Unbalanced nulling interferometer with four-quadrant phase mask

One of the problems for direct observation of extrasolar planets is the speckle noise due to a wave-front error. Therefore, high-accuracy adaptive optics is required for realizing a wavefront quality of λ/10000 rms. An unbalanced nulling interferometer has a possibility to assist high-accuracy correction. In this paper, we propose the interferometer with a four-quadrant phase mask in which an optical path is common. By using the mask, we succeed in stabilizing the interference and taking measurements of wavefront errors with 10-times higher sensitivity. In this way, we expect to construct high-accuracy adaptive optics which is more stable.