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