{TAIL ~ The optical signal propagates} along the tube toward a second end of {HEAD ~ the tube} via one or more reflections on the inner surface of the tube 
{ TAIL ~ The optical signal propagates } along the tube toward a second end of { HEAD ~ the tube } via one or more reflections on the inner surface of the tube
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH