a process is described for making a doped - silica ingot useful in the manufacture of optical fibers . at least a silicon compound and a titanium compound are decomposed in the flame of the induction plasma burner in the presence of a determined supply of hydrogen and are reacted with the oxygen contained in the burner feed gas and / or in the vector gas to form sio 2 and h 2 o against a heat - stable support . as a result silica and titanium oxide are deposited on the support in the form of a homogeneous vitreous mass exhibiting a selected concentration of hydroxyl groups between 10 and 50 parts per million . fluorine - doped silica is deposited radially in the same way on the resulting ingot . the resulting semifinished product is a cylinder consisting of a titanium - doped silica core , whose tio 2 concentration by weight is about 0 . 1 to 8 %, covered by a sheath of fluorine - doped silica , whose fluorine concentration is about 0 . 1 to 3 %.