Three-dimensional common-path interferometer is proposed to obtain achromatic nulling for the on-axial source; the off-axial source remains detectable. The 3D interferometer involves $\pm 90^{\circ }$ polarization rotations in each interferometer arm. That results in the achromatic 180$^{\circ}$ phase shift, so that the on-axial source interferes destructively. Depending on the source axial position, the light energy is split by different ratios between the Bright and the Nulled interferometer outputs. For the linearly polarized on-axial source, all the energy at nearly 100% is directed to the Bright port. For the off-axial source, the light is split by the ratio at nearly 50%/50% between the Bright and the Nulled ports. The common-path scheme compensates effectively the optical path difference (OPD) and it remains stable to mechanical vibrations. Theory, simulations and preliminary breadboard experiments are shown to be in reasonable agreement.
