Abstract: A furnace body for a glass preform elongating apparatus which makes an elongated body by passing the glass preform through the furnace body and elongating the glass preform while heating the glass preform, the furnace body comprising a furnace core tube shaped like a cylinder through which the glass preform passes and having so adequate length in an axial direction that the elongated body may not bend or distort its form, a heating member disposed at an outer peripheral portion of the furnace core tube, a thermal insulator enveloping the furnace core tube and the heating member from outside in circumferential and axial directions thereof, and a furnace body outer shell holding the thermal insulator therein, wherein a through hole is disposed near the heating member downstream thereof in an advancing direction of the glass preform so as to penetrate through the furnace core tube, thermal insulator, and furnace body outer shell in the direction orthogonal to the furnace core tube.

Abstract: A device for drawing down an optical fiber preform (2) connected at one end to a feed rod (3) having an equal or smaller diameter. A chamber (11) in which the preform is drawn has an upper part (11A) with an opening (12). The opening engages the preform to seal the chamber. An enclosure (5) has a seal (6) at its upper part (5A) for sealing around the feed rod. A lower part (5B) of the enclosure contacts the upper part of the chamber to seal the chamber while the feed rod is passing through the opening.

Abstract: A method for manufacturing an optical fiber (11), wherein the fiber (11) is drawn from one end of a preform (9) which is heated to above the glass softening temperature. The preform (9) is surronded by a protective gas (14) whose flow direction corresponds to the fiber drawing direction. The flow of the gas surrounding the preform (9) in the drawing area (10) is stabilized by an additional flushing gas (16).