METHOD FOR PRODUCING A HOLLOW CORE FIBER AND FOR PRODUCING A PREFORM FOR A HOLLOW CORE FIBER

Description

Technical background

[0001] The invention relates to a method for producing an antiresonant hollow core fiber comprising a hollow core extending along a fiber longitudinal axis and a sheath surrounding the hollow core, which includes a number of antiresonant elements, with the following process steps: (a) Providing a sheath tube having an inner bore and a longitudinal axis along which a sheath tube wall bounded by an inner and an outer surface extends; (b) Providing a number of tubular antiresonant element preforms; (c) Arranging the antiresonant element preforms at predetermined positions on the inner surface of the sheath tube wall to form a primary preform having a hollow core region and an inner sheath region; (d) Further processing the primary preform into a secondary preform from which the hollow core fiber is drawn, the further processing comprising elongation and a single execution of the following hot forming process: collapsing additional sheath material in the form of a fused silica overlay cylinder and simultaneous elongation; and (e) Drawing the secondary preform into the hollow core fiber. where a primary preform is used in the elongation process, which has an outer diameter in the range of 20 to 70 mm.

[0002] Furthermore, the invention relates to a method for producing a preform for an antiresonant hollow core fiber, which has a hollow core extending along a longitudinal fiber axis and a sheath region surrounding the hollow core, comprising several antiresonance elements, with the following process steps: (a) Providing a sheath tube having an inner bore and a longitudinal axis along which a sheath tube wall bounded by an inner and an outer surface extends; (b) Providing a number of tubular antiresonance element preforms; (c) Arranging the antiresonance element preforms at predetermined positions on the inner surface of the sheath tube wall to form a primary preform having a hollow core and an inner sheath; and (d) Further processing the primary preform into a secondary preform for the hollow core fiber, the further processing comprising elongation and a single execution of the following hot forming process: collapsing additional sheath material in the form of a cap cylinder as quartz glass and simultaneous elongation, the elongation process using a primary preform having an outer diameter in the range of 20 to 70 mm.

[0003] Conventional solid-material single-mode optical fibers have a glass core surrounded by a cladding layer of glass with a lower refractive index. Light transmission is based on total internal reflection between the core and cladding. However, the interactions of the guided light with the solid material are associated with increased latency in data transmission and relatively low damage thresholds to high-energy radiation.

[0004] These disadvantages are avoided or reduced by "hollow core fibers", in which the core comprises an evacuated cavity filled with gas or liquid. In In hollow-core fibers, the interaction of light with the glass is less pronounced than in solid-core fibers. The refractive index of the core is lower than that of the cladding, so light transmission by total internal reflection is not possible, and the light would normally escape from the core into the cladding. InDepending on the physical mechanism of light transmission, hollow core fibers are divided into "photonic band gap fibers" and "antiresonance reflection fibers".

[0005] In "photonic bandgap fibers," the hollow core is surrounded by a cladding in which small hollow channels are arranged periodically. The periodic structure of these hollow channels in the cladding causes the effect known as the "photonic bandgap," a term borrowed from semiconductor technology. This effect means that light of certain wavelengths scattered by the cladding structures interferes constructively in the central cavity due to Bragg reflection and cannot propagate transversely within the cladding.

[0006] In the embodiment of the hollow-core fiber known as "antiresonant hollow-core fiber" (AR-HCF), the hollow core is surrounded by an inner cladding in which so-called "antiresonant elements" (AREs) are arranged. The walls of the antiresonant elements, evenly distributed around the hollow core, can act as Fabry-Perot cavities operating in antiresonance, reflecting the incident light and guiding it through the fiber core.

[0007] This fiber technology promises low optical attenuation, a very broad transmission spectrum (also in the UV or IR wavelength range) and low latency in data transmission.

[0008] Potential applications of hollow core fibers lie in the field of data transmission, high-performance beam guidance, for example for material processing, modal filtering, nonlinear optics, especially for supercontinuum generation, from the ultraviolet to infrared wavelength range. State of the art

[0009] One disadvantage of antiresonant hollow core fibers is that higher order modes are not necessarily suppressed, so that over long transmission distances they are often not purely single-mode and the quality of the output beam deteriorates.

[0010] In Francesco Poletti's paper "Nested antiresonant nodeless hollow core fiber"; Optics Express, Vol. 22, No. 20 (2014); DOI: 10.1364 / OE 22.023807, a fiber design is proposed in which antiresonance elements are not implemented as a simple, singular structural element, but rather as several nested structural elements. These nested antiresonance elements are designed such that higher-order nuclear modes are phase-matched to the cladding modes and are suppressed, but not the fundamental nuclear mode. This ensures the propagation of the fundamental nuclear mode at all times, and the hollow core fiber can be effectively made single-mode over a limited wavelength range.

