| 摘要 |
PROBLEM TO BE SOLVED: To shorten a cooling item and to improve the working efficiency of equipment by vitrifying a porous glass preform to transparent glass, then cooling the transparent glass while introducing inert gas into a furnace core tube and/or into a vacuum vessel on the outside of this furnace core tube. SOLUTION: The porous glass preform 1 supported at a seed rod 17 is housed into the furnace core tube 2. The porous glass preform 1 is then heated and vitrified to the transparent glass by means of a heater 3 disposed on the outer periphery of the furnace core tube 2 and a thermally insulating material 4 disposed at the outer periphery of this heater 3. Next, cylinders 8, 9 are driven to form through-parts 10, 11 at two points, upper and lower of the thimble insulating material 4. The inert gas is introduced into furnace core tube 2 and/or into the vacuum vessel 7 on the outside of the furnace core tube 2 from vacuum pumps 19a, 19b and a supply port 5a and/or 5b while the pressure in a furnace body 7 is controlled. The transparent glass is cooled while the pressure in the furnace core tube 2 and/or the vacuum vessel 7 is raised to about 10<4> to 10<5> Pa. As a result, the time required for cooling in the cooling stage after the formation of the transparent glass is shortened. |
