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 further processing the primary preform in order to form a secondary preform, including a process of elongating the primary preform in order to directly form the hollow-core fiber or to form the 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 primary preform with an outer diameter ranging from 20 to 70 mm is used for the elongation process.

Claims

1. A method for producing an anti-resonant hollow-core fiber having a hollow core extending along a longitudinal axis of the hollow-core fiber and an inner sheath surrounding the hollow core, which inner sheath includes a plurality of anti-resonance elements, the method comprising the steps of: (a) providing a cladding tube having an inner bore and a longitudinal axis, along which a cladding tube wall delimited by an inner side and an outer side extends; (b) providing a plurality of tubular anti-resonance element preforms; (c) arranging the anti-resonance element preforms at desired positions of the inner side of the cladding tube wall to form a primary preform having an outer diameter in the range of 20 to 70 mm, a hollow core region, and an inner sheath region; (d) further processing of the primary preform into a secondary preform from which the hollow-core fiber is drawn, wherein the further processing includes elongating the primary preform and, optionally, a single or repeated performance of one or more of the following hot-forming processes: (i) collapsing the primary preform, (ii) collapsing and simultaneously elongating the primary preform, (iii) collapsing additional sheath material, (iv) collapsing additional sheath material and subsequently elongating the primary preform, (v) collapsing additional sheath material and simultaneously elongating the primary preform, and (e) drawing the secondary preform to form the hollow-core fiber.

2. The method according to claim 1, further comprising, during the elongation of the primary preform, using a temperature-controlled heating element, the temperature of which is kept precisely at +/−0.1° C.

3. The method according to claim 1, further comprising, during the elongation of the primary preform, feeding the primary preform to a heating zone at a feed rate set to yield a throughput of at least 0.8 g/min and an average dwell time in the heating zone of less than 25 min.

4. The method according to claim 1, wherein a draw-down ratio during the elongation of the primary preform is set to a value in the range of 1.05 to 10.

5. The method according to claim 1, further comprising fixing and/or sealing using a sealing or bonding compound containing amorphous SiO.sub.2 particles to arrange the anti-resonance element preforms and/or elongate the primary preform and/or draw the hollow-core fiber.

6. The method according to claim 5, wherein 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 of the anti-resonance element preforms are sealed by the sealing or bonding compound when the primary preform is elongated and/or when the hollow-core fibers are drawn.

7. The method according to claim 1, further comprising machining the inner side of the cladding tube and/or the outer side of the cladding tube.

8. The method according to claim 1, further comprising machining at the desired positions to provide the inner side of the cladding tube with a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube.

9. The method according to claim 1, wherein, when the anti-resonance element preforms are arranged at desired positions on the inside of the cladding tube wall, upper face ends of the anti-resonance element preforms are positioned at the desired positions using a positioning template.

10. The method according to claim 9, wherein the cladding tube has a pair of end faces and the positioning template is used in the region of at least one of the end faces.

11. The method according to claim 1, wherein the primary preform is made of quartz glass having a plurality of constituents and at least one dopant that lowers the viscosity of quartz glass and, when the primary preform is elongated according to method step (d) and/or when the hollow-core fiber is drawn according to method step (e), the plurality of constituents of the primary preform made of quartz glass are heated together and softened.

12. The method according to claim 11, wherein the cladding tube and the additional sheath material are each made of quartz glass, the additional sheath material is collapsed in accordance with method step (d), and the quartz glass of the cladding tube at a measuring temperature of 1250° C. has a viscosity at least 0.5 dPa.Math.s higher than the quartz glass of the additional sheath material (with the viscosity given as a logarithmic value in dPa.Math.s).

13. The method according to claim 1, further comprising fixing and/or sealing using a sealing or bonding compound containing amorphous SiO.sub.2 particles to arrange the anti-resonance element preforms according to method step (c) and/or to draw the hollow-core fiber according to method step (d).

