The Post Office

British Telecommunications Corporation

George R Newns

Keith J Beales

Anthony G Dunn

Paul W France

Clive R Day

William J Duncon

William J Duncan

A range of alkali metal borosilicate glass compositions modified by the addition of alkaline earth metal oxides has been found to be especially suitable for the production of graded index optical fibre using the double crucible method. A core glass from this range may be paired with a suitable cladding glass, which may be another glass from the same range or an unmodified alkali metal borosilicate, so that in the nozzle of the double crucible thermal diffusion takes place to give a composition gradient of alkaline earth metal oxide and hence a refractive index gradient. The fibres thus obtained have better refractive index profiles if the proportions of the oxides in the core and cladding glasses are chosen so as to exclude a simple ion-exchange mechanism. Fibres having numerical apertures of up to 0.268 and best loss values as low as 5.0 dB/km have been prepared.

Graded index optical fibres

In order to provide improved resistance to attack by atmospheric water etc. a clad glass optical fibre has a plastic coating which is in turn coated with a metal coating. The metal coating may in turn be coated with a second plastic coating. The metal coating is formed by drawing the plastic coated fibre through a globule of molten metal, the surface of which is exposed to an oxygen rich atmosphere.

Optical fibres and coatings therefor

A graded index optical fiber is produced in a double crucible with an elongate heated exit nozzle. The core and cladding glasses of the fiber diffuse into one another in the nozzle.

Method for manufacture of graded index optical fibres

A range of alkali metal borosilicate glass compositions modified by the addition of alkaline earth metal oxides has been found to be especially suitable for the production of graded index optical fibre by thermal diffusion using the double crucible method. A core glass chosen from this range may be paired with a suitable cladding glass so that, in the nozzle of the double crucible, alkaline earth metal exchange occurs to give a composition gradient of alkaline earth metal oxide and hence a gradation of refractive index. Using these glasses, fibres have been produced having refractive index profiles that approximate closely to the ideal parabolic distribution, numerical apertures of up to 0.21, and best loss values as low as 6.4 dB/km at 850 nm.

Optical fibres and glasses

A particular range of glass compositions in the soda-boro-silicate glass system has been found to be particularly suitable for ultra-low loss dielectric optical waveguides. The composition range is indicated in FIG. 1 of the specification. By preparing such glasses in a reducing atmosphere (0-2% CO in CO.sub.2) and adding about 1% As.sub.2 O.sub.3 to the melt attenuation coefficients due to copper and iron can be minimized. The water content of the glass can be reduced by bubbling dry CO in CO.sub.2 through the molten glass. Glasses containing less than 30% Na.sub.2 O by weight have the lowest losses. Using glasses of this type dielectric optical waveguides having an insertion loss of less than 14 dBKm.sup.-.sup.1 have been made.

Making a dielectric optical waveguide glass

A particular range of glass compositions in the soda-boro-silicate glass system has been found to be particularly suitable for ultra-low loss dielectric optical waveguides. The composition range is indicated in FIG. 1 of the specification. By preparing such glasses in a reducing atmosphere (0-20% CO in CO.sub.2) and adding about 1% As.sub.2 O.sub.3 to the melt attenuation coefficients due to copper and iron can be minimised. The water content of the glass can be reduced by bubbling dry CO in CO.sub.2 through the molten glass. Glasses containing less than 30% Na.sub.2 O by weight have the lowest losses. Using glasses of this type dielectric optical waveguides having an insertion loss of less than 14 dBKm.sup.-.sup.1 have been made.

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Hintlesham, United Kingdom

George R Newns