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