Nulling stellar coronagraph using liquid-crystal device

Nulling stellar coronagraph has been proposed to detect faint objects very close to a bright point-like star, especially extra-solar planets. The principle of the nulling stellar coronagraph is to cause destructive interference for the light from a star. There have been proposed several methods for nulling interferometry. The key point of the nulling interferometry is the way to produce pi-phase shift over wide range of wavelength. Here we propose a method for realizing achromatic pi-phase shift utilizing polarization interference. The phase difference between two light beams that pass through different polarizers (azimuth angle of 0degrees and 90degrees, respectively) is pi radians when these polarizers are placed between mutually orthogonal polarizer and analyzer (i.e., azimuth angle of +/-45degrees). We adopt a ferroelectric liquid-crystal (FLC) device to convert the polarization direction of the incident beam. The FLC device is regarded as a birefringent device with retardation pi, namely a half wave plate. The FLC device forms four-quadrant structure and is placed between the polarizer and the analyzer. By fixing the optic axes of the four-quadrant FLC suitably, it can rotate the incident linearly polarized light in parts by +/-45degrees.