METHOD AND INTERMEDIATE PRODUCT FOR PRODUCING A MULTI-CORE FIBER WITH A MARKER

Description

Technical background

[0001] The present invention relates to a method for producing a multi-core fiber with a marker zone or a preform for such a multi-core fiber, comprising the formation of a semi-finished product which has a cladding glass region made of a cladding glass with a plurality of core glass regions made of a core glass embedded therein and at least one marker element, wherein the multi-core fiber or the preform is obtained by elongating the semi-finished product, and wherein the production of the cladding glass region comprises a method step in which a cladding material layer is deposited on an outer cladding of a target rod having a target rod longitudinal axis using an external deposition method, wherein the target rod has a first glass region extending along the target rod longitudinal axis and wherein the semi-finished product comprises the cladding material layer and the target rod.

[0002] Furthermore, the invention relates to an intermediate product for producing a multi-core fiber or a preform therefor, comprising a target rod made of glass, a marker element and a cladding material layer containing SiO 2 soot, which surrounds the target rod and the marker element and which is connected to the target rod in a frictional, positive and / or materially locking manner.

[0003] In multi-core fibers, several optical waveguides (hereinafter also referred to as "signal cores") are integrated into a single fiber. The signal cores extend along the fiber's longitudinal axis. They are surrounded by cladding material with a lower refractive index, enabling essentially independent light guidance. This fiber design promises high signal transmission capacity because, when combined in a single optical fiber, different signals can be transmitted simultaneously in each of the spatially separated signal cores. This method of signal transmission is also referred to as "spatial multiplexing," which can particularly increase data transmission capacity in optical telecommunications. Multi-core fibers are also considered components of fiber optic sensors in measurement and medical technology, as well as for illumination and imaging purposes in microscopic or endoscopic devices. State of the art

[0004] Multicore fibers are produced by elongating a solid preform or an assembly of components. These are often made of synthetically produced quartz glass (SiO 2 ), which can be doped or undoped. The production of synthetic quartz glass includes, for example, plasma or CVD deposition methods known as OVD, VAD, MCVD, PCVD, or FCVD processes. In these processes, a liquid or gaseous silicon-containing starting material is subjected to a chemical reaction (hydrolysis, pyrolysis, or oxidation), and the reaction product—particulate SiO 2—is deposited as a solid from the gas phase onto a deposition surface. The starting material is, for example, silicon tetrachloride (SiCl 4 ) or a chlorine-free silicon compound such as a polyalkylsiloxane. The reaction zone is, for example, a furnace, a burner flame, or an electric arc (plasma).

[0005] In the so-called "stack-and-draw" process, core rods and glass cylinders of different diameters are stacked together to achieve a relatively high packing density and a certain degree of symmetry. The cylindrical components are inserted into a cladding tube and spatially fixed therein. This assembly is drawn into the multi-core fiber or it is further processed into a preform from which the multi-core fiber is subsequently drawn. This method is described, for example, in US Pat. No. 6,154,594.

[0006] The stack-and-draw process requires extensive adjustment effort and easily leads to dimensional errors and the introduction of contaminants due to the large proportion of exposed component surfaces. Furthermore, the elongated preform often exhibits different radius values in the azimuthal direction due to differences in radial packing density, which must be compensated for by cylindrical grinding.

[0007] In the process known from US 2015 / 0307387 A1, a preform for multi-core fibers is produced by drilling through-holes into a rod-shaped base material made of glass, into which core glass rods are then inserted. The base body is a hollow cladding glass cylinder in the form of a soot body produced using the OVD process based on SiO2 (SiO2 soot body). In the OVD process, the deposition surface is generally the outer shell of a rod- or tubular deposition mandrel rotating about its longitudinal axis. A substantially cylindrical soot body is deposited by reversing the back and forth movement of the reaction zone. After the deposition process is completed, the deposition mandrel is removed, leaving a central bore in the center axis of the cylindrical soot body. Longitudinal bores are drilled into the hollow cladding glass cylinder to accommodate the core glass rods.In addition, a further longitudinal bore is created by mechanical drilling in the area near the edge of the cladding glass hollow cylinder to accommodate a marker element in the form of a marker rod.

[0008] The marker rod is elongated into a marker zone when the multicore fiber is pulled. The marker zone is used, particularly in symmetrical fiber designs, to break symmetry, allowing the signal cores of the multicore fiber to be identified and clearly assigned in terms of their positions relative to each other and to the fiber's centerline. Identifying and assigning the signal cores is necessary, for example, so that two multicore fibers can be joined at their end faces using conventional splicing methods with low loss and correctly assigned signal cores.

[0009] US 2016 / 075590 A1 relates to the production of a multi-core fiber by drawing from a preform. The preform has a cladding glass region, several core rods, and a rod-shaped marker element. The core rods contain a core glass region surrounded by an inner cladding glass. The core rods are arranged in a glass lathe with their longitudinal axes parallel to one another, and the intermediate regions are filled with SiO2 soot using an external deposition process. The marker element is also embedded in this process. After vitrification, the SiO2 soot material forms a cladding glass region consisting of an outer cladding glass that surrounds the core rods and the marker element, which differs from the outer cladding glass in its refractive index or color.

[0010] US 2022 / 0003921 A1 discloses another method for producing a multi-core fiber by drawing from a preform. In one method, a preform is produced with a marker rod and four core rods. The core rods contain a core glass region surrounded by an inner cladding glass. They are embedded in a cylinder made of an outer cladding glass and evenly distributed around the cylinder's central axis. The outer cladding glass has a lower softening temperature than the inner cladding glass. The core rods and the outer cladding glass can be produced using an external deposition process. The cladding glass cylinder is provided with a central bore, directly adjacent to four bores for receiving the core rods, so that all bores form a common longitudinal opening. A rod with a square cross-section, consisting of the outer cladding glass, is inserted into the center of the longitudinal opening.The marker rod is inserted into the gap between this cladding glass rod and one of the core rods. When the assembly of cladding glass cylinder, core rods, and marker rod is heated, the outer cladding glass softens and closes the gaps in the area of the longitudinal opening. Technical task

[0011] The production of cladding glass hollow cylinders using external deposition processes, particularly the OVD process, is cost-effective compared to other manufacturing methods, particularly the VAD (Vapor Phase Axial Deposition) process. However, it has the disadvantage that after removal of the deposition mandrel, the central bore remains, which, upon collapse, can lead to mechanical stresses and asymmetric deformations and destroy the fiber design. Furthermore, collapse reduces the cross-sectional area of the cladding glass portion. These disadvantages can be at least partially eliminated by inserting a filler rod to close the central bore. The filler rod can be made of glass that has essentially the same refractive index and thermal expansion coefficient as the cladding glass hollow cylinder.But even with this measure, mechanical stresses arise when collapsing onto the filler rod, which may require subsequent annealing of the composite of the cladding glass hollow cylinder and the filler rod.

[0012] The marker zone, which is additionally incorporated into the fiber design, should have the smallest possible cross-section to counteract undesirable effects, such as the influence on signal transmission, stresses induced in the fiber, or so-called fiber curl. "Fiber curl" is defined as the degree of curvature along a specific length of the fiber. The curvature results from thermal stresses that arise during fiber manufacturing. High fiber curl creates optical losses due to microbends and complicates low-loss splicing of the multi-core fiber.

[0013] To minimize these disadvantages, a thin marker zone is desired, which in turn requires the smallest possible diameter of the channel for accommodating the marker element at the level of the cladding glass hollow cylinder. Typically, the channel diameter in the cladding glass hollow cylinder is less than 15 mm, associated with a high aspect ratio above 65 (with a hollow cylinder length of approximately 1 m).

[0014] The precise manufacturing and exact alignment of such thin channels in a cladding glass hollow cylinder is difficult, even with the use of high-precision drilling machines. Furthermore, it has been shown that drilling thin channels is particularly prone to cracks in the channel walls. The cost of manufacturing the cladding glass hollow cylinder from synthetic quartz glass is high, and the loss is particularly painful if the small hole for accommodating the marker element leads to the rejection of the otherwise finished cladding glass hollow cylinder.

[0015] Further complicating the situation is the fact that the cutting and splicing of the multi-core fiber can occur at any position according to specific requirements during intended use, so a consistent geometry along the entire fiber length must be relied upon to avoid reliance on measurements. At the level of the cladding glass hollow cylinder, this means that the longitudinal axes of the marker element and the cladding glass hollow cylinder must be as parallel as possible.

[0016] The invention is therefore based on the object of providing a method for producing multi-core fibers with a marker zone, which method utilizes the cost advantages of the external deposition method, but reduces the disadvantages and difficulties associated with the filler rod and the marker element, and in which the risk of rejects is also reduced.

[0017] Furthermore, the invention is based on the object of providing an intermediate product which is suitable for the cost-effective production of a multi-core fibre with a marker element, which is characterized in particular by a low fibre curl. General description of the invention

[0018] With regard to the method, this object is achieved according to the invention, starting from the method mentioned at the outset, in that the target rod has a marker region which extends along the longitudinal axis of the target rod and adjoins the first glass region, which marker region contains the marker element or which provides a hollow channel which either forms the marker element or which is designed to receive the marker element.

[0019] The multi-core fiber is obtained by elongating the preform or semi-finished product. The semi-finished product can be, for example, a primary preform for the multi-core fiber or an assembly of components that is directly drawn into the multi-core fiber or further processed into a preform for the multi-core fiber.

[0020] The creation of the cladding glass region of the semi-finished product comprises a process step in which a cladding material layer is created using an external deposition process. External deposition processes include, for example, thermal spraying or vapor phase deposition.

[0021] In thermal spraying, oxidic or easily oxidizable silicon-containing or silicon dioxide-containing starting powders in the form of a fluid mass, such as a flowable SiO2 powder, sol, dispersion, or slurry, are fed to an energy source, melted therein, and projected at high speed onto the outer surface of a deposition mandrel rotating about its longitudinal axis. The energy source can be, for example, a combustible gas-oxygen flame, a plasma jet, an arc, or a laser beam. The plasma spraying process is particularly preferred, as it allows for a comparatively high energy input and high speeds when projecting the molten starting powder particles.

[0022] In vapor-phase deposition processes, SiO2 particles are generated in situ by hydrolysis, pyrolysis, or oxidation of a silicon-containing precursor and deposited as a SiO2-containing layer on the outer surface of a deposition mandrel rotating about its longitudinal axis. Examples include the OVD (outside vapor phase deposition) and the POD (plasma outside deposition) processes. At sufficiently high temperatures in the area of the deposition mandrel surface, the SiO2 particles vitrify immediately, a process also known as "direct vitrification." In contrast, in the soot process, the temperature during deposition of the SiO2 particles is so low that a porous SiO2 soot layer is obtained.

[0023] The porous SiO2 soot layer obtained as the cladding material layer in the soot process is vitrified in a separate process step to form a cladding glass region made of transparent quartz glass. In the external deposition processes with direct vitrification and in the thermal spray processes, a glassy layer is directly obtained as the cladding material layer, which forms the cladding glass region of the semi-finished product or at least part of it.

[0024] The production of the cladding glass area of the semi-finished product using external deposition processes is cost-effective compared to other production methods.

[0025] In the method according to the invention, a target rod is used as the deposition mandrel for the external deposition process. After completion of the external deposition process, the target rod remains in the deposited cladding material layer and thus forms part of the semi-finished product for producing the multi-core fiber. The target rod consists of glassy material that becomes a component of the multi-core fiber. This differs from other external deposition processes in which the deposition mandrel is removed after completion of the external deposition process, leaving a central through-opening in the cladding material layer. Subsequent filling of the through-opening with a filler rod is thus unnecessary, and thus the difficulties associated with filling to ensure straightness, dimensional accuracy, and the greatest possible freedom from stress, as well as the risks of failure and the expenses for adjustment, time, and materials, are also eliminated.

[0026] The target rod has at least a first glass region and an adjacent marker region. The marker region is formed in or on the target rod. The target rod is thus used to insert a marker element into the semi-finished product in addition to the target rod material. In addition to the first glass region, the target rod can have one or more glass regions that differ in their chemical composition from the first glass region.

[0027] The marker element forms, for example, a hollow channel in the target rod, which can be filled with air, one or more cylindrical components, or with a powder bed. The marker element is arranged on an outer wall of the target rod and is designed as (at least one) cylindrical component or as a coating or mass adhering to the outer wall. The hollow channel can also be filled with a marker material after completion of the external deposition process.

[0028] Since the marker element is located in or on the target rod, there is no need to adapt the cladding material layer to accommodate the marker element, for example, by mechanical processing and, in particular, by creating a hole in the cladding material layer to accommodate the marker element. The effort and risk of damage associated with such an adaptation of the cladding material layer are thus eliminated.

