Patent GB-1413891-A (GB)
Title: Optical glass bodies
IPC: C03C13/04
Filed: 1973-03-07
Abstract: 1413891 Optical glass body having a refractive index gradient NIPPON SELFOC CO Ltd 7 March 1973 [10 March 1972 30 March 1972] 11125/73 Heading C1M An optical glass body comprises at its central axis 50-70 mol. per cent SiO 2 , 10-30 mol per cent B 2 O 3 , 1-25 mol per cent Tl 2 O, and R 2 O, wherein R is at least one of the alkali metals, in an amount of up to 29 mol per cent such that the total proportion of Tl 2 O and R 2 O is 10-30 mol per cent, and optionally other compatible ingredients, e.g. ZnO in an amount 0-12 mol per cent, the concentration of the thallium ions contained in said glass body decreasing from its central axis to the peripheral surface in such a manner that the refractive index of the glass body continuously decreases from the central axis of its body to its peripheral surface. The glass body may contain at least one of Li<SP>+</SP>, Na<SP>+</SP>, K<SP>+</SP>, Rb<SP>+</SP> and Cs<SP>+</SP> ions in a concentration which increases from the central axis of the body towards the peripheral surface thereof. The preferred refractive index distribution is parabolic satisfying the equation N=N 0  (1 - ar<SP>2</SP>) wherein N 0  is the refractive index at the central axis of the body, N is the refractive index at a distance r from the central axis of the body in a direction perpendicular to the central axis and a is a positive constant. The glass body may be produced by contacting a glass body comprising 50-70 mol per cent SiO 2 , 10-30 mol per cent B 2 O 3 , 1-25 mol per cent Tl 2 O, and R 2 O, wherein R is at least one of the alkali metals, in an amount up to 29 mol per cent such that the total proportion of Tl 2 O and R 2 O is 10-30 mol per cent and optionally other compatible ingredients, said glass body having a uniform refractive index, with a source of at least one of sodium, potassium, lithium, rubidium and caesium ions at 400-600‹ C. In order to prevent devitrification of the glass when heated the glass composition preferably contains at least one of the compounds of formulµ R<SP>1</SP>O, R<SP>11</SP> 2 O 3  (apart from B 2 O 3  and R<SP>111</SP>O 2  (apart from SiO 2 ) where R<SP>1</SP> is a divalent atom such as Mg, Ca, Ba, Zn or Pb(II); R<SP>11</SP> is a trivalent atom such as Al; and R<SP>111</SP> is a tetravalent atom such as Sn(IV) or Ti 3  in a total proportion of 1-20 mol. per cent. The incorporation of 2-15 mol per cent ZnO is particularly suitable in this respect. The glass body may be suitably drawn or cut to form an optical fibre or a lens respectively. A typical glass composition suitable for use in the production of an optical fibre comprises 1À0-5À5 mol per cent Tl 2 O, 50-70 mol per cent SiO 2 , 10-30 mol per cent B 2 O 3 , 6À5-29 mol per cent R 2 O such that the total proportion of Tl 2 O and R 2 O is 10-30 mol per cent, and optionally at least one of the oxides MgO, CaO, BaO, ZnO, PbO, Al 2 O 3 , SnO 2  and TiO 2 , the total amount of said oxides being at most 15 mol per cent. A typical glass composition for use in the production of the present invention comprises 5À5-25 mol per cent TiO 2 , 50-65 mol per cent SiO 2 , 10-25 mol per cent B 2 O 3 , 6À5-24À5 mol per cent R 2 O such that the total amount of Tl 2 O and R 2 O is 15-30 mol per cent, and optionally at least one of the oxides MgO, CaO, BaO, ZnO, PbO, Al 2 O 3 , SnO 2  and TiO 2 , the total amount of said oxides being 1-20 mol per cent. The glass may also contain traces of AS 2 O 3  as fining agent. Fig. 1 indicates the concentration of the ions at a radial distance from the central axis in a direction perpendicular to the central axis of the rod during the ion exchange:-curve 1 being the concentration of T1<SP>+</SP> during the initial stage of ion exchange, the ion exchange occurring near the surface of the glass rod; curve 1<SP>1</SP> being the concentration of ions diffusing from the external ion source, e.g. K<SP>+</SP>, during the initial stage (i.e. at the central rod concentration of K<SP>+</SP> is zero) ; curves 2 and 2<SP>1</SP> being the concentrations of T1<SP>+</SP> and K+ when the K<SP>+</SP> ions have almost reached the centre of the rod; and curves 3 and 3<SP>1</SP> being the concentrations of TI<SP>+</SP> and K<SP>+</SP> when the ion exchange has proceeded still further. It can be seen that the concentration of thallium ions at the centre of the rod is lower than that before ion exchange, while the concentration of potassium ions at the centre of the rod is higher. In many cases the concentration of thallium ions at the peripheral surface of the glass rod is almost zero. The preferred diameter of the glass body of the invention is 0À03-10 mm. and the difference in refractive indices between the central axis and the peripheral surface is preferably 0À005-0À15. The glass body may also be produced by a process comprising melting the glass in the inside pot of a double platinum pot while melting in the outside pot a different class of glass containing an alkali metal ion, and then effecting the mutual heatdiffusion of the thallium ions contained in the inside pot and the alkali metal ions contained in the outer pot during the spinning of a glass fibre from the double platinum pot.