The structure includes a first polymeric sleeve having a refractive index numerically lower than the refractive index of {HEAD ~ the exterior cylindrical surface} of {TAIL ~ the glass core} 
of
The structure includes a first polymeric sleeve having a refractive index numerically lower than the refractive index of { HEAD ~ the exterior cylindrical surface } of { TAIL ~ the glass core }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL