PRODUCTION OF LIGHT-CONDUCTING GLASS FIBERS

1. A process for producing light-conducting glass fibers wherein a glass fiber comprising a glass fibre core containing first ions of high degree of contribution with respect to the increase of the refractive index and a glass fiber coating layer disposed peripherally around said core and containing second ions of low degree of contribution to the increase of the refractive index, each of said core and said coating layer being such that the first ions contained in the core can be substituted by the second ions contained in the coating layer by mutual thermal diffusion between said first and second ions through the boundary surface between said core and said coating layer, is heated thereby to cause said mutual thermal diffusion through said boundary surface with the result that in the vicinity of the optical axis of the glass fiber, the refractive index in any cross section of the fiber perpendicular to the optical axis decreases progressively from the fiber center toward the periphery, wherein the glass is thus heated by immersion thereof in a molten medium maintained at a high temperature and having such a specific gravity that the difference between the specific gravities of said glass fiber and said molten medium at said high temperature is less than 0.4 thereby to accomplish said mutual thermal diffusion rapidly whilst deformation of the glass fiber, which would otherwise be caused by the softening of the glass fiber arising from high temperature heating, is prevented.

2. A process as claimed in claim 1 in which said glass fiber core is a glass containing thallium (Tl) ions as said first ions.

3. A process as claimed in claim 1 or 2 in which said glass fiber coating layer is a glass containing as said second ions of at least one alkali metal.

4. A process as claimed in any one of the preceding claims in which said molten medium is selected from inorganic salts of metals, mixtures thereof, metals, and metal oxides and is a substance in liquid state which is stable and has little or no erosiveness with respect to the glass fiber.

5. A process as claimed in any one of the preceding claims in which said high temperature is higher than the temperature at which the viscosity of the glass fiber becomes 10¹⁰ poise.

6. A process as claimed in any one of the preceding claims in which the glass fiber is thus heated by immersion at a temperature and for a time period such that the refractive index of the glass fiber in any cross section perpendicular to the optical axis, at least in the glass fiber thus drawn by said drawing the vicinity of the central axis, decreases sub- device thereby to cause the glass fiber to be stantially as the square (second power) of the immersed in said bath; a heating device for distance from the central axis. heating and maintaining the bath at said high 7. A process as claimed in claim 1 which temperature thereby to heat the glass fiber to is continuously carried out by first continuously cause said mutual thermal diffusion through drawing said glass fiber and then continuously said boundary surface thereby to establish a passing the fiber thus drawn through the refractive index distribution in the glass fibre interior of a heating furnace containing the wherein the refractive index in any cross secmolten medium thereby to heat the glass fiber, tion of the fiber perpendicular to the optical 8. A process as claimed in claim 7 in which axis thereof in the vicinity of the optical axis the glass fiber is thus continuously drawn by decreases progressively outward from the feeding a glass rod longitudinally at a constant centre of the fiber toward the periphery speed through a heating furnace, said glass thereof; and a withdrawing device for conrod comprising a glass core structure con- tinuously conveying the glass fibre thus heated taining said first ions and a coating layer out of the bath.

containing said second ions and disposed con- 11. Apparatus as claimed in claim 10 in centrically around said core structure, and which said molten medium is selected from drawing one end away from the remainder of inorganic salts of metals, mixtures thereof, the glass rod at a speed greater than said metals, and metal oxides and is a substance constant speed. in liquid state which is stable and has little 9. A process as claimed in claim 7 in which or no erosiveness with respect to the glass the glass fiber is thus continuously drawn by fiber.

causing a molten glass containing said first 12. Apparatus as claimed in any one of ions to flow through a central nozzle to form claims 10 or 11 in which said bath is of said glass fiber core and causing a molten glass sufficient length for causing the resulting glass containing said second ions to flow through fiber immersed therein to acquire thereby a an annular nozzle disposed coaxially around refractive index in any cross section perpensaid central nozzle thereby to form said glass dicular to the optical axis thereof which, at fiber coating layer. least in the vicinity of central axis, decreases 10. Apparatus when used for carrying out as the square (second power) of the distance the process claimed in claim 1 comprising in from the central axis.

combination; a drawing device for continuously 13. A process for producing light-conductdrawing a glass fiber comprising a glass fiber ing glass fibers substantially as hereinbefore core containing first ions of high degree of described with reference to the figure of the contribution with respect to the increase of accompanying drawing,

the refractive index and a glass fibre coating 14. A process for producing light-conductlayer disposed peripherally around said core ing glass fibers substantially as hereinbefore .and containing second ions of low degree of described with reference to the Example.

contribution to the increase of the refractive 15. Apparatus as claimed in claim 10 and index, each of said core and said coating layer substantially as hereinbefore described with being such that the first ions contained in the reference to and as illustrated by the figure of core can be substituted by the second ions the accompanying drawing,

contained in the coating layer by their mutual 16. A glass fiber whenever prepared by a thermal diffusion through the boundary sur- process as claimed in any one of claims 1 to face between said core and said coating layer; 9, 13 and 14.

a bath of a molten medium maintained at a For the Applicants:

high temperature and having such a specific F. J. CLEVELAND & COMPANY, gravity that the difference between the specific Chartered Patent Agents, .gravities of said glass fiber and said molten Lincoln’s Inn Chambers, medium at said high temperature is less than 40-43, Chancery Lane,

0.4; a feeding device for continuously feeding London, W.C.2.

Printed for Her Majesty’s Stationery Office by the Courier Press, Leamington Spa, 1973.

Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1ΑΥ, from which copies may be obtained.