Achromatic four-quadrant phase mask (FQPM) coronagraphy using natural beam splitter phase shifts

The four-quadrant phase mask (FQPM) is an exciting new approach to coronagraphy, a classic astronomical technique for detecting faint companions very close to bright stars. Starlight rejection is potentially very high, and inner working distances are substantially smaller than those achieved with classical Lyot coronagraphy. The key component of the original FQPM scheme is a transparent mask divided into quadrants delivering relative phase shifts alternately 0/pi/0/pi, inserted in an intermediate focal plane of the telescope. Monochromatic masks of this kind have been successfully demonstrated in laboratory and telescope tests. Fabrication of masks with achromatic pi phase shifts is challenging but of great interest for optimum astronomical sensitivity. In this paper I present a novel concept for achromatic FQPM operation that utilizes intrinsic phase relationships between transmitted and reflected beams in a dielectric beam splitter.