METHOD AND SEMI-COATED PRODUCT FOR THE PRODUCTION OF A MULTI-CORE FIBER

Description

Technical background

[0001] The present invention relates to a method for producing a multi-core fiber, comprising a process step in which a component ensemble is formed into the multi-core fiber or into a preform for the multi-core fiber, comprising a hollow cylinder having a central bore and a longitudinal axis, which has a cladding glass area made of cladding glass and several core glass areas covered with a core glass, wherein at least a part of the central bore is covered by a glass filler rod having a filler rod longitudinal axis and a filler rod outer cladding surface.

[0002] Furthermore, the invention relates to a semi-finished product for the production of a multi-core fiber, comprising a hollow cylinder having a central bore and a longitudinal axis, which has a cladding glass area made of cladding glass and several core glass areas within the cladding glass area covered with a core glass, wherein at least a part of the central bore is covered by a glass filler rod having a filler rod longitudinal axis and a filler rod outer cladding surface.

[0003] In multicore fibers, several optical core regions (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 independent light transmission. This fiber design promises high signal transmission capacity because different signals can be transmitted simultaneously in each of the spatially separated signal cores within a single optical fiber. This method of signal transmission is also known as "spatial multiplexing," which can significantly increase data transmission capacity in optical telecommunications.Multicore fibers are also considered key components for energy transmission in material processing, as part of fiber optic sensors in measurement and medical technology, and 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 often consist of synthetically produced fused silica (SiO₂), which can be doped or undoped. The production of synthetic fused silica 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—particle-like SiO₂—is deposited as a solid from the gas phase onto a deposition surface. The starting material is, for example, silicon tetrachloride (SiCl₄) 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 sheath tube and spatially fixed within it. This assembly is then drawn to form the multicore fiber, or it is first processed into a preform from which the multicore fiber is subsequently drawn.

[0006] The "stack-and-draw" process requires a high degree of adjustment 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 variations in radial packing density, which must be corrected by cylindrical grinding.

[0007] US Patent 2022 / 003921 A1 describes a semi-finished product and a process for manufacturing a multi-core fiber by creating five circular openings in a solid glass cladding cylinder through heating or powder forming processes. A centrally located opening is uniformly surrounded and overlapped by four surrounding openings. The four outer openings are filled with core glass rods. A rod made of a low-viscosity glass with a polygonal cross-section is inserted into the remaining open central opening, and a marker rod is inserted into a void next to this rod. In a later stage of the process, all components of the assembly are heated, with the rod made of the low-viscosity glass softening first and filling the gaps.

[0008] From US patent 2016 / 347645 A1, a semi-finished product and a method for producing a multi-core fiber by elongating the semi-finished product are known. The semi-finished product forms a multi-part fiber preform, which consists of a one-piece sheath tube made of cladding glass, designed to receive and axially guide a stack of three identical cylindrical stack pieces made of cladding glass, each having through-holes and a longitudinal groove on its cylindrical outer surface, several core rods for insertion into the through-holes, and several marker rods for insertion into the longitudinal grooves. Within the sheath tube bore, the stack pieces are arranged one above the other so that the through-holes and the longitudinal grooves are aligned. The marker zone of the fiber preform, formed by the marker rods, runs along the inner outer surface of the sheath tube.

[0009] The US 8,532,454 B2 reveals a multi-core fiber with seven core regions.

[0010] In another known method for producing a preform for a multicore fiber, as described, for example, in US 2021 / 0300812 A1, a large number of through-holes are created by longitudinally perforating a cladding cylinder made of synthetic SiO₂. These through-holes run along the cylinder's longitudinal axis. A core rod containing a core material with a higher refractive index than the cladding cylinder is inserted into each through-hole. With a large number of through-holes, a thin and easily breakable wall can remain between adjacent holes. To reduce the risk of breakage, it is proposed to create the necessary number of through-holes in the cladding cylinder successively, and in the meantime, to fill and fuse at least some of the through-holes with a core rod.

[0011] The cylinder of the glass casing is elongated. The through-holes have a large aspect ratio (ratio of length to diameter), which fundamentally complicates their precise manufacturing and exact alignment parallel to the longitudinal axis of the hollow cylinder of the glass casing.

[0012] In the processes known from US 2015 / 284286 A1 and US 2015 / 0307387 A1, a hollow cylinder in a shell glass is used in the form of a SiO₂-based soot body produced by the OVD process (SiO₂ soot body), which has a density between 0.8 g / cm³ and 1.6 g / cm³. In the OVD process, the deposition surface is generally the outer shell surface of a rod- or tube-shaped deposition mandrel rotating about its longitudinal axis. A substantially cylindrical soot body is deposited by reversing the reaction zone. After completion of the deposition process, the deposition mandrel is removed, leaving a central through-hole in the central axis of the cylindrical soot body. Longitudinal bores for receiving core glass rods are made in the resulting hollow cladding glass cylinder, whereby the lower density of the SiO2 soot body compared to quartz glass facilitates the production of dimensionally accurate longitudinal bores.The central "OVD through-hole" remaining from the manufacturing process can lead to asymmetrical deformations during collapse, destroying the fiber design. Furthermore, collapse reduces the cross-sectional area of ​​the cladding portion. These disadvantages can be prevented by inserting a filler bar that closes the OVD through-hole. The filler bar consists of glass with essentially the same refractive index as the cladding hollow cylinder. The filler bar can also be produced using an OVD process, by compression molding, or by a combination of compression molding and OVD processes. Additionally, a channel for receiving a marker element is created in the cladding hollow cylinder. This channel is produced by mechanical drilling in the area near the edge of the cladding hollow cylinder.

[0013] The marker elements form continuous, line-like marker zones within the multicore fiber and serve to break symmetry, enabling the unambiguous identification and assignment of the signal cores and their positions relative to each other and to the fiber's central axis. This is necessary, for example, to allow two multicore fibers to be joined at their ends using conventional splicing methods with minimal attenuation.

