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