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