METHOD AND SEMI-PRODUCED PRODUCT FOR MANUFACTURING A MULTI-CORE FIBER - Patent application

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 multicore fiber, comprising method steps in which components are reshaped to form a multicore fiber, or into 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 portion of the central bore being occupied by a glass filler rod comprising a filler rod longitudinal axis and a filler rod outer cladding surface.

[0002] The present invention further relates to a semi-finished product for manufacturing 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 within the cladding glass region, at least a portion of the central bore being occupied by a glass filling rod comprising a filling rod longitudinal axis and a filling rod outer cladding surface.

[0003] In multicore fibers, several light wave conducting optical core regions (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 with a lower refractive index, allowing the light to be guided independently of each other. This fiber design ensures a high signal transmission capacity, since 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 "spatial multiplexing" and can increase the data transmission capacity, especially in optical communications. Multicore fibers are also considered as important components for transmitting energy for material processing, as components of optical fiber sensors in measurement and medical technology, and for illumination and imaging purposes in microscopy or endoscopic devices. [Background technology]

[0004] Multicore fibers are produced by stretching solid preforms or groups of components. They often consist of synthetically produced quartz glass (SiO2), which may be doped or undoped. The production of synthetic quartz glass includes, for example, plasma or CVD deposition methods known under the names OVD, VAD, MCVD, PCVD or FCVD methods. Liquid or gaseous silicon-containing starting materials are subjected to chemical reactions (hydrolysis, pyrolysis or oxidation) and the reaction products, i.e. particulate SiO2, are deposited as solids from the gas phase on the deposition surface. The starting materials are, for example, silicon tetrachloride (SiCl4) or chlorine-free silicon compounds, such as polyalkylsiloxanes. The reaction zone is, for example, 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 some degree of symmetry. The cylindrical components are inserted into a sheath tube and spatially fixed therein. This group is either drawn to form a multicore fiber or is further pre-processed to form a preform from which the multicore fiber is drawn.

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

[0007] US 2022 / 003921 describes a semi-finished product and a method for producing a multicore fiber by producing five circular openings in a cladding glass solid cylinder by a heating or powder molding process, the centrally located opening being uniformly surrounded by and overlapping with the four surrounding openings. The four outer opening areas are occupied by core glass rods, while in the still free central opening area a rod with a polygonal cross section made of low-viscosity glass is inserted, and in the free space adjacent to this rod a marker rod is inserted. In a later method step, all components of the group are heated, and the rod made of low-viscosity glass softens first to fill the gaps.

[0008] A semi-finished product and a method for stretching the semi-finished product to produce a multicore fiber are known from US 2016 / 347645 A1. The semi-finished product forms a multi-part fiber preform consisting of an integral cladding tube made of cladding glass, designed to receive and axially guide a stack of three structurally identical cylindrical stack pieces made of cladding glass, each of the stack pieces comprising in its cylindrical cladding surface a through bore and a longitudinal groove, a number of core rods for insertion in the through bores, and a number of marker rods for insertion in the longitudinal grooves. In the cladding tube bore, the stack pieces are placed one on top of the other such that the through bores and the longitudinal grooves are aligned. The marker zone of the fiber preform formed by the marker rods runs on the inner cladding surface of the cladding tube.

[0009] US Pat. No. 8,532,454 discloses a multicore fiber having seven core regions.

[0010] In another known procedure for producing a preform for a multicore fiber, as described, for example, in US Patent Application Publication No. 2021 / 0300812, a number of through holes are produced by longitudinal drilling of a synthetic SiO2-based glass cladding cylinder, which extends in the direction of the cylinder's longitudinal axis. In each through hole, a core rod is inserted, which contains a core material with a higher refractive index than the glass of the glass cladding cylinder. In the case of a large number of through holes, thin and easily breakable walls may remain between adjacent holes. To reduce the risk of breakage, it has been proposed to produce the required number of through holes in the glass cladding cylinder in succession, while occupying and fusing at least a part of the through holes with a core rod.

[0011] The glass cladding cylinder is elongated, and therefore the through-hole has a large aspect ratio (ratio of length to diameter), which fundamentally makes it difficult to manufacture the through-hole with dimensional precision and to align it precisely parallel to the longitudinal axis of the hollow glass cladding cylinder.

[0012] In the methods known from US 2015 / 284286 and US 2015 / 0307387, a hollow glass cladding cylinder in the form of a SiO2-based soot body (SiO2 soot body) produced using an OVD method is used, which has a density of 0.8 g / cm 3 ~1.6g / cm 3In the OVD method, the deposition surface is generally the outer cladding surface of a rod-shaped or tubular deposition mandrel rotating around its longitudinal axis. A substantially cylindrical soot body is deposited by reversing movements back and forth through the reaction zone. After the deposition process is completed, the deposition mandrel is removed so that a central through-opening remains at the central axis of the cylindrical soot body. In the hollow glass cladding cylinder thus produced, a longitudinal bore is introduced to receive the core glass rod, and the low density of the SiO2 soot body compared to quartz glass makes it easier to produce a dimensionally accurate longitudinal bore. The central "OVD through-opening" remaining due to the manufacturing process can lead to asymmetric deformations when it collapses destroying the fiber design. The collapse also reduces the cross-sectional area of ​​the cladding glass portion. These defects can be prevented by inserting a filling rod that closes the OVD through-opening. The filling rod is made of a glass having substantially the same refractive index as the hollow glass cladding cylinder. The filling rod can also be manufactured by OVD or press molding or by a combination of press molding and OVD. Furthermore, a channel for receiving the marker element is introduced into the hollow glass cladding cylinder. The channel is created by mechanical drilling in the area of ​​the hollow glass cladding cylinder close to the edge.

[0013] The marker elements are used to form a continuous linear marker zone in the multicore fiber and to break the symmetry so that the signal cores and their positions relative to each other and to the fiber central axis can be clearly identified and assigned, which is necessary, for example, so that two multicore fibers can be joined together with low attenuation by their end faces using conventional splicing methods.

[0014] To splice multi-core fibers, the fiber ends to be connected are arranged with their end faces facing each other. In a known method, light is simultaneously fed into all signal cores at the opposite fiber end and collectively detected at the fiber end of the other multi-core fiber by a photodetector and a power meter. In particular, light is irradiated from the side of a multi-core fiber when one end of the multi-core fiber is not available for feeding light. By relatively displacing the multi-core fiber end faces in the horizontal and vertical directions and relatively rotating them in the azimuth 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 fused together in this position. Summary of the Invention [Problem to be solved by the invention]

[0015] The higher the number of multi-cores, the greater the theoretical increase in data transmission capacity compared to an optical fiber with a single fiber core (single-mode fiber or multimode fiber). On the other hand, in principle, each of the multiple cores must have an optical attenuation that approximately corresponds 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 cores as interference signals. However, this can be caused by so-called "crosstalk" between the multiple cores, especially if they are too close to each other. This effect therefore requires that a certain minimum distance be maintained between the fiber cores. For these reasons, the available cross-sectional area in the radial cross-section of the multi-core fiber must be used as completely as possible for occupation by the multiple cores.

[0016] Therefore, the additional marker zone introduced in the fiber design should occupy a small percentage of the fiber cross section. A small size of the marker zone is also advantageous to counteract other undesirable effects such as the so-called fiber curl or stresses induced in the fiber. The diameter of the channel for receiving the marker element is therefore small, generally significantly smaller than the diameter of the bore for receiving the core rod. Typically, the channel diameter of the hollow glass cladding cylinder before the fiber drawing process is less than 15 mm with an aspect ratio of more than 65 (for a hollow cylinder length of about 1 m).

[0017] The dimensionally accurate manufacture and precise alignment of such thin channels in hollow glass cladding cylinders is difficult even when using high-precision drilling machines. Moreover, it has been shown that cracks are increasingly generated in the channel walls during drilling of particularly thin channels. The cost of manufacturing hollow glass cladding cylinders from synthetic quartz glass is high, and losses are particularly painful, especially when a small bore for receiving a marker element leads to rejecting an otherwise completed hollow glass cladding cylinder.

[0018] To make matters worse, cutting and splicing of multicore fibers can be performed at any location according to the specific requirements when used as intended, so that one must rely on a consistent geometry along the entire fiber length so that one does not have to rely on measurements.

[0019] It is therefore an object of the present invention 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, it is an object of the present invention to provide a semi-finished product suitable for the production of a multicore fiber without a central signal core, which is in particular characterized by a low "fiber curl".

[0021] A property of glass fibers, defined as the degree of curvature over a particular length of the fiber, is called "fiber curl." The curvature is due to thermal stresses induced during fiber manufacturing. Large "fiber curl" makes low attenuation splices of multicore fibers more difficult. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] With regard to this method, this object is achieved by the invention in that, based on the method described at the beginning, a recess extending in the direction of the longitudinal axis of the filling rod is created in or on the filling rod and a marker element made of marker glass is inserted or formed into the recess.

[0023] A hollow glass cladding cylinder with 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 the hollow glass cladding cylinder using the OVD method is cost-effective compared to other manufacturing methods, in particular compared to the VAD (Vapor Phase Axial Deposition) method. However, it has the disadvantage that the aforementioned central bore remains. Said central bore can be completely or partially closed by glass filler material provided by multiple 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 in the multicore fiber forms part of the optical cladding. In embodiments where the chemical composition of the glass filler material and the cladding glass are different, the glass filler material of the filler rod in the multicore fiber can have additional functions, for example, it can act as a "stress zone" that creates and / or compensates for compressive or tensile stresses acting radially in the fiber.

[0025] The method according to the invention is used to manufacture a multicore fiber without a central signal core, the central bore being used to insert a marker element in addition to the glass filling material of the filling rod, the glass filling material not including 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, into which recess the marker element is inserted or which recess forms the marker element.

[0027] The marker elements are present within the components as components, as coatings on the components, or as cavities, forming continuous linear marker zones of marker material or air within the multicore fiber that can be used, for example, during splicing for symmetry breaking and for unambiguous identification of the signal cores and their positions relative to each other and to the central fiber axis.

[0028] The recess may be, for example, a hollow channel extending through the filler rod along the longitudinal axis of the filler rod, in which case the filler rod forms the edge of the marker element in the form of an elongated air-filled cavity, or the marker element may be disposed within a recess on the outer cladding surface of a cylindrical filler rod.

[0029] Since the marker element is located in or on the glass filled rod and thus in the central bore of the hollow glass cladding cylinder, but not in the cladding glass of the hollow cylinder, the need to adapt the hollow glass cladding cylinder for the purpose of inserting the marker element, for example by mechanical processing, in particular by creating a bore in the hollow glass cladding cylinder for receiving the marker element, is thus eliminated, thus eliminating the risk of damage associated with such adaptation of the hollow glass cladding cylinder.

