Method and device for producing a quartz glass body

In a known procedure for manufacture of a quartz glass body, a glass starting material and fuel gas are fed to a rotationally symmetrical deposition burner ( 1 ) having several annular gap nozzles ( 7 - 9 ) and being formed by coaxial arrangement of a number of quartz glass tubes ( 2 - 5 ), such glass starting material in a burner flame forming SiO 2 particles which, under back and forth motion of the deposition burner ( 1 ) along the longitudinal axis of a rotating mandrel ( 12 ), are deposited on such rotating mandrel under formation of an essentially cylindrical porous blank. To enable replacement of such deposition burner without major efforts in terms of work and costs, the procedure of the invention proposes to use a deposition burner ( 1 ) the annular gap nozzles ( 7 - 9 ) of which have gap widths with a maximum dimensional deviation of 0.1 mm, and that the deposition burner ( 1 ) is co-axially encompassed and aligned in a given direction of space by means of an alignment unit ( 27; 32 ) engaging the burner's outer surface ( 35 ), and that the alignment unit ( 27; 32 ) is connected to a shifting device ( 28 ) for positioning the former within a horizontal plane. In a suitable device for implementation of this procedure, a deposition burner ( 1 ) is manufactured to possess annular gap nozzles ( 7 - 9 ) with gap width deviations of no more than 0.1 mm, the outer surface ( 35 ) of such burner being coaxially engaged by an alignment unit ( 27; 32 ) capable of rotating at least in a first plane and connected to a shifting unit ( 28 ) capable of being positioned within a second horizontal plane (FIG. 2 ).

FIG. 1 serves to illustrate a suitable procedure for the determination of gap widths and dimensional deviations of deposition burners. The schematic depiction shows a top view of the front face of a rotationally symmetrical deposition burner 1 . Deposition burner 1 consists of at least four quartz glass tubes 2 , 3 , 4 , 5 in coaxial arrangement. Central quartz glass tube 2 surrounds central nozzle 6 ; separation gas nozzle 7 is situated between the adjacent quartz glass tubes 2 and 3 ; quartz glass tubes 3 and 4 surround fuel gas nozzle 8 , and quartz glass tubes 4 and 5 surround external nozzle 9 . In the following, separation gas nozzle 7 is used as an example to illustrate the procedure for determination of the dimensional deviation of the gap width. For illustrative purposes, quartz glass tubes 2 - 5 are depicted with shape and positional errors, e.g. uneven wall thickness, non-circular cross-sections and eccentric arrangement. The ideal case is denoted by two dotted concentric lines 12 and 13 extending coaxial to longitudinal axis 14 of burner 1 . Concentric line 12 with external radius, R A2 , surrounds the external wall of quartz glass tube 2 , whereas concentric line 13 with internal radius, R 13 , tangentially touches the inner surface of quartz glass tube 3 . Since both concentric lines 12 and 13 are concentric with respect to longitudinal axis 14 , the gap width between these lines is constant in all places. The nominal gap width of separation gas nozzle 7 in the embodiment shown is 0.8 mm. Variations in the wall thickness and diameter as well as eccentricities of the adjacent quartz glass tubes 2 and 3 , and eccentric arrangement lead to deviations from the stated nominal gap width. Designation number 10 symbolizes the real maximum gap width and designation number 11 symbolizes the real minimum gap width. The first step in determining the dimensional deviation is to calculate the annular gap width according to the following equation: S &equals;(internal diameter of outer quartz glass tube 3 ) minus(outer diameter of inner quartz glass tube 2 ) As the two quartz glass tubes each are afflicted by dimensional deviations, extreme tolerance values, S max (maximal deviation) and S min (minimal deviation), are calculated. In the embodiment shown, the maximal value of the internal diameter of quartz glass tube 3 is 4.7 mm&plus;0.05 mm&equals;4.75 mm, whereas the minimal value of the external diameter of quartz glass tube 2 is 3.1 mm−0.05 mm&equals;3.05 mm. From these values, the maximal deviation S max is calculated: (4.75 mm−3.05 mm)/2&equals;0.85 mm. Using a similar calculation the minimal value of the internal diameter of quartz glass tube 3 is determined to be 4.7 mm−0.05 mm&equals;4.65 mm, and the maximal value of the external diameter of quartz glass tube 2 to be 3.1 mm&plus;0.05 mm&equals;3.15 mm. From these values, the minimal deviation S max is calculated to be (4.65 mm−3.15 mm)/2&equals;0.75 mm. The dimensional accuracy as defined for the purpose of this invention is calculated as the difference of the minimal and maximal deviations: S max −S min &equals;0.85 mm−0.75 mm&equals;0.1 The dimensional deviations of fuel gas nozzle 8 and external nozzle 9 are then determined accordingly. The dimensional deviation of neither of the annular gap nozzles exceeds the permissible value of 0.1 mm. FIG. 2 depicts a suitable device for implementation of the procedure according to the invention. The device consists of a deposition burner 1 , a swivel table 27 and a shifting table 28 . Deposition burner 1 is a four-nozzle burner as shown in the schematic top view of burner mouth 31 depicted in FIG. 1 . Hereinafter, any reference to equivalent components of deposition burner 1 will be made using the designation numbers of FIG. 1 . Deposition burner 1 is essentially rotationally symmetrical about its longitudinal axis 14 . The