Nulling stellar coronagraph using liquid-crystal device

摘要

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 π-phase shift over wide range of wavelength. Here we propose a method for realizing achromatic π-phase shift utilizing polarization interference. The phase difference between two light beams that pass through different polarizers (azimuth angle of 0°and 90°, respectively) is π radians when these polarizers are placed between mutually orthogonal polarizer and analyzer (i.e., azimuth angle of ±45°). 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 π, 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 ±45°.