Method for producing a preform for producing a multicore fiber, as well as preform and multicore fiber

Abstract

Method for manufacturing a preform for manufacturing a multicore fiber, wherein the multicore fiber is an optical waveguide, comprising the following steps: - centrally cutting out a first partial pipe segment with a first partial pipe segment cross-sectional area from a receiving pipe with a receiving pipe inner diameter, wherein a web remains at each end of the first partial pipe segment to the respective end of the receiving pipe, so that the receiving pipe has a first core rod receiving recess, - axial insertion of a central filler rod with a filler rod outer diameter into the receiving tube, wherein the filler rod outer diameter is smaller than the receiving tube inner diameter, so that the receiving tube has the central filler rod, - Inserting a first core rod with a first core rod cross-sectional area into the core rod receiving recess, so that the receiving tube contains the core rod, - axial insertion of the receiving tube containing the first core rod and the central filler rod into a jacket tube, so that a jacket tube containing the receiving tube is present, and - Fusing the jacket tube containing the receiving tube to form a preform, so that the preform is present.

Description

[0001] The invention relates to a method for producing a preform for producing a multi-core fiber with several steps as well as a preform and a multi-core fiber.

[0002] To increase the transmission capacity of optical fibers, the options available include temporal multiplexing, wavelength division multiplexing, and spatial multiplexing, as well as mode group multiplexing for multimode fibers. However, the possibilities for increasing the transmission capacity of conventional single-core fibers are slowly reaching their technical and physical limits. Limited possibilities for increasing capacity exist by reducing the core diameter or the numerical aperture.

[0003] Multicore fibers are being developed for spatial multiplexing processes. Due to the number of cores, multicore fibers have a transmission capacity many times greater than that of a single-core fiber. The core diameter of the individual cores of multicore fibers is generally less than 50 µm. Due to the reduced number of core modes propagating in such cores, higher bandwidths are possible with full or even limited excitation than with conventional multimode fibers.

[0004] Multicore fibers are manufactured using the so-called "stack-and-draw" process. In this process, core preforms and pure quartz rods of different diameters are stacked in a relatively complex manner to achieve a certain symmetry and a relatively high packing density. The rods are then spatially fixed and inserted into a jacketing tube and fused to form a final preform, either before or during fiber drawing. To achieve the desired high packing density, a relatively large number of filler rods of different diameters must be used in addition to the waveguiding core rods. Stacking the individual rods in the desired packing structure and symmetry is relatively complex and can easily lead to stacking errors, thus rendering the fused secondary preform or the final fiber unusable.Since the achievable packing density varies in the radial direction, the secondary preform after melting without remaining openings will generally have different radius values ​​in the azimuthal direction, which must be compensated accordingly on the secondary preform by cylindrical grinding or lead to out-of-roundness on the fiber.

[0005] Due to the use of a large number of individual rods of different diameters, the stacked preform has numerous interfaces prior to fusion. These interfaces, due to possible contamination or surface inhomogeneities, can lead to bubbles in the fused secondary preform or unwanted bubbles in the fiber during fusion. Remaining bubbles at the interfaces or even foreign particles lead to diameter disturbances in the fiber and, due to light scattering, to increased attenuation values ​​and impair fiber strength. Therefore, these interfaces must be avoided at all costs.

[0006] Another state-of-the-art manufacturing method for producing multi-core fibers involves drilling holes axially into a glass cylinder and inserting core rods into these holes. The resulting secondary preform can be fused either before drawing or during fiber drawing. The problems with this manufacturing method are that it is difficult to drill sufficiently deep holes axially into the quartz rod with sufficient accuracy in terms of parallelism to the longitudinal axis. Ultrasonic drilling techniques are typically used to drill the holes. The ultrasonically drilled holes must then be polished or at least fire-polished to reduce the roughness of the inner hole surface.The production of multi-core preforms with sufficiently long preform lengths and a large number of cores leads to high production costs and, accordingly, to undesirably high geometric errors with regard to the achievable eccentricity of the cores with respect to their target positions.

[0007] US 2010 / 0 316 088 A1 relates to a thermal measuring fiber grating for combined optical and electronic transmission with multiple measuring fibers, wherein each thermal measuring fiber has an internal semiconducting element and at least one pair of conductive electrodes which are in contact with the semiconducting element along the fiber length. Furthermore, the measuring fiber has a surrounding insulator and is connected to an electronic circuit. To produce a preform for the thermal measuring fiber, four slits are first made in a sheath made of insulating material. Conductive materials are inserted into these slits to form the electrodes. Subsequently, a rod made of semiconducting material is inserted into the interior of the insulating sheath and a thin foil of insulating material is wrapped around the exterior of the cylindrical sheath.The preform is then thermally consolidated and drawn, with the different materials being laminated at the material transitions.

[0008] CN 1 01 943 767 A discloses a method for producing a multi-core fiber with square fiber cores. First, single-mode fiber cores with different refractive indices are produced in equal lengths using the MCVD process. Subsequently, a high-purity quartz glass rod, a high-purity quartz glass inner sleeve tube, and a high-purity quartz glass outer sleeve tube are selected. The inner sleeve tube is then radially severed to form three high-purity glass elements, which are inserted between the outer sleeve tube and the quartz glass rod. One of the three prefabricated fiber cores is inserted into each of the remaining three square gaps between each two high-purity glass elements. One front end of the assembled preform is sintered, and then the opposite unsintered end of the assembled preform is drawn.

[0009] The object of the invention is to improve the state of the art.

[0010] The problem is solved by a method for producing a preform for producing a multi-core fiber, wherein the multi-core fiber is an optical fiber, with the following steps: - centrally separating a first partial pipe segment with a first partial pipe segment cross-sectional area from a receiving pipe with a receiving pipe inner diameter, wherein a web remains at each end of the first partial pipe segment to the respective end of the receiving pipe, so that the receiving pipe has a first core rod receiving recess, - axially inserting a central filler rod with a filler rod outer diameter into the receiving tube, wherein the filler rod outer diameter is smaller than the receiving tube inner diameter, so that the receiving tube has the central filler rod, - Inserting a first core rod with a first core rod cross-sectional area into the core rod receiving recess so that the receiving tube has the core rod, - axially inserting the receiving tube comprising the first core rod and the central filler rod into a casing tube so that a casing tube comprising the receiving tube is formed, and - Melting the jacket tube containing the receiving tube into a preform, so that the preform is produced.

[0011] This significantly improves the geometry of the stacked and / or fused secondary preforms or the final fiber. Furthermore, stacking the otherwise usual large number of filler and core rods is eliminated, and stacking faults are eliminated or at least significantly reduced in probability.

[0012] If the core rod, or subsequently the core rods, are fitted precisely between the edges of the remaining circular ring sectors and thus the recesses, a high degree of accuracy can be maintained with regard to both the azimuthal angular position and the radial position. Due to the fixed position for the core rod, displacement of the core rod during further processing is prevented. Furthermore, the core rods can be positioned according to a predefined radial pattern. Furthermore, laser fibers can be produced with a base core that deviates from cylindrical symmetry.

[0013] A core of the invention is therefore that the receiving tube is designed in such a way that the core rod can be aligned and / or fixed in a defined manner. The following terminology is explained:

[0014] A "preform" is also called a preform. Such preforms are used in the manufacture of optical fibers. The long and, compared to the preform, very thin optical waveguide is produced by drawing the preform. The preform can be made of a glassy material such as quartz. Individual elements can be doped or have a certain coating to ensure the conditions for total internal reflection within the fiber. The preform is typically approximately 1 meter long and has a diameter between 10 and 50 millimeters. Furthermore, the preform essentially already possesses the refractive index profile of the final fiber.

[0015] A “multi-core fiber” is an optical fiber, also called a fiber optic cable, in which not just one core but several cores are used, so that light signals can be transported spatially parallel across the cores at the same time.

[0016] The "receiving tube" forms an essential component of the final preform. This receiving tube is made of high-purity quartz (SiO2), for example. This receiving tube forms a framework for the core rod to be inserted, especially after the central section of the tube segment has been cut out.

[0017] The "central separation" can be achieved by milling or laser cutting. "Central" means that a web remains at each end of the partial tube segment, connecting it to the respective end of the receiving tube. This ensures that the receiving tube remains essentially stable. The separation can be performed in such a way that the core rod can be secured in the core rod holder.

[0018] The "partial tube segment" is the part of the receiving tube that is formed, in particular, by cutting it out of the receiving tube. After removing the partial tube segment from the receiving tube, access to the interior of the receiving tube is possible not only from the sides at the end of the receiving tube, but also from the long side. Thus, after removal by cutting out the partial tube segment, a "core rod receiving recess" is created in the receiving tube.

[0019] An “axial insertion” is the insertion of a component at one end of the receiving tube in the longitudinal direction along the longitudinal axis.

[0020] The “partial pipe segment cross-sectional area” is the area of ​​the partial pipe segment that is formed when cut in the radial direction.

[0021] The “central filler rod” is used, for example, to store the core rod or to fill the receiving tube with the corresponding fiber material.

[0022] The "receiving tube inner diameter" is the inner diameter of the receiving tube and is designed so that the central filler rod can be inserted axially with its "outer diameter." When the central filler rod is inserted axially, a gap forms between the filler rod and the receiving tube due to the different diameters.

[0023] The “insertion” of the “first core rod” takes place in particular in the radial direction from the outside, whereby, for example, after insertion, a cylindrical surface of the core rod rests on the cylindrical surface of the filler rod.

[0024] The “core rod” is designed in such a way that it guides the light in the later multi-core fiber.

[0025] The “core rod cross-sectional area” is in particular the area of ​​the core rod that is formed when a radial section is made through the core rod.

[0026] A "jacketing tube" is also called a "jacketing tube." The jacket tube, the central filler rod, the receiving tube, and the core rod are "fused" together to form the preform. During the fusion process, the structure becomes viscous by applying the appropriate temperature, causing the individual components to bond together. This can be done, for example, under vacuum and using a suitable mold, resulting in a round, polygonal, or rectangular preform.

[0027] In order to arrange several cores in the preform, the process can include the following additional steps before fusing to form the preform. - centrally separating a second partial tube segment with a second partial tube segment cross-sectional area, a third partial tube segment with a third partial tube segment cross-sectional area, a fourth partial tube segment with a fourth partial tube segment cross-sectional area and / or further partial tube segments each with a partial tube segment cross-sectional area from the receiving tube, so that the receiving tube has a second core rod receiving recess, a third core rod receiving recess, a fourth core rod receiving recess and / or further core rod receiving recesses and the respective core rod receiving recesses are spaced azimuthally from one another by material of the receiving tube, - Inserting a second core rod, a third core rod, a fourth core rod or further core rods into the second core rod receiving recess, into the third core rod receiving recess, into the fourth core rod receiving recess and / or into the further core rod receiving recesses, so that the receiving tube has the respective core rods and - axial insertion of the receiving tube comprising the core rods and the central filler rod into the casing tube so that the casing tube comprises the receiving tube.

[0028] In essence, the process for the first core rod is extended to further core rods.

[0029] In particular, in order to ensure a cross-sectional area that is as circular as possible and / or a homogeneous arrangement of the core rods, the respective core rods can be designed or selected such that the respective core rod cross-sectional areas essentially correspond to the respective partial tube segment cross-sectional area.

[0030] This can be achieved, for example, by measuring the cross-sectional area of ​​the partial tube segment and the core rod cross-sectional area and selecting a round core rod with the appropriate area. The deviations of the core rod cross-sectional area from the partial tube segment cross-sectional area should be less than + / - 5%, in particular less than + / - 2%, and especially less than + / - 0.5%. The cross-sectional area of ​​the partial circular segment can also be determined mathematically.

[0031] In a further embodiment, one or more filler rods are inserted into the respective core rod receiving recesses before or after the insertion of the core rod and / or the further core rods.

[0032] This allows the corresponding free spaces to be provided with appropriate fiber material, so that deviations from a round shape after fusion are reduced. In addition, defined positioning can be achieved using the filler rods. The material of the filler rod corresponds in particular to the material of the receiving tube. The filler rod or filler rods can be designed either as inner filler rods or as outer filler rods. In particular, the inner filler rods have a smaller diameter than the outer filler rods. The inner filler rods are arranged in particular between the central filler rod and the respective core rod, and the outer filler rod or rods are arranged between the core rod and the jacketing tube.

[0033] In a related embodiment, the inner filler rod or all inner filler rods of the respective core rod holder and / or the outer filler rod or all outer filler rods of the respective core rod holder and the respective core rods are designed such that their total cross-sectional area substantially corresponds to the respective partial pipe segment cross-sectional area.

[0034] The cross-sectional area of ​​the partial pipe segment can be determined either metrologically or mathematically, and the areas can be adjusted to each other by summing the cross-sectional area of ​​the core rods and the filler rods. "Substantially" in this case means, in particular, that the deviations of the total cross-sectional area from the partial pipe segment cross-sectional area are less than + / - 1.5%, in particular less than + / - 0.8%.

[0035] To increase the number of cores, the first core rod and / or the subsequent core rods can themselves have multiple cores. In this case, a core rod can be manufactured in the same way as the preform.

[0036] In order to obtain particularly homogeneous surfaces that do not require pre-polishing and / or post-treatment, the partial pipe segment or the partial pipe segments can be cut out by laser cutting, in particular by means of a CO2 laser.

[0037] In addition, the fusion process to form the preform can be performed before or during the drawing of a preform into a multi-core fiber. Especially when drawing and fusion are performed simultaneously, the energy required to produce the multi-core fiber can be reduced. This is possible, in particular, because the corresponding cross-sectional areas of the filler and core rods correspond to the cross-sectional areas of the partial tube segments, thus achieving high geometries and eliminating the need for post-treatment of the preform.

[0038] Furthermore, the problem is solved by a preform which was manufactured according to a previously described process.

[0039] In addition, the task is solved by a multi-core fiber, which is created by drawing the preform or by simultaneously fusing and drawing the preform to form the multi-core fiber.

[0040] The invention will be explained below using an exemplary embodiment. Fig. 1 a highly schematic radial cross-sectional view of a jacketing tube with a receiving tube, wherein four core rods and a central filler rod are arranged in the receiving tube, Fig. 2 a highly schematic partial section of a preform with a core rod and filler rods before melting and Fig. 3 a highly schematic three-dimensional representation of a receiving tube with a core rod receiving recess.

[0041] A quartz receiving tube 103 is slit open in the middle using a CO2 laser, and a partial tube segment (not shown) is removed. The remaining opening 105 serves as a core rod receiving recess. Cutting out the opening creates a cut edge 106. Due to the edge pieces 102, the stability of the receiving tube 103 is essentially maintained.

[0042] In total, four partial tube segments are removed from the receiving tube 103 in a radially evenly distributed manner, so that four core rod receiving recesses 105 are provided.

[0043] In a first alternative, a central filler rod 107 is fully inserted axially laterally into the receiving tube 103. A core rod 109 is inserted laterally into the respective core rod receiving recess 105. After all four core rod receiving recesses 105 have been equipped with core rods 109, the receiving tube 103 with the inserted central filler rod 107 and the core rods 109 resting on it is inserted axially laterally into a jacketing tube 111. This forms a preform 101 prior to melting (see Fig. 1).

[0044] Remaining tube segments 104 are provided between the respective core rods as part of the receiving tube 103. The core rods 109 each rest at two points on the cut edges 106. A jacket tube clearance 115 is provided between the receiving tube 103 and the jacketing tube 111. Similarly, a core rod clearance is provided between the central filler rod 107 and the receiving tube 103.

[0045] The preform is then fused so that all free spaces are closed, and at the same time or subsequently the fused preform is drawn into a multi-core fiber.

[0046] In a second alternative, after the receiving tube 103 has been provided with the central filler rod 107, two inner filler rods 117 are first inserted into each core rod receiving recess 105, followed by the core rod 109 and then two outer filler rods 119. Subsequently, the thus equipped receiving tube 103 is inserted axially laterally into the jacketing tube 111.

