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) forming a number of preforms or preforms for antiresonance elements at predetermined positions of the sheath tube wall, forming a primary preform for the hollow core fiber, wherein at least some of the preforms for antiresonance elements are tubular antiresonance element preforms, preferably composed of several nested structural elements comprising an ARE outer tube and an ARE inner tube inserted therein; and (c) elongating the primary preform to form the hollow core fiber or further processing the primary preform to form a secondary preform from which the hollow core fiber is drawn.where further processing comprises 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 shell material, (v) collapse of additional shell material followed by elongation, (vi) collapse of additional shell material and simultaneous elongation.

[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) forming a number of preforms or preforms for antiresonance elements at predetermined positions of the sheath tube wall, forming a primary preform for the hollow core fiber, wherein at least some of the preforms for antiresonance elements are tubular antiresonance element preforms, preferably composed of several nested structural elements comprising an ARE outer tube and an ARE inner tube inserted therein; and (c) further processing the primary preform into a secondary preform for the hollow core fiber, wherein the further processing comprises 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.

[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. 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. 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] Ben Sherlock et al. Described 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 method for producing an antiresonant element hollow core fiber, comprising providing a primary preform including a cladding tube, forming a number of preforms for antiresonant elements at predetermined positions on the cladding tube wall, and further processing the primary preform into a secondary preform from which the hollow core fiber is drawn. In the fiber drawing process, the secondary preform is capped with a fluoro-doped quartz glass cylinder. The lower viscosity of the fluoro-doped quartz glass allows for a lower drawing temperature, which contributes to better preservation of the predetermined fiber structure.

[0015] Gregory T. Jasion et al.: "Fabrication of tubular anti-resonant hollow core fibers: modelling, draw dynamics and process optimization", OPTICS EXPRESS, Vol. 27, No. 15, (2019-07-10), pages 20567-20582, DOI: 10.1364 / OE.27.020567, discloses a method for manufacturing an antiresonant hollow core fiber. The method comprises the following steps: providing a sheath tube with an outer diameter in the range of 20 to 30 mm; arranging a number of tubular antiresonant element preforms at predetermined positions on the inner surface of the sheath tube wall to form a primary preform; and further processing of the primary preform. This further processing includes elongation to a secondary preform (cane) with an outer diameter of approximately 3 mm, from which the hollow core fiber is drawn. Technical task

[0016] 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.

[0017] 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 element outer tube (ARE outer tube) and an antiresonance element 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.

[0018] 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.

[0019] 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 antiresonance elements in the fiber can be achieved reproducibly in a sufficiently stable and reproducible manner. Furthermore, disadvantages of the classical "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 easily achieved, should be avoided as far as possible. Summary of the invention

[0020] With regard to the production of the antiresonant hollow core fiber, this problem is solved according to the invention, starting from a process of the aforementioned type, by jointly heating and softening components of the primary preform made of quartz glass and optionally components surrounding the primary preform made of quartz glass, wherein the quartz glass of at least one of the primary preform components and optionally the quartz glass of at least one of the components surrounding the preform contains at least one dopant which lowers or increases the viscosity of quartz glass, so that the antiresonant element preforms consist of quartz glass which, at a measurement temperature of 1250 °C, has a viscosity at least 0.4 dPa·s higher than that of the quartz glass of the sheathing tube.

[0021] Components of the preform include the casing tube and the antiresonance element preforms, as well as any additional casing material produced on the outer surface of the casing tube. Components surrounding the preform include, for example, a capping cylinder or cylinders that surround the preform during the hot forming process, onto which it collapses to form additional casing material. For the sake of simplicity, the term "components" of the preform will be used in the following text to refer to these components surrounding the preform.

[0022] At least one of the preform components contains at least one dopant that either decreases or increases the viscosity of fused silica. A type of doping that decreases the viscosity of fused silica is referred to below as "de-doping," and a type of doping that increases the viscosity of fused silica is referred to below as "up-doping." Fluorine, chlorine, and / or hydroxyl groups are preferably used as dopants that decrease the viscosity of fused silica. Al₂O₃ and / or nitrogen are suitable dopants that increase the viscosity of fused silica.

[0023] 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 one or more capping cylinders, 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.

[0024] The addition of sheathing material is achieved, for example, by 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 remains unchanged. In this process, the antiresonant element preforms are either altered in their shape or arrangement, or they remain unchanged.

