  We present a two-waveguide coupler which, realizes complete achromatic
all-optical switching. The coupling of the waveguides has a hyperbolic-secant
shape while the phase mismatch has a sign flip at the maximum of the coupling.
We derive an analytic solution for the electric field propagation using coupled
mode theory and show that the light switching is robust again small-to-moderate
variations in the coupling and phase mismatch. Thus, we realize an achromatic
light switching between the two waveguides. We further consider the extended
case of three coupled waveguides in an array and pay special attention to the
case of equal achromatic light beam splitting.
