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