A time delay unit is coupled to {HEAD ~ the coupler interface point} for receiving {TAIL ~ the optical signal}, directing it along a path, and reflecting the signal back to the coupler interface point 
A time delay unit is coupled to { HEAD ~ the coupler interface point } for receiving { TAIL ~ the optical signal } , directing it along a path , and reflecting the signal back to the coupler interface point
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH