Abstract: A glass suitable for use in optical fibre cores has a refractive index of 1.540-1.610, Na.sub.2 O and B.sub.2 O.sub.3 in a molar ratio of 1.05-2:1, 2-12 mole percent BaO, 40-63 mole percent SiO.sub.2, and 1.5-15 mole percent ZrO.sub.2, the proportion of SiO.sub.2 and ZrO.sub.2 together not exceeding 65 mole percent. The glass can be prepared in a highly reduced state by the use of CO and a redox buffering agent. The glass is especially suitable for the production of high numerical aperture wide cored fibres.

Abstract: An oxide glass is dried by passing through a melt of the glass fluorine or another perhalogenated compound including fluorine atoms. For example, C.sub.3 F.sub.8 (e.g. after pre-cracking) can be used to reduce the OH content of a borosilicate glass, and the loss of an optical fibre drawn from such a glass, to low levels.

Abstract: A glass suitable for use as the core glass in a high numerical aperture optical fibre has a composition selected from the range of glasses which comprise(i) from 15 to 25 mole percent of soda,(ii) from 25 to 65 mole percent of silica and germania, taken together, the content of germania not exceeding 42 mole percent,(iii) up to 22.5 mole percent of boric oxide and barium oxide, taken together, the content of boric oxide not exceeding 20 mole percent, and the content of barium oxide not exceeding 12 mole percent, and(iv) from 0.01 to 1 weight percent of a redox buffering agent. The glass can be prepared by controlling the partial oxygen pressure of the melt so that it is approximately 10.sup.-5 atmospheres. An optical fibre can be produced, by using a soda-boro-silicate clad glass, and a core glass having the composition in the above range.