METHOD AND INTERMEDIATE PRODUCT FOR MANUFACTURING A MULTI-CORE FIBER HAV

Abstract

A known method for producing a multicore fiber with a marker zone or a preform for this type of multicore fiber comprises forming a semi-finished product comprising a glass cladding region made of cladding glass in which a plurality of glass core regions made of core glass are embedded, and drawing the semi-finished product to form a multicore fiber or a preform thereof. The formation of the glass cladding region can be performed using an external deposition method in which a cladding material layer is deposited on the outer surface of a target rod. According to the invention, in order to utilize the cost advantages of the external deposition method while avoiding the disadvantages that arise when forming a central hole with the external deposition method, the target rod comprises a first glass region extending along the longitudinal axis of the target rod and a marker region extending along the longitudinal axis of the target rod and adjacent to the first glass region, the marker region comprising a marker element or providing a hollow channel designed to form or receive a marker element, and the semi-finished product comprises a cladding material layer and a target rod.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a multicore fiber with a marker zone or a preform for such a multicore fiber, comprising forming a semi-finished product comprising a glass cladding region made of a cladding glass in which a plurality of core glass regions made of a core glass are embedded, and at least one marker element, wherein the multicore fiber or preform is obtained by drawing the semi-finished product, and the manufacturing of the glass cladding region comprises the method step of depositing, using an external deposition method, a layer of cladding material on the outer surface of a target rod having a longitudinal axis of the target rod.

[0002] Furthermore, the invention relates to an intermediate product for producing a multicore fiber or a preform thereof.

[0003] In multicore fibers, several light-wave conducting optical core regions (hereafter also called "signal cores") are integrated into a common fiber. The signal cores extend along the longitudinal axis of the fiber. They are surrounded by a cladding material with a lower refractive index, allowing substantially independent light guiding. This fiber design ensures a high signal transmission capacity, since different signals coupled into a single optical fiber can be transmitted simultaneously in each of the spatially separated signal cores. This signal transmission method, also called "spatial multiplexing", can increase the data transmission capacity, especially in optical communications. Multicore fibers are also considered as components of optical fiber sensors in measurement and medical technology, and are considered for illumination and imaging purposes in microscopic or endoscopic devices. [Background technology]

[0004] Multicore fibers are produced by drawing solid preforms or groups of components. They often consist of synthetically produced quartz glass (SiO2), which may be doped or undoped. The production of synthetic quartz glass includes, for example, plasma or CVD deposition methods known under the names OVD, VAD, MCVD, PCVD or FCVD methods. Liquid or gaseous silicon-containing starting materials are subjected to chemical reactions (hydrolysis, pyrolysis or oxidation) and the reaction product, particulate SiO2, is deposited as a solid from the gas phase on a deposition surface. The starting materials are, for example, silicon tetrachloride (SiCl4) or chlorine-free silicon compounds such as polyalkylsiloxanes. The reaction zone is, for example, a furnace, a burner flame or an arc (plasma).

[0005] In the so-called "stack and draw" method, core rods and glass cylinders of different diameters are stacked, which creates a relatively high packing density and has a certain degree of symmetry. The cylindrical components are inserted into a sheath tube and are spatially fixed therein. This group is drawn to form a multicore fiber, or is further processed beforehand to form a preform from which the multicore fiber is drawn. This method is described, for example, in US Pat. No. 6,154,594 (A).

[0006] The "stack and draw" method requires a high level of conditioning effort and is prone to errors in dimensional stability and impurity inclusions due to the large percentage of free component surfaces. Furthermore, due to differences in radial packing density, stretched preforms often have different azimuthal radius values, which need to be compensated for by cylindrical grinding.

[0007] In the method known from US 2015 / 0307387 A1, a preform for a multicore fiber is produced by drilling a through hole in a rod-shaped substrate made of glass and subsequently inserting a core glass rod into the through hole. A hollow glass cladding cylinder in the form of a SiO2-based soot body produced according to the OVD method is used as substrate (SiO2 soot body). In the OVD method, the deposition surface is generally the outer surface of a rod-shaped or tubular deposition mandrel rotating about its longitudinal axis. The substantially cylindrical soot body is deposited by reversing the back and forth movement of the reaction zone. After completion of the deposition process, the deposition mandrel is removed, so that a central intermediate bore remains in the central axis of the cylindrical soot body. A longitudinal bore for receiving the core glass rod is introduced into the hollow glass cladding cylinder. Furthermore, a further longitudinal bore for receiving a marker element in the form of a marker rod is formed by mechanical drilling in the region of the hollow glass cladding cylinder close to the edge.

[0008] When the multi-core fiber is drawn, the marker rod is stretched to form a marker zone, which serves to break the symmetry, especially in the case of a symmetric fiber design, in order to be able to identify the signal cores of the multi-core fiber and to unambiguously assign them with respect to their positions relative to each other and with respect to the fiber central axis. Identification and assignment of the signal cores is necessary, for example, to be able to splice two multi-core fibers together via an end face by conventional splicing methods, which have low attenuation and where the signal cores are correctly assigned. Summary of the Invention [Problem to be solved by the invention]

[0009] The production of hollow glass clad cylinders using external deposition methods, especially based on the OVD method, is cost-effective compared to other manufacturing methods, especially compared to the VAD (vapor axial deposition) method. However, this has the disadvantage that after removal of the deposition mandrel, a central bore remains, which leads to mechanical stresses and asymmetric deformations during collapse and can also destroy the fiber design. Furthermore, this collapse reduces the cross-sectional area of ​​the glass clad section. These disadvantages can be at least partially eliminated by inserting a filling rod that closes the central bore. The filling rod can consist of a glass that has substantially the same refractive index and thermal expansion coefficient as the hollow glass clad cylinder. However, even with this measure, mechanical stresses can occur in the filling rod during collapse, which may require a subsequent tempering of the composite formed by the hollow glass clad cylinder and the filling rod.

[0010] The marker zones additionally inserted in the fiber design should have the smallest possible cross section to counteract unwanted effects such as influence on signal transmission, stresses induced in the fiber, or so-called fiber curl. The degree of curvature over a certain length of fiber is defined as "fiber curl". The curvature is caused by thermal stresses that occur during fiber manufacturing. A large "fiber curl" leads to optical losses due to microbending, making low attenuation splicing of multicore fibers more challenging.

[0011] To minimize these drawbacks, a thin marker zone is required, which requires that the diameter of the channel for receiving the marker element in the plane of the hollow glass-clad cylinder be as small as possible. Typically, the channel diameter in a hollow glass-clad cylinder is less than 15 mm, with a high aspect ratio of more than 65 (the length of the hollow cylinder is about 1 m).

[0012] The dimensionally accurate production and precise alignment of such thin channels in hollow glass-clad cylinders is difficult, even when using high-precision drilling machines. Moreover, it has been shown that cracks increasingly occur in the channel walls during drilling, especially of thin channels. The cost of producing hollow glass-clad cylinders from synthetic quartz glass is high, and the loss is particularly painful when an otherwise completed hollow glass-clad cylinder is rejected due to, among other things, the small bore for receiving the marker element.

[0013] Furthermore, since cutting and splicing of a multicore fiber can be performed at any desired location according to the specific requirements of the intended use, a consistent shape along the entire fiber length must be achieved in a measure-independent manner, which means that the longitudinal axes of the marker element and the hollow glass-clad cylinder must run as parallel as possible in the plane of the hollow glass-clad cylinder.

[0014] It is therefore an object of the present invention to identify a method for manufacturing a multicore fiber having a marker zone, which utilizes the cost advantages of external deposition methods, while reducing the drawbacks and difficulties associated with filling rods and marker elements, and also reduces the risk of rejection.

[0015] Furthermore, it is an object of the present invention to provide an intermediate product suitable for the cost-effective production of multicore fibers with marker elements, which is characterized by a particularly low fiber curl. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] With regard to this method, this object is achieved according to the invention starting from the method mentioned at the beginning, in which the target rod has a first glass region extending along the longitudinal axis of the target rod and a marker region extending along the longitudinal axis of the target rod and adjacent to the first glass region, the marker region containing a marker element or providing a hollow channel designed to form or receive a marker element, and the semi-finished product comprises a cladding material layer and the target rod.

[0017] The multicore fiber is obtained by drawing a preform or semi-finished product, which can be, for example, a primary preform for a multicore fiber or a group of components that are directly drawn to form a multicore fiber or that are further processed to form a preform for a multicore fiber.

[0018] The production of the glass clad region of the semifinished product comprises the method step of producing a clad material layer using an external deposition method, for example a thermal spraying method or a gas phase deposition method.

[0019] In thermal spraying, an oxidizable or slightly oxidizable silicon-containing or silicon dioxide-containing starting powder in the form of a fluid mass such as a flowable SiO2 powder, sol, dispersion or slurry is fed to an energy source, melted therein, and spun at high speed onto the outer surface of a deposition mandrel rotating around its longitudinal axis. The energy source can be, for example, a combustible gas-oxygen flame, a plasma jet, an arc, or a laser beam. In this case, plasma spraying methods are particularly preferred, which allow relatively high energy input and high speed during spin coating of the molten starting powder particles.

[0020] In the vapor deposition method, SiO2 particles are produced by in situ hydrolysis, pyrolysis, or oxidation of silicon-containing precursors, which are deposited as a SiO2-containing layer on the outer surface of a deposition mandrel rotating around a longitudinal axis. Examples include the OVD method (external vapor deposition) and the POD method (plasma external deposition). At a sufficiently high temperature in the region of the deposition mandrel surface, the SiO2 particles are directly vitrified, which is also known as "direct vitrification". In contrast, in the method known as the "soot method", the temperature during deposition of the SiO2 particles is low, resulting in a porous SiO2 soot layer.

[0021] The porous SiO2 soot layer obtained as cladding material layer in the soot process is vitrified in a separate process step to form a glass cladding region made of transparent quartz glass. In the case of external deposition and thermal spray processes with direct glazing, a glass layer is obtained directly as cladding material layer and forms the glass cladding region of the semifinished product or at least part of it.

[0022] The production of the glass clad region of the semi-finished product using external deposition techniques is cost-effective compared to other manufacturing methods.

[0023] In the method according to the invention, a target rod is used as a deposition mandrel for the outside deposition method. After the completion of the outside deposition method, the target rod remains in the deposited cladding material layer, thereby forming part of the semi-finished product for manufacturing a multicore fiber. The target rod consists of a glassy material that is part of the multicore fiber. This is different from other outside deposition methods, which leave a central through-opening in the cladding material layer by removing the deposition mandrel after the completion of the outside deposition method. Subsequent filling of the through-opening with a filling rod is therefore not necessary, and therefore the difficulties involved in filling to ensure straightness, dimensional stability and maximum relief from stress are also eliminated, and the risk of failure as well as adjustments, time and material costs are also reduced.

[0024] The target rod has at least one first glass region and a marker region adjacent thereto. The marker region is formed in or on the target rod. The target rod is thus used to insert a marker element into the workpiece in addition to the target rod material. In addition to the first glass region, the target rod can have a further glass region or multiple glass regions that differ from the first glass region in terms of chemical composition.

[0025] In the target rod, the marker elements form hollow channels that can be filled, for example, with air, a cylindrical element or elements, or a powder bed, or the marker elements are disposed on the outer wall of the target rod, where they are designed as (at least one) cylindrical element or as a coating or mass that adheres to the outer wall. After completion of the external deposition method, the hollow channels can also be filled with the marker material.

[0026] Because the marker element is disposed in or on the target rod, there is no need to adapt the cladding material layer for the purpose of inserting the marker element, for example by machining, in particular by forming a bore in the cladding material layer to receive the marker element, thus eliminating the effort and risk of damage associated with such adaptation of the cladding material layer.

[0027] The dimensional stability and straightness of the target rod is relatively easy to ensure, if necessary, by mechanical round grinding and / or by a drawing process (subjecting a starting cylinder to a drawing process to draw a cylindrical strand therefrom and forming a target rod from the cylindrical strand or cutting a plurality of target rods to length from the cylindrical strand). To avoid damage to the cylindrical strand surface, the drawing process is preferably performed without the use of a forming tool acting on the drawn cylindrical strand.

[0028] Axially parallel alignment of the marker element is facilitated by the marker element being adjacent to the target rod or its first glass region, which straightness is relatively easy to achieve and also facilitates axially parallel alignment of the marker element, particularly in the particularly preferred procedure of placing the marker region between the first glass region and the glass cladding region.

[0029] The component group is thus provided with marker elements without the need to form separate bores in the glass clad region with the associated risks and difficulties explained above, while at the same time ensuring high precision despite the high aspect ratio, which is manifested, for example, in the semi-finished product by deviations in the axial parallelism of the marker elements of less than 0.3 mm / m.

[0030] The target rod may consist of a single glass cylinder or may be composed of multiple glass cylinders connected together. The multiple glass cylinders may have the same composition or may differ from each other. The target rod may consist entirely or partially of cladding and / or core glass, or of separate and / or other glasses.

[0031] The chemical compositions of the first glass region and the adjacent marker region are different from each other. In a multicore fiber, the marker region forms a continuous linear marker zone made of marker material or air. The marker zone can be useful, for example, to break symmetry during splicing and to clearly identify the signal cores and their positions relative to each other and to the fiber central axis.

[0032] In the cladding material layer, a plurality of core rod bores are formed in the usual manner, each for receiving a core rod. At least one core rod bore for receiving a core rod may also be introduced in the target rod.

[0033] The semi-finished product thus produced, with multiple core glass regions, is reshaped and either directly drawn to form a multicore fiber or consolidated to form a preform for a multicore fiber, the consolidation process may involve simultaneous drawing. The "consolidated preform" thus produced is optionally drawn to form a multicore fiber or further processed to form a "secondary preform". Further processing to form a "secondary preform" may include, for example, forming additional bores in the glass cladding regions and coating the bores with core glass or other glasses, or one or more of the following hot forming processes: collapse, collapse, draw, collapse and simultaneously draw additional cladding material. The multicore fiber is drawn from the secondary preform produced by further processing.

[0034] The marker element is preferably provided in the form of a cylindrical component of marker material, or in the form of a layer or mass of marker material connected to the target rod.

[0035] In the case of a marker element comprising at least one cylindrical component, this component runs parallel to the target rod and is connected to the target rod at least locally, preferably over the entire length of the component. The at least one cylindrical marker element component is, for example, a tube, preferably a rod. In the case of a marker element in the form of a tube, the tube wall may comprise a material with a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process, but is maintained as a cavity ("airline") in the finished multicore fiber.

[0036] In the case of a marker element in the form of a layer or mass connected to the target rod, the layer or mass may, for example, be disposed within a hollow channel of the target rod, and is preferably attached to a region of the outer surface of the target rod.

[0037] By attaching the target rod, the marker element benefits, inter alia, from its straightness and alignment, and these properties are transferred to some extent to the marker element, for example, by frictional engagement, integral bonding, and / or form-fitting between the target rod and the marker element.

[0038] The cross-sectional shape of the target rod is generally circular, but may also deviate from circular shapes, such as oval, elliptical, or polygonal, and the enclosing circle that encloses the cross-sectional contour may have a diameter in the range of, for example, 36 mm to 76 mm.

[0039] The target rods are produced, for example, by axial deposition from the gas phase according to the so-called VAD or OVD method, followed by collapse of a central through-opening, or by a pressing method, in which a bed of SiO2 particles is introduced into a mold cavity and pressure is applied to the bed to form a pressed blank, which is then vitrified to form a glass rod.

[0040] In a preferred procedure, the target rod has a recess extending along the longitudinal axis of the target rod, the recess forming or having the marker element disposed within the recess.

[0041] The recess is designed, for example, as a bore through the target rod, preferably as a longitudinal groove (longitudinal channel) on the outer surface of the target rod. The groove-like recess is filled, for example, with a cylindrical component made of marker material or with particulate marker material. The particulate marker material can have a certain dimensional stability due to thermal compression or addition of a binder. The recess thereby ensures a shape fit between the marker element and the target rod. The marker element and the target rod can also be connected to each other beforehand (i.e. before the deposition of the cladding material layer) by integral bonding, for example by sintering or fusion, which is also referred to herein as "integration". Any edges and protrusions can be rounded off during integration. After integration, the marker material preferably fills the recess as completely as possible.

[0042] In this connection, the procedure of providing the target rod with longitudinal grooves is particularly preferred. This is because, on the one hand, longitudinal grooves are particularly easy to produce geometrically accurate on the outer surface of the target rod, for example by milling with a mechanical milling machine or by laser ablation, in comparison with the bore. On the other hand, the longitudinal grooves produced in this way are just as accurate and straight as the target rod itself. Furthermore, the depth or the opening width of the longitudinal grooves can be made substantially as small as desired, for example both less than 15 mm, preferably less than 10 mm, particularly preferably less than 5 mm. During the external deposition method, the longitudinal grooves can be filled with the marker material. Thus, small-volume, geometrically accurate marker elements can be produced in a simple manner, which have an axial deviation in the semi-finished product of less than 0.3 mm / m and thus form correspondingly small and highly accurate marker zones in the multicore fiber. The longitudinal grooves are filled with the marker material, for example, by inserting a cylindrical component (rod or tube) made of the marker material, or by introducing a bed of marker material particles, or by coating the inside of the longitudinal grooves with the marker material. The cylindrical component, internal coating, or floor formed of the marker material may be further secured to the longitudinal groove by fusing.

[0043] In a preferred procedure, the fused composite of the target rods and the marker elements is produced by drawing a preliminary product. The preliminary product is present, for example, as a composite of the target rods and the marker elements or forms a group of target rods and marker element starting bodies that are not connected to each other or are only locally connected to each other. The preliminary product has a larger transverse dimension than the produced fused composite of the target rods and the marker elements. The drawing of the preliminary product reduces, on the one hand, any geometric errors and dimensional deviations, and on the other hand, this drawing contributes to the straightening of the fused composite. The desired fused composite formed from the target rods and the marker elements is cut to a predetermined length from the drawn fused composite strand for use in the production of a multicore fiber. In this way, marker elements with small transverse dimensions can be produced with high dimensional stability, even with particularly large aspect ratios.

[0044] In a particularly preferred method variant, the cladding material layer produced using the external deposition method is formed as a SiO2-based soot layer.

[0045] The SiO2 soot layer is obtained by hydrolysis, pyrolysis or oxidation of silicon-containing starting compounds in an oxygen-containing reaction zone and deposition of the SiO2 particles formed during this process on the outer surface of a rotating target rod, where the temperature in the region of the outer surface of the target rod is kept low so that the deposited cladding material layer does not remain densely vitrified but rather becomes porous. The temperature is, for example, in the range of 800°C to 1250°C, and the specific density is typically around 0.6 g / cm3. 3 ~1.8g / cm 3 The range is.

[0046] The porosity of the SiO2 cladding material layer allows the OH groups contained therein to be subsequently removed by treatment in a dry atmosphere and doping with a gaseous dopant, for example fluorine.

