LOW-WATER OPTICAL FIBER PREFORM AND PROCESS FOR MAKING IT

摘要

An improved flame-hydrolysis process is disclosed, for manufacturing large optical fiber core preforms, useful in the production of low water, single-mode optical fibers. In one feature of the invention, the preform is manufactured using a vapor axial deposition (VAD) apparatus to form an inner core and an outside vapor deposition (OVD) apparatus to form an outer core, wherein the VAD and OVD apparatus both are located within the same housing. In another feature of the invention, the concentration of a germanium dopant in the inner and outer cores is substantially uniform, but increases immediately adjacent the outer surface of the outer core. Further, the concentration of a fluorine dopant in a surrounding cladding layer decreases from a maximum value immediately adjacent to the outer core. These variations in dopant concentrations are selected to offset the effect on refractive index caused by diffusion of the germanium dopant while the preform is subsequently being processes.