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...http://www.google.de/patents/US4452508?utm_source=gb-gplus-sharePatent US4452508 - Graded index optical fibres