[0047] In all embodiments, the cross-sectional area of ​​the partial segment tube was determined by measurement. In the first alternative, the circular area of ​​the core rod corresponds to the partial segment tube cross-sectional area, and in the second alternative, the sum of the cross-sectional areas of the two inner filler rods 117, the two outer filler rods 119, and the core rod 109 corresponds to the determined partial segment tube cross-sectional area.

Claims

[1] A method for producing a preform for producing a multi-core fiber, wherein the multi-core fiber is an optical fiber, comprising the following steps: - centrally separating a first partial pipe segment with a first partial pipe segment cross-sectional area from a receiving pipe with a receiving pipe inner diameter, wherein a web remains at each end of the first partial pipe segment to the respective end of the receiving pipe, so that the receiving pipe has a first core rod receiving recess, - axially inserting a central filler rod with a filler rod outer diameter into the receiving tube, wherein the filler rod outer diameter is smaller than the receiving tube inner diameter, so that the receiving tube has the central filler rod, - Inserting a first core rod with a first core rod cross-sectional area into the core rod receiving recess so that the receiving tube has the core rod, - axially inserting the receiving tube comprising the first core rod and the central filler rod into a casing tube so that a casing tube comprising the receiving tube is formed, and - Melting the jacket tube containing the receiving tube into a preform, so that the preform is produced. [2] Method according to claim 1, characterized by that the following steps are carried out before melting into the preform: - centrally separating a second partial tube segment with a second partial tube segment cross-sectional area, a third partial tube segment with a third partial tube segment cross-sectional area, a fourth partial tube segment with a fourth partial tube segment cross-sectional area and / or further partial tube segments each with a partial tube segment cross-sectional area from the receiving tube, so that the receiving tube has a second core rod receiving recess, a third core rod receiving recess, a fourth core rod receiving recess and / or further core rod receiving recesses and the respective core rod receiving recesses are spaced azimuthally from one another by material of the receiving tube, - Inserting a second core rod, a third core rod, a fourth core rod or further core rods into the second core rod receiving recess, into the third core rod receiving recess, into the fourth core rod receiving recess and / or into the further core rod receiving recesses, so that the receiving tube has the respective core rods and - axial insertion of the receiving tube comprising the core rods and the central filler rod into the casing tube so that the casing tube comprises the receiving tube. [3] Method according to one of the preceding claims, characterized by that the respective core rods are designed in such a way that the respective core rod cross-sectional area essentially corresponds to the respective partial tube segment cross-sectional area. [4] Method according to one of the preceding claims, characterized bythat before or after the insertion of the first core rod and / or the further core rods, one or more filler rods are inserted into the respective core rod receiving recess. [5] Method according to claim 4, characterized by that before inserting the first core rod and / or the further core rods, one inner filler rod, two inner filler rods or several inner filler rods are inserted into the respective core rod receiving recess. [6] Method according to one of claims 4 or 5, characterized by that before the axial insertion of the receiving tube into the casing tube, one outer filler rod, two outer filler rods or several outer filler rods are inserted into the respective core rod receiving recess provided with core rods. [7] Method according to one of claims 4 to 6, characterized bythat the inner filler rod or all inner filler rods of the respective core rod holder and / or the outer filler rod or all outer filler rods of the respective core rod holder and the respective core rods are designed in such a way that their total cross-sectional area essentially corresponds to the respective partial pipe segment cross-sectional area. [8] Method according to one of the preceding claims, characterized by that the first core rod and / or the further core rods have or have multiple core rods. [9] Method according to one of the preceding claims, characterized by that the cutting out of the partial pipe segment or the partial pipe segments is carried out by laser cutting, in particular by means of a CO2 laser. [10] Method according to one of the preceding claims, characterized by that the melting to form the preform takes place before the preform is drawn into the multi-core fiber. [11] Method according to one of claims 1 to 9, characterized bythat the fusion to the preform takes place during drawing of the preform into the multi-core fiber. [12] Preform produced by a method according to any one of claims 1 to 10. [13] Multi-core fiber, wherein the multi-core fiber is an optical waveguide, which is produced by drawing the preform according to claim 12 or by a method according to claim 11.

Images