[0047] The soot layer is preferably vitrified by heating to a vitrification temperature.

[0048] Vitrification is carried out, for example, in a sintering furnace under vacuum and / or in a helium atmosphere by heating to temperatures above 1400° C. In this way, a cladding material layer made of transparent quartz glass is obtained.

[0049] In one procedure, prior to vitrification, the soot layer undergoes a doping treatment in an atmosphere containing a dopant and / or a dehydration treatment in a halogen-containing atmosphere or under vacuum.

[0050] The doping or dehydration treatment can be carried out just before the vitrification process or in the same furnace. The dehydration treatment includes, for example, a helium purge followed by a high-temperature chlorination process at a temperature of about 900° C. In the alternative drying under vacuum, the SiO2 soot layer is treated in a vacuum furnace at a pressure of 0.1 mbar or less and at a temperature of at least 1150° C. This reduces the hydroxyl group concentration to less than 1 ppm by weight.

[0051] The doping process may, for example, involve loading a SiO2 soot layer with fluorine, in which the target rod together with the cladding material layer is introduced into a doping furnace and exposed to an atmosphere at a temperature above 980° C., the atmosphere including a fluorine-containing substance, for example silicon tetrachloride, thus loading the SiO2 soot layer with fluorine, for example, allowing an average fluorine content of at least 1500 ppm by weight after consolidation.

[0052] In a preferred method variation, a core rod bore for receiving a core glass rod within the layer of cladding material is formed after vitrification of the soot layer.

[0053] The low density of the cladding material layer made from SiO2 soot compared to high density quartz glass facilitates the formation of bores for receiving the core glass rods. In the cladding material layer, two or more, for example 4 to 7, longitudinal bores (core rod bores) are made in a conventional manner, their longitudinal axes extending parallel to the longitudinal axis of the target rod. The core rod bores are through bores or blind bores and in each case serve to receive at least one core rod made of core glass.

[0054] The composition of the core glass is uniform in the radial direction or varies gradually or stepwise. It differs from that of the cladding glass so as to ensure light guidance in the core glass region. In the simplest case, all the core rods have the same dimensions and consist of the same core glass. However, the core rods may also differ with respect to their dimensions and / or the composition of the corresponding core glass.

[0055] To minimize the risk of damage to the core rod, the core rod is preferably inserted into the core rod bore after vitrification of the porous cladding material layer and formation of the glass cladding region from transparent quartz glass.

[0056] The marker element forms an elongated hollow channel filled with air or preferably comprises a marker material that differs from the adjacent cladding glass and / or adjacent first glass region of the target rod in at least one physical and / or chemical property, preferably selected from refractive index, color, fluorescence, and / or glass specific density.

[0057] The characteristic (or characteristics) that distinguishes the marker element from the glass of the component group, in particular, affects the visual appearance of the marker element and is preferably detectable by an optical sensor. For example, the glass composition of the marker glass, as well as the glass filling material, can be based on quartz glass. The refractive index of the quartz glass can be modified by doping. For example, doping the marker quartz glass with fluorine reduces the refractive index compared to undoped quartz glass. If the marker quartz glass is contaminated with carbon, it can become black. Depending on the oxidation state, if the marker quartz glass is doped with titanium, it will become gray-blue. If the marker quartz glass is doped with rare earth metals or germanium oxide, it will fluoresce at wavelengths specific to the dopant. The specific glass density of the marker element can be modified by pores, which is manifested as a reduction in light transmission compared to glass without bubbles.

[0058] In a preferred procedure, the preparation of the building block group comprises the following method steps: (a) providing a target rod including a marker region; (b) providing a plurality of core rods comprising a core glass; (c) depositing a layer of cladding material on an exterior surface of the target rod using an external deposition technique; (d) forming a core rod bore in at least the cladding material layer and optionally the first glass region of the target rod; (e) inserting a core rod into the core rod bore.

[0059] The semi-finished product thus produced includes a target rod (having at least one marker element and a layer of cladding material formed thereon) and a core rod. The list numbers (a)-(d) do not indicate the order of the method steps. The semi-finished core rod is also referred to herein as a core rod if it has already been fused in the corresponding core rod bore.

[0060] In a variation of the preferred method, the marker region is formed as a hollow channel and a cylindrical marker element is inserted into the hollow channel either before or after the core rod is inserted into the core rod bore.

[0061] The above technical problem is solved by the present invention with respect to an intermediate product for manufacturing a multicore fiber or a preform thereof, the intermediate product comprising a target rod made of glass, a marker element formed on or in the target rod, and a cladding material layer comprising SiO2 soot, the cladding material surrounding the target rod and the marker element and connected to the target rod in a frictional engagement, form-fitting and / or integrally bonded manner.

[0062] The intermediate product according to the invention can appear as such in connection with the manufacture of a multicore fiber or the manufacture of a preform for a multicore fiber on the basis of the method according to the invention, in which application it is a particularly preferred intermediate product.

[0063] The intermediate product is a cylindrical joint composite including a cladding material layer with a central region covered by a glass target rod and formed on or in at least one marker element. The cladding material layer is present as a porous soot layer based on SiO2. It is produced by an outside deposition method (OVD method) and forms a glass cladding region made of cladding glass in the final multicore fiber.

[0064] The target rod extends along a longitudinal axis of the cylinder and defines a central volume region of the cylinder. The central volume region also includes at least one marker element adjacent to the first glass region of the target rod.

[0065] In embodiments where the chemical composition of the first glass region of the target rod corresponds to the chemical composition of the cladding glass of the multicore fiber, the corresponding volumetric region in the multicore fiber forms a part of the optical cladding. In embodiments where the chemical composition of the first glass region of the target rod and the chemical composition of the cladding glass of the multicore fiber are different from each other, additional functions can be added to the target rod in the multicore fiber, for example, the target rod can act as a "stress zone" that creates and / or compensates for compressive or tensile stresses acting radially in the fiber.

[0066] The target rod may be monolithic or may consist of multiple parallel cylinders. In addition to the first glass region, the target rod may have further glass regions or multiple glass regions that differ from the first glass region in terms of chemical composition. In particular, the target rod may include at least one core glass region, which will form the signal core in the final multicore fiber.

[0067] The target rod is used to provide, in addition to the first glass region, a marker element made of the marker glass, which is present in the intermediate product, for example, as an elongated cavity or as an integral or non-integral component made of the marker material or as a coating of such a component with the marker material, to form a continuous linear marker zone made of the marker material or an air-filled hollow channel in the multicore fiber.

[0068] The marker element is located in or on the target rod. It is, for example, located in a hollow channel extending parallel to the longitudinal axis of the cylinder or attached to the outer surface of the target rod. Since the marker element is located in or on the target rod but not completely in the cladding material layer produced by the external deposition method, this cladding material layer does not have to be adapted for the purpose of inserting the marker element, for example by machining, in particular by forming a separate bore in the cladding material layer for receiving the marker element. Thus, the risk of damage associated with such adaptation of the cladding material layer is eliminated.

[0069] The dimensional stability and straightness of the target rod can be ensured by simple measures, such as machining and / or drawing processes of the target rod. Machining can optionally be an external machining method, which is generally much less complicated than an internal machining method.

[0070] The marker element is adjacent to the target rod, which facilitates axially parallel alignment of the marker element. Straightness of the target rod is relatively easy to achieve, which also facilitates axially parallel alignment of the marker element. This is especially true in the particularly preferred embodiment where the marker element is located between the first glass region and the glass cladding region of the target rod.

[0071] Thus, the intermediate product comprises marker elements, for which there is no need to form separate bore regions in the cladding material layer associated with the risks and difficulties explained above, and at the same time, despite the high aspect ratio, a high degree of precision can be ensured, as manifested, for example, by the deviation of the axial parallelism of the marker elements in the intermediate product being less than 0.3 mm / m.

[0072] The target rod can consist of a single glass cylinder or can be made up of several glass cylinders connected to each other. The several glass cylinders can have the same composition or can be different from each other. The target rod can consist completely or partially of cladding glass or core glass or another glass. The target rod can have, for example, a central core made of core glass, which is surrounded by a cladding region made of cladding glass.

[0073] The marker elements may, for example, be present as rods extending parallel to the target rod.

[0074] In further embodiments, the marker element is present as a layer of marker material attached within a hollow channel of the target rod or within a region of the outer surface of the target rod, Again, the marker element layer and the target rod can optionally be present in an integrated, i.e. fused, form with one another.

[0075] The marker elements extend along the longitudinal axis of the target rod, preferably along its entire length, and may be locally, but preferably continuously, attached to the target rod.

[0076] By attaching to the target rod, the marker element benefits from its straightness and alignment, and these properties are transferred to some extent to the marker element, for example, by frictional engagement, integral bonding, and / or form-fitting between the target rod and the marker element.

[0077] The marker element is preferably designed in the form of a cylindrical component of the marker material or in the form of a layer or mass of the marker material connected to the target rod. At least one cylindrical marker element component is, for example, a tube, preferably a rod. In the case of a marker element in the form of a tube, the tube wall can comprise a material with a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process, but remains as a cavity ("airline") in the finished multicore fiber.

[0078] The cross-sectional shape of the target rod is generally circular, but may also deviate from circular shapes, such as oval, elliptical, or polygonal, and the enclosing circle that encloses the cross-sectional contour may have a diameter in the range of, for example, 36 mm to 76 mm.

[0079] In a preferred embodiment, the target rod has a recess extending along the longitudinal axis of the target rod, the recess forming or having the marker element disposed within the recess.

[0080] The recess is preferably designed as a bore through the target rod or as a longitudinal groove (longitudinal channel) on the outer surface of the target rod. It forms a hollow channel filled with air or is filled, for example, with a cylindrical component made of marker material or with particulate marker material. The particulate marker material can have a certain degree of dimensional stability due to partial compression or the addition of a binder. The marker material fills the recess as completely as possible. Thereby, the recess ensures a shape match between the marker element and the target rod. The marker element and the target rod can also be connected to each other in advance (i.e. before the production of the cladding material layer) by integral bonding, for example by sintering or fusing together.

[0081] In this connection, the embodiment in which the target rod is provided with longitudinal grooves is particularly preferred. This is because, on the one hand, longitudinal grooves are particularly easy to manufacture on the outer surface of the target rod, for example by milling with a mechanical milling machine or by laser ablation, in comparison with the bore. On the other hand, the longitudinal grooves thus manufactured are just as accurate and straight as the target rod itself. Furthermore, the depth or the opening width of the longitudinal grooves can be substantially as small as desired, for example both less than 15 mm, preferably less than 10 mm, particularly preferably less than 5 mm. This allows the manufacture in a simple manner of small-volume, geometrically accurate marker elements, which have an axial deviation in the intermediate product of less than 0.3 mm / m, thus forming correspondingly small and highly accurate marker zones in the multicore fiber.

[0082] The longitudinal grooves are filled with the marker material, for example, by inserting a cylindrical component (rod or tube) made of the marker material, or by introducing a bed of marker material particles, or by coating the interior of the longitudinal groove with the marker material. The cylindrical component, the internal coating, or the bed formed of the marker material can further be fixed to the longitudinal groove by fusing.

[0083] The marker element forms an elongated hollow space (channel) or channels filled with air or preferably comprises a marker material having at least one physical and / or chemical property different from the adjacent first glass region of the target rod and / or the cladding glass obtained by vitrification of the cladding material layer of the intermediate product, the property being selected from refractive index, color, fluorescence, and / or glass specific density.

[0084] Based on the method according to the invention or using the intermediate body according to the invention, a multicore fiber is obtained having a plurality of signal cores and crossed by at least one continuous linear marker zone, which serves to break the symmetry and to clearly identify the signal cores and their position relative to each other and relative to the fiber central axis.

[0085] Definitions and Measurement Methods The individual terms in the above description are further defined below. The definitions are part of the description of the present invention. For terms and measurement methods not specifically defined herein, the interpretations given by the International Telecommunication Union (ITU) apply. In the event of a discrepancy between one of the definitions below and the rest of this specification, the statements made elsewhere in this specification shall prevail.

[0086] Clad Glass / Glass Clad Area The glass cladding region includes a cladding glass. At least a portion of the glass cladding region is manufactured by an external deposition method. A core glass region designed for signal transmission is formed within the glass cladding region. The cladding glass may, for example, consist of undoped silica glass or include at least one dopant that reduces the refractive index of the silica glass. Fluorine and boron are dopants that can reduce the refractive index of the silica glass.

[0087] Core rod / core glass area The core rods comprise a core glass having a radially uniform or non-uniform refractive index profile. The core glass of each core rod forms a core glass region. The core rod may comprise a region made of a core glass having a relatively high refractive index and at least one further region made of another glass having a relatively low refractive index, for example quartz glass doped with fluorine and / or chlorine. The glass with the highest refractive index is generally located at the central axis of the core rod. It consists, for example, of quartz glass to which at least one dopant is added to increase the refractive index. In a multicore fiber, the core rod forms at least one signal core, in which the signal to be transmitted is mainly transmitted. The signal core may be adjacent to other glass regions having a smaller refractive index, also provided by the core rod.

[0088] Target Rod The target rod comprises at least one first glass region and a marker region formed in or on the target rod adjacent to the at least one first glass region. In addition to the first glass region, the target rod may comprise a further glass region or regions that differ from the first glass region in terms of chemical composition. The target rod is made of a glass that is part of a multicore fiber. In particular, the target rod may comprise a glass region that serves as a signal core in the final multicore fiber. The composition of the first glass region may correspond to or deviate from the composition of the cladding glass in order to impart additional properties to the multicore fiber.

[0089] The target rod serves as a deposition mandrel for carrying out an external deposition process to produce a cladding material layer from the target rod. The cladding material layer consists of quartz glass or is completely or partially present as a SiO2 soot layer. The target rod thus serves to insert marker elements in the region of the cladding material layer close to the axis in addition to the target rod material.

[0090] Marker elements / marker materials / marker glass The marker element comprises air and / or a marker material, in particular at least one marker glass, the chemical composition of which is different from that of the first target rod glass region to which it is adjacent and / or the density of the marker material is different from that of the cladding glass and the first target rod glass region. The marker element is present in the workpiece as a component or as a layer or mass on the component and forms an optically detectable marker zone in the multicore fiber.

[0091] Component Group / Integrated Preform / Secondary Preform / Semi-finished Product / Intermediate Product A "component group" includes a target rod with at least one marker element and a cladding material layer deposited on the target rod with a core rod bore, into which one core rod is inserted in each case. A "preform", also called "consolidated preform" in this specification, is obtained by fixing the core rod in the core rod bore, for example by narrowing the hollow glass cladding cylinder end or by collapsing and fusing. The component group or the (consolidated) preform is drawn to form a "secondary preform" or to directly form a multicore fiber. The term "semi-finished product" here encompasses the component group, the consolidated preform and the secondary preform. The cylindrical bonded composite is called an intermediate product, which is the glass target rod, the marker element and the SiO2 soot-containing cladding material layer surrounding the target rod and the marker element.

[0092] Quartz glass Here, quartz glass is a highly siliceous glass with an SiO2 content of at least 80 mol%, preferably at least 90 mol%. The quartz glass is undoped or contains one or more dopants. The quartz glass is, for example, a melt product from naturally occurring SiO2 raw materials (natural quartz glass) or is synthetically produced (synthetic quartz glass) or consists of a mixture of these quartz glass types. Synthetic transparent quartz glass is obtained, for example, by flame hydrolysis or oxidation of synthetically produced silicon compounds, by polycondensation of organic silicon compounds by the so-called sol-gel method, or by hydrolysis and precipitation of inorganic silicon compounds in liquids.

[0093] Integration / fusion / vitrification / disintegration When referring to glass components or SiO2 masses, fusion is understood to mean that the components or SiO2 masses melt together on the contact surfaces. Fusion is performed by heating the components or SiO2 masses at least in the region of the contact surfaces with a heat source such as a furnace, burner or laser. During collapse, the gaps between the components close. Vitrification refers to a hot process to convert the porous soot material into a dense glass. Consolidation can include fusion, vitrification and collapse processes. The result is a thermally compacted and manageable semi-finished product such as a preform or a group of components firmly joined by fusion.

[0094] Position display:Top / Bottom These designations relate to positions during the drawing process and / or the fiber drawing process: "bottom" indicates a position in the direction of the drawing process, and "top" indicates a position opposite to the direction of the drawing process.

[0095] cross section Cross sections were taken perpendicular to the longitudinal / longitudinal axis.

[0096] Longitudinal Section Sections were taken parallel to the longitudinal / longitudinal axis.

[0097] Boa The terms "bore", "central bore", "internal bore" or "longitudinal bore" mean a hole having a cylindrical shape or any other internal geometric shape, which may be produced, for example, by a drilling process or by depositing a layer of material on the exterior surface of a mandrel by a deposition or pressing process and then removing the mandrel.

[0098] Axial parallel alignment / Axial parallelism The reference axis is in each case the longitudinal axis of the semi-finished product, the preform, or the central axis of the multicore fiber. [Brief description of the drawings]

[0099] Exemplary embodiments The invention will now be explained in more detail with reference to exemplary embodiments and to the drawings, which in particular show in a schematic diagram: [Figure 1] FIG. 1 shows a cross-sectional view of a semi-finished product for producing a multicore fiber having a hollow glass cladding cylinder, a core rod, and a marker element according to the prior art. [Diagram 2] FIG. 2 shows processing steps (a)-(d) in a variation of the first method for producing a target rod having marker elements for use as a deposition mandrel in an external deposition process. [Diagram 3] FIG. 3 shows processing steps (a)-(b) of a variant of the second method for producing a target rod having marker elements for use as a deposition mandrel in an external deposition process. [Figure 4] FIG. 4 illustrates the deposition of a soot layer on a target rod containing the marker element of FIG. [Diagram 5] FIG. 5 shows a component composite on a target rod including a marker element and a vitrified soot layer. [Figure 6] FIG. 6 shows a cross section of the composite component of FIG. 5 after forming the longitudinal bore. [Figure 7] FIG. 7 shows a consolidated preform of the component composite of FIG. 6 with a core rod inserted and fused into the longitudinal bore. [Figure 8] FIG. 8 shows a further embodiment of the consolidated preform in cross section.

[0100] FIG. 1 shows a schematic cross section of a hollow cylinder 1 made of cladding glass according to the prior art, which serves as a substrate for the manufacture of a multicore fiber. The hollow cylinder 10 is manufactured in a known manner using the OVD method, in which SiO2 soot particles are deposited from the gas phase onto the outer surface of a cylindrical deposition mandrel rotating around its longitudinal axis, so that a SiO2 soot body is formed on the outer surface of the deposition mandrel. After completion of the deposition process, the deposition mandrel is removed, leaving an internal bore 20. The SiO2 soot body is then vitrified to form the hollow cylinder 10. Four longitudinal bores 40, each for receiving a core rod 30, and a further smaller longitudinal bore 60 for receiving a marker rod 70 are introduced in the wall of the hollow cylinder 10. During the integration of this component group, a filling rod 80 is inserted into the central bore 20, which is, for example, made of the same material as the hollow cylinder cladding glass.

[0101] The manufacture of multiple longitudinal bores (40, 60) in the hollow cylinder cladding glass entails considerable effort and risk of failure. In particular when manufacturing the smaller longitudinal bores 60 for receiving the marker rods 70, unacceptable deviations from axial parallelism can occur, leading to cracks in the inner walls of the bores, which can lead to the breakage of the hollow cylinder 1 manufactured in a complex manner. The filling and collapse of the central bore 20 as well as the custom manufacture of the filling rods 80 also entails effort and risk of failure and can easily lead to dimensional deviations. These drawbacks are avoided in the method according to the invention, which will be described below with reference to figures 2 to 8.

[0102] 2 and 3 show diagrammatically the method steps for manufacturing a target rod 1 with marker elements 5 or 5a.

[0103] The target rod 1, shown diagrammatically in FIG. 2a, consists of synthetically produced undoped quartz glass, commercially available under the name F300. Known techniques are suitable for this purpose, for example the VAD (vapor axial deposition), OVD (external vapor deposition), MCVD (modified chemical vapor deposition) or powder pressing methods. It serves as a deposition mandrel in the OVD external deposition method, which is further described below with reference to FIG. 4. The target rod 1 is about 1800 mm long and has an outer diameter of about 42 mm set by circular grinding. The circular grinding eliminates possible disturbances and bends in the outer surface 2. Alternatively or additionally, the adaptation of the diameter and the improvement of the surface are achieved by drawing in a tool-free drawing process.

[0104] FIG. 2b shows that longitudinal grooves 3 are milled into the outer surface of the target rod 1. The longitudinal grooves 3 extend over the entire length of the target rod 1. They have a U-shape with a rounded lower part and straight side walls. Their opening width and depth are in each case 6 mm. In a subsequent method step, hollow channels can be produced from the longitudinal grooves 3, which form marker elements within the meaning of the present invention.

[0105] FIG. 2c shows the insertion of a marker rod 4 into the longitudinal groove 3. The marker rod 4 has a diameter of 5 mm. It consists of synthetically produced quartz glass doped with fluorine and is commercially available under the name F320. Both the viscosity and the refractive index of the fluorine-doped quartz glass of the marker rod 4 are smaller than those of the undoped quartz glass constituting the target rod 1. The marker rod 4 is obtained by drawing a starting cylinder made of F320 quartz glass in a tool-free manner. It has a smooth surface formed in the molten mass and is characterized by high dimensional stability and straightness, and can therefore be precisely fitted and inserted into the narrow longitudinal groove 3 without difficulty.

[0106] The marker rod 4 inserted in the longitudinal groove 3 is heated over its entire length by a burner, so that the fluorine-doped quartz glass softens and deforms due to its relatively low viscosity. Figure 2d shows the marker glass mass 5 after softening, deformation and fusion with the target rod 1. The glass volume of the original marker rod 4 matches the internal volume of the longitudinal groove 3, so that the marker glass mass 5 just completely fills the longitudinal groove 3.

[0107] In the alternative procedure shown in Fig. 3, on the one hand the production of the longitudinal grooves is omitted and on the other hand a composite of target rod 1 and marker element 5a is produced on the basis of an upstream drawing process. As shown in Fig. 3a, the starting or preliminary product for the drawing process is on the one hand a target rod starting cylinder 1a made of undoped quartz glass (F300) with a diameter of 63 mm and a length of 800 mm and a non-circular starting marker rod 4a whose outer peripheral edge is flat or slightly concave (inwardly curved) in cross section. The starting marker rod 4a is made of fluorine-doped quartz glass (F320) and has a diameter of 8 mm. The starting marker rod 4a is fused to the starting target rod 1a in parallel axial alignment, with the aforementioned peripheral edge resting on the outer surface 2a of said target rod. During drawing, the composite of starting target rod 1a and marker rod 4a is drawn to a length of 1800 mm. In this case, the starting cylinders (1a, 4a) are further fused to each other and the low-viscosity quartz glass of the marker rod 4a flows slightly onto the target rod side 2 and, after cooling, forms a flat glass bead 5a fixedly connected to the outer surface 2 of the target rod 1. This is shown diagrammatically in Fig. 3b. The drawing process, on the one hand, reduces any geometrical errors and dimensional deviations of the starting cylinders (1a, 4a) and, on the other hand, directs the final molten composite (1, 5a) straight forward.

[0108] FIG. 4 shows diagrammatically the use of the target rod 1 thus prepared and filled with marker glass chunks 5 as a deposition mandrel in the OVD external deposition process. Here, a high-purity SiO2 starting material, for example silicon tetrachloride, is fed to a deposition burner 9 and a burner flame 8 and converted into solid SiO2 particles 11. These SiO2 particles 11 are deposited from the gas phase onto the outer surface 2 of the target rod 1 rotating about its longitudinal axis 1b (directional arrow R), while the deposition burner 9 performs an inverse back and forth movement along the longitudinal axis 1b of the target rod (which in the cross-sectional view of FIG. 4 extends perpendicular to the sheet plane). A SiO2 soot layer 17 is formed on the outer surface 2 of the target rod 1. The intermediate product 18 obtained after the completion of the OVD external deposition process comprises a cylindrical joint composite formed from the target rod 1, the marker glass chunks 5 and the SiO2 soot layer 17 surrounding the target rod 1 and the marker glass chunks 5.

[0109] The intermediate product 17 is dehydrated in a chlorine-containing atmosphere at a temperature of 850° C., and immediately thereafter the SiO2 soot layer 17 is vitrified in vacuum at a temperature of 1450° C.

[0110] FIG. 5 shows the composite 15 subsequently obtained and formed from the target rod 1, which includes the marker glass mass 5 and the glass cladding region 12 formed from undoped synthetic quartz glass and obtained after integration of the SiO2 soot layer. The dashed circular line 12c represents the circumference of the target rod 1. The outer diameter of the composite 15 is set to a value of 200 mm by external round grinding. The glass cladding region 12 extends along the outer surface 2 of the target rod 1 and has a usable length, excluding the round end caps, of about 1500 mm.