[0011] Effective mode suppression depends on the center wavelength of the transmitted light and on structural parameters of the fiber design, such as the radius of the hollow core and the diameter difference of nested ring structures in the antiresonance elements.

[0012] From EP 3 136 143 A1, an antiresonant hollow core fiber is known (referred to there as a "hollow core fiber without a band gap") in which the core can conduct modes in addition to the fundamental mode. For this purpose, it is surrounded by an inner sheath with "non-resonant elements" that provide phase matching of antiresonant modes with the higher modes. The hollow core fiber is manufactured using a so-called "stack-and-draw" technique, in which the starting elements are arranged into an axis-parallel ensemble and fixed to form a preform, which is then elongated. A sheath tube with a hexagonal inner cross-section is used, and six so-called "ARE preforms" (anti-resonance element preforms) are fixed in the inner edges of the sheath tube. The sheath tube has an outer diameter of 28 mm. This preform is drawn into a hollow core fiber in two stages.

[0013] From WO 2018 / 169487 A1, a method for producing a preform for antiresonant hollow-core fibers is known, in which a first sheath section comprises a plurality of rods and a second sheath section comprises a plurality of tubes surrounded by an outer cladding tube. Rods, tubes, and cladding tube are assembled to form a preform using a "stack and draw" technique. Before elongating the preform, the preform end is sealed by applying a sealing compound. A UV adhesive, for example, is used as the sealing compound.

[0014] A.F. Kosolapov et al., "Hollow-core revolver fibre with a double-capillary reflective cladding", Quantum Electronics, Vol. 46, No. 3, March 29, 2016 (2016-03-29), Pages 267-270, DOI: 10.1070 / QEL15972, describes a method for producing an antiresonant hollow-core fiber by providing a cladding tube and a number of antiresonant element preforms, which are assembled as a double tube consisting of an outer ARE tube and an inner ARE tube welded to its inner wall. The double tube is elongated in a drawing furnace and subsequently exhibits an oval cross-section. Five of the oval double tubes are mounted on the inside of the cladding tube. The double tubes are then welded to the inside of the cladding tube. The cladding tube has an outer diameter of 25 mm. The primary preform thus produced is elongated to form a secondary preform with an outer diameter of 6 mm. The draw-out ratio is approximately 4. The hollow core fiber is then drawn from the secondary preform.

[0015] US patent 2005 / 0226578 A1 describes the fabrication of a photonic optical fiber by creating a tubular ensemble of tubular and rod-shaped elements, with the spaces between the elements being filled with a filler material. The filler material is deposited from the gas phase (CVD process; SiO₂ sol) or produced from the liquid phase (SiO₂-containing colloidal sol).

[0016] Ben Sherlock et al. describe in "Tunable fibre-coupled multiphoton microscopy with a negative curvature fibre*", Journal of Biophotonics, Vol. 9, No. 7, March 15, 2016 (2016-03-15), pages 715-720, DOI: 10.10027jbio.201500290; a preform for an antiresonant element hollow core fiber that features a capping cylinder made of fluorine-doped fused silica. The lower viscosity of the fluorine-doped fused silica allows for a lower drawing temperature and better structural preservation. Technical task

[0017] Antiresonant hollow-core fibers, and especially those with nested structural elements, have complex internal geometries, which complicates their precise and reproducible manufacturing. This is all the more true because, in order to maintain resonance or antiresonance conditions, even small dimensional deviations on the order of the operating wavelength of the guided light are unacceptable. Deviations from the target geometry can originate in the configuration of the fiber preform, and they can also occur due to unintended non-scale deformations during the fiber drawing process.

[0018] In the well-known "stack-and-draw" technique, many elements must be assembled with precise positioning. For example, to produce the hollow-core fiber in the "NANF" design known from the aforementioned paper, six antiresonance element preforms, each consisting of an antiresonance preform outer tube (ARE outer tube) and an antiresonance preform inner tube (ARE inner tube) welded to one side of the inner surface of the ARE outer tube, must be attached to the inside of a sheath tube.

