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