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