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

Abstract

A known method for manufacturing a multicore fiber with a marker zone close to the edge comprises a method step in which a group of components is reshaped to form a multicore fiber or to form a preform for a multicore fiber, the group of components comprising a cylinder with a cylinder longitudinal axis and an outer surface and with a glass cladding region made from cladding glass, a plurality of core glass regions provided with core glass and extending in the cylinder longitudinal axis direction, surrounded by the cladding glass, and at least one marker element extending in the direction of the cylinder longitudinal axis. On this basis, in order to provide a method for manufacturing a multicore fiber with a marker zone close to the edge with a reduced risk, according to the invention, a marker element is arranged on the outer surface of the glass cladding cylinder, a longitudinal groove is created in the outer surface of the glass cladding region and extends in the direction of the cylinder longitudinal axis, and the marker element is melted in the longitudinal groove before reshaping to form the preform or the multicore fiber.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a multicore fiber, comprising the method steps of reshaping a group of components to form a multicore fiber or to form a preform for a multicore fiber, the group of components comprising: a cylinder having a cylinder longitudinal axis and an outer surface, the cylinder having a glass cladding region made from a cladding glass; a plurality of core glass regions, each of which is surrounded by a cladding glass and extends in the direction of the longitudinal axis of the cylinder; at least one marker element extending in the direction of the cylinder longitudinal axis; The present invention relates to a method comprising the steps of:

[0002] Additionally, the present invention relates to a semi-finished product for manufacturing a multicore fiber, the semi-finished product including a glass cladding cylinder having a cylinder longitudinal axis and an outer surface, the glass cladding region made from a cladding glass including a plurality of openings for receiving a core rod made from a core glass.

[0003] In a multicore fiber, 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 substantially independent light guiding. 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, also called "spatial multiplexing", 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 fabricated by stretching a solid preform or group of components. These are made of synthetically produced fused silica (SiO 2 ), which may be doped or undoped. The production of synthetic quartz glass involves, 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 a chemical reaction (hydrolysis, pyrolysis or oxidation) which produces a reaction product, namely fine-grained SiO 2 is deposited as a solid from the gas phase on the deposition surface. The starting material is, for example, silicon tetrachloride (SiCl 4 ) or chlorine-free silicon compounds such as polyalkylsiloxanes. The reaction zone is, for example, a burner flame, an electric arc (plasma) or a furnace.

[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 cladding 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 level 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] 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 arranged one on top of the other such that the through bores and the longitudinal grooves are aligned. A marker zone of the fiber preform formed by the marker rods extends on the inner cladding surface of the cladding tube.

[0008] US 2016 / 070058 discloses a preform and a method for producing a multicore fiber by drawing the preform. The preform has a flattened portion on its outer surface that can function as a marker element. In the fiber drawing process, a plastic coating is applied to the multicore fiber.

[0009] In the methods known from US 2015 / 284286 and US 2015 / 0307387, SiO 2 produced by the OVD method is 2 Soot body based on (SiO 2 The soot body is used as a glass cladding cylinder, and the density is 0.8 g / cm 3 ~1.6g / cm 3In the OVD process, the deposition surface is generally the outer surface of a rod-like or tubular deposition mandrel that rotates about its longitudinal axis. A substantially cylindrical soot body is deposited by reversing movement back and forth through the reaction zone. After the deposition process is completed, the deposition mandrel is removed so as to leave a central through opening at the central axis of the cylindrical soot body. The hollow glass cladding cylinder thus produced introduces a longitudinal bore to receive the core glass rod and is filled with SiO 2 as compared to quartz glass. 2 The low density of the soot body makes it easy to manufacture a dimensionally accurate longitudinal bore. The central "OVD through opening" that remains due to the manufacturing process can lead to asymmetric deformation during collapse, destroying the fiber design. Also, the collapse reduces the cross-sectional area of ​​the proportion of cladding glass. These drawbacks can be avoided by using a component that closes the OVD through opening. This can be, for example, a core rod or a filling rod. The filling rod consists of a glass with substantially the same refractive index as the hollow glass cladding cylinder. The filling rod can also be manufactured by the OVD method or by press molding, or by a combination of press molding and OVD methods. Furthermore, a channel for receiving the marker element is formed in 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.

[0010] In multicore fibers, the marker elements serve to form a continuous linear marker zone, breaking the symmetry around the signal core and allowing their positions to be clearly identified and unambiguously assigned to each other and to the fiber central axis, 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.

