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