A method for producing an optical fiber preform, an optical fiber preform and an optical fiber

Having: (1) the thermal diffusion process, average particle size in this step is equal to or less than the diameter of 1mm alkali metal raw material, heating the starting material to generate the alkali metal salt of an alkali metal salt vapor and carrier gas together with supplying a glass inner end side from the silica-based glass, using a glass tube in the longitudinal direction of the relative movement of the heat source for heating the glass tube, the alkali metal oxidation reaction is diffused toward the inside of the glass tube heat; ( 2) the solid core of the step, the step of solid core glass manipulation after the thermal diffusion step to prepare the core rod; (3) an additional step of cladding portion, in the step by step production of the solid core an additional cladding portion around the core rod.