Fiber preform method with a hot drill-in step for a Ge-doped tube and an F-doped rod

A method of making an optical fiber perform for dispersion-shifted optical fiber having a center core portion and ring core portion surrounding said center core portion and having higher refractive index than said center core portion comprising:

• a soot body synthesizing step of synthesizing a rod-shaped soot body of GeO2 and SiO2;
• a transparent glass forming step of dehydrating and consolidating said soot body so as to form a transparent glass body having a refractive index difference relative to pure silica glass within the range of 0.2 to 1.5%;
• a pipe forming step of heating said transparent glass body to a softening temperature or higher and inserting a rod having a melting point higher than said softening temperature into said transparent glass body along the center axis thereof so as to prepare a glass pipe for said ring core portion;
• an insertion step of inserting a silica glass rod for said center core portion having a cylindrical form doped with F element having a refractive index difference relative to pure silica glass within the range of −1.0 to 0.0% inside said glass pipe;
• and
• a heat-collapsing step of heating and collapsing said glass pipe and said silica glass rod which are in an inserted state brought into by said insertion step.