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