The directional coupler structure comprises {HEAD ~ two waveguides}, wherein {TAIL ~ the waveguides} arranged exhibit different effective diffractive index 
wherein
The directional coupler structure comprises { HEAD ~ two waveguides } , wherein { TAIL ~ the waveguides } arranged exhibit different effective diffractive index
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH