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 a cladding tube is provided with an outer diameter ranging from 90 to 250 mm and a length of at least 1 m; tubular structural elements are provided, at least some of which have a wall thickness ranging from 0.2 to 2 mm and a length of at least 1 m; and the structural elements are arranged in the cladding tube inner bore while the cladding tube longitudinal axis is vertically oriented, the upper end face of each structural element being positioned at the target position.

Claims

1. Method for producing an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and an inner sheath region surrounding the hollow core, which sheath region comprises several anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (1) comprising an inner bore (6) of the cladding tube and a longitudinal axis of the cladding tube, along which a cladding tube wall delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (5) composed of several nested tubular structural elements and comprising an ARE outer tube (5a) and an ARE inner tube (5b) inserted therein, wherein the structural elements have a structural elements longitudinal axis, (c) arranging the anti-resonance element preforms (5) at desired positions on the inner side of the cladding tube wall to form a primary preform (8) for the hollow-core fiber, which comprises a hollow core region and an inner sheath region, and (d) elongating the primary preform (8) to form the hollow-core fiber or further processing the primary preform (8) to form a secondary preform 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 a cladding tube (1) having an outer diameter in the range of 90 and 250 mm and a length of at least 1 m is provided, and that tubular structural elements (5a; 5b) are provided, at least a portion of which has a wall thickness in the range of 0.2 and 2 mm and a length of at least 1 m, and that the structural elements (5a; 5b) are arranged in the inner bore of the cladding tube in accordance with method step (c) with a vertically oriented longitudinal axis of the cladding tube, wherein the structural elements (5a; 5b) are each positioned at the desired position at their upper face end.

2. Method according to claim 1, characterized in that a cladding tube (1) having an outer diameter in the range of 120 to 200 mm is provided, and that tubular structural elements (5a; 5b) are provided, at least a portion of which has a wall thickness in the range of 0.25 and 1 mm.

3. Method according to claim 1 or 2, characterized in that at least one of the face ends of the anti-resonance element preforms (5) is sealed prior to drawing the hollow-core fiber in accordance with method step (d).

4. Method according to any of the preceding claims, characterized in that the inner side of the cladding tube and/or the outer side of the cladding tube and/or the inner side of the ARE outer tube and/or the outer side of the ARE outer tube is produced by machining, in particular by drilling, milling, grinding, honing, and/or polishing

5. Method according to any of the preceding claims, characterized in that the inner side of the cladding tube is provided with a longitudinal structure (3) extending in the direction of the longitudinal axis of the cladding tube by machining in the region of the desired positions.

6. Method according to any of the preceding claims, characterized in that the upper face ends of the structural elements (5a; 5b) are positioned at the desired position by means of a positioning template.

7. Method according to claim 6, characterized in that the positioning template is used in the region of a cladding tube end face, preferably in the region of both cladding tube end faces.

8. Method according to any of the preceding claims, characterized in that, when the hollow-core fiber is drawn in accordance with method step (d), several components of the preform (8) made of quartz glass are heated together and softened, wherein the quartz glass of at least some of the preform components contains at least one dopant that lowers the viscosity of quartz glass.

9. Method according to claim 8, characterized in that additional sheath material is collapsed in accordance with method step (d), and in that the quartz glass of the cladding tube (1) at a measured temperature of 1250° C. has a viscosity at least 0.5 dPa.Math.s higher, preferably a viscosity at least 0.6 dPa.Math.s higher, than the quartz glass of the additionally applied sheath material (if the viscosity is given as a logarithmic value in dPa.Math.s).

10. Method according to any of the preceding claims, characterized in that the provision of the primary preform (8) comprises arranging the anti-resonance element preforms (5) at desired positions of the inner side of the cladding tube wall, wherein the arranging of the anti-resonance element preforms (5) and/or the drawing of the hollow-core fiber in accordance with method step (d) comprises a fixing measure and/or a sealing measure using a sealing or bonding compound containing amorphous SiO.sub.2 particles.

11. 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 an inner sheath region surrounding the hollow core, which sheath region comprises several anti-resonance elements, comprising 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 delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (5) composed of several nested tubular structural elements and comprising an ARE outer tube (5a) and an ARE inner tube (5b) inserted therein, wherein the structural elements have a structural elements longitudinal axis, (c) arranging the anti-resonance element preforms at desired positions on the inner side of the cladding tube wall to form a primary preform (8) for the hollow-core fiber, which comprises a hollow core region and an inner sheath region, and (d) optionally further processing the primary preform (8) to form a secondary preform 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 a cladding tube (1) having an outer diameter in the range of 90 and 250 mm and a length of at least 1 m is provided, and that tubular structural elements (5a; 5b) are provided, at least a portion of which has a wall thickness in the range of 0.2 and 2 mm and a length of at least 1 m, and that the structural elements (5a; 5b) are arranged in the inner bore of the cladding tube in accordance with method step (c) with a vertically oriented longitudinal axis of the cladding tube, wherein the structural elements (5a; 5b) are each positioned at the desired position at their upper face end.

Description

EXEMPLARY EMBODIMENT

[0097] 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:

[0098] FIG. 1 a machined cladding tube provided with longitudinal grooves for use in the method according to the invention in a cross-sectional view,

[0099] FIG. 2 a detail of the longitudinal structure and anti-resonance element from FIG. 1 in an enlarged view, and

[0100] FIG. 3 a detail corresponding to FIG. 2 with another embodiment of the anti-resonance element.

[0101] Cladding tubes whose wall is provided with longitudinal grooves in the region of the inner sheath surface or whose wall has longitudinal slots are used. The longitudinal grooves or longitudinal slots are distributed uniformly around the inner circumference of the respective cladding tube in an odd or even symmetry, for example, and they serve to precisely position the anti-resonance element preforms at the desired positions in the quartz glass cladding tube.

