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