Method for manufacturing optical fibers having eccentric longitudinal index inhomogeneity

Claim 1

In a method of manufacturing a preform which is intended to be subsequently drawn into an optical fiber, said method being of the type that includes the steps of: flowing a vapor mixture including at least one compound, glass-forming, precursor together with an oxidizing medium through a hollow, cylindrical substrate; and heating said substrate and contained vapor mixture with a moving heat source, external to the substrate, such that a suspension of particulate, oxidic reaction product material is produced within the substrate, said particulate material traveling downstream and coming to rest on the inner surface of said substrate and thereat being fused to form a continuous glassy deposit on said inner surface, the improvement which comprises: terminating the vapor-flowing and substrate-heating steps recited above before said continuous glassy deposit has attained the desired thickness; inserting at least one longitudinally extending rod into said substrate, said rod having a refractive index differing from the refractive index of said glassy deposit and being eccentrically positioned within said substrate; restarting said vapor-flowing and substrate-heating steps to resume the production of said glassy deposit, said deposit forming on the glassy deposit layer previously formed and on said rod; and then terminating said vapor-flowing and substrate-heating steps when said glassy deposit has attained the desired thickness whereby a preform is obtained which, when subsequently collapsed and pulled, yields an optical fiber having an eccentric, longitudinal index inhomogeneity close to the axis of the fiber.