Patent GB-1287300-A (GB)
Title: Continuous production of light-conducting glass filaments
IPC: C03C21/00
Filed: 1969-11-13
Abstract: 1287300 Glass fibre optics manufacture NIPPON SELFOC KK 13 Nov 1969 [18 Nov 1968] 55627/69 Heading C1M In the continuous production of a lightconducting glass filament having in any crosssection a distribution of refractive index which decreases continuously from the centre axis outwardly, a bath of molten glass from which the filament is drawn has in contact therewith first and second electrodes of opposite polarity, the first electrode being in contact with and surrounding an area of the surface from which the filament is drawn and either the (1) first or (2) second electrode containing ions which can exchange with ions within the molten glass to produce glass with a (1) decreased or (2) increased refractive index, an electric voltage being applied across the electrodes and through the glass so that (1) the polarity of the first electrode or (2) that of the second electrode is positive with respect to the other to promote transfer and diffusion of ions. As shown in Fig. 4, molten glass 10 flows in an annular space between a hollow central anode 11 and surrounding cathode 12, both of electroconductive porous material, and is spun into a glass filament 13 through nozzle 15a. A molten metal alloy 14 in anode 11 contains a metal to be dissolved in to the glass to give a greater refractive index in the centre of the filament and a vessel 15 surrounding cathode 12 contains a molten metal alloy 16 into which a further metal from the glass is dissolved when voltage is applied to the electrodes 11, 12. The electrodes 11, 12 may be of porous graphite or perforated platinum plate. In place of molten alloys, molten metals or molten salts may be used, or, preferably, a molten salt is used at the anode and molten alloy at the cathode. In a suggested modification, the polarities of electrodes 11, 12 may be reversed and, with a glass of high refractive, the material 16 at the anode being such as to lower the refractive index. In a modification of Fig. 4, the electrode 11 is of solid metal or alloy and the porous material is not used as an electrode, a separate electrode being positioned in the molten material outside. As shown in Fig. 6, molten glass 32 on a layer of molten metal alloy 31 such as tin alloy in a vessel 30 is drawn upwardly to a filament 33 in the space through an annular electrode 34 to which an annular body 35 of a molten salt such as KNO 3  clings and is in contact with the glass, direct current being passed from the anode 34 through the glass to the molten alloy 31 used as a cathode. In a modification, the electrode 34 and molten salt cover the top surface of the molten glass and the filament is drawn through a central aperture extending through the alloy 31 and base of the vessel 30. The initial glass may be silicate, borate, phosphate or other oxide glasses and may have the composition in percentages by weight:- SiO 2  48; Na 2 O 12; Tl 2 O 16; PbO 24. Cations in the order of magnitude of contribution to increased refractive index are, for monovalent ions, Tl>Li>K = Na = Rb and for divalent ions, Pb>Ba>Cd>Sr>Ca>Zn>Be>Mg, and any of these may be used as modifying oxides and/or ion substitutes. Specification 1,211,854 is referred to.