I Scientific Highlights 065

We propose a novel nulling interferometer based on polarization interferometry, which we call a polarization interferometric nulling coronagraph (PINC), for direct detection of extrasolar planets[1]. The PINC is a fully-symmetric modified Michelson interferometer using polarizing beam splitters (PBS), in which achromatic half-wave plates (HWP) such as Fresnel rhombs are installed. Two crossed polarizers ( ± 45°) are also installed in front of and behind the interferometer. The HWPs provide an achromatic π-phase difference between two light beams from subapertures (SA1, SA2) extracted from a telescope pupil, and eliminate an on-axis star light (Fig. 1, top). From Jones calculus, the achievable contrast of the PINC has been estimated to be about 10−10 at 5 λ/D over a wavelength range from 1.6 to 2.2 μm. We also carried out laboratory experiments on the PINC using two laser light sources (wavelengths of λ = 532 and 671 nm). Figure 1(bottom) shows a picture of the laboratory simulator of the PINC. As the HWPs, we used commercially-available Fresnel rhombs made of BK7. Figure 2a,b shows results of the laboratory experiments. As a result, we obtained a halo suppression level of about 10−6 at 5 λ/D for both wavelengths. The experimentally acquired contrast curves (Fig. 2a,b) were well fitted to those of the computer simu-lations assuming a phase aberration of 3 nm rms and an optical-path difference (OPD) error of 3.7 nm (Fig. 2a’,b’). Thus it is expected that higher contrasts will be realized by introducing a high-performance OPD control and an extreme adaptive optics system. Polarization Interferometric nulling coronagraph for high-contrast imagin