Method and semi-finished product for manufacturing multicore fibers

Abstract

A known method for manufacturing a multicore fiber comprises method steps in which a group of components is reshaped to form a multicore fiber or a preform for a multicore fiber, comprising a hollow cylinder comprising a central bore and a hollow cylinder longitudinal axis, the hollow cylinder comprising a cladding glass region made from cladding glass and a plurality of core glass regions occupied by core glass, at least a part of the central bore being occupied by a glass filling material. To provide a method for manufacturing a multicore fiber without a central signal core, in which the risk of rejection during completion of the hollow glass cladding cylinder is reduced, it is proposed that a marker element made of marker glass is used, adjacent to the glass filling material extending along the longitudinal axis of the central bore.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a multi-core fiber, comprising a method step of reforming a group of components to form a multi-core fiber comprising a hollow cylinder including a central bore and a longitudinal axis of the hollow cylinder, or to a perform for a multi-core fiber. The hollow cylinder includes a cladding glass region made of cladding glass and a plurality of core glass regions occupied by core glass, and at least a part of the central bore is occupied by a glass filling rod including a filling rod longitudinal axis and an outer cladding surface of the filling rod.

[0002] Furthermore, the present invention relates to a semi-finished product for manufacturing a multi-core fiber comprising a hollow cylinder including a central bore and a longitudinal axis of the hollow cylinder. The hollow cylinder includes a cladding glass region made of cladding glass and a plurality of core glass regions occupied by core glass within the cladding glass region, and at least a part of the central bore is occupied by a glass filling rod including a filling rod longitudinal axis and an outer cladding surface of the filling rod.

[0003] In a multi-core fiber, a plurality of light core regions of the optical wave guiding type (hereinafter also referred to as "signal cores") are integrated in a common fiber. The signal cores extend along the longitudinal axis of the fiber. They are surrounded by a cladding material having a lower refractive index, enabling light to be guided independently of each other. This fiber design ensures a high signal transmission capacity because different signals coupled to a single optical fiber can be transmitted simultaneously in each of the spatially separated signal cores. This signal transmission method is also called "space multiplexing" and can particularly increase the data transmission capacity in optical communication. Multi-core fibers are also regarded as important components for transmitting energy for material processing as components of optical fiber sensors in measurement and medical technology, and are considered for the purposes of illumination and imaging in microscopes or endoscope devices.

Background Art

[0004] Multicore fibers are manufactured by stretching a solid preform or a group of components. These often consist of synthetically produced quartz glass (SiO2), which may be doped or undoped. The production of synthetic quartz glass includes plasma or CVD deposition methods, known, for example, as OVD, VAD, MCVD, PCVD, or FCVD. Liquid or gaseous silicon-containing starting materials are subjected to a chemical reaction (hydrolysis, thermal decomposition, or oxidation), and the reaction product, i.e., fine particles of SiO2, is deposited as a solid from the gas phase on the deposition surface. Starting materials include, for example, silicon tetrachloride (SiCl4) or chlorine-free silicon compounds, such as polyalkylsiloxanes. The reaction zone is, for example, an oven, a burner flame, or an arc (plasma).

[0005] In the so-called "stack-and-draw" method, core rods and glass cylinders of different diameters are stacked to create a relatively high packing density and a degree of symmetry. The cylindrical components are inserted into a sheath tube and spatially fixed within it. This group is then drawn out to form multicore fibers, or further pre-processed to form a preform from which the multicore fibers are drawn.

[0006] The "stack-and-draw" method requires a high degree of adjustment effort and easily leads to dimensional stability errors and the introduction of impurities resulting from a large proportion of free-form surface areas. Furthermore, due to differences in radial packing density, elongated preforms often have different radial values ​​in the azimuthal direction, which must be compensated for by cylindrical grinding.

[0007] U.S. Patent Application Publication No. 2022 / 003921 describes a semi-finished product and method for producing a multicore fiber by manufacturing five circular openings in a cladding glass solid cylinder by a heating or powder molding process, the centrally located opening being uniformly surrounded and overlapping by four surrounding openings. The four outer opening regions are occupied by core glass rods, and a rod with a polygonal cross-section made of low-viscosity glass is inserted into the still-free central opening region, with marker rods inserted into the empty spaces adjacent to this rod. In a later method step, all components of the group are heated, and the rods made of low-viscosity glass soften first to fill the gaps.

[0008] A semi-finished product and a method for producing a multicore fiber by stretching the semi-finished product are known from U.S. Patent Application Publication No. 2016 / 347645. The semi-finished product forms a multipart fiber preform consisting of a single cladding tube made of cladding glass, and is designed to receive and axially guide a stack of three structurally identical cylindrical stack pieces made of cladding glass, each of which includes through bores and longitudinal grooves on its cylinder cladding surface, a plurality of core rods for insertion into the through bores, and a plurality of marker rods for insertion into the longitudinal grooves. Within the cladding tube bore, the stack pieces are arranged overlapping each other so that the through bores and longitudinal grooves are aligned. Marker zones of the fiber preform formed by the marker rods run on the inner cladding surface of the cladding tube.

[0009] U.S. Patent No. 8,532,454 discloses a multicore fiber having seven core regions.

[0010] Another known procedure for manufacturing preforms for multicore fibers involves, for example, the creation of multiple through-holes by longitudinal drilling of a glass-clad cylinder based on synthetic SiO2, as described in U.S. Patent Application Publication No. 2021 / 0300812, which extend in the direction of the longitudinal axis of the cylinder. A core rod containing a core material with a refractive index higher than that of the glass-clad cylinder is inserted into each through-hole. When there are many through-holes, thin, easily breakable walls may remain between adjacent holes. To reduce the risk of breakage, it has been proposed to create the required number of through-holes in a glass-clad cylinder in sequence, occupying and fusing at least a portion of the through-holes with core rods in between.

[0011] Glass-clad cylinders are elongated. Therefore, through holes have a large aspect ratio (ratio of length to diameter), which basically makes it difficult to manufacture through holes with dimensional accuracy and to align them precisely parallel to the longitudinal axis of the hollow glass-clad cylinder.

