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