The inner sleeve is heat bondable to the glass core and has {TAIL ~ a refractive index} lower than {HEAD ~ the glass core} 
The inner sleeve is heat bondable to the glass core and has { TAIL ~ a refractive index } lower than { HEAD ~ the glass core }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD