A new radiation-type waveguide-to-waveguide coupler is proposed. The c
oupling is realized through gratings embedded on the dielectric wavegu
ides involved. Compared to the conventional hybrid-mode coupler, this
type of coupler has advantages in that the two waveguides can be quite
dissimilar as well as being separated by large distances. It is demon
strated that the coupling performance of this radiation-type coupler c
an be enhanced significantly by using photonic bandgap (PBG) structure
s.