WO2001040125A1

WO2001040125A1 - Quartz glass rod for a preform for optical fibers and method for producing the same

Info

Publication number: WO2001040125A1
Application number: PCT/EP2000/011805
Authority: WO
Inventors: Heinz Fabian
Original assignee: Heraeus Tenevo Ag; Shin-Etsu Quartz Products Co., Ltd.
Priority date: 1999-12-03
Filing date: 2000-11-27
Publication date: 2001-06-07
Also published as: DE19958289C1

Abstract

The invention relates to a method for economically producing a quartz glass rod consisting of synthetic quartz glass with a radially and axially homogenous refractive index curve. According to the invention, a whole cylinder consisting of quartz glass is coated with a coating tube consisting of porous glass, the latter shrinking onto the whole cylinder during sintering. The coating tube has a predetermined refractive index which ensures that there is a maximum increase in the refractive index value of 0.001 in the area of a contact surface between the whole cylinder and the shrank-on coating cylinder. A solid quartz glass rod produced in this way is characterized by a homogenous refractive index distribution in an axial and radial direction and comprises a whole cylinder consisting of synthetic quartz glass which is encased in a quartz glass layer formed by a coating tube consisting of porous quartz glass that is sintered and shrunk on with a maximum increase in the refractive index of 0.001 in the area of a contact surface. The inventive quartz glass rod is used for economically producing a preform for multimode fiber optical waveguides.

Description

^Q UARZGLASSSTAB FOR A PREFORM FOR OPTICAL FIBERS AND METHOD FOR THE PRODUCTION THEREOF

The invention relates to a method for producing a quartz glass rod, comprising providing a full cylinder made of synthetic quartz glass with a predetermined refractive index with a radially and axially homogeneous refractive index curve.

The invention also relates to a quartz glass rod with a radially and axially homogeneous refractive index curve.

Furthermore, the present invention relates to a preform for a multimode step index optical fiber with a core made of quartz glass and with a jacket radially enveloping the core.

Methods for producing a quartz glass rod from synthetic quartz glass are generally known. In a common method for producing synthetic quartz glass by flame hydrolysis of silicon compounds, SiO ₂ particles are formed in a deposition burner and deposited in layers on a substrate. Depending on the temperature during the deposition, the deposited particles are sintered directly, so that the quartz glass has no residual porosity - one speaks in this case of a "direct process" - or, during the deposition, a porous quartz glass body (a so-called "soot body") is produced, the Properties can be modified in a subsequent gas phase treatment, and which is then sintered into a quartz glass rod. This process is referred to as the "soot process".

There are several variants of the soot process. On the one hand, this is the so-called outside vapor deposition process (OVD), in which the SiO ₂ particles be deposited on the cylindrical surface of a dome rotating about its longitudinal axis, which is removed after the deposition process and leaves a corresponding opening. This has to be collapsed to produce a quartz glass rod, which causes additional costs. It is also possible to use a quartz glass rod as the mandrel, which forms part of the end product and therefore does not have to be removed. However, this results in process-related disadvantages.

On the other hand, the vapor axial deposition method (VAD) is used, as is known from EP-A2 401 845. Si0 ₂ particles are formed by flame hydrolysis of SiCI using a detonating gas separating burner and deposited on the lower end of a vertically oriented glass rod rotating about its longitudinal axis. This is deducted upwards in accordance with the thickness growth of the deposited layer. After reaching the predetermined length, the porous quartz glass rod thus obtained is sintered to form a full cylinder made of quartz glass.

Since a soot body without an axial bore is obtained in this case, this method variant is in principle well suited for producing a quartz rod with a radially and axially homogeneous refractive index curve. However, the economy of the process is limited due to the relatively small separation area.

A preform of the type specified at the outset is known from US Pat. No. 4,251,251. This describes the production of a quartz glass preform for an optical fiber using the so-called OVD process. In a first process step, a porous Si0 ₂ cylinder (hereinafter referred to as “soot cylinder”) is produced, which consists of a SiCV inner layer doped with germanium oxide (25% by weight) and boron oxide (5% by weight), which layer consists of a with boron oxide (2 wt .-%) doped Si0 ₂ -Außenschicht is surrounded. The Sootzylinder is formed by flame hydrolysis of SiCl ₄ (or of GeCl ₄ and BCl ₃₎ by generating Si0 ₂ particles by means of deposition and in a first process step to form the doped Si0 ₂ inner layer in layers on the outer surface of a its longitudinal axis rotating mandrel can be deposited from quartz glass. In a second process step, the SiO ₂ outer layer is deposited on the SiC inner layer produced in this way. The quartz glass mandrel is then pulled out of the soot cylinder. The soot cylinder thus has a bore, the inside diameter of which corresponds to the outside diameter of the dome. The preform is obtained from the soot cylinder thus produced by sintering the porous soot cylinder and simultaneously collapsing the bore. The preform has a core made of quartz glass doped with boron oxide and germanium oxide, the refractive index of which is 1.476 and which is surrounded by a jacket made of quartz glass with a refractive index of 1.457.

The present invention is based on the object of proposing a method by means of which a quartz glass rod, in particular a thick quartz glass rod, can be produced inexpensively, to provide a solid quartz glass rod produced by the method and having a homogeneous refractive index distribution in the axial and radial directions, and one using a to provide such inexpensive preform for multimode optical fibers produced by such quartz glass rods.

With regard to the method, this object is achieved, based on the method mentioned at the outset, in that the full cylinder is overlaid with a flashing tube made of porous quartz glass by sintering the flashing tube and thereby onto the full cylinder to form a

Quartz glass rods shrink on, with the flashing tube showing the specified refractive index with a radially homogeneous and axially homogeneous refractive index curve over at least most of its wall thickness - seen radially from the inside, with the proviso that there is a contact area between the solid cylinder and the shrunk-on and sintered flashing tube Refractive index jump of maximum 0.001 is generated.

In a first process step, a quartz glass full cylinder with an axially and radially homogeneous refractive index curve is produced, which is then overlaid in a second process step by a flash tube made of porous quartz glass.

The production of the full cylinder can be carried out according to one of the methods mentioned above (OVD or VAD method) take place, with a relatively thin full cylinder is generally obtained with regard to the economy and efficiency of the method. The method according to the invention aims to use such a thin, comparatively inexpensive solid cylinder in order to obtain a thicker solid cylinder, the production of which would otherwise be less effective and inexpensive.

For this purpose, in the second process step, the flash tube is provided in the form of a porous Si0 ₂ hollow cylinder with an axial opening. The porous flash tube can be produced simply and inexpensively by oxidizing or hydrolyzing a silicon compound to form Si0 ₂ particles, and depositing the Si0 ₂ particles in layers on a carrier rod, and then removing the latter. The flash tube produced in this way is characterized by an exact inner bore, predetermined by the outer diameter of the support rod. Because of this exact geometric dimensions of the flash tube, additional process steps with the aim of obtaining predetermined nominal dimensions are not necessary. Such post-processing would be necessary, for example, if the flash tube were sintered before the full cylinder was flashed over.

Therefore, the porous flash tube is suitable for carrying out the following process step according to the invention, namely for shrinking onto the full cylinder, directly - that is, without mechanical reworking. The full cylinder is inserted into the axial opening of the porous flash tube, which is then sintered and shrunk onto the full cylinder and fuses with it. A contact surface is formed between the full cylinder and the flash tube.

So that the shrunk-on flashing tube to produce a thicker one

Contributes quartz glass rods with axially and radially homogeneous refractive index, it is necessary that after sintering at least that part of the flash tube facing the solid cylinder has the same refractive index as the solid cylinder. In practice, the condition of exactly the same refractive indices can hardly be realized. For most applications of the quartz glass rod, however, it is sufficient if in There is a jump in the refractive index of at most 0.001 in the area of the contact surface between the full cylinder and the shrunk-on and sintered flashing tube. For this purpose, the flashing tube has the same refractive index as the solid cylinder, at least over the largest part of its wall thickness - seen radially from the inside out - with a radially homogeneous and axially homogeneous refractive index curve. A homogeneous refractive index curve is understood here to mean that the minimum and maximum refractive index differ over the entire curve by at most 0.001. The specification "the largest part of the wall thickness of the flash tube" includes a length specification (no volume specification). The jump in refractive index is preferably a maximum of 0.0003.

It has proven to be advantageous to adjust the refractive index of the porous quartz glass to the refractive index of the solid cylinder by treatment in an atmosphere containing a dopant before sintering. This makes it possible to minimize the jump in refractive index in the area of the contact surface.

To produce a thick quartz glass rod, it is advantageous if a flash tube with a radially homogeneous refractive index curve is used. As a result, both the full cylinder and the flash tube contribute with their entire wall thickness to the production of the quartz glass rod.

