A coupler includes input, output and {HEAD ~ interface points} for routing {TAIL ~ the optical signal} 
for routing
A coupler includes input , output and { HEAD ~ interface points } for routing { TAIL ~ the optical signal }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL