PURPOSE: To provide a preform having a uniform refractive index distribution of a side core part and having good reproducibility of dispersion characteristics at the time of reproducing the porous glass preform for the double core type optical fiber by a VAD method and by controlling a burner for a side core in specific range.
CONSTITUTION: A burner for a center core for forming a center core part and the burner 2 for a side core for forming a side core part are disposed and the numerical value of equation I consisting of a distance X(m) between a combustible gas blow-off port 3 of the burner 2 for the side core and the deposition surface of the side core part of a soot preform 1, the linear velocity of flow V(m/sec) of the combustible gas and the inside diameter D(m) of the combustible gas blow-off port 3 are so set as to attain the range of formula II. Gaseous raw materials and the combustible gas are passed to the burners for the center core and the side core and the combustible gas is ignited to form flames. The gaseous raw materials are then flame hydrolyzed in the flames to form start flow. The soot flow is thus deposited on the rotating soot preform 1 by integrating the center core part and the side core part.
KOIDE HIROYUKI
OGINO TAKESHI
HIRASAWA HIDEO
