The physical properties of asymmetric
photonic-crystal directional couplers are studied under the
tight-binding model, which assumes that the field distribution of
photonic-crystal waveguides is localized around periodic defects.
The couplings of nearby defects between two asymmetric waveguides
cause two dispersion relations to split further whereas the
couplings of nearby defects within an individual waveguide cause the
sinusoidal modulations of the dispersion curves. The field-envelope
distributions of the coupler are expressed by using the eigenmodes
of the tight-binding equations and are consistent with the comparing
results from finite-difference time-domain simulations.