[0111] Figure 6 shows a composite 15 formed from the glass cladding layer 12 and the prepared target rod 1 after four bores 13 have been formed in a predetermined (here rectangular) configuration in the glass cladding region 12 in the direction of the longitudinal axis 1b of the target rod by mechanical drilling. The bores 13 serve to receive the core rod 14 (Figure 7) and have a diameter of 30 mm. The bores 13 extend over the entire usable length of the glass cladding region 12 (through bores). In an alternative embodiment, the bores are designed as blind bores.

[0112] Furthermore, four core rods 14 made of Ge-doped quartz glass are manufactured, each having a length of approximately 1500 mm and an outer diameter of approximately 28 mm. Known techniques, such as the MCVD method (Improved Chemical Vapor Deposition), are also suitable for this purpose.

[0113] The core rod 14 is inserted into the bore 12. The component group formed from the composite 15 and the core rod 14 is then heated, so that the annular gap around the core rod 14 is closed and all the components of the group are fused together. Figure 7 shows diagrammatically a preform 16 thus consolidated, which is made up of the composite 15 of the target rod 1, the marker glass mass 5, the glass cladding region 12 and also the core rod 14.

[0114] The consolidated preform 16 is then drawn to form a secondary preform, in which the preform 16 is held in a drawing apparatus by a holder with the longitudinal axis 1b of the target rod aligned vertically. The secondary preform thus produced is finally drawn in a drawing apparatus in a conventional manner to form a multicore fiber.

[0115] In this exemplary embodiment, the marker element is present as a marker glass mass 5 produced by reshaping the original marker rod 4 in the longitudinal groove 3. In an alternative procedure, a capillary is inserted into the longitudinal groove 3 during the outside deposition method. During the subsequent integration process, a complete collapse of the capillary is prevented by creating and maintaining an overpressure in it. In this way, a cavity is formed that extends along the longitudinal axis 1b and is present in the multicore fiber as a hollow channel filled with air. Since the refractive index of air is significantly different from that of the cladding glass, the hollow channel can function as a marker zone.

[0116] Except for the smaller radial dimension, the cross section of the multicore fiber substantially corresponds to the cross section of the consolidated preform 16. The core glass region (14) of the original core rod forms the signal core extending along the longitudinal axis of the fiber, the original target rod (1) forms part of the glass cladding region, and the original marker element (5) forms a visually easily detectable marker zone. The marker zone (5) is characterized by its small size and therefore results in low stress on the multicore fiber during the fiber drawing process, resulting in low fiber curl.

[0117] In contrast to Fig. 7, in the embodiment of the consolidated preform 26 for a multicore fiber shown in Fig. 8, the center of the target rod 1 is occupied by a core rod 14a. The bore for receiving the core rod 14a is formed in and along the longitudinal axis 1b of the target rod in one operation together with the other core rod bores 13 (Fig. 6) after vitrification of the soot layer. All the core rods 14, 14a have the same diameter. The original target rod 1 thus has a core glass region, which is surrounded by a glass cladding region adjacent to the marker zone 5.

Claims

1. 1. A method for manufacturing a multicore fiber having a marker zone or a preform for such a multicore fiber, comprising forming a semi-finished product comprising a glass cladding region made of a cladding glass in which a plurality of core glass regions made of a core glass are embedded, and at least one marker element, wherein the multicore fiber or the preform is obtained by drawing the semi-finished product, and the manufacturing of the glass cladding region comprises a method step of depositing, using an external deposition method, a cladding material layer on an outer surface of a target rod having a longitudinal axis of the target rod, the target rod comprising a first glass region extending along the longitudinal axis of the target rod and a marker region extending along the longitudinal axis of the target rod and adjacent to the first glass region, the marker region comprising the marker element or providing a hollow channel designed to form or receive the marker element, and wherein the semi-finished product comprises the cladding material layer and the target rod.

2. The method of claim 1 , wherein the marker region is disposed between the first glass region and the glass cladding region.

3. 3. The method of claim 1 or 2, characterized in that the marker element forms at least one cylindrical component or layer or mass connected to the target rod.

4. 4. The method of claim 3, wherein the target rod includes a recess extending along a longitudinal axis of the target rod, the recess forming the marker element or the marker element being disposed within the recess.

5. The method of claim 4 , wherein the recess comprises a longitudinal groove in the outer surface of the target rod.

6. The method according to any one of claims 3 to 5, characterized in that the marker element is attached to the target rod before carrying out the external deposition method.

7. The cladding material layer produced using the external deposition method is preferably SiO ２ 7. The method according to claim 1, wherein the soot layer is formed based on .

8. 8. The method of claim 7, wherein the soot layer is vitrified by heating to a vitrification temperature.

9. 9. The method according to claim 8, characterized in that, prior to vitrification, the soot layer is subjected to a doping treatment in an atmosphere containing at least one dopant and / or a dehydration treatment in a halogen-containing atmosphere or under vacuum.

10. 9. A method according to claim 7 or 8, characterized in that a core rod bore for receiving a core glass rod is produced after vitrification of the soot layer.

11. 11. The method according to claim 1, characterized in that the marker element is designed as a hollow channel or comprises a marker material that differs from the adjacent cladding glass and / or the adjacent first glass region of the target rod in at least one physical and / or chemical property selected from refractive index, color, fluorescence and / or glass specific density.

12. The production of said semi-finished product comprises the following method steps: (a) providing the target rod including the marker region; (b) providing a plurality of core rods comprising a core glass; (c) depositing the layer of cladding material on the outer surface of the target rod using the outside deposition method; (d) forming a core rod bore in at least the layer of cladding material and optionally the first glass region of the target rod; The method of any one or more of the preceding claims, comprising the step of: (e) inserting the core rod into the core rod bore.

13. 13. The method according to claim 12, characterized in that the marker area is designed as a hollow channel and that a cylindrical marker element is inserted into the hollow channel before or after the insertion of the core rod.

14. An intermediate product for manufacturing a multicore fiber or a preform thereof, comprising a glass target rod, a marker element formed on or within the target rod, and a SiO ２ a cladding material layer comprising soot, said layer surrounding said target rod and said marker element and connected to said target rod in a frictional engagement, form fitting and / or integrally bonded manner.

15. 15. The intermediate product of claim 14, characterized in that the target rod comprises a first glass region adjacent to which the marker element is preferably disposed between the first glass region and the cladding material layer, and very preferably at least partially filling a recess in an outer surface of the target rod.

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