WO2001032572A1

WO2001032572A1 - Method for producing a quartz glass blank for optical waveguides and quartz glass blank produced according to this method

Info

Publication number: WO2001032572A1
Application number: PCT/EP2000/010665
Authority: WO
Inventors: Heinz Fabian
Original assignee: Heraeus Quarzglas Gmbh & Co. Kg; Shin-Etsu Quartz Products Co., Ltd.
Priority date: 1999-11-02
Filing date: 2000-10-30
Publication date: 2001-05-10
Also published as: DE19952821B4; DE19952821A1

Abstract

According to a known method for producing a quartz glass blank for optical waveguides, a core rod with a quartz glass core with a first refractive index and a jacket which radially delimits the core and which has at least one inner jacket layer consisting of a quartz glass with a second refractive index is produced, said inner jacket layer bordering the core. The core rod is coated with a coating tube consisting of porous quartz glass, said coating tube being sintered and hereby shrunk onto the core rod. The aim of the invention is to develop an economical and reproducible method for producing a quartz glass blank for optical waveguides with low optical damping, based on this known method, and to provide a corresponding blank. To this end, the invention provides that the core (1) and the inner jacket layer (2; 3; 7) are produced according to the OVD, MCVD, PCVD or VAD method and that the core rod (4) is coated with a coating tube (5) produced according to the OVD method.

Description

Process for producing a quartz glass preform for optical fibers and quartz glass preform produced by the process

The present invention relates to a method for producing a quartz glass preform for optical fibers by providing a core rod which has a core made of quartz glass with a first refractive index and a cladding which radially delimits the core and which has at least one inner core adjoining the core

Has a cladding layer made of quartz glass with a second refractive index, and overlaying the core rod with an overlay tube.

Furthermore, the present invention relates to a quartz glass preform produced by the method, comprising a core rod which has a core made of quartz glass with a first refractive index and a cladding radially surrounding the core, the at least one inner cladding layer made of quartz glass with a second Has refractive index, and with a second cladding layer enveloping the core rod.

The use of optical fibers for data transmission has gained in economic importance in the past 20 years. After the optical fibers were initially improved in terms of their optical attenuation and fiber strength, cost reduction is now a central issue. Possible starting points for this are increasing the transmission capacity per optical fiber and reducing the manufacturing costs of the optical fibers.

Optical fiber preforms for commercial applications are essentially manufactured using the known OVD (Outside Vapor Deposition), MCVD (Modified Chemical Vapor Deposition, PCVD (Plasma Induced Chemical Vapor Deposition) and VAD (Vapor Axial Deposition) process. This process uses First a core rod is produced, which essentially forms the core and the cladding of the later optical fiber. The cladding of the optical fiber is referred to below as "optical cladding". Typical diameter ratios of the core rod to core diameter are between 2 and 6. This diameter ratio is known as the so-called "d / dκ ratio", where d _{M is} the diameter of the core rod and d «the diameter of the core. Since commercially used single-mode optical fibers have typical core diameters of approx. 8 μm to 9 μm and a fiber diameter of 125 μm, additional quartz glass must be applied to the core rod in order to achieve these geometrical relationships. This additional quartz glass, which encases the core rod, is also called

The quality of the jacket material is important for the mechanical strength of the optical fiber, while its influence on the optical properties has so far played only a minor role.

With a typical d / d _κ ratio of 4, the core rod contributes just under 10% to the total fiber cross section. The remaining 90% is provided by the jacket material. With regard to the cost optimization of preform production, the costs for the production and application of the jacket material are of central importance.

A method and a preform of the type specified at the outset are known from DE-A 39 11 745. It is proposed for the production of a core rod to provide a core layer of quartz glass with a cover layer made of powder-ceramic material by forming a tube from the powder-ceramic material and applying this in a combined doping and sintering process in a gas phase containing a dopant Kemglas rod is shrunk. By repeatedly using this method, multilayered structures can be produced from glasses with different optical behavior.

In connection with the manufacture of the core rod, it has been shown that even with great care, the interface between the jacket tube and the core glass rod is not free of defects which may result during the subsequent fusion Defects. These defects have a particularly disadvantageous effect at the interface between the core and the jacket. In addition, due to the high temperature when the core glass rod is melted and the powder-ceramic top view of the porous jacket tube, there is a risk that the thin core glass rod will bend. In addition, the production of the cover layer formed from powder-ceramic material has disadvantages with regard to the production costs for the quartz glass preform and has proven to be not very reproducible.

A similar process is also known from US-A 4,675,040. In the method described therein for producing a quartz glass preform for so-called monomode fibers, a core rod is produced in a first process step by enveloping and fusing a core glass rod made of quartz glass with a jacket tube. The core glass rod consists of undoped, synthetic quartz glass, while the casing tube is in the form of a so-called “SiO ₂ soot body”, which is obtained by flame hydrolysis of SiCI and layer-by-layer deposition of SiO ₂ particles on a substrate. The surface of the The core glass rod is polished and heated to reduce the OH content in a plasma flame. The porous jacket tube is doped with fluorine to lower the refractive index and then sintered at a temperature of 1650 ° C. and thereby onto the core glass rod to form a core rod with a The core of the core rod produced in this way has a diameter of 8 mm and is encased in a cladding (“optical cladding”) with a smaller refractive index, the difference in the refractive indices being indicated as Δ = 0.30 ( Δ = (nι-n ₂ ) / nι x100, with n-ι = 1, 4585). In a second process step, the core rod is overlaid by a jacket tube made of undoped quartz glass by collapsing the jacket tube onto the core rod. The combination of core rod and flashing tube forms a quartz glass preform, from which a monomode fiber is then drawn ,

In the known method, the jacket material is provided in the form of a flash tube made of quartz glass. In the case of synthetic quartz glass, the flash tube is usually produced by one Silicon compound, such as SiCl, is oxidized or hydrolyzed to form SiO ₂ particles and the SiO ₂ particles are deposited in layers on a carrier rod, this is then removed and the tube obtained from porous soot material is densely sintered. The production of such flash pipes includes a large number of process steps.

In connection with the production of the core rod, it has been shown that the interface between the jacket tube and the core glass rod is not free of defects, even with great care, which can lead to defects during the subsequent fusion. These defects have a particularly disadvantageous effect at the interface between the core and the jacket. In addition, due to the high temperature when the core glass rod and porous jacket tube are fused, there is a risk that the thin core glass rod will bend. This danger is exacerbated if the core glass additionally contains a dopant, such as germanium oxide, which lowers the viscosity of quartz glass. Bending of the core glass rod can result in bending of the preform or an eccentricity of the fiber core in the optical fiber.

The invention is therefore based on the object of specifying an inexpensive and reproducible method for producing a quartz glass preform for optical fibers with low optical attenuation and to provide a corresponding preform.

With regard to the method, this object is achieved, based on the method mentioned at the outset, in that the core and the inner cladding layer are produced by the OVD, MCVD, PCVD or VAD method, and in that the core rod is produced by one by the OVD Process manufactured flashover pipe is overlaid.

In a first process step, a core rod is produced, which is overlaid in a second process step by a flash tube in the form of a porous SiO ₂ cylinder. The core rod supplies the quartz glass for the fiber core and at least part of the optical cladding of the later optical fiber. It is essential that a core rod is provided in which at least the core and the inner cladding layer which radially delimits the core are produced by the OVD, MCVD, PCVD or VAD method. Because only when a core rod produced in this way is used can the problems associated with the core rod production in the generic method with regard to crystal formation at the interface between core and cladding and the deflection of the core glass rod be avoided. In order to achieve a reproducible result, it is also essential that an overlay tube manufactured according to the OVD process is used to overlay the core rod.

The core rod comprises the core, which is delimited radially by the inner cladding layer. In addition to the inner jacket layer, the core rod can have one or more further cladding layers which envelop the first cladding layer. This jacket material can also be applied according to one of the methods mentioned above (OVD, MCVD, PCVD or VAD) or by means of the so-called rod-in-tube technique.