[0014] To splice a multi-core fiber, the fiber ends to be joined are arranged so that their end faces are aligned. In a known method, light is simultaneously injected into all signal cores at the opposite fiber end and collectively detected at the end of the other multi-core fiber using a photodetector and power meter. Particularly in cases where one end of the multi-core fiber is unavailable for light injection, this fiber is illuminated laterally. By relative displacement of the multi-core fiber end faces in the horizontal and vertical directions and by relative rotation in the azimuthal direction, the fiber ends are automatically aligned in a fusion splicing machine until the signal cores are correctly assigned and the collective received light power is maximized, and then fused together in this position. Technical task

[0015] The greater the number of multiple cores, the greater the theoretical increase in data transmission capacity compared to a single-core optical fiber (single-mode or multi-mode fiber). However, it is essential that each multiple core has an optical attenuation approximately equivalent to that of a single-core optical fiber. This requires that the fiber design does not introduce additional attenuation or interfere with the independent information transmission of the signal cores. This can be caused by crosstalk between the multiple cores, particularly if they are too close together. Therefore, maintaining a certain minimum distance between the fiber cores is crucial.For these reasons, there is a need to utilize the cross-sectional area available in the radial cross-section of the multicore fiber as completely as possible for the covering with the multiple cores.

[0016] The marker zones to be additionally incorporated into the fiber design should therefore occupy a small proportion of the fiber cross-section. A small marker zone size is also advantageous to counteract other undesirable effects, such as fiber curl or fiber-induced stresses. The diameter of the channel for receiving the marker element is therefore small and generally significantly smaller than the diameter of the bores for the core rods. Typically, the channel diameter in the cladding hollow cylinder before the fiber drawing process is less than 15 mm, accompanied by an aspect ratio above 65 (with a hollow cylinder length of approximately 1 m).

[0017] The precise manufacturing and exact alignment of such thin channels within a hollow cylinder made of synthetic quartz glass is difficult, even when using high-precision drilling machines. Furthermore, it has been shown that drilling thin channels is particularly prone to causing cracks in the channel walls. The cost of manufacturing the hollow cylinder from synthetic quartz glass is substantial, and the loss is especially painful when the small hole required for the marker element leads to the rejection of the otherwise completed cylinder.

[0018] To make matters more complicated, the cutting and splicing of the multicore fiber can take place at any position according to the specific requirements of its intended use, so that a consistent geometry along the entire fiber length must be relied upon to avoid having to rely on measurements.

[0019] It is therefore an object of the invention to provide a method for producing multi-core fibers without a central signal core, which reduces the disadvantages of the known methods, and in which, in particular, the risk of rejects during the completion of the cladding glass hollow cylinder is reduced.

[0020] Furthermore, the invention is based on the objective of providing a semi-finished product suitable for the production of a multi-core fiber without a central signal core, which is characterized in particular by a low "fiber curl".

[0021] Fiber curl is a property of optical fibers defined as the degree of curvature over a specific length of the fiber. This curvature results from thermal stresses that arise during fiber production. A high fiber curl makes low-loss splicing of multicore fibers more difficult. General description of the invention

[0022] With regard to the method, this problem is solved according to the invention, starting from the method mentioned at the outset, by creating a recess in or on the filling rod extending in the direction of the longitudinal axis of the filling rod, into which a marker element made of marker glass is inserted or which forms the marker element.

[0023] A hollow glass cylinder with a central bore is used. Such hollow cylinders are obtained, for example, and preferably, using the OVD (Outside Vapor Deposition) process after the deposition mandrel has been removed. The production of hollow glass cylinders using the OVD process is cost-effective compared to other manufacturing methods, especially compared to the VAD (Vapor Phase Axial Deposition) process. However, it has the disadvantage that the aforementioned central bore remains. This bore can be completely or partially sealed by means of a glass filler material, which is provided by several filler rods or by a single filler rod containing the glass filler material.

[0024] In embodiments where the chemical composition of the glass filler rod corresponds to that of the cladding glass, the glass filler material of the filler rod forms part of the optical cladding in the multicore fiber. In embodiments where the chemical compositions of the glass filler material and the cladding glass differ, the glass filler material of the filler 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 radially acting compressive or tensile stresses within the fiber.

[0025] The method according to the invention serves to produce a multi-core fiber without a central signal core. The central bore is used to insert a marker element in addition to the glass filler material of the core rod. The glass filler material does not contain a core region suitable for signal transmission.

[0026] At least one recess extending in the direction of the longitudinal axis of the filler rod is created in or on the filler rod, into which the marker element is inserted or which forms the marker element.

[0027] The marker element exists within the component assembly as a component itself, as a coating on a component, or as a cavity, and forms a continuous, line-like marker zone in the multicore fiber, consisting of a marker material or air. This marker zone can, for example, serve to break symmetry during splicing and to uniquely identify the signal cores, as well as their positions relative to each other and to the fiber's central axis.

[0028] The recess is, for example, a hollow channel extending through the filler rod along its longitudinal axis. In this case, the filler rod forms the outline of a marker element in the form of an air-filled, elongated cavity. Alternatively, the marker element may be arranged in a recess on the outer surface of a cylindrical filler rod.

[0029] Since the marker element is located in or on the glass filler rod, and thus within the central bore of the hollow cylinder's outer glass, but not within the outer glass of the hollow cylinder itself, there is no need to modify the hollow cylinder's outer glass to accommodate the marker element. This modification would require, for example, mechanical processing and, in particular, the creation of a bore for the marker element within the hollow cylinder's outer glass. Consequently, the risk of damage associated with such modification of the hollow cylinder's outer glass is eliminated.

[0030] The dimensional accuracy and straightness of the center bore of the shell glass hollow cylinder can be easily ensured by the OVD manufacturing process itself and, if necessary, by subsequent measures. Suitable subsequent measures include, for example, mechanical post-processing of the center bore and / or an elongation process to which the initial hollow cylinder produced in the OVD deposition process is subjected in order to elongate it into a tube strand from which the shell glass hollow cylinder is produced or from which several shell glass hollow cylinders are cut to length. The elongation process is preferably carried out without the use of a forming tool that engages the drawn tube strand in order to avoid damage to the tube strand surface.

[0031] Similarly, the dimensional accuracy and straightness of a filler rod that fits precisely into the center bore can be ensured relatively easily through mechanical machining and / or by means of such an elongation process. This mechanical machining can, if necessary, be an external machining process, which is generally significantly less complex than an internal machining process.

[0032] An axis-parallel alignment of the marker element is facilitated by forming it from the recess of the filler rod that extends in the direction of the filler rod's longitudinal axis, or by inserting it into the recess extending in the direction of the filler rod's longitudinal axis.

[0033] The marker element is inserted into the recess, for example, by inserting a cylindrical component (rod or tube) made of marker glass that extends parallel to the outer surface of the filler rod, or by introducing a bed of particles from the marker glass, or by coating the inside of the recess with the marker glass.

