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

Abstract

Methods are known for producing an antiresonant hollow-core fiber having a hollow core extending along a fiber longitudinal axis and an inner sheath surrounding the hollow core, comprising several antiresonant elements. The known methods comprise the following steps: providing a sheath tube having an inner bore and a longitudinal axis along which a sheath wall bounded by an inner and an outer surface extends; providing a number of tubular antiresonant element preforms; 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 and an inner sheath; elongating the primary preform to form the hollow-core fiber or further processing the primary preform to form a secondary preform.In order to achieve high precision and exact positioning of the antiresonance elements in a sufficiently stable and reproducible manner, it is proposed that a secondary preform be formed which has an outer diameter in the range of 30 to 90 mm, and that at least one of the end faces of the antiresonance element preforms be closed before fiber drawing.

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 primary preform for the hollow core fiber, comprising a hollow core region and a sheath region comprising at least one sheath tube having an inner bore and a longitudinal axis along which a sheath tube wall bounded by an inner and outer surface extends, wherein a number of tubular and / or hollow channel antiresonant element preforms are arranged in the sheath region; (b) further processing the primary preform into a secondary preform comprising a single or repeated execution of one or more of the following hot forming processes: (i) elongation; (ii) collapse and simultaneous elongation; (iii) collapse of additional sheath material followed by elongation; (iv) collapse of additional sheath material and simultaneous elongation; and (c) drawing the hollow core fiber from the secondary preform.

[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 primary preform for the hollow core fiber, comprising a hollow core region and a sheath region comprising at least one sheath tube having an inner bore and a longitudinal axis along which a sheath wall bounded by an inner sheath surface and an outer sheath surface extends, wherein a number of tubular and / or hollow-channel antiresonant element preforms are arranged in the sheath region; and (b) further processing the primary preform into a secondary preform comprising a single or repeated execution of one or more of the following hot forming processes: (i) elongation; (ii) collapse and simultaneous elongation; (iii) collapse of additional sheath material followed by elongation; (iv) collapse of additional sheath material and simultaneous elongation.

[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 hollow-core fibers, the interaction of light with the glass is less 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. Depending on the physical mechanism of light transmission, hollow-core fibers are subdivided into photonic bandgap fibers and antiresonant 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.

[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. 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, which are surrounded by an outer sheath tube. Rods, tubes, and sheath 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. Technical task

[0014] 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.

[0015] 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 a sheath tube and an inner capillary welded to one side of the inner sheath tube's surface, must be attached to the inside of a sheath tube.

[0016] 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 outer casing 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.

[0017] 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.

[0018] 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

[0019] 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 forming a secondary preform which has an outer diameter in the range of 30 to 90 mm, and by sealing at least one of the end faces of the antiresonant element preforms before drawing the hollow core fiber according to process step (c).

[0020] The starting point for the production of the antiresonant hollow core fiber is a preform, also referred to here as a "cane" or "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 typically added to the primary preform to create a preform referred to here as a "secondary preform." If necessary, the hollow core fiber is produced by elongating the secondary preform. Alternatively, the primary or secondary preform is surrounded by a cane or multiple canes, forming a coaxial ensemble of components, and this coaxial ensemble is then directly elongated to form the hollow core fiber.The general term "preform" is used here to refer to the component or coaxial ensemble of components from which the hollow core fiber is ultimately drawn.

[0021] The addition of mantle material involves collapsing a capping cylinder onto the primary preform. The coaxial arrangement of the primary preform and capping cylinder is either elongated during the collapse of the capping cylinder, or it is not. This process either changes the shape or arrangement of the antiresonant element preform, or it does not.

[0022] 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.

[0023] The preform used in the fiber drawing process according to the invention is characterized by an outer diameter in the range of 30 to 90 mm. This is a large outer diameter compared to the current state of the art. Since the absolute geometric errors in fiber drawing are scaled down more significantly with increasing outer diameter of the preform, this fundamentally enables more precise production of the hollow core fiber.