[0012] The method described in U.S. Patent Publication No. 2015 / 284286 and U.S. Patent Publication No. 2015 / 0307387 uses a hollow glass cladding cylinder in the form of an SiO2-based soot (SiO2 soot) manufactured using the OVD method, at a concentration of 0.8 g / cm³. 3 ~1.6g / cm 3It has a density of . In the OVD method, the deposition surface is generally the outer cladding surface of a rod-shaped or tubular deposition mandrel that rotates around its longitudinal axis. A substantially cylindrical soot body is deposited by reversal movement back and forth through the reaction zone. After the deposition process is complete, the deposition mandrel is removed so that a central through-hole remains on the central axis of the cylindrical soot body. A longitudinal bore is introduced into the hollow glass cladding cylinder thus produced to receive the core glass rod, and because the density of the SiO2 soot body is lower than that of quartz glass, it is easier to produce a dimensionally accurate longitudinal bore. The central "OVD through-hole" remaining due to the manufacturing process can result in asymmetric deformation when it collapses and destroys the fiber design. Collapse also reduces the cross-sectional area of ​​the cladding glass portion. These drawbacks can be prevented by inserting a filling rod that closes the OVD through-hole. The filling rod is made of glass having substantially the same refractive index as the hollow glass cladding cylinder. The filling rod can also be manufactured by the OVD method, press forming, or a combination of press forming and the OVD method. Furthermore, a channel for receiving the marker element is introduced into the hollow glass cladding cylinder. The channel is created by mechanically boring out a region of the hollow glass cladding cylinder near the edge.

[0013] Marker elements are used to break symmetry within a multicore fiber by forming continuous linear marker zones, allowing for clear identification and assignment of signal cores and their positions relative to each other and the fiber's central axis. This is necessary, for example, to allow two multicore fibers to be joined together with low attenuation by their end faces using conventional joining methods.

[0014] To join multicore fibers, the fiber ends to be connected are positioned so that their end faces face each other. In known methods, light is simultaneously supplied to all signal cores at the opposite fiber end and collectively detected at the fiber end of the other multicore fiber by a photodetector and power meter. In particular, if one end of a multicore fiber is unavailable for light supply, light is shone onto this multicore fiber from the side. By displacing the multicore fiber end faces in the horizontal and vertical directions and rotating them relative to each other in the azimuthal direction, the fusion splicer automatically aligns the fiber ends until the signal cores are correctly assigned and the collective light-receiving power is maximized, and then they are fused together at this position. [Overview of the project] [Problems that the invention aims to solve]

[0015] Theoretically, the more multi-cores there are, the greater the increase in data transmission capacity compared to an optical fiber with a single fiber core (single-mode or multi-mode fiber). On the other hand, in principle, each of the multiple cores must have optical attenuation that is approximately equivalent to the optical attenuation of an optical fiber with a single core. This requires that the fiber design does not cause additional attenuation or interfere with the independent information transmission of the signal core as an interfering signal. However, this can be caused by so-called "crosstalk" between multiple cores, especially if they are too close to each other. Therefore, this effect requires that a certain minimum distance be maintained between fiber cores. For these reasons, the available cross-sectional area in the radial section of a multi-core fiber should be used as completely as possible for occupation by the multi-cores.

[0016] Therefore, the additional marker zone introduced into the fiber design should occupy only a small proportion of the fiber cross-section. A small marker zone size is also advantageous in counteracting other undesirable effects such as so-called fiber curl or stress induced within the fiber. Thus, the diameter of the channel for receiving the marker element is small, generally significantly smaller than the diameter of the bore for receiving the core rod. Typically, the channel diameter of a hollow glass cladding cylinder before the fiber stretching process is less than 15 mm, with an aspect ratio greater than 65 (for a hollow cylinder length of approximately 1 m).

[0017] Dimensionally accurate manufacturing and precise alignment of such thin channels within hollow glass-clad cylinders are difficult, even when using high-precision bore drilling machines. Furthermore, it has been shown that cracks are increasingly generated in the channel walls, especially during the bore drilling of thin channels. The cost of manufacturing hollow glass-clad cylinders from synthetic quartz glass is high, and the loss is particularly painful if, among other things, a small bore to accept a marker element leads to the rejection of the otherwise finished hollow glass-clad cylinder.

[0018] Even worse, the cutting and splicing of multicore fibers can be performed at any point according to specific requirements when used as intended, and consequently, must rely on a consistent geometric shape along the entire fiber length so as not to rely on measurements.

[0019] Therefore, an object of the present invention is to provide a method for manufacturing a multicore fiber without a central signal core, which reduces the drawbacks of known methods and, in particular, reduces the risk of rejection during the completion of the hollow glass cladding cylinder.

[0020] Furthermore, an object of the present invention is specifically to provide a semi-finished product suitable for manufacturing multi-core fibers without a central signal core, characterized by low "fiber curl".

[0021] A property of glass fibers, defined as the degree of curvature over a specific length of the fiber, is called "fiber curl." This curvature is caused by thermal stress that occurs during fiber manufacturing. A large fiber curl makes low-attenuation bonding of multicore fibers more difficult. [Modes for carrying out the invention]

[0022] With respect to this method, the objective is achieved by the present invention in that, based on the method described earlier, a recess extending in the direction of the longitudinal axis of the filling rod is generated inside or on the filling rod, and a marker element made from marker glass is inserted into or formed within the recess.

[0023] A hollow glass cladding cylinder having a central bore is used. Such a hollow cylinder is obtained, for example, and preferably, using the OVD (Outside Vapor Deposition) method after the deposition mandrel has been removed. The production of hollow glass cladding cylinders using the OVD method is cost-effective compared to other manufacturing methods, particularly the VAD (Vapor Phase Axial Deposition) method. However, it has the disadvantage of leaving the aforementioned central bore. This central bore can be completely or partially closed by glass filling material provided by multiple filling rods, or by a single filling rod containing glass filling material.

[0024] In embodiments where the chemical composition of the glass-filled rod corresponds to the chemical composition of the cladding glass, the glass-filled material of the filling rod within the multicore fiber forms part of the optical cladding. In embodiments where the chemical compositions of the glass-filled material and the cladding glass differ, the glass-filled material of the filling rod within the multicore fiber can have additional functions, 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 present invention is used to manufacture a multi-core fiber without a central signal core. The central bore is used to insert a marker element in addition to the glass filling material of the filling rod. The glass filling material does not include a core region suitable for signal transmission.

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

[0027] The marker element exists within the component group as a component, as a coating of the component, or as a cavity, and forms a continuous linear marker zone composed of a marker material or air within the multi-core fiber. The marker zone can be used, for example, during joining for symmetry breaking and for clear identification of the signal cores and their positions relative to each other and relative to the fiber central axis.

[0028] The recess is, for example, a hollow channel extending through the filling rod along the longitudinal axis of the filling rod. In this case, the filling rod forms the edge of the marker element in the form of an elongated cavity filled with air. Or, the marker element is arranged within a recess on the outer cladding surface of the cylindrical filling rod.

[0029] The marker element is arranged within or on the glass filling rod, and thus within the central bore of the hollow glass cladding cylinder, but not within the cladding glass of the hollow cylinder. Therefore, the need to adapt the hollow glass cladding cylinder for the purpose of inserting the marker element is eliminated, for example, by mechanical processing, especially by generating a bore for receiving the marker element within the hollow glass cladding cylinder. Therefore, the risk of damage associated with adapting the hollow glass cladding cylinder in this way is eliminated.

