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 cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall bounded by an inner side and an outer side extends, (b) Providing a number of tubular anti-resonance element preforms, (c) Arranging the anti-resonance element preforms at desired positions on the inner side of the cladding tube wall to form a primary preform having a hollow core region and an inner cladding region, (d) Further processing of the primary preform into a secondary preform from which the hollow core fiber is drawn, wherein the further processing comprises elongation and optionally a single or repeated execution of one or more of the following hot forming processes: (i) collapsing, (ii) collapsing and simultaneous elongation, (iii) collapsing additional cladding material, (iv) collapsing additional cladding material and subsequent elongation,(v) collapsing additional sheath material and simultaneously elongating it, and (e) drawing the secondary preform to the hollow core fiber, wherein after elongation according to process step (d), at least a part of the former tubular anti-resonance element preforms of the primary preform has an oval outer cross-sectional shape, with a longest cross-sectional axis A L and a shortest cross-sectional axis A K , where the axial length ratio A L / A K in the range between 1.01 and 1.27, and where the shortest cross-sectional axis A K runs in a radial direction as seen from the cladding tube's longitudinal axis.

[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 cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall bounded by an inner side and an outer side extends, (b) Providing a number of tubular anti-resonance element preforms, (c) Arranging the anti-resonance element preforms at desired positions on the inner side of the cladding tube wall to form a primary preform having a hollow core region and an inner cladding region, and (d) Further processing the primary preform into a secondary preform for the hollow core fiber, wherein the further processing comprises elongation and optionally a single or repeated performance of one or more of the following hot forming processes: (i) Collapsing, (ii) Collapsing and simultaneous elongation, (iii) Collapsing additional cladding material, (iv) Collapsing additional cladding material and subsequent elongation,(v) Collapse of additional mantle material and simultaneous elongation, , wherein after elongation according to process step (d), at least a part of the former tubular anti-resonance element preforms of the primary preform has an oval outer cross-sectional shape, with a longest cross-sectional axis A L and a shortest cross-sectional axis A K , where the axial length ratio A L / A K in the range between 1.01 and 1.27, and where the shortest cross-sectional axis A K runs in a radial direction as seen from the cladding tube's longitudinal axis.

[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 a 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] Gregory T. Jasion et al., "Fabrication of tubular anti-resonant hollow core fibers: modeling, 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 anti-resonant hollow core fiber having a hollow core extending along a fiber's longitudinal axis and an inner cladding region surrounding the hollow core, which contains a plurality of anti-resonant elements. The method comprises the following steps: providing a cladding tube with an outer diameter in the range of 20 to 30 mm; arranging a number of tubular anti-resonant element preforms at desired positions on the inside of the cladding tube wall to form a primary preform; and further processing the primary preform. Further processing involves elongation to a secondary preform (cane) with an outer diameter of approximately 3 mm, from which the hollow core fiber is drawn.In the secondary preform, the anti-resonance element preforms have an oval outer cross-sectional shape, with the shortest cross-sectional axis running radially. During the fiber drawing process, the secondary preform (cane) is covered by an outer jacket tube with an outer diameter in the range of 10 to 30 mm.

[0015] Nawazuddin et al.: "Lotus Shaped Negative Curvature Hollow Core Fiber with 10.5 dB / km at 1550 nm Wavelength", 2017 EUROPEAN CONFERENCE ON OPTICAL COMMUNICATION (ECOC), IEEE, September 17, 2017, pages 1-3, DOI: 10.1109 / ECOC.2017.8346101; also discloses a method for producing an antiresonant hollow core fiber, which has a hollow core extending along a fiber's longitudinal axis and an inner cladding region surrounding the hollow core, which contains several antiresonant elements. A primary preform is produced with a cladding tube, on the inside of which six large ARE outer tubes and six small capillaries are arranged as spacer elements. A secondary preform (cane) is drawn from the primary preform, after which the ARE outer tubes and the capillaries have an oval cross-section.

[0016] Fei Yu: "Hollow core negative curvature fibers"; in: "University of Bath PhD.", (2013-12-31), pages 59-74, discloses a method for producing an antiresonant hollow-core fiber having a hollow core extending along a fiber's longitudinal axis and an inner cladding region surrounding the hollow core, which contains a plurality of antiresonance elements. The production of the hollow-core fiber comprises the following process steps: providing a cladding tube; arranging a number of tubular antiresonance element preforms at desired positions on the inside of the cladding tube wall to form a primary preform; and further processing the primary preform, including elongation to form a secondary preform (cane) from which the hollow-core fiber is drawn. In the secondary preform, the antiresonance element preforms have a non-circular cross-section.During the subsequent drawing of the hollow core fiber, different areas of the secondary preform are subjected to different pressures. Technical task

[0017] 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.

[0018] 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.

[0019] 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.

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

[0021] 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-resonance elements and exact positioning at specified azimuthal positions, are not easy to achieve, should be avoided as far as possible. Summary of the invention

[0022] With regard to the method for producing 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 elongation the primary preform is continuously fed to a heating zone at a feed rate, softened zone by zone in the heating zone, and withdrawn from the heating zone at a withdrawal rate, wherein the feed rate is adjusted so that a throughput of at least 0.8 g / min and an average residence time in the heating zone in the range of 5 to 25 min result, and wherein a temperature-controlled heating zone is used whose target temperature is maintained to an accuracy of + / - 0.1 °C.

[0023] The starting point for the production of the antiresonant hollow-core fiber is a preform, also referred to here as the "primary preform." The production of the primary preform typically involves the installation and connection of antiresonant element preforms to a cladding tube to form a so-called "cane." 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 the "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.

[0024] The addition of sheath material is achieved, for example, by collapsing a casing cylinder onto the primary preform. The coaxial assembly 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.

[0025] The production of the secondary preform comprises a number of process steps in which starting elements of the hollow core fiber are manufactured and positioned relative to one another, and at least one hot-forming step. Each of the starting elements exhibits a certain deviation from its target geometry, and each positioning and forming step inevitably leads to geometric deviations that add up to an absolute geometric error in the finished preform. In particular, the hot forming of glass can lead to unwanted and irreproducible deformation due to the slightest deviations from an ideal, usually cylindrically symmetrical temperature profile of the heating zone. Such deformations can also be due to pressure and temperature conditions during the elongation or fiber drawing process, which are difficult to circumvent or avoid.

[0026] The method according to the invention aims to compensate for the deforming effect of such pressure and temperature conditions during the elongation and fiber drawing process.

[0027] For this purpose, it is provided that the elongation of the primary preform is carried out in such a way that, after completion of the elongation step, at least a part of the former tubular and round-section anti-resonance element preforms of the primary preform have an oval outer cross-sectional shape.

[0028] During the elongation of the primary preform, additional sheath material is applied, or no additional sheath material is applied. The result of the elongation step is a secondary preform or a different semi-finished product, which may require further processing to create the secondary preform.

[0029] It has proven advantageous if most – preferably all – of the originally round structural elements of the former primary preform are geometrically modified during the elongation process so that they have a non-round, oval, and ideally elliptical radial cross-sectional shape after the elongation process. This is because a further geometric change occurs in the subsequent fiber drawing process, transforming cavities with an oval radial cross-section into cavities with a round radial cross-section.

[0030] It was found that a favorable degree of ovality in the cross-sectional shape of the elongated structural elements for this compensation can be described by the lengths of their longest and shortest axes (in the cross-section). The ratio of the longest cross-sectional axis AL in the cross-section and the shortest cross-sectional axis AK in the cross-section ranges between 1.01 and 1.27. The orientation of the ovals on the inner surface of the former cladding tube is important: namely, such that the shortest cross-sectional axis AK runs in the radial direction, and the longest cross-sectional axis AL runs tangentially to the former inner surface.

[0031] The antiresonance element preforms can be composed of a single structural element, such as a simple tube element, or of multiple structural elements. In the case of multiple structural elements, these are, for example, nested together and, in the simplest case, form an ARE outer tube into which an ARE inner tube is inserted.

[0032] During elongation, the primary preform is continuously fed into a heating zone at a feed rate, softened zone by zone in the heating zone, and withdrawn from the heating zone at a withdrawal rate.

[0033] Excessively high feed rates can lead to temperature gradients in the primary preform, which can cause the anti-resonance element preforms distributed at different radial positions to exhibit different elongation behavior. Excessively low feed rates can lead to undesirable deformation of the anti-resonance element preforms.

[0034] A suitable compromise has been found to be if the feed rate is adjusted so that a throughput of at least 0.8 g / min and an average residence time in the heating zone in the range of 5 to 25 min is achieved.

[0035] The temperature of the heating zone during the hot forming process should be as constant as possible. Therefore, a temperature-controlled heating element is used in the hot forming process according to process step (d), whose target temperature is maintained to within + / - 0.1 °C.

[0036] This allows temperature fluctuations in the hot forming process to be limited to less than + / - 0.5°C.

[0037] In a particularly preferred procedure relating to anti-resonance element preforms made of a simple tubular structural element, the tubular elements have an oval, preferably elliptical, outer cross-sectional shape with the axial length ratio AL / AK in the range between 1.07 and 1.27.

[0038] Another particularly preferred procedure relates to anti-resonance element preforms which are composed of several nested structural elements and comprise at least one ARE outer tube and one ARE inner tube running in the ARE outer tube and parallel to the ARE outer tube longitudinal axis.

[0039] In such anti-resonance element preforms, the ARE inner tube is fixed to the inside of the ARE outer tube wall. If the contact area between the ARE inner and outer tubes is too small, this can lead to detachment and decentering of the ARE inner tube within the ARE outer tube, also due to surface tension during a hot-forming process. To counteract this, it has proven advantageous for the ARE outer tubes to have an oval, preferably elliptical, outer cross-sectional shape with an axial length ratio AL / AK in the range between 1.07 and 1.27, and for the ARE inner tubes to have an oval, preferably elliptical, outer cross-sectional shape with an axial length ratio AL / AK in the range between 1.01 and 1.05.

[0040] The comparatively large ovality of the ARE outer tubes results in an increase in the contact area with the ARE inner tubes.

[0041] The primary preform preferably has an outer diameter in the range of 20 to 70 mm before elongation.

[0042] The larger the diameter, the slower the tracking speed during elongation and the longer each axial section of the preform is exposed to the high temperature of the heating zone. However, if the tracking speed during elongation is too slow, the structural elements of the anti-resonance element preforms will deform. Therefore, the diameter of the primary preform is preferably a maximum of 70 mm. And the diameter of the primary preform is preferably at least 20 mm. It has been shown that with smaller diameters, the thermal inertia of the preform is too low to compensate for any temperature fluctuations in the heating zone.

[0043] If the feed speed is too high, temperature gradients can occur in the primary preform, which can lead to different elongation behavior of the anti-resonance element preforms distributed at different radial positions. A feed speed that is too low can lead to undesirable deformation of the anti-resonance element preforms. Adjusting the feed speed to achieve a throughput in the range of 3.3 g / min to 85 g / min has proven to be a suitable compromise.

[0044] To reduce absolute geometric errors, a high draw ratio during elongation is desirable. On the other hand, a high draw ratio is associated with correspondingly large forming processes and material movements, which can easily lead to undesirable deformations in the delicate structural elements of the anti-resonance element preforms.

[0045] A suitable compromise has proven advantageous if the draw ratio during elongation is set to a value in the range of 1.05 to 10, preferably to a value in the range of 1.05 to 5.

[0046] In a preferred method variant, the arrangement of the anti-resonance element preforms and / or the elongation of the primary preform, and / or the drawing of the hollow core fiber comprises a fixing measure and / or a sealing measure using a sealing or bonding compound containing amorphous SiO 2 particles.

[0047] The sealing or bonding compound used for sealing or fixing contains amorphous SiO2 particles, which are suspended, for example, in a dispersion liquid. This compound is applied between the surfaces to be bonded or sealed and is usually pasty during use. Upon drying at low temperature, the dispersion liquid is partially or completely removed and the compound solidifies. The sealing or bonding compound, and in particular the solidified SiO2-containing sealing or bonding compound obtained after drying, meets the requirements for fixing and compaction. The temperature required for drying is below 300°C, which promotes compliance with the dimensional stability of the preform and avoids thermal impairments.Heating to higher temperatures around 800 °C, for example, during elongation of the preform into a hollow-core fiber, leads to further thermal strengthening of the sealing or bonding compound, which is also suitable for forming opaque or transparent glass. This occurs through sintering or vitrification, whereby sintering to opaque glass requires comparatively lower temperatures and / or shorter heating times than vitrification to complete transparency. The sealing or bonding compound can thus be fully densified by heating and vitrified by heating during the hot-forming process.

[0048] During the hot-forming process, the sealing or bonding compound does not decompose and releases few impurities. It is thus characterized by thermal stability and purity during the hot-forming process and avoids deformation due to different thermal expansion coefficients.

[0049] The sealing and bonding compound can also be used advantageously to wear open ends of the anti-resonance element preforms and / or individual structural elements of the anti-resonance element preforms and / or any annular gap between tube elements during elongation of the primary preform and / or during drawing of the hollow core fiber.

[0050] In this way, the individual components of the primary preform and / or secondary preform can be exposed to different internal pressures during the elongation or fiber drawing process.

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

[0052] Compared to other known forming techniques using heat and pressure, these processing techniques produce more precise and delicate structures and avoid surface contamination by forming tools such as nozzles, presses or melt molds.

[0053] The machining preferably also includes structuring the cladding tube interior in the region of the target positions of the anti-resonance element preforms by providing them with a longitudinal structure extending in the direction of the cladding tube's longitudinal axis. This longitudinal structure comprises, for example, longitudinal slots and / or longitudinal grooves in the cladding tube's interior wall, which run parallel to the cladding tube's longitudinal axis and are preferably created by drilling, sawing, milling, cutting, or grinding.

[0054] The longitudinal structure extending along the cladding tube's longitudinal axis serves as a positioning aid for the anti-resonance element preforms. It facilitates the anti-resonance element preforms' adoption of predefined positions on the inside of the cladding tube.

[0055] The accuracy of positioning the preforms on the inner surface of the cladding tube is improved if the upper front ends of the structural elements are positioned at the desired position using a positioning template.

[0056] 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.

[0057] The star-shaped arrangement of the retaining elements, as specified by the design, facilitates the precise positioning of the anti-resonance element preforms at the respective target positions and their fixation, for example, using the sealing or bonding compound described above. The positioning template is preferably used exclusively in the area of ​​the cladding tube end faces, preferably in the area of ​​both cladding tube end faces.

[0058] A procedure has also proven successful in which, during elongation of the primary preform according to process step (d) and / or during drawing of the hollow core fiber according to process step (e), several components of the preform made of quartz glass are heated and softened together, wherein the quartz glass of at least some of the preform components contains at least one dopant which reduces the viscosity of quartz glass.

[0059] Components of the primary preform include the cladding tube and the anti-resonance element preforms arranged therein. The secondary preform contains additional cladding material, which is provided, for example, in the form of one or more overlay cylinders and collapsed onto the primary preform.

[0060] Fluorine, chlorine and / or hydroxyl groups are preferably used as dopants to reduce the viscosity of quartz glass.

[0061] Doping allows the thermal expansion coefficients of adjacent preform components to be adjusted to avoid or reduce stresses. It can also be used to reduce the thermal stability of one component to benefit the stability of a neighboring component.

[0062] For example, 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, at a measuring temperature of 1250 °C than the quartz glass of additionally applied cladding material (when the viscosity is specified as a logarithmic value in dPas).

[0063] In particular with regard to low optical attenuation and a large optical transmission bandwidth of the hollow core fiber, it has proven particularly advantageous if the antiresonance elements are arranged around the hollow core with an odd symmetry.

[0064] 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 elongation the primary preform is continuously fed to a heating zone at a feed rate, softened zone by zone in the heating zone, and withdrawn from the heating zone at a withdrawal rate, wherein the feed rate is adjusted so that a throughput of at least 0.8 g / min and an average residence time in the heating zone in the range of 5 to 25 min result, and wherein a temperature-controlled heating zone is used whose target temperature is maintained to an accuracy of + / - 0.1°C.

[0065] During elongation of the primary preform, additional cladding material is applied, or no additional cladding material is applied. The result of the elongation step is a secondary preform or a different semi-finished product in the form of the elongated primary preform, which may require further processing into the secondary preform. In either case, the elongated primary preform is characterized by at least some of the anti-resonance element preforms having an oval cross-sectional shape. This is suitable for counteracting deformation that can occur during elongation of the secondary preform into the hollow core fiber, and can also enlarge the contact areas between nested structural elements, thus contributing to greater mechanical stability. This process enables more precise production of the hollow core fiber.

[0066] Measures for producing the preform are explained above in the context of the production of the hollow core fiber and these explanations are hereby incorporated. Definitions

[0067] 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

[0068] 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

[0069] 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).

