Abstract: Disclosed is an optical waveguide fiber preform and an optical waveguide fiber drawn therefrom, in which the density and thus the effective refractive index of the clad layer is caused to change in a pre-selected way axially along the waveguide preform and the associated waveguide fiber. The axial change in density of the clad layer is due to the fraction of the clad volume that is air or a glass of a composition different from that of the base clad glass. The axially variation in clad index changes the signal mode power distribution, thereby changing key waveguide fiber parameters such as magnitude and sign of dispersion, cut off wavelength and zero dispersion wavelength. The invention includes methods of making the structures having an axially varying clad layer. The invention relates to preforms and associated waveguide fibers which guide light due to difference in refractive index between core and clad.

Abstract: Disclosed is an optical waveguide fiber preform which is an assembly of one or more core rods surrounded by a plurality of clad rods. The clad rods have a central portion and a surrounding layer, in which the refractive index of the central portion is lower than that of the surrounding layer. The preform is drawn into an optical waveguide fiber that has a two component clad layer. Proper choice of the central portion and surrounding layer provide a waveguide fiber that is endlessly single mode. Alternative embodiments include fibers containing dopants that enhance fiber photosensitivity or which provide stress-induced or asymmetric-refractive-index-induced birefringence in the waveguide fiber.