METHODS FOR PRODUCING A HOLLOW-CORE FIBER AND FOR PRODUCING A PREFORM FOR A HOLLOW-CORE FIBER

Abstract

Methods are known for producing an anti-resonant hollow-core fiber which has a hollow core extending along a fiber longitudinal axis and an inner jacket region that surrounds the hollow core, said jacket region comprising multiple anti-resonant elements. The known methods have the steps of: providing a cladding tube that has a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall extends that is delimited by an interior and an exterior; providing a number of tubular anti-resonant element preforms; arranging the anti-resonant element preforms at target positions of the interior of the cladding tube wall, thereby forming a primary preform which has a hollow core region and an inner jacket region; and elongating the primary preform in order to form the hollow-core fiber or further processing the primary preform in order to form a secondary preform. The aim of the invention is to achieve a high degree of precision and an exact positioning of the anti-resonant elements in a sufficiently stable and reproducible manner on the basis of the aforementioned methods. This is achieved in that the step of providing the cladding tube includes a processing measure, in which the cladding tube wall is machined with a longitudinal structure extending in the direction of the cladding tube longitudinal axis in the region of the target positions.

Claims

1. Method for producing an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and a sheath region surrounding the hollow core and comprising a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (1) comprising an inner bore (16) of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall (2) delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (5), (c) arranging the anti-resonance element preforms (5) at desired positions of the inner side of the cladding tube wall (2) to form a primary preform (17) for the hollow-core fiber which comprises a hollow core region and a sheath region, and (d) elongating the primary preform (17) to form the hollow-core fiber or further processing the primary preform (17) into a secondary preform (18) from which the hollow-core fiber is drawn, wherein the further processing comprises a single or repeated performance of one or more of the following hot-forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that the provision of the cladding tube (1) according to method step (a) comprises a machining measure in which by machining in the region of the desired positions, the cladding tube wall (2) is provided with a longitudinal structure (3) extending in the direction of the longitudinal axis of the cladding tube.

2. Method according to claim 1, characterized in that the longitudinal structure (3) of the cladding tube wall (2) is produced by drilling, sawing, milling, cutting or grinding.

3. Method according to claim 1 or 2, characterized in that a cladding tube (1) with a circular inner cross-section is provided, and that the longitudinal structure (3) is designed as a longitudinal groove on the inner side of the cladding tube wall or as a longitudinal slot.

4. Method according to any one of the preceding claims, characterized in that the cladding tube (1) comprises end-face ends (12), and that the longitudinal structure (3) ends before the end-face ends.

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

6. Method according to any one of the preceding claims, characterized in that the longitudinal structure (3) has a maximum width SB in the circumferential direction of the inner side of the cladding tube and that the anti-resonance element preforms (5) have a diameter that is less than the maximum width SB of the longitudinal structure (3).

7. Method according to claim 6, characterized in that the anti-resonance element preforms (5) are at least partially enclosed by the longitudinal structure (3).

8. Method according to claim 7, characterized in that an insert tube (8) is inserted into the inner bore of the cladding tube, and that the anti-resonance element preforms (5) are in each case enclosed in a pressure space between the longitudinal structure (3) and the insert tube (8).

9. Method according to claim 8, characterized in that wall sections of the insert tube (8) are deformed in the region of the longitudinal structure (3) by applying an internal pressure in the pressure space when a process according to method step (d) is carried out.

10. Method according to any one of claims 1 to 5, characterized in that the longitudinal structure (3) has longitudinal slots distributed about the circumference of the cladding tube wall (2), and that the anti-resonance element preforms (5) are arranged on one longitudinal slot in each case.

11. Method according to claim 10, characterized in that the longitudinal slots have parallel longitudinal edges (3a; 3b) and a maximum slot width S.sub.B, and the anti-resonance element preforms (5) are connected to the longitudinal edges (3a; 3b).

12. Method according to claim 11, characterized in that the anti-resonance element preforms (5) are connected to the longitudinal edges (3a; 3b) by softening, preferably accompanied by simultaneous lengthening.

13. Method according to any one of the preceding claims, characterized in that the anti-resonance element preforms (5) each have at least one anti-resonance element (5a; 5b) and at least one capillary (9) connected to the anti-resonance element (5a; 5b), wherein the capillary (9) is accommodated in a recess of the longitudinal structure (3), and when a process according to method step (d) is carried out, the capillary (9) is deformed by applying an internal pressure in the inner bore of the capillary.

14. Method according to any one of the preceding claims, characterized in that the anti-resonance elements (5) are arranged about the hollow core with an odd-numbered symmetry.

15. Method for producing a preform for an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and a sheath region surrounding the hollow core, said sheath region comprising a plurality of anti-resonance elements, with the method steps of: (a) providing a cladding tube (1) comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall (2) delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (5), (c) arranging the anti-resonance element preforms (5) at desired positions of the inner side of the cladding tube wall (2) to form a primary preform (17) for the hollow-core fiber which comprises a hollow core region and a sheath region, and (d) optionally further processing the primary preform (17) into a secondary preform (18) for the hollow-core fiber, wherein the further processing comprises a single or repeated performance of one or more of the following hot-forming processes: (i) elongation, (ii) collapse, (iii) collapse and simultaneous elongation, (iv) collapse of additional sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that the provision of the cladding tube (1) according to method step (a) comprises a machining measure in which, by machining in the region of the desired positions, the cladding tube wall (2) is provided with a longitudinal structure (3) extending in the direction of the longitudinal axis of the cladding tube.

