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