  In this work we present the results of a study of the possibility of using a
homogeneous basis and a new generalization of coupled modes theory to describe
non-periodic structured waveguides. It was shown that for the studied
inhomogeneities the single wave approximation gives good results. Within the
framework of single wave approximation, the fields are represented as the sum
of two components, one of which is associated with the forward eigen wave, and
the second with the backward eigen wave. But this second component is not a
backward wave. Because of the coupling, characteristics of this component are
determined by the first component, which is significantly larger than the
second.
