Method of manufacturing porous glass preform for optical fiber

A method of manufacturing a porous glass preform for an optical fiber, comprising:

• forming glass particle deposition layers by depositing glass particles on an outer periphery of a target rod while the target rod rotates and while a burner for synthesizing glass particles and the target rod that is rotating are relatively reciprocated, the glass particles being synthesized by a glass making feedstock gas and a combustion gas being supplied to the burner, the glass particle deposition layers increasing an outer diameter of the target rod;
• reducing a rotating speed of the target rod r [rpm] and a relative moving speed between the target rod and the burner V [mm/min.];
• and
• maintaining a variation in sweeping pitch P [mm], expressed as V/r, to be within a range of a central value±10% while the outer diameter satisfies a relation
• 0.5 L≤R≤0.8 L
• wherein a final outer diameter of the porous glass preform for an optical fiber is L [mm], an outer diameter of a glass particle deposition body in a middle of the porous glass preform is R [mm], and
• wherein a central value is an average value of a maximum value and a minimum value of the sweeping pitch when the outer diameter of a glass particle deposition body in a middle of synthesis is within a range where 0.5 L≤R≤0.8 L.