We describe an alternative design for the 4-quadrant phase mask
coronagraph described recently by Rouan et al. (2000). Based on
the same principle, i.e. producing a very efficient nulling by
mutually destructive interferences of the coherent light from the
main source, our mask realises the π phase shift using some
properties of ZOGs (Zeroth Order Gratings) and according to an
original scheme respecting the 4-quadrant symmetry. When the
period of the one-dimension grating structure is smaller than the
wavelength of the incident light, the structure becomes
birefringent. The effective refractive indices depend on the
wavelength. Using this feature, we can design a mask whose nulling
efficiency is maintained within a wide wavelength range. Numerical
simulations were made according to the RCWA (Rigorous Coupled Wave
Analyses).