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 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 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 for the hollow core fiber having a hollow core region and a cladding region, and (d) Further processing the primary preform into a secondary preform from which the hollow core fiber is drawn, wherein the further processing comprises a single or repeated execution of one or more of the following hot forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapsing additional cladding material, (v) collapsing additional cladding material and subsequent elongation,(vi) Collapse of additional mantle material and simultaneous elongation.

[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 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 for the hollow core fiber having a hollow core region and a cladding region, and (d) Optionally further processing the primary preform into a secondary preform for the hollow core fiber, wherein the further processing comprises a single or repeated execution of one or more of the following hot forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapsing additional cladding material, (v) collapsing additional cladding material and subsequent elongation,(vi) Collapse of additional mantle material and simultaneous elongation,

[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 reflective 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

[0009] Material processing, modal filtering, nonlinear optics, especially for supercontinuum generation, from the ultraviolet to infrared wavelength range. State of the art

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] GB 2 566 466 A discloses a method for producing a preform for an antiresonant hollow-core optical fiber, comprising the method steps (a) to (d) mentioned above. Arranging the ARE outer tubes (antiresonant cladding tubes) at predefined peripheral locations within the cladding tube involves the use of spacer elements, each of which is in contact with two adjacent ARE outer tubes. The radial distance r 2 between these contact points and the cladding tube longitudinal axis is greater than the radial distance r 1 between the longitudinal axes of the ARE outer tube and the cladding tube longitudinal axis. The spacer elements can form an integral structure with the cladding tube. The cladding tube can be shaped to include radially projecting portions, for example, by molding or by machining the inner wall. For this purpose, glass material is removed to create concave regions from which the spacer elements protrude as convex projections.In one example, the spacers have a rectangular cross-section that protrudes from the concave inner wall of the cladding tube. In another example, the inner wall is shaped with a series of alternating concave areas to accommodate the ARE outer tubes and convex protrusions that serve as spacers.

[0016] WO 2002 / 072489 A2 describes a method for producing a microstructured optical fiber in which a glass tube with longitudinal grooves or slots is used to form hollow channels. The longitudinal grooves or slots can be created by mechanical processing. 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 anti-resonant hollow core fiber, this object is achieved according to the invention, starting from a method of the type mentioned at the outset, in that the provision of the cladding tube according to method step (a) comprises a processing measure in which the cladding tube wall is provided by machining in the region of the target positions with a longitudinal structure extending in the direction of the cladding tube longitudinal axis, which has longitudinal slots distributed around the circumference of the cladding tube wall and in that the anti-resonance element preforms are each arranged on a longitudinal slot.

[0023] The starting point for the production of the antiresonant hollow-core fiber is a preform, also referred to here as a "cane" or "primary preform." It comprises a 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, but 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.

[0024] The accuracy of the positioning of the preforms is improved by structuring the cladding tube in advance using machining.

[0025] Machining refers to mechanical machining techniques that remove material, such as turning, cutting, drilling, sawing, milling, or grinding. Compared to other known forming techniques using heat and pressure, these machining techniques produce more precise and delicate structures and avoid surface contamination from forming tools such as nozzles, presses, or melt molds. The longitudinal structure of the cladding tube's inner wall is preferably created by drilling, sawing, milling, cutting, or grinding.

[0026] This processing creates a longitudinal structure extending along the cladding tube's longitudinal axis, which 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. For this purpose, the longitudinal structure is accessible at least from the inside of the cladding tube; it can also extend through the entire cladding tube wall to the outside.

[0027] The longitudinal structure has longitudinal slots distributed around the circumference of the cladding tube wall, with the anti-resonance element preforms being arranged at a longitudinal slot.

[0028] The longitudinal slots penetrate the cladding tube wall from the inside out, with the exception of the two end regions of the cladding tube. They have parallel longitudinal edges and a maximum slot width SB, with the anti-resonance element preforms preferably being connected to the longitudinal edges.

[0029] The anti-resonance element preforms have a width dimension that is larger than the maximum width SB , so that they do not fully immerse themselves in the longitudinal slots.

[0030] With the invention, antiresonant hollow core fibers and preforms for them can be manufactured precisely and reproducibly.

[0031] Preferably, a cladding tube with a circular inner cross-section is provided.

[0032] The fact that a cladding tube with a round inner cross-section is provided for the purpose of structuring the cladding tube interior results in lower manufacturing costs for the cladding tube itself, for example in comparison to a cladding tube with an inner bore in the form of a hexagon socket.

