Achromatic Four Quadrant Phase Mask using the Dispersion of Form Birefringence

摘要

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 RCWT (Rigorous Coupled Wave Theory).