[0030] The dimensional stability and straightness of the central bore of the hollow glass cladding cylinder can be easily ensured by the OVD manufacturing method itself and, if necessary, by subsequent measures. Suitable subsequent measures include, for example, a mechanical reworking and / or stretching process of the central bore, whereby the initial hollow cylinder manufactured in the OVD deposition process is provided for stretching the tube strand from which the hollow glass cladding cylinder is manufactured, or from which a plurality of hollow glass cladding cylinders are cut to length. The stretching process is preferably carried out without the use of a forming tool engaging the drawn tube strand, in order to avoid damage to the tube strand surface.

[0031] Similarly, the dimensional stability and straightness of the filler rod to be accurately inserted into the central bore can be relatively easily ensured by means of machining and / or such elongation processes, which may optionally be external machining methods that are generally significantly less complex than internal machining methods.

[0032] Axially parallel alignment of the marker element is facilitated by being formed by a recess in the filling rod extending in the direction of the filling rod's longitudinal axis or by being inserted into a recess extending in the direction of the filling rod's longitudinal axis.

[0033] The marker element is inserted into the recess, for example, by inserting a cylindrical component (rod or tube) made from marker glass running parallel to the outer cladding surface of the filler 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 in 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 element made of marker material or with particulate marker material. The particulate marker material can have a certain dimensional stability by hot compression or by adding a binder. The marker material fills the recess completely or partially. Thereby, the recess 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 beforehand (i.e. before insertion into the central bore), for example by material bonding by sintering or fusion.

[0035] The procedure of providing the filling rod with longitudinal grooves is particularly preferred. On the one hand, the longitudinal grooves in the filling rod outer cladding surface can be produced particularly easily and geometrically accurately, for example, in comparison with the bore of the filling rod by milling with a mechanical milling cutter or by laser ablation. On the other hand, the longitudinal grooves produced in this way are just as accurate and straight as the filling rod itself. Furthermore, the depth or the opening width of the longitudinal grooves can be made substantially as small as desired, for example both less than 15 mm, preferably less than 10 mm, particularly preferably less than 5 mm. The longitudinal grooves can be reshaped as walls of hollow channels filled with air, which can serve as marker elements, or the longitudinal grooves can also be filled with marker material. This means that geometrically accurate marker elements with small volumes can be produced in a simple manner, with deviations in their axial alignment in the preform or component group of less than 0.3 mm / m, thus forming correspondingly small and highly accurate marker zones in the multicore fiber.

[0036] The longitudinal grooves are filled with the marker elements, for example by inserting a cylindrical component (rod or tube) made of the marker material, or by introducing a bed of particles from the marker material, or by internally coating the longitudinal grooves with the marker material. The cylindrical component consisting of the marker material or respectively a bed from the marker material can further be fixed in the longitudinal grooves by fusion.

[0037] The relatively easy to achieve straightness of the filler rod and central bore also facilitates axially parallel alignment of the marker element, which applies in particular to the particularly preferred procedure in which the marker element extends along the filler rod longitudinal axis and is fused into the recess prior to reshaping to form a preform or multicore fiber.

[0038] The marker element (component, bed, layer) is thereby fixed in the recess by fusion. For this purpose, it is melted into the recess over at least a part of its length, preferably locally at several points distributed over its length, ideally over its entire length. The filling rod filled with marker element material is also called "modified filling rod" in the following.

[0039] By melting the marker element into the recess before reshaping the component group to form the multicore fiber or preform, it can be ensured that the side edges of the recess filled with the melted marker element form an almost stepless continuous transition to the outer cladding surface of the modified filling rod, thus avoiding structural defects during the fiber drawing process. This has a positive effect on the dimensional stability of the multicore fiber. The marker element, for example, fills the recess completely and ideally has a curvature that matches the outer contour of the filling rod outer cladding surface.

[0040] Thus, a preferred procedure provides that the marker element has a length and that melting occurs completely, partially or at specific points along at least 80% of this length, preferably at least 90% of this length.

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

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

[0043] In this way it is much better and easier to fill the recesses uniformly, preferably completely, than if, for example, the marker elements and recesses were vertically oriented during the melting process.

[0044] The marker elements melted into the recesses are fixed relative to the filler rod, which simplifies handling of the modified filler rod in further fiber manufacturing processes. During melting, a certain degree of rounding of the marker material and therefore conformity to the contour of the outer cladding surface of the filler rod can be achieved by surface tension. After melting the marker elements, the accuracy and quality of melting of the modified filler rod can be checked and improved if necessary.

[0045] In this way, the component group is equipped with marker elements without the need to generate separate holes in the hollow glass cladding cylinder, which is associated with the risks and difficulties mentioned above. At the same time, despite the high aspect ratio, a high level of precision can be guaranteed, which is evident, for example, in the preform or component group 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 elements are present as rods extending parallel to the filler rod or filler rods within the central bore.

[0047] In a further preferred procedure, the marker element is present as a layer which is applied to the recess of the filler rod.

[0048] Two or more, for example 4 to 7, additional longitudinal bores (core rod bores) are usually made in the hollow cylinder, whose longitudinal axis runs parallel to the longitudinal axis of the central bore. The core rod bores are through or blind bores and serve in each case to receive at least one core rod made from a core glass. Viewed in the radial direction, the composition of the core glass is uniformly homogeneous or changes gradually or in steps. It differs from the cladding glass in that light guidance in the core glass region is guaranteed.

[0049] The desired number of core rod bores are produced in one operation, occupied in each case 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 in each case by at least one core rod, and the core rod bores occupied by the core rods are crushed (this reshaping process is also called "consolidation" in this specification) before the remaining or further distribution of core rod bores is produced in a second or further operation, which distribution is also occupied in each case by at least one core rod 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 can also differ with respect to their dimensions and / or the composition of the corresponding core glass.

[0050] The filling of the central bore with the filler rod containing the marker element can be done before or after all the 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 the filler rod and the marker element, then the filled central bore is collapsed 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 bores is produced, mated 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 after that a second portion of the core rod bores is produced.

[0051] The components thus produced can be reshaped and drawn directly to form a multicore fiber or consolidated to form a preform for a multicore fiber, the consolidation process being associated with a simultaneous elongation process. The "consolidated preform" thus produced is optionally drawn to form a multicore fiber or further processed to form a "secondary preform". Further processing to form a "secondary preform" can include, for example, creating further bores in the glass cladding region and populating them with the core glass or other glasses, or performing one or more of the following hot forming processes once or repeatedly: collapse, collapse, stretch, collapse and simultaneous stretch on additional cladding material. The multicore fiber is drawn from the secondary preform produced by further processing.

[0052] In a preferred procedure, the manufacture of the building blocks comprises the following method steps: (a) providing a hollow cylinder comprising a cladding glass; (b) providing a plurality of core rods comprising a core glass; (c) providing a filler rod including a filler rod longitudinal axis and including a glass filler material; (d) creating at least one recess on an outer cladding surface of the filler rod; (e) providing a marker element; (f) placing and fusing a marker element into the groove; (g) creating a core rod bore extending along a hollow cylinder longitudinal axis; (h) introducing a filler rod and a marker element into the central bore; (i) introducing a core rod into a core rod bore to form a component group;

[0053] The component group thus produced includes a hollow cylinder, at least one modified filler rod fused to at least one marker element, and a core rod. The list letters (a)-(g) merely specify a preferred, but not mandatory, order of the method steps. The core rods of the component group are also referred to herein as core rods if they are already melted in the 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 filler rod end to the second filler rod end.

[0055] By fusing with the filler rod, the marker element benefits in particular 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 comprise a material having a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process, but remains as a cavity ("airline") in the finished multicore fiber.

[0057] The cross-sectional shape, optionally the outer diameter and length of the filling rod are adapted to the shape and length of the hollow cylinder central bore. The central bore is preferably filled leaving a circumferential gap with a gap width of less than 2 mm, 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.

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

[0059] The property (or properties) that distinguish the marker element from the glasses of the component group, in particular, influences the visual appearance of the marker element and is preferably detectable by an optical sensor. The glass composition of the marker glass, for example also the glass filling material of the filling rod, can be based on quartz glass. The refractive index of the quartz glass can be changed by doping. For example, doping the marker quartz glass with fluorine reduces the refractive index with respect to undoped quartz glass. The incorporation of carbon into the marker quartz glass can result in a black coloration. Depending on the oxidation state, doping the marker quartz glass with titanium results in a gray or blue coloration. Doping the marker quartz glass with rare earth metals or germanium oxide results in fluorescence at dopant-specific wavelengths. The specific glass density of the marker element can be changed by pores, which is manifested in a reduced light transmission with respect to glass without bubbles.

[0060] With regard to a semi-finished product for producing a multicore fiber, the above-mentioned technical object is achieved according to the invention starting from the semi-finished product mentioned at the beginning in that the filling rod comprises a recess extending in the direction of the filling rod longitudinal axis, in which a marker element made of marker glass is inserted or a marker element is formed which extends along the central bore longitudinal axis and along the filling rod longitudinal axis.

[0061] The semi-finished product according to the invention is used to manufacture a multicore fiber without a central signal core. It comprises a hollow cylinder, which comprises 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 filler rod made of a glass filler material. At least one recess is arranged in or on the filler rod extending in the direction of the filler rod longitudinal axis, into which a marker element is inserted or which recess forms the marker element.

[0062] The central bore longitudinal axis and the filler rod longitudinal axis extend coaxially with one another in the blank.

[0063] The number of core glass regions is at least two, preferably between four and seven. For example, they are each designed as a core rod bore extending parallel to the hollow cylinder longitudinal axis in the cladding glass region of the hollow cylinder and each occupied by a core rod. The filling rod is inserted into the central bore of the hollow cylinder and at least partially filled. At least one marker element is also arranged in the central bore of the hollow cylinder. The marker element is connected to the filling rod by being formed as a recess in the filling rod or by at least partially filling a recess in or on the filling rod. The filling rod filled in this way is also referred to herein as a "modified filling rod". The filling rod does not include any core region suitable for signal transmission.

[0064] The semi-finished product forms a component group when all or some of the core rods, filler rods, and marker elements are merely inserted into their respective bores or recesses and have not yet been fused therein. The semi-finished product forms a preform (also called a "consolidated preform") when all of the bores or recesses have collapsed and the core rods, filler rods, and marker elements are fused to each other or to the hollow cylinder.

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

[0066] In embodiments where the chemical composition of the glass filling rod corresponds to that of the cladding glass, the glass filling material of the filling rod in the multicore fiber forms part of the optical cladding. In embodiments where the chemical composition of the glass filling rod and the cladding glass are different, the glass filling material of the filling rod in the multicore fiber can have additional functions, for example, it can act as a "stress zone" that creates and / or compensates for compressive or tensile stresses acting radially in the fiber.

[0067] The central bore is used to receive a marker element made from a marker glass in addition to the filling rod, which may be present in the semi-finished product, for example, as an elongated cavity or as a consolidated or non-consolidated component made from the marker material or as a coating of such a component with the marker material, forming in the multicore fiber a continuous linear marker zone or an air-filled hollow channel made from the marker material.

