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 anti-resonant hollow core fiber, which has a hollow core extending along a fiber longitudinal axis and a cladding region surrounding the hollow core, which comprises a number of anti-resonance elements, with the method steps: (a) Providing a primary preform for the hollow core fiber, which comprises at least one cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall delimited by an inner side and an outer side extends, (b) Forming a number of precursors or preforms for anti-resonance elements at desired positions of the cladding tube wall, and (c) Further processing of the primary preform to form a secondary preform from which the hollow core fiber is drawn, wherein the further processing comprises a single or repeated collapse of additional cladding material in the form of an overlay cylinder, wherein the overlay cylinder has an overlay cylinder inner side facing the cladding tube, an overlay cylinder outer side and an overlay cylinder wall therebetween.

[0002] Furthermore, the invention relates to a method for producing a preform for an anti-resonant hollow core fiber, which has a hollow core extending along a fiber longitudinal axis and a cladding region surrounding the hollow core, which comprises a plurality of anti-resonance elements, with the method steps: (a) Providing a primary preform for the hollow core fiber, which comprises at least one cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall delimited by an inner side and an outer side extends, (b) Forming a number of precursors or preforms for anti-resonance elements at desired positions of the cladding tube wall, and (c) Further processing of the primary preform to form a secondary preform for the hollow core fiber, wherein the further processing comprises a single or repeated collapse of additional cladding material in the form of an overlay cylinder, wherein the overlay cylinder has an overlay cylinder inner side facing the cladding tube, an overlay cylinder outer side and an overlay cylinder wall therebetween.

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

[0004] These disadvantages are avoided or reduced by hollow-core fibers, in which the core contains 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 divided into photonic bandgap fibers and antiresonance reflection fibers.

[0005] In "photonic bandgap fibers," the hollow core region is surrounded by a cladding in which small hollow channels are periodically arranged. The periodic structure of the hollow channels in the cladding causes the effect referred to in semiconductor technology as the "photonic bandgap," whereby light of certain wavelength ranges scattered by the cladding structures experiences constructive interference in the central cavity due to Bragg reflection and cannot propagate transversely in the cladding.

[0006] In the design known as "antiresonant hollow-core fibers" (AR-HCF), the hollow core region is surrounded by an inner cladding region in which so-called "antiresonant elements" (or "antiresonant elements"; abbreviated to "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 (including in the UV or IR wavelength range) and low latency in data transmission.

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

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

[0010] In the paper by Francesco Poletti, "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 antiresonant elements are not formed as a simple, singular structural element, but are composed of multiple, nested structural elements. The nested antiresonant elements are designed such that higher-order core modes are phase-matched to the cladding modes and are suppressed, but not the fundamental core mode. This ensures the propagation of the fundamental core mode at all times, and the hollow-core fiber can be effectively made single-mode over a limited wavelength range.

[0011] The 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] EP 3 136 143 A1 discloses an antiresonant hollow-core fiber (referred to therein as a "hollow-core fiber with no band gap") in which the core can conduct additional modes in addition to the fundamental mode. For this purpose, it is surrounded by an inner cladding 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 in an axis-parallel ensemble and fixed to a preform, which is then elongated. A cladding 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 cladding tube. This preform is drawn into a hollow-core fiber in two stages.

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

[0014] DE 10 2014 011 041 A1 discloses a method for producing an optical flow-measuring hollow fiber in which the light is guided using a double-antiresonant guiding principle. This is intended to prevent the formation of higher-order modes. The central hollow core has a hollow core wall with a polygonal cross-section and is surrounded by air chambers with air clamp walls that are connected to the corners of the polygon. The manufacturing method for the optical flow-measuring hollow fiber comprises stacking capillaries to form a packing and encasing them with an inner cladding tube, pressurized stretching and sintering of the packing to form a first preform, over-cladding the first preform with an outer cladding tube to form a second preform, and pressurized warping of the preform to form the optical flow-measuring hollow fiber.Quartz glass is proposed as the material for the capillary and the jacket tubes, which can be partially or fully doped, with fluorine being one of the dopants mentioned.

[0015] In "Tunable fiber-coupled multiphoton microscopy with a negative curvature fiber*," Journal of Biophotonics, Vol. 9, No. 7, March 15, 2016 (2016-03-15), pages 715-720, ISSN: 1864-063X, Ben Sherlock et al. describe a preform for an antiresonance element hollow-core fiber that features a casing cylinder made of fluorine-doped fused silica. The lower viscosity of the fluorine-doped fused silica allows for a lower pull temperature and better structural retention. Technical task

[0016] Antiresonant hollow-core fibers, especially those with nested structural elements, have complex internal geometries, making their precise and reproducible manufacturing difficult. This is all the more true because, in order to maintain resonance or antiresonance conditions, even small dimensional deviations on the order of magnitude of the operating wavelength of the guided light are intolerable. Deviations from the target geometry can be caused by the configuration of the fiber preform, and they can also occur due to unintentional off-scale deformations during the fiber drawing process.

[0017] With 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 paper mentioned above, six antiresonance element preforms, each consisting of an antiresonance element outer tube (abbreviated to ARE outer tube) and an antiresonance element inner tube (abbreviated to ARE inner tube) welded to one side of the inner surface of the ARE outer tube, must be attached to the inside of a cladding tube.

[0018] To achieve low attenuation values ​​and wide transmission ranges, in addition to a uniform wall thickness of the antiresonance elements, the azimuthal positioning of the antiresonance elements within the cladding tube is also important. This cannot be easily achieved using 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 aim of the invention is to provide a method for producing an anti-resonant hollow core fiber and a preform for anti-resonant hollow core fibers, with which a high precision of the structural elements and an exact positioning of the anti-resonant elements in the fiber can be achieved in a sufficiently stable and reproducible manner. In addition, disadvantages of the classic "stack and draw" technique, with which the required structural accuracies, in particular a uniform wall thickness of the anti-resonant elements and an exact positioning at predetermined azimuthal positions, cannot be easily achieved, are to be avoided as far as possible. Summary of the invention

[0020] With regard to the production of the antiresonant hollow core fiber, this object is achieved according to the invention on the basis of a method of the type mentioned at the outset in that, during the further processing of the primary preform according to method step (c), a trapping cylinder is used which has a radial viscosity profile in which the viscosity in the trapping cylinder wall increases from a viscosity minimum η(Zmin) towards the trapping cylinder inner side.

[0021] 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 cladding tube in which or on which precursors or preforms for forming antiresonant elements in the hollow-core fibers (referred to here briefly as "antiresonant elements") are contained. The primary preform can be elongated to form the hollow-core fiber; however, additional cladding material is usually added to the primary preform to create a preform referred to here as a "secondary preform." If necessary, the hollow-core fiber is created by elongating the secondary preform. Alternatively, the primary preform or the secondary preform are surrounded by one or more overwrapped cylinders to form a coaxial assembly of components, and the coaxial assembly is elongated directly to form the hollow-core fiber.The general term "preform" is understood here to refer to the component or coaxial ensemble of components from which the hollow core fiber is ultimately drawn.

[0022] The addition of sheath material is achieved, for example, by collapsing a casing cylinder onto the primary preform. The coaxial arrangement of primary preform and casing cylinder is either elongated or not elongated when the casing cylinder collapses. In this process, the shape or arrangement of the antiresonance element preforms is either changed or not changed.

[0023] Carrying out one of the hot forming processes mentioned in process step (c) (hereinafter also referred to as "thermal processing") can lead to deformation and structural deviations 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 one another.

[0024] The required processing temperature is generally determined by the component with the largest surface area; this is typically the outer shell area of ​​the preform. Smaller components (such as the anti-resonance element preforms and their individual structural elements) are subject to greater deformation at the same temperature. Since the preform is heated from the outside inward in the heating zone during thermal processing, a radial temperature gradient develops across the preform volume, with a minimum in the preform 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 anti-resonance hollow-core fibers.

[0025] The viscosity of single-component glass, especially quartz glass, can be modified by doping. By using materials with different doping levels and thus adjusted viscosity, structural preservation during thermal processing can be ensured. Doping allows the viscosities of adjacent preform components to be adjusted. It can be used, in particular, to reduce the thermal stability of one component to benefit the stability of a neighboring component.

[0026] However, when using a casing cylinder to overcast a primary preform, a step-like viscosity change can occur at the contact surface between the primary preform and the casing cylinder. These abrupt changes can lead to internal mechanical stresses in the finished (secondary) preform as well as in the final hollow core fiber, which negatively impact both the optical and mechanical properties. These include deviations of the optical parameters from the design properties (refractive index fluctuations due to mechanical stress) and an increasing tendency to fracture.