[0011] 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 to 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, when one end of a multi-core fiber is not available for feeding light, the multi-core fiber is irradiated with light laterally. 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 collected light power received is maximized, and then fused to each other in this position.

[0012] In this splicing method, it is assumed that the multicore fiber is pre-spliced, for example in a factory. To enable easy splicing of the multicore fiber at the installation site, US Pat. No. 9,541,707 proposes a multicore fiber design in which a number of signal cores are arranged in a glass cladding region and a marking zone is exposed on the outer surface of the multicore fiber. This particular characteristic of the fiber design is also referred to below as "marker zone close to the edge".

[0013] To produce a multicore fiber with a marker zone close to the edge, a number of through bores are produced in the glass cladding cylinder, extending in the direction of the longitudinal cylinder axis, specifically a number of core rod bores and marker rod bores. The marker rod bores are located as close as possible to the outer surface of the glass cladding cylinder. A core rod is inserted in each of the core rod bores and a marker rod is inserted in each of the marker rod bores. The outer periphery of the glass cladding cylinder is then ground until a part of the marker rod is exposed. From the components thus produced, a multicore fiber is drawn with the surface on which the marker zone is exposed. Technical Problem

[0014] The glass cladding cylinder is elongated, and therefore the through holes have a large aspect ratio (ratio of length to diameter), which in principle makes dimensionally accurate manufacture and precise alignment parallel to the longitudinal axis of the glass cladding cylinder more difficult.

[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 any additional attenuation or impair 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] In addition, the marker zones are always imperfections in the multicore fiber, which in principle should be as small as possible, but as large as necessary to ensure their detectability. 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.

[0017] Therefore, the marker zone, which is additionally introduced into the fiber design, should occupy as small a percentage of the fiber cross section as possible. 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 glass cladding cylinder before the fiber drawing process is less than 15 mm, with an aspect ratio of more than 65 (cylinder length is about 1 m).

[0018] Even when high-precision boring machines are used, dimensionally accurate manufacture and precise alignment of such thin channels in a glass cladding cylinder is difficult. The cost of manufacturing a glass cladding cylinder from synthetic quartz glass is large, and losses are especially painful, especially for small bores to accommodate marker elements that would otherwise result in the finished glass cladding cylinder being rejected.

[0019] To make matters worse, for marker rod bores close to the edges, only a thin residual wall remains, which can easily be damaged both during manufacture and further processing of the through bore, especially when the marker rod is inserted.

[0020] Grinding of the outer circumference of the glass-clad cylinder according to US Patent No. 9,541,707 is carried out when the through-bore is completely equipped with the core rod and the marker rod. Apart from the fact that grinding must be carried out with the utmost care and therefore requires a lot of time and expense, this method step involves the risk of total loss.

[0021] It is therefore an object of the present invention to provide a method for manufacturing a multicore fiber having a marker zone close to its edge, which reduces the drawbacks of known methods, in particular in that the risk of rejection is reduced.

[0022] Furthermore, it is an object of the present invention to provide a semi-finished product suitable for carrying out this method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] With regard to this method, this object is achieved according to the invention and based on the method described at the beginning in that a marker element is arranged on the outer surface of a glass cladding cylinder, a longitudinal groove is created on the outer surface of the glass cladding region extending in the direction of the cylinder longitudinal axis, and the marker element is melted in the longitudinal groove before reshaping to form a preform or a multicore fiber.

[0024] At least one longitudinal groove is created on the outer surface of the glass cladding region extending in the direction of the longitudinal cylinder axis, and a marker element is placed in this groove and melted in. Placing the marker element on the outer surface of the glass cladding cylinder includes, for example, attaching the marker element in the form of a component in the longitudinal groove of the outer surface, and applying, depositing or pressing the marker element in the form of a layer or mass in the longitudinal groove of the outer surface.