According to the invention, the flash tube is provided in the form of a porous SiO ₂ cylinder with an axial opening. The porous flash tube can be produced simply and inexpensively by oxidizing or hydrolyzing a silicon compound to form SiO ₂ particles, and depositing the SiO ₂ particles in layers on a carrier rod, and then removing the latter. The flash tube thus produced is characterized by an exact inner bore, which is predetermined by the outer diameter of the support rod, and is therefore directly suitable for carrying out the method according to the invention. The process steps additionally required in the known method for producing the “jacket tube” from quartz glass, which include, in particular, the reworking of the inner bore after collapsing and the division of the desired geometry, are therefore omitted. Accordingly, the manufacturing costs for the so-called “jacket Material "of the preform compared to the previous method less. Since the "jacket material" accounts for by far the largest part of the preform volume, the savings have a clear impact on the production costs for the preform. ~ ",""-, PCT / EP00 / 10665 O 01/32572

- 6 -

To produce the preform, the core rod is inserted into the axial opening of the porous flash tube, which is then sintered, shrunk onto the core rod and fuses with it. At the interface between the core rod and the flashing tube, any possible defects occur in the later optical fiber at the edge or outside the light-guiding area and therefore do not have any effect on the transmission of the optical fiber.

The method according to the invention thus enables the cost-effective production of a quartz glass preform that meets high demands on its optical properties.

It has proven to be advantageous to use a flash tube that has a density in the range between 0.4 and 0.7 g / cm ³ . Sintering onto the core rod is facilitated by a high density of the flash tube. In view of this, a density of the flash tube of at least 0.5 g / cm ^{3 has} proven particularly useful. The upper limit of the specified density range results from the requirement of sufficient reactivity and permeability of the porous flashing tube, which enables effective cleaning in a gas atmosphere after the deposition process. The density is determined as the mean of several measurements using mercury porosimetry.

In a preferred procedure, the difference between the first and second refractive index is set to at least 0.003. The difference in refractive index influences the light guidance in the core. The greater the refractive index difference, the better the light is guided in the core and the less the influence of the cladding ("optical cladding") and in particular the interface between the core rod and the flashing tube. A large refractive index difference between the core and the inner cladding layer of 0.003 and more allows this interface (core rod /

Overflow tube) close to the light-guiding area of the optical fiber and thus to reduce the volume fraction of the "optical cladding" in favor of the much cheaper "jacket material" of the overlay tube.

In this regard, it has also proven to be advantageous to use a core rod in which the ratio of the core rod outer diameter and core diameter O 01/32572

is three or less (dιvι / dκ ratio <3). According to the invention, the wall thickness of the jacket is a maximum of twice the core diameter. The core diameter of an optical fiber is generally predetermined by its intended use. In the case of a single-mode fiber, for example, it is approximately 8 μm. The remaining radial fiber cross-section is formed by the "optical cladding" and the "jacket material". A reduction in the dM / d «ratio thus also works in the sense that the volume fraction of the“ optical cladding ”is reduced in favor of the“ jacket material ”of the flashing tube, which according to the invention can be produced much more cheaply.

With a view to reducing costs, a flash tube with an inner diameter of at least 30 mm is preferably used. The core rod is designed with a correspondingly adapted, large outer diameter, so that a large-volume and thus - in relation to the length of the optical fiber to be drawn - inexpensive preform can be produced. The wall thickness of the covering tube, which defines the outer diameter of the preform and the wall thickness of the “jacket material” of the optical fiber, is generally between 60 mm and 500 mm (with an average density (measured over the radial cross section) in the range of 0.4 g / cm ³ to 0.7 g / cm ³ ). With a wall thickness of at least 60 mm it is ensured that the porous quartz glass body has sufficient mechanical strength. Wall thicknesses above 500 mm make sintering more difficult due to possibly diffusing gases and extend the diffusion paths with a gas phase treatment of the flash tube.

In a first preferred method variant, the core rod is formed from the core and the inner jacket layer. Process steps for the formation of additional cladding layers between the core and the flashing tube are thus avoided.

In an alternative, but equally suitable process variant, the core rod is overlaid with a quartz glass cladding tube before overlaying. In this case, the core rod comprises, in addition to the core and the inner cladding layer, at least one further cladding layer which is provided by the cladding tube. The cladding tube consists of undoped quartz glass or of a dopant Quartz glass. It can contribute to the refractive index profile of the preform and in particular to the “optical cladding” of the optical fiber. The cladding tube is produced using an inexpensive process and in some cases replaces the more expensive jacket material Way enlarged inexpensively

To avoid contamination of the core rod, the cladding tube is preferably made of synthetic quartz glass.