[0019] To achieve low attenuation values ​​and wide transmission ranges, in addition to a uniform wall thickness of the antiresonance elements, the azimuthal position of the antiresonance elements within the cladding tube is also important. This is not readily achievable with the "stack-and-draw" technique. The aim of the invention is to provide a method for the cost-effective production of an antiresonant hollow-core fiber that avoids the limitations of conventional manufacturing processes.

[0020] In particular, the object of the invention is to provide a method for producing an antiresonant hollow core fiber and a preform for antiresonant hollow core fibers, with which a high precision of the structural elements and an exact positioning of the antiresonant elements in the fiber can be achieved reproducibly in a sufficiently stable and reproducible manner.

[0021] Furthermore, the disadvantages of the classic "stack and draw" technique, with which the required structural accuracies, in particular a uniform wall thickness of the antiresonance elements and an exact positioning at predetermined azimuthal positions, are not easy to achieve, should be avoided as much as possible. Summary of the invention

[0022] With regard to the method for producing the antiresonant hollow core fiber, this problem is solved according to the invention, starting from a method of the aforementioned type, by the fact that the secondary preform obtained after elongation has an outer diameter in the range of 20 to 70 mm.

[0023] The starting point for the production of the antiresonant hollow core fiber is a preform, also referred to here as the "primary preform." It comprises a sheath tube containing preforms or components for forming antiresonant elements within the hollow core fibers (referred to here as "antiresonance elements"). The primary preform can be elongated to form the hollow core fiber. However, additional sheath material is added to the primary preform to create a preform referred to here as the "secondary preform." The hollow core fiber is then produced by elongating the secondary preform. The general term "preform" is used here to refer to the component or coaxial assembly of components from which the hollow core fiber is ultimately drawn.

[0024] The addition of sheathing material is achieved by collapsing a capping cylinder onto the primary preform. The coaxial arrangement of the primary preform and capping cylinder is elongated during the collapse of the capping cylinder. This process may or may not alter the shape or arrangement of the antiresonant element preforms.

[0025] The production of the preform comprises a number of process steps in which the initial elements of the hollow-core fiber are manufactured and positioned relative to each other, and at least one hot forming step. Each of the initial elements exhibits a certain deviation from its target geometry, and each step of positioning and forming inevitably leads to geometric deviations that accumulate into an absolute geometric error in the finished preform. In particular, the hot forming of glass can lead to unwanted and non-reproducible deformation even with the slightest deviations from an ideal, usually cylindrically symmetrical, temperature profile of the heating zone.

[0026] The primary preform used in the inventive method for the purpose of elongation – with simultaneous collapse of additional sheath material – is characterized by a diameter in the range of 20 to 70 mm, preferably by an outer diameter in the range of 30 to 60 mm. This is a comparatively large outer diameter.

[0027] Since the existing absolute geometric errors during fiber drawing are scaled down more significantly with increasing diameter of the preform, a more precise manufacturing of the hollow core fiber is fundamentally made possible.

[0028] However, it has been shown that any increase in the diameter of the primary preform does not automatically lead to a more precise hollow core fiber, but rather that the following boundary conditions must be met to comply with a maximum relative geometric error of 3.5% in the wall thickness of the hollow core fiber. I. The diameter of the primary preform is a maximum of 70 mm. The larger the diameter, the slower the elongation rate and the longer each axial segment of the preform is exposed to the high temperature of the heating zone. However, if the elongation rate is too slow, the structural elements of the antiresonance element preforms will deform. II. The diameter of the primary preform is at least 20 mm. It has been shown that with smaller diameters, the thermal inertia of the preform is too low to compensate for any temperature fluctuations in the heating zone. Furthermore, as the preform diameter decreases, the benefit of reducing an existing absolute defect through elongation diminishes.

[0029] The hollow core and the material for the inner shell of the secondary preform are determined by the primary preform. The primary preform comprises the hollow core and an inner shell. Increasing the outer diameter of the primary preform can be achieved either by increasing the size of the hollow core (resulting in reduced damping) or by decreasing the outer diameter of the final hollow core fiber (resulting in less material usage). An outer diameter of the inner shell of the secondary preform in the range of 7 mm to 50 mm represents a suitable compromise.

[0030] During elongation, the primary preform is continuously fed into a heating zone at a feed rate, softened zone by zone within the heating zone, and withdrawn from the heating zone at a withdrawal rate. The forming process preferably takes place without contact between the softened gas and a forming tool, and particularly preferably in a vertical drawing process.

[0031] The temperature of the heating zone during the hot forming process should be as constant as possible. Therefore, a temperature-controlled heating element is advantageously used in the hot forming process according to process step (d), the target temperature of which is maintained to an accuracy of + / - 0.1°C.