[0027] The invention specifically aims to enable the use of differently doped starting materials for the overlay cylinder and the cladding tube in order to counteract deformation, particularly of delicate structural elements, during thermal processing, without negatively affecting the optical and mechanical fiber and preform properties. This should achieve greater accuracy in the geometric shape and positioning of the anti-resonance elements in the hollow-core fiber.

[0028] To this end, at least the overlay cylinder, and preferably also the cladding tube, exhibits a dopant concentration gradient toward the common contact surface. In the overlay cylinder, this is achieved by having a radial viscosity profile in which the viscosity in the overlay cylinder wall increases from a viscosity minimum η(Z min ) toward the inside of the overlay cylinder.

[0029] This allows for viscosity differences between the overlay cylinder and the cladding tube to be maintained without creating an excessively large step at the contact surface. This creates a transition zone with an adjusted viscosity at the contact surface, which counteracts deformation and contributes to the structural integrity of the anti-resonance elements.

[0030] The viscosity profile in the overlay cylinder wall increases from a viscosity minimum η(Z min ) towards the overlay cylinder inside.

[0031] A radial viscosity profile with a viscosity minimum in the cylinder wall is achieved, for example, by introducing dopants into the glass of the casing cylinder. These dopants reduce the viscosity of the glass and can at least partially escape upon heating. Fluorine, chlorine, and / or hydroxyl groups are preferably used here. The casing cylinder is therefore preferably made of quartz glass, with the viscosity profile of the casing cylinder being generated by at least one dopant that reduces the viscosity of quartz glass, the dopant preferably containing fluorine, chlorine, and / or hydroxyl groups.

[0032] In a preferred procedure, it is provided that the viscosity in the area of ​​the inside of the casing cylinder has a viscosity value η(Z), and that the cladding tube in the area of ​​the outside of the cladding tube has a viscosity value η(M), whereby at a measuring temperature of 1250 °C the following applies to the viscosity values ​​(when the viscosity is specified as a logarithmic value in dPa s): η(M) = η(Z) ±0.5 dPa s, preferably η(M) = η(Z) ±0.3 dPa s.

[0033] The viscosity value η(M) at the outer surface of the cladding tube lies within the narrowest possible range around the viscosity value η(Z) of the overlay cylinder in the area of ​​its inner surface. Preferably, η(M) is also greater than η(Z).

[0034] It has proven particularly advantageous if the quartz glass of the casing cylinder contains fluorine as a dopant in a concentration between 500 and 8000 ppm by weight.

[0035] It has also proven advantageous if there is a large difference between the viscosity minimum and the viscosity of the jacket tube in the contact area, as this represents a measure of the otherwise desired viscosity difference between the cladding tube and the casing cylinder (with the exception of the contact area). Thus, the viscosity minimum η(Z min ) at a measurement temperature of 1250 °C (when viscosity is specified as a logarithmic value) is preferably: η(M) - η(Z min ) > 0.8 dPa s, preferably > 1 dPa s.

[0036] In another preferred method variant, the cladding tube has a radial viscosity profile between its cladding tube outer side and its cladding tube inner side, in which the viscosity gradually increases towards the inner side.

[0037] To adjust the viscosity gradient within the cladding tube wall, it has proven effective if the cladding tube is made of quartz glass and fluorine is used as the dopant in a concentration between 500 and 8000 wt. ppm.

[0038] In this context, it proves to be advantageous if the cladding tube has a viscosity profile with a viscosity maximum η(M max ) and that at least some of the precursors for anti-resonance elements are present as tubular anti-resonance element preforms, which are preferably composed of several nested structural elements, comprising an ARE outer tube and an ARE inner tube inserted therein, and that the anti-resonance element preforms consist of quartz glass which, at a measuring temperature of 1250 °C, has a viscosity that is at least 0.4 dPas higher than the maximum viscosity η(M max ) of the quartz glass of the cladding tube, preferably a viscosity that is at least 0.5 dPas higher (when the viscosity is specified as a logarithmic value).

[0039] The casing cylinder and cladding tube are preferably made of quartz glass. Fluorine doping of the additional cladding material provided by the casing cylinder enables a significant reduction in viscosity compared to the quartz glass of the cladding tube, even if the quartz glass of the cladding tube itself does not contain any dopant. It has proven advantageous if the quartz glass of the cladding 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 additional cladding material at a measurement temperature of 1250 °C. The viscosity differences are given here and below as logarithmic viscosity values ​​in dPa s.

[0040] In an advantageous process, all preform components consist of different quartz glass grades, with the viscosity of the quartz glass grades increasing, as a first approximation, from the outside to the inside. Besides fluorine, other dopants such as Al 2 O 3 , nitrogen, chlorine, and hydroxyl groups can be used to adjust the viscosity. In the simplest case, however, it is sufficient if only the additional cladding material consists of fluorine-containing quartz glass.

[0041] In a further preferred procedure, at least some of the precursors for anti-resonance elements are in the form of tubular anti-resonance element preforms, which are preferably composed of several interleaved structural elements, comprising an ARE outer tube and an ARE inner tube inserted therein, wherein the anti-resonance element preforms are made of quartz glass which, at a measuring temperature of 1250 °C, has a viscosity that is at least 0.4 dPa s higher than the quartz glass of the cladding tube, preferably at least 0.5 dPa s higher.

[0042] The quartz glass of the ARE outer tube can contain a viscosity-increasing dopant, such as nitrogen or Al 2 O 3. However, it has proven particularly advantageous if the cladding tube is made of quartz glass containing a viscosity-reducing dopant.

[0043] With regard to a high thermal stability of the ARE inner tubes in the case of nested structural elements, it has proven useful if at least part of the ARE inner tubes, preferably all ARE inner tubes, are made of quartz glass, which 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 at a measuring temperature of 1250 °C.

[0044] The cladding tube is preferably manufactured using a vertical drawing process without a forming tool and a two-stage elongation process. In the first stage, a starting hollow cylinder made of glass is mechanically machined to set the final dimensions of the starting hollow cylinder. In a first elongation process, the starting cylinder is continuously fed to a heating zone with a first heating zone length, softened in certain areas therein, and an intermediate cylinder is drawn from the softened area. In a second elongation process, with a vertically oriented longitudinal axis, this intermediate cylinder is continuously fed to another heating zone with a second, shorter heating zone length, softened in certain areas therein, and a tube strand is drawn from the softened area. The cladding tube is obtained from the tube strand by cutting it to length.

[0045] By using preform components with low viscosity, especially in the outer shell area, the method according to the invention enables the use of comparatively large preforms for thermal processing.

[0046] In this regard, it is preferred to form a secondary preform having an outer diameter in the range of 30 to 90 mm and / or to form a primary preform having an outer diameter in the range of 20 mm to 70 mm.

[0047] The outer diameter in the range of 30 to 90 mm is large compared to the current state of the art. Since the existing absolute geometric error during fiber drawing is downscaled more significantly with increasing outer diameter of the preform, the use of larger preforms also fundamentally enables more precise production of the hollow core fiber. For diameters larger than 90 mm, however, temperature gradients form within the preform volume during the fiber drawing process, which can result in deviations in the wall thickness of the anti-resonance elements in the hollow core fiber. For preform outer diameters of less than 30 mm, there is no longer any significant contribution from downscaling the geometric error. Furthermore, it is advantageous to form 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 state of the art, the outer diameters of the primary preforms (canes) are typically 4 to 6 mm.

[0048] In a preferred method variant, the formation of preforms for anti-resonance elements according to method step (b) comprises arranging the anti-resonance element preforms at desired positions on the inside of the cladding tube wall, wherein a positioning template is used for arranging which has holding elements for positioning the anti-resonance element preforms at the desired positions.

[0049] The positioning template has, for example, a shaft projecting into the inner bore of the cladding tube, which is provided with holding elements in the form of several radially outward-pointing holding arms.

[0050] The design-specific star-shaped arrangement of the retaining elements facilitates the precise positioning of the anti-resonance element preforms at the respective target positions and their fixation. The positioning template is preferably used exclusively in the area of ​​the cladding tube end faces, preferably in the area of ​​both cladding tube end faces.

[0051] The accuracy of positioning the preforms on the inner surface of the cladding tube is improved by machining the inner surface of the cladding tube, in particular by drilling, milling, grinding, honing and / or polishing.

[0052] With regard to the production of the preform for the hollow core fiber, the above-mentioned technical problem is solved according to the invention on the basis of a method of the type mentioned at the outset in that, during the further processing of the primary preform according to method step (c), an overlay cylinder is used which has a radial viscosity profile in which the viscosity in the overlay cylinder wall increases from a viscosity minimum η(Z min ) towards the inside of the overlay cylinder.

[0053] The preform is the starting point for the production of the antiresonant hollow-core fiber. By elongating the preform, either the antiresonant hollow-core fiber is drawn directly or a semi-finished product is first created, from which the antiresonant hollow-core fiber is subsequently drawn. Preform production involves collapsing additional cladding material onto the primary preform using a trapping cylinder with a radial viscosity profile, with viscosity increasing toward the inside of the trapping cylinder.

[0054] This allows for viscosity differences between the overlay cylinder and the cladding tube to be accommodated without excessively large steps occurring at the contact surface. Measures for producing the preform are explained above in connection with the production of the hollow core fiber, and these explanations are incorporated here. Definitions

[0055] Individual process steps and terms from the above description are defined in more detail below. These definitions are part of the description of the invention. In the event of a factual contradiction between one of the following definitions and the rest of the description, the statement in the description shall prevail. Anti-resonance elements

[0056] 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 (flat, straight). They are typically made of a material that is transparent to the working light, for example, glass, particularly doped or undoped SiO2, a plastic, particularly a polymer, a composite material, or a crystalline material. Anti-resonance element preform / anti-resonance element precursor

[0057] Anti-resonance element preforms are components or parts of the preform that essentially become anti-resonance elements in the hollow core fiber through simple elongation during the fiber drawing process. Anti-resonance element precursors are components or parts of the preform that only become anti-resonance element preforms through forming or directly become anti-resonance elements. The anti-resonance element preforms can be simple or nested components, to which additional positioning aids can be attached. They are originally present in the primary preform (cane).

[0058] Further processing of the primary preform, particularly through hot forming steps, can produce intermediate products in which the original anti-resonance element preforms are present in a modified form compared to the original. This modified form is also referred to here as an anti-resonance element preform or anti-resonance element precursor. Preform / primary preform / secondary preform

[0059] The preform is the component from which the antiresonant hollow-core fiber is drawn. It is a primary preform or a secondary preform created 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 and subsequent elongation, (vi) collapse of additional mantle material and simultaneous elongation. Elongation / Collapse

[0060] During elongation, the primary preform is elongated. Elongation 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. During elongation, the primary preform can also be drawn out of scale, and its geometry can be altered.

[0061] Collapse involves narrowing an internal bore or closing or narrowing annular gaps between tubular components. Collapse is usually accompanied by elongation. Hollow core / Inner shell area / Outer shell area / Cane

[0062] The assembly consisting of at least one cladding tube and loosely or firmly fixed preforms or precursors for anti-resonance elements is also referred to here as the "primary preform" or "cane." The primary preform (cane) comprises the hollow core and a cladding region. This cladding region is also referred to as the "inner cladding region" if there is also an "outer cladding region," created, for example, by collapsing onto the cane, and if a distinction is to be made between these cladding regions. The terms "inner cladding region" and "outer cladding region" are also used for the corresponding regions in the hollow core fiber or in intermediate products obtained by further processing the primary preform.

[0063] The term "pipe inside" is also used as a synonym for "pipe inner surface," and the term "pipe outside" is also used as a synonym for "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

[0064] This refers to mechanical manufacturing processes for the separation of a workpiece, in particular turning, cutting, drilling, sawing, milling, and grinding. This machining creates a longitudinal structure extending along the longitudinal axis of the cladding tube, which serves as a positioning aid for the anti-resonance element preforms. The longitudinal structure is accessible from the inside of the cladding tube; it can also extend through the entire cladding tube wall to the outside. Particle size and particle size distribution

[0065] The particle size and particle size distribution of the SiO2 particles are characterized using the D50 values. These values ​​are taken from particle size distribution curves, which show the cumulative volume of the SiO2 particles as a function of the particle size. The particle size distributions are often characterized using the respective D10, D50, and D90 values. The D10 value indicates the particle size not reached by 10% of the cumulative volume of the SiO2 particles, and accordingly the D50 and D90 values ​​indicate 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

[0066] The invention is explained in more detail below with reference to an embodiment and a drawing. In detail, a schematic representation figure 1 shows a coaxial tube arrangement consisting of a casing cylinder and a primary preform, which is composed of a cladding tube and anti-resonance element preforms positioned and fixed therein, based on a view of the radial cross-section, Figure 2 shows a diagram showing the radial progression of the fluorine concentration and viscosity in the casing cylinder and the cladding tube, and Figure 3 shows a sketch to explain an ideal radial concentration or viscosity profile of a preform for a hollow core fiber.

[0067] During the production of the hollow core fiber or the preform for the hollow core fiber, a large number of components must be joined together. In addition, it can be helpful to seal existing gaps or channels in the preform during hot forming processes. For joining or sealing, an SiO 2 -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 grinding quartz glass grains. This slurry contains amorphous SiO 2 particles with a particle size distribution characterized by a D 50 value of approximately 5 µm and a D 90 value of approximately 23 µm. Additional amorphous SiO 2 grains with an average grain size of approximately 5 µm are mixed into the base slurry. The slip used as a bonding compound has a solids content of 90%, which consists of at least 99.9 wt.% SiO2.

[0068] Figure 1shows schematically the coaxial tube arrangement 1 with a casing cylinder 2, a cladding tube 3 with a cladding tube wall, on whose inner surface anti-resonance element preforms 4 are fixed at previously defined azimuthal positions at equal intervals; in the exemplary embodiment, there are six preforms 4, in another preferred embodiment not shown, there is an odd number of preforms.

[0069] The cladding tube 3 has an outer diameter of 27 mm and an inner diameter of 20 mm. The antiresonance element preforms 4 are 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 at 0.3 mm. All tubular components 2, 3, 4a, 4b have a length of 700 mm.

[0070] The anti-resonance element preforms 4 are fixed to the inner wall of the cladding tube 3 using the SiO 2 -based bonding compound. The bonding compound is applied locally to the cladding tube's inner surface in the area of ​​the front ends, and the anti-resonance element preforms 4 are placed thereon using a positioning template with a structurally predetermined star-shaped arrangement of holding arms for the individual anti-resonance element preforms 4. The effect of the positioning template is limited to the area around the two front ends of the cladding tube. This method creates a precise and reproducible connection between the cladding tube 3 and the anti-resonance element preforms 4. Solidification of the bonding compound at a low temperature below 300 °C is sufficient for fixing, thus avoiding excessive heating of the surrounding areas and thus deformation of the anti-resonance element preforms 4.

[0071] The resulting primary preform is overclad with the quartz glass overlay cylinder 2. The overlay cylinder 2 has an outer diameter of 63.4 mm and a wall thickness of 17 mm. As the overlay cylinder 2 collapses onto the cladding tube 3, the coaxial tube arrangement is simultaneously elongated. For this purpose, the coaxial arrangement of cladding tube 3 and overlay cylinder 2 is fed from below into a temperature-controlled heating zone with its longitudinal axis oriented vertically. There, it is softened zone by zone, starting with the upper end of the arrangement. The heating zone is maintained at a target temperature of 1580 °C with a control accuracy of + / - 0.1 °C. This allows temperature fluctuations during the hot forming process to be limited to less than + / - 0.5 °C.

[0072] The secondary preform formed in the collapsing and elongation process has an outer diameter of approximately 50 mm and a cladding wall thickness of 16.6 mm, consisting of the outer cladding and inner cladding. 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 upward during the fiber drawing process. This end face is connected to a quartz glass holding tube, which also serves as a gas connection. The holder is fixed to the overlay cylinder 2 and the cladding tube 3 using the sealing or bonding compound.

[0073] During the fiber drawing process, the secondary preform is fed from above into a temperature-controlled heating zone with its longitudinal axis oriented vertically. It is softened zone by zone, starting at the bottom. At the same time, gas is introduced into 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 allows temperature fluctuations during the hot forming process to be limited to less than + / - 0.5 °C.

[0074] By drawing the preform into a hollow core fiber, the existing absolute geometric error is scaled down so that in the hollow core fiber, the antiresonance elements obtained from the antiresonance element preforms have a maximum deviation of less than 3.5% in wall thickness (relative to an average wall thickness).

[0075] The small error in wall thickness is attributed, on the one hand, to the use of the comparatively large secondary preform and the associated downscaling of the original absolute geometric deviations, and, on the other hand, to the comparatively low processing temperatures during the hot forming processes (elongation and collapse, fiber drawing). The lower processing temperatures are, in turn, due to the fact that the overlay cylinder 2 and the cladding tube 3 are made of fluorine-doped quartz glass. These components represent the largest surface areas in the coaxial arrangement 1 and significantly determine the processing temperature.By doping the components of the secondary preform with fluorine with the largest surface area, the necessary processing temperature can be lowered and thus the relative stiffness and thermal stability of the anti-resonance element preforms 4 located further inside can be indirectly improved by exposing them to a lower temperature during the hot forming process.

[0076] The following Table 1 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 Overlay cylinder Fluorine-doped quartz glass 10,000 ppm wt. 3 cladding tube Fluorine-doped quartz glass 2,700 ppm wt. 4a ARE outer pipe undoped quartz glass 4b ARE inner tube undoped quartz glass

[0077] The fluorine-doped quartz glass tubes (2; 3) exhibit a fluorine concentration profile with a maximum fluorine concentration in the center of the tube wall. The data on the fluorine concentration of the quartz glass listed in the "Material" column of Table 1 are average values.

[0078] The diagram of Figure 2 shows measured fluorine concentration profiles C (in ppm by weight) for a cladding tube CF (M) and for a casing cylinder CF (Z)", as well as viscosity profiles η (in 1g dPa s) calculated from the concentration profiles for a temperature of 1250 °C along the radial spatial coordinate (position P (in mm)).

[0079] The fluorine concentration profile in quartz glass 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 quartz glass (η = 11.8 dPa s (corresponding to 100%)) using the following formula: Viscosity decrease at 1250 °C: 12% (±2%) per wt.% fluorine.

[0080] Table 2 shows viscosity values ​​for fluorine contents of commercially available quartz glass grades (for a measuring 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

[0081] The diagram of Figure 2 shows that the viscosity of the casing cylinder η(Z) is lower than that of the cladding tube η(M). For both quartz glass tubes, the viscosity has a minimum in the center of the tube, which is approximately 10 11.45< dPa s for the cladding tube and approximately 10 10.65< dPa s for the casing cylinder. The viscosity difference between the minima (in 1 g dPa s) is thus approximately 0.80 dPa s. The difference between the viscosity of the cladding tube in the area of ​​the cladding tube exterior (approximately 10 11.5< dPa s) and the viscosity minimum for the casing cylinder is approximately 0.85 (in 1 g dPa s).

[0082] In the preform, the outer surface of the cladding tube and the inner surface of the overlay cylinder form a common contact surface. The local position of the contact surface, transferred to the viscosity profiles, is indicated in the diagram by the two rectangles "K." At these positions, the following values ​​result for the viscosities of the overlay cylinder and cladding tube: Sheath tube: about 11.5 Ig(dPa·s) Overlay cylinder: about 11.15 Ig(dPa·s)

[0083] The viscosity difference in the area of ​​the contact surface is therefore approximately 0.35 (in 1g dPa.s).

[0084] The structural elements (4a; 4b) of the antiresonance element preforms (4) consist of undoped quartz glass and have a viscosity of approximately 10 11.8< dPa s.

[0085] The diagram of Figure 3shows the radial dopant concentration profile across the wall of the secondary preform in 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 casing cylinder is ideally as high as the concentration CF (M) of the fluorine-doped quartz glass originating from the cladding tube. The corresponding viscosity profile of the cladding tube and the casing 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-resonant elements, comprising the method steps of: (a) providing a primary preform (1) for the hollow-core fiber, which preform has at least one sheath (3) comprising a sheath inner bore and a sheath longitudinal axis along which a sheath wall delimited by an inner side and an outer side extends, (b) forming a number of precursors or preforms (4) for anti-resonance elements at desired positions of the sheath wall, and (c) further processing the primary preform (1) to form a secondary preform from which the hollow-core fiber is drawn, the further processing comprising a single or repeated collapse of additional casing material in the form of an overlay cylinder (2), the overlay cylinder (2) comprising an overlay cylinder inner side facing the sheath (3), an overlay cylinder outer side and an overlay cylinder wall therebetween, characterized in that during the further processing of the primary preform (1) according to method step (c), an overlay cylinder (2) is used which has a radial viscosity profile in which the viscosity in the overlay cylinder wall increases from a viscosity minimum η(Zmin) toward the inner side of the overlay cylinder.

2. The method according to claim 1, characterized in that the viscosity in the region of the inner side of the overlay cylinder has a viscosity value η(Z), and in that the sheath has, in the region of the sheath outer side, a viscosity value η(M), where, at a measuring temperature of 1250°C, the following applies to the viscosity values (if the viscosity is given as a logarithmic value in dPa·s): η(M) = η(Z) ±0.5 dPa·s, preferably η(M) = η(Z) ±0.3 dPa s.

3. The method according to claim 1, characterized in that for the viscosity minimum η(Zmin), at a measuring temperature of 1250°C, the following applies (if the viscosity is given as a logarithmic value): η(M) - η(Zmin) > 0.8 dPa s, preferably > 1 dPa s.

4. The method according to any of the preceding claims, characterized in that the sheath (3) has, between its sheath outer side and its sheath inner side, a radial viscosity profile, in which the viscosity gradually increases toward the inner side.

5. The method according to claim 4, characterized in that the sheath (3) has a viscosity profile with a viscosity maximum η(Mmax), and in that at least some of the precursors for anti-resonance elements are in the form of tubular anti-resonance element preforms (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 antiresonance element preforms (4) are made of quartz glass, which, at a measuring temperature of 1250°C, has a viscosity that is at least 0.4 dPa·s higher, preferably at least 0.5 dPa·s higher (when the viscosity is specified as a logarithmic value), than the maximum viscosity η(Mmax) of the quartz glass of the sheath (3).

6. The method according to any of the preceding claims, characterized in that the overlay cylinder (2) consists of quartz glass, and in that the viscosity profile of the overlay cylinder (2) is produced by at least one dopant which reduces the viscosity of quartz glass, the dopant preferably containing fluorine, chlorine, and / or hydroxyl groups.

7. The method according to any of the preceding claims, characterized in that the sheath (3) consists of quartz glass, and in that the viscosity profile of the sheath (3) is produced by at least one dopant which reduces the viscosity of quartz glass, the dopant preferably containing fluorine, chlorine, and / or hydroxyl groups.

8. The method according to claim 6 and claim 7, characterized in that the quartz glass of the overlay cylinder (2) and the sheath (3) contains fluorine as a dopant in a concentration between 500 and 8000 ppm by weight.

9. The method according to any of the preceding claims, characterized in that the sheath (3) is produced in a vertical drawing process without a mold with a two-stage elongation process, where in the first stage a starting hollow cylinder made of glass is mechanically processed to set the final dimensions of the starting hollow cylinder, the starting cylinder is continuously fed to a first heating zone with a first heating zone length in a first elongation process with a vertically oriented longitudinal axis, softened therein in regions, and an intermediate cylinder is drawn off from the softened region, the intermediate cylinder is continuously fed to a second heating zone with a second, shorter heating zone length in a second elongation process with a vertically oriented longitudinal axis, softened therein in regions, and a tube strand is drawn off from the softened region, and the sheath (3) is obtained from the tube strand by cutting to length.

10. 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 (1) is formed which has an outer diameter in the range of 20 mm to 70 mm.

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

12. A method for producing a preform for an anti-resonant hollow-core fiber which comprises a hollow core extending along a longitudinal axis of the fiber and an inner casing region that surrounds the hollow core and comprises a plurality of anti-resonant elements, comprising the method steps of: (a) providing a primary preform (1) for the hollow-core fiber, which preform has at least one sheath (3) comprising a sheath inner bore and a sheath longitudinal axis along which a sheath wall delimited by an inner side and an outer side extends, (b) forming a number of precursors or preforms (4) for anti-resonance elements at desired positions of the sheath wall, and (c) further processing the primary preform (1) to form a secondary preform for the hollow-core fiber, the further processing comprising a single or repeated collapse of additional casing material in the form of an overlay cylinder (2), the overlay cylinder (2) comprising an overlay cylinder inner side facing the sheath (3), an overlay cylinder outer side and an overlay cylinder wall therebetween, characterized in that during the further processing of the primary preform (1) according to method step (c), an overlay cylinder (2) is used which has a radial viscosity profile in which the viscosity in the overlay cylinder wall increases from a viscosity minimum η(Zmin) toward the inner side of the overlay cylinder.