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