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