The structure 2 wherein the index of refraction of {HEAD ~ the guiding core} is smaller than {TAIL ~ the index} of refraction of the surrounding distributed gratings 
The structure 2 wherein the index of refraction of { HEAD ~ the guiding core } is smaller than { TAIL ~ the index } of refraction of the surrounding distributed gratings
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH