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