EP1084085A1

EP1084085A1 - Method and device for drawing an optical fibre from a preform

Info

Publication number: EP1084085A1
Application number: EP99929206A
Authority: EP
Inventors: Hans-Jürgen Lysson; Manuela Hahn; Frank Lisse; Jean-François Bourhis; Marc Jean-Pierre Léon Claude NICOLARDOT; Eric Lainet; Joel Jacques Lupette; André Yves Marie DAGORNE
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel CIT SA; Alcatel Lucent SAS
Priority date: 1998-06-13
Filing date: 1999-06-02
Publication date: 2001-03-21
Also published as: WO1999065834A1; CN1274337A

Abstract

The present invention relates to a furnace for producing an optical fibre (2) from a preform (1), wherein said furnace comprises an inner space (4) with a geometrical axis along which the preform (1) is capable of displacement. The inner space includes a region (10) which extends along said axis and in which the melting temperature of the preform (1) material can be adjusted, wherein said temperature decreases at the axial ends of the inner space (4). This invention is characterised in that the generation of heat in the furnace and/or the emission of heat outside the inner space (4) varies along the axis. This invention further relates to a method for producing optical fibres.

Description

Apparatus and method for drawing an optical fiber from a preform

description

The invention relates to a furnace for producing an optical fiber from a preform, the furnace having an interior with a geometric axis along which the preform is movable and in which a temperature can be set in a region along the axis at which the preform material melts, the temperature dropping to the axial ends of the interior. A method for producing the fiber is also described.

Optical fibers, in particular glass fibers, have a refractive index profile in their interior for guiding the light, which is generally achieved by a combination of different or differently doped materials. The special design of the profile depends on the type of fiber, which can be, for example, a multimode gradient fiber or a single-mode fiber. Since the direct production of the required refractive index profile is impractical due to the small fiber cross section, the fiber is drawn from an approximately cylindrical preform with a radius in the range of a few cm in the prior art. A large number of different processes are known for the production of preforms, for example the inner coating of a quartz glass tube from the gas phase with subsequent collapse (CVD process), the outer coating of a glass core from the gas phase (OVD process), and the axial coating of a substrate from the gas phase (VAD method) or inserting a glass core into a glass tube with subsequent fusion (rod-tube method). One end of the preform is brought into a viscous state by melting, which occurs in the range of approximately 1900 to 2200 K in conventional quartz glasses. In this state, the glass can be drawn out into a thread of small thickness, which forms the optical fiber. Drawing furnaces with a rotationally symmetrical interior are customary for heating the preform, in which the preform is advanced in the axial direction in accordance with the amount of material drawn out as fiber. The melted part of the preform, the so-called drawing onion, is located in the area along the axis of the furnace whose temperature is at a maximum. On the other hand, the temperature drops in the direction of the end of the preform facing away from the drawing bulb and in the direction of the fibers.

The furnace is heated, for example, with a hollow cylindrical resistance element made of graphite, which surrounds the interior and is heated by a direct or alternating current which is usually applied parallel to the axis. Also common are induction furnaces, the interior of which is also surrounded by a tubular element which, for. B. consists of zirconium oxide or graphite. The current flow from the field of a coil surrounding the tube is excited by induction. The heating elements of the furnace are expediently designed as an exchangeable insert. Known drawing yards have a structure which is symmetrical in the axial direction with respect to their area of maximum temperature, in particular with regard to the arrangement and shape of the heating elements. Correspondingly, a symmetrical temperature profile forms in this area, especially on the surface of the wall of the interior, with the onion being in the center of the furnace.

To produce fibers with special properties, such as optical damping or mechanical strength, it is necessary to set a predetermined cooling rate of the fiber and / or geometry of the onion. This may require both rapid cooling of the fiber and keeping the drawing temperature as long as possible. In the prior art, an extension is attached to the drawing furnace, the temperature of which can be adjusted if necessary. However, the possibility of influencing the fiber temperature in this way is limited. In particular, the cooling rate cannot be varied in the immediate vicinity of the onion. In addition, the attachment of an extension requires sufficient installation space at the fiber exit of the drawing furnace, which is often not available when arranged on a fiber drawing tower. Another problem with the manufacture of optical fibers is the increasing diameter of the preforms in order to increase the fiber length that can be produced in one operation. So that the temperatures necessary for drawing are also reached in this case, the power of the furnace heating is increased in the prior art, for example the feed power of the induction coil. As a rule, this requires an exchange of the power supply unit and is therefore expensive. Furthermore, the power loss increases, so that an improved cooling of the furnace is necessary.

Against this background, the invention has set itself the task of developing a drawing furnace and a method for pulling the fibers, which offer improved possibilities for influencing the cooling rate of the fiber and the shape of the onion and enable a low heating power of the furnace.

This object is achieved in that the heat generation of the furnace and / or the heat flow from the interior varies along the axis of the furnace. In an advantageous method for producing an optical fiber from a preform, which is heated to the end in a region of maximum temperature and is drawn to the fiber, the temperature falling in the direction of the longitudinal axis of the fiber on both sides of the region of maximum temperature, the heating power and / or the heat dissipation from the heated area along the fiber longitudinal axis varies and is preferably set asymmetrically to the area of maximum temperature.

The central idea of the invention is to set a defined temperature profile along its axis in the furnace. This makes it possible to influence the cooling rate of the fiber in the oven and to adjust the shape of the onion. For this purpose, the heat generation or the heat flow along the axis of the interior varies, often a combination of both measures is advantageous. In addition, an improved heating efficiency can be achieved by a reduced heat flow, preferably in connection with a specific heat generation in the area of the onion. This means that preforms with a larger diameter, for example in the range of around 10 cm, can be used to pull the fiber without increasing the maximum heating power of the furnace.

In an advantageous embodiment of the invention, an asymmetrical temperature distribution is set in the axial direction, in particular on the wall of the interior. The cooling rate in the interior of the furnace can thus be influenced, the maximum temperature of the furnace generally is unchanged. The area of maximum temperature is preferably arranged displaced to an axial end of the furnace. As a result, an asymmetrical temperature profile is created that drops faster towards the end of the furnace that is closer to the maximum temperature area, while the drop toward the opposite end is slower. Alternatively or additionally, the temperature gradient in the vicinity of the area of maximum temperature can be of different sizes in both axial directions. For example, the temperature may drop from the area of maximum temperature to one end of the furnace predominantly near the edge of the interior, while the drop to the other end occurs continuously over an extensive area of the interior. The temperature profile also changes the geometry of the onion, which depends on the temperature distribution in its environment.

The position of the area of maximum temperature is determined by the temperature of the walls of the interior, that is to say by influencing the local heating power and the heat discharges from the wall. The temperature of individual areas of the preform, the onion and the fiber is mainly influenced by radiative heat transfer from the opposite wall sections. It is therefore possible, in a known manner, to conduct a laminar protective gas flow inside the furnace, which protects the fiber from dirt and the furnace from oxidation. The shielding gas flow has a considerably smaller influence on the temperature of the preform and the fiber than the wall temperature, since its speed depends on the geometric parameters of the furnace and the heat transport in the furnace takes place predominantly by radiation. As a result, the gas flow to protect the fiber and preform from contamination does not result in any significant changes in the set temperature profile.

As a result, the possibilities for varying the cooling rate of the fiber and the geometry of the onion are expanded considerably, so that the fiber properties can be adjusted to a greater extent. In many cases, there is no need to attach an extension to the drawing furnace.

The furnace is preferably heated with an electrically conductive layer, for example made of graphite or zirconium oxide, which surrounds the interior in the radial direction. The conductive layer forms part of the furnace wall or is inserted into the furnace as a tubular element and is preferably exchangeable. A current flow through the layer can be done by applying an external voltage, preferably in the axial direction of the furnace, as well as inductively. It is conceivable that the layer is divided parallel to the direction of the current flow.

In order to vary the heat generation or the heat dissipation, the electrical resistance or the thermal conductivity of the conductive layer expediently changes along the axis of the furnace. Due to the close physical relationships, the two variables generally influence each other. For example, the electrical resistance decreases with increasing layer thickness, while the thermal conductivity increases parallel to the layer surface, in particular in the axial direction of the furnace.

The variation in the thickness of the conductive layer is suitable for setting electrical resistance or thermal conductivity. If the furnace is heated by a voltage applied to the layer, the heat is generated in particular in the areas along its longitudinal axis in which the layer thickness is small. On the other hand, in inductive heating, areas with a greater layer thickness are warmed because the coupling to the electromagnetic field of the induction coil is optimized.

Alternatively or additionally, the material of the conductive layer can vary along the longitudinal axis of the furnace. Different doping or compositions of the material are conceivable for this purpose, for example.

In the area of the maximum temperature of the furnace, a high thermal conductivity of the conductive layer parallel to the walls of the interior is preferred in order to ensure a uniform temperature. In contrast, the thermal conductivity to the axial ends of the interior expediently decreases, so that energy losses due to heat conduction in the axial direction are avoided.

An asymmetrical temperature profile in the interior of the furnace can be generated by the conductive layer being designed asymmetrically to the axial central plane of the furnace. For this purpose, the conductive cross-section of the layer can vary asymmetrically to the central plane, for example by the layer having one or more cross-sectional constrictions on one side outside the central plane. Alternatively, it is conceivable that the local electrical resistance of the layer varies asymmetrically to the central plane, for example due to a doping that varies in the axial direction. In the case of induction heating of the furnace, one or more elements are present which generate a variable, magnetic flux in an electrically conductive layer around the interior. The elements are preferably induction coils operated at high or medium frequency in the furnace wall. Since the local flow in the layer decreases with increasing distance from the generating elements, an asymmetrical temperature profile can be set in the interior by arranging the elements asymmetrically to the central plane of the furnace. It is conceivable that the elements are axially displaceable relative to the interior, so that different temperature profiles can be set in a simple manner by changing their position.

In order to reduce the energy required for heating, the furnace wall generally comprises a layer of insulating material which surrounds the interior with the elements for heating it in the radial direction. If the thermal conductivity of this layer varies along the axis of the furnace, the temperature profile forms on the wall and in the interior of the furnace depending on it. Therefore, instead of or in addition to the measures described above, there is in particular the possibility of forming an asymmetrical temperature profile with respect to the central plane, in that the radial heat flow from the interior is asymmetrical. For example, the thickness of the insulation layer can increase linearly or non-linearly towards an axial end of the interior. It is also conceivable that the specific thermal conductivity of the material of the insulating layer varies in the axial direction of the interior.

In general, both axial end faces of the furnace are provided with a cover which reduces the escape of protective gas and heat losses from its interior and prevents the ingress of dirt. The covers have openings through which the preform can be inserted into the interior and the fiber exits. An asymmetrical temperature profile can be generated in the interior by the covers having different temperatures from one another. In the simplest case, they consist of materials of different thermal conductivity, ie one cover made of a material with good thermal conductivity, such as copper or brass, and the other of poorly conductive material, such as steel. Likewise, a cover for achieving a high temperature of its surface can have a coating or lining with a material with low thermal conductivity, for example ceramic or quartz, or the thickness of such a layer on the two covers can be different. It is advantageous to cover the interior with cooling devices, the output of which can be set differently to generate an asymmetrical temperature profile in the furnace. For example, the amount or inlet temperature of the coolant may differ with liquid-cooled covers. If both covers are flowed through in series by a coolant circuit, the cover temperatures can be set by selecting the direction of flow.

Another way to adjust special properties of the fibers is that the oven has two or more heating zones, i.e. H. Has temperature maxima along its axis. In this case, the conductive layer expediently consists of a corresponding number of sections which are spaced apart from one another in the axial direction of the furnace by material of low thermal conductivity and are heated inductively.

In addition to the measures described above for influencing the temperature professional in the interior of the oven, there is the possibility that the oven has an axial extension which surrounds the fiber radially. In the case of a passive extension without the possibility of temperature control, the cooling of the fiber is delayed after it leaves the oven. If the temperature of the extension can be actively adjusted, for example by heating or cooling, the fiber can be cooled particularly slowly or rapidly after it has left the oven.

In the following part of the description, exemplary embodiments of the invention are explained in more detail with reference to the drawing. It shows a schematic representation of cross sections through different drawing yards.

Fig. 1: Oven in the prior art Fig. 2: Oven with an asymmetrical heating layer Fig. 3: Oven with an asymmetrically arranged induction coil Fig. 4: Oven with asymmetrical insulation.

Fig. 5: Inductively heated oven with optimized heating efficiency

Figures 1 to 5 show different drawing yards, in each of which a preform (1) is heated at the end and drawn into a fiber (2). The furnace is usually arranged on a fiber drawing tower, its axis, along which the fiber (2) runs, being oriented vertically. The furnace is heated by a conductive layer (3) which radially surrounds its approximately cylindrical interior (4). In the layer (3) an electrical current flow is generated by applying an external voltage by means of leads, not shown, or by induction. On the outside, the heated layer (3) is surrounded by insulation (5), which is arranged in the housing (6) of the furnace and reduces heat losses. The interior (4) is closed at its end faces by covers (7, 8), openings in the covers (7, 8) allowing the preform (1) and the fiber (2) to be carried out. Further openings in the covers (7, 8) are conceivable for the entry and exit of a laminar protective gas flow through the interior (4), so that contamination of the surfaces of the preform (1) and fiber (2) is reduced.

The onion (9), d. H. the melted area at the lower end of the preform (1) is in all cases in the area (10) of the interior (4) in which the temperature T is maximum. On the other hand, the temperature T always drops to the axial ends of the furnace in order to avoid premature softening of the preform (1) and to allow the fiber (2) to harden. The schematic course of the temperature T along the interior (4) of the furnace can be seen in each case from the temperature profiles on the right-hand side of FIGS. 1 to 4.

In the prior art (FIG. 1), the furnace has a mirror-symmetrical structure with respect to its central plane (11), which runs perpendicular to the axis of the furnace. A temperature distribution symmetrical to the central plane (11) thus arises in the interior (4), the area (10) of maximum temperature coinciding with the central plane (11). One way of influencing the cooling rate of the fiber (2) is by placing an extension (12), indicated by dashed lines, on the lower cover (8) of the furnace. The insulating effect of the extension (12) delays the temperature decrease on the fiber, as is also shown in dashed lines in the right part of the figures. Heating or cooling of the extension (12) is also conceivable. The variation in the cooling rate of the fiber (2) and thus its material properties that can be achieved in this way is, however, limited.

Considerably expanded possibilities result from the setting of a temperature profile in the interior (4) which is asymmetrical to the central plane (11). Fig. 2 shows a furnace in which for this purpose the layer (3) for heating has a cross-sectional constriction (13), which is arranged below the central plane (11), ie asymmetrically to it. When current flows through the layer (3) parallel to the longitudinal axis of the furnace, the

Cross-sectional constriction (13) heated disproportionately, so that area (10) maximum temperature. The region (10) of maximum temperature which now deviates from the central plane (11) results in an asymmetrical temperature profile with a different gradient of the temperature on both ends of the furnace. In the example shown, the temperature T of the furnace drops to its lower end much more quickly. As in all the exemplary embodiments, the cooling rate of the fiber (2) can also be influenced in this case by placing an extension (12) on the lower cover (8).

In the inductively heated furnace shown in FIG. 3, the electrically conductive layer (3) is heated by a current flow which is generated by an induction coil (14) through which a high-frequency alternating current flows. The layer (3) is to be regarded as a short-circuited secondary winding for the induction coil (14). Since the amount of the inductive flow decreases with increasing distance from the induction coil (14), the induced current is greatest in the area of the layer (3) which is closest to the induction coil (14), that is to say in particular inside the coil. Accordingly, the temperature of the wall (15) of the interior (4) is highest there. By arranging the induction coil (14) asymmetrically to the central plane (11) of the furnace, approximately completely above or below the central plane (11), the asymmetrical temperature profile in the interior (4) can be generated.

Instead of or in addition to the asymmetrical heating of the interior (4) or its wall (15), an asymmetrical temperature profile can also be set by asymmetrical heat flow from the interior (4). Correspondingly, the exemplary embodiment in FIG. 4 has insulation (5), the thickness of which increases linearly from the upper cover (7) towards the lower cover (8). The axial drop in temperature T to the lower cover (8) thus also takes place much more slowly than to the upper cover (7).

The furnace shown in Fig. 5 allows the preform (1) to be heated with minimal power of the induction coil (14). The thickness of the conductive layer (3) varies along the axis of the furnace. In the area of the induction coil (14), the thickness of the layer (3), which preferably consists of graphite, is large in order to improve the thermal conductivity parallel to the walls (15) of the interior (4). This means that a constant, high temperature can be set in this range. In addition, the coupling to the electromagnetic field of the can be adjusted by adjusting the layer thickness

Optimize induction coil (14). In the area of the axial ends of the interior (4) The thickness of the layer (3), on the other hand, is considerably smaller, so that its thermal conductivity in the axial direction is also low. The heat flow along the walls (15) of the interior (4) from the area (10) of maximum temperature to the generally cooled covers (7, 8) is thus considerably reduced. Insulation (5) prevents heat loss in the radial direction. In this way, fibers can be drawn with a comparatively low heating power of the furnace and little cooling of the covers (7, 8).

The result is an oven for drawing an optical fiber, which offers improved heating efficiency and considerably increased possibilities for influencing the cooling rate of the fiber and the geometry of the onion, and thus allows a wide variation in the properties of the optical fiber.

Claims

5 claims

1. Oven for producing an optical fiber (2) from a preform (1), the oven having an interior (4) with a geometric axis along which the preform (1) can be moved and in which in a region (10 ) a temperature can be set along the axis at which the material of the preform (1) melts, the

Temperature drops to the axial ends of the interior (4), characterized in that the heat generation of the furnace and / or the heat flow from the interior (4) varies along the axis of the furnace. 5 2. Oven according to claim 1, characterized in that the temperature distribution of the interior (4) along the axis asymmetrically to the region (10) of maximum temperature is adjustable.

3. Oven according to claim 2, characterized in that the area (10) of maximum o temperature outside the central plane perpendicular to the axis (11) of the

Interior (4) is adjustable.

4. Oven according to one of the preceding claims, characterized in that the interior (4) is surrounded by an electrically conductive layer (3). 5

5. Oven according to claim 4, characterized in that the electrical resistance and / or the thermal conductivity of the conductive layer (3) varies along the axis of the furnace. 0

6. Oven according to claim 4 or 5, characterized in that the thickness of the conductive layer (3) varies.

7. Oven according to one of claims 4 to 6, characterized in that the material of the conductive layer (3) varies. 5

8. Oven according to one of claims 4 to 7, characterized in that the

Thermal conductivity of the conductive layer (3) is greater in the area of the maximum temperature of the furnace than in the area of its axial ends.

9. Oven according to one of claims 4 to 8, characterized in that the conductive layer (3) is asymmetrical to the central plane (11) of the interior (4).

10. Oven according to one of claims 4 to 9, characterized in that the interior (4) is surrounded by an inductively heatable layer (3) and the elements for generating the inductive flow asymmetrically to the central plane (11) of the interior

(4) are arranged.

11. Oven according to one of the preceding claims, characterized in that the interior (4) with the elements for heating it is surrounded by insulation (5), the thermal conductivity of which varies along the axis of the interior (4).

12. Oven according to one of claims 2 to 11, characterized in that the interior (4) is provided on both ends with a cover (7, 8), the thermal conductivity of which deviates from one another.

13. Oven according to claim 12, characterized in that the covers (7, 8) consist of materials of different thermal conductivity or have coatings of different thermal conductivity.

14. Oven according to claim 12 or 13, characterized in that the covers (7, 8) have cooling devices whose cooling capacity can be adjusted differently.

15. Furnace according to one of the preceding claims, characterized in that the furnace has a plurality of temperature maxima along its axis.

16. Oven according to one of the preceding claims, characterized in that the oven has an axial extension (12) which engages around the fiber (2).

17. Oven according to claim 16, characterized in that the temperature of the extension (12) is adjustable.

18. A method for producing an optical fiber (2) from a preform (1) which is heated in a region (10) of maximum temperature at the end to the melt and drawn out to the fiber (2), the temperature being in the direction of the longitudinal axis of the fiber (2) drops on both sides of the area (10) of maximum temperature, characterized in that the heating power and / or the heat dissipation from the heated area varies along the longitudinal fiber axis.

19. The method according to claim 18, characterized in that the heating power along the longitudinal fiber axis and / or the heat dissipation from the heated area is set asymmetrically to the area (10) of maximum temperature.