[0032] This allows temperature fluctuations in the hot forming process to be limited to less than + / - 0.5°C.

[0033] If the feed rate is too high, temperature gradients can occur in the primary preform, which can cause the antiresonance element preforms distributed at different radial positions within it to exhibit different elongation behavior. An insufficient feed rate can lead to undesirable deformation of the antiresonance element preforms. A suitable compromise has proven to be set to achieve a feed rate of at least 0.8 g / min, preferably in the range of 0.8 g / min to 85 g / min, and particularly preferably in the range of 3.3 g / min to 85 g / min, and an average residence time in the heating zone of less than 25 min, preferably in the range of 5 to 25 min.

[0034] To reduce absolute geometric errors, a large draw-out ratio during elongation is desirable. On the other hand, a large draw-out ratio is accompanied by correspondingly large forming processes and material movements, which can easily lead to undesirable deformations in the delicate structural elements of the antiresonance element preforms.

[0035] A suitable compromise has proven advantageous if the draw-out ratio during elongation is set to a value in the range of 1.05 to 10, preferably to a value in the range of 1.05 to 5.

[0036] In a preferred method variant, the arrangement of the antiresonance element preforms and / or the elongation of the primary preform, and / or the drawing of the hollow core fiber, comprises a fixing measure and / or a sealing measure using a sealing or bonding compound containing an amorphous SiO2 particles.

[0037] The sealing or bonding compound used for sealing or fixing contains amorphous SiO₂ particles, which are suspended, for example, in a dispersion fluid. This compound is applied between the surfaces to be joined or sealed and is typically pasty in its initial state. During drying at low temperatures, the dispersion fluid is partially or completely removed, and the compound hardens. The sealing or bonding compound, and in particular the hardened SiO₂-containing sealing or bonding compound obtained after drying, meets the requirements for fixing and sealing. The drying temperature required is below 300 °C, which promotes the dimensional accuracy of the preform and prevents thermal deterioration.Heating to higher temperatures around 800 °C, for example during the elongation of the preform into a hollow-core fiber, leads to further thermal hardening of the sealing or bonding compound, which is also suitable for forming opaque or transparent glass. This occurs through sintering or vitrification, whereby sintering to opaque glass requires comparatively lower temperatures and / or shorter heating times than vitrification to complete transparency. The sealing or bonding compound can thus be completely densified by heating and vitrified by heating during the hot forming process. In this process, the sealing or bonding compound behaves like quartz glass; it becomes viscous and deformable.

[0038] During the hot forming process, the sealing or bonding compound does not decompose and releases few impurities. It is therefore characterized by thermal stability and purity during the hot forming process and prevents deformation due to differing coefficients of thermal expansion.

[0039] The sealing and bonding compound can also be advantageously used to wear down open ends of the antiresonance element preforms and / or individual structural elements of the antiresonance element preforms and / or any annular gap between tube elements when elongating the primary preform and / or when drawing the hollow core fiber.

[0040] In this way, the individual components of the primary preform and / or secondary preform can be exposed to different internal pressures during elongation or the fiber drawing process.

[0041] In the case of antiresonance element preforms, each having at least one ARE outer tube and / or at least one ARE inner tube, the accuracy of the positioning of the preforms on the inner surface of the casing tube is further improved by machining the casing tube inner surface and / or the casing tube outer surface and / or the ARE outer tube inner surface and / or the ARE outer tube outer surface, in particular by drilling, milling, grinding, honing and / or polishing.

[0042] These processing techniques, compared to other known forming techniques using heat and pressure, produce more accurate and intricate structures and avoid surface contamination from forming tools such as nozzles, presses or melt molds.

[0043] The machining process preferably also includes structuring the inner surface of the casing tube in the area of ​​the target positions of the antiresonance element preforms by providing it with a longitudinal structure extending in the direction of the casing tube's longitudinal axis. This longitudinal structure comprises, for example, longitudinal slots and / or longitudinal grooves in the inner wall of the casing tube, which run parallel to the casing tube's longitudinal axis and which are preferably produced by drilling, sawing, milling, cutting, or grinding.

[0044] The longitudinal structure extending along the longitudinal axis of the casing tube serves as a positioning aid for the antiresonance element preforms. It facilitates the antiresonance element preforms assuming predetermined, defined positions on the inside of the casing tube.

[0045] The accuracy of positioning the preforms on the inner surface of the casing tube is improved if the upper end faces of the structural elements are positioned at the target position using a positioning template.

