Common-path achromatic interferometer-coronagraph: nulling of polychromatic light

A three-dimensional common-path interferometer is proposed, which can achromatically null out an on-axis source while it maintains the detectability of an off-axis source. A geometric phase in the three-dimensional interferometer introduces an achromatic pi-phase shift to the light from the on-axis source, such that destructive interference nulls out the axial light at one of the ports of the interferometer. Light from the off-axis source, which is exempt from the pi-phase shift, comes out from both ports with equal intensity. The common-path scheme makes the system highly immune to environmental disturbances.