[0025] This procedure has several advantages over the prior art. (1) The effort of creating a bore in the glass cladding cylinder to receive the marker element is eliminated, along with the associated risk of failure. (2) The marker elements melted in the longitudinal grooves are exposed on the outer surface, thus eliminating the effort and risk of failure in removing the cladding glass material to expose the marker glass, in contrast to the method known from US Pat. No. 9,541,707. (3) Axis-parallel alignment of the marker elements is achieved by the fact that the marker elements are adjacent to the outer surface. Dimensional stability and straightness of the outer surface of the glass cladding cylinder can be ensured relatively easily. For example, a starting cylinder is stretched by machining or a stretching process to form a cylinder strand from which a glass cladding cylinder is produced or from which multiple glass cladding cylinders are cut to length. To prevent damage to the surface, the stretching process is preferably performed without the use of a forming tool that engages the stretched cylinder strand. (4) It is possible to omit creating a bore for receiving the marker element. The arrangement on the outer surface allows for particularly small marker elements with an aspect ratio of virtually any size. (5) By melting the marker elements into the longitudinal grooves before reshaping the component group to form the multicore fiber or preform, it can be ensured that the side edges of the longitudinal grooves filled with the melted marker elements form a continuous transition to the outer surface of the glass cladding region with almost no gradient, 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 optionally fill the longitudinal grooves completely and ideally have a curvature that matches the outer contour of the outer surface. If the spatial volume of the longitudinal groove and the volume occupied by the marker element after melting are approximately the same size, this contributes to a complete filling of the longitudinal groove. Achieving this is simplified if, before melting, the marker element can protrude to any desired extent relative to the upper edge of the longitudinal groove and the height of this protrusion is not limited by an outer cladding tube or the like. Such limitations can be omitted in the method according to the invention. (6) During the bonding process, the peripheral markers can be detected optically with relative ease. (7) In addition, the azimuthal alignment precision of the multicore fiber is high for the splicing process because the peripheral position of the marker is particularly sensitive to angular deviations.

[0026] The edge location of the marker zone in a multicore fiber results from a similar peripheral location of the marker element on the outer cladding of the glass cladding region of the component group or on the outer cylindrical cladding surface of the preform, where "peripheral" means that the marker element is part of the outer surface of the glass cladding region of the component group or the outer surface of the preform.

[0027] The component groups are equipped with marker elements without a separate bore, which has to be created for that purpose in the glass cladding cylinder, in connection with the above-mentioned risks and difficulties. At the same time, a high dimensional stability can be ensured, which manifests itself, for example, in the preform or the component group, in that the axial parallelism of the marker elements has a deviation of less than 0.3 mm / m.

[0028] The longitudinal grooves are filled with marker elements, for example by inserting cylindrical components (rods or tubes) made of marker glass that run parallel to the outer surface of the glass cladding cylinder, or by introducing a bed of marker glass particles, or by coating the interior of the longitudinal grooves with marker glass.

[0029] The marker element in the form of a tube may be open on both sides or at least on one side (an opening above the softening zone will suffice), which allows to prevent complete collapse of the tube inner bore during reshaping of the preform or the group of components to form the multicore fiber by applying pressure so that hollow channels ("airlines") remain in the finished multicore fiber. Alternatively or additionally, the formation of hollow channels in the multicore fiber can also be promoted by the tube wall comprising a material with a higher viscosity than the cladding glass, so that the bore does not completely collapse during the fiber drawing process.

[0030] The marker elements (components, beds, layers) arranged in the longitudinal grooves are fixed therein by fusion, for this purpose they are melted in the longitudinal grooves over at least a part of their length, preferably locally at a number of points distributed over their length, ideally over their entire length.

[0031] Thus, a preferred procedure provides that the marker element has a length and that the marker is completely melted in an area or point along at least 80% of this length, preferably at least 90% of this length.

[0032] Melting the marker element preferably includes a method step in which a glass cladding cylinder is mounted with the cylinder longitudinal axis oriented horizontally so that the longitudinal groove is located at the top, and the material of the marker element is heated and softened by a heat source.

[0033] By melting a marker element with a cylindrical longitudinal axis oriented horizontally, the material of the marker element sinks by gravity as soon as it is locally heated and softened, for example by a burner or laser, thereby filling the cavity remaining in the longitudinal groove. Surface tension can result in rounding of the free surface area adjacent to the atmosphere.

[0034] In this manner, it is substantially easier to uniformly, and preferably completely, fill the longitudinal grooves than if, for example, the marker elements and longitudinal grooves were vertically oriented during the melting process.

[0035] The marker elements melted in the longitudinal grooves are fixed relative to the glass cladding cylinder, which simplifies their handling during later stages of the fiber manufacturing process. During melting, some rounding of the marker material and therefore conformity to the contour of the outer surface of the glass cladding cylinder can be achieved as a result of surface tension. After melting the marker elements, the absence of defects and the quality of the melting process can be monitored and improved if necessary.

[0036] Two or more, for example 4 to 7, longitudinal bores (core rod bores) are made in the glass cladding cylinder in a conventional manner, 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.

[0037] 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, occupied in each case by at least one core rod, and the core rod bore occupied by the core rod is collapsed (this reshaping process is also called "consolidation" in this specification) before the remaining distribution or a further distribution of core rod bores is produced in a second or further operation, this distribution also occupied in each case by at least one core rod, and optionally collapsed. 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.

[0038] Placing the marker element on the outer surface and fusing it into the longitudinal groove can occur before or after all of the core rod bores are created and / or filled, or before or after a portion of the core rod bores are created and / or filled. In a preferred procedure, the marker element is placed on the outer surface and fused into the longitudinal groove and then the desired core rod bores are created.

[0039] 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" includes, for example, creating further bores in the glass cladding region and its coating with core glass or other glasses, or performing one or more of the following hot forming processes once or repeatedly: collapsing, stretching, collapsing, and simultaneous elongation. The multicore fiber is drawn from the secondary preform produced by the further processing, in which the peripheral marker elements of the components form a peripheral marker zone.

[0040] Due to the peripheral location of the marker zone, the fiber splicing device can identify the marker zone more quickly and more accurately. Due to the relatively simple detectability of the peripheral marker zone, the marker zone can be made particularly small so that the multicore fiber is given a relatively low fiber curl during the fiber drawing process. A property of glass fiber is defined as the degree of curvature over a certain length of the fiber, called "fiber curl". The curvature is due to thermal stresses that arise during fiber manufacturing. A large "fiber curl" makes low attenuation splicing of multicore fibers more difficult.

[0041] In comparison with the bores, the longitudinal grooves are particularly easy to produce on the outer surface, for example by milling with a mechanical milling machine or by laser ablation. On the other hand, the longitudinal grooves produced in this way are as accurate and straight as the glass cladding cylinder 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. Thus, by filling the longitudinal grooves with marker glass, geometrically accurate marker elements with small volumes can be produced in a simple manner, which have an axial deviation 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.

[0042] The marker glass completely or partially fills the longitudinal grooves. In a particularly preferred method variant, the cladding glass volume V M A cladding glass having a volume V is removed from the glass cladding cylinder to create a longitudinal groove. E A marker element having a V E =V M + / -0.1xV M It is.

[0043] The marker glass volume is sized such that the molten marker glass fills the longitudinal groove open volume as completely as possible, ideally with a curvature that matches the contour of the outer surface. This avoids as much as possible asymmetries and defects during the fiber drawing process. The cross-sectional contours of the longitudinal groove and the marker element do not need to match for this purpose. The longitudinal groove preferably has a circular lower portion.

[0044] In a preferred procedure, the manufacture of the building blocks comprises the following method steps: (a) providing a glass cladding cylinder comprising cladding glass; (b) providing a core rod comprising a core glass; (c) providing a marker element; (d) creating at least one longitudinal groove on the outer surface of the glass cladding cylinder; (e) creating a core rod bore extending along the cylinder longitudinal axis; (f) disposing and fusing a marker element within the longitudinal groove; (g) introducing a core rod into the core rod bore, thereby forming a core glass region;

[0045] The components thus produced include a glass cladding cylinder, a marker element, and at least two core rods.The list designations (a)-(g) merely designate a preferred, but not required, order of the method steps.

[0046] By attaching the marker element to the glass cladding cylinder, the marker element benefits from its straightness and alignment: these properties are pseudo-transferred to the marker element, which extends along the cylinder longitudinal axis of the glass cladding cylinder, preferably over its entire length.

[0047] The marker glass preferably differs from the cladding glass and any glass filler material in at least one physical and / or chemical property selected from refractive index, color, fluorescence and / or specific glass density.

[0048] The characteristic (or characteristics) that distinguishes the marker element from the glasses 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 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-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.

[0049] The glass cladding cylinder is preferably a hollow cylinder having a central bore.

[0050] Such hollow cylinders are obtained, for example, by the OVD (Outside Vapor Deposition) method after the deposition mandrel has been removed. The production of hollow glass cladding cylinders based on the OVD process is cost-effective compared to other manufacturing methods, especially compared to the VAD (Vapor Axial Deposition) process. However, it has the disadvantage that the aforementioned central bore may remain. This can be completely or partially closed by a core rod or glass rod containing another glass filling material.