[0046] The positioning template, for example, has a shaft projecting into the inner bore of the casing tube, which is provided with retaining elements in the form of several radially outward-pointing retaining arms.

[0047] The inherent star-shaped arrangement of the retaining elements facilitates the precise positioning of the antiresonance element preforms at their respective target positions and their fixation, for example, using the sealing or bonding compound described above. The positioning template is preferably used exclusively in the area of ​​the sheathing tube ends, and preferably in the area of ​​both sheathing tube ends.

[0048] Furthermore, a procedure has proven effective in which, during the elongation of the primary preform according to process step (d) and / or during the drawing of the hollow core fiber according to process step (e), several components of the preform made of quartz glass are heated and softened together, wherein the quartz glass of at least some of the preform components contains at least one dopant which lowers the viscosity of quartz glass.

[0049] Components of the primary preform include the sheath tube and the antiresonant element preforms arranged within it. The secondary preform contains additional sheath material, provided in the form of one or more overlay cylinders, which is collapsed onto the primary preform.

[0050] Fluorine, chlorine and / or hydroxyl groups are preferably used as dopants to lower the viscosity of quartz glass.

[0051] Doping allows for the adjustment of the coefficients of thermal expansion of adjacent preform components to avoid or reduce stresses. It can also be used to decrease the thermal stability of one component in favor of the stability of a neighboring component.

[0052] For example, it has proven advantageous if the quartz glass of the casing tube has a viscosity at least 0.5 dPa.s higher, preferably at least 0.6 dPa.s higher, at a measuring temperature of 1250 °C than the quartz glass of additionally applied jacket material (when the viscosity is specified as a logarithmic value in dPas).

[0053] Particularly with regard to low optical attenuation and a large optical transmission bandwidth of the hollow core fiber, it has proven especially advantageous if the antiresonance elements are arranged around the hollow core with an odd-numbered symmetry.

[0054] With regard to the production of the preform for the hollow core fiber, the above-mentioned technical problem is solved according to the invention, starting from a method of the aforementioned type, by the fact that the secondary preform obtained after elongation has an outer diameter in the range of 20 to 70 mm.

[0055] During elongation, the primary preform is continuously fed into a heating zone at a feed rate, softened zone by zone in the heating zone, and withdrawn from the heating zone at a withdrawal rate.

[0056] The primary preform is used to produce an intermediate product in the form of a secondary preform for the antiresonant hollow core fiber. A primary preform with a larger diameter is produced compared to previous state-of-the-art methods, allowing the absolute geometric error present in the primary preform to be scaled down more significantly during elongation.

[0057] This method enables more precise manufacturing of the hollow core fiber. Measures for producing the preform are explained above in the context of hollow core fiber production, and these explanations are hereby incorporated. Definitions

[0058] Individual process steps and terms from the above description are defined below. These definitions form part of the description of the invention. In case of a substantive contradiction between one of the following definitions and the rest of the description, the definition in the description shall prevail. Anti-resonance elements

[0059] The antiresonance elements can be simple or nested structural elements of the hollow-core fiber. They have at least two walls that, viewed from the direction of the hollow core, have a negative curvature (convex) or no curvature (planar, straight). They are generally made of a material that is transparent to the working light, for example, glass, in particular doped or undoped SiO₂, a plastic, in particular a polymer, a composite material, or a crystalline material. Antiresonance element preform / Antiresonance element precursor

[0060] Antiresonance element preforms are components or parts of the preform that are essentially transformed into antiresonance elements in the hollow core fiber through simple elongation during the fiber drawing process. Antiresonance element preforms are components or parts of the preform that only become antiresonance element preforms or antiresonance elements directly through forming. The antiresonance element preforms can be simple or nested components, to which positioning aids may also be attached. They are originally present in the primary preform.

[0061] Further processing of the primary preform, particularly through hot forming steps, can produce intermediate products in which the original antiresonance element preforms exist in a modified form compared to the original shape. This modified form is also referred to here as an antiresonance element preform or antiresonance element stage. Preform / primary preform / secondary preform

[0062] The preform is the component from which the antiresonant hollow core fiber is drawn. It is either a primary preform or a secondary preform produced by further processing the primary preform. Further processing of the primary preform into a secondary preform, from which the hollow core fiber is drawn, may involve a single or repeated execution of one or more of the following hot forming processes: (i) Elongation, (ii) Collapse, (iii) Collapse and simultaneous elongation, (iv) Collapse of additional mantle material, (v) Collapse of additional mantle material followed by elongation, (vi) Collapse of additional mantle material and simultaneous elongation. Elongate / Collapse

[0063] Elongation involves lengthening the primary preform. This lengthening can occur without simultaneous collapse. Elongation can be performed to scale, so that, for example, the shape and arrangement of components or parts of the primary preform are reflected in the elongated final product. However, during elongation, the primary preform can also be stretched out of scale, thus altering its geometry.

[0064] Collapse occurs when an internal bore narrows or annular gaps between tubular components close or narrow. Collapse is generally accompanied by elongation. Hollow core / Inner shell area / Outer shell area

[0065] The assembly consisting of at least one sheath tube and preforms or precursors for antiresonance elements, either loosely held or firmly fixed within it, is also referred to here as the "primary preform." The primary preform comprises the hollow core and a sheath region. This sheath region is also referred to as the "inner sheath region" if there is also an "outer sheath region," which is produced, for example, by collapse, and if a distinction is to be made between these sheath regions. The terms "inner sheath region" and "outer sheath region" are also used for the corresponding regions in the hollow core fiber or in intermediate products obtained by further processing of the primary preform.

[0066] The term "pipe inside" is also used synonymously with "pipe inner surface," and the term "pipe outside" is also used synonymously with "pipe outer surface." The term "internal bore" in connection with a pipe does not imply that the internal bore was created by a drilling process. Machining

[0067] This includes subtractive mechanical manufacturing processes for machining a workpiece, in particular turning, cutting, drilling, sawing, milling, and grinding. This machining creates a longitudinal structure extending along the longitudinal axis of the casing tube, which serves as a positioning aid for the antiresonance element preforms. The longitudinal structure is accessible from the inner surface of the casing tube; it can also extend through the entire casing tube wall to the outer surface. Particle size and particle size distribution

[0068] The particle size and size distribution of SiO₂ particles are characterized using the D₅₀ values. These values ​​are derived from particle size distribution curves, which show the cumulative volume of SiO₂ particles as a function of particle size. The particle size distributions are often characterized using the respective D₁₀, D₅₀, and D₅₀ values. The D₁₀ value represents the particle size not reached by 10% of the cumulative volume of SiO₂ particles, and the D₅₀ and D₅₀ values ​​represent the particle sizes not reached by 50% and 90% of the cumulative volume of SiO₂ particles, respectively. The particle size distribution is determined by light scattering and laser diffraction spectroscopy according to ISO 13320. Example of implementation

[0069] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The drawing shows a schematic representation. Figure 1 a primary preform with a sheath tube and antiresonance element preforms positioned and fixed therein for the production of a preform for a hollow core fiber based on a top view of the radial cross-section.

[0070] In the production of the hollow core fiber or the preform for the hollow core fiber, a large number of components must be joined together. Furthermore, it can be helpful to seal existing gaps or channels in the preform during hot forming processes. For joining or sealing, a SiO₂-based sealing or bonding compound is used, as known from DE 10 2004 054 392 A1. In this process, an aqueous slurry is produced by wet-milling quartz glass granules. This slurry contains amorphous SiO₂ particles with a particle size distribution characterized by a D₅₀ value of approximately 5 µm and a D₅₀ value of approximately 23 µm. Additional amorphous SiO₂ granules with an average particle size of approximately 5 µm are added to the base slurry. The slurry used as a bonding agent has a solids content of 90%, consisting of at least 99.9 wt% SiO2.

[0071] Figure 1 Figure 1 schematically shows a primary preform 3 with a sheath tube 1 having a sheath tube wall 2, on the inner surface of which antiresonance element preforms 4 are fixed at previously defined azimuthal positions at uniform intervals; in the exemplary embodiment there are six preforms 4, in another preferred embodiment not shown there is an odd number of preforms.

[0072] The inner sheath 1 is made of quartz glass and has a length of 1000 mm, an outer diameter of 27 mm, and an inner diameter of 20 mm. The antiresonance element preforms 4 are arranged as an ensemble of nested structural elements consisting of an ARE outer tube 4a and an ARE inner tube 4b. The ARE outer tube 4a has an outer diameter of 6.2 mm, and the ARE inner tube 4b has an outer diameter of 2.5 mm. The wall thickness of both structural elements (4a; 4b) is the same and is 0.3 mm. The lengths of the ARE outer tube 4a and the ARE inner tube 4b correspond to the sheath length 1.

[0073] The antiresonance element preforms 4 are fixed to the inner wall of the sheathing tube 1 by means of the SiO 2-based bonding compound 5.