A procedure in which the flash tube is subjected to a chlorine treatment by heating in a chlorine-containing atmosphere at a temperature above 700 ° C. has proven to be particularly favorable. The chlorine treatment removes impurities and reduces the OH content of the SiO ₂ cylinder. Chlorine and chlorine compounds with a hygroscopic effect at the treatment temperature are suitable for chlorine treatment.

For the chlorine treatment, the outer diameter of the core rod and the inner diameter of the flashing tube are preferably selected such that an annular gap remains between the flashing tube and the core rod inserted therein during the chlorine treatment. The chlorine treatment after inserting the core rod and before sintering the flash tube cleans both the surface of the core rod and the annular gap between the core rod and flash tube. In addition, the annular gap ensures that the chlorine-containing

Atmosphere can affect the flash tube both from its inner wall and from the outer wall, so that the diffusion paths are halved and thus the treatment times are shortened. The width of the ring span is at least 3 mm.

Fluorine doping reduces the refractive index of quartz glass. It has proven useful to dope the flash tube with fluorine before sintering and after the chlorine treatment. This simultaneously expels chlorine from the porous quartz glass.

A procedure is preferred in which a flash tube is used, the wall thickness of which is excessive in such a way that after shrink fitting of the flash tube, the quartz glass preform has a diameter that is larger than a target diameter specified for the preform. The wall thickness of the flash tube is greater than the wall thickness that would be sufficient to produce the preform with its target diameter. This ensures that the preform diameter can be reworked to the target size in the event that unprocessed preform geometries should require this. If necessary, the post-processing is carried out inexpensively by mechanical processing, such as by grinding.

With regard to the quartz glass preform for optical fibers, the above-mentioned object is achieved on the basis of the generic preform according to the invention in that the core and the inner cladding layer are produced by the OVD, MCVD, PCVD or VAD process, and in that at least one part the second cladding layer is formed from a porous quartz glass sintered from porous quartz glass and shrunk onto the core rod during sintering and produced by the OVD process

In the preform according to the invention, the core rod provides the quartz glass for the fiber core and at least part of the optical cladding of the later optical fiber. It is essential that at least the core and the inner cladding layer are produced by the OVD, MCVD, PCVD or VAD process.

The core rod is surrounded by a second cladding layer, at least part of which is made of a flash tube sintered from porous quartz glass and shrunk onto the core rod during sintering. In order to achieve a reproducible result, it is essential that at least a part of the second cladding layer is formed by an overlay tube manufactured according to the OVD process.

With regard to the production of the porous covering tube and the preform and the advantages of this procedure with regard to the manufacturing costs of the preform, reference is made to the above explanations regarding the method according to the invention.

The quartz glass preform is preferred for the production of monomode fibers used.

The invention is explained in more detail below with the aid of an exemplary embodiment and a drawing. In the drawing show in detail

FIG. 1 shows a radial cross section of a typical preform according to the invention in a schematic illustration,

FIG. 2 shows a first exemplary embodiment for carrying out the method according to the invention using a flow chart, and

Figure 3 shows a second embodiment for performing the method according to the invention using a flow chart.

In the illustration according to FIG. 1, reference numeral 6 denotes a preform

Quartz glass. The preform 6 comprises a core 1, an inner cladding layer 2 surrounding the core 1, a cladding tube 3 (or a substrate tube 7 - see description of FIG. 3 below). Core 1 and inner cladding layer 2 are either produced by the OVD process (see example 1) or by the MCVD process (see example 2). The core-jacket rod 1, 2 forms, together with the cladding tube 3, a core rod 4. An overlay tube 5 made of sintered, porous quartz glass is shrunk onto the core rod 4.

Sheath layer 2 and cladding tube 3 (or substrate tube 7) form the essential part of the "optical cladding" in an optical fiber to be drawn from the preform 6. The outer diameter of the cladding tube 3 (or the substrate tube 7) is less than three times the diameter of the Core 1.

Exemplary embodiments of the method according to the invention are explained in more detail below with reference to FIGS. 2 and 3. Insofar as reference numbers are mentioned in the following, these designate components of the preform, as shown in FIG. 1.

Example 1 :

According to FIG. 2, a core consisting of core 1 and cladding layer 2 is first Sheathed rod 1, 2 produced by external deposition using the OVD process. The core 1 consists of quartz glass doped with germanium dioxide with a diameter of 12 mm, while the cladding layer 2, which directly adjoins the core 1, is formed from undoped quartz glass with an outer diameter of 18 mm. This results in a dιy dκ ratio of 1.5 for the core-jacket rod 1, 2. The refractive index of the core glass (quartz glass of the core) is 0.005 higher than the refractive index of the cladding glass surrounding it (quartz glass of the inner cladding layer). The mean OH content in core 1 (measured over the radial cross section) is below 30 ppb by weight.