[0033] The positioning points can have different shapes. Longitudinal structures designed as radially continuous longitudinal slots can be manufactured easily and precisely, for example, by milling, drilling, or cutting. The internal geometry of the slot can be semicircular, semi-oval, V-shaped, U-shaped, or polygonal, for example. It can also form a depression or recess that narrows from the inside to the outside.

[0034] Particularly in the case of continuous longitudinal slots, it is advantageous if the cladding tube has front ends, with the longitudinal structure ending in front of the front ends.

[0035] Depending on the geometry of the longitudinal structure and the shape and size of the anti-resonance element preforms, the fixation between the anti-resonance element preform and the longitudinal structure results in one contact point, several contact points, or larger contact areas. A procedure in which the anti-resonance element preforms, when arranged in the desired position according to process step (c), touch the longitudinal structure at two edges each has proven particularly effective.

[0036] This results in a more stable position than with only a single contact line, and reliably prevents the anti-resonance element preform from rolling away during elongation. The outer dimension (e.g., the outer diameter) of the preform can be larger than the maximum width SB of the longitudinal structure, so that the preform rests on the longitudinal slot and is not completely submerged. Below the contact edges, a cavity may be created in the longitudinal slot, into which a gas can be introduced or from which a gas can be extracted.

[0037] A process variant has proven successful in which the anti-resonance element preforms each have at least one anti-resonance element and at least one capillary connected to the anti-resonance element, wherein the capillary is received in a recess of the longitudinal structure and is deformed during elongation of the preform and / or the semi-finished product by applying an internal pressure in the capillary inner bore.

[0038] The capillary serves not only to fix the anti-resonance element preform to the longitudinal structure, which has a recess with dimensions sufficient to accommodate the capillary, but also to apply additional pressure to the capillary during the elongation process, so that, as a result of its deformation and expansion, it positions the actual anti-resonance element of the preform. The wall of the deformed capillary can form an additional anti-resonance element of the preform.

[0039] To ensure a more precise geometric structure, the bond between the anti-resonance element preform and the respective longitudinal edges is created by softening and simultaneously elongating the composite. For this purpose, the assembly of cladding tube and anti-resonance element preforms, also referred to as the "cane" in the following, is elongated. The anti-resonance element preforms are connected along their entire length to the cut edges inside the cladding tube.

[0040] The area below the longitudinal edges forms a cavity into which a gas can be introduced or from which a gas can be extracted.

[0041] 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 element preforms are arranged around the hollow core with an odd symmetry.

[0042] 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.

[0043] The sealing or bonding compound used for sealing or fixing contains amorphous SiO2 particles, which are, for example, suspended 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 the provision of the cladding tube according to method step (a) comprises a processing measure in which the cladding tube wall is provided by machining in the region of the desired position with a longitudinal structure extending in the direction of the cladding tube longitudinal axis, which has longitudinal slots distributed around the circumference of the cladding tube wall and in that the anti-resonance element preforms are each arranged on a longitudinal slot.

[0048] The preform is the starting point for the production of the antiresonant hollow-core fiber. By elongating the primary preform, the antiresonant hollow-core fiber is either drawn directly, or the primary preform is further processed to create another semi-finished product, also referred to here as the "secondary preform," from which the antiresonant hollow-core fiber can be drawn.

[0049] In any case, preform manufacturing involves installing and bonding antiresonance element preforms to a cladding tube. Preform positioning accuracy is improved by pre-structuring the cladding tube. Preform manufacturing steps are explained above in the context of hollow-core fiber manufacturing, and these explanations are incorporated herein. Definitions

[0050] 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

[0051] 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

[0052] 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).

[0053] 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

[0054] 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

[0055] 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.

[0056] 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

[0057] 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.

[0058] 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

[0059] 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

[0060] 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

[0061] The invention is explained in more detail below with reference to an embodiment and a drawing. In detail, a schematic representation Figures 1 to 3 Cladding tubes for use in a method for producing a preform for a hollow core fiber with longitudinal structures for positioning anti-resonance element preforms in several embodiments, all of which are reference examples not belonging to the invention, Figures 4 and 5 Sheath tubes with longitudinal structures and anti-resonance element preforms positioned therein based on contact points and contact surfaces, all of which are reference examples not belonging to the invention, Figures 6 and 7 further embodiments of cladding tubes with longitudinal structures and anti-resonance element preforms positioned therein, all of which are reference examples not belonging to the invention, Figures 8 and 9an embodiment of a cladding tube with longitudinal structures and anti-resonance element preforms positioned therein, which are equipped with a capillary for the purpose of adjustable positioning, which is a reference example not belonging to the invention, Figure 10 Process steps for producing a preform for a hollow core fiber using a slotted cladding tube, Figure 11 a section of the longitudinal structure and anti-resonance element from Figure 10c in enlarged view, and Figure 12 a Figure 11 corresponding section with a different embodiment of the anti-resonance element.