[0034] The recess is designed, for example, as a bore in the filler rod and preferably as a longitudinal groove on the outer surface of the filler 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 be given a certain degree of dimensional stability through thermal compaction or the addition of a binder. The marker material completely or partially fills the recess. The recess ensures a positive fit between the marker element and the filler rod. The marker element and filler rod can also be pre-bonded (i.e., before being inserted into the central bore), for example, by sintering or fusing.

[0035] A particularly preferred method involves providing a longitudinal groove to a filler rod. Firstly, a longitudinal groove in the outer surface of the filler rod is much easier and geometrically precise to produce than a bore in the rod; for example, by milling with a mechanical cutter or by laser ablation. Secondly, the longitudinal groove produced in this way is just as precise and straight as the filler 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 formed as the wall of an air-filled hollow channel, which can serve as a marker element, or the longitudinal groove can also be filled with a marker material.Thus, geometrically precise marker elements with small volumes can be produced in a simple way, which in the preform or in the component ensemble have a deviation of their axial orientation of less than 0.3mm / m and which accordingly form correspondingly small and highly accurate marker zones in the multicore fiber.

[0036] 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 inside of the longitudinal groove with the marker material. The cylindrical component or bed of marker material can be further secured in the longitudinal groove by fusing.

[0037] The relatively easy-to-achieve straightness of the filler bar and center bore also facilitates the axial alignment of the marker element. This applies particularly to a preferred method in which the marker element extends along the longitudinal axis of the filler bar and is melted into the recess before forming the preform or the multicore fiber.

[0038] The marker element (component, fill, layer) is fixed in the recess by melting. For this purpose, it is melted into the recess over at least a portion of its length, preferably locally at several points distributed along its length, and ideally over its entire length. The filler rod filled with the marker element material is subsequently also referred to as the "modified filler rod".

[0039] By melting the marker element into the recess before forming the component assembly into a multicore fiber or a preform, it can be ensured that the side edges of the recess filled with the melted marker element form largely stepless, continuous transitions to the outer surface of the modified filler bar, thus avoiding structural defects during the fiber drawing process. This has a positive effect on the dimensional accuracy of the multicore fiber. The marker element, for example, completely fills the recess and ideally has a curvature adapted to the outer contour of the filler bar's outer surface.

[0040] Thus, in a preferred method, the marker element has a length and the melting is carried out completely, partially or selectively along at least 80% of this length, preferably along at least 90% of this length.

[0041] The melting of the marker element preferably comprises a process step in which the filler rod with a horizontally oriented filler rod longitudinal axis is positioned such that the recess is located on a top surface of the filler rod's outer shell surface, wherein the material of the marker element inserted in the recess is heated and softened by means of a heating source.

[0042] By melting the marker element with the filler rod's longitudinal axis horizontally, the marker element material, once locally heated and softened (e.g., by a torch or laser), sinks downwards due to gravity, filling any remaining voids in the recess. Surface tension can lead to a rounding of the free surface area adjacent to the atmosphere.

[0043] In this way, it is much easier and more efficient to fill the recess evenly and preferably completely than would be the case, for example, if the marker element and the recess were oriented vertically during the melting process.

[0044] The marker element, melted into the recess, is fixed in relation to the filler rod, which simplifies handling of the modified filler rod in the subsequent fiber manufacturing process. During melting, surface tension causes a certain degree of rounding of the marker material, thus adapting it to the contour of the filler rod's outer surface. After the marker element has been melted in, the accuracy and quality of the melting of the modified filler rod can be checked and improved if necessary.

[0045] Thus, the component assembly is equipped with a marker element without requiring a separate bore in the hollow cylinder of the cladding glass – which involves the risks and difficulties described above. At the same time, high accuracy can be ensured despite the high aspect ratio, which manifests itself, for example, in the preform or the component assembly in that the axial parallelism of the marker element has a deviation of less than 0.3 mm / m.

[0046] In a preferred method, the marker element is in the form of a rod extending parallel to a filler rod or to several filler rods within the central bore.

[0047] In another preferred method, the marker element is present as a layer that is applied in the recess of the filler rod.

[0048] Two or more, for example four to seven, additional longitudinal bores (core rod bores) are drilled into the hollow cylinder in the usual manner, their longitudinal axes running parallel to the longitudinal axis of the central bore. The core rod bores are either through holes or blind holes and serve to accommodate at least one core rod made from the core glass. The composition of the core glass is either consistently homogeneous in the radial direction or changes gradually or in steps. It differs from that of the outer glass in such a way as to ensure light transmission within the core glass region.

[0049] The desired number of core rod bores is either produced in a single operation and each is fitted with at least one core rod. Alternatively, only one core rod bore, or only a portion of the desired number of core rod bores, is produced beforehand, each fitted with at least one core rod, and the core rod bores are collapsed (this forming process is also referred to as "consolidation") before the remaining portion, or a further portion, of the core rod bores is produced in a second or subsequent operation, and each of these is also fitted with at least one core rod and collapsed if necessary. 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.

[0050] The filling of the central bore with the filler rod, including the marker element, can be carried out before or after all core rod bores have been created and / or filled, or it can be carried out before or after a portion of the core rod bores have been created and / or filled. In a preferred method, the central bore is filled with the filler rod and the marker element, then the filled central bore is collapsed by heating, and only then are the desired core rod bores created. In another preferred method, a first portion of the desired number of core rod bores is created and fitted with at least one core rod, the central bore is filled with the filler rod and the marker element, then the filled core rod bores and the filled central bore are consolidated by heating, and only then are a second portion of the core rod bores created.

[0051] The component assembly produced in this way is formed and either drawn directly into the multicore fiber or consolidated into a preform for the multicore fiber, whereby consolidation may be accompanied by simultaneous elongation. The resulting "consolidated preform" is then either drawn into the multicore fiber or further processed into a "secondary preform." Further processing into a "secondary preform" includes, for example, creating additional bores in the cladding glass area 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 elongation. The multicore fiber is then drawn from the secondary preform produced by this further processing.

[0052] In a preferred method, the production of the component assembly comprises the following process steps: (a) Providing the hollow cylinder containing the shell glass, (b) Providing several core rods containing the core glass, (c) Providing a filler rod having a longitudinal axis and containing the glass filler material, (d) Creating the at least one recess on the outer shell surface of the filler rod, (e) Providing the marker element, (f) Arranging and fusing the marker element into the recess, (g) Creating core rod bores extending along the longitudinal axis of the hollow cylinder, (h) Inserting the filler rod and the marker element into the central bore, and (i) Inserting the core rods into the core rod bores to form the component assembly.

[0053] The resulting component assembly comprises the hollow cylinder, at least one modified filler rod fused with at least one marker element, and core rods. List symbols (a) to (i) indicate only a preferred, but not a mandatory, sequence of the process steps. Core rods of the component assembly are also defined here as those core rods that have already been melted into their respective core rod bores.

[0054] The marker element extends along the longitudinal axis of the filler rod, preferably over its entire length, i.e., from the first end of the filler rod to the second end of the filler rod.

[0055] By merging with the filler rod, the marker element particularly benefits from its straightness and alignment; these properties are essentially transferred to the marker element.

[0056] The marker element 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 contain a material with a higher viscosity than the cladding glass, so that during the fiber drawing process the bore does not completely collapse and remains as a cavity ("airline") in the finished multicore fiber.

[0057] The cross-sectional geometry, and optionally the outer diameter and length of the filler rod, are adapted to the geometry and length of the hollow cylinder's central bore. The central bore is preferably filled while leaving a circumferential gap with a gap width of less than 2 mm, particularly preferably a maximum of 1 mm. For example, with a central bore in the range of 38 mm to 78 mm, the filler rod diameter is in the range of 36 mm to 76 mm.

[0058] The marker element forms an air-filled, elongated cavity (channel) or it contains a marker material that preferably differs in at least one physical and / or chemical property from the cladding glass and from the glass filling material of the filling 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 assembly particularly affects the optical appearance of the marker element and is preferably detectable by means of an optical sensor. The glass composition of a marker glass—like, for example, the glass filler material of the filler rod—can be based on quartz glass. The refractive index of quartz glass can be altered by doping. For example, doping a marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. Incorporation of carbon into the marker quartz glass can lead to a blackening. Doping the marker quartz glass with titanium results in a gray or blue coloration, depending on the oxidation state. Doping the marker quartz glass with rare earth metals or germanium oxide manifests as fluorescence at dopant-specific wavelengths.The specific glass density of the marker element can be changed by pores and manifests itself in a reduction of optical transparency compared to bubble-free glass.

[0060] With regard to the semi-finished product for the production of a multi-core fiber, the above-mentioned technical problem is solved according to the invention, starting from the semi-finished product mentioned at the outset, by the fact that the filler rod has a recess extending in the direction of the filler rod longitudinal axis, into which a marker element made of marker glass is inserted or which forms the marker element, extending along the central bore longitudinal axis and the filler rod longitudinal axis.

[0061] The semi-finished product according to the invention serves to manufacture a multi-core fiber without a central signal core. It comprises a hollow cylinder with a cladding glass section and several core glass sections coated with a core glass. The central bore of the hollow cylinder is completely or at least partially covered by a filler rod made of glass filler material. At least one recess extending in the direction of the longitudinal axis of the filler rod is provided in or on the filler rod, into which the marker element is inserted or which forms the marker element.

[0062] The longitudinal axis of the center bore and the longitudinal axis of the filler bar run coaxially to each other in the semi-finished product.

[0063] The number of core glass regions is at least two, preferably four to seven. They are each configured, for example, as core rod bores running parallel to the longitudinal axis of the hollow cylinder in the cladding glass region of the hollow cylinder, and each is fitted with a core rod. The filler rod is inserted into the central bore of the hollow cylinder and at least partially fills it. At least one marker element is also located in the central bore of the hollow cylinder. This marker element is connected to the filler rod either by being formed as a recess in the filler rod or by at least partially filling a recess in or on the filler rod. The filler rod thus filled is also referred to here as a "modified filler rod." The filler rod does not contain a core region suitable for signal transmission.

[0064] The semi-finished product forms a component assembly when all or some core bars, filler bars, and marker elements are merely inserted into their respective bores or recesses but not yet fused together. The semi-finished product forms a preform (also referred to as a "consolidated preform") when all bores or recesses have collapsed and the core bars, filler bars, and marker elements have fused together or with the hollow cylinder.

[0065] The hollow cylinder made of a glass jacket has a central bore. Such hollow cylinders can be obtained cost-effectively, for example, using the OVD process after the deposition mandrel has been removed. The central bore is completely or partially sealed with a glass filler material, which is provided by several filler rods or, preferably, by a single filler rod.

[0066] In embodiments where the chemical composition of the glass filler rod corresponds to that of the cladding glass, the glass filler material of the filler rod forms part of the optical cladding in the multicore fiber. In embodiments where the chemical compositions of the glass filler rod and the cladding glass differ, the glass filler material of the filler 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 radially acting compressive or tensile stresses within the fiber.

[0067] The central bore is used to accommodate a marker element made of marker glass in addition to the filler rod. The marker element is present in the semi-finished product, for example, as an elongated cavity, as a consolidated or unconsolidated component made of a marker material, or as a coating of such a component with the marker material. Within the multicore fiber, it forms a continuous, linear marker zone made of the marker material or an air-filled hollow channel.

[0068] The marker element is located in or on the filler rod. For example, it may be in the form of air or another gas in a recess designed as a hollow channel extending through the filler rod along its longitudinal axis, or it may be attached in a recess on the outer surface of the filler rod.

[0069] Since the marker element is located in or on the filling rod, and thus within the central bore of the hollow cylinder's outer glass, but not within the cylinder's outer glass, there is no need to modify the hollow cylinder's outer glass to accommodate the marker element. This modification would require, for example, mechanical processing and, in particular, the creation of a separate bore for the marker element within the hollow cylinder's outer glass. Consequently, the risk of damage associated with such modifications to the hollow cylinder's outer glass is eliminated.

[0070] The dimensional accuracy and straightness of the center bore can be easily ensured by the OVD manufacturing process itself and, if necessary, by subsequent measures. Suitable subsequent measures for achieving dimensional accuracy and straightness include, for example, mechanical post-processing of the center bore and / or an elongation process to which the initial hollow cylinder produced in the OVD deposition process is subjected. This process elongates the tube strand from which the shell glass hollow cylinder is produced or from which several shell glass hollow cylinders are cut to length. The elongation process is preferably carried out without the use of a forming tool that engages the drawn tube strand to avoid damaging the tube strand surface.Similarly, the dimensional accuracy and straightness of the filler rod, which is precisely inserted into the center bore, can be ensured relatively easily through mechanical machining and / or by means of such an elongation process. This mechanical machining can, if necessary, be an external machining process, which is generally significantly less complex than an internal machining process.

