  We use a phenomenological Hamiltonian approach to derive a set of coupled
mode equations that describe light propagation in waveguides that are
periodically side-coupled to microcavities. The structure exhibits both Bragg
gap and (polariton like) resonator gap in the dispersion relation. The origin
and physical significance of the two types of gaps are discussed. The
coupled-mode equations derived from the effective field formalism are valid
deep within the Bragg gaps and resonator gaps.