The core-jacket rod 1, 2 is then encased with a cladding tube 3 made of undoped, synthetic quartz glass to form a core rod 4 with an outer diameter of 30 mm. This results in ad _M / dκ ratio of 2.5 for core rod 4

A flash tube 5 in the form of an undoped, porous quartz glass body (“soot body”) is then produced using the OVD method by depositing SiO ₂ particles on an Al ₂ O ₃ tube that has an outer diameter of 38 mm The Al ₂ O ₃ tube is removed after the deposition process, and the flash tube 5 has an average density of approximately 0.5 g / cm ³ .

In order to keep the number of defects at the interface between the flash tube 5 and the core rod 4 as low as possible, the core rod 4 is flame polished.

Subsequently, the core rod 4 is inserted into the bore of the flash tube 5 and centered therein by holding devices to form an annular gap with a width of approximately 4 mm. The holding devices ensure that core rod 4 and flashing tube 5 melt together concentrically. Subsequently, the arrangement of flash tube 5 and core rod 4 is heated to 1000 ° C. in an oven and held at this temperature for about 60 minutes. The furnace atmosphere consists of a helium-chlorine mixture, whereby the flash tube 5 is dehydrated and in addition the surface of the core rod 4 is cleaned. Finally, the flash tube 5 is sintered at approximately 1400 ° C. by supplying the flash tube 5 and core rod 4 arrangement to a heating zone and starting from the bottom is softened zone by zone, the flash tube 5 sintered tightly and melting onto the core rod 4.

The quartz glass preform 6 thus produced is given its predetermined final dimensions by grinding. Then their diameter is approx. 170 mm. The interface between the core rod 4 and the flash tube 5 has no bubbles or other defects due to the chlorine treatment. Due to the high geometric precision of the core rod 4 and the flashing tube 5 and the central mounting during the sintering process, the optical fibers drawn from the preform 6 are characterized by a core eccentricity of less than 0.35 μm.

Example 2:

Another exemplary embodiment of the method according to the invention is described below with reference to FIG. 3. In a first method step, a so-called substrate tube 7 is provided. The substrate tube 7 consists of fluorine-doped quartz glass, which is produced by the OVD method. The substrate tube 7 has an inner diameter of 20 mm and a wall thickness of 3 mm. The fluorine content of the quartz glass is 2500 ppm by weight.

On the inner wall of the bore of the substrate tube 7, an inner cladding layer 2 made of fluorine-doped quartz glass is produced by internal deposition using the MCVD method. The fluorine content of the cladding layer 2 is adjusted to approximately 2500 ppm by weight. A layer of germanium-containing quartz glass is deposited on the cladding layer 2. The germanium content of this layer is 4.5% by weight. The substrate tube 7 coated on the inside in this way is then collapsed to form a core rod 4. The germanium-containing layer forms the core 1 with a diameter of 7 mm. The wall thickness of the cladding layer 2 is 3 mm in the core rod 4 and the diameter of the substrate tube 7 is 19 mm. The dw / dκ ratio is thus 2.7.

The difference in the refractive indices of core 1 and cladding layer 2 is approximately 0.005. The average OH content in the core rod 4 (measured over the radial cross section) is below 30 ppb by weight. The surface of the core rod 4 is flame polished using oxyhydrogen burners.

In a further process step, a flash tube 5 in the form of an undoped, porous quartz glass body (“soot body”) is produced by the OVD method by depositing SiO ₂ particles on an Al ₂ O ₃ tube that has an outer diameter of 38 mm After the deposition process, the Al ₂ O ₃ tube is removed and the flash tube 5 has an average density of about 0.5 g / cm ³ .

Subsequently, the core rod 4 is inserted into the bore of the flash tube 5 and centered therein by holding devices to form an annular gap with a width of 4 mm. The holding devices ensure that core rod 4 and flashing tube 5 melt together concentrically. Subsequently, the arrangement of flash tube 5 and core rod 4 is heated to 1000 ° C. in an oven and held at this temperature for about 60 minutes. The furnace atmosphere consists of a helium-chlorine mixture, whereby the flash tube 5 is dehydrated and in addition the surface of the core rod 4 is cleaned. For the purpose of one

Fluorine doping of the porous flash tube is used to replace the chlorine gas stream with a fluorine-containing gas stream for a further 60 min.