[0025] The execution of one of the hot forming processes mentioned in process step (c) (hereinafter also referred to as "thermal processing") can lead to deformation and structural deviation of the desired fiber geometry. This is particularly the case when both thick-walled and delicate preform components made of the same material are located close together or adjacent to each other.

[0026] The necessary processing temperature is generally determined by the component with the largest surface area; this is typically the outer shell of the preform. Smaller components (such as the antiresonant element preforms and their individual structural elements) are subject to greater deformation at the same temperature. Since the preform is heated from the outside in during thermal processing, a radial temperature profile develops across the preform volume, with a minimum in the center. This can exacerbate the aforementioned deformation problem if delicate components are arranged on a preform radius that is closer to the heating zone than a less delicate component, which is regularly the case with preforms for antiresonant hollow core fibers.

[0027] The invention aims to mitigate this problem and thereby achieve greater accuracy in the geometric shape and positioning of the anti-resonance elements in the hollow core fiber. For this purpose, the quartz glass contains at least one of the preform components, specifically at least one dopant that lowers or increases the viscosity of the quartz glass.

[0028] Doping allows for the adjustment of the viscosities of adjacent preform components. In particular, it can be used to reduce the thermal stability of one component in favor of the stability of a neighboring component. Specifically, by doping the preform component with the largest surface area, the required processing temperature can be lowered, thereby indirectly improving the relative stiffness and thermal stability of components located further inside, by exposing them to a lower temperature during the hot forming process.

[0029] At least some of the precursors for antiresonance elements are in the form of tubular antiresonance element preforms, which are preferably composed of several nested structural elements, comprising an ARE outer tube and an ARE inner tube inserted therein, wherein the antiresonance element preforms consist of quartz glass which, at a measurement temperature of 1250 °C, has a viscosity at least 0.4 dPa·s higher, preferably at least 0.5 dPa·s higher, than the quartz glass of the sheathing tube.

[0030] The quartz glass of the ARE outer tube can contain a viscosity-increasing dopant, such as Al₂O₃ or nitrogen. However, it has proven particularly advantageous if the cladding tube consists of quartz glass containing a viscosity-reducing dopant.

[0031] Ideally, a component in the outer shell of the preform is doped with an ab-type material. This is particularly the outermost shell of the preform. This allows for a reduction in the processing temperature, thereby reducing deformation during the hot forming process.

[0032] Thus, in a preferred method, the optional further processing of the primary preform includes the collapse of additional jacket material, and the additional jacket material consists of quartz glass containing a dopant that reduces the viscosity of quartz glass, wherein the dopant is preferably fluorine and is present in a concentration between 500 and 14,500 ppm by weight, preferably between 2,000 and 10,000 ppm by weight.

[0033] Fluorine doping of the additional jacket material in this area enables a significant reduction in viscosity compared to the quartz glass of the outer casing, even if the quartz glass of the outer casing itself contains no dopant. It has proven advantageous if the quartz glass of the outer casing exhibits a viscosity at least 0.5 dPa·s higher, preferably at least 0.6 dPa·s higher, than the quartz glass of the additional jacket material at a measurement temperature of 1250 °C. Viscosity differences are expressed here and subsequently as logarithmic viscosity values ​​in dPa·s.

[0034] In an advantageous process, all preform components consist of different grades of quartz glass, with the viscosity of the components increasing, to a first approximation, from the outside to the inside. Besides fluorine, other dopants such as Al₂O₃, nitrogen, chlorine, and hydroxyl groups can be used to adjust the viscosity. Al₂O₃ increases the viscosity of quartz glass up to a concentration of approximately 15 ppm by weight. In the simplest case, however, it is sufficient if only the additional jacket material contains a dopant and consists of fluorine-containing quartz glass.

[0035] With regard to high thermal stability of the ARE inner tubes in the case of nested structural elements, it has proven advantageous if at least some of the ARE inner tubes, preferably all ARE inner tubes, are made of quartz glass which, at a measurement temperature of 1250 °C, has a viscosity at least 0.4 dPa·s higher, preferably at least 0.5 dPa·s higher, than the quartz glass of the ARE outer tube.

[0036] The casing is preferably manufactured using a vertical drawing process without a die, employing a two-stage elongation process. In the first stage, a starting hollow glass cylinder is mechanically machined to achieve the desired final dimensions. In a first elongation process, the starting cylinder is continuously fed with its longitudinal axis oriented vertically into a heating zone of a defined length. Within this zone, it is partially softened, and an intermediate cylinder is drawn from the softened area. This intermediate cylinder is then continuously fed with its longitudinal axis oriented vertically into a second, shorter heating zone. Within this zone, it is partially softened, and a tube strand is drawn from the softened area. The casing is then obtained from this tube strand by cutting it to length.

[0037] By using preform components with low viscosity, especially in the outer shell area, the inventive method enables the use of comparatively large preforms for thermal processing.

[0038] In this regard, it is preferably preferred that a secondary preform is formed which has an outer diameter in the range of 30 to 90 mm, and / or that a primary preform is formed which has an outer diameter in the range of 20 mm to 70 mm.

[0039] The outer diameter, ranging from 30 to 90 mm, is large compared to current state-of-the-art technology. Since the absolute geometric error during fiber drawing is scaled down more significantly with increasing outer diameter of the preform, using larger preforms generally enables more precise manufacturing of the hollow core fiber. However, with diameters larger than 90 mm, temperature gradients develop within the preform volume during the fiber drawing process, which can result in wall thickness variations in the antiresonance elements of the hollow core fiber. For preform outer diameters of less than 30 mm, there is no significant benefit from scaling down the geometric error. Furthermore, it is advantageous to create a large primary preform with an outer diameter in the range of 20 to 70 mm. This is a comparatively large outer diameter.In the prior art, the outer diameters of the primary preforms (canes) are typically 4 to 6 mm.

[0040] In a preferred method variant, the formation of preforms according to process step (b) comprises arranging the antiresonance element preforms at predetermined positions on the inside of the sheathing tube wall, wherein a positioning template is used for arranging, which has retaining elements for positioning the antiresonance element preforms at the predetermined positions.

[0041] 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.

[0042] The inherent star-shaped arrangement of the retaining elements facilitates the precise positioning and fixation of the antiresonance element preforms at their respective target positions. 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.

[0043] The accuracy of the positioning of 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.

[0044] 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 jointly heating and softening components of the primary preform made of quartz glass and optionally components of the primary preform made of quartz glass surrounding the primary preform, wherein the quartz glass of at least one of the primary preform components and optionally the quartz glass of at least one of the components surrounding the preform contains at least one dopant that lowers or increases the viscosity of quartz glass, so that the antiresonance element preforms consist of quartz glass which, at a measurement temperature of 1250 °C, has a viscosity at least 0.4 dPa·s higher than that of the quartz glass of the sheathing tube.

[0045] The preform is the starting point for the production of the antiresonant hollow core fiber. By elongating the preform, the antiresonant hollow core fiber is either drawn directly, or a semi-finished product is first produced from which the antiresonant hollow core fiber is subsequently drawn. The production of the preform involves forming components of the primary preform from quartz glass containing a dopant that reduces the viscosity of quartz glass. This allows for a reduction in the processing temperature during the hot forming process and enables the use of comparatively large preforms. The measures for producing the preform are explained above in the context of hollow core fiber production, and these explanations are hereby incorporated. Definitions

[0046] 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

[0047] 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

[0048] 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).

[0049] 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

[0050] 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

[0051] 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.

[0052] 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

[0053] 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" or "cane." The primary preform (cane) comprises the hollow core and a sheath section. This sheath section is also referred to as the "inner sheath section" if there is also an "outer sheath section," which is created, for example, by collapsing onto the cane, and if a distinction is to be made between these sheath sections. The terms "inner sheath section" and "outer sheath section" are also used for the corresponding areas in the hollow core fiber or in intermediate products obtained by further processing of the primary preform.

[0054] 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

[0055] 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 inside of the casing tube; it can also extend to the outside through the entire casing tube wall. Particle size and particle size distribution

[0056] 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

[0057] 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 coaxial tube arrangement consisting of a capping cylinder and a primary preform, comprising a sheath tube and antiresonance element preforms positioned and fixed therein, as shown in a view of the radial cross-section, Figure 2 a diagram showing the radial profile of fluorine concentration and viscosity in the capping cylinder and the sheathing tube, and Figure 3 A sketch illustrating an ideal radial concentration or viscosity profile of a preform for a hollow core fiber.

[0058] 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.

[0059] Figure 1Figure 1 schematically shows the coaxial tube arrangement 1 with a cap cylinder 2, a sheath tube 3 with a sheath tube wall, 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.

[0060] The sheathing tube 3 has 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. All tubular components 2, 3, 4a, 4b have a length of 700 mm.

[0061] The antiresonance element preforms 4 are fixed to the inner wall of the casing 3 using a SiO₂-based bonding compound. The bonding compound is applied locally to the inner surface of the casing in the area of ​​the end faces, and the antiresonance element preforms 4 are placed onto it using a positioning template with a predefined star-shaped arrangement of retaining arms for the individual antiresonance element preforms 4. The influence of the positioning template is limited to the area around the two end faces of the casing. This method creates a precise and reproducible connection between the casing 3 and the antiresonance element preforms 4. For fixation, the bonding compound only needs to be cured at a low temperature below 300 °C, thus preventing excessive heating of the surrounding areas and consequently deformation of the antiresonance element preforms 4.

[0062] The primary preform thus obtained is encased with the quartz glass encapsulation cylinder 2. The encapsulation cylinder 2 has an outer diameter of 63.4 mm and a wall thickness of 17 mm. As the encapsulation cylinder 2 collapses onto the sheathing tube 3, the coaxial tube assembly is simultaneously elongated. For this purpose, the coaxial assembly of sheathing tube 3 and encapsulation cylinder 2, with its longitudinal axis oriented vertically, is fed from below into a temperature-controlled heating zone and softened zone by zone, starting at the upper end of the assembly. The heating zone is maintained at a setpoint temperature of 1580 °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.

[0063] The secondary preform formed during the collapsing and elongating 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. It is then drawn into the antiresonant hollow-core fiber. Prior to this, all antiresonant element preforms are sealed with the sealing or bonding compound. The sealing compound is applied only to the end face of the antiresonant element preforms that faces upwards during the fiber drawing process. This end face is connected to a quartz glass holder tube, which also serves as a gas connection. The holder is fixed to the capping cylinder 2 and the sheathing tube 3 using the sealing or bonding compound.

[0064] In the fiber drawing process, the secondary preform, with its longitudinal axis oriented vertically, is fed from above into a temperature-controlled heating zone 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. The heating zone is maintained at a target temperature of approximately 2080 °C with a control accuracy of + / - 0.1 °C. This limits temperature fluctuations in the hot forming process to less than + / - 0.5 °C.

[0065] 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 (based on a mean wall thickness).

[0066] The small error in wall thickness is attributed, on the one hand, to the use of the comparatively large secondary preform and the associated scaling down of the original absolute geometric deviations, and on the other hand, to comparatively low processing temperatures during the hot forming processes (elongation and collapse, fiber drawing). The lower processing temperatures, in turn, are due to the fact that the capping cylinder 2 and the casing tube 3 are made of fluorine-doped quartz glass. These components represent the largest surface area components in the coaxial arrangement 1 and significantly determine the processing temperature.By doping the largest surface areas of the secondary preform with fluorine, the necessary processing temperature can be lowered, thereby indirectly improving the relative stiffness and thermal stability of the inner antiresonance element preforms 4 by exposing them to a lower temperature during the hot forming process.

[0067] Table 1 below summarizes information on the materials of the components of the coaxial arrangement and the secondary preform. Table 1 Reference number in Fig. 1 Name / Function material 2 Overlap cylinder Fluorine-doped quartz glass 10,000 wt. ppm 3 sheathing tube Fluorine-doped quartz glass 2,700 wt. ppm 4a ARE outer tube undoped quartz glass 4b ARE inner tube undoped quartz glass

[0068] The fluorine-doped quartz glass tubes (2; 3) have a fluorine concentration profile with a maximum fluorine concentration in the middle of the tube wall. The fluorine concentration data for the quartz glass listed in the "Material" column of Table 1 are mean values.

[0069] The diagram of Figure 2 shows measured fluorine concentration profiles C (in wt. ppm) for a sheathing tube CF (M) and for a capping cylinder CF (Z)", as well as viscosity profiles η (in Ig dPa·s) calculated from the concentration profiles for a temperature of 1250 °C along the radial spatial coordinate (position P (in mm)).

