The Post Office

British Telecommunications Corporation

British Telecommunications Public Limited Company

Keith J Beales

George R Newns

Clive R Day

Anthony G Dunn

Benjamin J Ainslie

Sally Partington

Paul W France

James D Rush

William J Duncan

William J Duncon

Steve F Carter

An improved method of manufacturing an optical fibre by vapor phase deposition of layers of cladding and core material for an optical fibre on the inside of a tubular substrate, followed by heating of the tube to cause it to collapse into a preform and drawing of the preform to form the fibre, includes performing the collapse with the introduction of chlorine gas acting as a drying agent. An alternative to chlorine gas is a vapor which decomposes in situ to yield chlorine without producing a solid deposit.

Glass fibres and optical communication

An optical fibre capable of monomode transmission in, and having a wavelength of zero dispersion in, the 1.55 .mu.m window and having a core comprising silica and germanium dioxide and a cladding comprising silica is drawn from an appropriate preform at a temperature in the range from 1900.degree. C. to 2000.degree. C. Fibres thus produced have low loss compared with fibres drawn at higher temperature.

Optical fibres

An oxide glass is dried by passing through a melt of the glass fluorine or another perhalogenated compound including fluorine atoms. For example, C.sub.3 F.sub.8 (e.g. after pre-cracking) can be used to reduce the OH content of a borosilicate glass, and the loss of an optical fibre drawn from such a glass, to low levels.

Method for drying oxide glasses

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

A glass suitable for use as the core glass in a high numerical aperture optical fibre has a composition selected from the range of glasses which comprise

Glass optical fibres and glass compositions 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.

Growing community of inventors

Ipswich, United Kingdom

Keith J Beales