[0029] The dimensional accuracy and straightness of the target rod are relatively easy to ensure, if necessary, by mechanical cylindrical grinding and / or by an elongation process, to which a starting cylinder is subjected to elongation into a cylinder strand, from which the target rod is produced or from which several target rods are cut to length. To avoid damage to the cylinder strand surface, the elongation process is preferably carried out without the use of a forming tool that engages the drawn cylinder strand.

[0030] An axially parallel alignment of the marker element is facilitated by the fact that it is adjacent to the target rod or its first glass region. The relatively easy-to-achieve straightness of the target rod also facilitates the axially parallel alignment of the marker element. This applies in particular to a particularly preferred procedure in which the marker region is arranged between the first glass region and the cladding glass region.

[0031] Thus, the component assembly is equipped with a marker element without the need to create a separate hole in the cladding glass area—associated with the risks and difficulties discussed above. At the same time, high accuracy can be ensured despite the high aspect ratio. This is evident, for example, in the semi-finished product, in the fact that the axial parallelism of the marker element has a deviation of less than 0.3 mm / m.

[0032] The target rod may consist of a single glass cylinder or it may be composed of several interconnected glass cylinders. The multiple glass cylinders may have the same composition or they may differ in composition. The target rod may consist entirely or partially of the cladding glass and / or the core glass, or of another glass and / or other glasses.

[0033] The chemical compositions of the first glass region and the adjacent marker region differ from each other. In multicore fibers, the marker region forms a continuous, linear marker zone made of a marker material or air. The marker zone can be used, for example, during splicing to break symmetry and clearly identify the signal nuclei and their positions relative to each other and to the fiber's central axis.

[0034] Typically, several core rod holes are created in the cladding material layer, each accommodating a core rod. At least one core rod hole can also be created in the target rod to accommodate a core rod.

[0035] The semi-finished product thus produced, with multiple core glass regions, is formed and either drawn directly into the multi-core fiber or consolidated into a preform for the multi-core fiber, whereby consolidation may be accompanied by simultaneous elongation. The "consolidated preform" thus produced is then drawn into the multi-core fiber, if necessary, or further processed into a "secondary preform." Further processing into the "secondary preform" includes, for example, creating additional holes in the cladding glass region and filling them with core glass or other glasses, or performing one or more of the following hot-forming processes once or repeatedly: collapsing additional cladding material, collapsing, elongating, collapsing, and simultaneous elongating. The multi-core fiber is drawn from the secondary preform produced by further processing.

[0036] The marker element is preferably provided in the form of a cylindrical component made of a marker material or in the form of a layer or mass of the marker material connected to the target rod.

[0037] In a marker element containing at least one cylindrical component, this component extends 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 a marker element in the form of a tube, the tube wall can contain a material with a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process and remains as a cavity ("airline") in the finished multi-core fiber.

[0038] In the case of a marker element in the form of a layer or mass connected to the target rod, this is arranged, for example, within a hollow channel in the target rod and is preferably attached in the region of the outer jacket of the target rod.

[0039] By attaching the marker element to the target rod, it particularly benefits from its straightness and alignment; these properties are essentially transferred to the marker element. Attachment is based, for example, on frictional engagement, material bonding, and / or positive engagement between the target rod and the marker element.

[0040] The cross-sectional geometry of the target rod is usually circular. It can also have a non-circular shape, such as an oval, elliptical, or polygonal geometry. For example, an enveloping circle surrounding the cross-sectional contour has a diameter in the range of 36 mm to 76 mm.

[0041] The target rod is manufactured, for example, by axial vapor deposition using the so-called VAD process or the OVD process, where the central through-hole is subsequently collapsed, or by a pressing process. In the pressing process, a bed of SiO2 particles is introduced into a mold cavity, and pressure is applied to the bed to form a compacted blank, which is then vitrified into a glass rod.

[0042] In a preferred procedure, the target rod has a recess extending along the longitudinal axis of the target rod, which recess forms the marker element or in which the marker element is arranged.

[0043] The recess is designed, for example, as a bore through the target rod and preferably as a longitudinal groove (longitudinal notch) on the outer jacket of the target rod. The groove-shaped recess is filled, for example, with a cylindrical component made of a marker material or with a particulate marker material. The particulate marker material can have a certain dimensional stability through thermal compaction or the addition of a binding agent. The recess ensures a positive connection between the marker element and the target rod. The marker element and target rod can also be bonded together in advance (i.e., before the jacket material layer is deposited) by means of a material bond, for example, by sintering or fusing, which is also referred to here as "consolidation." During consolidation, any edges and protrusions may become rounded. After consolidation, the marker material preferably fills the recess as completely as possible.

[0044] Particularly preferred in this context is the method in which a target rod is provided with a longitudinal groove. Firstly, a longitudinal groove in the outer surface of the target rod is particularly easy to produce and geometrically precise compared to a bore, for example, by milling using a mechanical milling cutter or by laser ablation. Secondly, the longitudinal groove created in this way is just as precise and straight as the target rod itself. Furthermore, the depth or opening width of the longitudinal groove can be kept almost arbitrarily small, for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. The longitudinal groove can be filled with a marker material during the external deposition process.This makes it easy to produce geometrically precise, low-volume marker elements that have an axial alignment deviation of less than 0.3 mm / m in the semi-finished product, thus forming correspondingly small and highly precise marker zones in the multi-core fiber. The longitudinal groove is filled with the marker material, for example, by inserting a cylindrical component (rod or tube) made of the marker material, 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 interior coating, or the bed of marker material can be additionally fixed in the longitudinal groove by fusing.

[0045] In a preferred process, a fused composite of target rod and marker element is created by elongating a precursor. The precursor is present, for example, as a composite of target rod and marker element, or it forms an ensemble of target rod and marker element starting bodies that are not or only locally connected to one another. The precursor has larger lateral dimensions than the fused composite of target rod and marker element to be produced. The elongation of the precursor not only downscales any geometric errors and dimensional deviations, but also contributes to straightening the fused composite. The desired fused composite of target rod and marker element is cut to length from the elongated fused composite strand for use in multi-core fiber production. In this way, marker elements with small lateral dimensions can be produced with high dimensional accuracy, even with particularly large aspect ratios.

[0046] In a particularly preferred process variant, the cladding material layer produced by the external deposition process is formed as a soot layer based on SiO 2.