[0070] 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

[0071] 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

[0072] 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.

[0073] During collapse, an internal bore is narrowed or annular gaps between tubular components are closed or narrowed. Collapse is usually accompanied by elongation. Hollow core / Inner shell area / Outer shell area / Cane

[0074] 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.

[0075] 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

[0076] 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 cladding tube's inner surface; it can also extend through the entire cladding tube wall to the outer surface. Particle size and particle size distribution

[0077] 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

[0078] The invention is explained in more detail below using an exemplary embodiment and a drawing. In detail, Figure 1 a primary preform with a cladding tube and anti-resonance element preforms positioned and fixed therein for producing a preform for a hollow core fiber based on a view of the radial cross-section in a schematic representation, Figure 2 a photograph of a secondary preform with preformed antiresonance element preforms in a radial cross-section view, Figure 3 Processing stages from the primary preform to the antiresonant hollow core fiber, and Figure 4 A sketch to explain force relationships during hot processing of nested anti-resonance elements.

[0079] 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.

[0080] Figure 1 schematically shows a cane (primary preform) 3 with a cladding tube 1 with a cladding tube wall 2, 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.

[0081] The cladding tube 1 is made of quartz glass and has a length of 1000 mm, an outer diameter of 27 mm, and an inner diameter of 20 mm. The antiresonance element preforms 4 are present 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, amounting to 0.3 mm. The lengths of the ARE outer tube 4a and the ARE inner tube 4b correspond to the cladding tube length.

[0082] The cladding tube 1 is manufactured in a vertical drawing process without a forming tool using 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. According to the final dimensions, the outer diameter is 90 mm and the diameter ratio of the outer to inner diameter is 2.5. In a first elongation process, the starting cylinder is continuously fed into a heating zone with a heating zone length of 200 mm with a vertically oriented longitudinal axis, where it is softened in certain areas, 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 into another heating zone with a heating zone length of 100 mm, where it is softened in certain areas, and a tube strand is drawn from the softened area. The cladding tube is obtained from the tube strand by cutting it to length.

[0083] The anti-resonance element preforms 4 are fixed to the inner wall of the cladding tube 1 by means of the SiO 2 -based bonding compound 5.

[0084] The bonding compound 5 is applied locally to the cladding tube's inner surface in the area of ​​the front ends, and the anti-resonance element preforms 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 positioning template is limited to the area around the two front ends of the cladding tube.

[0085] This method creates a precise and reproducible connection between the cladding tube 1 and the anti-resonance element preforms 4. For fixation, solidification of the bonding compound 5 at a low temperature below 300 °C is sufficient, thus avoiding excessive heating of the surrounding areas and thus deformation of the anti-resonance element preforms 4.

[0086] The primary preform 3 is overclad with a quartz glass overcladding cylinder, which collapses onto the cladding tube 1, and simultaneously elongates the tube assembly into a secondary preform. The overcladding cylinder has an outer diameter of 63.4 mm and a wall thickness of 17 mm.

[0087] During the collapsing and elongation process, the coaxial arrangement of cladding tube 1 and overlay cylinder is fed from below into a temperature-controlled heating zone with the longitudinal axis oriented vertically and is softened zone by zone, starting with the upper end of the arrangement.

[0088] The heating zone is maintained at a target temperature of 1600 °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.

[0089] During the collapsing and elongating step, an oval outer cross-sectional shape is imprinted on the former tubular anti-resonance element preforms 4 of the primary preform 3 by generating and maintaining the same internal pressure in the hollow core region and in the tubular anti-resonance element preforms 4. Figure 4outlines the resulting force relationships. Ambient pressure pa prevails both in the hollow core area 40 and in the inner bore of the ARE inner tube 4b during elongation. At an overpressure of 2.36 mbar in the cavity of the ARE inner tube 4b (compared to the pressure outside it), the ARE inner tube 4a would be stable during elongation. Without the internal pressure, forces act as indicated by the directional arrows F. The ratio of the normal force F N1 at the upper vertex "S" to the resulting normal force F N2 on the sides of the ARE inner tube wall is between 1.07 and 1.27 (1.07 < F N1 / F N2 < 1.27). As a result of this force ratio, the ARE inner tube 4b becomes oval and approximately elliptical during elongation, with the long axis of the ellipse tangential to the inner wall of the ARE outer tube 4a and the shorter axis perpendicular to it.

