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