Achromatic four quadrant phase mask using the dispersion of form birefringence

We describe an alternative design for the 4-quadrant phase mask coronagraph described recently by Rouan et al. 2000.(1) 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 reallses the pi 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 RCWT (Rigorous Coupled Wave Theory).