[0074] The bonding compound 5 is applied locally to the inner surface of the sheathing tube in the area of ​​the end faces, and the antiresonance element preforms 4 are placed on top of it using a positioning template with a structurally predetermined star-shaped arrangement of retaining arms for the individual antiresonance element preforms 4. The positioning template is limited to the area around the two end faces of the sheathing tube.

[0075] This method creates a precise and reproducible connection between the sheathing tube 1 and the antiresonance element preforms 4. For fixation, it is sufficient to solidify the bonding compound 5 at a low temperature below 300 °C, thus preventing excessive heating of the surrounding areas and consequently deformation of the antiresonance element preforms 4.

[0076] The primary preform 3 is overlaid with a quartz glass overlay cylinder, whereby the overlay cylinder collapses onto the sheathing tube 1, and simultaneously the tube assembly elongates to form a secondary preform. The overlay cylinder has an outer diameter of 63.4 mm and a wall thickness of 17 mm.

[0077] During the collapsing and elongating process, the coaxial arrangement of the sheathing tube 1 and the capping cylinder is fed from below into a temperature-controlled heating zone with a vertically oriented longitudinal axis and is softened zone by zone, starting with the upper end of the arrangement.

[0078] The heating zone is maintained at a target temperature of 1600 °C with a control accuracy of + / - 0.1 °C. This allows temperature fluctuations in the hot forming process to be limited to less than + / - 0.5 °C.

[0079] The secondary preform formed in the collapsing and elongating process has an outer diameter of approximately 50 mm and consists of an outer shell and an inner

[0080] The composite sheath wall thickness is 16.6 mm. The maximum wall thickness variation (largest value minus smallest value) of the antiresonant element preforms is less than 4 µm. The secondary preform is then drawn to form the antiresonant hollow core fiber.

[0081] The following table lists the take-off parameters for different outer diameters before (BEFORE) and after (AFTER) the forming process. Table 1 Outer diameter BEF [mm] Outer diameter AFTER [mm] Sheathing tube length [mm] Feed rate [mm / min] Deduction [mm / min] 90 70 1000 15 9,80 80 70 1000 15 4,59 40 20 1000 5 15 25 20 1000 10 5,63

[0082] The heating zone has a length of 100 mm. For example, a casing tube with an outer diameter of 90 mm and a wall thickness of 10 mm results in a throughput of 27.6 g / min into the heating zone at a feed rate of 5 mm / min, and 83 g / min at a feed rate of 15 mm / min. At feed rates of 5 mm / min and 15 mm / min, a tube with an outer diameter of 25 mm and a wall thickness of 1 mm results in throughputs of 0.8 g / min and 2.49 g / min, respectively.

[0083] The following table summarizes further dimensions of preforms depending on the desired diameter ratio (OD / ID) between the outer and inner diameter of the shell area of ​​the hollow core fiber. Table 2 Nr. OD / ID Fiber outer diameter / inner diameter Preform outer diameter (mm) Cane inner diameter (mm) Cane outer diameter (mm) 1 2,3 230 / 98 90 38 46 2 2,9 230 / 80 90 31 39 3 2,0 200 / 98 90 44 53 4 3,0 230 / 98 50 16,8 22,2 5 2,3 230 / 98 25 11 13 6 2,3 230 / 98 100 58 75

[0084] The maximum wall thickness deviation of the antiresonance element preforms in the preform is approximately 4 µm in all embodiments. Hollow core fibers with an outer diameter of 200 µm or 230 mm were drawn from the preforms, as specified in the table above, and the wall thicknesses of the antiresonance elements were determined. Example No. 4 in the table corresponds to the embodiment described in detail above. Examples 5 and 6 are comparative examples. No hollow core fibers with optimal geometry were obtained in the fiber drawing process using the preforms from the comparative examples. This is attributed to a primary preform that was either too large or too small during the elongation process.

Claims

1. A method for producing an anti-resonant hollow-core fiber which comprises a hollow core that extends along a fiber longitudinal axis and an inner casing region that surrounds the hollow core and comprises a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (1) which comprises a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (2) delimited by an inner face and an outer face extends, (b) providing a number of tubular anti-resonance element preforms (4), (c) arranging the anti-resonance element preforms (4) at target positions on the inner face of the cladding tube wall (2) to form a primary preform (3), which comprises a hollow core region and an inner casing region, wherein the primary preform (3) has an outer diameter in the range of 20 to 70 mm, (d) further processing the primary preform (3) to form a secondary preform, which has an outer diameter in the range of 20 to 70 mm and from which the hollow core fiber is drawn, wherein the further processing comprises elongation and performing the following hot-forming process a single time: collapsing additional casing material in the form of an overlay cylinder made of quartz glass and simultaneous elongation, and (e) drawing the secondary preform to form the hollow-core fiber.

