British Telecommunications Corporation

William J Duncan

Keith J Beales

George R Newns

Anthony G Dunn

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.

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Ipswich, United Kingdom

William J Duncan