  We propose an analytical approach to design flattened wavelength splitters
with cascaded Mach-Zehnder interferometers when wavelength dependence of the
directional couplers cannot be neglected. We start from a geometrical
representation of the action of a doubly point symmetrical filter, assuming no
wavelength dependence of the couplers. Next we derive the analytical formulas
behind its working principle and we extend them to the wavelength dependent
case. We also show how the geometrical representation allows to broaden the
class of working structures.
