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