burner consists of four quartz glass tubes 2 , 3 , 4 , 5 in coaxial arrangement with its central nozzle 6 surrounded by three annular gap nozzles (separation gas nozzle 7 , fuel gas nozzle 8 , and external nozzle 9 ) in coaxial arrangement. The cross-sections of central nozzle 6 , separation gas nozzle 7 , fuel gas nozzle 8 , and external nozzle 9 correspond to a ratio of 1:5:15:40, respectively. Each of the nozzles ( 6 - 9 ) is fitted with a gas inlet 30 a , 30 b , 30 c , 30 d . The front faces of the upper quartz glass tubes near burner mouth 31 are polished and the edges smoothened by hydrofluoric acid etching. Deposition burner 1 is suspended in vertical alignment by means of an alignment unit engaging a bracket 32 . Bracket 32 has a bore hole 25 through which deposition burner 1 extends. The upper and lower areas of bore hole 25 are designed to have a screw thread each to surround and thus engage deposition burner 1 . By screwing a union nut 34 onto screw thread 24 truncated cone-shaped surface 23 on the inside of union nut 34 is pressed against flexible coaxial ring 33 such that this ring is pressed against front face 22 of bracket 32 and outer surface 35 of deposition burner 1 . By tightening the two union nuts 34 outer surface 35 of deposition burner 1 is held in centric suspension in two places providing for axial guidance. Swivel axis 21 , extending perpendicular with respect to longitudinal axis 14 and running on bearings on swivel table 28 , engages in the center of bracket 32 . Rotation around swivel axis 21 causes deposition burner 1 to swivel by swivel angle “&bgr;” (designation number 36 ; perpendicular to the plane shown in the drawing). Locking screw 20 serves to fix swivel axis 21 in position, whereas adjusting screw 19 acting on swivel table 27 facilitates the swivelling of deposition burner 1 around axis 37 by a swivel angle of “&agr;” (designation number 38 ). Axis 37 connects swivel table 27 to a bearing block 26 attached to a commercial shifting table 28 . Spindle 39 provides for linear motion of shifting table 28 which is screwed onto an extension arm 40 . In the following, the procedure of the invention is illustrated on the example of a preform for optical fibers and on the basis of the device shown in FIG. 2 : The first procedural step consists of the manufacture of deposition burner 1 through known methods of the glass-making industry using suitably selected and carefully manufactured quartz glass tubes. Subsequently, the dimensions of burner mouth 31 are measured with a profile projector in order to determine the dimensional deviations of the three annular gap nozzles, as shown in FIG. 1 above. In the embodiment shown, the dimensional deviation values are 0.1 mm, 0.006 mm, and 0.07 mm for the three nozzles proceeding from inside to outside, respectively. Thus, deposition burner 1 complies with the required maximal deviation of gap widths of no more than 0.1 mm for any of the fuel gas nozzles. Subsequently, deposition burner 1 is lifted up into bore hole 25 and mounted and fixed in bracket 32 such that the arrangement ensures accurate axial guidance of deposition burner 1 by flexible coaxial rings 33 engaging on outer surface 35 of the burner. Through the use of swivel axis 21 and axis 37 deposition burner 1 is aligned such that longitudinal burner axis 14 is in a vertical position. The fixed and aligned deposition burner 1 is then shifted in a horizontal plane by means of shifting table 27 until longitudinal axis 14 of deposition burner 1 intersects the longitudinal axis of mandrel 12 (in FIG. 2 the longitudinal axis of mandrel 12 runs perpendicular to the plane shown in the drawing). After manufacture, alignment, and positioning, deposition burner 1 shows unique, but reproducible burner properties. These characteristics are reproduced in a replacement burner replacing deposition burner 1 provided said replacement burner is manufactured, aligned, and positioned in accordance with the procedure of the invention, such that no work- and cost-intensive adjustment of process parameters is required. This also holds true, if deposition burner 1 is one out of a series of deposition burners in a burner group. To manufacture a GeO 2 -doped core layer in accordance with the OVD procedure, soot particles are deposited by moving deposition burner 1 back and forth along mandrel 12 , which rotates around its longitudinal axis. For this purpose, SiCl 4 , GeCl 4 , and carrier gas oxygen are supplied to central nozzle 6 of deposition burner 1 . The two starting components (SiCl 4 &plus;GeC 4 ) and the flow of carrier gas oxygen are supplied at a molar ratio of 1:1. Separation gas oxygen, hydrogen, and fuel gas oxygen are supplied through separation gas nozzle 7 , fuel gas nozzle 8 , and external nozzle 9 , respectively. The four gas flows, i.e. SiCl 4l &plus;GeCl 4 &plus;carrier gas oxygen, separation gas oxygen, hydrogen, and fuel gas oxygen, are supplied at a volume ratio of 1:1:10:5, respectively. Once the core layer has grown to the nominal size, a first SiO 2 coat layer is deposited on the core layer. For this purpose, the supply of GeCl 4 to deposition burner 1 is stopped, from whence non-doped SiO 2 particles are deposited under formation of a coating glass layer. Finally, mandrel 12 is removed and the successively formed green body is cleaned, sintered, and collapsed into a core rod using generally known procedures. To complete the manufacture of the preform for optical fiber production, the core rod is subsequently coated with additional coating glass layers.