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