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