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