Method of modifying the index profile of an optical fiber preform in the longitudinal direction

Abstract

The refractive index profile of a multimode optical preform, fabricated by MCVD for example, is modified in the longitudinal direction by removing selected amounts of core material from the wall of a central hole through the preform prior to collapse. The amount of core material to be removed is determined by measuring the refractive index profiles of optical fibers drawn from a number of precursor preforms at various locations along the length of the drawn fibers, averaging their profiles, and then comparing those averages with a desired profile. The refractive index profile is indirectly measured by bandwidth measurements, which are related to the alpha value a selected parameter (e.g., bandwidth) at various locations along the length of a fiber drawn from a previously fabricated preform. This parameter is then compared with desired values of that parameter at the various locations, and the differences are used to calculate the amount of core material that needs to be removed. Selective core material removal is accomplished by moving a torch along the length of the preform and flowing an etchant gas, such as a mixture of oxygen and SF , through the central hole in the preform. Selective etching is accomplished by varying the flow rate and/or the concentration of the etchant gas and/or the torch speed as a function of the location of the torch along the length of the preform.