Manufacturing method of porous glass base material for optical fiber

A manufacturing method of porous glass base material for optical fiber by depositing glass fine particles on a starting material, comprising:

• supplying an organic siloxane raw material supplied from a raw material tank to a vaporizer;
• vaporizing the raw material while combining the raw material and a carrier gas in the vaporizer;
• and externally depositing SiO2 fine particles on the starting material through combustion reaction by supplying the combined gas of the raw material and the carrier gas to a burner in a manufacturing apparatus of porous glass base material for optical fiber, wherein
• before starting to supply the raw material to a raw material gas supply pipe of the burner, the burner and the inside of the raw material gas supply pipe of the burner are pre-heated by flowing purge gas of 60° C. or higher into the raw material gas supply pipe and supplying combustible gas and combustion supporting gas to the burner to form an oxyhydrogen flame at the burner outlet so that an outer surface temperature of the burner is 60° C. or higher, and
• the purge gas flowing into the raw material gas supply pipe flows at a flow rate that satisfies Q/V>350[min−1] when the volume in the raw material gas supply pipe from a burner inlet of the raw material gas to the burner outlet in the burner is V[L] and the gas flow rate flowing into the raw material gas supply pipe in the burner before the start of raw material supply is Q[SLM].