[0062] 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.

[0063] The Figures 1 to 8 Each schematically shows a top view of the cladding tube end face. The cladding tubes 1 each have longitudinal grooves 3 on the inside of the cladding tube wall 1a. The longitudinal grooves 3 are evenly distributed around the inner circumference of the respective cladding tube 1 in sixfold symmetry.

[0064] The cladding tubes 1 are made of quartz glass. They have a length of 500 mm, an outer diameter of 73 mm, and an inner diameter of 24 mm.

[0065] Figure 1 shows deep, narrow, and in cross-section almost closed circular longitudinal grooves 3, created by drilling. The maximum depth is 3 mm and the inner diameter is 4 mm.

[0066] Figure 2 shows flat, narrow and in cross-section semicircular longitudinal grooves 3, which are produced by milling.

[0067] Figure 3 shows flat, wide, bowl-shaped longitudinal grooves 3 in cross-section, which are also produced by milling.

[0068] The longitudinal grooves 3 serve as positioning aids for antiresonance element preforms 5. These can be present as an ensemble of nested elements consisting of an ARE outer tube 5a and an ARE inner tube 5b, as shown in the Figures 4 and 5 shown.

[0069] Figure 4 shows deep, narrow, and in cross-section almost closed, circular longitudinal grooves 3, which were created by drilling and against which the anti-resonance element preforms 5 rest on two edges 3a, 3b. The ARE outer tube has an outer diameter of 73 mm. The wall thickness of the ARE inner tube and ARE outer tube are approximately the same at 0.35 mm.

[0070] Figure 5shows flat, wide, and bowl-shaped longitudinal grooves 3, created by milling, against which the anti-resonance element preforms 5 rest on larger contact surfaces 3c. The ARE outer tube has an outer diameter of 73 mm. The wall thickness of the ARE inner tube and ARE outer tube is approximately the same at 0.35 mm.

[0071] Figure 6 shows deep, narrow, and in cross-section almost closed, circular longitudinal grooves 3, which are created by drilling and in which the anti-resonance element preforms 5 are accommodated. The anti-resonance element preforms 5 are designed as simple capillary tubes in this case.

[0072] Figure 7shows deep, narrow, and in cross-section almost closed circular longitudinal grooves 3, which are created by drilling and in which the anti-resonance element preforms 5 are accommodated. The anti-resonance element preforms 5 are designed as simple capillary tubes. An insert tube 8 is inserted into the cladding tube's inner bore coaxially to the cladding tube's longitudinal axis.

[0073] The outer diameter of the insert tube 8 is similar to the inner diameter of the cladding tube 1 (as Fig. 1) and its wall thickness is similar to that of the anti-resonance element preforms. Before the fiber drawing process, the cladding tube 1 is collapsed onto the insert tube 8, closing the longitudinal grooves 3. The closed longitudinal grooves form hollow channels, which are subsequently inflated (e.g., during elongation of the preform). This creates protrusions with convex surfaces on the insert tube 8, directed inward toward the inner bore, which act as an additional anti-resonance boundary layer.

[0074] In one process variant (not shown in the figures), almost closed circular longitudinal grooves are created by drilling. An insert tube is inserted into the cladding tube's inner bore coaxially to the cladding tube's longitudinal axis. The outer diameter of the insert tube 8 is similar to the inner diameter of the cladding tube (as Fig. 1) and its wall thickness is similar to that of the anti-resonance element preforms. Before the fiber drawing process, the cladding tube is collapsed onto the insert tube, closing off the longitudinal grooves. The closed longitudinal grooves form hollow channels, which are subsequently inflated (e.g., during preform elongation). This creates protrusions with convex surfaces on the insert tube, directed inward toward the inner bore, which act as anti-resonance elements.

[0075] At the Figure 8In the process variant shown, the anti-resonance element preforms 5 are equipped with anti-resonance elements and additionally with a positioning capillary 9. The positioning capillaries 9 are each inserted into the longitudinal grooves 3, which are almost circular in cross-section and created by drilling. The positioning capillaries 9 form hollow channels that can be inflated when the assembly softens, for example when the preform elongates, so that the anti-resonance elements fixed to them can be pushed closer to the center (the core area). The extent of the displacement can be easily adjusted and regulated via the internal pressure in the hollow channels (positioning capillaries 9). This allows not only a precise adjustment of the azimuthal position but also the radial position of the anti-resonance elements and thus the core diameter of the anti-resonant hollow-core fibers.

[0076] Figure 9shows two different embodiments of anti-resonance element preforms 5, each equipped with a positioning capillary 9.

[0077] The Figures 10 to 12 show schematically process steps for the production of hollow core fibers, whereby a quartz glass cladding tube 1 with longitudinal slots 3 is used for the precise positioning of the antiresonance element preforms. As in Figure 10(a)As shown, the wall of the cladding tube 1 is cut longitudinally at predefined azimuthal positions at equal intervals, for example, by means of a mechanical saw, water jet cutting, laser, or the like. The number of longitudinal cuts 3 corresponds to the number of anti-resonance element preforms 5 to be positioned; in the exemplary embodiment, there are six preforms 5. The longitudinal cuts 3 end before the cladding tube ends, so that the front end regions 12 remain circumferentially closed and connect the remaining webs 14. The cut edges are then glazed.

[0078] The cutting width of the longitudinal slots 3 is uniform and amounts to 2 mm. The anti-resonance element preforms 5 have a substantially round outer cross-section with a diameter of 7.4 mm.

[0079] From the top view of the cross section of the intermediate pipe 10 along the section line AA in Figure 10(b)It can be seen that the six longitudinal slots 3 are evenly distributed around the tube wall 1a, and it shows the anti-resonance element preforms 5, which rest against the longitudinal slots 3 at two contact lines each, the cut edges 3a; 3b, and protrude into the cladding tube's inner bore 16. For fixation, the two ends of the anti-resonance element preforms 5 are, for example, glued to the inside of the cladding tube 1 or fused thereto. Alternatively and preferably, the anti-resonance element preforms are bonded to the cladding tube's inner surface using the SiO 2 -containing sealing and bonding compound described above. Local fixation, particularly in the two end regions, is sufficient for this purpose.

[0080] By subsequently elongating this assembly, the anti-resonance element preforms 5 are connected over their entire length to the cut edges 3a; 3b inside the cladding tube 1. By applying overpressure to the inner bore of the cladding tube 1, it can be checked whether the cut joints 3 are completely closed by the anti-resonance element preforms 5. The longitudinal slots 3 thus serve as an exact positioning aid, against which each anti-resonance element preform 5 can be precisely positioned and fixed. The assembly 17 consisting of the cladding tube 1 and the anti-resonance element preforms 5 positioned therein is often also referred to as a "cane" (17).

[0081] In Figure 10(c)It is shown that the cane 17 is subsequently overclad with an overcladding tube 15 to add additional sheath material and to adjust the core-to-sheath diameter ratio specified for the hollow core fiber in the preform 18. In the preform 18 thus obtained, an annular gap 19 remains between the overcladding tube 15 and the cane 17.

[0082] When the preform 18 is elongated to form the hollow core fiber, gas can be introduced into or withdrawn from the hollow channels formed in the cut joints 3 between the overlay tube 15 and the molten anti-resonance element preforms 5 via the annular gap 19 in order to generate overpressure or underpressure in the hollow channels.

[0083] If necessary or desired, the radial position of the anti-resonance elements 5 in the cladding tube inner bore 16 can be changed and corrected, as shown in Figure 11Sketch (a) shows an antiresonance element preform 5 with an ARE outer tube 5a and a nested inner element (ARE inner tube 5b) in the initial position. Sketch (b) shows the ARE outer tube wall, deformed by pressure and heat and partially inverted, with the ARE inner tube 5b fixed to the inversion in a radial position different from the initial position. This allows fiber designs to be realized that, unlike the classic "stack-and-draw" technique, also have non-hexagonal symmetry and, in particular, non-integer symmetry.

[0084] By applying pressure in the hollow channels, it is also possible to "fold over" a wall section of an anti-resonance element preform 5 towards the interior of the anti-resonance element preform. Figure 12(a)shows an antiresonance element preform 5 with a simple ARE outer tube 5a (without an additional nested ARE inner tube) in the initial position. Under the influence of heat and pressure, the wall section between the two contact lines is inflated inward. As shown in Figure 12b As shown, a further glass membrane 5c with a negatively (convex) curved surface is created within the ARE outer tube 5a, which can replace a nested inner element such as the ARE inner tube 5b ("nested element").