[0070] The fluorine concentration profile in fused silica is determined by infrared spectroscopy. The viscosity scales with the fluorine concentration for a given temperature and is calculated from a base value for undoped fused silica (η = 11.8 dPa·s (corresponding to 100%)) using the following formula: decrease in viscosity at 1250 °C: 12% (±2%) per wt% fluorine.

[0071] Table 2 lists viscosity values ​​for fluorine contents of commercially available quartz glass grades (for a measurement temperature of 1250 °C). Table 2 Fluorine content [wt. ppm] Ig η @ 1250 °C [dPa*s] 0 11,80 4800 11,00 10000 10,50 13000 9,80

[0072] The diagram of Figure 2 This shows that the viscosity of the receiving cylinder η(Z) is lower than that of the cladding tube η(M). In both quartz glass tubes, the viscosity has a minimum in the center of the tube, which is approximately 1011.45 dPa·s for the cladding tube and approximately 1010.65 dPa·s for the receiving cylinder. The viscosity difference between the minima (in Ig dPa·s) is therefore approximately 0.80 dPa·s. The difference between the viscosity of the cladding tube in the region of the outer surface (approximately 1011.5 dPa·s) and the viscosity minimum of the receiving cylinder is approximately 0.85 (in Ig dPa·s).

[0073] In the preform, the outer surface of the casing tube and the inner surface of the capping cylinder form a common contact area. The position of this contact area, as represented by the viscosity profiles, is indicated in the diagram by the two rectangles "K". At these positions, the following viscosities of the capping cylinder and casing tube are obtained: Sheathing tube: approximately 11.5 Ig(dPa·s) Overcap cylinder: approximately 11.15 Ig(dPa·s)

[0074] The viscosity difference in the area of ​​the contact surface is therefore approximately 0.35 (in Ig dPa·s).

[0075] The structural elements (4a; 4b) of the antiresonance element preforms (4) consist of undoped quartz glass and have a viscosity of about 10 11.8< dPa·s.

[0076] The diagram of Figure 3The figure shows the radial dopant concentration profile across the wall of the secondary preform in an idealized form. The y-axis plots the fluorine concentration CF (in relative units) against the spatial coordinate P (in relative units). At the contact surface "K", the dopant concentration CF(Z) of the fluorine-doped quartz glass originating from the case cylinder is ideally equal to the concentration CF(M) of the fluorine-doped quartz glass originating from the casing tube. The corresponding viscosity profile of the casing tube and the case cylinder therefore shows the same viscosity on both sides at the contact surface K.

Claims

1. A method for producing an anti-resonant hollow-core fiber which comprises a hollow core extending 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 sheath tube (3) which comprises a sheath tube inner bore and a sheath tube longitudinal axis, along which a sheath tube wall extends, which wall is delimited by an inner face and an outer face, (b) forming a number of precursors or blank molds (4) for anti-resonance elements at desired positions of the sheath tube wall, in order to form a primary preform (1) for the hollow-core fiber, at least some of the precursors for anti-resonance elements being in the form of tubular anti-resonance element blank molds (4) which are preferably composed of a plurality of nested structural elements comprising an ARE outer tube (4a) and an ARE inner tube (4b) inserted therein, and (c) elongating the primary preform (1) to form the hollow-core fiber or further processing the primary preform (1) to form a secondary preform from which the hollow-core fiber is drawn, the further processing comprising carrying out one or more of the following hot-forming processes once or repeatedly: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional casing material, (v) collapse of additional casing material and subsequent elongation, (vi) collapse of additional casing material and simultaneous elongation, characterized in that, in carrying out a process according to method step (c), components (3; 4) of the primary preform (1) made of fused silica, and optionally components (2) surrounding the primary preform (1) made of fused silica, are heated and softened together, the fused silica of at least one of the primary preform components (3; 4), and optionally the fused silica of at least one of the components (2) surrounding the preform, containing at least one dopant which reduces or increases the viscosity of fused silica, so that the anti-resonance element blank molds (4) consist of fused silica which, at a measurement temperature of 1250°C, has a viscosity at least 0.4 dPa·s higher than that of the fused silica of the sheath tube (3).

2. The method according to claim 1, characterized in that the dopant which reduces the viscosity of fused silica comprises fluorine, chlorine and / or hydroxyl groups, and in that the viscosity-increasing dopant comprises Al2O3 and / or nitrogen.

