The optical signal is optically coupled into {HEAD ~ a first end} of {TAIL ~ a nontransparent hollow tube} 
of
The optical signal is optically coupled into { HEAD ~ a first end } of { TAIL ~ a nontransparent hollow tube }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL