Patent ID: 8039410

Claim:
A core glass in the alkali-zinc-silicate system for a step index fiber, which contains, in Mol % on an oxide basis: SiO 2 54.5-65 ZnO 18.5-30 Σ alkali metal oxides 8-20 La 2 O 3 0.5-3 ZrO 2 2-5 HfO 2 0.02-5 Σ ZrO 2 + HfO 2 2.02-5 TiO 2 0-0.5 BaO 0.4-6 SrO 0-6 MgO 0-2 CaO 0-2 Σ alkaline earth metal oxides 0.4-6 Li 2 O 0.5-3 Σ SiO 2 + ZrO 2 + HfO 2 >58.5; wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, but with the proviso that the core glass contains said Li 2 O in an amount that is not more than 25% of a sum total amount, in Mol % on said oxide basis, of said alkali metal oxides present in the core glass.