The present invention is a rare earth (RE) relates to a method of producing a doped optical fiber, and more particularly to a stable dispersion of the RE oxide coating at ambient temperature silica nanoparticles (sol) using the precursor material of RE oxide coated silica nanoparticles a production and a silica glass tube the inner surface onto the Ge, Al, suitable dopant dip coating a silica sol containing selected from P, or the like, or according to any other conventional method the thin film coating, then the coated tube by MCVD method, and processed by an optical preform, to a method for manufacturing an optical fiber comprising an optical fiber made of a predetermined type. Novelty of the present invention is a rare earth ion porous flexible layer to then perform one and solution doping method or other conventional method for omitting the step of forming a porous flexible layer inside a fused silica glass tube at a high temperature of the CVD method to a core layer formed and that there is a need to introduce, by the addition of RE oxides directly in the sol is microcrystalline and cluster formation of rare earth ions is suppressed, and the result is composition comprising a RE concentration variability of the core prevents the reproducibility and reliability of the method greatly is increased, by the addition of Ge (OET) ₄ at ambient temperature in the silica sol, the amount of GeCl ₄ needed to achieve a predetermined value at a high temperature is decreased perforation.