Low Loss Multimode Optical Fibers via Fictive Temperature Reduction by Means of Outer-Cladding Na Doping

The reduction of the intrinsic Rayleigh scattering of the optical fibers by slightly altering the outer-cladding composition via the introduction of alkali is the goal of this paper. In this view, multimode fibers with Al- and Na-doped outer-cladding were fabricated via the PCVD process. In optimal conditions (typ. 10-80 ppm Na), we obtained a reduction of 0.06 dB/km at 850 nm for 50/125 multimode fibers corresponding to an attenuation decrease from 2.21 dB/km down to 2.15 dB/km. This is attributed to Rayleigh scattering reduction due to a fictive temperature lowering. In addition, the minor amounts of Na incorporated in the outer-cladding results in much lower NBOHC defects related absorption at 650 nm.