[0090] Corresponding considerations for the ARE outer tube 4a result in an oval outer cross-sectional shape, which is characterized by an axial length ratio AL / AK that lies between 1.07 and 1.27 (1.07 < AL / AK < 1.27).

[0091] The secondary preform formed in this way during the collapsing and elongation process has an outer diameter of approximately 50 mm and a wall thickness of 16.6 mm, consisting of the outer shell and inner shell. The photo of Figure 2 shows the oval-shaped ARE outer tubes. The ARE inner tubes are also measurably oval-shaped, but to such a small extent that the deformation is not visually noticeable.

[0092] The secondary preform is elongated into an antiresonant hollow-core fiber. For this purpose, all structural elements of the former antiresonant element preforms, i.e., the ARE outer tube 4a and the ARE inner tube 4b, are sealed with the aforementioned 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.

[0093] The same end face is then connected to a quartz glass holding tube, which also serves as a gas connection. The holder is fixed to the overlay cylinder and the cladding tube using the sealing or bonding compound. 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 with the bottom end. The heating zone is maintained at a target temperature of approximately 2100°C with a control accuracy of + / - 0.1°C. This allows temperature fluctuations during the hot forming process to be limited to less than + / - 0.5°C. At the same time, gas is supplied to the core area (hollow core), creating an internal pressure of 4 mbar in the core area.

[0094] Through the fiber drawing process carried out in this way, the oval cross-sectional shape of the former structural elements of the anti-resonance element preforms becomes a round cross-sectional shape, so that an anti-resonant hollow core fiber with anti-resonance elements embedded therein is obtained, which have a round cross-sectional shape.

[0095] Figure 3 schematically shows the process steps. From the primary preform 30 with anti-resonance element preforms 34 with a round cross-section, an elongation and collapse process 37 is used to create a secondary preform 32 with elongated anti-resonance element preforms 35 with an oval cross-section (characterized by the short cross-sectional axis AK and the long cross-sectional axis AL). This preform is elongated in a fiber drawing process 38 to form an anti-resonance hollow-core fiber 33 containing anti-resonance elements 36 with a round cross-section.

Claims

1. A method for producing an antiresonant hollow-core fiber (33) comprising a hollow core extending along a fiber longitudinal axis and an inner sheath region which surrounds the hollow core and comprises a plurality of antiresonance elements, comprising the method steps of: (a) providing a cladding tube (1) which comprises a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (2) bounded by an inner side and an outer side extends, (b) providing a number of tubular antiresonance element preforms (4; 34), (c) arranging the antiresonance element preforms (4; 34) at target positions on the inner side of the cladding tube wall (2) to form a primary preform (3; 30) which comprises a hollow core region and an inner sheath region, (d) further processing the primary preform (3; 30) to form a secondary preform (32) from which the hollow-core fiber (33) is drawn, wherein the further processing comprises elongation and optionally a single or repeated performance of one or more of the following hot-forming processes: (i) collapse, (ii) collapse and simultaneous elongation, (iii) collapse of additional sheath material, (iv) collapse of additional sheath material and subsequent elongation, (v) collapse of additional sheath material and simultaneous elongation, and (e) drawing the secondary preform (32) to form the hollow-core fiber (33), wherein, after the elongation according to method step (d), at least a portion of the former tubular antiresonance element preforms (4; 34) of the primary preform (3; 30) has an oval outer cross-sectional shape, with a longest cross-sectional axis AL and a shortest cross-sectional axis AK, wherein the axis length ratio AL / AK is in the range between 1.01 and 1.27, and wherein the shortest cross-sectional axis AK runs in the radial direction when viewed from the cladding tube longitudinal axis, characterized in that during elongation the primary preform (3; 30) is continuously fed to a heating zone at a feed rate, is softened zone by zone in the heating zone, and is withdrawn from the heating zone at a withdrawal rate, wherein the feed rate is set such that a throughput of at least 0.8 g / min and an average dwell time in the heating zone in the range of 5 to 25 min is achieved, and wherein a temperature-controlled heating zone is used, the target temperature of which is maintained to an accuracy of + / - 0.1°C.