Claims

1. A method for producing an anti-resonant hollow-core fiber which comprises a hollow core extending along a longitudinal axis of the fiber and a casing region that surrounds the hollow core and comprises a plurality of anti-resonant elements, comprising the method steps of: (a) providing a sheath tube (1) which comprises a sheath tube inner bore (16) and a sheath tube longitudinal axis, along which a sheath tube wall (1a) extends which is delimited by an inner face and an outer face, (b) providing a number of anti-resonant element preforms (5), (c) arranging the anti-resonant element preforms (5) at desired positions on the inner face of the sheath tube wall (1a) to form a primary preform (17) for the hollow-core fiber, which primary preform comprises a hollow core region and a casing region, and (d) further processing the primary preform (17) to form a secondary preform (18) from which the hollow-core fiber is drawn, the further processing comprising carrying out one or more of the following hot-forming processes once or repeatedly: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional casing material, (v) collapse of additional casing material and subsequent elongation, (vi) collapse of additional casing material and simultaneous elongation, characterized in that the provision of the sheath tube (1) according to method step (a) comprises a machining measure in which the sheath tube wall (1a) is provided, by machining in the region of the target positions, with a longitudinal structure (3) which extends in the direction of the sheath tube longitudinal axis and has longitudinal slots distributed around the circumference of the sheath tube wall (1a) and in that the anti-resonant element preforms (5) are each arranged on a longitudinal slot.

2. The method according to claim 1, characterized in that the longitudinal structure (3) of the sheath tube inner wall (1a) is created by drilling, sawing, milling, cutting or grinding.

3. The method according to claim 1 or 2, characterized in that a sheath tube (1) having a circular inner cross section is provided.

4. The method according to one of the preceding claims, characterized in that the sheath tube (1) has front ends (12), and in that the longitudinal structure (3) ends before the front ends.

5. The method according to one of the preceding claims, characterized in that the anti-resonant element preforms (5) each touch the longitudinal structure (3) on two edges (3a; 3b) when arranged at the desired position according to method step (c).

6. The method according to claim 1, characterized in that the longitudinal slots comprise parallel longitudinal edges (3a; 3b) and a maximum slot width SB and the anti-resonant element preforms () are connected to the longitudinal edges (3a; 3b).

7. The method according to claim 6, characterized in that the anti-resonant element preforms (5) are connected to the longitudinal edges (3a; 3b) by softening, preferably accompanied by simultaneous lengthening.

8. The method according to one of the preceding claims, characterized in that the anti-resonant element preforms (5) each comprise at least one anti-resonant element (5a; 5b) and at least one capillary (9) connected to the anti-resonant element (5a; 5b), the capillary (9) being received in a cavity of the longitudinal structure (3), and the capillary (9) being deformed by applying an internal pressure in the capillary inner bore when carrying out a process according to method step (d).

9. The method according to one of the preceding claims, characterized in that the anti-resonant elements (5) are arranged around the hollow core with an uneven symmetry.

10. 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 a casing region that surrounds the hollow core and comprises a plurality of anti-resonant elements, comprising the method steps of: (a) providing a sheath tube (1) which comprises a sheath tube inner bore and a sheath tube longitudinal axis, along which a sheath tube wall (1a) extends which is delimited by an inner face and an outer face, (b) providing a number of anti-resonant element preforms (5), (c) arranging the anti-resonant element preforms (5) at desired positions on the inner face of the sheath tube wall (1a) to form a primary preform (17) for the hollow-core fiber, which primary preform comprises a hollow core region and a casing region, and (d) optionally further processing the primary preform (17) to form a secondary preform (18) for the hollow-core fiber, the further processing comprising carrying out one or more of the following hot-forming processes once or repeatedly: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional casing material, (v) collapse of additional casing material and subsequent elongation, (vi) collapse of additional casing material and simultaneous elongation, characterized in that the provision of the sheath tube (1) according to method step (a) comprises a machining measure in which the sheath tube wall (1a) is provided, by machining in the region of the target positions, with a longitudinal structure (3) which extends in the direction of the sheath tube longitudinal axis and has longitudinal slots distributed around the circumference of the sheath tube wall (1a) and in that the anti-resonant element preforms (5) are each arranged on a longitudinal slot.