The flash tube 5 is then sintered at 1400 ° C. by supplying the flash tube 5 and core rod 4 arrangement to a heating zone and softening it zone by zone, starting from the bottom. The quartz glass of the flash tube 5 sinters and shrinks to form the quartz glass preform 6 on the core rod 4.

The preform 6 thus produced is due to its radial refractive index profile

Production of optical fibers of the "depressed-clad" type is suitable. It obtains its predetermined final dimensions by grinding off the surface of the cylinder. Then its diameter is approximately 100 mm. The interface between the

Core rod 4 and the flash tube 5 have none due to the chlorine treatment

Bubbles or other optical defects. Because of the high geometric

Precision of the core rod 4 and the flashing tube 5 and the central holder during the sintering process are characterized by the optical fibers drawn from the preform 6 and a core eccentricity of less than 0.35 μm.

Claims

1 / 32572- 14 -patent claims

1. A method for producing a quartz glass preform for optical fibers by providing a core rod which comprises a core made of quartz glass with a first refractive index and a cladding which radially delimits the core and which comprises at least one inner cladding layer adjacent to the core

Has quartz glass with a second refractive index, and overlaying the core rod with an overlay tube made of porous quartz glass by sintering the overlay tube and thereby shrinking onto the core rod, characterized in that the core (1) and the inner cladding layer (2; 3; 7) are produced by the OVD, MCVD, PCVD or VAD process, and that the core rod (4) is overlaid by an overflow pipe (5) produced by the OVD process.

2. The method according to claim 1, characterized in that the flash tube (5) has a density in the range between 0.4 and 0.7 g / cm ³ .

3. The method according to claim 2, characterized in that the density of the flash tube (5) is at least 0.5 g / cm ³ .

4. The method according to any one of the preceding claims, characterized in that the difference between the first and second refractive index is at least 0.003.

5. The method according to any one of the preceding claims, characterized in that a core rod (4) is used in which the ratio of the core rod outer diameter and core diameter is three or less.

6. The method according to any one of the preceding claims, characterized in that a flash tube (5) with an inner diameter of at least 30 mm is used.

7. The method according to any one of the preceding claims, characterized in that the core rod (4) from the core and the inner cladding layer (2; 3; 7) is formed.

8. The method according to any one of the preceding claims, characterized in that the core rod is coated with a quartz glass cladding tube before overlaying.

9. The method according to any one of the preceding claims, characterized in that the flash tube (5) by a chlorine treatment

Heating in a chlorine-containing atmosphere at a temperature above 700 ° C is subjected.

10. The method according to claim 9, characterized in that the outer diameter of the core rod (4) and the inner diameter of the flash tube (5) are selected such that an annular gap between the flash tube (5) and the core rod (4) inserted therein during the chlorine treatment remains.

11. The method according to any one of the preceding claims, characterized in that the flash tube (5) is doped with fluorine.

12. The method according to claim 9 or 10 and 11, characterized in that the fluorine doping takes place before the sintering and after the chlorine treatment.

13. The method according to any one of the preceding claims, characterized in that an overlay tube (5) is used, the wall thickness of which is excessive such that a quartz glass preform is obtained after the shrink-fit of the overlay tube (5)

Has a diameter that is larger than a predetermined diameter specified for the preform (6).

14. The method according to claim 13, characterized in that the target diameter of the preform (6) is adjusted by mechanical processing.

15. quartz glass preform for optical fibers, produced according to one of claims 1 to 14, comprising a core rod, the core made of quartz glass with a first refractive index and a radially surrounding sheath, the at least one inner cladding layer adjacent to the core Has quartz glass with a second refractive index, and with a second cladding layer enveloping the core rod, characterized in that the core (1) and the inner cladding layer (2; 3; 7) according to the OVD, MCVD, PCVD or VAD method are produced, and that at least part of the second cladding layer is formed from a flash tube (5) sintered from porous quartz glass and shrunk onto the core rod (4) during sintering and produced by the OVD method from porous quartz glass.