[0102] FIG. 1 schematically shows the cross-section of a thick-walled quartz glass cladding tube 1 with longitudinal grooves 3 on the inner sheath surface. The inner wall of the cladding tube 1 is mechanically brought to the predetermined final dimension by drilling, grinding, and honing. Thereafter, the longitudinal grooves 3 are milled into the inner sheath surface at defined azimuthal positions at a uniform distance. The number of longitudinal grooves 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 grooves 3 extend from one cladding tube end to the other so as to penetrate the end faces. Next, the cut edges (3a; 3b) are vitrified.

[0103] The cut width and the cut depth of the longitudinal grooves 3 are uniform and are each 2 mm. The anti-resonance element preforms 5 to be positioned thereon have a substantially round outer cross-section with a diameter of, for example, 7.4 mm. They lie on the two cut edges 3a; 3b of the longitudinal grooves 3 and project into the cladding tube inner bore 6. For fixing, the two ends of the anti-resonance element preforms 5 are adhered in the region of the cladding tube end faces using a sealing and bonding compound containing SiO.sub.2. 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 6 of the cladding tube 1, it can be checked whether the longitudinal grooves 3 are completely closed by the anti-resonance element preforms 5. The longitudinal grooves 3 thus serve as an exact positioning aid on which each anti-resonance element preform 5 can be precisely positioned and fixed.

[0104] Instead of the thick-walled cladding tube 1, a cladding tube with a smaller wall thickness can also first be equipped with the anti-resonance element preforms 5 and additional sheath material can be applied to the primary preform thus produced, in particular by overlaying with an overlay cylinder brought to final dimension by machining.

[0105] During elongation of the primary preform 8 to form the hollow-core fiber or to form another precursor of the hollow-core fiber, gas can be introduced into or withdrawn from the hollow channels that have formed in the longitudinal grooves 3 and the fused anti-resonance element preforms 5, in order to produce positive pressure or negative pressure in the hollow channels.

[0106] If required or desired, the radial position of the anti-resonance elements 5 in the inner bore 6 of the cladding tube can thus be modified and corrected, as outlined in FIG. 2. Sketch (a) shows an anti-resonance element preform 5 having an ARE outer tube 5a and a nested (ARE inner tube 5b in the starting position. Sketch (b) shows the outer tube wall, which is deformed by pressure and heat and which is inverted inward in regions, with the ARE inner tube 5b fixed to the inversion in a radial position that is modified in comparison 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.

[0107] 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. FIG. 3(a) shows an anti-resonance element preform 5 with a simple 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 inward. As shown in FIG. 3(b), a further glass membrane 5c with a negatively (convexly) curved surface thus arises within the outer tube 5a, which curved surface can replace a nested inner element (such as the ARE inner tube 5b).

[0108] The wall thickness of the individual structural elements 5a, 5b of the anti-resonance element preforms 5 is in the range of 0.2 and 2 mm, and the outer diameter of the sheath 1 is in the range of 90 and 250 mm. The length of the components is the same and is 1 m.

[0109] A small deflection of the longitudinal axes of the structural elements is achieved by the mass of the cladding tube and the comparatively large-volume, tubular and long structural elements, assisted by the positioning of the anti-resonance element preforms with vertically oriented longitudinal axis. A maximum angular deviation of 0.3 degrees was measured.

[0110] Table 1 shows dimensions of these components for an anti-resonant hollow-core fiber in which the wall thickness (WT) of the structural elements for the anti-resonance elements in the final fiber is 0.55 μm. The column “Fiber” specifies further dimensions of the hollow-core fibers to be produced:

TABLE-US-00001 TABLE 1 Preform Preform Fiber OD 90 OD 220 WT = 0.55 μm (μm) (mm) (mm) Cladding OD 230 90 250 tube ID 98 38 107 OD/ID 2.3 ARE OD 29 11.3 31.5 ID 27.9 10.9 30.3 WT 0.55 0.22 0.60 ID/OD 0.96 0.96 0.96 OD/ID 1.04 1.04 1.04 NE OD 8.8 3.4 9.6 ID 7.7 3 8.4 WT 0.55 0.22 0.60 ID/OD 0.75 0.88 0.88 OD/ID 1.33 1.14 1.14 Core D 40 15.7 43.5 d d 5.5 2.15 6.98 Here: OD Outer diameter ID Inner diameter ARE: ARE outer tube 5a of the anti-resonance element preform NE ARE inner tube 5b of the anti-resonance element preform Core D Inner diameter of the hollow core d Wall thickness

[0111] Table 2 shows the dimensions for an anti-resonant hollow-core fiber in which the wall thickness (WT) of the structural elements in the final fiber is 1.10 μm. The column “Fiber” specifies the dimensions of the hollow-core fibers to be produced. The short terms used in Table 1 and explained therein are used.

TABLE-US-00002 TABLE 2 Preform Preform Fiber OD 90 OD 220 WT = 1.10 μm (μm) (mm) (mm) Cladding OD.sub.Sheath 230 90 250 tube ID.sub.Cane 98 38 107 OD/ID 2.3 ARE OD 29 11.3 31.5 ID 26.8 10.5 29.1 WT 1.10 0.43 1.20 ID/OD 0.92 0.92 0.92 OD/ID 1.08 1.08 1.08 NE OD 8.8 3.4 9.6 ID 6.6 2.6 7.2 WT 1.10 0.43 1.20 ID/OD 0.75 0.75 0.75 OD/ID 1.33 1.33 1.33 Core D 40 15.7 43.5 d d 5.5 2.15 5.98 z/R 0.90