DE10157306C1 - Production of a component made from doped quartz glass used in the manufacture of optical fibers comprises forming a sealing zone having a specified soot density in the region of a contact surface - Google Patents

Abstract

Production of a component made from doped quartz glass comprises forming a sealing zone having a soot density of at least 27% of the theoretical density of quartz glass in the region of a contact surface. An Independent claim is also included for a tubular semi-finished product comprising a radial soot layer sequence consisting of an inner porous SiO2 soot layer and an outer porous SiO2 soot layer joined by a contact surface. Preferred Features: A soot density of at least 29, preferably at least 31% of the theoretical density of quartz glass is adjusted in the region of the sealing zone. The sealing zone has a radial expansion in the region of 50 mu m to 3 mm, preferably 100 mu m to 1 mm. The soot layers are doped with TiO2, Geo2. TaO2, Al2O3, P2O5, B, F and/or Cl.

Description

The invention relates to a method for producing a component made of doped quartz glass for the production of optical fibers, which has a plurality of radially successive layers of quartz glass, which differ in their refractive index, the quartz glass layers being produced by on one around its longitudinal axis rotating carrier is deposited a radial soot layer sequence, comprising at least one inner, porous SiO ₂ soot layer and an adjacent, outer, porous SiO ₂ soot layer, the adjacent soot layers differing in their doping and in each case connected to one another via a cylinder-shaped contact surface and the soot layer sequence is then dried in a halogen-containing atmosphere and glazed to form the quartz glass layers.

Furthermore, the invention relates to a tubular semi-finished product for producing a component made of doped quartz glass for the production of optical fibers, comprising a radial soot layer sequence which has at least one inner, porous SiO ₂ soot layer and an adjacent, outer, porous SiO ₂ soot layer, wherein neighboring soot layers differ from one another in their doping and are each connected to one another via a cylindrical jacket-shaped contact surface.

Components made of doped quartz glass are used as the starting material in the form of tubes or rods for the production of preforms of optical fibers. The preforms generally have a core which is encased by a jacket made of a material with a lower refractive index. For the production of the core, procedures have become established which are known as VAD (vapor-phase axial deposition; axial deposition from the vapor phase), OVD (outside vapor-phase deposition; external vapor deposition) and MCVD processes (modified chemical vapor-phase de position; internal deposition from the vapor phase) and PCVD (plas ma chemical vapor-phase deposition; plasma-assisted deposition from the vapor phase). In all of these procedures, the core glass is produced in that SiO ₂ layers are deposited on a substrate. In VAD and OVD processes, the core glass is deposited from the outside on a substrate; in MCVD and PCVD processes on the inner wall of a so-called substrate tube. Depending on the fiber design, the substrate tube consists of doped or undoped quartz glass. In addition, the manufacture of preforms according to the so-called rod-in-tube technique is known in which a rod made of core glass is inserted into a tube made of cladding glass and fused with it becomes. By elongating the preform, optical fibers are obtained.

Depending on the procedure, the cladding glass is made in a separate process manufactured (OVD, plasma process, rod-in-pipe technology) or the cladding glass and the core glass are produced at the same time, as in the so-called VAD procedure is common. The difference in refractive index between core glass and Cladding glass is adjusted by adding suitable dopants. fluorine and boron lower the refractive index of quartz glass while increasing the refractive index a large number of dopants are suitable for quartz glass, in particular Germanium, phosphorus or titanium.

With a simple fiber design for an optical fiber, the core is off Quartz glass with a first refractive index from a quartz glass jacket coated with a second, lower refractive index. In the course of the optimization tion of optical fibers, especially for the simultaneous transmission of several However, wavelengths with high transmission rates are using fiber designs developed significantly more complex refractive index profiles. For example, in EP-A1 785 448 an optical fiber made of quartz glass with a fiber design described as "double-core + double-cladding" (double core + dop pelter coat) is called, and that to reduce the so-called Polarization mode dispersion should contribute.  

With the widespread MCVD internal deposition of core glass layers with increasing number and thickness of the layers a corresponding veren supply of the inner bore of the substrate tube, and thus a reduction in inner surface. This increases the effectiveness of the deposition Course of the process. This can be done by enlarging the inside diameter water and wall thickness of the substrate tube can only be counteracted to a limited extent, because the temperature required for the deposition within the sub strat tube is usually generated by heating from the outside. A ver Larger inner diameter or wall thickness of the substrate tube is required However, an increase in the outside temperature changes in order to conditions inside the pipe. But this is through softening and limited plastic deformation of the substrate tube. In addition, that will Collapse with thick-walled or large substrate tubes and with thick In layers are becoming increasingly difficult. The MCVD technology comes up against this to a limit.

In the OVD process using oxyhydrogen burners, porous SiO ₂ soot layers are generally first produced, which are then dried and glazed. Such a method is described in US 4,810,276 (A), from which a generic tubular semi-finished product is also known. It describes the production of a preform for an optical step index fiber, whereby a so-called core rod is first provided, in which a core glass made of SiO ₂ and GeO ₂ from a cladding glass made of SiO ₂ and a dopant reducing the refractive index of quartz glass (such as Fluorine or B ₂ O ₃ ) is enveloped. The core rod is produced by means of the OVD method by flame hydrolysis of SiCl ₄ and GeCl ₄ , soot particles being deposited in layers on a rotating support to form an inner porous soot layer. The outer soot layer forming the jacket is then also deposited on the inner soot layer by means of the OVD method. Inner and outer soot layer are connected to each other via a cylindrical contact surface. After the carrier has been removed, the porous soot tube thus obtained is dried by heating in a halogen-containing atmosphere, glazed and the inner bore is collapsed to form the core rod.

Ideally, a defined refractive index level is formed between the core glass and the cladding glass. Production-related, for example by leaching GeO ₂ during the dehydration treatment of the soot tube in a chlorine-containing atmosphere, by diffusion or other matter transport processes, such as chemical transport reactions, however, there is a depletion of GeO _2, especially in the peripheral areas of the doping, and This leads to a flattening of the refractive index level and to a deterioration in the fiber properties.

In order to avoid this, US Pat. No. 4,810,276 (A) proposes to compensate for the depletion of GeO ₂ in the edge regions of the doping by depositing soot particles with a higher GeO ₂ concentration there. This creates an inner soot layer with a GeO ₂ concentration profile, which is characterized by concentration maxima in the area of the inner wall and in the area of the contact surface with the outer soot layer. The concentration maxima should be leveled by the GeO ₂ depletion in the dehydration treatment of the soot tube in a chlorine-containing atmosphere in such a way that a core rod with a step-like refractive index profile is obtained after glazing and collapsing.

However, since the GeO ₂ depletion depends on a large number of parameters, the ultimately resulting distribution of the refractive index is not very reproducible, and the success of the process can only be checked at a late process stage - namely in the core rod. The known method also has the after part that each parameter change, such as the soot tube geometry, the thickness of the individual soot layers, the dopant concentration or parameters in the dehydration treatment (temperature, duration, gas phase composition), a complex optimization of the radial GeO ₂ - Concentration course required.

Another method for producing a preform for optical fibers by means of the OVD method is known from DE 100 29 151 C1. In order to facilitate the removal of the carrier, it is proposed to deposit the innermost layers adjacent to the carrier of the GeO _2- doped core region of the pre-form at elevated temperature, so as to produce a higher soot density in this region, which is less sensitive to injury Pull out the carrier. The higher density area begins at the inner wall of the soot body and it has a total thickness of a maximum of 500 µm, preferably a maximum of 150 µm. The density in this range is 20% to 30% of the density of quartz glass. The measures proposed in DE 100 29 151 C1 have no significant effect on the radial GeO ₂ concentration curve in the core area of the preform, in particular they do not contribute to a defined refractive index level in the area of the contact area between the core and the cladding glass.

In DE 100 50 324 C1, which belongs to the prior art according to § 3 PatG, an OVD method is proposed for the production of a GeO ₂ -doped tube with radially homogeneous distribution of the dopant, in which a rich inside and outside of the tube higher soot density is generated. As a result, the depletion of dopant by diffusion or leaching out of the tube is minimized in the later treatment stages, and the desired homogeneous dopant distribution over the tube wall is achieved. The tube is used as a substrate tube in the manufacture of a preform according to the MCVD process, or it serves as an overlay tube for applying additional core material to a so-called core rod. The measures proposed in DE 100 50 324 C1 are not suitable for producing a defined refractive index level within a soot body or for forming a defined, inhomogeneous radial refractive index curve over the soot tube wall. In particular, the outer regions of higher density also have no effect on the refractive index of the quartz glass, which is brought into contact with the tube in the following processing steps.

The invention is therefore based on the object, a simple and inexpensive Process for the production of a component made of doped quartz glass for the Manufacture of optical fibers, specifying a given radial Distribution of one or more dopants within a radia len soot sequence can be set reproducibly.  

Furthermore, the invention is based on the object of providing a tubular semi-finished product made of porous SiO ₂ containing a dopant or a plurality of dopants, which can be subjected to a dehydration treatment without the preset dopant distribution over the tube wall changing significantly.

With regard to the method, this task is based on a procedure ren of the type mentioned according to the invention solved in that A compression zone in the area of the contact surface of adjacent soot layers with a soot density of at least 27% of the theoretical density of quartz glass is produced.

In the method according to the invention, two or more radially successive, adjacent SiO ₂ soot layers are produced which differ in their doping and which are connected to one another by a common cylindrical jacket-shaped contact surface. The soot layers contain different dopants or they contain a dopant in different concentrations, whereby the concentration can also be "zero". The soot layers overall are also referred to as "soot layer sequence".

A compression zone is provided in the area of the common contact area see within a soot density of at least 27% of theoretical Density of quartz glass is present. The compression zone is either internal half of the inner soot layer or within the other outer soot layer formed, being in the cases on the common contact surface ends, or it extends over the common contact surface into the inner one and into the outer soot layer. It should be noted that the density does not increase abruptly within the compression zone due to production, but in a gradient, albeit as steep as possible. The radial end expansion of the respective compression zone results from the area within which the stated minimum soot density of 27% is present. For the he averaging the density, either samples are taken and using mercury berporosimetry measured or the density is determined by X-ray averages.  

Through the compression zone between the individual soot layers in the an out-diffusion following the soot separation treatment steps of dopant from one soot layer into the adjacent soot layer prevents or diminishes. Furthermore, the leaching described above mechanism by halogen-containing substances through the compression zone with special needs.

This can ensure that even after the treatment steps, a predetermined dopant distribution over the radial soot layer sequence is present, so that from it a preform with a given radial refractive index profile - be it a stepped refractive index profile with steep Flanks or a profile with a gradient in the refractive index curve - who creates that can.

Especially in the case that the innermost soot layer of the soot layer sequence contains a dopant, it makes sense, also in the area of the inner wall Set the soot layer sequence to a higher soot density to ensure diffusion and reduce leaching of the dopant over the free surface. The minimum soot density in the compression zone is 27%. With one soot density, the described effect of the compression zone occurs insufficient reduction of diffusion and leaching Dimensions.

In view of this, a soot density of at least 29%, preferably at least 31%, of the theoretical density of quartz glass is advantageously set in the region of the compression zone. The information on the relative density within the soot wall is based on a quartz glass density of 2.21 g / cm ³ .

With increasing density and increasing strength of the compression zone takes their effect on the hindrance of out-diffusion or Leaching of dopant increases, but so does the drying the halogen-containing atmosphere during dehydration treatment decreases. The hydroxyl group content affects the optical attenuation Unfavorable fiber. In view of this, there is an advantageous process variant  ante in that the compression zone within the outer soot layer is generated. Since the material of the outer soot layer from the later Core of the fiber is further away than the material of the inner soot layer the hydroxyl group content of the outer soot layer is less dependent on the opti damping of the fiber.

In the method according to the invention, the competing effects are too note that on the one hand by the compression zone (or the compression tion zones) reduces the diffusion and leaching of dopant and there is facilitated by local fixation of the dopant, but others drying is difficult. The density in the compression zone and their radial extent is therefore as small as possible, but as large to hold as necessary. It has proven to be particularly favorable if the ver sealing zone a radial expansion in the range between 50 µm and 3 mm, preferably between 100 microns and 1 mm. The soot density within a compression zone near the core of the future fiber should be 35% do not exceed the theoretical density of quartz glass. In the core Be compression areas with high soot density are less harmful. Similar Like the density of the compression zone, its radial expansion also has an effect both hindering the diffusion and leaching of Do. animal substance, but also on the effectiveness of subsequent treatment steps, such as a dehydration process. The thickness range mentioned has proved to be a suitable compromise.

A smaller one is preferably used in the area outside the compression zone Soot density set to a maximum of 30% of the density of quartz glass. Through the compared to the compression zone, the soot density is lower these areas relieved by dehydration treatment. Especially a soot density of is preferred in the region outside the compression zone a maximum of 28%, preferably a maximum of 26% of the density of quartz glass provides.

It has proven useful to generate the compression zone by increasing the surface temperature of the SiO ₂ soot layer during deposition. The density is significantly influenced by the surface temperature when the soot layer is deposited. An additional process step for after compression is not necessary. A large number of measures are suitable for increasing the surface temperature. The following measures should be mentioned only by way of example: setting a higher flame temperature of the deposition burner, changing the distance between the deposition burner and soot layer, reducing the speed of the relative movement between the deposition burner and soot layer. Due to a higher surface temperature, the SiO ₂ soot particles become more cross-linked, so that a layer with greater diffusion inhibition results.

The doping of the soot layers is advantageously set by one or by several of the dopants TiO ₂ , GeO ₂ , TaO ₂ , Al ₂ O ₃ , P ₂ O ₅ , boron, fluorine and / or chlorine.

The method according to the invention is particularly suitable for producing a radial refractive index curve with one refractive index level or with several Refractive index levels. Here the concentration of a dopant changes from an inner soot layer to the outer soot layer in steps. In one of the Soot layers can also have a zero concentration of the dopant The method is equally advantageous for fixation a radial concentration gradient of a dopant. Included here the inner soot layer and the outer soot layer have the same dopant, wherein the inner soot layer has a radial gradient in the dopant concentration which continues into the outer soot layer. The radial concentrate onsgradient of the dopant is through a compression zone or fixed by several compression zones.

In a particularly preferred embodiment of the method according to the invention, the inner soot layer is doped with GeO ₂ , and the outer, initially undoped soot layer is loaded with fluorine in a gas phase treatment subsequent to the deposition. Fluorine doping of the inner soot layer doped with GeO ₂ is prevented by the compression zone between the inner and outer soot layer.

With regard to the tubular semi-finished product for producing a component from do quartz glass for the production of optical fibers is given above bene task starting from a semi-finished product of the Gat mentioned tion solved according to the invention in that in the area of the contact surface Compression zone with a soot density of at least 27% of the theoretical Density of quartz glass is provided.

The tubular semifinished product according to the invention comprises two or more radially successive, adjacent SiO ₂ soot layers, and is also referred to below as a soot tube. The soot layers are connected to one another by a common cylindrical jacket-shaped contact surface and they contain different dopants or they contain a dopant in different concentrations, wherein the concentration can also be “zero”.

A compression zone is provided in the area of the common contact area see within a soot density of at least 27% of theoretical Density of quartz glass is present. The compression zone is either internal half of the inner soot layer or within the other outer soot layer formed, being in the cases on the common contact surface ends, or it extends over the common contact surface into the inner one and into the outer soot layer. The radial extent of each Compression zone results from the area within which the ge mentioned minimum soot density of 27%.

The compression zone prevents that during heat treatment steps the semi-finished product is subjected to dopant from one soot layer into the neighboring soot layer diffuses out. In particular, the compression reduces treatment zone during a dehydration treatment of the soot tube in halogen content the atmosphere between the halogen and the dopant and thus reduces the effect of dopant leaching.

This can ensure that even after the treatment a predetermined dopant distribution over the radial soot layer sequence is present, so that a preform with a predetermined radial refractive index profile - be it a step-shaped refractive index  profile with steep flanks or a profile with a gradient in the refractive index ver run can be generated.

Therefore, the soot tube according to the invention - as a semifinished product - is for such treatment steps are particularly suitable. Because even after the said Treatment steps have the soot tube a predetermined - in particular a radially homogeneous - dopant distribution over its entire interior get rich. Especially in the case that the innermost soot layer of the soot layer sequence contains a dopant, it makes sense that also in the area an area of higher soot density is provided in the inner wall of the soot tube is so that outdiffusion and leaching of the dopant over the free Surface is reduced in subsequent heat treatment steps.

The minimum soot density in the compression zone is 27%. With one soot density, the described effect of the compression zone occurs insufficient reduction of diffusion and leaching Dimensions.

Advantageous further developments of the semifinished product according to the invention result from the subclaims. The characteristics of the subclaims correspond with those of the procedural claims already explained above, so that to that extent reference is made to these explanations.

The invention is described below with the aid of exemplary embodiments and a Drawing explained in more detail. The drawing shows a schematic representation in detail:

Fig. 1a a first radial refractive index profile across the wall of a core rod for optical fibers in an idealized form,

1b shows an embodiment for a the refractive index profile of Figure 1a at Fitted density profile across the wall of a porous SiO ₂ -Sootrohres invention prior to vitrification..;

Fig. 2a shows another radial refractive index profile over the wall of a core rod for optical fibers in an idealized form, and

FIG. 2b shows an embodiment for the refractive index profile of FIG. 2a fitted density profile across the wall of a porous SiO ₂ -Sootrohres invention prior to vitrification.

In FIGS. 1a and 2a are each Ideal gradients radial refractive index profiles of core rods for the production of preforms for optical fibers shown specific automatically. The refractive index difference to undoped quartz glass is plotted on the y-axis (the value "0" corresponds to the refractive index of pure quartz glass). The radial position "P" is plotted on the x-axis in relative units in% (based on the core rod radius). The value P = 0% denotes the position of the central axis of the core rod and the value P = 100% the position of the core rod outer jacket.

In Fig. 1b, the density profile through the wall of a porous soot body from which a core rod according to Fig. 1a is obtained, shown in a process stage prior to cleaning, vitrification and collapsing. FIG. 2b shows a corresponding density profile of a porous soot tube for a core bar according to Fig. 2a. The relative soot density "d" in% (based on the theoretical density of quartz glass) is plotted on the y axis. The x-axis denotes the position "P" within the pipe wall in relative units (in%, based on the total wall thickness of the soot pipe). The position "P = 0%" corresponds to the position of the inner tube wall and the position "P = 100%" to the outer surface of the soot tube.

Fig. 1a shows a radial refractive index profile of a core rod with a homogeneous GeO ₂ doping of 5 wt .-% in a core area 1 , the inner layer 2 from undoped quartz glass and from an outer layer 3 with a homogeneous GeO _2nd Content of 1% by weight, is coated. This is a step profile in which the individual areas ( 1 ; 2 ; 3 ) are separated from each other by steep refractive index steps (ideally rectangular steps). The contact surfaces of the core rod regions 1 , 2 , 3 and the outer boundary of the outer cladding layer 3 are identified by dotted position lines 4 ′, 4 ″, 4 ″ ″ and transferred to the corresponding radial positions P in the SiO ₂ soot tube in FIG. 1b ,

From Fig. 1b, one embodiment results in a density profile for a soot tube according he invention, which facilitates the manufacture of the radial refractive index profile of FIG. 1a. This soot tube has a total of four compression zones 5 , 6 , 7 , 8 over its wall thickness, in which the soot density profile has a relative maximum. The inner maximum with a relative density of approximately 27% is assigned to an inner compression zone 5 which is immediately adjacent to the inner bore. The adjacent maximum density with a relative density of approximately 29% results from a compression zone 6 in the area of the contact area 4 'between the GeO ₂ -doped core area 1 and the inner cladding layer 2 . A further density maximum with a relative density of approximately 29% is provided adjacent to this, which results from a compression zone 7 in the area of the contact area 4 ″ between the two cladding layers 2 , 3. And the outer density maximum is determined by a compression zone 8 generated in the area of the outer wall of the soot tube.

The relative density here is about 31%. In the areas 9 , 10 , 11 between the compression zones 5 , 6 , 7 , 8 , the relative soot density is constant at about 22%.

The radial extent of the inner compression zone 5 is 800 microns and that of the other compression zones 6 , 7 and is about 1500 microns.

The manufacture of a soot tube with the density profile shown in FIG. 1b and the manufacture of a core rod with the refractive index profile shown in FIG. 1a are explained by way of example below:
Soot particles are deposited in layers on a carrier rotating about its longitudinal axis by reciprocating a separating burner. For this purpose, the deposition burner SiCl ₄ and GeCl _{4 are} fed and hydrolyzed in a burner flame in the presence of oxygen to SiO ₂ and GeO ₂ . The ratio of SiCl ₄ and GeCl ₄ is kept constant during the deposition of the inner compression zone 5 and the area 9 , so that the predetermined homogeneous GeO ₂ concentration of 5 mol% is established over this part of the wall thickness of the soot tube.

To set the higher density of 27% in the region of the inner compression zone 5 , a comparatively high surface temperature is generated when the first twenty soot layers are deposited. Then - during the deposition of the soot layers which form the region 9 - the temperature of the flame of the deposition burner is reduced by reducing the feed rates of the fuel gases hydrogen and oxygen by 15%. This results in an average soot density of about 22% in area 9 .

As soon as the soot layers that form region 9 are deposited, the supply of GeCl ₄ to the deposition burner is stopped. Then the compression zone 6 is generated by the temperature of the flame of the separator debrenner is increased again. For this purpose, the feed rates of the fuel gases hydrogen and oxygen are increased to such an extent that a relative density of 29% is established in the compression zone 6 . Then - in from the soot layers divorce, which form the region 10 - the temperature of the flame of the deposition is reduced again, so that also in the rich Be 10 an average soot density of about 22% is established. Before the deposition of the GeO _2- containing soot layers (approx. 20 soot layers in front), which form the jacket layer 3 in the core rod, the temperature of the flame of the deposition burner is increased again. Then the supply of GeCl ₄ to the deposition burner resumes at a constant flame temperature. The compression zone 7 is further developed, which thus extends partially in the area of the soot layers for the inner cladding layer 2 , and partially in the area of the soot layers which form the outer cladding layer 3 . The maximum relative density in the area of compression zone 7 is 30%.

To generate the soot layers that form the region 11 , the temperature of the burner flame is reduced again, so that a relative density of approximately 22% is established in the region 11 . Finally, with constant supply of GeCl ₄ to the deposition burner, the compression zone 8 is generated by increasing the temperature of the flame by increasing the feed rates of the fuel gases hydrogen and oxygen to such an extent that a relative density of 31% is established in the region of the compression zone 8 .

After completion of the deposition process and removal of the carrier, a soot tube with the density profile shown in FIG. 1b is obtained. Using this soot tube, a core rod is produced with the refractive index profile shown in FIG. 1a, as explained below by way of example:
The soot tube is subjected to a dehydration treatment in order to remove the hydroxyl groups introduced due to the production process, by introducing it into a dehydration oven in a vertical orientation and first treating it at a temperature in the range from 800 ° C. to about 1000 ° C. in a chlorine-containing atmosphere. The duration of treatment is about six hours. This gives a hy droxyl group concentration of less than 100 ppb in the areas 9 , 10 and 11 of the soot tube. The compression zones 5 , 6 , 7 , 8 hinder the leaching of GeO ₂ and ensure that the preset concentration profile according to FIG. 1a is largely retained in the core rod.

The soot tube treated in this way is glazed in a glazing furnace at a temperature in the range around 1350 ° C. and the inner bore collapses, so that a core rod with the refractive index profile shown in FIG. 1a for the ideal case is obtained.

In fact, the core region 1 and the outer jacket region 3 have a homogeneous GeO ₂ concentration which only slightly decreases at the respective edges. The core rod shows a defined refractive index profile with steep flanks and an overall low hydroxyl group concentration, so that it is particularly suitable as a core material for a preform for optical fibers. The slope of the flank between the core area 1 and the inner cladding area 2 can be described quantitatively on the basis of the optical fiber obtained using the core rod. In the fiber with an outside diameter of 125 µm, the core region 1 is formed into a core diameter of 8.5 µm. with a refractive index difference of 5 × 10 ^{-3 there} is a refractive index level to the jacket 2 with a radial expansion of less than 0.25 microns.

In the radial refractive index profile shown in FIG. 2a, the GeO ₂ doping in a core region 12 decreases from 5% by weight to zero in several stages 22 . In the exemplary embodiment, six stages 22 are provided for this purpose, which form a quasi-continuous gradient in the refractive index, as is shown schematically by the dotted line 23 . The finer the gradation, the more accurate the approximation to a continuous course of the refractive index profile.

The core region 12 is encased by an inner cladding layer 13 made of undoped quartz glass and by an outer cladding layer 14 with a homogeneous fluorotoping (2000 ppm by weight). The contact surfaces of the core rod regions 12 , 13 , 14 are identified by dotted position lines 15 ', 15 "and transferred to the corresponding radial positions P in the SiO ₂ soot tube in FIG. 2b.

FIG. 2b shows an exemplary embodiment of a density profile in a soot tube, which facilitates the production of a core rod with the radial refractive index profile according to FIG. 2a. The soot tube has a large number of compression zones 16 , 17 , 18 with relative maxima of the soot density over its wall thickness. The compression zones 16 are formed within the soot layers, which form the core region 12 of the core rod. They each have a maximum with a relative density of about 27% and are separated from one another by regions 19 of lower density (22%). The radial dimensions of the compression zones 16 Ver are about 800 microns, the areas 19 in between Be low density about 4000 microns. Adjacent to the outer compression zone 16 , a maximum density with a relative density of about 27% is provided, which results from a compression zone 17 in the region of the contact surface 15 'between the GeO ₂ -doped core region 12 and the inner cladding layer 13 . This is followed on the outside by a further relative maximum density with a density of approximately 31%, which results from a compression zone 18 in the region of the contact surface 15 ″ between the two cladding layers 13 and 14 .

In the areas 19 , 20 , 21 between the compression zones 16 , 17 , 18 , the relative soot density is constant at about 22%.

The radial extent of the compression zones 17 , 18 is approximately 1500 μm.

The preparation is described of a soot tube with the density profile in the Figure 2b and shown with a core rod in Fig refractive index profile shown by way of example 2a..:
Soot particles are deposited in layers on a carrier rotating about its longitudinal axis by reciprocating a separating burner. For this purpose, the deposition burner SiCl ₄ and GeCl _{4 are} fed and hydrolyzed in a burner flame in the presence of oxygen to SiO ₂ and GeO ₂ . The proportion of GeCl ₄ is continuously reduced during the deposition of the soot layers forming the later core region 11 , so that the gradient of the GeO ₂ concentration decreases from 5 mol% to 0 mol% over this part of the wall thickness of the soot tube. results.

To set the higher density of 27% in the area of the compression zones 16 , a comparatively high surface temperature is generated, while the deposition of the soot layers which form the areas 19 - the temperature of the flame of the deposition burner is reduced by the feed rates of Fuel gases hydrogen and oxygen can be reduced by 15%.

Once the last soot layers are deposited in the region 19, the supply of GeCl ₄ is completely stopped at the deposition and for further deposition of the compression zones 17 and 18 and the portions 20 and 21 forming soot layers the deposition only SiCl ₄ as Glass starting material (without a dopant) supplied.

The compression zone 17 is produced by increasing the feed rates of the fuel gases hydrogen and oxygen to the separation burner to such an extent that a relative density of 27% is established in the area of the compression zone 17 . Subsequently - during deposition of the soot layers forming the Be ranging 20 - the temperature of the flame of the deposition is lowered as the, so that even in the area 20 have an average soot density of about 22% is obtained.

Likewise, the soot layers are deposited, which form the compression zone 18 and the region 21 , the temperature of the burner flame being increased by increasing the supply of the combustion gases, hydrogen and oxygen, to the extent that the compression zone 18 is produced, so that the range increases Compression zone 18 sets a relative density of at most 31%.

After completion of the deposition process and removal of the carrier, the soot tube is subjected to a dehydration treatment as described above to remove the hydroxyl groups introduced due to the production process. The outer cladding region 14 is then doped with fluorine. For this purpose, the soot tube is introduced in a vertical orientation into a dehydration and doping furnace and is first treated at a temperature in the range from 700 ° C to about 900 ° C in a fluorine-containing atmosphere (C ₂ F ₆ or CF ₄ ). The treatment lasts about four hours. As a result, a hydroxyl group concentration of less than 100% by weight is obtained in the areas 12 to 14 of the soot tube, and at the same time a fluorine doping is generated in the area of the outer jacket area 14 . The compression zones 16 , 17 and 18 thereby reduce the transport and leaching of GeO ₂ from the respective areas 19 , so that a GeO ₂ concentration curve results, which leads to the refractive index curve shown in FIG. 2a. In particular, the compression zone 18 hinders diffusion of fluorine in the inner cladding region 13 . The compression zones 16 , 17 , 18 thus ensure that the preset concentration profile according to FIG. 2a is essentially retained in the core rod.

Using the soot tube obtained in this way, a core rod with the idealized refractive index profile shown in FIG. 2a is produced by vitrifying the soot tube in a glazing furnace at a temperature in the region of 1350 ° C.

The quartz glass tube thus obtained is characterized by a defined refractive index profile and an overall low hydroxyl group concentration, so that it is particularly suitable as a core material for a preform for optical fibers is.  

To complete the preform for optical fibers, the so produced In any case, core rod finally with additional cladding glass layers give.

Claims (18)

1. A method for producing a component made of doped quartz glass for the production of optical fibers, which has a plurality of radially successive layers of quartz glass, which differ in their refractive index, the quartz glass layers being produced by a carrier rotating on its longitudinal axis radial soot layer sequence comprising at least one inner, porous SiO ₂ soot layer and an adjoining outer, porous SiO ₂ soot layer, is deposited, whereby adjacent soot layers differ in their doping and are each connected to one another via a cylinder-shaped contact surface, and the soot layers then being dried in a halogen-containing atmosphere and glazed to form the quartz glass layers, characterized in that in the region of the contact surface ( 4 ', 4 ", 4 ''', 15 ', 15 ", 15 ''') a compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) with a soot at least 27% of the theoretical density of quartz glass. 2. The method according to claim 1, characterized in that a soot density of at least 29%, preferably at least 31% of the theoretical density of quartz glass is set in the region of the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ). 3. The method according to claim 1 or 2, characterized in that the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) is generated within the respective outer soot layer. 4. The method according to any one of the preceding claims, characterized in that the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) has a radial extent in the range between 50 microns and 3 mm, preferably between 100 microns and 1st mm. 5. The method according to any one of the preceding claims, characterized in that in the area ( 9 , 10 , 11 , 19 , 20 , 21 ) outside the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) an in Compared to an adjacent compression zone, a lower soot density of up to 30% of the density of quartz glass is set. 6. The method according to claim 5, characterized in that in the area ( 9 , 10 , 11 , 19 , 20 , 21 , 22 ) outside the compression zone ( 9 , 10 , 11 , 19 , 20 , 21 ) a soot density of at most 28% , preferably a maximum of 26% of the density of quartz glass. 7. The method according to any one of the preceding claims, characterized in that the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) is generated by the surface temperature of the SiO ₂ soot layer is increased during deposition. 8. The method according to any one of the preceding claims, characterized in that the doping of the soot layers by one or more of the dopants TiO ₂ , GeO ₂ , TaO ₂ , Al ₂ O ₃ , P ₂ O ₅ , boron, fluorine and / or Chlorine is set. 9. The method according to any one of the preceding claims, characterized records that the inner soot layer and the outer soot layer densel ben dopant contain, and that the inner soot layer a radial Has gradient in the dopant concentration, which is in the outer Soot shift continues. 10. The method according to claim 9, characterized in that the inner soot layer is doped with GeO ₂ , that the outer, initially undoped soot layer is loaded with fluorine in a gas phase treatment subsequent to the deposition. 11. A tubular semi-finished product for producing a component made of doped quartz glass for the production of optical fibers, comprising a radial soot layer sequence, which has at least one inner, porous SiO ₂ soot layer and an adjacent, outer, porous SiO ₂ soot layer, with adjacent soot layers in differ from each other in their doping and are each connected via a cylindrical jacket-shaped contact area, characterized in that in the area of the contact area a compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) with a soot density of at least 27% of the theoretical density of Quartz glass is provided. 12. Semi-finished product according to claim 11, characterized in that a soot density of at least 29%, preferably at least 31% of the theoretical density of quartz glass is set in the region of the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ). 13. Semi-finished product according to claim 11 or 12, characterized in that the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) is provided within the respective outer soot layer. 14. Semi-finished product according to one of claims 11 to 13, characterized in that the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) has a radial expansion in the range between 50 µm and 3 mm, preferably between 100 µm and 1 mm. 15. Semi-finished product according to one of claims 11 to 14, characterized in that in the area ( 9 , 10 , 11 , 19 , 20 , 21 ) outside the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) a smaller one Soot density of maximum 30% of the density of quartz glass is present. 16. Semi-finished product according to claim 15, characterized in that in the area ( 9 , 10 , 11 , 19 , 20 , 21 ) outside the compression zone ( 5 , 6 , 7 , 8 , 16 , 17 , 18 ) a soot density of at most 28% , preferably at most 26% of the density of quartz glass. 17. Semi-finished product according to one of claims 11 to 16, characterized in that the soot layers are doped with one or more of the dopants TiO ₂ , GeO ₂ , TaO ₂ , Al ₂ O ₃ , P ₂ O ₅ , boron, fluorine and / or chlorine are. 18. Semi-finished product according to one of claims 11 to 17, characterized in that that the inner soot layer and the outer soot layer are the same do contain animal substance, and that the inner soot layer a radial Gra served in the dopant concentration, which is in the outer Soot shift continues.