In optical fibre preform mfr. by internal tube CVD and then collapsing to a preform, the collapsing heat treatment is carried out along the glass tube according to a temp. profile determined by an upper region of max. glass temp. (hot zone) and a lower region at the glass softening temp. (soft zone) along the glass tube section being treated. The hot zone has a uniform temp. level and a width along the glass tube section which is similar to the soft zone width.
