-mask (EOPM) coronagraph is one of promising concepts because of its high-contrast performance, small inner working angle, and simple optical configuration[1]. The EOPM divides a stellar image into eight-octant sectors and provides π-phase difference between adjacent sectors. Then a destructive interference occurs inside pupil area on a reimaged pupil plane, and the stellar light is totally diffracted outside the pupil area. The diffracted stellar light is blocked by a diaphragm called Lyot stop. On the other hand, a planetary image, formed at an off-axis position on the EOPM, will not be totally blocked by the Lyot stop and can be detected at a final focal plane. For detecting extremely faint planetary signal and carrying out spectroscopic characterizations, achromatic high-contrast imagers will be required. We manufactured a coronagraphic mask based on photonic-crystal technology for realizing a fully achromatic EOPM[2]. The photonic crystal holds periodic nanostructure of refractive indices, and exhibits anisotropic properties of effective dielectric constants. The manufactured photonic-crystal mask consists of an eight-octant half-wave plates with fast axes of ± 45°. As shown in figure 1, extremely small central defect, an order of submicron, was realized by the photoniccrystal technology. The photonic-crystal mask operates as an fully-achromatic EOPM when the mask is placed between two crossed polarizers with axes of 0° and 90°. We carried out laboratory demonstration of the EOPM coronagraph using two laser light sources with different wavelengths as a simulated star (DPSS laser with λ = 532 nm, and He-Ne laser with λ = 633 nm). Figure 2 shows the experimental results. The light from the simulated star was strongly suppressed, and peak-topeak contrasts of 7 × 10−6 and 3 × 10−6 were obtained for the DPSS and the He-Ne lasers, respectively. In a halo region, contrasts of about 10−6 and 10−7 were obtained for the both lasers at 3 and 13 λ/D, respectively. We expect improved coronagraphic performance with a use of extreme adaptive optics systems, because we assume that the residual speckle noise mainly comes from phase aberrations of the optical components. An achromatic eight-octant phase-mask coronagraph using photonic crystal