[0047] The SiO 2 soot layer is produced by hydrolysis, pyrolysis, or oxidation of a silicon-containing starting compound in an oxygen-containing reaction zone and deposition of the resulting SiO 2 particles on the outer shell of the rotating target rod. The temperature in the region of the target rod's outer shell is kept so low that the deposited shell material layer is not densely vitrified but remains porous. This temperature is, for example, in the range of 800°C to 1250°C, and the specific density is typically in the range of approximately 0.6 g / cm 3 to 1.8 g / cm 3 .

[0048] The porosity of the SiO 2 cladding material layer allows subsequent removal of the OH groups contained therein by treatment in a drying atmosphere and doping with a gaseous dopant, such as fluorine.

[0049] The soot layer is preferably vitrified by heating to a vitrification temperature.

[0050] Vitrification is achieved, for example, by heating in a sintering furnace to a temperature above 1400°C under vacuum and / or in a helium atmosphere. This results in a cladding material layer made of transparent quartz glass.

[0051] In one procedure, the soot layer is subjected to a doping treatment in an atmosphere containing a dopant and / or a dehydration treatment in a halogen-containing atmosphere or under vacuum before vitrification.

[0052] The doping or dehydration treatment can immediately precede the vitrification process and can take place in the same furnace. The dehydration treatment, for example, includes a helium purge followed by a hot chlorination process at a temperature of around 900°C. In the alternative vacuum drying process, the SiO2 soot layer is treated in a vacuum furnace at a temperature of at least 1150°C under a pressure of 0.1 mbar or less. This reduces the hydroxyl group concentration to less than 1 ppm by weight.

[0053] The doping treatment includes, for example, loading the SiO2 soot layer with fluorine. This involves placing the target rod, together with the cladding material layer, in a doping furnace and exposing it to an atmosphere containing fluorine-containing substances, such as silicon tetrafluoride, at a temperature above 980°C for several hours. This enables fluorine loading of the SiO2 soot layer, resulting, for example, in an average fluorine content of at least 1500 wt. ppm after consolidation.

[0054] In a preferred process variant, the core rod holes for receiving core glass rods are created in the cladding material layer after the soot layer has been vitrified.

[0055] The lower density of the SiO2 soot cladding layer compared to dense quartz glass facilitates the creation of holes for core glass rods. Typically, two or more, for example, four to seven longitudinal holes (core rod holes) are drilled into the cladding layer, with their longitudinal axes running parallel to the target rod's longitudinal axis. The core rod holes are through-holes or blind holes and each serve to accommodate at least one core rod from the core glass.

[0056] The composition of the core glass is either consistently homogeneous in the radial direction, or it changes gradually or in steps. It differs from that of the cladding glass in such a way that light is guided in the core glass region. In the simplest case, all core rods have the same dimensions and are made of the same core glass. However, the core rods can also differ in their dimensions and / or in the composition of the respective core glass.

[0057] To minimize the risk of damage to the core rods, the core rods are preferably inserted into the core rod bores after the porous cladding material layer has been vitrified and the cladding glass area has been formed from transparent quartz glass.

[0058] The marker element forms an air-filled, elongated hollow channel or it contains a marker material which preferably differs in at least one physical and / or chemical property from the adjacent cladding glass and / or from the adjacent first glass region of the target rod, wherein the property is preferably selected from: refractive index, color, fluorescence and / or specific glass density.

[0059] The property (or properties) that distinguishes the marker element from the glasses of the component ensemble particularly affects the optical appearance of the marker element and is preferably detectable using an optical sensor. The glass composition of a marker glass can be based on quartz glass, as can the glass filler material, for example. The refractive index of quartz glass can be changed through doping. For example, doping a marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. The incorporation of carbon into the marker quartz glass can lead to a blackening. Doping the marker quartz glass with titanium results in a gray-blue coloration, depending on the oxidation state. Doping the marker quartz glass with rare earth metals or germanium oxide results in fluorescence at dopant-specific wavelengths.The specific glass density of the marker element can be changed by pores and is reflected in a reduction in optical transparency compared to bubble-free glass.

[0060] In a preferred procedure, the production of the component ensemble comprises the following process steps: (a) providing the target rod having the marker region, (b) providing a plurality of core rods containing a core glass, (c) depositing the cladding material layer on the outer cladding of the target rod using the outer deposition process, (d) creating core rod bores at least in the cladding material layer and optionally in the first glass region of the target rod, and (e) inserting the core rods into the core rod bores.

[0061] The semi-finished product thus produced comprises the target rod with at least one marker element and the cladding material layer produced thereon, as well as the core rods. List symbols (a) to (d) do not specify a sequence of the process steps. Core rods of the semi-finished product also include those core rods that have already been melted into their respective core rod bores.

[0062] In a preferred method variant, the marker region is designed as a hollow channel, wherein a cylindrical marker element is inserted into the hollow channel before or after the core rods are inserted into the core rod bores.

[0063] With regard to the intermediate product for producing a multi-core fiber or a preform therefor, the above-mentioned technical problem is solved according to the invention starting from an intermediate product with the features mentioned at the outset in that the marker element is formed on or in the target rod.

[0064] The intermediate product according to the invention can be used as such in the production of a multicore fiber or in the production of a preform for the multicore fiber using the process according to the invention. It is a particularly preferred intermediate product for this application.

[0065] The intermediate product is a cylindrical joint comprising a cladding material layer with a central region occupied by a glass target rod, and on or in which at least one marker element is formed. The cladding material layer is a porous SiO2-based soot layer. It is produced by an external vapor deposition (OVD) process and forms a cladding glass region made of a cladding glass in the final multi-core fiber.

[0066] The target rod extends along the cylinder's longitudinal axis and forms the cylinder's central volume region. The central volume region also includes at least one marker element, which borders a first glass region of the target rod.

[0067] In embodiments where the chemical composition of the first glass region of the target rod corresponds to that of the cladding glass of the multicore fiber, the corresponding volume region in the multicore fiber forms part of the optical cladding. In embodiments where the chemical compositions of the first glass region of the target rod and the cladding glass of the multicore fiber differ, the target rod in the multicore fiber can have an additional function, for example, it can act as a "stress zone" that generates and / or compensates for compressive or tensile stresses acting in the radial direction within the fiber.

[0068] The target rod can be a single piece or consist of several parallel cylinders. In addition to the first glass region, it can have another glass region or several glass regions that differ in their chemical composition from the first glass region. In particular, the target rod can contain at least one core glass region, with the core glass region forming a signal core in the final multi-core fiber.

[0069] The target rod is used to provide a marker element made of marker glass in addition to the first glass region. The marker element is present in the intermediate product, for example, as an elongated cavity or as a consolidated or unconsolidated component made of a marker material, or as a coating of such a component with the marker material. It forms a continuous, linear marker zone made of the marker material or an air-filled hollow channel in the multi-core fiber.

[0070] The marker element is arranged in or on the target rod. It is located, for example, in a hollow channel extending parallel to the cylinder's longitudinal axis, or it is attached to the outer jacket of the target rod. However, since the marker element in or on the target rod is not located entirely within the jacket material layer produced by external deposition processes, there is no need to adapt this jacket glass area for the purpose of inserting the marker element, for example, by mechanical processing and, in particular, by creating a separate bore for receiving the marker element. The risk of damage associated with such adaptation of the jacket material layer is thus eliminated.

[0071] The dimensional accuracy and straightness of the target rod can be ensured through simple measures. These measures include, for example, mechanical machining of the target rod and / or an elongation process. Mechanical machining can, if necessary, be an external machining process, which is generally significantly less complex than an internal machining process.

[0072] An axially parallel alignment of the marker element is facilitated by its abutment against the target rod. The relatively easy-to-achieve straightness of the target rod also facilitates the axially parallel alignment of the marker element. This applies in particular to a particularly preferred embodiment in which the marker element is arranged between the first glass region of the target rod and the cladding glass region.

[0073] Thus, the intermediate product is equipped with a marker element that eliminates the need to create a separate hole in the cladding material layer—associated with the risks and difficulties discussed above. At the same time, despite the high aspect ratio, high accuracy can be ensured, which is evident, for example, in the intermediate product, in the axial parallelism of the marker element with a deviation of less than 0.3 mm / m.

[0074] The target rod may consist of a single glass cylinder or it may be composed of several interconnected glass cylinders. The several glass cylinders may have the same composition or they may differ in composition. The target rod may consist entirely or partially of the cladding glass, the core glass, or another glass. For example, the target rod may have a central core of the core glass surrounded by a cladding region of the cladding glass.

[0075] The marker element is, for example, a rod that extends parallel to the target rod.

[0076] In a further embodiment, the marker element is present as a layer of a marker material, which is arranged in a hollow channel in the target rod or in the area of the outer casing of the target rod. Here, too, the marker element layer and the target rod can optionally also be present in a consolidated, i.e., fused, form.

[0077] The marker element extends along the longitudinal axis of the target rod, preferably over its entire length, and it can be attached locally, but preferably continuously, to the target rod.

[0078] By attaching the marker element to the target rod, it benefits from its straightness and alignment; these properties are essentially transferred to the marker element. Attachment is based, for example, on frictional engagement, material bonding, and / or positive engagement between the target rod and the marker element.

[0079] The marker element is preferably in the form of a cylindrical component made of a marker material or in the form of a layer or mass of the marker material bonded to the target rod. The at least one cylindrical marker element component is, for example, a tube, preferably a rod. In a marker element in the form of a tube, the tube wall can contain a material with a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process and remains as a cavity ("airline") in the finished multi-core fiber.

[0080] The cross-sectional geometry of the target rod is usually circular. It can also have a non-circular shape, such as an oval, elliptical, or polygonal geometry. For example, an enveloping circle surrounding the cross-sectional contour has a diameter in the range of 36 mm to 76 mm.

[0081] In a preferred embodiment, the target rod has a recess extending along the target rod longitudinal axis, which forms the marker element or in which the marker element is arranged.

[0082] The recess is preferably designed as a bore through the target rod or as a longitudinal groove (longitudinal notch) on the outer jacket of the target rod. It forms an air-filled hollow channel or it is filled, for example, with a cylindrical component made of a marker material or with a particulate marker material. The particulate marker material can have a certain dimensional stability through partial compaction or the addition of a binding agent. The marker material fills the recess as completely as possible. The recess ensures a positive connection between the marker element and the target rod. The marker element and the target rod can also be bonded together in advance (i.e., before the jacket material layer is created) by means of a material bond, for example by sintering or fusing.

[0083] Particularly preferred in this context is the embodiment in which a target rod is provided with a longitudinal groove. Firstly, a longitudinal groove in the target rod's outer jacket is particularly easy to produce compared to a bore; for example, by milling using a mechanical milling cutter or by laser ablation. Secondly, the longitudinal groove created in this way is just as precise and straight as the target rod itself. Furthermore, the depth or opening width of the longitudinal groove can be kept almost arbitrarily small; for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. This makes it easy to produce geometrically precise, low-volume marker elements that have an axial alignment deviation of less than 0.3 mm / m in the intermediate product and thus form correspondingly small and highly precise marker zones in the multi-core fiber.

[0084] The longitudinal groove is filled with the marker material, for example, by inserting a cylindrical component (rod or tube) made of the marker material, by introducing a fill of marker material particles, or by coating the interior of the longitudinal groove with the marker material. The cylindrical component, the interior coating, or the fill of marker material can be additionally fixed in the longitudinal groove by fusing.

[0085] The marker element forms an air-filled, elongated cavity (channel) or channel or it contains a marker material which preferably differs in at least one physical and / or chemical property from the adjacent first glass region of the target rod and / or from the cladding glass obtained by vitrifying the cladding material layer of the intermediate product, wherein the property is selected from: refractive index, color, fluorescence and / or specific glass density.

[0086] Using the method according to the invention or the intermediate product according to the invention, a multicore fiber is obtained that has several signal nuclei and is traversed by at least one continuous, linear marker zone. The marker zone serves to break symmetry and to clearly identify the signal nuclei and their positions relative to one another and to the fiber's central axis. Definitions and measurement methods

[0087] Individual terms from the above description are defined below for additional information. These definitions are part of the description of the invention. For terms and measurement methods not specifically defined in the description, the interpretation according to the International Telecommunication Union (ITU) applies. In the event of a conflict between one of the following definitions and the rest of the description, the remainder of the description prevails. Cladding glass / cladding glass area

[0088] The cladding glass region contains a cladding glass. At least a portion of the cladding glass region is created using an external deposition process. Core glass regions designed for signal transmission are created in the cladding glass region. The cladding glass consists, for example, of undoped quartz glass or it contains at least one dopant that lowers the refractive index of quartz glass. Fluorine and boron are dopants that can lower the refractive index of quartz glass. Core rods / core glass area

[0089] The core rods contain a core glass that has a homogeneous or non-homogeneous refractive index profile in the radial direction. The core glass of each of the core rods forms a core glass region. The core rods can contain a region made of a core glass with a comparatively high refractive index and at least one further region made of another glass with a comparatively low refractive index; for example, a quartz glass doped with fluorine and / or chlorine. The glass with the highest refractive index is usually located in the central axis of the core rod. It consists, for example, of quartz glass to which at least one dopant has been added to increase the refractive index. In the multi-core fiber, the core rod forms at least one signal core, in which the signal to be transmitted is primarily transported.The signal core may be adjacent to other glass regions with a lower refractive index, which have also been provided by the core rod. Target rod

[0090] The target rod has at least a first glass region and an adjacent marker region formed in or on the target rod. In addition to the first glass region, it can have another glass region or multiple glass regions that differ in their chemical composition from the first glass region. The target rod is made of glass that becomes a component of the multicore fiber. In particular, the target rod can have a glass region that serves as the signal core in the final multicore fiber. The composition of the first glass region can correspond to that of the cladding glass, or it can differ from that of the cladding glass in order to impart an additional property to the multicore fiber.

[0091] The target rod serves as a separating mandrel for carrying out a

[0092] External deposition process by which a cladding material layer is created from the target rod. The cladding material layer consists of quartz glass or is present entirely or partially as a SiO2 soot layer. The target rod thus serves to insert a marker element into the near-axis region of the cladding material layer in addition to the target rod material. Marker element / marker material / marker glass

[0093] The marker element contains air and / or a marker material, in particular at least one marker glass. The chemical composition of the marker material differs from that of the adjacent first target rod glass region to which it is adjacent, and / or the density of the marker material differs from that of the cladding glass and the first target rod glass region. The marker element is present in the semi-finished product as a component or as a layer or mass on a component and forms an optically detectable marker zone in the multi-core fiber. Component ensemble / consolidated preform / secondary preform / semi-finished product / Intermediate product

[0094] The "component assembly" comprises the target rod with at least one marker element and the cladding material layer deposited on the target rod with core rod bores, each of which contains a core rod inserted into each of which. By securing the core rods in the core rod bores—for example, by narrowing the end of a cladding glass hollow cylinder or by collapsing and fusing them—a "preform" is obtained, which is also referred to here as a "consolidated preform." The component assembly or the (consolidated) preform is elongated into a "secondary preform" or directly into the multi-core fiber. The term "semi-finished product" here encompasses the component assembly, the consolidated preform, and the secondary preform. The intermediate product is a cylindrical joining composite comprising a glass target rod, a marker element, and a cladding material layer containing SiO2 soot, which surrounds the target rod and the marker element. Quartz glass

[0095] Quartz glass is understood here to mean glass with a high silicic acid content and an SiO2 content of at least 80 mol%, preferably at least 90 mol%. The quartz glass is undoped or contains one or more dopants. It is, for example, molten from naturally occurring SiO2 raw material (natural quartz glass), or it is synthetically produced (synthetic quartz glass), or it consists of mixtures 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 using the so-called sol-gel process, or by hydrolysis and precipitation of inorganic silicon compounds in a liquid. Consolidate / Merge / Glaze / Collapse

[0096] In the context of components or a SiO2 mass made of glass, fusion means that the components or the SiO2 mass are fused together at a contact surface. Fusion occurs by heating the components or the SiO2 mass, at least in the area of the contact surface, using a heat source such as a furnace, a burner or a laser. Collapse closes the gaps between the components. Vitrification describes the hot process for converting porous soot material into dense glass. Consolidation can include fusion, vitrification and collapse processes. The result is a thermally strengthened, easy-to-handle semi-finished product, such as a preform or a component ensemble firmly joined by fusion. Position information: top / bottom

[0097] The information refers to positions during the elongation process or fiber drawing process. "Bottom" refers to the position in the drawing direction, "top" to the position opposite to the drawing direction. Cross-section

[0098] The cut perpendicular to the longitudinal direction / longitudinal axis. Longitudinal section

[0099] A cut parallel to the longitudinal direction / longitudinal axis. drilling

[0100] The terms "bore," "center bore," "inner bore," or "longitudinal bore" refer to holes with a cylindrical but otherwise arbitrary internal geometry. They are created, for example, by a drilling process or by depositing a layer of material on the outer surface of a mandrel through a deposition or pressing process, followed by the removal of the mandrel. Axis-parallel alignment / axis parallelism

[0101] The reference axis is the longitudinal axis of the semi-finished product, the preform or the central axis of the multi-core fiber. Example

[0102] The invention is explained in more detail below with reference to an embodiment and a drawing. In detail, a schematic representation Figure 1 a cross-section of a semi-finished product for the production of a multi-core fiber with a cladding glass hollow cylinder, core rods and a marker element according to the prior art, Figure 2 Processing steps (a) to (d) in a first process variant for producing a target rod with a marker element for use as a deposition mandrel in an external deposition process, Figure 3 Processing steps (a) and (b) in a second process variant for producing a target rod with a marker element for use as a deposition mandrel in an external deposition process, Figure 4the deposition of a soot layer on the target rod including a marker element of Figure 1 , Figure 5 the component assembly on the target rod including marker element and vitrified soot layer, Figure 6 a cross-section of the component composite of Figure 5 after creating longitudinal holes, Figure 7 a consolidated preform of the component composite of Figure 6 with core rods inserted and melted into the longitudinal bores, and Figure 8 another embodiment of a consolidated preform in cross section.

[0103] Figure 1shows a schematic cross-section of a hollow cylinder 10 made of a cladding glass according to the prior art, which serves as a base body for the production of a multi-core fiber. The hollow cylinder 10 is produced in a known manner using the OVD method. In this process, SiO 2 soot particles are deposited from the gas phase onto the outer shell of a cylindrical deposition mandrel rotating about its longitudinal axis, so that an SiO 2 soot body forms on the outer shell of the deposition mandrel. After the deposition process is completed, the deposition mandrel is removed, leaving an inner bore 20. The SiO 2 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 made in the wall of the hollow cylinder 10.When consolidating this component ensemble, a filler rod 80 is inserted into the central bore 20, which, for example, is made of the same material as the hollow cylinder cladding glass.

[0104] The multitude of longitudinal bores (40; 60) in the hollow cylinder cladding glass is associated with high effort and risk of failure. In particular, when creating the smaller longitudinal bore 60 for receiving the marker rod 70, unacceptable deviations from axial parallelism and cracks on the inner wall of the bore can easily occur, which can lead to the failure of the elaborately produced hollow cylinder 1. The filling and collapsing of the central bore 20 and the precise production of the filler rod 80 are also associated with effort and risk of failure and can easily lead to dimensional deviations. These disadvantages are avoided by the method according to the invention, which is described below with reference to the Figures 2 to 8 is explained.

[0105] Figure 2 and Figure 3show schematically process steps for preparing a target rod 1 with a marker element 5 or 5a.

[0106] The Figure 2a The schematically shown target rod 1 consists of synthetically produced, non-doped quartz glass, which is commercially available under the designation F300. Known methods are suitable for its production, such as VAD (Vapor Phase Axial Deposition), OVD (Outside Vapor Deposition) or MCVD (Modified Chemical Vapor Deposition) or powder pressing. It serves as a deposition mandrel in an OVD external deposition process, which is described below with reference to Figure 4The target rod 1 has a length of approximately 1800 mm and an outer diameter adjusted to approximately 42 mm by cylindrical grinding. Cylindrical grinding eliminates any defects in the outer shell 2 and any bends. Alternatively or additionally, the diameter adjustment and surface improvement are achieved by elongation in a tool-free elongation process.

[0107] Figure 2b shows that a longitudinal groove 3 has been milled into the outer surface of the target rod 1. The longitudinal groove 3 extends over the entire length of the target rod 1. It is U-shaped with a rounded bottom and straight side walls. Its opening width and depth are each 6 mm. In a subsequent process step, a hollow channel can be created from the longitudinal groove 3, which forms a marker element according to the invention.

[0108] Figure 2cshows the longitudinal groove 3 with a marker rod 4 inserted therein. The marker rod 4 has a diameter of 5 mm. It is made of synthetically produced quartz glass that is doped with fluorine and is commercially available under the designation F320. Both the viscosity and the refractive index of the fluorine-doped quartz glass of the marker rod 4 are lower than the undoped quartz glass from which the target rod 1 is made. The marker rod 4 is obtained by elongating a starting cylinder made of F320 quartz glass in a tool-free process. It has a smooth surface produced in the molten state and is characterized by high dimensional accuracy and straightness, so that it can be inserted easily and precisely into the narrow longitudinal groove 3.

[0109] The marker rod 4 inserted into the longitudinal groove 3 is heated over its entire length by means of a burner, so that the fluorine-doped quartz glass softens and deforms due to its comparatively low viscosity. Figure 2d shows the resulting marker glass mass 5 after softening, deformation and fusion with the target rod 1. The glass volume of the former marker rod 4 is adapted to the internal volume of the longitudinal groove 3 in such a way that the marker glass mass 5 just completely fills the longitudinal groove 3.

[0110] At the Figure 3In the alternative procedure shown, on the one hand, the creation of a longitudinal groove is dispensed with and, on the other hand, the composite of target rod 1 and marker element 5a is created using a preceding elongation process. The starting or precursor product for the elongation process is, on the one hand, a target rod cylinder 1a made of undoped quartz glass (F300) with a diameter of 63 mm and a length of 800 mm and a non-circular marker rod 4a, in which a peripheral section of the outer shell is flat or slightly concave (bent inwards) in cross-section, as shown in Figure 3ais indicated. The starting marker rod 4a consists of fluorine-doped quartz glass (F320) and has a diameter of 8 mm. The starting marker rod 4a is fused in parallel axial alignment with the starting target rod 1a, with the aforementioned peripheral section resting on its outer jacket 2a. During elongation, the composite of the starting target rod 1a and marker rod 4a is stretched to a length of 1800 mm. The starting cylinders (1a; 4a) fuse further together, and the lower-viscosity quartz glass of the marker rod 4a flows out slightly onto the target rod surface 2 and, after cooling, forms a flat glass bead 5a, which is firmly bonded to the outer jacket 2 of the target rod 1. This schematically shows the Figure 3b . The elongation process causes, on the one hand, the downscaling of any geometric errors and dimensional deviations in the initial cylinders (1a; 4a) and, on the other hand, it straightens the final melt composite (1; 5a).

[0111] Figure 4 shows schematically the use of the thus prepared target rod 1 filled with the marker glass mass 5 as a deposition mandrel in an OVD external deposition process. In this process, a high-purity SiO 2 starting material, for example silicon tetrachloride, is fed to a deposition burner 9 and a burner flame 8, in which it is converted into solid SiO 2 particles 11. These SiO 2 particles 11 are deposited from the gas phase on the outer shell 2 of the prepared target rod 1, which rotates about its longitudinal axis 1b (directional arrow R), with the deposition burner 9 performing a reversing back and forth movement along the longitudinal axis 1b of the target rod (this runs in the cross-sectional view of Figure 4perpendicular to the plane of the page). A SiO 2 soot layer 17 forms on the outer shell 2 of the target rod 1. The intermediate product 18 obtained after completion of the OVD external deposition process comprises a cylindrical joint composite of the target rod 1, the marker glass mass 5 and the SiO 2 soot layer 17, which surrounds the target rod 1 and the marker glass mass 5.

[0112] The intermediate product 17 is subjected to a dehydration treatment in a chlorine-containing atmosphere at a temperature of 850°C and immediately thereafter the SiO 2 soot layer 17 is vitrified under vacuum at a temperature of 1450°C.

[0113] Figure 5shows the resulting composite 15 consisting of target rod 1, including marker glass mass 5, and the cladding glass region 12 made of undoped, synthetically produced quartz glass, obtained after consolidation of the SiO 2 soot layer. The dashed circular line 12c represents the circumference of target rod 1. The outer diameter of composite 15 is adjusted to a value of 200 mm by external cylindrical grinding. The cladding glass region 12 extends along the outer cladding 2 of target rod 1 and, minus rounded end caps, has a usable length of approximately 1500 mm.

[0114] Figure 6 shows the composite 15 of cladding glass layer 12 and prepared target rod 1 after four holes 13 in a predetermined (here square) configuration have been created in the cladding glass region 12 by mechanical drilling in the direction of the target rod's longitudinal axis 1b. The holes 13 serve to accommodate core rods 14 ( Figure 7) and have a diameter of 30 mm. The holes 13 extend through the entire usable length of the cladding glass area 12 (through holes). In an alternative embodiment, the holes are designed as blind holes.

[0115] In addition, four core rods 14 made of germanium-doped quartz glass with a length of approximately 1500 mm and an outer diameter of approximately 28 mm are manufactured. Known techniques are also suitable for this, such as the MCVD (Modified Chemical Vapor Deposition) process.

[0116] The core rods 14 are inserted into the bores 12. The component ensemble consisting of the composite 15 and the core rods 14 is then heated so that the annular gaps around the core rods 14 close and all components of the ensemble are fused together. Figure 7shows schematically the thus consolidated preform 16, which is composed of the composite 15 of target rod 1, marker glass mass 5 and cladding glass region 12 as well as the core rods 14.

[0117] The consolidated preform 16 is then elongated into a secondary preform. The preform 16 is held in an elongation device by means of a holder in a vertical alignment with the target rod's longitudinal axis 1b. The secondary preform thus produced is then drawn into a multi-core fiber in a conventional manner in a drawing device.

[0118] In this embodiment, the marker element is present as a marker glass mass 5, which was created by reshaping the original marker rod 4 within the longitudinal groove 3. In an alternative procedure, a capillary is inserted into the longitudinal groove 3 during the external deposition process. During the subsequent consolidation processes, the complete collapse of the capillary is prevented by generating and maintaining excess pressure within it. In this way, a cavity is created that extends along the longitudinal axis 1b and is present in the multi-core fiber as an air-filled hollow channel. The hollow channel can serve as a marker zone because the refractive index of air differs significantly from that of the cladding glass.

[0119] Apart from the smaller radial dimensions, the cross-section of the multicore fiber essentially corresponds to the cross-section of the consolidated preform 16. The core glass regions (14) of the former core rods form signal cores that extend along the fiber's longitudinal axis; the former target rod (1) forms part of the cladding glass region, and the former marker element (5) forms a visually easily recognizable marker zone. The marker zone (5) is characterized by its small size, so that it imposes low tension on the multicore fiber during the fiber drawing process, resulting in low fiber curl.

[0120] In contrast to Figure 7 is at the Figure 8In the embodiment of a consolidated preform 26 for a multi-core fiber shown, the center of the target rod 1 is also occupied by a core rod 14a. The bore for receiving the core rod 14a is formed in and along the target rod longitudinal axis 1b and in one operation with the remaining core rod bores 13 ( Figure 6 ) is generated after vitrification of the soot layer. The diameter of all core rods 14, 14a is the same. The former target rod 1 therefore has a core glass region surrounded by a cladding glass region, which in turn is adjacent to the marker zone 5.

Claims

1. A method for producing a multi-core fiber with a marker zone or a preform for such a multi-core fiber, comprising the formation of a semi-finished product (16; 26) which comprises a glass cladding region (12) made of a cladding glass with a plurality of core glass regions (14; 14a) made of a core glass embedded therein, and at least one marker element (5), the multi-core fiber or the preform being obtained by elongation of the semi-finished product (16; 26), and the production of the glass cladding region (12) comprising a method step in which a cladding material layer (17) is deposited on an outer surface (2; 2a) of a target rod (1) having a target rod longitudinal axis (1b) using an outside deposition method, the target rod (1) comprising a first glass region extending along the target rod longitudinal axis (1b), and the semi-finished product (26) comprising the cladding material layer (17) and the target rod (1), characterized in that the target rod (1) also comprises a marker region extending along the target rod longitudinal axis (1b) and adjacent to the first glass region, which marker region contains the marker element (5) or provides a hollow channel which either forms the marker element (5) or is designed to receive the marker element (5).

2. The method according to claim 1, characterized in that the marker region is arranged between the first glass region and the glass cladding region (12).

3. The method according to claim 1 or 2, characterized in that the marker element (5) forms at least one cylindrical component (4) or a layer (4a) or mass (4a) connected to the target rod.

4. The method according to claim 3, characterized in that the target rod (1) comprises a recess (3) extending along the target rod longitudinal axis (1b), which recess forms the marker element or in which recess the marker element (5) is arranged.

5. The method according to claim 4, characterized in that the recess (3) comprises a longitudinal groove in the outer surface (2) of the target rod (1).

6. The method according to one of claims 3 to 5, characterized in that the marker element (5) is attached to the target rod (1) before the outside deposition method is carried out.

7. The method according to one or more of the preceding claims, characterized in that the cladding material layer (12; 17) produced using the outside deposition method is formed as a soot layer (17) based on SiO2.

8. The method according to claim 7, characterized in that the soot layer (17) is vitrified by heating to a vitrification temperature.

9. The method according to claim 8, characterized in that, prior to vitrification, the soot layer (17) is subjected to a doping treatment in an atmosphere containing at least one dopant and / or to a dehydration treatment in a halogen-containing atmosphere or under vacuum.

10. The method according to claim 7 or 8, characterized in that core rod bores (13) for receiving core glass rods (14; 14a) are produced after the soot layer (17) is vitrified.

11. The method according to one or more of the preceding claims, characterized in that the marker element (5) is designed as a hollow channel, or in that it contains a marker material which differs from the adjacent cladding glass and / or from the adjacent first glass region of the target rod (1) in at least one physical and / or chemical property, the property being selected from: refractive index, color, fluorescence, and / or specific glass density.

12. The method according to one or more of the preceding claims, characterized in that the production of the semi-finished product comprises the following method steps: (a) providing the target rod (1) comprising the marker region, (b) providing a plurality of core rods (14; 14a) containing a core glass, (c) depositing the cladding material layer (12; 17) on the outer surface (2) of the target rod (1) using the outside deposition method, (d) producing core rod bores (13) at least in the cladding material layer (12; 17) and optionally in the first glass region of the target rod (1), and (e) inserting the core rods into the core rod bores.

13. The method according to claim 12, characterized in that the marker region is designed as a hollow channel, and in that a cylindrical marker element is added into the hollow channel before or after the core rods are inserted.

14. An intermediate product for producing a multi-core fiber or a preform for same, comprising a target rod (1) made of glass, a marker element (5), and a cladding material layer (17) which contains SiO2 soot, surrounds the target rod (1) and the marker element (5) and is frictionally, form-fittingly and / or integrally connected to the target rod (1), characterized in that the marker element (5) is formed on or in the target rod (1).

15. The intermediate product according to claim 14, characterized in that the target rod (1) comprises a first glass region which is adjacent to the marker element (5), the marker element (5) preferably being arranged between the first glass region and the cladding material layer (17) and very preferably at least partially filling a recess (3) in an outer surface (2) of the target rod (1).