[0071] An axis-parallel alignment of the marker element is facilitated by forming it from the recess of the filler rod that extends in the direction of the filler rod's longitudinal axis, or by inserting it into the recess extending in the direction of the filler rod's longitudinal axis.

[0072] The recess is designed, for example, as a bore in the filler rod and preferably as a longitudinal groove on the outer surface of the filler rod. The groove-shaped recess is filled—forming a "modified filler rod"—for example, with a cylindrical component made of a marker material or with a particulate marker material. The particulate marker material can be given a degree of dimensional stability through thermal compaction or the addition of a binder. The marker material completely or partially fills the recess. The recess ensures a positive fit between the marker element and the filler rod. The marker element and filler rod can also be pre-bonded (i.e., before being inserted into the central bore), for example, by sintering or fusing.

[0073] A particularly preferred embodiment in this context is one in which the recess in the filler bar is designed as a longitudinal groove. Firstly, a longitudinal groove in the outer surface of the filler bar is significantly easier and geometrically precise to manufacture compared to a bore in the filler bar. Secondly, the longitudinal groove produced in this way is just as precise and straight as the filler bar itself. Furthermore, the depth and / or the 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. Thus, geometrically precise marker elements with a small volume are available, which in the preform or in the component assembly exhibit a deviation in their axial orientation of less than 0.3 mm / m and which accordingly form correspondingly small and highly accurate marker zones in the multicore fiber.

[0074] The marker material is present, for example, in the form of a cylindrical component (rod or tube) made of the marker material, or it is formed by a bed of particles made of the marker material or by coating the longitudinal groove with the marker material. The cylindrical component made of the marker material or the bed of marker material can be additionally fixed in the longitudinal groove by fusing.

[0075] The relatively easy-to-achieve straightness of the filler rod and center bore also facilitates the axial alignment of the marker element. This applies particularly to a preferred embodiment of the semi-finished product in which the marker element extends along the longitudinal axis of the filler rod and is fused into the recess.

[0076] The marker element (component, fill, layer) is fixed in the recess by fusion. For this purpose, it is fused into the recess over at least part of its length, preferably locally at several points distributed along its length, and ideally over its entire length. The result is a modified filler rod filled with the marker element material.

[0077] By melting the marker element into the recess, it can be ensured that the side edges of the recess filled with the melted marker element form largely stepless, continuous transitions to the outer surface of the filler rod, thus avoiding structural defects during the fiber drawing process. This has a positive effect on the dimensional accuracy of the multicore fiber. The marker element, for example, completely fills the recess and ideally has a curvature adapted to the outer contour of the filler rod's outer surface.

[0078] Thus, in a preferred embodiment, the marker element has a length and is completely, partially or selectively fused into the recess along at least 80% of this length, preferably along at least 90% of this length.

[0079] The marker element, melted into the recess, is fixed in relation to the filler rod, which simplifies its handling in the subsequent fiber manufacturing process. During melting, surface tension allows for a certain degree of rounding of the marker material, thus adapting it to the contour of the filler rod's outer surface. After the marker element has been melted in, the accuracy and quality of the melting process can be checked and improved if necessary.

[0080] Thus, the semi-finished product is equipped with a marker element, eliminating the need to create a separate bore in the hollow cylinder of the jacket glass – with the risks and difficulties described above. At the same time, high accuracy can be guaranteed despite the high aspect ratio, which is manifested, for example, in the semi-finished product by the fact that the axial parallelism of the marker element has a deviation of less than 0.3 mm / m.

[0081] In a preferred embodiment, the recess comprises a bore and / or a longitudinal groove in the outer surface of the filler rod, wherein the semi-finished product according to the invention further comprises: the hollow cylinder having the central bore, at least two core rods containing the core glass and forming the core glass areas, the filler rod arranged in the central bore and at least one marker element placed in the recess of the filler rod.

[0082] The marker element is, for example, a rod extending parallel to one or more filler rods within the central bore. The marker element rod and filler rods may also be in a consolidated form, i.e., fused with their surroundings.

[0083] Two or more, for example four to seven, additional longitudinal bores (core rod bores) are machined into the hollow cylinder, their longitudinal axes running parallel to the longitudinal axis of the central bore. The core rod bores are either through holes or blind holes and serve to accommodate at least one core rod made of the core glass. 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 outer glass in such a way as to ensure light guidance within the core glass area. 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.

[0084] The semi-finished product is either drawn directly into the multicore fiber or consolidated into a preform for the multicore fiber, whereby consolidation may be accompanied by simultaneous elongation. The resulting "consolidated preform" is then either drawn into the multicore fiber or further processed into a "secondary preform" from which the final multicore fiber is drawn.

[0085] The marker element extends along the longitudinal axis of the filler rod, preferably over its entire length, i.e. from the first end of the filler rod to the second end of the filler rod, and it is preferably attached to the filler rod.

[0086] By attaching to the filler rod, the marker element benefits from its straightness and alignment; these properties are essentially transferred to the marker element. The attachment is based, for example, on friction, material adhesion, and / or a form fit between the filler rod and the marker element.

[0087] 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 connected to the filler rod. The at least one cylindrical component is, for example, a tube and preferably a rod. In the case of a marker element in the form of a tube, the tube wall can contain a material that has a higher viscosity than the cladding glass, so that during the fiber drawing process the bore does not completely collapse and remains as a cavity ("airline") in the finished multicore fiber.

[0088] The cross-sectional geometry, and optionally the outer diameter and length of the filler rod, are adapted to the geometry and length of the hollow cylinder's central bore. The central bore is preferably filled while leaving a circumferential gap with a gap width of less than 2 mm, particularly preferably a maximum of 1 mm. For example, with a central bore in the range of 38 mm to 78 mm, the filler rod diameter is in the range of 36 mm to 76 mm.

[0089] The marker element forms an air-filled, elongated cavity or channel, or it contains a marker material that preferably differs from the cladding glass and the glass filling material of the filling rod in at least one physical and / or chemical property, wherein the property is selected from: refractive index, color, fluorescence and / or specific glass density.

[0090] The property (or properties) that distinguishes the marker element from the glasses of the component assembly particularly affects the optical appearance of the marker element and is preferably detectable by means of an optical sensor. The glass composition of a marker glass—like, for example, the glass filler material—can be based on quartz glass. The refractive index of quartz glass can be altered by doping. For example, doping the marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. Incorporation of carbon into the marker quartz glass can lead to a blackening. Doping the marker quartz glass with titanium results in a gray or blue coloration, depending on the oxidation state. Doping the marker quartz glass with rare earth metals or germanium oxide manifests as fluorescence at dopant-specific wavelengths.The specific glass density of the marker element can be changed by pores and manifests itself in a reduction of optical transparency compared to bubble-free glass.

[0091] Using the inventive method or the inventive semi-finished product, a multi-core fiber is obtained in which the signal cores are arranged outside an envelope around the fiber's central axis and the marker zone is arranged inside the envelope around the fiber's central axis.

[0092] The type of multicore fiber corresponds to that of a "multicore fiber without a central signal core". All signal cores are located outside the fiber's central axis and completely outside any envelope around the fiber's central axis.

[0093] The multi-core fiber is traversed by at least one continuous, line-like marker zone. The marker zone serves to break symmetry and to uniquely identify the signal nuclei and their positions relative to each other and to the fiber's central axis.

[0094] Viewed in the fiber cross-section, the marker zone—preferably the only one—lies within the aforementioned envelope defined by the signal nuclei, and thus in a region between the signal nuclei (and not outside of them), which also encompasses the fiber's central axis. Surprisingly, it has been found that with this positioning of the marker zone, the multicore fiber exhibits a comparatively small fiber curl during the fiber drawing process. Without committing to this theory, it can be assumed that positioning the marker zone in the inner region of the fiber cross-section has less influence on the symmetry of the fiber design than positioning it in the outer region. Apparently, this results in lower radially acting forces being generated in the multicore fiber during the fiber drawing process. Definitions and measurement methods

[0095] Individual terms from the above description are further defined below. These definitions form 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) shall apply. In the event of a conflict between any of the following definitions and the rest of the description, the wording of the remaining description shall prevail. Jacketed glass hollow cylinder / jacketed glass area

[0096] The hollow cylinder contains a cladding glass. The cladding glass forms a cladding region with a central bore, outside of which the core glass regions designed for signal transmission are produced. The cladding glass consists, for example, of undoped fused silica or it contains at least one dopant that lowers the refractive index of fused silica. Fluorine and boron are dopants that can lower the refractive index of fused silica. The cladding glass hollow cylinder is elongated and essentially cylindrical. Deviations from the cylindrical shape may be present in the region of the end faces. The cladding glass cylinder is preferably manufactured using the OVD process. Core rods / core glass area

[0097] The core rods contain a core glass that exhibits a homogeneous or non-homogeneous refractive index profile in the radial direction. The core glass of each core rod forms a core glass region. The core rods can contain one region of a core glass with a comparatively high refractive index and at least one other region of a different glass with a comparatively low refractive index; for example, fused silica doped with fluorine and / or chlorine. The glass with the highest refractive index is typically located along the central axis of the core rod. It consists, for example, of fused silica to which at least one dopant has been added to increase the refractive index. In the multicore fiber, the core rod forms at least one signal core in which the signal to be transmitted is primarily carried.The signal core can border other glass areas with a lower refractive index, which have also been provided by the core rod. Filling rod / modified filling rod

[0098] The filler rod contains a glass core material. Within the multicore fiber, the filler rod does not form a signal core usable for signal transmission. In the simplest case, the composition of the glass core material corresponds to that of the cladding glass. However, the composition can also differ from that of the cladding glass to impart an additional property to the multicore fiber. For example, it can have a lower coefficient of thermal expansion than the cladding glass.

[0099] The modified filling rod is connected to a marker element, forming a manageable unit. It contains glass filling material and marker material. Marker element / marker material / marker glass

[0100] The marker element contains a marker material or consists partly of air or another gas. In particular, the marker element contains at least one marker glass. The chemical composition of the marker material differs from that of the cladding glass and the glass filler material of the core rod, and / or the density of the marker material differs from that of the cladding glass and the glass filler material. The marker element is present in the preform and in the component assembly as a component or as a layer or mass on a component and forms an optically detectable marker zone in the multicore fiber. Component ensemble / consolidated preform / secondary preform / semi-finished product

[0101] The "component ensemble" comprises the hollow glass cylinder with the core rods inserted within it, at least one filler rod, and at least one marker element. By fixing the core rods in the core rod bores—for example, by narrowing one end of the hollow glass cylinder or by collapse and fusion—a "preform" is obtained, also referred to here as a "consolidated preform." The component ensemble or the (consolidated) preform is then elongated to form a "secondary preform" or directly to the multicore fiber. The term "semi-finished product" encompasses the component ensemble, the consolidated preform, and the secondary preform. The forming of the component ensemble includes elongation to form the multicore fiber or the formation of the consolidated preform. Quartz glass

[0102] Quartz glass, for example, is melted 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. Merge

[0103] In the context of glass components, fusion refers to the process of fusing the components together at a contact surface. This fusion is achieved by heating the components, at least in the area of ​​the contact surface, using a heat source such as a furnace, a burner, or a laser. Position information: top / bottom

[0104] The information refers to positions during the elongation process and the fiber drawing process. "Bottom" indicates the position in the drawing direction, "Top" the position against the drawing direction. cross-section

[0105] The cut is perpendicular to the longitudinal direction / longitudinal axis. Longitudinal section

[0106] A cut parallel to the longitudinal direction / longitudinal axis. Drilling

[0107] The terms "bore", "center bore", "internal 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 onto the outer surface of a mandrel through a deposition or pressing process, after which the mandrel is removed. Axis parallel alignment / Axis parallelism

[0108] The reference axis is, in each case, the longitudinal axis of the cladding glass hollow cylinder or the central axis of the multicore fiber. Example of implementation

[0109] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. Specifically, a schematic representation is shown. Figure 1 a cross-section of a cladding glass hollow cylinder with a central bore and through bores for receiving core rods, Figure 2 Processing steps (a) to (d) for the production of a filling rod with marker element for insertion into the central bore of the mantel glass hollow cylinder of Figure 1 , Figure 3 the component ensemble consisting of the jacketed glass hollow cylinder and the inserted filling rod including the marker element, Figure 4 a cross-section of a consolidated preform made of a hollow glass cylinder, inserted filler rod including marker element and core rods inserted into the through-holes and Figure 5a cross-section of a preform of Figure 4 drawn multicore fiber.

[0110] Figure 1 Figure 1 schematically shows a cross-section of a hollow cylinder 1 made of a cladding glass, which serves as a base body for the production of a multi-core fiber.

[0111] The hollow cylinder 1 is produced in a known manner using the OVD method. In this process, SiO₂ soot particles are formed by passing a high-purity SiO₂ starting material, for example silicon tetrachloride, through a separator burner and into a burner flame, where it is oxidized to solid SiO₂. This solid SiO₂ is deposited from the gas phase as fine SiO₂ soot particles onto the outer surface of a cylindrical deposition mandrel rotating about its longitudinal axis. The separator burner performs a reversing back-and-forth motion along the longitudinal axis of the deposition mandrel. An SiO₂ soot body forms on the outer surface of the deposition mandrel. After completion of the deposition process, the deposition mandrel is removed, leaving an inner bore 2. The SiO₂ soot body is then vitrified in a vacuum furnace.

[0112] The resulting hollow cylinder 1 consists of undominated, synthetically produced quartz glass, which forms the cladding glass region 1b. The hollow cylinder has a length of 1500 mm and is adjusted to a nominal outer diameter of 200 mm by cylindrical grinding and to an inner diameter of 42 mm by drilling and honing. Four bores 3 are produced in a predetermined (here square) configuration by mechanical drilling in the direction of the hollow cylinder's longitudinal axis 1a, as shown in the illustration of Figure 1 perpendicular to the plane of the sheet. The bores 3 serve to accommodate core rods ( Figure 4 ) and have a diameter of 30 mm. The bores extend through the entire hollow cylinder 1 (through bores). In an alternative embodiment, the bores are designed as blind holes.

[0113] Figure 2 schematically shows the process steps for manufacturing a filling rod 5. The in Figure 2(a)The infill rod 5 shown is preferably made of the same glass as the hollow cylinder 1, i.e., undoped quartz glass. Known methods are suitable for its production, such as VAD (Vapor Phase Axial Deposition), OVD (Outside Vapor Deposition), or MCVD (Modified Chemical Vapor Deposition). It serves to fill the inner bore 2 of the hollow cylinder and has a length of approximately 1500 mm and an initial outer diameter of approximately 45 mm, which is reduced to approximately 40 mm by cylindrical grinding. Cylindrical grinding eliminates any defects on the outer surface 5b and any bends. Alternatively or additionally, the diameter adjustment and surface improvement are achieved by elongation in a tool-free elongation process. In this illustration, the longitudinal axis 5a of the infill rod also runs perpendicular to the plane of the blade.

[0114] Figure 2(b)Figure 1 shows that a longitudinal groove 6 has been milled into the outer surface of the filler rod 5. The longitudinal groove 6 extends over the entire length of the filler rod 5. It has a U-shape 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 produced from the longitudinal groove (6), which forms a marker element according to the invention. This will be shown below with reference to Figure 4 explained in more detail.

[0115] Figure 2(c)Figure 6 shows the longitudinal groove 6 with a marker rod 7 inserted therein. The marker rod 7 has a diameter of 5 mm. It is made of synthetically produced fluorine-doped quartz glass, commercially available under the designation F320. Both the viscosity and the refractive index of the fluorine-doped quartz glass of the marker rod 7 are lower than those of the undoped quartz glass from which the hollow cylinder 1 and the filler rod 5 are made. The marker rod 7 is obtained by elongating a starting cylinder of F320 quartz glass using a tool-free process. It has a smooth, molten surface and is characterized by high dimensional accuracy, allowing it to be inserted easily and precisely into the narrow longitudinal groove 6.

[0116] The filler rod 5 and the marker rod 7 are then fused together. The filler rod 5 is positioned with its longitudinal axis 5a oriented horizontally, such that the longitudinal groove 6 is located on its upper side. The marker rod 7, inserted into the longitudinal groove 6, is first heated at specific points using a torch. This heat fixes it in place at three approximately evenly spaced fixing points located at the ends and in the middle of the marker rod 7, distributed over 95% of its length. The marker rod 7 is then heated evenly with the torch until the fluorine-doped quartz glass, due to its relatively low viscosity, softens and deforms, allowing it to sink into and fill the longitudinal groove 6.The surface of the softened glass mass adjacent to the free atmosphere exhibits a certain bulging due to surface tension, thus avoiding a pronounced step between the lateral edges of the longitudinal groove 6 and the outer surface 5b of the filling rod 5.

[0117] Figure 2(d) Figure 8 shows the marker glass mass 8 after softening, deformation, and fusing with the former filler rod 5, as well as the resulting modified filler rod 5c filled with marker glass mass 8. The glass volume of the former marker rod 7 is matched to the inner volume of the longitudinal groove 6 such that the marker glass mass 8 just completely fills the longitudinal groove 6.

[0118] The modified filling rod 5c, thus filled with the marker glass mass 8, is inserted into the inner bore 2 of the hollow cylinder 1. Figure 3 schematically shows the component ensemble 9 consisting of hollow cylinder 1 and modified filling rod 5c with the marker glass mass 8.

[0119] In addition, four core rods 4 made of germanium-doped quartz glass with a length of approximately 1500 mm and an outer diameter of approximately 28 mm are produced. Well-known techniques are also suitable for this, for example the MCVD process (Modified Chemical Vapor Deposition).

[0120] The core rods 4 are inserted into the bores 3. Subsequently, the component ensemble 9, consisting of the cladding glass hollow cylinder 1, the modified filler rod 5c, and the core rods 4, is heated so that the inner bore 2 and the annular gap close around the core rods 4 and all components of the ensemble 9 are fused together.

[0121] Figure 4Figure 1 schematically shows the component assembly, fixed in this way, consisting of the cladding glass hollow cylinder 1, the modified filler rod 5c, and the core rods 4, which forms the consolidated preform 10. All core rods 4 form separate, circular core glass areas 4a, which are located entirely outside an envelope 11, whereas the marker element 8 is located entirely inside this envelope 11.

[0122] The consolidated preform 10 is then elongated to form a secondary preform. In this process, the preform 10 is held in an elongation device by means of a holder in a vertical orientation along the longitudinal axis 1a of the hollow cylinder. The secondary preform thus produced is then drawn in a drawing device to form a multicore fiber 20 in the usual manner.

[0123] In this embodiment, the marker element 8 is present as marker glass mass 8, which was produced by forming the original marker rod 7. In an alternative procedure, the longitudinal groove 6 is unfilled (no rod or tube is inserted) when the component assembly 9 is consolidated into the preform 10, and the complete collapse of the longitudinal groove 6 is prevented by creating and maintaining overpressure within it. In this way, a cavity is created that extends along the longitudinal axis 1a and exists in the multicore fiber as an air-filled hollow channel. The hollow channel can serve as a marker zone, since the refractive index of air differs significantly from that of the cladding glass 1b.

[0124] Figure 5Figure 1 schematically shows the cross-section of the multi-core fiber 20. Apart from the smaller radial dimensions, this corresponds essentially to the cross-section of the consolidated preform 10. The core glass regions (4a) of the former core rods (4) form signal cores 4b, which extend along the fiber's longitudinal axis 20a; the former filler rod (5) has become part of the cladding glass region 1b and is no longer visually distinguishable from it, and the former marker element (8) forms a marker zone 8a. All signal cores 4b are located entirely outside the envelope 11a, whereas the marker zone 8a is located entirely within this envelope 11a. The marker zone 8a is characterized by its small size, so that it exerts a low stress on the multi-core fiber 20 during the fiber drawing process, resulting in a low fiber curl.

Claims

1. A method for producing a multicore fiber (20), comprising a method step in which a component group (9) is reshaped to form the multicore fiber (20) or a preform (10) for the multicore fiber (20) and comprises a hollow cylinder (1) comprising a central bore (2) and a hollow cylinder longitudinal axis (1a), which hollow cylinder comprises a cladding glass region (1b) made of cladding glass and a plurality of core glass regions (4a) provided with a core glass, wherein at least a part of the central bore (2) is occupied by a glass filling rod (5) which comprises a filling rod longitudinal axis (5a) and a filling rod outer cladding surface (5b), characterized in that a recess (5) extending in the direction of the filling rod longitudinal axis (2) is produced in or on the filling rod (5), into which recess a marker element (6; 7; 8) made of marker glass is inserted or which forms the marker element (6; 7; 8).

2. The method according to claim 1, characterized in that the marker element (6; 7; 8) extends along the filling rod longitudinal axis (5a) and is melted into the recess (6) prior to reshaping to form the preform (10) or the multicore fiber (20).

3. The method according to claim 1 or 2, characterized in that the marker element (6; 7; 8) has a length and in that melting takes place along at least 80% of this length, preferably along at least 90% of this length, completely, in parts or at certain points.

4. The method according to one or more of claims 1 to 3, characterized in that the melting of the marker element (6; 7; 8) comprises a method step in which the filling rod (5) with the horizontally oriented filling rod longitudinal axis (5a) is mounted in such a way that the recess (6) is located on an upper side of the filling rod outer cladding surface (5b), wherein the material of the marker element (6; 7; 8) is heated and softened by means of a heat source.

5. The method according to one or more of the preceding claims, characterized in that the production of the component group (9) comprises the following method steps: (a) providing the hollow cylinder (1) containing the cladding glass, (b) providing a plurality of core rods (4) containing the core glass, (c) providing a filling rod (5) comprising a filling rod longitudinal axis (5a) and containing the glass filling material, (d) producing the at least one recess (6) on the outer cladding surface (5b) of the filling rod (5), (e) providing the marker element (6; 7; 8), (f) arranging and melting the marker element (6; 7; 8) into the recess (6), (g) producing core rod bores (3) extending along the hollow cylinder longitudinal axis (1a), (h) introducing the filling rod (5) and the marker element (6; 7; 8) into the central bore (2), and (i) introducing the core rods (4) into the core rod bores (3), forming the component group (9).

6. The method according to one or more of the preceding claims, characterized in that the marker element (7; 8) is provided in the form of a cylindrical component or in the form of a layer or mass connected to the filling rod (5).

7. The method according to one or more of the preceding claims, characterized in that the recess (6) comprises a bore and / or a longitudinal groove in the outer cladding surface of the filling rod (5).

8. The method according to one or more of the preceding claims, characterized in that the marker element (6; 7; 8) forms an air-filled, elongate cavity or in that it contains a marker material which differs in at least one physical and / or chemical property from the cladding glass and from the glass filling material, wherein the property is selected from: refractive index, color, fluorescence, and / or specific glass density.

9. A semifinished product for producing a multicore fiber, comprising a hollow cylinder (1) comprising a central bore (1a), which hollow cylinder comprises a cladding glass region (1b) made of cladding glass, and a hollow cylinder longitudinal axis (1a), and a plurality of core glass regions (4a) provided with a core glass within the cladding glass region (1b), wherein at least a part of the central bore (2) is occupied by a glass filling rod (5) which comprises a filling rod longitudinal axis (5a) and a filling rod outer cladding surface (5b), characterized in that the filling rod (5) comprises a recess (5) which extends in the direction of the filling rod longitudinal axis (2) and into which a marker element (6; 7; 8) made of marker glass is inserted, or which forms the marker element (6; 7; 8) which extends along the central bore longitudinal axis (1a) and the filling rod longitudinal axis (2b).

10. The semifinished product according to claim 9, characterized in that the marker element (6; 7; 8) has a length and in that it is melted into the recess (5) completely, in parts or at certain points along at least 80% of this length, preferably along at least 90%.

11. The semifinished product according to claim 9 or 10, characterized in that the marker element comprises a channel filled with a gas.

12. The semifinished product according to one or more of claims 9 to 11, characterized in that the recess (6) comprises a bore and / or a longitudinal groove in the outer cladding surface (5b) of the filling rod (5), and in that the semifinished product further comprises: the hollow cylinder (1) comprising the central bore (2), at least two core rods (4) containing the core glass and forming the core glass regions (4a), the filling rod (5) arranged in the central bore (2), and at least one marker element (6; 7; 8) attached in the recess (6) of the filling rod (5).

13. The semifinished product according to one or more of claims 9 to 12, characterized in that the marker element (7; 8) is present in the form of a cylindrical component or in the form of a layer or mass connected to the filling rod.

14. The semifinished product according to one or more of claims 9 to 13, characterized in that the marker element (6) forms an air-filled, elongate cavity, or in that it contains a marker material which differs in at least one physical and / or chemical property from the cladding glass, the core glass and the glass filling material, wherein the property is selected from: refractive index, color, fluorescence, and / or specific glass density.