Core glass in alkali-zinc-glass system for a fibre optic cable and fibre optic cable with same

Core glass (I) in alkali-zinc-silicate-glass system, for a step-index fiber, comprises (in mol%): (a) silicon dioxide (54.5-65); (b) zinc oxide (18.5-30); alkali oxide (8-20); lanthanum(III) oxide (0.5-3); (d) zirconium oxide (2-5); (e) hafnium oxide (0.02-5); (f) zirconium oxide and hafnium oxide (2.02-5); (g) barium oxide (0.4-6); (h) strontium oxide (0-6); (i) magnesium oxide (0-2); (j) calcium oxide (0-2); (k) alkaline earth metal oxide (0.4-6); and (l) lithium oxide (0.5-3). Core glass (I) in alkali-zinc-silicate-glass system, for a step-index fiber, comprises (in mol%): (a) silicon dioxide (54.5-65); (b) zinc oxide (18.5-30); (c) sum of alkali oxide (8-20); (d) lanthanum(III) oxide (0.5-3); (e) zirconium oxide (2-5); (f) hafnium oxide (0.02-5); (g) zirconium oxide and hafnium oxide (2.02-5); (h) barium oxide (0.4-6); (i) strontium oxide (0-6); (j) magnesium oxide (0-2); (k) calcium oxide (0-2); (l) sum of alkaline earth metal oxide (0.4-6); and (m) lithium oxide (0.5-3), where the amount of = 25 mol.% of the total alkali oxide is greater than 58.5 mol.% of the sum of silicon oxide, zirconium oxide and hafnium oxide and the ratio of zinc oxide to the alkaline earth metal oxide is greater than 3.5:1. An independent claim is included for a step-index fiber comprising a core and a mantle, where the core glass is (I).