METHOD FOR PRODUCING A SILICA GLASS PREFORM FOR OPTICAL FIBRES

摘要

A method for producing a silica glass preform for an optical fiber includes processes for producing a silica glass soot body having a core portion added with a positive dopant and an intermediate portion having a refractive index lower than that of the core portion, heating the silica glass soot body at a temperature for transparently vitrifying the soot body in a helium atmosphere containing a negative dopant raw material and forming a first core rod having the intermediate portion to at least a portion of which a negative dopant is added, giving a silica glass soot layer as a trench portion to an outer circumference of the first core rod, heating the soot layer at a temperature for transparently vitrifying the soot layer in the helium atmosphere and forming a second core rod having the trench portion to entire of which the negative dopant is added, and giving silica glass as a cladding portion to an outer circumference of the second core rod.