[0068] The marker element is disposed in or on the filler rod, for example in the form of air or another gas in a recess designed as a hollow channel extending through the filler rod along the filler rod longitudinal axis, or attached in a recess on the outer cladding of the filler rod.

[0069] Since the marker element is located in or on the glass filler rod and thus in the central bore of the hollow glass cladding cylinder, but not in the cladding glass of the hollow cylinder, the need to adapt the hollow glass cladding cylinder for the purpose of inserting the marker element, for example by mechanical processing, in particular by creating a separate bore in the hollow glass cladding cylinder for receiving the marker element, is eliminated, thus eliminating the risk of damage associated with such adaptation of the hollow glass cladding cylinder.

[0070] The dimensional stability and straightness of the central bore can be easily ensured by the OVD manufacturing process itself and, if necessary, by subsequent measures. Suitable subsequent measures for adjusting the dimensional stability and straightness include, for example, mechanical reworking of the central bore and / or a stretching process in which the initial hollow cylinder produced in the OVD deposition process is provided for stretching the tube strand from which the hollow glass cladding cylinder is produced or from which a number of hollow glass cladding cylinders are cut to length. The stretching process is preferably carried out without the use of a forming tool that engages the drawn tube strand, in order to avoid damaging the tube strand surface. Similarly, the dimensional stability and straightness of the filling rod that is accurately inserted into the central bore can be relatively easily ensured by machining and / or such a stretching process. This machining can optionally be an external machining method, which is generally significantly less complicated than an internal machining method.

[0071] Axially parallel alignment of the marker element is facilitated by being formed by a recess in the filling rod extending in the direction of the filling rod's longitudinal axis or by being inserted into a recess extending in the direction of the filling rod's longitudinal axis.

[0072] The recess is designed, for example, as a bore in 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 of marker material or particulate marker material to form a "modified filling rod". The particulate marker material can have a certain dimensional stability by thermal compression or by the addition of a binder. The marker material fills the recess completely or partially. The recess thereby 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), for example by material bonding by sintering or fusion.

[0073] In this connection, the embodiment in which the recess of the filling rod is designed as a longitudinal groove is particularly preferred. On the one hand, the longitudinal groove of the filling rod outer cladding surface is particularly easy to manufacture and geometrically accurate compared to the bore of the filling rod. On the other hand, the longitudinal groove thus manufactured is just as accurate and straight as the filling rod itself. Furthermore, the depth or the opening width of the longitudinal groove can be made substantially as small as desired, for example both less than 15 mm, preferably less than 10 mm, particularly preferably less than 5 mm. This means that geometrically accurate marker elements with small volumes are available, which have an axial misalignment of less than 0.3 mm / m in the preform or component group, thus forming a correspondingly small and highly accurate marker zone 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 formed by a bed of particles from the marker material or by internally coating the longitudinal groove with the marker material. The cylindrical component consisting of the marker material or respectively a bed from the marker material can also be fixed in the longitudinal groove by fusion.

[0075] The relatively easy to achieve straightness of the filler rod and central bore also facilitates axially parallel alignment of the marker element, which applies in particular to the particularly preferred embodiment of the semi-finished product in which the marker element extends along the filler rod longitudinal axis and is melted into the recess.

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

[0077] By melting the marker elements into the recesses, it can be ensured that the side edges of the recesses filled with the melted marker elements form a large, stepless, continuous transition to the outer cladding surface of the filling rod, thus avoiding structural defects during the fiber drawing 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 matches the outer contour of the filling rod outer cladding surface.

[0078] Thus, in a preferred embodiment, it is provided that the marker element has a length and that the marker element is fused into the recess completely, partially or at a particular point 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 relative to the filler rod, which simplifies its handling in the further fiber manufacturing process. During melting, a certain degree of rounding of the marker material and therefore conformity to the contour of the outer cladding surface of the filler rod can be achieved by surface tension. After melting the marker element, the absence of defects and the quality of the melting process can be monitored and improved if necessary.

[0080] As a result, the semi-finished product is equipped with marker elements without the need to manufacture a separate bore in the hollow glass cladding cylinder, with the associated risks and difficulties mentioned above. At the same time, despite the high aspect ratio, a high level of precision can be ensured, which is manifested in the semi-finished product, for example, in that the axial parallelism of the marker elements 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 cladding surface of the filler rod, and the semi-finished product according to the invention further comprises a hollow cylinder comprising a central bore, at least two core rods comprising a core glass and forming a core glass region, a filler rod arranged in the central bore, and at least one marker element mounted in the recess of the filler rod.

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

[0083] Two or more, for example 4 to 7, additional longitudinal bores (core rod bores) are made in the hollow cylinder, whose longitudinal axis runs parallel to the longitudinal axis of the central bore. The core rod bores are through or blind bores and serve to receive at least one core rod made in each case from a core glass. Viewed in the radial direction, the composition of the core glass is uniformly homogeneous or changes gradually or in stages. It differs from the cladding glass in that light guidance in the core glass region is guaranteed. In the simplest case, all core rods have the same dimensions and consist of the same core glass. However, the core rods can also differ with regard to their dimensions and / or the composition of the corresponding core glass.

[0084] The semi-finished product is either directly drawn to form a multicore fiber or consolidated to form a preform for a multicore fiber, which consolidation may be accompanied by simultaneous elongation. The "consolidated preform" thus produced is optionally drawn to form a multicore fiber or is further processed to form a "secondary preform" from which the MFK is finally drawn.

