METHOD AND SEMI-FINISHED PRODUCT FOR PRODUCING A MULTICORE FIBRE

Abstract

Methods for producing a multicore fiber comprise a method step in which a component group is reshaped to form the multicore fiber or a pre-form for the multicore fiber, which comprises a hollow cylinder comprising a central bore and a hollow cylinder longitudinal axis, which hollow cylinder comprises a cladding glass region made of cladding glass and a plurality of core glass regions occupied by a core glass, wherein at least part of the central bore is occupied by a glass filling material. In order to provide a method for producing multicore fibers without central signal core, in which the risk of rejects during the completion of the hollow glass cladding cylinder is reduced, a marker element made of marker glass adjacent to the glass filling material is used, which extends along the longitudinal axis of the central bore.

Claims

1. A method for producing a multicore fiber, comprising a method step in which a component group is reshaped to form the multicore fiber or a pre-form for the multicore fiber, which comprises a hollow cylinder comprising a central bore and a hollow cylinder longitudinal axis, which hollow cylinder comprises a cladding glass region made of cladding glass and a plurality of core glass regions provided with a core glass, wherein at least a part of the central bore is occupied by a glass filling rod comprising a filling rod longitudinal axis and a filling rod outer cladding surface, wherein a recess extending in the direction of the filling rod longitudinal axis is produced in or on the filling rod, into which recess a marker element made of marker glass is inserted or which forms the marker element.

2. The method according to claim 1, wherein the marker element extends along the filling rod longitudinal axis and is melted into the recess prior to reshaping to form the pre-form or the multicore fiber.

3. The method according to claim 1, wherein the marker element has a length and in that melting takes place along at least 80% of this length, preferably along at least 90% of this length, completely, in sections or at certain points.

4. The method according to claim 1, wherein melting of the marker element comprises a method step in which the filling rod with the horizontally oriented filling rod longitudinal axis is mounted in such a way that the recess is located on an upper side of the filling rod outer cladding surface, wherein the material of the marker element is heated and softened by means of a heat source.

5. The method according to claim 1, wherein the production of the component group comprises the following method steps: (a) providing the hollow cylinder containing the cladding glass, (b) providing multiple core rods containing the core glass, (c) providing a filling rod comprising a filling rod longitudinal axis and containing the glass filling material, (d) producing the at least one recess on the outer cladding surface of the filling rod, (e) providing the marker element, (f) arranging and melting the marker element into the recess, (g) producing core rod bores extending along the hollow cylinder longitudinal axis, (h) introducing the filling rod and the marker element into the central bore, and (i) introducing the core rods into the core rod bores, forming the component group.

6. The method according to claim 1, wherein the marker element is provided in the form of a cylindrical component or in the form of a layer or mass connected to the filling rod.

7. The method according to claim 1, wherein the recess comprises a bore and/or a longitudinal groove in the outer cladding surface of the filling rod.

8. The method according to claim 1, wherein the marker element forms an air-filled, elongate cavity or in that it contains a marker material which differs in at least one physical and/or chemical property from the cladding glass and from the glass filling material, wherein the property is selected from: refractive index, color, fluorescence, and/or specific glass density.

9. A semi-finished product for producing a multicore fiber, comprising a hollow cylinder comprising a central bore, which hollow cylinder comprises a cladding glass region made of cladding glass and a hollow cylinder longitudinal axis, and a plurality of core glass regions provided with a core glass within the cladding glass region, wherein at least a part of the central bore is occupied by a glass filling rod, which comprises a filling rod longitudinal axis and a filling rod outer cladding surface, wherein the filling rod comprises a recess which extends in the direction of the filling rod longitudinal axis and into which a marker element made of marker glass is inserted, or which forms the marker element, which extends along the central bore longitudinal axis and the filling rod longitudinal axis.

10. The semi-finished product according to claim 9, wherein the marker element has a length and in that it is melted into the recess, completely, in sections or at points, along at least 80% of this length, preferably along at least 90%.

11. The semi-finished product according to claim 9, wherein the marker element comprises a channel filled with a gas.

12. The semi-finished product according to claim 9, wherein the recess comprises a bore and/or a longitudinal groove in the outer cladding surface of the filling rod, and in that the semi-finished product further comprises: the hollow cylinder comprising the central bore, at least two core rods containing the core glass and forming the core glass regions, the filling rod arranged in the central bore, and at least one marker element attached in the recess of the filling rod.

13. The semi-finished product according to claim 9, wherein the marker element is present in the form of a cylindrical component or in the form of a layer or mass connected to the filling rod.

14. The semi-finished product according to claim 9, wherein the marker element forms an air-filled, elongate cavity, or in that it contains a marker material which differs in at least one physical and/or chemical property from the cladding glass, the core glass and the glass filling material, wherein the property is selected from: refractive index, color, fluorescence, and/or specific glass density.

Description

EXEMPLARY EMBODIMENT

[0118] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. In detail, in a schematic representation,

[0119] FIG. 1 shows a cross-section of a hollow glass cladding cylinder with a central bore and through-bores for receiving core rods,

[0120] FIG. 2 shows processing steps (a) to (d) for the production of a filling rod with a marker element for insertion into the central bore of the hollow glass cladding cylinder of FIG. 1,

[0121] FIG. 3 shows the component group of hollow glass cladding cylinder and inserted filling rod including marker element,

[0122] FIG. 4 shows a cross-section of a consolidated pre-form of hollow glass cladding cylinder, inserted filling rod including marker element and core rods inserted into the through-bores, and

[0123] FIG. 5 shows a cross-section of a multicore fiber drawn from the pre-form of FIG. 4.

[0124] FIG. 1 schematically shows a cross-section of a hollow cylinder 1 made of a cladding glass, which serves as a base body for the production of a multicore fiber.

[0125] The hollow cylinder 1 is produced in a known manner using the OVD method. In this method, SiO.sub.2 soot particles are formed by a high-purity SiO.sub.2 starting material, for example silicon tetrachloride, being passed through a deposition burner and supplied to a burner flame in which it is oxidized to solid SiO.sub.2. This is deposited in the form of fine SiO.sub.2 soot particles from the gas phase on the outer cladding surface of a cylindrical deposition mandrel rotating about its longitudinal axis, wherein the deposition burner executes a reversing back and forth movement along the deposition mandrel longitudinal axis. An SiO.sub.2 soot body forms on the outer cladding surface of the deposition mandrel. After completion of the deposition process, the deposition mandrel is removed so that an inner bore 2 remains. The SiO.sub.2 soot body is then vitrified in a furnace under vacuum.

[0126] The resulting hollow cylinder 1 consists of undoped, synthetically produced quartz glass, which forms the cladding glass region 1b. The hollow cylinder has a length of 1500 mm and is adjusted to a nominal outer diameter of 200 mm by cylindrical grinding and to an inner diameter of 42 mm by drilling and honing. Four bores 3 are produced in a predetermined (here quadratic) configuration by mechanical drilling in the direction of the hollow cylinder longitudinal axis 1b, which runs perpendicular to the sheet plane in the illustration of FIG. 1. The bores 3 are used to receive core rods (FIG. 4) and have a diameter of 30 mm. The bores extend through the entire hollow cylinder 1 (through-bores). In an alternative embodiment, the bores are designed as blind bores.

[0127] FIG. 2 schematically shows method steps for producing a filling rod 5. The filling rod 5 shown in FIG. 2(a) preferably consists of the same glass as the hollow cylinder 1, i.e., of undoped quartz glass. The known methods, such as VAD (Vapor Phase Axial Deposition) methods, OVD (Outside Vapor Deposition) methods or MCVD (Modified Chemical Vapor Deposition) methods are suitable for its production. It is used to fill the hollow cylinder inner bore 2 and has a length of about 1500 mm and an initial outer diameter of about 45 mm, which is reduced to about 40 mm by cylindrical grinding. Possible disturbances of the outer cladding surface 5b and bends are eliminated by the cylindrical grinding. Alternatively or additionally, the diameter adaptation and surface improvement is achieved by elongation in a tool-free elongation process. In this illustration, the filling rod longitudinal axis 5a also runs perpendicular to the sheet plane.

[0128] FIG. 2(b) shows that a longitudinal groove 6 has been milled into the outer cladding surface of the filling rod 5. The longitudinal groove 6 extends over the entire length of the filling rod 5. It has a U-shape with a rounded base and straight side walls. Its opening width and depth are each 6 mm. In a subsequent method step, a hollow channel can be produced from the longitudinal groove (6), which forms a marker element within the meaning of the invention. This is explained in more detail below with reference to FIG. 4.

[0129] FIG. 2(c) shows the longitudinal groove 6 with a marker rod 7 inserted therein. The marker rod 7 has a diameter of 5 mm. It consists of synthetically produced quartz glass doped with fluorine and can be obtained commercially under the name F320. Both the viscosity and the refractive index of the fluorine-doped quartz glass of the marker rod 7 are smaller than in the undoped quartz glass from which the hollow cylinder 1 and the filling rod 5 are made. The marker rod 7 is obtained by elongating a starting cylinder made of the F320 quartz glass using a tool-free method. It has a smooth surface produced in the melt flow and is characterized by high dimensional stability, so that it can be inserted into the narrow longitudinal groove 6 without any difficulty and with a precise fit.

[0130] The filling rod 5 and the marker rod 7 are then fused together. The filling rod 5 is mounted with its longitudinal axis 5a oriented horizontally in such a way that the longitudinal groove 6 is located on its upper side. The marker rod 7 inserted into the longitudinal groove 6 is first heated at certain points by means of a burner so that it is fixed in the longitudinal groove at three approximately evenly distributed fixing points, which are located at the ends and in the middle of the marker rod 7 and which are distributed over 95% of its length. It is then heated evenly by means of the burner until the fluorine-doped quartz glass softens and deforms due to its comparatively low viscosity, so that it sinks into the longitudinal groove 6 and fills it up. Due to the surface tension, the surface of the softened glass mass adjacent to the free atmosphere shows a certain bulge, so that a pronounced step between the lateral edges of the longitudinal groove 6 and the outer cladding surface 5b of the filling rod 5 is avoided.

[0131] FIG. 2(d) shows the marker glass mass 8 after softening, deforming, and fusing with the former filling rod 5 and the resulting modified filling rod 5c filled with marker glass mass 8. The glass volume of the former marker rod 7 is matched to the inner volume of the longitudinal groove 6 such that the marker glass mass 8 just completely fills the longitudinal groove 6.

[0132] The modified filling rod 5c filled in this way with the marker glass mass 8 is inserted into the inner bore 2 of the hollow cylinder 1. FIG. 3 schematically shows the component group 9 of hollow cylinder 1, and modified filling rod 5c with the marker glass mass 8.

[0133] Moreover, four core rods 4 made of germanium-doped quartz glass with a length of about 1500 mm and an outer diameter of about 28 mm are produced. Known techniques are also suitable for this purpose, for example the MCVD (Modified Chemical Vapor Deposition) method.

[0134] The core rods 4 are inserted into the bores 3. Subsequently, the component group 9 of hollow glass cladding cylinder 1, modified filling rod 5c, and the core rods 4 is heated, so that the inner bore 2 and the annular gaps around the core rods 4 close and all components of group 9 are fused together.

[0135] FIG. 4 schematically shows the component group fixed in this way of hollow glass cladding cylinder 1, modified filling rod 5c, and the core rods 4, which forms the consolidated pre-form 10. All core rods 4 form separate, circular core glass regions 4a, which are located completely outside of a cladding circle 11, whereas the marker element 8 is located completely within this cladding circle 11.

[0136] The consolidated pre-form 10 is then elongated to form a secondary pre-form. Thereby, the pre-form 10 is held in an elongating device by means of a holder in a vertical alignment of the hollow cylinder longitudinal axis 1a. The secondary pre-form produced in this way is finally drawn in the usual manner in a drawing device to form a multicore fiber 20.

[0137] In this embodiment, the marker element 8 is present as a marker glass mass 8, which has been produced by reshaping the original marker rod 7. In an alternative procedure, the longitudinal groove 6 is unfilled (no rod or tube is inserted) when the component group 9 is consolidated into the pre-form 10, and the longitudinal groove 6 is prevented from collapsing completely by generating and maintaining an overpressure in it. In this way, a cavity is produced which extends along the longitudinal axis 1a and which is present in the multicore fiber as an air-filled hollow channel. The hollow channel can serve as a marker zone, since the refractive index of air differs significantly from that of the cladding glass 1b.

[0138] FIG. 5 schematically shows the cross-section of the multicore fiber 20. Apart from the smaller radial dimensions, this substantially corresponds to the cross-section of the consolidated pre-form 10. The core glass regions (4a) of the former core rods (4) form signal cores 4b, which extend along the fiber longitudinal axis 20a; the former filling rod (5) has become part of the cladding glass region 1b and no longer can be distinguished visually therefrom, and the former marker element (8) forms a marker zone 8a. All signal cores 4b are located completely outside of the cladding circle 11a, whereas the marker zone 8a is located completely within this cladding circle 11a. The marker zone 8a is characterized by a small size, so that it imposes a low tension on the multicore fiber 20 during the fiber drawing process, and, as a result, a low fiber curl occurs.