14. A method for producing a preform for an anti-resonant hollow-core fiber having a hollow core extending along a longitudinal axis of the hollow-core fiber and an inner sheath surrounding the hollow core, which inner sheath includes a plurality of anti-resonance elements, the method comprising the steps of: (a) providing a cladding tube having an inner bore and a longitudinal axis, along which a cladding tube wall delimited by an inner side and an outer side extends; (b) providing a plurality of tubular anti-resonance element preforms; (c) arranging the anti-resonance element preforms at desired positions of the inner side of the cladding tube wall to form a primary preform having an outer diameter in the range of 20 to 70 mm, a hollow core region, and an inner sheath region; and (d) further processing the primary preform into a secondary preform for the hollow-core fiber, wherein the further processing includes elongating the primary preform and optionally a single or repeated performance of one or more of the following hot-forming processes: (i) collapsing the primary preform, (ii) collapsing and simultaneously elongating the primary preform, (iii) collapsing additional sheath material, (iv) collapsing additional sheath material and subsequently elongating the primary preform, (v) collapsing additional sheath material and simultaneously elongating the primary preform.

15. The method according to claim 3, wherein the feed rate is set to yield a throughput in the range from 0.8 g/m in to 85 g/m in.

16. The method according to claim 15, wherein the feed rate set to yield a throughput in the range from 3.3 g/m in to 85 g/m in.

17. The method according to claim 3, wherein the average dwell time is in the range of 5 to 25 min.

18. The method according to claim 4, wherein the draw-down ratio during the elongation of the primary preform is set to a value in the range of 1.05 to 5.

19. The method according to claim 7, wherein the step of machining includes drilling, milling, grinding, honing, and/or polishing.

20. The method according to claim 10, wherein the positioning template is used in the region of both of the end faces.

21. The method according to claim 12, wherein the quartz glass of the cladding tube, at a measured temperature of 1250° C., has a viscosity higher by at least 0.6 dPa.Math.s than the quartz glass of the additional sheath material (with the viscosity given as a logarithmic value in dPa.Math.s).

Description

EXEMPLARY EMBODIMENT

[0104] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in schematic representation

[0105] FIG. 1 a primary preform with a cladding tube and anti-resonance element preforms positioned and fixed therein for producing a preform for a hollow-core fiber based on a view of the cross-section.

[0106] 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 and as disclosed in DE 10 2004 054 392 A1 is used. 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 D.sub.90 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.

[0107] FIG. 1 schematically shows a primary preform 3 with a cladding tube 1 having a cladding tube wall 2, on the inner sheath surface of which are fixed, at a uniform distance, anti-resonance element preforms 4 at previously defined azimuthal positions; in the exemplary embodiment, there are six preforms 4; in another preferred embodiment (not shown), there is an odd number of preforms.

[0108] The inner cladding tube 1 consists of quartz glass and has a length of 1000 mm, an outer diameter of 27 mm and an inner diameter of 20 mm. The anti-resonance element preforms 4 are present as an ensemble of nested structural elements consisting of an ARE outer tube 4a and an ARE inner tube 4b. The ARE outer tube 4a has an outer diameter of 6.2 mm and the ARE inner tube 4b has an outer diameter of 2.5 mm. The wall thickness of both structural elements (4a; 4b) is equal and is 0.3 mm. The lengths of ARE outer tube 4a and ARE inner tube 4b correspond to the cladding tube length 1.

[0109] The anti-resonance element preforms 4 are fixed to the inner wall of the cladding tube 1 by means of the bonding compound 5 based on SiO.sub.2.

[0110] The bonding compound 5 is applied locally to the inner sheath surface of the cladding tube in the region of the face ends, and the anti-resonance element preforms are placed thereon using a positioning template with a structurally predetermined star-shaped arrangement of holding arms for the individual anti-resonance element preforms 4. In this case, the positioning template is limited to the region around the two face ends of the cladding tube.

[0111] This method creates a precise and reproducible connection between the cladding tube 1 and the anti-resonance element preforms 4. Solidification of the bonding compound 5 at a low temperature below 300° C. is sufficient for fixing, so that an intense heating of the surrounding regions and thus a deformation of anti-resonance element preforms 4 is avoided.

[0112] The primary preform 3 is collected with a collecting cylinder made of quartz glass, wherein the collecting cylinder collapses onto the cladding tube 1, and at the same time, the tube ensemble is elongated to form a secondary preform. The collecting cylinder has an outer diameter of 63.4 mm and a wall thickness of 17 mm.

[0113] In the collapse and elongation process, the coaxial arrangement of the cladding tube 1 and the collecting cylinder coming from below in a vertically oriented longitudinal axis is fed to a temperature-controlled heating zone and softens therein zone-by-zone starting with the upper end of the arrangement.

[0114] The heating zone is kept at a desired temperature of 1600° C. with a control accuracy of +/−0.1° C. Temperature fluctuations in the hot-forming process can thereby be limited to less than +1-0.5° C.

[0115] The secondary preform formed in the collapse and elongation process has an outer diameter of approximately 50 mm and a sheath wall thickness of 16.6 mm composed of an outer sheath and an inner sheath. The maximum wall thickness variation (greatest value minus smallest value) of the anti-resonance element preforms is less than 4 μm. The secondary preform is subsequently drawn into the anti-resonant hollow-core fiber.

[0116] The following table mentions the removal parameters at different outer diameters before (BEFORE) and after (AFTER) the forming process (collapsing the outer sheath and elongation).

TABLE-US-00001 TABLE 1 Outer Outer diameter diameter Cladding BEFORE AFTER tube length Feed rate Removal [mm] [mm] [mm] [mm/min] [mm/min] 90 70 1000 15 9.80 80 70 1000 15 4.59 40 20 1000 5 15 25 20 1000 10 5.63

[0117] The heating zone has a length of 100 mm. For example, a cladding tube having an outer diameter of 90 mm and a wall thickness of 10 mm at a feed rate of 5 mm/min results in a throughput of 27.6 g/min into the heating zone; at a feed rate of 15 mm/min, the throughput is 83 g/min. At the feed rates of 5 mm/min or 15 mm/min, throughputs of 0.8 g/min or 2.49 g/min result for a tube having an outer diameter of 25 mm and 1 mm wall thickness.

[0118] In the following table, the outer diameters (OD) or inner diameters (ID) of preforms and components thereof are summarized as a function of the desired diameter ratio (OD/ID) between the outer and inner diameters of the sheath region of the hollow-core fiber.

TABLE-US-00002 TABLE 2 OD ID (former OD (former primary (secondary primary preform in Fiber preform preform) the cane No. OD/ID OD/ID (mm) (mm) (mm) 1 2.3 230/98 90 38 46 2 2.9 230/80 90 31 39 3 2.0 200/98 90 44 53 4 3.0 230/98 50 16.8 22.2 5 2.3 230/98 25 11 13 6 2.3 230/98 100 58 75 7 4.0 230/90 50 12.5 18.8

[0119] The value for OD/ID in table column 2 results from dividing the values of columns 4 (outer diameter of the secondary preform) and 5 (inner diameter of the former primary preform in the secondary preform).

[0120] The maximum deviation of the wall thickness of the anti-resonance element preforms in the preform is about 4 μm in all exemplary embodiments. Hollow-core fibers having outer diameters of 200 μm and 230 mm, respectively, were drawn from the preforms, as indicated in the table above, and the wall thicknesses of the anti-resonance elements were determined. Example no. 4 of the table corresponds to the exemplary embodiment described in detail above. Examples 5 and 6 are comparative examples. In the fiber-drawing process while using the preforms of the comparative examples, no hollow-core fibers with an optimal geometry were obtained. This is attributed to too large or too small a primary preform during the elongation process.