A procedure has proven to be particularly favorable in which the flash tube is subjected to a chlorine treatment by heating in a chlorine-containing atmosphere at a temperature above 700.degree. The chlorine treatment removes impurities and lowers the OH content of the porous quartz glass. Chlorine and chlorine compounds with a hygroscopic effect at the treatment temperature are suitable for chlorine treatment, which also has a favorable effect on the quality of the contact surface.

For the chlorine treatment, the outer diameter of the solid cylinder and the inner diameter of the flash tube are preferably selected such that an annular gap remains between the flash tube and the solid cylinder inserted therein during the chlorine treatment. The chlorine treatment after inserting the full cylinder and before sintering the flash tube both the surface of the solid cylinder and the annular gap between the solid cylinder and the flash tube are cleaned. In addition, the annular gap ensures that the chlorine-containing atmosphere can act on the flash tube both from its inner wall and from the outer wall, which halves the diffusion paths and thus shortens the treatment time.

To carry out the method according to the invention, it has proven to be advantageous to use a flashing tube with a wall thickness in the range from 50 mm to 500 mm. The greater the wall thickness of the flash tube, the more it can contribute to the quartz glass rod. With a wall thickness below 10 mm, the process is uneconomical. The upper limit mentioned is primarily determined by the technical difficulties in sintering and shrinking of the flashing tube, which increase with the wall thickness, and by long diffusion times during a gas phase treatment of the porous flashing tube.

In a particularly advantageous method variant for producing the quartz glass rod, a quartz glass rod with a radially and axially homogeneous refractive index curve is used as the solid cylinder, which has been successively built up using the method according to the invention.

With regard to the quartz glass rod with a radially and axially homogeneous refractive index curve, the above-mentioned object is achieved according to the invention based on the quartz glass rod described at the outset in that it comprises a full cylinder made of synthetic quartz glass, which is enveloped by at least one quartz glass layer formed by sintering and shrinking on a flash tube made of porous quartz glass and which has a refractive index jump of at most 0.001 in the area of a contact surface between the solid cylinder and the quartz glass layer.

The quartz glass rod is characterized overall by a radially and axially homogeneous refractive index curve. Reference is made to the above explanation of the term for this information.

According to the invention, the quartz glass rod comprises an axial quartz glass solid cylinder and a radially homogeneous refractive index curve. This is enveloped by at least one quartz glass layer formed by sintering and shrinking on a flashing tube made of porous quartz glass. This quartz glass layer is produced from the “flashing tube”, as is explained in more detail above on the basis of the description of the method. In this respect, the “flashing tube” corresponds to the “quartz glass layer” of the quartz glass rod Production of the quartz glass layer has already been pointed out in connection with the explanation of the method according to the invention.

The quartz glass rod thus has an outer quartz glass layer, which was formed by sintering and shrinking a flash tube made of porous quartz glass onto the full cylinder. The flash tube is suitable for the production of a quartz glass rod with an overall axially and radially homogeneous refractive index distribution only on the condition that the refractive index curve of the flash tube is matched to that of the full cylinder. As a yardstick for fulfilling this

A requirement in the sense of the invention is that the quartz glass rod has a refractive index jump of a maximum of 0.001 in the area of a contact surface between the full cylinder and the quartz glass layer. However, the jump in refractive index is preferably a maximum of 0.0003.

The suitability of the quartz glass rod according to the invention for producing a preform for optical fibers is improved if the OH content of the solid cylinder and the quartz glass layer is less than 1 ppm by weight. OH absorption bands and the associated optical attenuation are thus reduced.

The suitability of the quartz glass rod according to the invention for producing a preform for optical fibers can also be improved in that the OH content of the full cylinder and the quartz glass layer is greater than 100 ppm by weight. The high OH content can neutralize defects in the quartz glass. Furthermore, the high OH content increases the resistance of the optical fiber when transmitting high-energy UV light with wavelengths below 400 nm, since defect centers induced by UV light are neutralized. The solid cylinder and quartz glass layer are advantageously doped with fluorine. This lowers the viscosity of the quartz glass and facilitates the formation of a trouble-free contact surface between the full cylinder and the flash tube when collapsing. Doping with fluorine lowers the refractive index of quartz glass.

It is equally advantageous with regard to a reduction in viscosity if the full cylinder and quartz glass layer are doped with germanium. Doping with germanium increases the refractive index of quartz glass.

In view of an inexpensive manufacture of the quartz glass rod according to the invention, it has proven to be advantageous if the ratio of the

Outside diameter of quartz glass layer and solid cylinder is at least 2.5. The flash tube contributes significantly to the volume of the quartz glass rod under these conditions.

With regard to the preform for a multimode step index optical fiber, the task specified above is based on a preform of the aforementioned

Genus solved according to the invention in that the core is designed as a quartz glass rod with a radially and axially homogeneous refractive index profile, it comprising a solid cylinder made of synthetic quartz glass, which is encased by at least one quartz glass layer formed by sintering and shrinking on a flashing tube made of porous quartz glass, and in the region of one Contact area between full cylinder and quartz glass layer has a refractive index jump of a maximum of 0.001.

In the preform according to the invention, a quartz glass rod according to the present invention is used as the core material. The advantages of such a quartz glass rod in terms of manufacturing costs have already been mentioned above. These cost advantages apply equally to the preform made using the quartz glass rod.

The quartz glass rod according to the invention is particularly suitable for producing a preform in which the jacket is formed by external deposition and direct glazing fluorine-doped Si0 ₂ particles is produced using a plasma torch. Such preforms, with a jacket obtained by plasma deposition of fluorine-doped quartz glass particles, are known under the name "Fluosil preforms". These preforms typically have a relatively thin optical jacket, so that the preforms essentially consist of the inexpensive quartz glass rod according to the invention and are therefore also inexpensive to manufacture.

The invention is explained in more detail below using an exemplary embodiment and a drawing. In the drawing show a schematic representation in detail

Figure 1 shows an arrangement of a tube made of porous quartz glass and one

Solid cylinder made of quartz glass - before collapsing into a quartz glass rod in a radial section,

FIG. 2 shows an exemplary embodiment for producing a quartz glass rod on the basis of a flow diagram with individual method steps, and

Figure 3 shows an embodiment for producing a preform

Use of the quartz glass rod according to the invention.

In FIG. 1, the reference number 1 is assigned overall to a coaxial arrangement of a quartz glass tube 2 and a full cylinder 3. The full cylinder 3 consists of undoped quartz glass with a radially and axially homogeneous refractive index curve. The OH content in the entire solid cylinder 3 is below 30 ppb by weight. The diameter is 30 mm.

The quartz glass tube 2 consists of a hollow cylinder made of porous, undoped quartz glass (soot body). The bore diameter of the quartz glass tube 2 is 38 mm, so that in the coaxial arrangement 1 between the bore wall and the solid cylinder 3 there remains an annular gap 4 with a gap width of 4 mm. The outer diameter of the quartz glass tube 2 is 180 mm in the exemplary embodiment.

The method steps for producing the in FIG. 1 shows an arrangement of quartz glass tube 2 and solid cylinder 3 as well as further process steps for the manufacture of a solid cylinder and a quartz glass rod produced therefrom, the reference numerals from FIG. 1 being used to designate the same or equivalent parts of the quartz glass rod.

To carry out the method according to the invention, a solid cylinder 3 and a porous quartz glass tube 2 made of undoped quartz glass are provided. The quartz glass tube 2 is produced by flame hydrolysis of SiCl ₄ and external deposition of SiO ₂ particles on the outer surface of a dome rotating about its longitudinal axis (OVD process). An aluminum oxide tube with a diameter of 38 mm is used as the mandrel. For this purpose, SiCI is hydrolyzed using an oxyhydrogen gas burner to form SiO ₂ particles, which are deposited on the outer surface of the dome by the known method. The quartz glass tube 2 obtained after removal of the dome consists of porous quartz glass.

The full cylinder 3 is produced by flame hydrolysis of SiCI to form SiO ₂ particles and axial deposition of the SiO ₂ particles on the end face of a rotating rod using the VAD method. The solid cylinder 3 thus produced has a diameter of approximately 30 mm.

To manufacture the preform according to the invention, the solid cylinder 3 is coaxially fixed in the axial bore of the quartz glass tube 2 to form the arrangement 1 shown in FIG. The arrangement 1 is subjected to a chlorine treatment in a helium-chlorine atmosphere at a temperature of approximately 1000 ° C. in order to lower the OH content of the quartz glass tube 2 and to clean the surfaces delimiting the annular gap 4. Subsequently, the porous quartz glass tube 2 is melted onto the solid cylinder 3 by heating the arrangement to a temperature of 1400 ° C. in an oven. The annular gap 6 is closed without problems by zone-by-zone heating of the vertically oriented arrangement 1. The contact area between the original full cylinder 3 and the quartz glass tube 2 cannot be seen with the naked eye. In this way, a thick quartz glass rod 3b is obtained inexpensively. This consists of undoped, synthetic quartz glass and is characterized by an axially and radially homogeneous refractive index curve. The refractive index jump in the area of the contact area between the original full cylinder 3 and quartz glass tube 2 is below 0.0002.

The quartz glass rod 3b can be used for a variety of applications. The optional use of the quartz glass rod 3b for producing a preform for a multimode optical fiber is described below with reference to FIG.

The quartz glass rod 3b according to the invention forms the core of the preform. The cladding which contributes to the light guidance (the so-called “optical cladding”) is produced on the quartz glass rod 3b by a plasma OVD process, fluorine-doped SiO ₂ particles being deposited on the lateral surface of the quartz glass rod 3b using a plasma torch. The refractive index of the so manufactured cladding glass is typically about 0.015 to 0.025 lower than the refractive index of undoped quartz glass.

Claims

claims

1. A method for producing a quartz glass rod, comprising providing a full cylinder made of synthetic quartz glass with a predetermined refractive index with a radially and axially homogeneous refractive index curve, characterized in that the full cylinder (3) with a flash tube (2) made of porous quartz glass is overlaid by the Flashing tube (2) is sintered and shrunk onto the full cylinder (3) to form a quartz glass rod, the flashing tube (2) over at least most of its wall thickness - seen radially from the inside out - the specified refractive index with radially homogeneous and axially homogeneous

Refractive index curve, with the proviso that a jump in refractive index of a maximum of 0.001 is generated in the area of a contact surface between the full cylinder (3) and the shrunk-on and sintered flashing tube (2).

2. The method according to claim 1, characterized in that the refractive index jump is a maximum of 0.0003.

3. The method according to claim 1 or 2, characterized in that the refractive index of the porous quartz glass is adjusted by treatment in an atmosphere containing a dopant before sintering to the refractive index of the full cylinder (3).

4. The method according to any one of the preceding claims, characterized in that a flash tube (2) is used with a radially homogeneous refractive index.

5. The method according to any one of the preceding claims, characterized in that the flash tube (2) by a chlorine treatment

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

6. The method according to claim 5, characterized in that the outer diameter of the solid cylinder (3) and the inner diameter of the flash tube (2) are chosen so that during the chlorine treatment between the porous flash tube (2) and the full cylinder (3) used therein Annular gap (4) remains.

7. The method according to any one of the preceding claims, characterized in that a flash tube (2) is provided with a wall thickness in the range of 50 mm to 500 mm.

8. A method for producing a quartz glass rod, characterized in that a quartz glass rod produced according to one of the preceding claims is provided as a solid cylinder (3).

9. quartz glass rod with a radially and axially homogeneous refractive index curve, characterized in that it comprises a solid cylinder (3) made of synthetic quartz glass, which is enveloped by at least one quartz glass layer formed by sintering and shrinking on a flashing tube (2) made of porous quartz glass, and in the region a contact surface between the full cylinder (3) and quartz glass layer has a refractive index jump of at most 0.001.

10. quartz glass rod according to claim 9, characterized in that the refractive index jump is a maximum of 0.0003.

11. quartz glass rod according to claim 9 or 10, characterized in that the OH content of the solid cylinder (3) and the quartz glass layer is less than 1 ppm by weight

12. quartz glass rod according to claim 9 or 10, characterized in that the OH content of the solid cylinder (3) and the quartz glass layer is greater than 100 ppm by weight.

13. quartz glass rod according to one of claims 9 to 12, characterized in that the solid cylinder (3) and the quartz glass layer are doped with fluorine.

14. quartz glass rod according to one of claims 9 to 13, characterized in that the solid cylinder (3) and the quartz glass layer are doped with germanium.

15. quartz glass rod according to one of claims 9 to 14, characterized in that the ratio of the outer diameter of the quartz glass layer and

Solid cylinder (3) is at least 2.5.

16. Preform for a multimode step index optical fiber with a core made of quartz glass and with a sheath radially enveloping the core, characterized in that the core is designed as a quartz glass rod according to one of claims 9 to 15.

17. Preform according to claim 16, characterized in that the jacket is produced by external deposition and direct glazing of fluorine-doped SiO ₂ particles using a plasma torch.