{HEAD ~ The optical signal} passing out of the second end of the nontransparent hollow tube is optically coupled onto {TAIL ~ an OEC} 
is optically coupled onto
{ HEAD ~ The optical signal } passing out of the second end of the nontransparent hollow tube is optically coupled onto { TAIL ~ an OEC }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL