WO2001098219A2

WO2001098219A2 - Method for producing an sio2 blank and blank so produced

Info

Publication number: WO2001098219A2
Application number: PCT/EP2001/006576
Authority: WO
Inventors: Wolfgang Krock; Klaus Ruppert; Peter Kleinsorge
Original assignee: Heraeus Tenevo Ag
Priority date: 2000-06-19
Filing date: 2001-06-11
Publication date: 2001-12-27
Also published as: WO2001098219A3; DE10029151C1

Abstract

According to known methods for producing SiO2 blanks, successive layers of SiO2 particles are deposited, by means of at least one deposition burner, on the peripheral surface of a support that rotates about its longitudinal axis, thereby forming the porous blank. The support is then removed. The aim of the invention is to facilitate removal of the support and to thereby allow the production of an SiO2 blank (1) whose inner walls are as flawless as possible. To this end, a hard layer sequence (5) which comprises the first layer is deposited, said hard layer sequence (5) having a higher density than the successive layers (6). The inventive porous SiO2 blank is characterized by a core glass zone (2) of porous quartz glass that consists of a plurality of success layers and that is coaxially surrounded by at least one peripheral glass zone (3) of porous quartz glass. Said core glass zone (2) has an inner core glass layer (5) that comprises the innermost layer and an outer core glass layer (6), the inner core glass layer (5) having a higher density than the outer core glass layer (6).

Description

Patent application

Process for the production of a SiO ₂ blank and blank produced by the process

The present invention relates to a process for the production of a SiO ₂ blank in that the porous blank is formed by depositing successive layers of SiO ₂ particles on the lateral surface of a support rotating about its longitudinal axis by means of at least one deposition burner and the support is removed.

Furthermore, the invention relates to a blank for the production of a preform for an optical fiber, which has a core glass region made of porous quartz glass which is formed from a plurality of successive layers and which is coaxially surrounded by at least one cladding glass region made of porous quartz glass.

A method and a blank of the type mentioned are known from US Pat. No. 4,362,545. For the production of a preform for optical fibers by means of a flame hydrolysis burner, SiO ₂ particles are deposited in layers on the cylindrical surface of a cylinder with both ends clamped in a lathe and rotating about its longitudinal axis, slightly conical dome. A reciprocating movement of the deposition burner along the longitudinal axis of the dome forms an elongated, porous blank made of SiO ₂ particles (soot body). This process is known as the OVD process (Outside Vapor Deposition). Aluminum oxide, quartz glass, graphite or silicon carbide are recommended as suitable materials for the manufacture of the dome.

The tubular, porous blank is used as a starting material for the production of a preform for optical fibers. It can be used to build the core and shell material of the preform. The blank according to US Pat. No. 4,362,545 becomes a so-called core rod is produced by sintering and collapsing the inner bore. This blank has a core glass layer made of SiO ₂ , which is surrounded by a B ₂ 0 ₃ -doped SiO ₂ layer.

For further processing, the blank is usually subjected to a treatment in a chlorination process and then glazed in a sintering process. This requires the inner bore to collapse if the blank is to form the central core area of the later preform. Otherwise, the porous or glazed blank is melted onto a further quartz glass cylinder arranged coaxially in the inner bore in the form of a rod or tube. In any case, the mandrel must be removed from the inner bore beforehand. In the case of a stuck mandrel, however, this can lead to damage to the inner wall of the blank, which requires complex reworking of the inner bore and thereby increase the manufacturing costs.

Various measures have been proposed to solve this problem, which are intended to facilitate the removal of the mandrel. For example, the use of a graphite mandrel (FR-A 2,178,177) or the preliminary coating of a mandrel with graphite powder (US Pat. No. 4,233,062) or with a so-called stratum layer made of a phosphorus-doped low viscosity glass (US Pat. No. 4,298,365).

However, these measures have other disadvantages, for example the use of graphite leads to changes in the redox conditions in the furnace space and to burning under oxidizing conditions, and the use of a stratum layer can leave impurities in the blank.

The present invention is therefore based on the object of specifying a simple and inexpensive method which enables the production of a SiO ₂ blank with an inner wall which is as defect-free as possible. Furthermore, the invention is based on the object of providing a blank for the production of a preform for an optical fiber which has an inner bore with an inner wall which is as defect-free as possible. With regard to the method, this object is achieved, based on the method mentioned at the outset, in that a hard layer sequence comprising the first layer is deposited, which has a higher density than layers following the hard layer sequence.

In the simplest case, the higher density of the hard layer sequence results from a higher surface temperature during the deposition of the first layers of SiO ₂ particles on the carrier. The higher density can also be set by heating individual layers of the hard layer sequence or the hard layer sequence as a whole. In any case, several comparatively harder layers are produced, which are referred to here as “hard layer sequence”. The hard layer sequence comprises the first layer and at least one further layer.

The hard layer sequence is characterized by the fact that it is mechanically much more resistant than softer SiO ₂ soot layers. However, this would also be associated with a comparatively stronger mechanical connection of the hard layer sequence with the carrier and, as a result, greater damage to the inner wall when removing the carrier. However, it has surprisingly been found that this effect does not occur if the hard layer sequence comprises at least the first layer and a further layer, and if softer SiO ₂ soot layers follow the hard layer sequence. In this case, the effect of the higher mechanical resistance of the hard layer sequence predominates, so that injuries to the inner wall of the blank when pulling out the carrier are avoided; extensive post-processing of the inner wall is therefore not necessary.

The tubular, porous blank can consist of homogeneous material, such as doped or undoped SiO ₂ ; it can also have areas of different doping. It is used as a starting material for the production of a preform for optical fibers, and it can be used to build up the core and cladding material of the preform. The blank is used in the form of a porous or glazed tube or in the form of a rod.

On the one hand, the total thickness of the hard layer sequence must be so great that the required mechanical resistance is achieved. On the other hand, a structure of the porous blank that is as open-pored as possible is to be maintained in order to ensure largely unimpeded access to treatment media in subsequent treatment steps. In view of this, it has proven to be advantageous if only the first to at most tenth layer, preferably the first to at most fifth layer, are deposited as the hard layer sequence.

A hard layer sequence with a total thickness of at most 500 μm, preferably at most 150 μm, is advantageously produced.

It has proven to be advantageous to generate a hard layer sequence with a density in the range from 20% to 35% of the density of quartz glass. This density is expediently set by the surface temperature when the hard layer sequence is deposited. The lower limit mentioned for the preferred density range is in turn predetermined by the desired mechanical resistance of the hard layer sequence, while the upper limit results from the desired residual porosity in this range.

A particularly simple procedure for producing the hard layer sequence results from the fact that a comparatively higher flame temperature of the deposition burner is set during its deposition. Accordingly, the surface temperature is reduced after the hard layer sequence has been deposited by lowering the flame temperature of the at least one deposition burner. There are, in turn, several suitable process variants for lowering the flame temperature, each of which can be used individually or in combination. The surface temperature is the temperature on the blank surface at the point of impact of the flame.

For example, the flame temperature is reduced by reducing the feed rate of burner gases to the at least one deposition burner relative to the feed rates of other gases supplied to the deposition burner. The burner gases are understood to be those gases whose exothermic reaction with one another essentially feeds the burner flame. A detonating gas burner is, for example, the burner gases oxygen and hydrogen. A Lowering the flame temperature is achieved either by reducing the feed rate of oxygen and / or hydrogen to the deposition burner or by feeding or increasing the feed rate of other gases, such as, for example, inert gas or starting materials for the formation of the SiO ₂ particles.

Alternatively or additionally, the surface temperature after the hard layer sequence has been deposited can also advantageously be reduced by increasing the distance between the at least one deposition burner and the surface of the blank being formed. The distance can be increased or decreased. Reducing or increasing the distance can lower the surface temperature if the burner flame becomes colder at the point of impact. This can be the case in particular with so-called focusing deposition burners. The distance is measured between the mouth of the deposition burner and the blank surface.

As an alternative or in addition to the possibilities described above, it has also proven useful to reduce the surface temperature after the hard layer sequence has been deposited by increasing the speed of the relative movement between the at least one deposition burner and the surface of the blank being formed.

With regard to the blank for the production of a preform for an optical fiber, the above-mentioned object is achieved according to the invention starting from the blank mentioned at the outset in that the core glass region has an inner core glass layer comprising the innermost layer and an outer core glass layer, the inner core glass layer being opposite the outer one Core glass layer has a higher density.

The blank according to the invention is used to produce a preform for optical fibers, the core glass area forming the central core of the later preform or the optical fiber after the inner bore of the blank has collapsed. The main part of the light is guided. Before the blank or the quartz glass tube obtained from it by sintering collapses, an there are absolutely defect-free inner holes. This is achieved in the blank according to the invention in that the core glass region has an inner core glass layer which consists of a hard layer sequence comprising the innermost SiO 2 soot layer. The hard layer sequence is formed in that the innermost SiO ₂ soot layer and at least one further soot layer have a higher density than layers following the hard layer sequence outwards, so that an area of higher density is produced compared to the layers located further out. The higher density goes hand in hand with a higher hardness and resistance, so that the wall of the inner bore of the blank is not impaired when the mandrel is pulled out and expensive reworking of the inner wall is avoided. Reference is made to the above explanations regarding the method according to the invention.

On the one hand, the total thickness of the inner core glass layer must be so great that the required mechanical resistance is achieved. On the other hand, a structure of the porous blank that is as open-pored as possible is to be maintained in order to ensure largely unimpeded access to treatment media in subsequent treatment steps. In this regard, it has proven to be advantageous if the inner core glass layer comprises only the first to at most tenth layer, preferably the first to at most fifth layer.

An embodiment of the blank according to the invention is preferred in which the inner core glass layer has a total thickness of at most 500 μm; preferably has a maximum of 150 microns.

A blank with a hard layer sequence that has a density in the range from 20% to 35% of the density of quartz glass has proven particularly useful. The invention is explained in more detail below on the basis of exemplary embodiments and a drawing. In the drawing, a schematic representation shows in detail:

FIG. 1 shows an embodiment of a blank according to the invention in a radial section,

FIG. 2 shows an exemplary embodiment for producing a preform using a blank according to the invention on the basis of a flow diagram of the individual method steps, and

FIG. 3 shows a section of a typical density curve over the radius in the case of a blank produced by the method according to the invention.

Figure 1 shows a blank 1 in the form of a tubular, porous SiO 2 soot body. The blank 1 has a core glass area, to which the overall reference number 2 is assigned and which is coaxially surrounded by a cladding glass area 3. The core glass area 2 consists of SiO ₂ soot, which is homogeneously doped with 5% by weight germanium dioxide, the cladding glass area 3 consists of undoped SiO ₂ soot. The diameter of the inner bore 4 is 5 mm; the outer diameter of the core glass area 2 is 50 mm and that of the cladding glass area 3 is 100 mm.

The core glass region 2 has a thin inner core glass layer 5 directly adjacent to the inner bore 4, which is surrounded by a thicker, outer core glass layer 6. The thickness of the inner core glass layer 5 is only 0.15 mm; in Figure 1, the thickness is shown enlarged for clarity. The remaining core glass area 2 is formed by the outer core glass layer 6.

Inner core glass layer 5 and outer core glass layer 6 nominally have the same chemical composition. The main difference between these two layers is that the inner core glass layer 5 has a density of 28% of the density of quartz glass, while the density of the outer core glass layer 6 is only 20% of the density of quartz glass. The density of the individual core glass layers 5, 6 is determined by means of mercury porosimetry. The method according to the invention for producing a tube made of synthetic quartz glass and its use for producing a preform for optical fibers is explained by way of example below with reference to FIG. 2.

To carry out the method according to the invention, a so-called core rod is first formed by means of a soot outer deposition method (OVD method) by flame hydrolysis of SiCI and / or GeCI, with corresponding oxide particles being deposited on the outer surface of a mandrel rotating about its longitudinal axis. An aluminum oxide tube with a diameter of 5 mm is used as the mandrel. The inner core glass layer 5 is first deposited by means of a detonating gas deposition burner, in addition to SiCI ₄ , GeCI _{4 is also} fed to the deposition burner in order to obtain the above-mentioned dopant concentration in the core glass region 2. The deposition burner is moved back and forth along the dome longitudinal axis in a predetermined movement cycle, one layer of SiO ₂ being produced one after the other on the mandrel or on the cylindrical surface of the blank being formed.

An essential method step of the method according to the invention is that a higher density is set in the inner core glass layer 5. This is achieved in that during the deposition of the inner core glass layer 5 a higher surface temperature is set in the area of the point of impact of the deposition burner than when the outer core glass layer 6 is deposited. The surface temperature is measured on the blank surface at the point of impact of the flame of the deposition burner. A commercially available thermal camera is used for this.

In the exemplary embodiment, the inner core glass layer 5 is formed by the three first SiO ₂ layers. When these layers are deposited, a surface temperature is set which brings about a specific density of the inner core glass layer 5 of 28% of the density of quartz glass. After the inner core glass layer 5 has been deposited, the outer core glass layer 6 is deposited, the surface temperature being reduced at the same time. There are several ways to do this.

On the one hand, the surface temperature after the core glass Layer 5 is reduced by lowering the flame temperature of the deposition burner. This is achieved by reducing the feed rates of the fuel gases hydrogen and oxygen to the separation burner by 15%. In a further exemplary embodiment, the surface temperature after the deposition of the inner core glass layer 5 is reduced in that the distance between the deposition burner and the surface of the blank being formed is increased from 130 mm to 150 mm.

In a further exemplary embodiment, the surface temperature after the deposition of the inner core glass layer 5 is reduced in that the speed of the relative movement between the deposition burner and the surface of the blank being formed is increased by 100% by the feed speed of the deposition burner along the mandrel being increased accordingly.

After the outer core glass layer 6 has been deposited, the supply of GeCI _{4 is} stopped and the cladding glass region 3 is deposited in layers on the core glass region 2 using the same method.

The mandrel is then removed. Due to the high strength of the inner core glass layer 5, this can be done without damaging the wall of the inner bore. The porous quartz glass tube thus obtained is dried in a chlorine-containing atmosphere and then sintered. To produce a preform for optical fibers, a so-called core rod is formed from it by collapsing the inner bore. In this way, a core rod with a diameter of 14 mm is obtained.

At the same time, a flashing tube made of undoped quartz glass (a so-called "jacket tube") is also produced by flame hydrolysis of SiCI ₄ with the formation of SiO ₂ particles and axial deposition of the SiO ₂ particles on a rotating mandrel The fiber obtained from the preform does not contribute significantly, the requirements for its purity and homogeneity are comparatively low. The jacket tube can therefore be produced inexpensively using several separating burners. Before sintering, the undoped, porous quartz glass is dried in a chlorine-containing atmosphere The jacket tube has an inner diameter during sintering of about 15 mm and an outer diameter of about 60 mm.

The jacket tube is then melted onto the core rod by heating the arrangement in an electrically heated furnace to a temperature of 2150 ° C. (furnace temperature). The ring gap is closed without problems by zone-by-zone heating of the vertically oriented arrangement.

The preform produced in this way then has an outer diameter of 100 mm. The fiber drawn from it shows an attenuation of 0.6 dB / km at a wavelength of 1385 nm.

FIG. 3 shows a section of a typical density curve over the radius in the case of a porous SiO ₂ blank produced by the method according to the invention. The density "δ" is plotted on the y-axis of the diagram and the radius "r" on the x-axis. Immediately adjacent to the inner bore 31, a region of higher density is provided, which is formed by the inner core glass layer 32. The outer core glass layer 33 adjoins the inner core glass layer 32 on the outside and the cladding region 34 adjoins this. The outer core glass layer 33 and the cladding region 34 do not differ in their density in this exemplary embodiment. The density of the outer core glass layer 33 is clearly below the density in the inner core glass layer 32. The interface between the inner core glass layer 32 and the outer core glass layer 33 is indicated by the dotted line 35. The boundary surface is defined as the cylinder jacket surface running parallel to the longitudinal axis, at which the difference in density between the inner core glass layer 32 and the outer core glass layer 33 is half of its maximum value.

Further information on the processes and devices relevant for the production of synthetic quartz glass for optical fibers by CVD deposition can be found in the following publications: US Pat. No. 5,788,730 describes a process and a deposition burner made of quartz glass with a central nozzle and described at least three annular gap nozzles for the production of a sot body with a homogeneous radial density distribution; DE-A1 197 25 955 teaches the use of a burner for feeding liquid glass starting material; and in DE-A1 195 01 733 a device is device for the simultaneous and uniform gas supply to a large number of separating burners using a pressure compensation vessel. In order to increase the efficiency of the soot deposition, DE-A1 196 29 170 proposes to create an electrostatic field between the deposition burner and the soot body; DE-A1 196 28 958 and DE-A1 198 27 945 specify measures for the homogenization of the soot separation when using an oscillating burner array. Methods and devices for handling the soot body during and after the deposition process are known from DE-A1 197 51 919 and DE-A1 196 49 935; and US Pat. No. 5,665,132, US Pat. No. 5,738,702 and DE-A1 197 36 949 result in measures for holding the soot body during glazing. The doping of quartz glass with fluorine and boron is described in EP-A 582 070; in US-A 5,790,736 a teaching is given to adjust the viscosity of the core and sheath material of a fiber; and DE 198 52 704 relates to a method for producing an optical fiber using doped substrate tubes according to the MCVD method. The post-processing of a glazed quartz glass hollow cylinder using a special drill is described in US-A 5,643,069. US Pat. No. 5,785,729 teaches the manufacture of large-volume preforms using the rod-in-tube technique; and DE-A1 199 15 509 describes a fume cupboard suitable for carrying out this technique. The subject of EP-A1 767 149 and DE-A1 196 29 169 is the manufacture of dimensionally accurate quartz glass tubes by a vertical drawing process.

Claims

claims

1. A process for the production of a SiO ₂ blank by the deposition of successive layers of SiO ₂ particles on the outer surface of a carrier rotating about its longitudinal axis by means of at least one deposition burner, and the carrier being removed, characterized in that a hard layer sequence (5) comprising the first layer is deposited, which has a higher density than layers (6) following the hard layer sequence (5).

2. The method according to claim 1, characterized in that the first to at most tenth layer are deposited as the hard layer sequence (5).

3. The method according to claim 1, characterized in that the first to a maximum of five layers are deposited as the hard layer sequence (5).

4. The method according to any one of the preceding claims, characterized in that the hard layer sequence (5) is generated with a total thickness of at most 500 microns, preferably at most 150 microns.

5. The method according to any one of the preceding claims, characterized in that the hard layer sequence (5) has a density in the range from 20% to 35% of the density of quartz glass.

6. The method according to any one of the preceding claims, characterized in that the surface temperature after the deposition of the hard layer sequence (5) is reduced by lowering the flame temperature of the at least one deposition burner.

7. The method according to claim 6, characterized in that the flame temperature is reduced by reducing the supply rate of burner gases to the at least one deposition burner relative to the supply rates of other gases supplied to the deposition burner.

8. The method according to claim 7, characterized in that the flame temperature temperature is reduced by supplying the separating burner with inert gas or increasing the supply of inert gas.

9. The method according to any one of claims 7 or 8, characterized in that the flame temperature is reduced by increasing the supply of starting materials for the formation of the SiO ₂ particles relative to the supply of oxygen and / or hydrogen.

10. The method according to any one of the preceding claims, characterized in that the surface temperature after the deposition of the hard layer sequence (5) is reduced by the distance between the at least one deposition burner and the surface of the blank (1) being formed is increased.

11. The method according to any one of the preceding claims, characterized in that the surface temperature after the deposition of the hard layer sequence (5) is reduced by the speed of the relative movement between the at least one deposition burner and the surface of the blank being formed (1) is increased.

12. Blank for the production of a preform for an optical fiber, which has a core glass region (2) made of porous quartz glass which is formed from a plurality of successive layers and which is coaxially surrounded by at least one cladding glass region (3) made of porous quartz glass, characterized in that the Core glass region (2) has an inner core glass layer (5) comprising the innermost layer and an outer core glass layer (6), the inner core glass layer (5) having a higher density than the outer core glass layer (6).

13. Blank according to claim 12, characterized in that the inner core glass layer (5) comprises the first to at most tenth layer.

14. Blank according to claim 12, characterized in that the inner core glass layer (5) comprises the first to a maximum of fifth layer.

15. Blank according to one of claims 12 to 14, characterized in that the inner core glass layer (5) has a total thickness of at most 500 microns, preferably at most 150 microns.

16. Blank according to one of claims 12 to 15, characterized in that the inner core glass layer (5) has a density m range of 20% to 35% of the density of quartz glass.