[0030] The dimensional stability and straightness of the central bore of a hollow glass cladding cylinder can be easily ensured by the OVD manufacturing method itself and, if necessary, by subsequent means. A suitable subsequent means includes, for example, a mechanical reworking and / or stretching process of the central bore from which a hollow glass cladding cylinder is manufactured, or from which a tube strand cut to a certain length is provided, to stretch the tube strand. The stretching process is preferably performed without using a forming tool that engages with the stretched tube strand to avoid damage to the tube strand surface.

[0031] Similarly, the dimensional stability and straightness of the filling rod, which is precisely inserted into the central bore, can be relatively easily ensured by machining and / or by means of such an elongation process. This machining may optionally be an external machining method, which is generally no more significantly complex than internal machining methods.

[0032] Alignment of the marker element parallel to the axial direction is facilitated by the formation of a recess in the filling rod extending in the direction of the longitudinal axis of the filling rod, or by insertion into a recess extending in the direction of the longitudinal axis of the filling rod.

[0033] Marker elements are inserted into recesses, for example, by inserting cylindrical components (rods or tubes) made of marker glass that extend parallel to the outer cladding surface of the filling rod, or by introducing a bed of particles from the marker glass, or by internally coating the recess with marker glass.

[0034] The recess is designed, for example, as a bore within the filling rod, preferably as a longitudinal slot (longitudinal groove) on the outer cladding surface of the filling rod. The groove-shaped recess is filled, for example, with a cylindrical component made from marker material or particulate marker material. Particulate marker material can have certain dimensional stability by thermal compression or by the addition of a binder. The marker material completely or partially fills the recess. This ensures a secure fit between the marker element and the filling rod. The marker element and the filling rod can also be connected to each other in advance (i.e., before insertion into the central bore) by material bonding, for example, by sintering or fusion.

[0035] The procedure of providing longitudinal grooves in the filling rod is particularly preferred. On the one hand, longitudinal grooves on the outer cladding surface of the filling rod can be manufactured particularly easily and geometrically accurately compared to the bore of the filling rod by, for example, milling with a mechanical milling cutter or laser ablation. On the other hand, longitudinal grooves manufactured in this manner are just as precise and linear as the filling rod itself. Furthermore, the depth or opening width of the longitudinal grooves can be substantially reduced as desired, for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. The longitudinal grooves can be reshaped as walls of air-filled hollow channels that can function as marker elements, or the longitudinal grooves can be filled with marker material. This means that geometrically accurate marker elements with small volume can be manufactured in a simple manner, and their axial misalignment in the preform or group of components is less than 0.3 mm / m, thus forming small, high-precision marker zones corresponding to the multicore fiber.

[0036] The longitudinal grooves are filled with marker elements, for example, by inserting cylindrical components (rods or tubes) made from marker material, by introducing a bed of particles from the marker material, or by internally coating the longitudinal grooves with marker material. The cylindrical components, each consisting of the marker material or a bed from the marker material, can be further fixed within the longitudinal grooves by fusion bonding.

[0037] The relatively easy-to-achieve straightness of the filling rod and central bore also facilitates the axial alignment of the marker element. This is particularly preferred in procedures where the marker element extends along the longitudinal axis of the filling rod and is fused into the recess before reshaping to form a preform or multicore fiber.

[0038] As a result, the marker elements (components, floors, layers) are fixed within the recess by fusion. For this purpose, they are melted within the recess over at least a portion of their length, preferably locally at several points distributed over their length, and ideally over their entire length. The filling rod filled with marker element material will also be referred to below as the “modified filling rod”.

[0039] By melting marker elements into recesses before reshaping the constituent groups to form multicore fibers or preforms, the side edges of the recesses filled with molten marker elements form a nearly stepless, continuous transition to the outer cladding surface of the modified filling rod, thus ensuring the avoidance of structural defects during the fiber stretching process. This has a positive effect on the dimensional stability of the multicore fibers. The marker elements, for example, completely fill the recesses and, ideally, have a curvature that conforms to the outer contour of the outer cladding surface of the filling rod.

[0040] Therefore, a preferred procedure results in a marker element having a certain length, and melting occurring completely, partially, or at a specific point along at least 80% of this length, preferably at least 90% of this length.

[0041] The melting of the marker element preferably involves a method step in which the filling rod is mounted with a horizontally oriented longitudinal axis of the filling rod such that the recess is located above the outer cladding surface of the filling rod, and the material of the marker element inserted into the recess is heated and softened by a heat source.

[0042] By aligning the longitudinal axis of the filling rod horizontally and melting the marker element, gravity causes the material of the marker element to settle downward as soon as it is locally heated and softened, for example by a burner or laser, thereby filling any remaining cavities within the recess. Surface tension can result in rounding of the free surface region adjacent to the atmosphere.

[0043] In this way, it is far better and easier to uniformly, and preferably completely, fill the recesses compared to, for example, when the marker elements and recesses are oriented vertically during the melting process.

[0044] The molten marker elements within the recesses are fixed to the filling rod, thereby facilitating the handling of the modified filling rod in further fiber manufacturing processes. During melting, a certain degree of roundness of the marker material, and therefore conformity to the contour of the outer cladding surface of the filling rod, can be achieved by surface tension. After melting the marker elements, the accuracy and quality of the melting of the modified filling rod can be checked and improved as needed.

[0045] In this way, the component group is equipped with marker elements without the need to create separate holes in the hollow glass cladding cylinder, which is associated with the risks and difficulties mentioned above. At the same time, a high level of precision can be guaranteed despite the high aspect ratio, which is evident in the preform or component group, for example, in that the axial parallelism of the marker elements has a deviation of less than 0.3 mm / m.

[0046] In a preferred procedure, the marker element exists as a filling rod within the central bore or as a rod extending parallel to multiple filling rods.

[0047] In a more preferred procedure, the marker element exists as a layer applied to the recess of the filling rod.

[0048] Two or more additional longitudinal bores (core rod bores), for example, four to seven, are typically fabricated within the hollow cylinder, their longitudinal axes extending parallel to the longitudinal axis of the central bore. The core rod bores are either through bores or dead bores, each serving to accommodate at least one core rod fabricated from the core glass. Radially, the composition of the core glass is either uniformly homogeneous or changes gradually or stepwise. This differs from cladding glass in that it ensures light induction within the core glass region.

[0049] A desired number of core rod bores are produced in a single operation, each occupied by at least one core rod, or only one core rod bore, or only a first distribution of the desired number of core rod bores is produced in advance, which is occupied by at least one core rod in each case, and the core rod bores occupied by the core rods are crushed (this reshaping process is also referred to herein as “compaction”) before the remaining distribution or further distribution of the core rod bores is produced in a second or further operation, which is also occupied by at least one core rod in each case and is optionally crushed. In the simplest case, all core rods have the same dimensions and consist of the same core glass. However, the core rods may also differ in terms of their dimensions and / or the composition of the corresponding core glass.

[0050] The filling of the central bore with a filling rod including marker elements can be performed before or after all core rod bores are produced and / or filled, or before or after some of the core rod bores are produced and / or filled. In a preferred method, the central bore is filled with a filling rod and marker elements, and the filled central bore is then compressed by heating, and then the desired core rod bore is produced. In another preferred method, a first distribution of the desired number of core rod bore is manufactured and fitted to at least one core rod, the central bore is filled with a filling rod and marker elements, and then the filled core rod bore and filled central bore are compacted by heating, and only then is the second portion of the core rod bore manufactured.

[0051] The components thus produced are reshaped and directly drawn to form multicore fibers, or compacted to form preforms for multicore fibers, the compaction process of which can be associated with a simultaneous extension process. The “compacted preforms” thus produced are optionally drawn to form multicore fibers, or further processed to form “secondary preforms.” Further processing to form “secondary preforms” includes, for example, generating additional bores within the glass cladding region and occupying them with core glass or other glass, or performing one or more of the following hot forming processes, i.e., collapse, collapse, extension, collapse and simultaneous extension on additional cladding material, once or repeatedly. The multicore fibers are drawn from the secondary preforms produced by the further processing.

[0052] In a preferred procedure, the manufacturing of the component group includes the following method steps: (a) A step of providing a hollow cylinder containing cladding glass, (b) A step of providing a plurality of core rods including core glass, (c) A step of providing a filling rod that includes a longitudinal axis of the filling rod and contains glass filling material, (d) A step of creating at least one recess on the outer cladding surface of the filling rod, (e) A process of providing marker elements, (f) A step of placing marker elements in the groove and melting them, (g) A process of generating a core rod bore that extends along the longitudinal axis of the hollow cylinder, (h) A step of introducing the filling rod and marker element into the central bore, (i) A step of introducing the core rod into the core rod bore to form a group of components.

[0053] The components thus manufactured include a hollow cylinder, at least one modified filling rod fused to at least one marker element, and a core rod. List letters (a) to (g) simply specify the preferred but not required order of the method steps. The core rod of the components is also referred to here as the core rod if it has already been molten within its respective core rod bore.

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

[0055] By fusing with the filling rod, the marker element benefits particularly from its straightness and alignment, and these properties are substantially 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 may contain a material with a higher viscosity than cladding glass, and as a result, the bore does not completely collapse during the fiber stretching process but is maintained as a cavity ("airline") within the finished multicore fiber.

[0057] The cross-sectional shape, and optionally the outer diameter and length of the filling rod, are matched to the shape and length of the hollow cylinder center bore. The center bore is preferably filled leaving a circumferential gap with a gap width of less than 2 mm, and particularly preferably a maximum of 1 mm. For example, for a center bore in the range of 38 mm to 78 mm, the diameter of the filling rod is in the range of 36 mm to 76 mm.

[0058] The marker element forms an elongated cavity (channel) filled with air, or preferably comprises a marker material having at least one physical and / or chemical property different from the cladding glass and the glass filling material of the filling rod, the property preferably selected from refractive index, color, fluorescence, and / or specific glass density.

[0059] The properties (or multiple properties) that distinguish marker elements from the constituent glass groups particularly affect the visual appearance of the marker elements and are preferably detectable by optical sensors. The glass composition of marker glass, for example, the glass filling material of the filling rod, can also be based on quartz glass. The refractive index of quartz glass can be altered by doping. For example, doping marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. Incorporating carbon into marker quartz glass can result in black coloration. Depending on the oxidation state, doping marker quartz glass with titanium produces gray or blue coloration. Doping marker quartz glass with rare earth metals or germanium oxide results in fluorescence at dopant-specific wavelengths. The specific glass density of marker elements can be altered by pores, which manifests as a decrease in light transmittance compared to bubble-free glass.

[0060] With respect to semi-finished products for manufacturing multicore fibers, the above technical objective is achieved by the present invention, starting from the semi-finished products described at the beginning, by including a filling rod with a recess extending in the direction of the longitudinal axis of the filling rod, into which a marker element made of marker glass is inserted, or by forming a marker element extending along the longitudinal axis of the central bore and the longitudinal axis of the filling rod.

[0061] A semi-finished product according to the present invention is used to manufacture a multi-core fiber without a central signal core. It includes a hollow cylinder comprising a cladding glass region made of cladding glass and a plurality of core glass regions occupied by core glass. The central bore of the hollow cylinder is fully or at least partially occupied by a filling rod made of glass filling material. At least one recess is located in or on the filling rod extending in the direction of the longitudinal axis of the filling rod, and a marker element is inserted into the recess or the recess forms a marker element.

[0062] The longitudinal axis of the central bore and the longitudinal axis of the filling rod extend coaxially with each other in the semi-finished product.

[0063] The number of core glass regions is at least two, preferably four to seven. For example, each is designed as a core rod bore extending parallel to the longitudinal axis of the hollow cylinder within the cladding glass region of the hollow cylinder, and each is occupied by a core rod. A filling rod is inserted into the central bore of the hollow cylinder and is at least partially filled. At least one marker element is also positioned within the central bore of the hollow cylinder. This marker element is connected to the filling rod by being formed as a recess within the filling rod, or by at least partially filling a recess within or on the filling rod. A filling rod thus filled is also referred to herein as a “modified filling rod”. The filling rod does not contain any core regions suitable for signal transmission.

[0064] A semi-finished product forms a set of components when all or some of the core rods, filling rods, and marker elements are merely inserted into their respective bores or recesses and have not yet fused together. A semi-finished product forms a preform (also called a "compacted preform") when all the bores or recesses collapse and the core rods, filling rods, and marker elements fused together with each other or into the hollow cylinder.

[0065] A hollow glass-clad cylinder has a central bore. Such a hollow cylinder can be obtained cost-effectively, for example, by using the OVD method after the deposition mandrel has been removed. The central bore is completely or partially closed by glass-filling material provided by multiple filling rods, or preferably by a single filling rod.

[0066] In embodiments where the chemical composition of the glass-filled rod corresponds to the chemical composition of the cladding glass, the glass-filled material of the filling rod within the multicore fiber forms part of the optical cladding. In embodiments where the chemical compositions of the glass-filled rod and the cladding glass differ, the glass-filled material of the filling rod within the multicore fiber can have additional functions, 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 receive marker elements made from marker glass in addition to the filling rod. Marker elements exist in the semi-finished product, for example, as elongated cavities, or as compacted or uncompacted components made from marker material, or as a coating of such components with marker material, and in multicore fibers, they form continuous linear marker zones or air-filled hollow channels made from marker material.

[0068] The marker element is positioned inside or on the filling rod. For example, it may be positioned in the form of air or another gas in a recess designed as a hollow channel extending through the filling rod along the longitudinal axis of the filling rod, or it may be mounted in a recess on the outer cladding of the filling rod.

[0069] Since the marker element is positioned within or on the filling rod, and therefore within the central bore of the hollow glass cladding cylinder, but not within the cladding glass of the hollow cylinder, the need to adapt the hollow glass cladding cylinder for the purpose of inserting the marker element is eliminated, for example, by mechanical processing, particularly by creating a separate bore within the hollow glass cladding cylinder to receive the marker element. Thus, the risk of damage associated with adapting the hollow glass cladding cylinder in this manner is eliminated.

[0070] The dimensional stability and straightness of the central bore can be easily ensured by the OVD manufacturing method itself and, if necessary, by subsequent means. Suitable subsequent means for adjusting dimensional stability and straightness include, for example, mechanical reworking of the central bore and / or a stretching process, in which an initial hollow cylinder manufactured in the OVD deposition process is provided to stretch a tube strand from which a hollow glass cladding cylinder is manufactured or from which multiple hollow glass cladding cylinders are cut to a certain length. The stretching process is preferably performed without using a forming tool that engages with the drawn tube strand in order to avoid damage to the tube strand surface. Similarly, the dimensional stability and straightness of the filling rod that is precisely inserted into the central bore can be relatively easily ensured by machining and / or such a stretching process. This machining may optionally be an external machining method that is generally no more significantly complex than an internal machining method.

[0071] Alignment of the marker element parallel to the axial direction is facilitated by the formation of a recess in the filling rod extending in the direction of the longitudinal axis of the filling rod, or by insertion into a recess extending in the direction of the longitudinal axis of the filling rod.

[0072] The recess is designed, for example, as a bore within the filling rod, preferably as a longitudinal slot (longitudinal groove) on the outer cladding surface of the filling rod. The groove-shaped recess is filled with a cylindrical component made, for example, from marker material or particulate marker material to form a "modified filling rod". Particulate marker material can have certain dimensional stability by thermal compression or by the addition of a binder. The marker material completely or partially fills the recess. This ensures a secure fit between the marker element and the filling rod. The marker element and the filling rod can also be connected to each other in advance (i.e., before being inserted into the central bore) by material bonding, for example, by sintering or fusion.

[0073] In this regard, embodiments in which the recess of the filling rod is designed as a longitudinal groove are particularly preferred. On the one hand, longitudinal grooves on the outer cladding surface of the filling rod are particularly easy to manufacture and geometrically accurate compared to bores of the filling rod. On the other hand, longitudinal grooves manufactured in this manner are just as precise and linear as the filling rod itself. Furthermore, the depth or opening width of the longitudinal groove can be substantially reduced as desired, for example, both less than 15 mm, preferably less than 10 mm, and particularly preferably less than 5 mm. This means that geometrically accurate marker elements with small volume are available, which have an axial misalignment of less than 0.3 mm / m in the preform or group of components, and thus form a small, high-precision marker zone corresponding to the multicore fiber.

[0074] The marker material may exist, for example, in the form of cylindrical components (rods or tubes) made from the marker material, or be formed by a bed of particles from the marker material, or by internally coating longitudinal grooves with the marker material. The cylindrical components, each consisting of the marker material or a bed from the marker material, can also be fixed within the longitudinal grooves by fusion.

[0075] The relatively easy achievement of straightness of the filling rod and central bore also facilitates the axial alignment of the marker element. This is particularly applicable to a particularly preferred embodiment of a semi-finished product in which the marker element extends along the longitudinal axis of the filling rod and is melted within the recess.

[0076] The marker elements (components, floors, layers) are fixed within the recesses by fusion. For this purpose, they are melted within the recesses over at least a portion of their length, preferably locally at several points distributed over their length, and ideally over their entire length. The result is a modified filling rod filled with the marker element material.

[0077] By melting the marker elements within the recesses, the side edges of the recesses filled with the molten marker elements form a large, continuous, and seamless transition to the outer cladding surface of the filling rod, thus ensuring the avoidance of structural defects during the fiber stretching process. This has a positive effect on the dimensional stability of the multicore fiber. The marker elements, for example, completely fill the recesses and, ideally, have a curvature that conforms to the outer contour of the outer cladding surface of the filling rod.

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

[0079] The molten marker elements within the recesses are fixed to the filling rod, thereby facilitating their handling in further fiber manufacturing processes. During melting, a certain degree of roundness of the marker material, and therefore conformity to the contour of the outer cladding surface of the filling rod, can be achieved by surface tension. After melting the marker elements, the absence of defects and the quality of the melting process can be monitored and improved as needed.

[0080] As a result, the semi-finished product is equipped with a marker element that eliminates the need to manufacture a separate bore within the hollow glass cladding cylinder, given the risks and difficulties mentioned above. At the same time, a high level of precision can be guaranteed despite the high aspect ratio, which is evident in the semi-finished product, for example, in 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 includes a bore and / or longitudinal groove on the outer cladding surface of the filling rod, and the semi-finished product according to the present invention further includes a hollow cylinder including a central bore, at least two core rods including a core glass and forming a core glass region, a filling rod disposed within the central bore, and at least one marker element mounted within the recess of the filling rod.

[0082] Marker elements exist, for example, as filling rods within a central bore or as rods extending parallel to multiple filling rods. The marker element rods and filling rods may also be in a compacted form, i.e., fused to their surroundings.

[0083] Two or more additional longitudinal bores (core rod bores), for example, four to seven, are fabricated within the hollow cylinder, their longitudinal axes extending parallel to the longitudinal axis of the central bore. The core rod bores are either through bores or dead bores, each serving to accommodate at least one core rod fabricated from the core glass. Viewed radially, the composition of the core glass is either uniformly homogeneous or changes gradually or stepwise. This differs from cladding glass in that light induction within the core glass region is guaranteed. 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 terms of their dimensions and / or the composition of their corresponding core glass.

[0084] The semi-finished product is either drawn directly to form a multicore fiber or compacted to form a preform for the multicore fiber, with compaction potentially accompanied by simultaneous elongation. The thus produced "compacted preform" is optionally drawn to form a multicore fiber or further processed to form a "secondary preform" from which the MFK is ultimately drawn.

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

[0086] By attaching the marker element to the filling rod, the marker element benefits from its straightness and alignment, and these properties are artificially transferred to the marker element. The attachment is based, for example, on frictional connection, material connection, and / or positive connection between the filling rod and the marker element.

[0087] The marker elements are preferably designed in the form of cylindrical components made from marker material, or in the form of layers or masses of marker material connected to a filling rod. At least one cylindrical component is, for example, a tube, preferably a rod. In the case of marker elements in the form of a tube, the tube wall may contain a material having a higher viscosity than cladding glass, so that the bore does not completely collapse during the fiber stretching process and is maintained as a cavity ("airline") within the finished multicore fiber.

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

[0089] The marker element forms an elongated cavity or channel filled with air, or preferably comprises a marker material having at least one physical and / or chemical property different from the glass filling material of the cladding glass and filling rod, the property being selected from refractive index, color, fluorescence, and / or specific glass density.

[0090] The properties (or multiple properties) that distinguish marker elements from the constituent glass groups particularly affect the visual appearance of the marker elements and are preferably detectable by optical sensors. The glass composition of marker glass, for example, the glass filler material, can also be based on quartz glass. The refractive index of quartz glass can be altered by doping. For example, doping marker quartz glass with fluorine lowers the refractive index compared to undoped quartz glass. Incorporating carbon into marker quartz glass may result in a black coloration. Depending on the oxidation state, doping marker quartz glass with titanium produces a gray or blue coloration. Doping marker quartz glass with rare earth metals or germanium oxide results in fluorescence at dopant-specific wavelengths. The specific glass density of marker elements can be altered by pores, which manifests as a decrease in light transmittance compared to bubble-free glass.

[0091] A multicore fiber is obtained based on the method according to the present invention, or using a semi-finished product according to the present invention, in which the signal core is located outside the cladding circle around the fiber central axis and the marker zone is located inside the cladding circle around the fiber central axis.

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

[0093] The multicore fiber is traversed by at least one continuous linear marker zone. The marker zone is used for symmetry breaking and to clearly identify the positions of the signal cores and their relative to each other and to the fiber's central axis.

[0094] In the fiber cross-section, preferably, the sole marker zone lies within the aforementioned cladding circle defined by the signal core, and therefore in the region between the signal cores (not outside the signal core), including the fiber central axis. Surprisingly, it has been shown that by positioning the marker zone in this manner during the fiber stretching process, a relatively low fiber curl is imprinted on the multicore fiber. While we do not wish to be bound by this theory, we can assume that positioning the marker zone in the inner region of the fiber cross-section has less impact on the symmetry of the fiber design than positioning it in the outer region. Clearly, this generates lower radial forces in the multicore fiber during the fiber stretching process.

[0095] Definition and measurement method The individual terms used in the above description are further defined below. These definitions are part of the description of the present invention. Terms and measurement methods not specifically defined in this specification shall be interpreted according to the International Telecommunication Union (ITU) interpretation. In the event of any inconsistency between one of the following definitions and the remainder of this description, the statement made elsewhere in this description shall prevail.

[0096] Hollow glass cladding cylinder / cladding glass area The hollow cylinder includes cladding glass. The cladding glass forms a cladding glass region with a central bore, and a core glass region designed for signal transmission is created outside of it. The cladding glass consists of, for example, undoped quartz glass or includes at least one dopant that reduces the refractive index of the quartz glass. Fluorine and boron are dopants that can reduce the refractive index of quartz glass. The hollow glass cladding cylinder has an elongated, substantially cylindrical shape. Deviations from the cylindrical shape may be present in the end face region. The glass cladding cylinder is preferably manufactured using the OVD method.

[0097] Core rod / core glass area The core rod includes core glass having a radially uniform or non-uniform refractive index profile. The core glass of each core rod forms a core glass region. The core rod may include a region made of core glass having a relatively high refractive index and at least one further region made of another glass having a relatively low refractive index, for example, fluorine and / or chlorine-doped quartz glass. The glass with the highest refractive index is generally located on the central axis of the core rod. It consists of, for example, quartz glass to which at least one dopant has been added to increase the refractive index. In a multicore fiber, the core rod forms at least one signal core, to which the transmitted signal is primarily transported. The signal core may be adjacent to other glass regions having a lower refractive index, similarly provided by the core rod.

[0098] Filling rod / Modified filling rod The filled rod contains a glass-filled material. The filled rod does not form a signal core in a multicore fiber that can be used for signal transmission. In its simplest form, the composition of the glass-filled material corresponds to the composition of the cladding glass. However, to impart additional properties to the multicore fiber, it can have a different composition from the cladding glass. For example, it can have a lower coefficient of thermal expansion than the cladding glass.

[0099] The modified filling rod is connected to the marker element, forming a manageable unit. This unit includes glass filling material and marker material.

[0100] Marker elements / Marker materials / Marker glass Marker elements contain marker material or are partially composed of air or another gas. In particular, marker elements contain at least one marker glass. The chemical composition of the marker material differs from that of the cladding glass and the glass-filling material of the filling rod, and / or the density of the marker material differs from that of the cladding glass and the glass-filling material. Marker elements exist within the preform and within the components, either as components or as layers or lumps on the components, forming optically detectable marker zones within the multicore fiber.

[0101] Component group / Compaction preform / Secondary preform / Semi-finished product The “component group” includes a hollow glass cladding cylinder into which a core rod is inserted, at least one filling rod, and at least one marker element. A “preform,” also referred to herein as a “compacted preform,” is obtained by fixing the core rod within the core rod bore, for example by narrowing the end of the hollow glass cladding cylinder, or by crushing and fusing. The component group or the (compacted) preform is stretched to form a “secondary preform” or to directly form multicore fibers. Here, the term “semi-finished product” encompasses the component group, the compacted preform, and the secondary preform. Reshaping the component group includes stretching to form multicore fibers or forming a compacted preform.

[0102] Quartz glass Quartz glass consists of, for example, melt products from naturally occurring SiO2 raw materials (natural quartz glass), synthetically produced materials (synthetic quartz glass), or mixtures of these types of quartz glass. Synthetic clear quartz glass can be obtained, for example, by flame hydrolysis or oxidation of synthetically produced silicon compounds, by polycondensation of organosilicon compounds by the so-called sol-gel method, or by hydrolysis and precipitation of inorganic silicon compounds in liquid.

[0103] Fusion When referring to components made from glass, fusion is understood to mean that the components fuse together at their contact surfaces. Fusion is achieved by heating the components, at least in the area of ​​their contact surfaces, with a heat source such as a furnace, burner, or laser.

[0104] Position display: top / bottom These indications relate to the position during the stretching process and / or fiber stretching process. "Bottom" indicates the position in the direction of the stretching process, and "Top" indicates the position opposite to the direction of the stretching process.

[0105] cross section A cross-section taken perpendicular to the longitudinal axis.

[0106] Longitudinal section A cross-section taken parallel to the longitudinal axis.

[0107] Boa The terms "bore," "central bore," "internal bore," or "longitudinal bore" refer to holes that are cylindrical but otherwise have any arbitrary internal shape. They are manufactured, for example, by a bore-punching process, or by depositing a layer of material onto the outer cladding surface of a mandrel using a deposition or pressing process, and then removing the mandrel.

[0108] Axial parallel alignment / Axial parallel In each case, the reference axis is either the longitudinal axis of the hollow glass-clad cylinder or the central axis of the multicore fiber. [Brief explanation of the drawing]

[0109] Exemplary Embodiments The present invention will be described in more detail below with reference to exemplary embodiments and drawings. [Figure 1] Figure 1 shows, in detail, a cross-sectional view of a hollow glass-clad cylinder having a central bore and a through bore for receiving a core rod, in a schematic diagram. [Figure 2] Figure 2 shows, in detail, a schematic diagram of the process steps (a) to (d) for manufacturing a filling rod having a marker element for insertion into the central bore of the hollow glass cladding cylinder of Figure 1. [Figure 3] Figure 3, in detail, shows a schematic diagram of the components of the inserted filling rod, including the hollow glass cladding cylinder and marker elements. [Figure 4] Figure 4, in detail, shows a schematic cross-sectional view of the compacted preform, the inserted filling rod including the marker element, and the core rod inserted into the through bore of the hollow glass cladding cylinder. [Figure 5] Figure 5, in detail, shows a cross-sectional view of the multicore fiber drawn from the preform in Figure 4, in a schematic diagram.

[0110] Figure 1 schematically shows a cross-section of a hollow cylinder 1 made of cladding glass, which serves as a substrate for manufacturing multicore fibers.

[0111] The hollow cylinder 1 is manufactured using a known method with respect to the OVD (Optical Vapor Deposition) method. In this method, SiO2 soot particles are formed by passing a high-purity SiO2 starting material, such as silicon tetrachloride, through a deposition burner and supplying it to a burner flame, where it is oxidized to solid SiO2. This is deposited from the gas phase in the form of fine SiO2 soot particles onto the outer cladding surface of a cylindrical deposition mandrel that rotates about its longitudinal axis, and the deposition burner performs reversal movement back and forth along the longitudinal axis of the deposition mandrel. The SiO2 soot body is formed on the outer cladding surface of the deposition mandrel. After the deposition process is complete, the deposition mandrel is removed so that the inner bore 2 remains. The SiO2 soot body is then vitrified in a furnace under vacuum.

[0112] The resulting hollow cylinder 1 is made of undoped synthetically produced quartz glass, forming a 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 bore drilling and honing. Four bores 3 are manufactured in a predetermined (here secondary) configuration by mechanically drilling bores in the direction of the longitudinal axis 1b of the hollow cylinder, which extends perpendicular to the seat surface in the illustration in Figure 1. The bores 3 are used to receive the core rod (Figure 4) and have a diameter of 30 mm. The bores extend through the entire hollow cylinder 1 (through bores). In another embodiment, the bores are designed as no-noise bores.

[0113] Figure 2 schematically illustrates the method steps for manufacturing the filling rod 5. The filling rod 5 shown in Figure 2(a) is preferably made of the same glass as the hollow cylinder 1, i.e., undoped quartz glass. Known methods, such as VAD (Vapor-Axial Deposition), OVD (Outer Vapor Deposition), or MCVD (Modified Chemical Vapor Deposition), are suitable for its manufacture. This is used to fill the bore 2 inside the hollow cylinder, having a length of about 1500 mm and an initial outer diameter of about 45 mm, which is reduced to about 40 mm by cylindrical grinding. This cylindrical grinding removes any potential external cladding surface 5b and bending interference. Alternatively or additionally, diameter adjustment and surface improvement are achieved by elongation in a tool-free elongation process. In this figure, the longitudinal axis 5a of the filling rod also extends perpendicular to the sheet plane.

[0114] Figure 2(b) shows that the longitudinal groove 6 is milled from the outer cladding surface of the filling rod 5. The longitudinal groove 6 extends along the entire length of the filling rod 5. It has a U-shape with a rounded base and straight side walls. Its opening width and depth are 6 mm, respectively. In a subsequent process step, a hollow channel forming a marker element within the meaning of the present invention can be manufactured from the longitudinal groove (6). This is described in more detail below with reference to Figure 4.

[0115] Figure 2(c) shows the longitudinal groove 6 into which the marker rod 7 is inserted. The diameter of the marker rod 7 is 5 mm. It is made of fluorine-doped synthetically produced quartz glass, which is commercially available under the name F320. The viscosity and refractive index of the fluorine-doped quartz glass of the marker rod 7 are both lower than those of the undoped quartz glass from which the hollow cylinder 1 and the filling rod 5 are made. The marker rod 7 is obtained by stretching a starting cylinder made from F320 quartz glass in a tool-free manner. It has a smooth surface produced by melt flow and is characterized by high dimensional stability, so it can be inserted into the narrow longitudinal groove 6 with a difficult and precise fit.

[0116] Next, the filling rod 5 and the marker rod 7 are fused to each other. The filling rod 5 is mounted with its longitudinal axis 5a oriented horizontally so that the longitudinal groove 6 is located above it. The marker rod 7, inserted into the longitudinal groove 6, is first heated at specific points by a burner, thereby fixing it in the longitudinal groove at three substantially evenly distributed fixing points located at the ends and center of the marker rod 7, distributed over 95% of its length. The fluorine-doped quartz glass is then uniformly heated by the burner until it softens and deforms due to its relatively low viscosity, resulting in it sinking into and filling the longitudinal groove 6. Due to surface tension, the surface of the softened glass mass adjacent to the free atmosphere exhibits a certain bulge, thus avoiding a noticeable step between the side edge of the longitudinal groove 6 and the outer cladding surface 5b of the filling rod 5.

[0117] Figure 2(d) shows the marker glass block 8 after softening, deformation, and fusion using the former filling rod 5, and the resulting modified filling rod 5c filled with the marker glass block 8. The glass volume of the former marker rod 7 matches the internal volume of the longitudinal groove 6, so that the marker glass block 8 completely fills the longitudinal groove 6.

[0118] In this way, the modified filling rod 5c, filled with the marker glass block 8, is inserted into the inner bore 2 of the hollow cylinder 1. Figure 3 schematically shows the components 9 of the hollow cylinder 1 and the modified filling rod 5c having the marker glass block 8.

[0119] In addition, four core rods 4, each approximately 1500 mm in length and 28 mm in outer diameter, are fabricated from germanium-doped quartz glass. Known techniques, such as MCVD (Modified Chemical Vapor Deposition), are also suitable for this purpose.

[0120] The core rod 4 is inserted into the bore 3. Subsequently, the group of components 9 of the hollow glass cladding cylinder 1, the modified filling rod 5c, and the core rod 4 are heated to close the annular gap around the inner bore 2 and the core rod 4, and to fuse all the components of group 9 together.

[0121] Figure 4 schematically shows the components fixed in this manner, including the hollow glass cladding cylinder 1, the modified filling rod 5c, and the core rod 4 that form the compacted preform 10. All core rods 4 form separate circular core glass regions 4a located entirely outside the cladding circle 11, while the marker elements 8 are located entirely within this cladding circle 11.

[0122] Next, the compacted preform 10 is stretched to form a secondary preform. This means the preform 10 is held within the stretching device by a holder in a vertical alignment with respect to the longitudinal axis 1a of the hollow cylinder. The secondary preform thus produced is finally stretched in a stretching device in a conventional manner to form the multicore fiber 20.

[0123] In this embodiment, the marker element 8 exists as a marker glass block 8 manufactured by reshaping the original marker rod 7. In an alternative procedure, the longitudinal groove 6 is not filled (no rod or tube is inserted) when the constituent group 9 is compacted within the preform 10, and the longitudinal groove 6 is prevented from collapsing completely by generating and maintaining overpressure within it. In this way, a cavity is created that exists within the multicore fiber as an air-filled hollow channel extending along the longitudinal axis 1a. Because the refractive index of air is significantly different from that of the cladding glass 1b, the hollow channel can function as a marker zone.

[0124] Figure 5 schematically shows a cross-section of the multicore fiber 20. Apart from the smaller radial dimensions, this substantially corresponds to the cross-section of the compacted preform 10. The core glass region (4a) of the former core rod (4) forms a signal core 4b extending along the fiber longitudinal axis 20a, the former filling rod (5) is 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 completely outside the cladding circle 11a, and the marker zone 8a is located completely within this cladding circle 11a. The marker zone 8a is characterized by its small size, which results in low tension on the multicore fiber 20 during fiber stretching, and consequently, less fiber curl.

Claims

1. A method for manufacturing a multicore fiber (20), comprising a step of reshaping a group of components (9) to form the multicore fiber (20) or a preform (10) for the multicore fiber (20), the multicore fiber (20) comprising a hollow cylinder (1) comprising a central bore (2) and a hollow cylinder longitudinal axis (1a), wherein the hollow cylinder comprises a cladding glass region (1b) made of cladding glass and a plurality of core glass regions (4a) provided with core glass, and at least a portion of the central bore (2) is occupied by a glass filling rod (5) comprising a filling rod longitudinal axis (5a) and a filling rod outer cladding surface (5b), and a recess (6) extending in the direction of the filling rod longitudinal axis (5a) is formed in or on the filling rod (5), and marker elements (7, 8) made of marker glass are inserted into or formed in the recess.

2. The method according to claim 1, characterized in that the marker elements (7, 8) extend along the longitudinal axis (5a) of the filling rod and are melted in the recess (6) before 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 elements (7, 8) have a certain length, and melting occurs completely, partially, or at a specific point along at least 80% of this length.

4. The method according to claim 1, wherein the marker elements (7, 8) are melted, and the filling rod (5) having a horizontally oriented longitudinal axis (5a) of the filling rod is mounted such that the recess (6) is located above the outer cladding surface (5b) of the filling rod, the material of the marker elements (7, 8) is heated and softened by a heat source.

5. The reshaping of the aforementioned group of components (9) is performed in the following method steps (a) A step of providing the hollow cylinder (1) including the cladding glass, (b) A step of providing a plurality of core rods (4) including the core glass, (c) A step of providing a filling rod (5) which includes a longitudinal axis (5a) of the filling rod and contains glass filling material, (d) A step of creating at least one recess (6) on the outer cladding surface (5b) of the filling rod (5), (e) A step of providing marker elements (7, 8), (f) A step of placing the marker elements (7, 8) in the recess (6) and melting them, (g) A step of generating a core rod bore (3) that extends along the longitudinal axis (1a) of the hollow cylinder, (h) A step of introducing the filling rod (5) and the marker elements (7, 8) into the central bore (2), (i) The step of introducing the core rod (4) into the core rod bore (3) to form the constituent elements (9), The method according to claim 1, characterized by including

6. The method according to claim 1, characterized in that the marker elements (7, 8) are provided in the form of cylindrical components or in the form of layers or masses connected to the filling rod (5).

7. The method according to claim 1, characterized in that the recess (6) includes a bore and / or longitudinal groove within the outer cladding surface of the filling rod (5).

8. The method according to claim 1, wherein the marker elements (7, 8) include a marker material having at least one physical and / or chemical property different from the glass filling material constituting the cladding glass and the filling rod, the property being selected from refractive index, color, fluorescence, and / or specific glass density.

9. A semi-finished product for manufacturing a multicore fiber, comprising a hollow cylinder (1) including a central bore (2), wherein the hollow cylinder includes a cladding glass region (1b) made of cladding glass and a hollow cylinder longitudinal axis (1a), and a plurality of core glass regions (4a) in which core glass is provided within the cladding glass region (1b), wherein at least a portion of the central bore (2) is occupied by a glass filling rod (5) including a filling rod longitudinal axis (5a) and a filling rod outer cladding surface (5b), wherein the filling rod (5) includes a recess (6) extending in the direction of the filling rod longitudinal axis (5a), and marker elements (7, 8) made of marker glass are inserted into or form the marker elements (7, 8) extending along the central bore longitudinal axis (1a) and the filling rod longitudinal axis (5a).

10. The semi-finished product according to claim 9, characterized in that the marker elements (7, 8) have a certain length and are melted completely, partially, or in a dot along at least 80% of this length in the recess (6).

11. The semi-finished product according to claim 9, wherein the recess (6) includes a bore and / or longitudinal groove within the outer cladding surface (5b) of the filling rod (5), and the semi-finished product further includes the hollow cylinder (1) including the central bore (2), at least two core rods (4) including the core glass and forming the core glass region (4a), the filling rod (5) disposed within the central bore (2), and at least one marker element (7, 8) mounted within the recess (6) of the filling rod (5).

12. The semi-finished product according to claim 9, characterized in that the marker elements (7, 8) exist in the form of cylindrical components or in the form of layers or masses connected to the filling rod.

13. The semi-finished product according to claim 9, wherein the marker elements (7, 8) include a marker material having at least one physical and / or chemical property different from the cladding glass, the core glass, and the glass filling material constituting the filling rod, and the property is selected from refractive index, color, fluorescence, and / or specific glass density.

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