  Direct detection of a planet around a star in the mid-IR, requires a nulling
interferometer featuring an achromatic phase shift of pi on broad range. A new
concept for designing such an achromatic phase shifter is presented here. The
major interest of this solution is that it allows a simple design, with
essentially one device per beam. The heart of the system consists in two
cellular mirrors where each cell has a thickness introducing for the central
wavelength, a phase shift of (2k+1)pi or of 2k pi on the fraction of the wave
it reflects. Each mirror is put in one of the collimated beams of the
interferometer. Because of the odd/even distribution, when recombining the two
beams, a destructive interference is produced on axis for the central
wavelength . If the distribution of cells thickness follows a rather simple
law, based on the Pascal's triangle, then the nulling is also efficient for a
wavelength not too far from the central wavelength. For instance, with two
mirrors of 64x64 cells, one reaches a nulling of 1.e-6 on more than one
complete octave. This could satisfy the specifications of space mission as
Darwin. We also show the way to distribute the cells in the plane of the pupil
for the optimum isolation of the planet image from the residual. We present the
nulling performances of those various configurations.