2. The method according to claim 1, characterized in that at least a portion of the tubular antiresonance element preforms (4; 34) are composed of a plurality of nested structural elements (4a; 4b) and comprise at least one ARE outer tube (4a) and one ARE inner tube (4b) extending in the ARE outer tube (4a) and parallel to the ARE outer tube longitudinal axis, wherein the ARE outer tube (4a) has an oval, preferably elliptical, outer cross-sectional shape with the axis length ratio AL / AK in the range between 1.07 and 1.27, and the ARE inner tube (4b) has an oval, preferably elliptical, outer cross-sectional shape with the axis length ratio AL / AK in the range between 1.01 and 1.05.

3. The method according to any of the preceding claims, characterized in that the primary preform (3; 30) has an outer diameter in the range of 20 to 70 mm.

4. The method according to any of the preceding claims, characterized in that during elongation the primary preform (3; 30) is continuously fed to a heating zone at a feed rate, is softened zone by zone in the heating zone, and is withdrawn from the heating zone at a withdrawal rate, wherein the feed rate is set such that a throughput in the range of 3.3 g / min to 85 g / min is achieved.

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

6. The method according to any of the preceding claims, characterized in that the fixing of the antiresonance element preforms (4) is carried out using a sealing or bonding compound (5) containing amorphous SiOz particles.

7. The method according to any of the preceding claims, characterized in that, during elongation of the primary preform (3; 30), open ends of the antiresonance element preforms (4) and / or individual structural elements (4a; 4b) of the antiresonance element preforms (4) and / or any annular gap between tubular elements are sealed by means of a sealing or bonding compound (5).

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

9. The method according to any of the preceding claims, characterized in that the upper endface ends of the structural elements (4a; 4b) are positioned at the target position by means of a positioning template.

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

11. The method according to any of the preceding claims, characterized in that, during elongation of the primary preform (3; 30) according to method step (d) and / or during drawing of the hollow-core fiber (33) according to method step (e), multiple components of the preform made of quartz glass are heated and softened together, wherein the quartz glass of at least some of the preform components contains at least one dopant which lowers the viscosity of quartz glass.

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

13. The method according to any of the preceding claims, characterized in that the arranging of the antiresonance element preforms (4; 34) at target positions on the inner side of the cladding tube wall (2) and / or the drawing of the hollow-core fiber (33) according to method step (d) comprises a fixing measure and / or a sealing measure using a sealing or bonding compound (5) containing amorphous SiOz particles.

14. A method for producing a preform (32) for an antiresonant hollow-core fiber comprising a hollow core extending along a fiber longitudinal axis and an inner sheath region which surrounds the hollow core and comprises a plurality of antiresonance elements, comprising the method steps of: (a) providing a cladding tube (1) which comprises a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (2) bounded by an inner side and an outer side extends, (b) providing a number of tubular antiresonance element preforms (4), (c) arranging the antiresonance element preforms (4; 34) at target positions on the inner side of the cladding tube wall (2) to form a primary preform (3; 30) which comprises a hollow core region and an inner sheath region, (d) further processing the primary preform (3; 30) to form a secondary preform (32) for the hollow-core fiber, wherein the further processing comprises elongation and optionally a single or repeated performance of one or more of the following hot-forming processes: (i) collapse, (ii) collapse and simultaneous elongation, (iii) collapse of additional sheath material, (iv) collapse of additional sheath material and subsequent elongation, (v) collapse of additional sheath material and simultaneous elongation, wherein, after the elongation according to method step (d), at least a portion of the former tubular antiresonance element preforms (4; 34) of the primary preform (3; 30) has an oval outer cross-sectional shape, with a longest cross-sectional axis AL and a shortest cross-sectional axis AK, wherein the axis length ratio AL / AK is in the range between 1.01 and 1.27, and wherein the shortest cross-sectional axis AK runs in the radial direction when viewed from the cladding tube longitudinal axis, characterized in that during elongation the primary preform (3; 30) is continuously fed to a heating zone at a feed rate, is softened zone by zone in the heating zone, and is withdrawn from the heating zone at a withdrawal rate, wherein the feed rate is set such that a throughput of at least 0.8 g / min and an average dwell time in the heating zone in the range of 5 to 25 min is achieved, and wherein a temperature-controlled heating zone is used, the target temperature of which is maintained to an accuracy of + / - 0.1°C.