[0051] With regard to the semi-finished product, the above technical object is achieved by a semi-finished product having the features of claim 8.

[0052] The semi-finished product according to the invention comprises a glass cladding cylinder having an opening for receiving a core rod, the outer surface of which has at least one recess extending in the direction of the cylinder longitudinal axis and designed as a longitudinal groove in which the marker element is melted.

[0053] The recess forms a longitudinal groove (longitudinal slot) on the outer surface of the glass cladding cylinder. The longitudinal groove on the outer surface is particularly easy to manufacture, for example by milling with a mechanical milling machine or by laser ablation, in comparison with the bore. On the other hand, the longitudinal groove thus manufactured is as accurate and straight as the glass cladding cylinder 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.

[0054] The marker element fused in the longitudinal groove is fixed relative to the glass cladding cylinder, which simplifies its handling during later stages of the fiber manufacturing process, for example, it is easy to check the freedom from defects and the quality of the fusion process before the opening of the glass cladding cylinder is filled with the core rod.

[0055] In a longitudinal groove, the marker element (component, bed, layer) is melted within the longitudinal groove locally over at least a portion of its length, preferably at multiple points distributed over its length, and ideally over its entire length.

[0056] Taking this into consideration, in a preferred embodiment of the semi-finished product, it is provided that the marker element has a length and that the marker element is melted partially or completely in spots into the longitudinal groove along at least 80% of this length, preferably along at least 90% of this length.

[0057] In a preferred embodiment of the workpiece, the marker element comprises a hollow channel that is filled with a gas.

[0058] Gases such as air or nitrogen exhibit a particularly high refractive index increase relative to the surrounding cladding glass and are therefore easy to detect even at small radial dimensions.

[0059] The longitudinal grooves are preferably designed to receive marker elements having a deviation in their axial alignment of less than 0.3 mm / m, thus forming a highly accurate marker zone in the multicore fiber obtained from the semi-finished product.

[0060] The glass cladding cylinder is preferably a hollow cylinder having a central bore.

[0061] The semi-finished product is provided for carrying out the method according to the invention and is suitable and designed for this purpose, the statements made regarding the glass cladding cylinder in relation to the method according to the invention also apply to the semi-finished product and are incorporated herein.

[0062] Definitions and Measurement Methods The individual terms in the above description are further defined below. The definitions are part of the description of the present invention. For terms and measurement methods not specifically defined herein, the interpretations given by the International Telecommunication Union (ITU) apply. In the event of a discrepancy between one of the definitions below and the remainder of the description, the statements made elsewhere in the description shall prevail.

[0063] Glass cladding cylinder / glass cladding area The glass cladding cylinder has an elongated, substantially cylindrical shape. Deviations from the cylindrical shape may be present in the region of the end faces. The glass cladding cylinder is designed as a solid cylinder or as a hollow cylinder. The glass cladding cylinder comprises a cladding glass forming a glass cladding region. 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 can reduce the refractive index of the quartz glass.

[0064] 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.

[0065] 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 composition of which differs from that of the cladding glass and / or the density of which differs from that of the cladding glass. 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.

[0066] Component Group / Consolidated Preform / Secondary Preform The "components" include a glass cladding cylinder into which a core rod is inserted and at least one marker element. By fixing the core rod in the core rod bore, for example by narrowing the glass cladding cylinder end or by collapsing and fusing, a "preform", also referred to herein as a "consolidated preform", is obtained. The components or (consolidated) preform is elongated to form a "secondary preform" or directly to form a multicore fiber. The term "semi-finished product" here encompasses components, consolidated preforms, and secondary preforms. Reshaping the components includes elongation or forming a consolidated preform to form a multicore fiber.

[0067] Quartz glass Silica glass is, for example, the naturally occurring SiO 2 It is either a melt product from 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.

[0068] 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.

[0069] 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.

[0070] cross section Longitudinal / Cross section taken perpendicular to the longitudinal axis.

[0071] Longitudinal section Longitudinal / Section taken parallel to the longitudinal axis.

[0072] Boa The terms "bore", "central bore", "inner bore" or "longitudinal bore" refer to a hole having any internal shape, for example, produced by a drilling bore process or by depositing a layer of material on the outer surface of a mandrel by a deposition or pressing process and then removing the mandrel. [Brief description of the drawings]

[0073] 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-section of a solid glass cladding cylinder having a longitudinal groove and a through bore in its outer surface. [Diagram 2] FIG. 2 in particular shows, in a schematic view, a group of components including the solid glass cladding cylinder of FIG. 1, a core rod inserted in the core rod bore, and a marker element inserted in the longitudinal groove. [Diagram 3] FIG. 3 in particular shows, in a schematic view, a cross-sectional view of a consolidated preform including a solid glass cladding cylinder, a marker element, and a core rod. [Figure 4] FIG. 4 in particular shows, in a schematic view, a cross-sectional view of a consolidated preform including a hollow glass cladding cylinder, a marker element, and a core rod. [Diagram 5] FIG. 5 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.

[0074] Example 1 FIG. 1 shows a schematic cross section of a cylinder 1 made of cladding glass, which serves as a substrate for producing a multicore fiber. The glass cladding cylinder 1 consists of undoped synthetically produced quartz glass. The quartz glass forms the glass cladding region 1a. The glass cladding cylinder 1 has a length of 1500 mm and is adjusted to a nominal outer diameter of 200 mm by cylindrical grinding. In the cylinder outer cladding 4, longitudinal grooves 5 are produced. Four bores 3 are produced in a predetermined (here secondary) configuration by mechanical drilling bores in the direction of the cylinder longitudinal axis 2, which in the illustration of FIG. 1 runs perpendicular to the sheet plane. The bores 3 serve to receive the core rods (FIG. 2) and have a diameter of 24 mm. The bores 3 pass through the entire cylinder 1 (through bores). In another embodiment, the bores are designed as blind bores.

[0075] A longitudinal groove 5 milled in the cylinder outer cladding 4 extends the entire length of the glass cladding cylinder 1. It is semicircular in cross section with an opening width of 10 mm and a depth of 5 mm.

[0076] FIG. 2 shows a component group 10 including the glass cladding cylinder 1, the core rod 7 and the marker rod 6 inserted in the longitudinal groove 5. The marker rod 6 is also 1500 mm long and 3.5 mm in diameter. 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 6 are smaller than the undoped quartz glass of which the cylinder 1 consists. The marker rod 6 is obtained by elongating in a tool-free manner a starting cylinder made from F320 quartz glass. It has a smooth and damage-free surface generated in the molten mass and is characterized by a high dimensional stability and straightness so that it can be inserted without difficulty in the longitudinal groove 5. The marker rod 6 inserted in the longitudinal groove 5 is melted in the longitudinal groove 5 by spot heating by a burner and is thereby fixed. The glass volume of the former marker rod 6 corresponds to the spatial volume of the longitudinal groove 5 so that the marker glass 11 fills the longitudinal groove 6 exactly and completely. In this case, the glass cladding cylinder 1 is mounted with the cylinder longitudinal axis oriented horizontally, so that the longitudinal groove 5 is located on its upper side. The glass material of the marker element 6 is heated and softened by the burner, thereby sinking into the longitudinal groove 5 and filling it completely. Due to surface tension, the surface of the softened glass mass adjacent to the free atmosphere shows a slight bulge.

[0077] In an alternative method variant, the marker rod 6 is melted in the longitudinal groove 5 over its entire length. This makes it possible to achieve a certain rounding of the marker material 11 as a result of surface tension and thus a conformity to the circular contour of the outer surface of the glass cladding cylinder 1, as can be seen in figure 3. After melting the marker rod 6, the absence of defects and the melt quality can be monitored and improved if necessary.

[0078] In a further alternative method variant, the tube is made of aluminum oxide (Al 2 O 3The marker element is made of silica glass doped with a dopant that increases the viscosity of the silica glass, such as . The inside diameter of the tube is 8 mm (or at least 10 mm) and the length is 1500 mm. Both sides are open. 2 O 3 During the melting of the doped quartz glass tubes into the longitudinal grooves 5, excess pressure is generated and maintained within the pipe bore, thus preventing collapse of the pipe bore. This pipe bore is also maintained during later stages of the manufacturing process so that air-filled hollow channels ("air lines") remain in the finished multicore fiber.

[0079] Further, in the embodiment, four core rods 7 made of Ge-doped quartz glass are manufactured, each having a length of about 1500 mm and an outer diameter of about 22 mm. Known techniques, such as MCVD (Modified Chemical Vapor Deposition), are also suitable for this purpose. Figure 2 shows the core rod 7 inserted in the bore 3 in a schematic manner. The core glass of the core rod 7 forms a core glass region 7a. The lower end of the glass cladding cylinder 1, to which the core rod 7 is attached, is then heated, causing the annular gap 8 around the core rod 7 to collapse.

[0080] 3 shows diagrammatically a preform 20 consolidated in this way, including the former group of components, namely the glass cladding cylinder 1, the core rod 7, and the marker rod 6, which form a marker glass mass 11 within the preform. The latter are exposed on the cylindrical outer cladding 4 and on a line 12 extending radially outward from the central point, which does not belong to the axis of symmetry of the fiber design.

[0081] The consolidated preform 20 is then elongated to form a secondary preform. In this case, the preform 20 is held in a drawing apparatus by a holder with its cylindrical longitudinal axis 2 aligned vertically. The secondary preform thus produced is finally elongated in a drawing apparatus in a conventional manner to form a multicore fiber. Except for the smaller radial dimension, its cross section substantially corresponds to the cross section of the consolidated preform 20 shown in FIG. 3. The former core rod 7 forms the signal core extending along the longitudinal axis of the fiber, and the former marker glass mass 11 forms a marker zone on the cylindrical cladding surface of the multicore fiber. The multicore fiber is characterized by a particularly low fiber curl and a particularly good splicing behavior.

[0082] Example 2 Thus, to the extent that the same reference numbers are used in Figures 4 and 5 as in Figures 1-3, the same or equivalent components or components of the semi-finished product are referenced as described in more detail above with reference to Example 1.

[0083] Figure 5 shows diagrammatically a cross-section of a hollow glass cladding cylinder 41 produced in a known manner using the OVD method. 2 The soot particles are made of high-purity SiO 2 The starting material, e.g., silicon tetrachloride, is passed through a deposition burner and fed to the burner flame, from which solid SiO 2 This is because fine SiO 2 The SiO 2 is deposited in the form of soot particles on the outer surface of a cylindrical deposition mandrel rotating about its longitudinal axis, the deposition burner performing a reversing movement back and forth along the longitudinal axis of the deposition mandrel. 2 A soot body is formed. After the deposition process is completed, the deposition mandrel is removed leaving a central inner bore 42. Thereafter, SiO 2 The soot body is vitrified in a furnace under vacuum and the central inner bore 42 does not collapse, ie, is maintained.

[0084] The hollow cylinder 41 thus obtained consists of undoped synthetically produced quartz glass, it has a length of 1500 mm, is adjusted by cylindrical grinding to an outer diameter of 200 mm and by boring and honing to an inner diameter of 42 mm.

[0085] A longitudinal groove 5 extending the entire length of the glass cladding cylinder 41, having a semicircular cross section, an opening width of 10 mm and a depth of 5 mm, is milled into the cylinder outer cladding 4.

[0086] By mechanical boring in the direction of the longitudinal axis 2, four evenly distributed further bores 3 with a diameter of 42 mm are produced around the central inner bore 42.

[0087] A marker rod made of synthetically produced quartz glass, likewise doped with fluorine and commercially available under the name F520, is inserted into the longitudinal groove 5. The length of the marker rod is 1500 mm and the diameter is 7 mm. It has a smooth and damage-free surface, obtained by elongating a starting cylinder made of F520 quartz glass in a tool-free manner and generated in a molten mass. The marker rod is characterized by a high dimensional stability so that it can be inserted into the longitudinal groove 5 without difficulties. The marker rod inserted into the longitudinal groove 5 is first fixed in the longitudinal groove 5 at three fixing points by point heating using a burner, the fixing points being distributed over 95% of its length at the ends and in the middle. It is then melted in the longitudinal groove 5 over its entire length. In this case, the fluorine-doped quartz glass of the marker rod melts and distributes in the longitudinal groove 5 and fills it completely. Surface tension results in a certain rounding of the marker glass gob 11 and therefore a conformity to the circular contour of the outer surface 4 of the hollow glass cladding cylinder 41. The absence of errors and the quality of the molten marker glass gob 11 are monitored.

[0088] The hollow glass cladding cylinder 41 thus modified serves as a semi-finished product for producing a multicore fiber. At a later stage of the manufacturing method, the bores 3 are filled with the same core rods 7, each having a diameter of 40 mm, and the central inner bore 42 of the hollow glass cladding cylinder 41 is filled with a filling rod made of cladding glass or another glass material. In an embodiment, the central bore 42 is filled with a core rod 7, also having a diameter of 40 mm. The lower end of the glass cladding cylinder 41 with the core rod 7 is then heated to collapse the annular gap around the core rod 7. FIG. 4 shows a schematic representation of a preform 40 consolidated using the modified hollow glass cladding cylinder 41 of FIG. 5.

[0089] It consists of the components of the former group, i.e. the hollow glass cladding cylinder 41 forming the glass cladding region 1a, the core rod forming the core glass region 7a and the marker rod forming the marker glass gob 11 in the preform 40, which is then elongated to form a secondary preform and finally drawn in a drawing apparatus to form the multicore fiber in the conventional manner. The marker zone close to the edge is particularly precise and has a small volume, so that the multicore fiber is characterized by a particularly low fiber curl and a particularly good splicing behavior.

Claims

1. A method for manufacturing a multicore fiber, comprising the method steps of reshaping a component group (10) to form said multicore fiber or to form a preform (20, 40) for said multicore fiber, said component group (10) comprising: a glass cladding cylinder (1, 41) having a cylinder longitudinal axis (2) and an outer surface (4) and having a glass cladding region (1a) made from cladding glass; a plurality of core glass regions (7a) surrounded by said cladding glass and provided with a core glass and extending in the direction of said cylinder longitudinal axis (2); at least one marker element (6) extending in the direction of said cylinder longitudinal axis (2); Including, a marker element (6) arranged on the outer surface (4) of the glass cladding cylinder (1, 41) and a longitudinal groove (5) is created in the outer surface (4) of the glass cladding region (1 a) and extends in the direction of the cylinder longitudinal axis (2), characterized in that the marker element (6) is melted in the longitudinal groove (5) prior to the re-molding process for forming the preform (20, 40) or the multicore fiber.

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

3. 3. The method according to claim 1 or 2, characterized in that the melting of the marker element (6) comprises the method steps in which the glass cladding cylinder (1, 41) is mounted with the cylinder longitudinal axis oriented horizontally so that the longitudinal groove (5) is located on the upper side of the glass cladding cylinder (1, 41) and the material of the marker element (6) is heated and softened by a heat source.

4. Cladding glass volume V Ｍ is removed from the glass cladding cylinder (1, 41) to create the longitudinal groove (5) and has a volume V Ｅ A marker element (6) having a length of 1 mm is received in the longitudinal groove (5), and V Ｅ = V Ｍ + / - 0.1xV Ｍ The method according to any one of claims 1 to 3, characterized in that

5. Said production of said component group (10) comprises (a) providing the glass cladding cylinder (1, 41) containing the cladding glass; (b) providing a core rod (7) comprising said core glass; (c) providing a marker element (6); (d) creating at least one longitudinal groove (5) on said outer surface (4) of said glass cladding cylinder (1, 41); (e) creating a core rod bore (3) extending along said cylinder longitudinal axis (2); (f) placing and fusing said marker element (6) in said longitudinal groove (5); (g) introducing the core rod (7) into the core rod bore (3) while forming the core glass region (7a); The method according to any one of claims 1 to 4, characterized in that it comprises:

6. The method according to any one of claims 1 to 5, characterized in that the marker element (6) is provided in the form of a cylindrical component, in particular as a solid rod or tube, or in the form of a layer or glass gob (11).

7. 7. The method according to claim 1, wherein a hollow cylinder (41) having a central bore (102) is used as the glass cladding cylinder (1, 41), comprising the glass cladding region made of cladding glass and a number of core glass regions provided with core glass.

8. 8. Method according to any one or more of the preceding claims, characterized in that the marker glass differs from the cladding glass in at least one physical and / or chemical property, said property being selected from the refractive index, the color, the fluorescence and / or the specific glass density.

9. 1. A semi-finished product for manufacturing a multicore fiber, comprising a glass cladding cylinder (1) having a cylinder longitudinal axis (2) and an outer surface (4) and having a glass cladding region made of cladding glass including a plurality of openings for receiving a core rod made of a core glass, the outer surface (4) of the glass cladding cylinder (1) having at least one recess (5) extending in the direction of the cylinder longitudinal axis (2) and designed as a longitudinal groove (5) in which a marker element (6) is melted.

10. 10. The semi-finished product according to claim 9, characterized in that the marker element has a length and that the marker element is partially or completely melted in spots along at least 80% of this length, preferably along at least 90% of this length.

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

12. Semi-finished product according to one or more of claims 9 to 11, characterized in that the glass cladding cylinder is a hollow cylinder with a central bore.

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