Method for producing a quartz glass body

The invention relates to a procedure for manufacture of a quartz glass body by deposition of SiO 2 particles on the outer surface of a cylindrical mandrel rotating around its longitudinal axis under formation of an essentially cylindrical porous blank with an inside bore hole, and by removing the mandrel and sintering of the blank, characterized in that the mandrel in the area of one of the ends of the successively forming blank is surrounded by a shaping element rotating at the same rotation velocity as the mandrel, such shaping element having a core area facing the blank which is at least partially removably embedded into the front face of the successively forming blank and which is removed before sintering after having widened the inside bore hole of the blank with a shape complementary to that of the core area.

Layers of SiO 2 particles are deposited by the action of a flame hydrolysis burner (not shown) onto an aluminum oxide carrier tube 1 capable of rotating around its longitudinal axis leading to the formation of porous hollow cylinder 3 . The external diameter of carrier tube 1 is 8 mm. A free end 4 of carrier tube 1 extends through a rotationally symmetrical quartz glass shaping cone designated in its entirety as reference number 5 . Shaping cone 5 consists of a bushing 7 from which protrudes an insert 6 , shaped like a truncated cone, pointing towards hollow cylinder 3 . Bushing 7 and truncated cone 6 are connected to form one unit. Truncated cone 6 contains a bore hole that extends coaxial with respect to longitudinal axis 2 and envelops carrier tube 1 . The truncated cone tapers off in the direction of hollow cylinder 3 from a maximal external diameter of approx. 20 mm to a minimal external diameter of 12 mm over a stretch of 14 mm. The length of bushing 7 is approx. 80 mm and the external diameter is 27 mm. Truncated cone 6 and part of bushing 7 are embedded in front end 8 of hollow cylinder 3 . The connection between shaping cone 5 and carrier tube 1 is friction-tight. This is implemented by two semi-spherical PTFE inserts 9 located inside bushing 7 which firmly hold carrier tube 1 in place. In the following, one embodiment of the procedure of the invention is illustrated in detail using FIG. 1 as an example. Truncated cone 5 is fixed to carrier tube 1 in the orientation depicted in the Figure by means of PTFE semi-spheres 9 . On the opposite end of carrier tube 1 , a bushing-shaped quartz glass holder is installed (not shown in FIG. 1 ) similar to that described in DE-A1 197 51 919 referred to above. Subsequently, carrier tube 1 is clamped into a lathe and rotated around its longitudinal axis. By moving the flame hydrolysis burner back and forth along carrier tube 1 , layers of SiO 2 particles are deposited onto the surface of the tube and the shaping cone (and on said bushing-shaped holder) rotating at the same speed as carrier tube 1 . During this process, the ends of shaping cone 5 and the holder become embedded in successively forming hollow cylinder 3 . Whereas on one of the ends a firm connection between hollow cylinder 3 and the holder is not only desired but required, the formation of a mechanical connection between hollow cylinder 3 and shaping cone 5 is to be prevented. Upon completion of the deposition process, carrier tube 1 is pulled out of hollow cylinder 3 and shaping cone 5 is removed simultaneously. Because of the rotationally symmetrical design and cone shape of the truncated cone shaping cone 5 is easier to remove. Hollow cylinder 3 is processed with a saw along dotted line 10 , if required. To forego this step, an alternative embodiment of the invention is designed to have a ridge at the position of dotted line 10 that is sufficiently high to prevent hollow cylinder 3 from growing from truncated cone 5 onto the cylindrical surface of bushing 7 during the deposition process. Once carrier tube 1 and shaping cone 5 are removed, the inside bore hole at end 8 of hollow cylinder 3 shows a rotationally symmetrical geometry complementary to the external geometry of truncated cone 6 , and successively widens as one progresses from inside to outside. Subsequently, hollow cylinder 3 is subjected to a cleaning and drying procedure using a halogen-containing atmosphere, and then sintered. During these steps, hollow cylinder 3 is held suspended in vertical direction in a treatment chamber (not shown in the Figure) using the afore-mentioned holder. After sintering, the inside bore hole has an internal diameter of approx. 3 mm; the widening of the inside bore hole at the end is maintained during the procedures at the same shrinking ratio. Subsequently, the front end (previously end 8 ) of the vitrified quartz glass tube (previously hollow cylinder 3 ) is attached to a tube pipe. For this purpose, front end ( 8 ) of quartz glass tube ( 3 ) is heated to the softening temperature. Because the front end was widened earlier in the procedure, the softened area of the inside bore hole does not collapse at this time. Once the tube pipe is attached, the inside bore hole of the quartz glass tube can be subjected to other generally known treatment steps for the production of optical fiber preforms—such as cleaning of the internal surface by introduction of a cleaning gas.