Description

[0088] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in detail in schematic representation:

[0089] FIGS. 1 to 3 cladding tubes for use in a method for producing a preform for a hollow-core fiber having longitudinal structures for positioning anti-resonance element preforms in a plurality of embodiments,

[0090] FIGS. 4 and 5 cladding tubes with longitudinal structures and anti-resonance element preforms positioned therein with the aid of contact points and contact surfaces,

[0091] FIGS. 6 and 7 further embodiments of cladding tubes with longitudinal structures and anti-resonance element preforms positioned therein,

[0092] FIGS. 8 and 9 an 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,

[0093] FIG. 10 method steps for producing a preform for a hollow-core fiber using a slotted cladding tube,

[0094] FIG. 11 a detail of the longitudinal structure and anti-resonance element from FIG. 10c in an enlarged view, and

[0095] FIG. 12 a detail corresponding to FIG. 11 with another embodiment of the anti-resonance element.

[0096] In the production of the hollow-core fiber or the preform for the hollow-core fiber, a plurality of components are to be connected together. In addition, it can be helpful to seal existing gaps or channels of the preform when carrying out hot-forming processes. For bonding or sealing, a sealing or bonding compound based on SiO.sub.2 is used, as is known from DE 10 2004 054 392 A1. In this case, an aqueous slip containing amorphous SiO.sub.2 particles having a particle size distribution characterized by a D.sub.50 value of about 5 μm and by a Dgo value of about 23 μm is produced by wet-milling silica glass grain. Further amorphous SiO.sub.2 grains with an average grain size of about 5 μm are mixed with the base slip. The slip used as a bonding compound has a solid content of 90%, which consists of at least 99.9 wt.% SiO.sub.2. FIGS. 1 to 8 each schematically show a plan view of the end face of the cladding tube. The cladding tubes 1 each have longitudinal grooves 3 on the inner side of the cladding tube wall 2. The longitudinal grooves 3 are distributed evenly in hexagonal symmetry about the inner circumference of the cladding tube 1.

[0097] The cladding tubes 1 are made of silica glass. They have a length of 500 mm, an external diameter of 73 mm and an internal diameter of 24 mm.

[0098] FIG. 1 shows deep, narrow, circular longitudinal grooves 3, which are almost closed in cross-section and are produced by drilling. The maximum depth is 3 mm and the internal diameter is 4 mm.

[0099] FIG. 2 shows shallow, narrow longitudinal grooves 3, which are semi-circular in cross-section and are produced by milling.

[0100] FIG. 3 shows shallow, narrow longitudinal grooves 3, which are dish-shaped in cross-section and are also produced by milling.

[0101] The longitudinal grooves 3 serve as positioning aids for anti-resonance element preforms 5. They can be present as an ensemble of nested elements that consists of an ARE outer tube 5a and an ARE inner tube 5b, as shown in FIGS. 4 and 5.

[0102] FIG. 4 shows deep, narrow circular longitudinal grooves 3, which are almost closed in cross-section and are produced by drilling and against which the anti-resonance element preforms 5 abut in each case at two edges 3a, 3b. The ARE outer tube has an external diameter of 73 mm in each case. The wall thickness of the ARE inner tube and of the ARE outer tube is about the same and is 0.35 mm.

[0103] FIG. 5 shows shallow, wide longitudinal grooves 3, which are dish-shaped in cross-section and are produced by milling and against which the anti-resonance element preforms 5 abut in each case at larger contact surfaces 3c. The ARE outer tube has an external diameter of 73 mm in each case. The wall thickness of the ARE inner tube and of the ARE outer tube is about the same and is 0.35 mm.

[0104] FIG. 6 shows deep, narrow, circular longitudinal grooves 3, which are almost closed in cross-section and are produced by drilling and in which the anti-resonance element preforms 5 are accommodated. In this case, the anti-resonance element preforms 5 are designed as simple capillary tubes.

[0105] FIG. 7 shows deep, narrow, circular longitudinal grooves 3, which are almost closed in cross-section and are produced by drilling and in which the anti-resonance element preforms 5 are accommodated. In this case, the anti-resonance element preforms 5 are designed as simple capillary tubes. An insert tube 8 is inserted coaxially with respect to the longitudinal axis of the cladding tube into the inner bore 7 of the cladding tube.