2. The method according to claim 1, characterized in that during elongation, a temperature-controlled heating element is used, the target temperature of which is kept to an accuracy of + / - 0.1°C.

3. The method according to claim 1 or 2, characterized in that during elongation, the primary preform (3) is continuously fed to a heating zone, with a feed rate being set such that a throughput of at least 0.8 g / min, preferably a throughput in the range of 0.8 g / min to 85 g / min, and particularly preferably a throughput in the range of 3.3 g / min to 85 g / min, and an average residence time in the heating zone of less than 25 min, preferably an average residence time in the range of 5 to 25 min, is achieved.

4. The method according to any of the preceding claims, characterized in that the drawdown ratio during elongation is set to a value in the range of 1.05 to 10, preferably to a value in the range of 1.05 to 5.

5. The method according to any of the preceding claims, characterized in that the arrangement of the anti-resonance element preforms (4) and / or the elongation of the primary preform (3) and / or the drawing of the hollow-core fiber comprises a fixing measure and / or a sealing measure using a sealing or bonding compound (5) containing amorphous SiO2 particles.

6. The method according to claim 5, characterized in that during elongation of the primary preform (3) and / or during drawing of the hollow-core fibers, open ends of the anti-resonance element preforms (4) and / or individual structural elements (4a; 4b) of the anti-resonance element preforms (4) and / or any annular gap between tubular elements are sealed by means of the sealing or bonding compound (5).

7. The method according to any of the preceding claims, characterized in that the cladding tube inner face and / or the cladding tube outer face and / or the inner face of the ARE outer tube and / or the outer face of the ARE outer tube is produced by machining, in particular by drilling, milling, grinding, honing and / or polishing.

8. The method according to any of the preceding claims, characterized in that the cladding tube inner face is provided, by machining in the region of the target positions, with a longitudinal structure that extends in the direction of the cladding tube longitudinal axis.

9. The method according to any of the preceding claims, characterized in that the upper endface ends of the structural elements are positioned at the target position by means of a positioning template.

10. The method according to claim 9, characterized in that the positioning template is inserted in the region of one cladding tube end face, preferably in the region of both cladding tube end faces.

11. The method according to any of the preceding claims, characterized in that during elongation of the primary preform (3) according to method step (d) and / or during drawing of the hollow-core fiber according to method step (e), a plurality of components of the preform (3) made of quartz glass are heated and softened together, the quartz glass of at least some of the preform components containing at least one dopant which lowers the viscosity of quartz glass.

12. The method according to claim 11, characterized in that additional casing material is collapsed according to method step (d), and in that the quartz glass of the cladding tube (1) at a measuring temperature of 1250°C has a viscosity which is at least 0.5 dPa·s higher, preferably a viscosity which is at least 0.6 dPa·s higher, than the quartz glass of the additionally applied casing material (when the viscosity is given as a logarithmic value in dPa· s).

13. The method according to any of the preceding claims, characterized in that the provision of the primary preform (3) comprises arranging the anti-resonance element preforms (4) at target positions on the inner face of the cladding tube wall (2), the arrangement of the anti-resonance element preforms (4) and / or the drawing of the hollow-core fiber according to method step (d) comprising a fixing measure and / or a sealing measure using a sealing or bonding compound (5) containing amorphous SiO2 particles.

14. A method for producing a preform for an anti-resonant hollow-core fiber which comprises a hollow core that extends along a fiber longitudinal axis and an inner casing region that surrounds the hollow core and comprises a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (1) which comprises a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (2) delimited by an inner face and an outer face extends, (b) providing a number of tubular anti-resonance element preforms (4), (c) arranging the anti-resonance element preforms (4) at target positions on the inner face of the cladding tube wall (2) to form a primary preform (3), which comprises a hollow core region and an inner casing region, wherein the primary preform (3) has an outer diameter in the range of 20 to 70 mm, and (d) further processing the primary preform (3) to form a secondary preform for the hollow-core fiber, wherein the secondary preform has an outer diameter in the range of 20 to 70 mm, wherein the further processing comprises elongation and performing the following hot-forming process a single time: collapsing additional casing material in the form of an overlay cylinder made of quartz glass and simultaneous elongation.