3. The method according to claim 1 or 2, characterized in that the optional further processing of the primary preform (1) comprises the collapse of additional casing material (2), and in that the additional casing material (2) consists of fused silica containing a dopant which reduces the viscosity of fused silica.

4. The method according to claim 3, characterized in that the fused silica of the additional casing material (2) contains fluorine as a dopant, in a concentration between 500 and 14,500 wt.ppm, preferably between 2,000 and 10,000 wt.ppm.

5. The method according to any of claims 2 to 4, characterized in that the fused silica of the sheath tube (3), at a measurement 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 fused silica of the additional casing material (2).

6. The method according to any of the preceding claims, characterized in that only the additional casing material (2) contains a dopant and consists of fluorine-containing fused silica.

7. The method according to any of the preceding claims, characterized in that at least some of the precursors for anti-resonance elements are in the form of tubular anti-resonance element blank molds (4) which are preferably composed of a plurality of nested structural elements comprising an ARE outer tube (4a) and an ARE inner tube (4b) inserted therein, and in that the anti-resonance element blank molds (4) consist of fused silica which, at a measurement temperature of 1250°C, has a viscosity at least 0.5 dPa·s higher than that of the fused silica of the sheath tube (3).

8. The method according to claim 7, characterized in that the sheath tube (3) consists of fused silica containing a dopant which reduces the viscosity of fused silica.

9. The method according to claim 7 or 8, characterized in that, in the case of nested structural elements, at least some of the ARE inner tubes (4b) consist of fused silica which, at a measurement temperature of 1250°C, has a viscosity which is at least 0.4 dPa·s higher, preferably a viscosity which is at least 0.5 dPa·s higher, than the fused silica of the ARE outer tube (4a).

10. The method according to any of claims 7 to 9, characterized in that the sheath tube (3), the ARE outer tube (4a), the ARE inner tube (4b) and / or an overlay cylinder (2) for collapsing additional casing material are produced using a vertical drawing process without using a form tool.

11. The method according to any of the preceding claims, characterized in that a secondary preform is formed which has an outer diameter in the range of 30 to 90 mm, and / or in that a primary preform is formed which has an outer diameter in the range of 20 mm to 70 mm.

12. The method according to any of the preceding claims, characterized in that the formation of blank molds (4) according to method step (b) comprises arranging the anti-resonance element blank molds (4) at desired positions of the inner face of the sheath tube wall, a positioning template being used for arranging, which template comprises holding elements for positioning the anti-resonance element blank molds at the desired positions.

13. The method according to claim 12, characterized in that a positioning template comprising a shaft that projects into the sheath tube inner bore is inserted, which shaft is provided with holding elements in the form of a plurality of holding arms pointing radially outward.

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

15. A method for producing a preform for an anti-resonant hollow-core fiber which comprises a hollow core extending 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 sheath tube (3) which comprises a sheath tube inner bore and a sheath tube longitudinal axis, along which a sheath tube wall extends, which wall is delimited by an inner face and an outer face, (b) forming a number of precursors or blank molds (4) for anti-resonance elements at desired positions of the sheath tube wall, in order to form a primary preform (1) for the hollow-core fiber, at least some of the precursors for anti-resonance elements being in the form of tubular anti-resonance element blank molds (4) which are preferably composed of a plurality of nested structural elements comprising an ARE outer tube (4a) and an ARE inner tube (4b) inserted therein, and (c) further processing the primary preform (1) to form a secondary preform for the hollow-core fiber, the further processing comprising carrying out one or more of the following hot-forming processes once or repeatedly: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional casing material, (v) collapse of additional casing material and subsequent elongation, (vi) collapse of additional casing material and simultaneous elongation, characterized in that, in carrying out a process according to method step (c), components of the primary preform (1) made of fused silica, and optionally components (2) surrounding the primary preform (1) made of fused silica, are heated and softened together, the fused silica of at least one of the primary preform components (3, 4), and optionally the fused silica of at least one of the components (2) surrounding the preform (3; 4), containing at least one dopant which reduces or increases the viscosity of fused silica, so that the anti-resonance element blank molds (4) consist of fused silica which, at a measurement temperature of 1250°C, has a viscosity at least 0.4 dPa·s higher than that of the fused silica of the sheath tube (3).