VORFORM MIT WASSERSTOFFUNDURCHLÄSSIGERLAGE BEI EINER OPTISCHEN FASER DIE AUS DIESER VORFORM HERGESTELLT WURDE UND HERSTELLUNGSVERFAHREN EINER SOLCHEN VORFORM

Optical fiber preform, has an external alumina-doped silica cladding (28) with a peripheral zone (25) of 0.08-2.2 R thicknes and of alumina content providing a relatively low equivalent alumina concentration throughout the external cladding. Silica-based optical fiber preform comprises a core (20) of radius (R), an internal optical cladding (21) in direct contact with the core and an external cladding (28) of alumina-doped silica. The novelty is that (a) the external cladding has an internal zo (27) in direct contact with the optical cladding and a peripheral zone (25) which is in direct contact with the internal zone an which consists mainly of alumina-doped silica; (b) the peripheral zone (25) has a thickness of 0.08-2.2 R; and (c) the quantity alumina in the peripheral zone is such that the equivalent concentration of alumina throughout the external cladding corresponds 100-1000 ppm aluminum by wt. of the silica. Independent claims are also included for: (1) a process for production of the above preform; and (2) an optical fiber manufactured from the above preform or a preform made by the above process