The gratings are positioned at different lengths along the optical fiber to provide for {HEAD ~ different path lengths} of {TAIL ~ light} reflected by the gratings and back down the optical fiber to an optical output  
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The gratings are positioned at different lengths along the optical fiber to provide for { HEAD ~ different path lengths } of { TAIL ~ light } reflected by the gratings and back down the optical fiber to an optical output
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH