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; forming a number of precursors for anti-resonant elements at target positions of the cladding tube wall; 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 from which the hollow-core fiber is drawn. 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 formation of the anti-resonant element precursors includes the formation of elongated pressure chambers, each of which adjoins a wall that can be deformed under pressure and heat in the region of the target positions of the anti-resonant elements and which cause a section of the deformable wall to protrude in the direction of the cladding tube inner bore under the effect of pressure and heat, thereby forming an anti-resonant element or a precursor for same, while carrying out a process according to step (c).

Claims

1. Method for producing an anti-resonant hollow-core fiber (29; 129) comprising a hollow core, extending along a longitudinal axis of the fiber, and a sheath region surrounding the hollow core and comprising a number of anti-resonance elements, having the method steps of: (a) providing a primary preform (26; 126) for the hollow-core fiber, which preform has at least one cladding tube having an inner cladding tube bore and a longitudinal cladding tube axis, along which a cladding tube wall (24; 124) delimited by an inner side and an outer side extends, (b) forming a number of precursors (25a; 25b; 125; 213) for anti-resonance elements (28a; 28b; 128) at setpoint positions of the cladding tube wall (24; 124), and (c) elongating the primary preform (26; 126) to form the hollow-core fiber (29; 129) or further processing the primary preform (26; 126) to form a secondary preform from which the hollow-core fiber is drawn, wherein the further processing comprises 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 sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that the formation of the anti-resonance element precursors (25a; 25b; 125; 213) comprises the formation of elongated pressure chambers (25a; 25b; 125; 213), which respectively adjoin, in the region of the setpoint positions of the anti-resonance elements (28a; 28b; 128), a wall (21; 22) that can be deformed under pressure and heat, and which, when a process according to method step (c) is carried out, cause a bulge in a section of the deformable wall (21; 22) in the direction of the inner cladding tube bore (16) as a result of pressure and heat, thereby forming an anti-resonance element (29; 129) or a precursor thereof.

2. Method according to claim 1, characterized in that the pressure chambers are designed as hollow channels (13; 25a; 25b; 125; 213) which are adjoined by the wall sections of a glass tube (21; 221).

3. Method according to claim 2, characterized in that the hollow channels (213) are formed within the tube wall of the glass tube (221) and run in parallel to the longitudinal axis of the glass tube.

4. Method according to claim 3, characterized in that a coaxial glass tube arrangement is formed, comprising an inner peripheral row of hollow channels in the tube wall of an inner glass tube and an outer peripheral row of hollow channels in the tube wall of an outer glass tube, wherein the hollow channels of the inner and outer peripheral rows lie on a common connecting line when viewed in the radial direction and are spatially separated from one another by at least one peripheral, inward-deformable glass wall.

5. Method according to claim 2, characterized in that the formation of the hollow channels (13; 25a; 25b; 125) comprises a measure in which an intermediate tube (10; 20) is arranged between the glass tube (21) and an outer tube (22; 23), said intermediate tube having a longitudinal axis of the intermediate tube, along which an intermediate tube wall delimited by an inner side and an outer side extends, and in that longitudinal slots (13) are introduced into the intermediate tube wall, wherein hollow channels (25a; 25b; 125) are formed from the longitudinal slots (13) when a process according to method step (c) is carried out.

6. Method according to claim 5, characterized in that an intermediate tube (10; 20) with a circular inner cross-section is provided and machined.

7. Method according to claim 5 or 6, characterized in that the intermediate tube (10; 20) has end-face ends, and in that the longitudinal slots (13) end prior to the end-face ends.

8. Method according to one of claims 5 to 7, characterized in that a coaxial tube arrangement (19) is formed, comprising an inner glass tube (21), an inner intermediate tube (10), an inner outer tube (22) that at the same time forms an outer glass tube (22), an outer intermediate tube (20), and an external outer tube (23), wherein the longitudinal slots (13) of the inner and outer intermediate tubes (10; 20) lie on a common connecting line in the radial direction and are spatially separated from one another by at least one peripheral, inward-deformable glass wall (22).

9. Method according to claim 2, characterized in that the formation of the hollow channels comprises a measure in which a glass tube and an intermediate tube are used, which coaxially surrounds the glass tube, wherein the glass tube has an outer sheath surface of the glass tube, into which surface longitudinal grooves are introduced that run in parallel to the longitudinal axis of the glass tube, and/or the intermediate tube has an inner sheath surface of the intermediate tube, into which surface longitudinal grooves are introduced that run in parallel to the longitudinal axis of the intermediate tube, wherein hollow channels are formed from the longitudinal grooves when carrying out a process according to method step (c), and wherein the hollow channels are deformed into the elongated bulges as a result of pressure and heat.

10. Method according to one of claims 5 to 9, characterized in that the longitudinal slots (13) or the longitudinal grooves are produced by machining—in particular, by cutting, drilling, sawing, milling, or grinding.

11. Method according to one of claims 5 to 10, characterized in that the longitudinal slots (13) and/or longitudinal grooves have longitudinal edges, and in that the longitudinal edges are fused with surrounding glass material by softening—preferably with simultaneous elongation.

12. Method according to one of claims 2 to 11, characterized in that the glass tube (21; 22) consists of a glass containing a dopant that lowers the viscosity.

13. Method for producing a preform for an anti-resonant hollow-core fiber having 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, having the method steps of: (a) providing a primary preform (26; 126) for the hollow-core fiber, which preform has at least one cladding tube having an inner cladding tube bore and a longitudinal cladding tube axis, along which a cladding tube wall (24; 124) delimited by an inner side and an outer side extends, (b) forming a number of precursors (25a; 25b; 125; 213) for anti-resonance elements (28a; 28b; 128) at setpoint positions of the cladding tube wall (24; 124), (c) optional further processing of the primary preform (26; 126) to form a secondary preform for the hollow-core fiber, wherein the further processing comprises 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 sheath material, (v) collapse of additional sheath material and subsequent elongation, (vi) collapse of additional sheath material and simultaneous elongation, characterized in that the formation of the anti-resonance element precursors (25a; 25b; 125; 213) comprises the formation of elongated pressure chambers (25a; 25b; 125; 213), which respectively adjoin, in the region of the setpoint positions of the anti-resonance elements (28a; 28b; 128), a wall (21; 22) that can be deformed under pressure and heat, and which, when a process according to method step (c) is carried out, cause a bulge in a section of the deformable wall (21; 22) in the direction of the inner cladding tube bore (16) as a result of pressure and heat, thereby forming an anti-resonance element (29; 129) or a precursor thereof.

Description

EXEMPLARY EMBODIMENT

[0096] 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:

[0097] FIG. 1 a first embodiment of an intermediate tube provided with longitudinal slots for use in the method according to the invention in a side view (a) and in a plan view (b) of a cross-section,

[0098] FIG. 2 method steps for producing a preform (b) for a hollow-core fiber using a tube ensemble (a) having several slotted, intermediate tubes, with reference to a first example,

[0099] FIG. 3 the fiber-drawing of the preform of FIG. 3 to form a hollow-core fiber, thereby forming anti-resonance elements in a cutout of the preform of FIG. 2(b) in an enlarged view,

[0100] FIG. 4 a second embodiment of an intermediate tube provided with longitudinal slots in a side view (a) and in a plan view (b) of a cross-section,

[0101] FIG. 5 method steps for producing a preform (b) for a hollow-core fiber using a tube ensemble (a) having a slotted, intermediate tube, with reference to a second example,

[0102] FIG. 6 the fiber-drawing of the preform of FIG. 5 to form a hollow-core fiber, thereby forming anti-resonance elements in a cutout of the preform of FIG. 5(b) in an enlarged view, and

[0103] FIG. 7 a coaxial arrangement of a glass tube with a thermally-deformable wall, which contains hollow channels, and an overlay cylinder in a plan view.

[0104] FIG. 1(a) shows an intermediate tube 10 in the wall of which, at previously defined azimuthal positions, longitudinal slots 13 are cut at regular intervals—for example, by means of a mechanical saw, water-jet cutting, laser, or the like. The longitudinal slots 13 serve to form anti-resonance elements in the finished hollow-core fiber or to form anti-resonance element preforms in a fiber preform, and the number of longitudinal slots 13 corresponds to the number of anti-resonance element preforms or anti-resonance elements that can be produced with the respective intermediate tube 10. In the exemplary embodiment, there are six anti-resonance element preforms or anti-resonance elements. The longitudinal slots 13 end prior to the tube ends so that the end-face end regions 12 continue to remain closed circumferentially and connect the remaining webs 14 to one another. The cut edges are subsequently vitrified. The cutting width of the longitudinal slots 13 is uniform and is 2 mm.

[0105] It can be seen from the plan view of the cross-section of the intermediate tube 10 along the sectional line A-A in FIG. 1(b) that the six longitudinal slots 13 are distributed evenly around the tube wall, and that they extend continuously from the outer wall of the intermediate tube to the inner wall of the intermediate tube up to the inner bore 16.

[0106] FIG. 2(a) schematically shows a plan view of a coaxial arrangement 19 of a total of five quartz glass tubes, including two intermediate tubes 10; 20, each with longitudinal slots 13. The coaxial tube arrangement 19 is composed of two coaxial stacks, which are each composed of a glass tube (21; 22) to be deformed, an intermediate tube (10; 20), and a sheath tube (22; 23). The tube with reference sign 22 has a dual function: in the inner stack, it forms a “sheath tube” and its wall becomes a constituent of the wall of hollow channels, and, in the outer stack, it forms a “glass tube” with a wall to be deformed.

[0107] Details on the dimensions and materials of the tubes are summarized in Table 1 below:

TABLE-US-00001 TABLE 1 Identifier/ RS Function Inner diameter Outer diameter Material 21 Glass tube 21 24 F-doped; to be deformed F320 10 Intermediate tube 25 31.5 Non-doped; 22 Glass tube or 32.5 34 F-doped; sheath tube F320 to be deformed 20 Intermediate tube 35 39 Non-doped 23 Sheath tube 40 60 Non-doped RS: Reference sign in FIG. 2a F-doped; F320: Quartz glass doped with fluorine/low viscosity Non-doped: Non-doped quartz glass/high viscosity

[0108] The materials used differ in terms of their viscosity. The unmachined tubes 21 and 22 consist of a commercially available quartz glass doped with fluorine (trade name: F320) and have a lower viscosity than the slotted, intermediate tubes 10, 20 and than the outermost sheath tube 23 (overlay tube).

[0109] FIG. 2(b) shows that the coaxial tube ensemble 19 is subsequently collapsed to form a primary preform 26 and is simultaneously elongated in the process. In this case, the annular gaps between the tubes disappear, and the tubes are fixedly connected to one another and form the cladding tube having a common cladding tube wall 24. Hollow channels are formed from the longitudinal slots 13 of the slotted intermediate tubes 10; 20 and can be used as front and rear pressure chambers 25a; 25b in the subsequent fiber-drawing process. In this case, two pressure chambers 25a, 25b each lie, one behind the other, in pairs when viewed in the radial direction. Before the fiber-drawing process, at least one of the closed, longitudinal, slot-free end regions of the primary preform 26 is removed so that pressure chambers 25a, 25b that are open at the end face are obtained, into which a compressed gas can be introduced. The primary preform 26 has a hollow core region 27 surrounded by a sheath (cladding tube wall 24). The pressure chambers 25a; 25b form precursors for anti-resonance elements of the hollow-core fiber to be drawn in the cladding tube wall 24.

[0110] The cutout of FIG. 3(a) shows in an enlargement the pressure chambers 25a; 25b produced from the original longitudinal slots in the cladding tube wall 24. When the preform thus obtained is drawn to form the fiber, a differential pressure is applied between the pressure chambers 25a, 25b and the hollow core region 27 so that the deformable wall regions of the original glass tubes 21; 22 adjoining the pressure chambers 25a; 25b are blown inwards along the pressure chambers 25a, 25b.

[0111] FIG. 3(b) shows that, in doing so, a first bulge 28a, which encloses a second bulge 28b, arises in the hollow-core fiber 29 on the inner side 17 of the former innermost glass tube. The first and second bulges 28a; 28b form a nested, anti-resonance element with two glass membranes having a negatively-curved surface.

[0112] Insofar as the same reference signs are used in the following description of FIGS. 4 to 7 as in FIGS. 1 to 3, identical or equivalent components or constituents are denoted thereby, as explained in more detail above with reference to these figures.

[0113] FIG. 4(a) shows another intermediate tube 110 in the wall of which, at previously defined azimuthal positions, longitudinal slots 13 are cut at regular intervals—for example, by means of a mechanical saw, water-jet cutting, laser, or the like. The longitudinal slots 13 serve to form anti-resonance elements in the finished hollow-core fiber or to form anti-resonance element preforms in a fiber preform, and the number of longitudinal slots 13 corresponds to the number of anti-resonance element preforms or anti-resonance elements that can be produced with the respective intermediate tube 10. In the exemplary embodiment, there are five anti-resonance element preforms or anti-resonance elements. The longitudinal slots 13 end prior to the tube ends so that the end-face end regions 12 continue to remain closed circumferentially and connect the remaining webs 14 to one another. The cut edges are subsequently vitrified. The cutting width of the longitudinal slots 13 is uniform and is 2 mm.

[0114] It can be seen from the plan view of the cross-section of the intermediate tube 110 along the sectional line A-A in FIG. 4(b) that the five longitudinal slots 13 are distributed evenly around the tube wall at a circumferential angle of 72 degrees, and that they extend continuously from the outer wall of the intermediate tube to the inner wall of the intermediate tube up to the inner bore 16.

[0115] FIG. 5(a) schematically shows a plan view of a coaxial ensemble of a total of three quartz-glass tubes, including the slotted, intermediate tube 110. The coaxial tube ensemble is composed of a glass tube 21 to be deformed, the intermediate tube 110 with the longitudinal slots 13, and a sheath tube 22.

[0116] Details on the dimensions and materials of the tubes are summarized in Table 2 below:

TABLE-US-00002 TABLE 2 Identifier/ Inner diameter Outer diameter RS Function [mm] [mm] Material 21 Glass tube 21 24 F-doped; to be deformed F320 110 Intermediate tube 25 31.5 Non-doped; 22 Sheath tube 32.5 60 Non-doped RS: Reference sign in FIG. 5a F-doped; F320: Quartz glass doped with fluorine/low viscosity Non-doped: non-doped quartz glass/high viscosity

[0117] The materials used differ in terms of their viscosity. The mechanically unmachined tube 21 consists of a commercially available quartz glass doped with fluorine (trade name: F320) and has a lower viscosity than the slotted intermediate tube 110 and the sheath tube 22 (overlay tube).

[0118] FIG. 5(b) shows that the coaxial tube ensemble subsequently collapses to form a primary preform 126. In this case, it is elongated at the same time, and the annular gaps between the tubes 21, 22, 110 disappear, so that they are firmly connected to one another, so that they form a common cladding tube wall 124. In doing so, hollow channels form in the cladding tube wall 124 from the longitudinal slots 13 of the slotted intermediate tube 110 and can be used as pressure chambers 125 in the subsequent fiber-drawing process. The primary preform 126 has a hollow core region 127 surrounded by a sheath (cladding tube wall 124). The pressure chambers 125 form precursors for anti-resonance elements in the sheath region of the hollow-core fiber to be drawn.

[0119] Before the fiber-drawing process, at least one of the closed, longitudinal slot-free end regions of the preform 126 is removed so that the pressure chambers 125 are open at the end face, and a compressed gas can be introduced.

[0120] The cutout of FIG. 6(a) shows a pressure chamber 125 produced from an original longitudinal slot 13 in an enlargement. When the preform 126 thus obtained is drawn to form the fiber, a differential pressure is applied between the pressure chambers 125 and the inner bore 16 so that the wall region adjoining the pressure chambers 125 is blown inwards along the pressure chamber 125.

[0121] FIG. 6(b) shows that a bulge 128, which forms an anti-resonance element with a glass membrane with a negatively-curved surface, arises in the hollow-core fiber 129 on the inner side 117 of the former glass tube.

[0122] Instead of the longitudinal slots 13, the intermediate tubes 10, 20, 110 can also be provided with longitudinal grooves on their inner sheath surface. If necessary, the longitudinal grooves are produced by mechanical milling in the inner sheath surface of the intermediate tube.

[0123] Instead of or in addition to the intermediate tubes provided with longitudinal slots or longitudinal grooves, the glass tubes 21; 22 may also be provided with a thermally-deformable wall with longitudinal grooves on its outer sheath surface. If necessary, the longitudinal grooves are produced by mechanical milling in the outer sheath surface of the glass tube.

[0124] Instead of or in addition to the aforementioned embodiments of glass tubes and/or intermediate tubes with longitudinal slots or longitudinal grooves, the glass tubes 21; 22 may also be provided with a thermally-deformable wall with hollow channels. With respect to this, FIG. 7 schematically shows an exemplary embodiment. The coaxial arrangement comprises a glass tube 221 with a thermally-deformable wall and an overlay cylinder 22 (sheath tube). Within the wall of the glass tube 221 and close to the inner sheath surface 221a, four hollow channels 213 distributed evenly around the circumference run in parallel to the longitudinal axis of the glass tube (which extends perpendicularly to the paper plane). The hollow channels 213 are produced by laser-cutting and continuous (they extend from one end of the wall to the other end). In the cross-section shown, they have a rectangular shape, wherein the long rectangular side runs tangentially to the adjacent wall section of the inner sheath surface 221a.

[0125] An intermediate tube can be dispensed with in this embodiment. Details on the dimensions and materials of the tubes are summarized in Table 3 below:

TABLE-US-00003 TABLE 3 Identifier/ Inner diameter Outer diameter RS Function [mm] [mm] Material 221 Glass tube 21 24 F-doped; to be deformed F320 22 Sheath tube 25 60 Non-doped RS: Reference sign in FIG. 7 F-doped; F320: Quartz glass doped with fluorine/low viscosity Non-doped: non-doped quartz glass/high viscosity

[0126] When carrying out a hot-forming process, an internal pressure can be generated in the hollow channels 213 by introducing a compressed gas, thereby deforming the wall sections of the glass tube 221, which delimits the hollow channels 213 inwards. In the process, elongated bulges pointing inwards, in the direction of the inner glass tube bore 16, form on the glass tube 221 and serve as anti-resonance element preforms.