Method of making halide fibers including casting under a reduced pressure
AssigneeBritish Telecommunications public limited company
InventorsPaul W. France, Steven F. Carter, John R. Williams
Life of this patent
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Abstract
Halide glass articles, e.g. rods, tubes and preforms for making fluoride glass fibres, are prepared by melting and/or casting the articles under a low pressure, e.g. 0.01 to 500 mbars and, during the low pressure regime, a gas flow rate of between 0.01 to 100 liters/min (measured at NTP) is maintained. It has been found that subjecting the melts to a low pressure reduces the attenuation of the fibre which eventually results from the melts.
Claims8
A method of preparing a halide glass wherein at least 90% mole of the halide of said halide glass is fluoride, which method comprises:
- (i) heating precursors of a halide glass until evolution of gas therefrom has substantially ceased;
- (ii) raising the temperature of the precursors and heating them at ambient pressure to ensure that all the precursors are melted and have passed into solution with each other;
- and
- (iii) reducing the temperature of the glass by at least 50 degrees C. and then substantially reducing the pressure to thereby form the halide glass.
A method of preparing a halide glass fibre which method comprises:
- casting a cladding precursor melt as a tube into a mould at a first pressure less than atmospheric pressure and thereafter casting a core precursor melt into the bore of said tube to thereby form a preform,
- the casting of said core precursor melt into said bore being done under a second pressure which is lower than said first pressure,
- the casting of said core precursor melt being done while at a second temperature which is lower than a first temperature at which the cladding precursor melt was cast, and
- drawing said preform into a fibre.
A method of preparing a fluoride glass fibre which fibre comprises a core of a second fluoride glass surrounded by and in contact with a cladding of a first fluoride glass wherein the refractive index of the second fluoride glass is higher than the refractive index of the first fluoride glass, which method comprises:
- (a) providing precursors of a first fluoride glass into a first crucible,
- (b) providing precursors of a second fluoride glass into a second crucible,
- (c) transferring each of said crucibles into a furnace and subjecting both crucibles simultaneously to a melting regime comprising, in the order specified,
- (c1) pre-heating said crucibles in the presence of a dry gas at ambient pressure until any evolution of gas evolved from the glass precursors substantially ceases,
- (c2) heating said crucibles to a temperature above the melting point of the glasses in the presence of a mixture of oxygen and inert gas in a mole ratio of from 1:0 to 1:100,
- (c3) reducing the temperature by at least 50.degree. C. but not a temperature below 600.degree. C. while maintaining a pressure in the range 5-150 millibars in said furnace,
- (d) transferring the crucibles to a casting zone, and
- (d1 ) casting said first fluoride glass as a tube wherein said casting includes pouring the melted precursors of said first fluoride glass into a mould at a temperature at which its viscosity is 0.01 to 1000 poise under a pressure of 2-100 millibars,
- (d2) casting said second fluoride glass into the bore of said tube whereby a fibre preform is produced, wherein the pouring of the melted precursors of said first fluoride glass into bore is carried out at a temperature which is at least 20.degree. C. lower than the temperature used in step (d1 ) and under a pressure in the range of 0.01-2 millibars, and
- (e) drawing the fibre preform into the fibre.
Dependencies are inferred from the claim text.