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

[0086] By attaching the marker element to the filler rod, the marker element benefits from its straightness and alignment, and these properties are pseudo-transferred to the marker element. The attachment can be based, for example, on a frictional connection, a material connection, and / or a positive connection between the filler rod and the marker element.

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

[0088] The cross-sectional shape, optionally the outer diameter and length of the filling rod are adapted 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, 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 elements form elongated cavities or channels filled with air or preferably comprise a marker material that differs in at least one physical and / or chemical property from the cladding glass and glass filling material of the filling rod, the property being selected from refractive index, color, fluorescence, and / or specific glass density.

[0090] The property (or properties) that distinguish the marker element from the glass of the component group, in particular, affects the visual appearance of the marker element and is preferably detectable by an optical sensor. The glass composition of the marker glass, as well as, for example, the glass filling material, can be based on quartz glass. The refractive index of the quartz glass can be changed by doping. For example, doping the marker quartz glass with fluorine reduces the refractive index with respect to undoped quartz glass. The incorporation of carbon into the marker quartz glass can result in a black coloration. Depending on the oxidation state, doping the marker quartz glass with titanium results in a gray or blue coloration. Doping the marker quartz glass with rare earth metals or germanium oxide results in fluorescence at dopant-specific wavelengths. The specific glass density of the marker element can be changed by pores, which is manifested in a reduced light transmission with respect to glass without bubbles.

[0091] Based on the method according to the invention or using the semi-finished product according to the invention, a multi-core fiber is obtained in which the signal cores are arranged outside the cladding circle around the central fiber axis and the marker zones are arranged within the cladding circle around the central fiber axis.

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

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

[0094] When viewed in the fiber cross section, preferably the only marker zone is in the aforementioned cladding circle defined by the signal cores, and thus in the region between the signal cores (and not outside the signal cores), which also includes the fiber central axis. Surprisingly, it has been shown that such placement of the marker zone during the fiber drawing process imprints a relatively low fiber curl on the multicore fiber. Without wishing to be bound by this theory, it can be assumed that placing the marker zone in the inner region of the fiber cross section has less impact on the symmetry of the fiber design than placing it in the outer region. Apparently, this generates a lower radial force in the multicore fiber during the fiber drawing process.

[0095] Definitions and Measurement Methods The individual terms in the above description are further defined below. The definitions are part of the description of the present invention. For terms and measurement methods not specifically defined in the specification, the interpretation given by the International Telecommunication Union (ITU) shall apply. In the event of a discrepancy between one of the definitions below and the rest of the description, the statement made elsewhere in the description shall prevail.

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

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

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

[0099] The modified filler rod is connected to the marker element to form therewith a manageable unit, which includes the glass filler material and the marker material.

[0100] Marker elements / marker materials / marker glass The marker elements comprise a marker material or consist partially of air or another gas. In particular, the marker elements comprise at least one marker glass. The chemical composition of the marker material is different from that of the cladding glass and the glass filling material of the filling rod and / or the density of the marker material is different from that of the cladding glass and the glass filling material of the filling rod. The marker elements are present in the preform and in the components as components or as layers or masses on the components and form optically detectable marker zones in the multicore fiber.

[0101] Component group / Consolidated preform / Secondary preform / Semi-finished product The "components" include a hollow glass cladding cylinder with a core rod inserted therein, at least one filler rod, and at least one marker element. A "preform", also referred to herein as a "consolidated preform", is obtained by fixing the core rod in the core rod bore, for example by narrowing the hollow glass cladding cylinder end or by crushing and fusing. The components or (consolidated) preform are stretched to form a "secondary preform" or to directly form a multicore fiber. The term "semi-finished product" here encompasses components, consolidated preforms, and secondary preforms. Reshaping the components includes stretching or forming a consolidated preform to form a multicore fiber.

[0102] Quartz glass Quartz glass is, for example, a melt product from naturally occurring SiO2 raw materials (natural quartz glass) or is synthetically produced (synthetic quartz glass) or consists of a mixture of these quartz glass types. Synthetic transparent quartz glass is obtained, for example, by flame hydrolysis or oxidation of synthetically produced silicon compounds, by polycondensation of organic silicon compounds by the so-called sol-gel process or by hydrolysis and precipitation of inorganic silicon compounds in liquids.

[0103] Fusion When referring to components made of glass, fusion is understood to mean that the components are fused to one another at their contact surfaces, which is achieved by heating the components at least in the region of the contact surfaces by a heat source such as a furnace, burner or laser.

[0104] Position display: top / bottom These designations relate to positions during the elongation process and / or fiber drawing process: "Bottom" denotes a position in the direction of the drawing process and "top" denotes a position opposite to the direction of the drawing process.

[0105] cross section Longitudinal / Cross section taken perpendicular to the longitudinal axis.

[0106] Longitudinal section Longitudinal / Section taken parallel to the longitudinal axis.

[0107] Boa The terms "bore", "central bore", "inner bore" or "longitudinal bore" refer to a hole that is cylindrical but has any other internal shape. They are manufactured, for example, by a drilling bore process or by depositing a layer of material onto the outer cladding surface of a mandrel by a deposition or pressing process and then removing the mandrel.

[0108] Axial parallel alignment / Axial parallel The reference axis in each case is the longitudinal axis of the hollow glass cladding cylinder or the central axis of the multicore fiber. [Brief description of the drawings]

[0109] Exemplary embodiments In the following, the invention is explained in more detail with reference to exemplary embodiments and drawings. [Figure 1] FIG. 1 in particular shows, in a schematic view, a cross-sectional view of a hollow glass cladding cylinder having a central bore and a through bore for receiving a core rod. [Diagram 2] FIG. 2 in detail shows, in a schematic diagram, process steps (a) to (d) for producing a filler rod having a marker element for insertion into the central bore of the hollow glass cladding cylinder of FIG. [Diagram 3] FIG. 3 in particular shows in a schematic diagram the components of the hollow glass cladding cylinder and the inserted filler rod containing the marker elements. [Figure 4] FIG. 4 in particular shows, in a schematic view, a cross-sectional view of a consolidated preform of a hollow glass cladding cylinder, an inserted filler rod containing a marker element, and a core rod inserted in the through bore. [Diagram 5] FIG. 5 specifically shows, in a schematic view, a cross-section of a multicore fiber drawn from the preform of FIG.

[0110] FIG. 1 shows diagrammatically a cross-section of a hollow cylinder 1 made of cladding glass, which serves as a substrate for producing a multicore fiber.

[0111] The hollow cylinder 1 is produced in a known manner using the OVD method. In this method, SiO2 soot particles are formed by passing a high-purity SiO2 starting material, for example silicon tetrachloride, through a deposition burner, feeding it into the burner flame, where it is oxidized to solid SiO2. It is deposited from the gas phase in the form of fine SiO2 soot particles on the outer cladding surface of a cylindrical deposition mandrel rotating about its longitudinal axis, the deposition burner performing a reversing movement back and forth along the deposition mandrel longitudinal axis. On the outer cladding surface of the deposition mandrel, a SiO2 soot body is formed. After completion of the deposition process, 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 consists of undoped synthetically produced quartz glass, which forms the cladding glass region 1b. The hollow cylinder has a length of 1500 mm and is adjusted to a nominal outer diameter of 200 mm by cylindrical grinding and to an inner diameter of 42 mm by drilling and honing. Four bores 3 are produced in a given (here secondary) configuration by mechanical drilling in the direction of the hollow cylinder longitudinal axis 1b, which runs perpendicular to the sheet plane in the illustration of FIG. 1. The bores 3 are used to receive the core rods (FIG. 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 blind bores.

[0113] FIG. 2 shows the method steps for manufacturing a filler rod 5 in a schematic way. The filler rod 5 shown in FIG. 2(a) is preferably made of the same glass as the hollow cylinder 1, i.e. undoped quartz glass. Known methods are suitable for its manufacture, for example the VAD (vapor axial deposition), OVD (outer vapor deposition) or MCVD (modified chemical vapor deposition) methods. It is used to fill the hollow cylinder bore 2 and has a length of about 1500 mm and an initial outer diameter of about 45 mm, which is reduced to about 40 mm by cylindrical grinding. This cylindrical grinding removes possible interference with the outer cladding surface 5b and bending. Alternatively or additionally, the adaptation of the diameter and the improvement of the surface are achieved by elongation in a tool-free elongation process. In this figure, the filler rod longitudinal axis 5a also runs perpendicular to the sheet plane.

[0114] FIG. 2(b) shows that a longitudinal groove 6 is milled into the outer cladding 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 base and straight side walls. Its opening width and depth are 6 mm, respectively. In a subsequent method step, a hollow channel forming a marker element within the meaning of the present invention can be produced from the longitudinal groove (6). This is explained in more detail below with reference to FIG. 4.

[0115] FIG. 2(c) shows the longitudinal groove 6 with the marker rod 7 inserted. The marker rod 7 has a diameter of 5 mm. It consists of synthetically produced quartz glass doped with fluorine and is commercially available under the name F320. Both the viscosity and the refractive index of the fluorine-doped quartz glass of the marker rod 7 are smaller than the undoped quartz glass from which the hollow cylinder 1 and the filling rod 5 are made. The marker rod 7 is obtained by elongating in a tool-free manner a starting cylinder made from F320 quartz glass. It has a smooth surface generated by melt flow and is characterized by a high dimensional stability, so that it can be inserted without difficulty and with a precise fit into the narrow longitudinal groove 6.

[0116] The filling rod 5 and the marker rod 7 are then fused together. The filling rod 5 is mounted with its longitudinal axis 5a oriented horizontally so that the longitudinal groove 6 is located on its upper side. The marker rod 7 inserted in the longitudinal groove 6 is first heated at certain points by a burner, whereby it is fixed in the longitudinal groove at three approximately evenly distributed fixing points located at the ends and in the center of the marker rod 7 and distributed over 95% of its length. It is then uniformly heated by the burner until the fluorine-doped quartz glass softens and deforms due to its relatively low viscosity, so that it sinks into the longitudinal groove 6 and fills it. Due to surface tension, the surface of the softened glass mass adjacent to the free atmosphere shows a certain bulge, so that a significant step between the side edges of the longitudinal groove 6 and the outer cladding surface 5b of the filling rod 5 is avoided.

[0117] 2(d) shows the marker glass gob 8 after softening, deformation and fusing with the former filler rod 5 and the resulting modified filler rod 5c filled with the marker glass gob 8. The glass volume of the former marker rod 7 matches the internal volume of the longitudinal groove 6 such that the marker glass gob 8 just completely fills the longitudinal groove 6.

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

[0119] Four core rods 4 are also fabricated, made of germanium-doped quartz glass, with a length of about 1500 mm and an outer diameter of about 28 mm. Known techniques, such as MCVD (Modified Chemical Vapour Deposition), are also suitable for this purpose.

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

[0121] 4 shows diagrammatically the components fixed in this way: the hollow glass cladding cylinder 1, the modified filler rod 5c, and the core rods 4 forming the consolidated preform 10. All the core rods 4 form separate circular core glass regions 4a located entirely outside the cladding circle 11, whereas the marker elements 8 are entirely located within this cladding circle 11.

[0122] The consolidated preform 10 is then elongated to form a secondary preform, whereby the preform 10 is held in a drawing apparatus by a holder in vertical alignment of the hollow cylinder longitudinal axis 1a. The secondary preform thus produced is finally drawn in a drawing apparatus in the usual way to form a multicore fiber 20.

[0123] In this embodiment, the marker elements 8 are present as marker glass chunks 8 produced by reshaping the original marker rods 7. In an alternative procedure, the longitudinal grooves 6 are not filled (no rods or tubes are inserted) when the component group 9 is consolidated into the preform 10, and the longitudinal grooves 6 are prevented from collapsing completely by creating and maintaining an overpressure therein. In this way, cavities are created that extend along the longitudinal axis 1a and are present in the multicore fiber as hollow channels filled with air. The hollow channels can function as marker zones, since the refractive index of air is significantly different from that of the cladding glass 1b.

[0124] FIG. 5 shows a schematic cross-section of a multicore fiber 20. Apart from the smaller radial dimension, this substantially corresponds to the cross-section of the consolidated preform 10. The core glass area (4a) of the former core rod (4) forms the signal cores 4b extending along the fiber longitudinal axis 20a, the former filling rod (5) has become part of the cladding glass area 1b and is no longer visually distinguishable therefrom, and the former marker elements (8) form the marker zone 8a. All the signal cores 4b are located completely outside the cladding circle 11a, while 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 a lower tension on the multicore fiber 20 during fiber drawing, which results in a smaller fiber curl.

Claims

1. 1. A method for manufacturing a multicore fiber (20), comprising the method steps of: a component group (9) being reshaped to form the multicore fiber (20) or a preform (10) for the multicore fiber (20) comprising a hollow cylinder (1) comprising a central bore (2) and a hollow cylinder longitudinal axis (1a), the hollow cylinder comprising a cladding glass region (1b) made from a cladding glass and a plurality of core glass regions (4a) provided with a core glass, at least a part of the central bore (2) being occupied by a glass filler rod (5) comprising a filler rod longitudinal axis (5a) and a filler rod outer cladding surface (5b), a recess (5) extending in the direction of the filler rod longitudinal axis (2) is made in or on the filler rod (5), and a marker element (6, 7, 8) made from a marker glass is inserted in the recess or a marker element (6, 7, 8) is formed in the recess.

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

3. 3. A method according to claim 1 or 2, characterized in that the marker element (6, 7, 8) has a length and melting occurs completely, partially or at specific points along at least 80% of this length, preferably along at least 90% of this length.

4. 4. The method according to claim 1, characterized in that melting the marker element (6, 7, 8) comprises a method step in which the filler rod (5) is mounted with the filler rod longitudinal axis (5a) oriented horizontally such that the recess (6) is located above the filler rod outer cladding surface (5b), and the material of the marker element (6, 7, 8) is heated and softened by a heat source.

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

6. The method according to any one or more of the preceding claims, 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 any 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 filler rod (5).

8. 8. The method according to claim 1, wherein the marker element (6, 7, 8) forms an elongated cavity filled with air or comprises a marker material having at least one physical and / or chemical property different from the cladding glass and from the glass filling material, said property being selected from refractive index, color, fluorescence and / or specific glass density.

9. 1. A semi-finished product for manufacturing a multicore fiber, comprising: a hollow cylinder (1) with a central bore (1a), said hollow cylinder comprising a cladding glass region (1b) made from cladding glass and a hollow cylinder longitudinal axis (1a), and a plurality of core glass regions (4a) in which core glass is provided within said cladding glass region (1b), at least a part of said central bore (2) being occupied by a glass filling rod (5) with a filling rod longitudinal axis (5a) and a filling rod outer cladding surface (5b), said filling rod (5) comprising a recess (5) extending in the direction of said filling rod longitudinal axis (2), in which a marker element (6, 7, 8) made from marker glass is inserted or forms said marker element (6, 7, 8) and extends along said central bore longitudinal axis (1a) and said filling rod longitudinal axis (2b).

10. 10. A semi-finished product according to claim 9, characterized in that the marker element (6, 7, 8) has a length and is melted completely, partially or at points into the recess (5) along at least 80% of this length, preferably along at least 90%.

11. 11. Semi-finished product according to claim 9 or 10, characterized in that the marker element comprises a gas-filled channel.

12. 12. A semi-finished product according to claim 9, characterized in that the recess (6) comprises a bore and / or a longitudinal groove in the outer cladding surface (5b) of the filler rod (5), and the semi-finished product further comprises the hollow cylinder (1) comprising the central bore (2), at least two core rods (4) comprising the core glass and forming the core glass region (4a), the filler rod (5) arranged in the central bore (2), and at least one marker element (6, 7, 8) mounted in the recess (6) of the filler rod (5).

13. Semi-finished product according to one or more of claims 9 to 12, characterized in that the marker elements (7, 8) are present in the form of cylindrical components or in the form of layers or masses connected to the filling rod.

14. 14. A semi-finished product according to any one or more of claims 9 to 13, characterized in that the marker element (6) forms an elongated cavity filled with air or comprises a marker material differing in at least one physical and / or chemical property from the cladding glass, the core glass and the glass filler material, said property being selected from the refractive index, the color, the fluorescence and / or the specific glass density.

Images