[0024] However, it has been shown that any increase in the preform outer diameter does not automatically lead to a more precise hollow core fiber, but rather that, in order to maintain a maximum relative geometric error of 3.5% in the wall thickness, the following boundary conditions must be met using antiresonance elements in the hollow core fiber. I. The preform outer diameter is a maximum of 90 mm. With larger diameters, temperature gradients form within the preform volume during the fiber drawing process, resulting in wall thickness deviations of more than 3.5% in the antiresonance elements of the hollow core fiber. The reference value for the percentage is the mean wall thickness. II. The preform outer diameter is at least 30 mm. It has been shown that antiresonance element preforms with a wall thickness deviation of approximately 4 µm are feasible. For preform outer diameters smaller than 30 mm, this absolute error in the preforms leads to a relative error in the wall thickness of the antiresonance elements in the finished hollow core fiber of more than 3.5%. III. All antiresonance element preforms, or at least some of them, form hollow channels and are generally open at both ends. The free inner diameter of the hollow channels is small and is typically in the range of a few millimeters in the preform.In the hot forming process, the preform is heated from the outside, creating a radial temperature gradient within the preform volume. This gradient—assuming otherwise identical process conditions—is greater the thicker the preform is. There is a risk that the hollow channels will shrink unevenly due to surface tension and depending on the local temperature. This risk increases with the size of the radial temperature gradient and the thickness of the preform. The temperature gradient, however, has no significant effect on the central hollow core. To counteract this effect in the relatively thick preforms according to the invention, the core area (hollow core) is left open during the fiber drawing process with a vertical orientation of the longitudinal axes, but the otherwise open upper end is closed in at least some of the antiresonance element preforms.The sealing of the antiresonance element preform(s) takes place before the fiber drawing process begins and remains intact throughout the process. Due to the sealing of the upper end, each hollow channel has an initial gas volume. During the fiber drawing process, the gas is heated, and the pressure in the hollow channels increases, causing them to expand from the bottom upwards. Since gas exchange is limited in the narrow hollow channels and the hot gas cannot escape upwards, the temperature difference between the lower and upper ends of the preform largely determines the degree of expansion, essentially independent of the original hollow channel diameter. This temperature difference is approximately the same for all hollow channels, regardless of their radial position, so that all hollow channels expand to approximately the same extent.This ensures that the original distribution of the hollow channel sizes in the thick preform is also maintained in the final hollow core fiber.

[0025] This concept is also suitable for a reproducible and precise manufacturing process for antiresonant hollow core fibers on an industrial scale. It is particularly well-suited for the precise production of antiresonant hollow core fibers with nested antiresonance elements that have significantly different inner diameters.

[0026] In a preferred method, a primary preform is formed in process step (a) which has an outer diameter in the range of 20 to 70 mm.

[0027] This is a comparatively large outer diameter. In the prior art, the outer diameters of the primary preforms (canes) are typically between 4 and 6 mm.

[0028] It has also proven advantageous if the primary preform forms an inner shell area in the secondary preform, which has an outer diameter in the range of 7 mm to 50 mm.

[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] It has also proven effective to set an internal pressure in the core area between 0.05 mbar and 20 mbar when drawing the hollow core fiber according to process step (c).

[0031] At an internal pressure of less than 0.05 mbar, the antiresonance element preforms or pre-stages may expand excessively. Conversely, an internal pressure of more than 20 mbar in the core area may result in insufficient gas pressure within the hollow channels of the antiresonance element preforms for them to expand sufficiently during the hot forming process.

[0032] 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.

[0033] This allows temperature fluctuations in the hot forming process to be limited to less than + / - 0.5°C.

[0034] In a preferred method variant, the provision of the primary preform comprises arranging the antiresonance element preforms at predetermined positions on the inside of the jacket tube wall, wherein the arranging of the antiresonance element preforms and / or the drawing of the hollow core fiber according to process step (c) comprises a fixing measure and / or a sealing measure using a sealing or bonding compound containing amorphous SiO2 particles.

[0035] The sealing or bonding compound used for sealing or fixing contains amorphous SiO2 particles, which are, for example, suspended 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 a low temperature, 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 required low temperature below 300°C promotes the dimensional accuracy of the preform and prevents thermal damage.By heating to higher temperatures, for example when elongating the preform into a hollow core fiber, the sealing or bonding compound is also suitable for forming opaque or transparent glass. This is achieved 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 densified by heating and vitrified by heating during the hot forming process.

[0036] 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.

[0037] In a preferred method variant, providing the primary preform according to process step (a) comprises arranging the antiresonance element preforms at predetermined positions on the inside of the jacket tube wall, wherein the arranging of the antiresonance element preforms is carried out by means of a positioning template to be inserted into the inner bore of the jacket tube, which has retaining elements for positioning the antiresonance element preforms at the predetermined positions.

[0038] 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.

[0039] The inherent star-shaped arrangement of the retaining elements facilitates the precise positioning and fixing of the antiresonance element preforms at their respective target positions, for example, using the sealing or bonding compound described above. The positioning template is preferably used exclusively in the area of ​​the casing tube ends, and preferably in the area of ​​both casing tube ends.

[0040] The accuracy of positioning the preforms on the inner surface of the casing tube is improved by machining the inside of the casing tube, in particular by drilling, milling, grinding, honing and / or polishing.

[0041] These processing techniques, compared to other known forming techniques using heat and pressure, produce more precise and intricate structures and avoid surface contamination from forming tools such as nozzles, presses or melt molds.

[0042] 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.

[0043] The longitudinal structure extending along the longitudinal axis of the casing tube serves as a positioning aid for the anti-resonance element preforms. It facilitates the anti-resonance element preforms assuming predetermined, defined positions on the inside of the casing tube.

[0044] Furthermore, a procedure has proven effective in which, during the drawing of the hollow core fiber according to process step (c), several components of the secondary 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.

[0045] Components of the preform include the casing tube and the antiresonance element preforms arranged therein, as well as additional casing material, which is provided, for example, in the form of one or more overlay cylinders and collapsed onto the primary preform. Fluorine, chlorine, and / or hydroxyl groups are preferably used as dopants to reduce the viscosity of the quartz glass.

[0046] 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.

[0047] For example, it has proven advantageous if the quartz glass of the jacket tube has a viscosity at least 0.5 dPa·s higher, preferably at least 0.6 dPa·s higher, than the quartz glass of additionally applied jacket material at a measuring temperature of 1250 °C (when the viscosity is specified as a logarithmic value in dPa·s).

[0048] 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.

[0049] 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 forming a secondary preform which has an outer diameter in the range of 30 to 90 mm and at least one of the end faces of the antiresonance element preforms is closed.

[0050] The secondary preform is the starting point for the production of the antiresonant hollow core fiber. The antiresonant hollow core fiber is drawn by elongating the preform.

[0051] A preform is produced which, compared to the previous state of the art, has a large outer diameter, so that the absolute geometric error present in the preform can be scaled down more during fiber drawing.

[0052] At least one of the end faces of the antiresonance element preforms is sealed before the fiber drawing process. The end face to be sealed is the one that forms the upper end when the preform is elongated with its longitudinal axis oriented vertically. The seal of the antiresonance element preform(s) remains intact throughout the fiber drawing process.

[0053] 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

[0054] 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

[0055] 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

[0056] 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 (cane).

[0057] 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

[0058] 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

[0059] 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.

[0060] 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 mantle area / Outer mantle area / Cane

[0061] The assembly consisting of at least one cane and preforms or precursors for antiresonance elements, either loosely held or firmly fixed within it, is also referred to here as the "primary cane." The primary cane comprises the hollow core and a cane section. This cane section is also referred to as the "inner cane section" if there is also an "outer cane section," which is created, for example, by collapsing onto the cane, and if a distinction is to be made between these cane sections. The terms "inner cane section" and "outer cane section" are also used for the corresponding areas in the hollow core fiber or in intermediate products obtained by further processing of the primary cane.

[0062] 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

[0063] 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

[0064] The particle size and size distribution of the SiO₂ particles are characterized using the D₅₀ values. These values ​​are derived from particle size distribution curves, which show the cumulative volume of the 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 the SiO₂ particles, and correspondingly, the D₅₀ and D₅₀ values ​​represent the particle sizes not reached by 50% and 90% of the cumulative volume of the SiO₂ particles, respectively. The particle size distribution is determined by light scattering and laser diffraction spectroscopy according to ISO 13320. Example of implementation

[0065] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. Specifically, a schematic representation is shown. 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, and Figure 2 the primary precursor of Figure 1 after sealing the antiresonance element preforms for the purpose of carrying out the fiber drawing process.

[0066] 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. This involves wet-milling quartz glass granules to produce an aqueous slurry containing 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.

[0067] Figure 1Figure 1 schematically shows a cane (primary preform) with a sheath tube 1 having a sheath tube wall 2, on the inner sheath 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.

[0068] The inner casing tube 1 is made of quartz glass and has a length of 700 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 outer tube 4a and an inner capillary 4b. The outer tube 4a has an outer diameter of 6.2 mm, and the inner capillary 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 outer tube 4a and the inner capillary 4b correspond to the length of the casing tube 1.

[0069] The antiresonance element preforms 4 are fixed to the inner wall of the casing tube 1 by means of the SiO 2 based bonding compound 5.

[0070] The bonding compound 5 is applied locally to the inner surface of the casing tube in the area of ​​the end faces, and the anti-resonance element preforms 4 are placed onto it using a positioning template with a structurally predetermined star-shaped arrangement of retaining arms for the individual anti-resonance element preforms 4. The positioning template is limited to the area around the two end faces of the casing tube.

[0071] This method creates a precise and reproducible connection between the outer casing 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.

[0072] The primary preform 1 is overmolded with a quartz glass overlay cylinder, whereby the overlay cylinder collapses onto the outer casing 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.

[0073] During the collapsing and elongating process, the coaxial arrangement of jacket tube 1 and capping cylinder is fed from below to 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.

[0074] 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.

[0075] The secondary preform formed during the collapse and elongation process has an outer diameter of approximately 50 mm and a sheath wall thickness of 16.6 mm, composed of the outer and inner sheaths. 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 into the antiresonant hollow core fiber.

[0076] Beforehand, all antiresonance element preforms are sealed with the sealing or bonding compound. This state is shown schematically in Figure 2 indicated by dark grey colored areas. The sealing compound 51 is applied only to the end face of the antiresonance element preforms 4 that faces upwards during the fiber drawing process.

[0077] The upward-facing end face is connected to a quartz glass retaining tube, which also serves as the gas connection. The retaining tube is fixed to the outer cylinder and the outer tube using a sealing or bonding compound.

[0078] In the fiber drawing process, the secondary preform is fed from above into a temperature-controlled heating zone with its longitudinal axis oriented vertically, and softened zone by zone, starting at the lower end. Simultaneously, gas is supplied to the core area (hollow core), creating an internal pressure of 4 mbar within the core.

[0079] The heating zone is maintained at a target temperature of approximately 2100 °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.

[0080] By drawing the preform to form a hollow core fiber, the existing absolute geometric error is scaled down, so that the antiresonance elements obtained from the antiresonance element preforms in the hollow core fiber have a maximum deviation of less than 3.5% in wall thickness.

[0081] The following table summarizes the 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 1 Nr. OD / Innendurchmesser 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 2,3 230 / 98 30 13 15 5 2,9 230 / 80 30 10 13 6 2,0 200 / 98 30 15 18 7 3,0 230 / 98 50 16,8 22,2 8 2,3 230 / 98 25 11 13 9 2,3 230 / 98 100 43 51

[0082] 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, as specified in the table above, were drawn from the preforms, and the wall thicknesses of the antiresonance elements were determined. In all examples, the error in the wall thickness of the antiresonance elements was less than 3.5% (based on the mean wall thickness).

[0083] Example No. 7 in the table corresponds to the embodiment described in detail above. Examples 8 and 9 are comparative examples. In the fiber drawing process using the preforms of the comparative examples, hollow core fibers were obtained in each case where the error in the wall thickness of the antiresonance elements was more than 4%. In comparative example 8, this unsatisfactory result is attributed to the comparatively small draw-out ratio, and in comparative example 9 to temperature gradients within the preform volume during the fiber drawing process.