[0106] The external diameter of the insert tube 8 is similar to the internal 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 so that the longitudinal grooves 3 are closed. The closed longitudinal grooves form hollow channels which are subsequently inflated (for example, when elongating the preform). As a result, protuberances with convex surfaces, which act as an additional anti-resonant boundary layer and are directed inwardly in the direction of the inner bore 7, are formed on the insert tube 8. In a method variant (not shown in the figures), circular longitudinal grooves, which are almost closed in cross-section, are produced by drilling. An insert tube is inserted coaxially with respect to the longitudinal axis of the cladding tube into the inner bore of the cladding tube. The external diameter of the insert tube 8 is similar to the internal 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 so that the longitudinal grooves are closed. The closed longitudinal grooves form hollow channels which are subsequently inflated (for example, when elongating the preform). As a result, protuberances with convex surfaces, which act as anti-resonance elements and are directed inwardly in the direction of the inner bore, are formed on the insert tube.

[0107] In the method variant shown in FIG. 8, 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 circular longitudinal grooves 3, which are almost closed in cross-section and produced by drilling. The positioning capillaries 9 form hollow channels which can be inflated when the ensemble softens, for example when the preform is elongated, so that the anti-resonance elements fixed thereto can be pushed closer to the center (the core region). The amount of displacement can easily be adjusted and regulated by the internal pressure in the hollow channels (positioning capillaries 9). In addition to a precise adjustment of the azimuthal position, this also allows an adjustment of the radial position of the anti-resonance elements and thus the core diameter of the anti-resonant hollow core fibers. FIG. 9 shows two different embodiments of anti-resonance element preforms 5, each equipped with a positioning capillary 9.

[0108] FIGS. 10 to 12 schematically show method steps for producing hollow-core fibers, wherein a silica glass cladding tube 1 with longitudinal slots 3 is used for precise positioning of the anti-resonance element preforms. As shown in FIG. 10(a), the wall of the cladding tube 1 is cut longitudinally at regular intervals at previously defined azimuthal positions, for example by means of a mechanical saw, water-jet cutting, laser or the like. The number of longitudinal incisions 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 incisions 3 end before the cladding tube ends so that the end-face end regions 12 continue to remain closed circumferentially and join the remaining webs 14. The cut edges are subsequently vitrified. The cutting width of the longitudinal slots 3 is uniform and is 2 mm. The anti-resonance element preforms 5 have a substantially round outer cross-section with a diameter of 7.4 mm. It can be seen from the top view of the cross-section of the intermediate tube 10 along the section line A-A in FIG. 10(b) that the six longitudinal slots 3 are evenly distributed around the tube wall 2, and it shows the anti-resonance element preforms 5 which abut against the longitudinal slots 3 in each case at two contact lines—the cut edges 3a; 3b—and protrude into the inner bore 16 of the cladding tube. For fastening, the two ends of the anti-resonance element preforms 5 are, for example, glued to the inner side of the cladding tube 1 or fused thereto. Alternatively and preferably, the anti-resonance element preforms are connected to the inner sheath surface of the cladding tube by means of the above-described sealing and bonding compound containing SiO.sub.2. Local fastening, in particular in the two end-face end regions, is sufficient for this purpose.

[0109] By means of a subsequent lengthening of this ensemble, 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 positive pressure in the inner bore of the cladding tube 1, a check can be made as to whether the kerfs 3 are completely closed by the anti-resonance element preforms 5. The longitudinal slots 3 thus serve as an exact positioning aid on which each anti-resonance element preform 5 can be precisely positioned and fixed.

[0110] FIG. 10(c) shows that the ensemble 17 consisting of cladding tube 1 and anti-resonance element preforms positioned therein is subsequently overlaid with an overlay tube 15 in order to add additional sheath material and to adjust the core-sheath diameter ratio prespecified for the hollow-core fiber in the secondary preform 18. In the secondary preform 18 thus obtained, an annular gap 19 remains between the buffer tube 15 and the ensemble 17. During the elongation of the preform 18 to form the hollow-core fiber, gas can be introduced into or withdrawn from the hollow channels via the annular gap 19, which hollow channels have formed in the kerfs 3 between the overlay tube 15 and the fused anti-resonance element preforms 5 in order to produce positive pressure or negative pressure in the hollow channels.

[0111] If required or desired, the radial position of the anti-resonance elements 5 in the inner bore 16 of the cladding tube can thus be modified and corrected, as outlined in FIG. 11. Sketch (a) shows an anti-resonance element preform 5 having an ARE outer tube 5a and nested inner element (ARE inner tube 5b) in the starting position. Sketch (b) shows the ARE outer tube wall, which is deformed by pressure and heat and which is inverted inwardly in regions, with the ARE inner tube 5b fastened to the inversion in a radial position that is modified compared to the starting position. Fiber designs which, in deviation from the classic “stack-and-draw technique,” even have non-hexagonal symmetry and in particular non-integral symmetry can thus also be realized. By applying pressure in the hollow channels, it is also possible to “fold over” a wall section of an anti-resonance element preform 5 toward the inside of the anti-resonance element preform.

[0112] FIG. 12(a) shows an anti-resonance element preform 5 with a simple ARE outer tube 5a (without an additional nested ARE inner tube) in the starting position. Under the influence of heat and pressure, the wall section between the two contact lines is inflated inwardly. As shown in FIG. 12b, a further glass membrane 5c with a negatively (convexly) curved surface thus arises within the ARE outer tube 5a, which curved surface can replace a nested inner element, such as the ARE inner tube 5b (also referred to in the technical literature as a “nested element”).