Claims

1. 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, comprising several antiresonance elements, comprising the process steps of: (a) providing a primary preform for the hollow core fiber, comprising a hollow core and a sheath comprising at least one sheath tube having an inner bore and a longitudinal axis along which a sheath wall bounded by an inner and outer surface extends, wherein a number of tubular and / or hollow channel antiresonance element preforms are arranged in the sheath; (b) further processing the primary preform into a secondary preform, comprising a single or repeated execution of one or more of the following hot forming processes: (i) elongation; (ii) collapse and simultaneous elongation.(iii) Collapse of additional sheath material and subsequent elongation, (iv) Collapse of additional sheath material and simultaneous elongation, and (c) Drawing the hollow core fiber from the secondary preform, , characterized by the fact that a secondary preform is formed which has an outer diameter in the range of 30 to 90 mm, and that before drawing the hollow core fiber according to process step (c) at least one of the end faces of the antiresonance element preforms is closed.

2. Method according to claim 1, characterized by the fact that according to process step (a) a primary preform is provided which has an outside diameter in the range of 20 mm to 70 mm.

3. Method according to claim 1 or 2, characterized by the fact that The primary preform in the secondary preform forms an inner shell area which has an outer diameter in the range of 7 mm to 50 mm.

4. Method according to any one of the preceding claims, characterized by the fact thatWhen drawing the hollow core fiber according to process step (c), an internal pressure in the core area is set in the range between 0.05 mbar and 20 mbar.

5. Method according to any one of the preceding claims, characterized by the fact that When carrying out a process according to process step (b), a temperature-controlled heating element is used whose target temperature is maintained to an accuracy of + / - 0.1 °C.

6. Method according to any one of the preceding claims, characterized by the fact that The provision of the primary preform comprises arranging the antiresonance element preforms at predetermined positions on the inside of the jacket tube wall, wherein the arranging of the antiresonance element preforms and / or the drawing of the hollow core fiber according to process step (c) comprises a fixing measure and / or a sealing measure using a sealing or bonding compound containing amorphous SiO2 particles.

7. Method according to any of the preceding claims, characterized by the fact thatThe provision of the primary preform according to process step (a) comprises arranging the antiresonance element preforms at designated positions on the inside of the jacket tube wall, wherein the arranging of the antiresonance element preforms is carried out by means of a positioning template to be inserted into the jacket tube inner bore, which has retaining elements for positioning the antiresonance element preforms at the designated positions.

8. Method according to claim 7, characterized by the fact that A positioning template is used with 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.

9. Method according to any one of the preceding claims, characterized by the fact that The inner surface of the casing tube is produced by machining, in particular by drilling, milling, grinding, honing and / or polishing.

10. Method according to any one of the preceding claims, characterized by the fact thatThe inside of the casing tube is provided with a longitudinal structure extending in the direction of the casing tube's longitudinal axis by machining in the area of ​​the target positions.

11. Method according to any of the preceding claims, characterized by the fact that In the drawing of the hollow core fiber according to process step (c), several components of the secondary 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.

12. Method according to claim 11, characterized by the fact thataccording to process step (b) additional jacket material is collapsed, and that the quartz glass of the jacket tube has a viscosity at least 0.5 dPa.s higher, preferably at least 0.6 dPa.s higher, than the quartz glass of the additionally applied jacket material at a measuring temperature of 1250 °C (when the viscosity is specified as a logarithmic value in dPa.s).

13. A method for producing a preform for an antiresonant hollow core fiber, comprising a hollow core extending along a fiber longitudinal axis and a sheath surrounding the hollow core, which includes several antiresonance elements, comprising the process steps of: (a) providing a primary preform for the hollow core fiber, which has a hollow core region and a sheath region comprising at least one sheath tube having an inner bore and a longitudinal axis along which a sheath wall bounded by an inner surface and an outer surface extends, wherein a number of tubular and / or hollow channel antiresonance element preforms are arranged in the sheath region; and (b) further processing the primary preform into a secondary preform.which includes a single or repeated execution of one or more of the following hot forming processes: (i) elongation, (ii) collapse and simultaneous elongation, (iii) collapse of additional shell material followed by elongation, (iv) collapse of additional shell material and simultaneous elongation, , characterized by the fact that A secondary preform is formed which has an outer diameter in the range of 30 to 90 mm, and at least one of the end faces of the antiresonance element preforms is closed.