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; further processing the primary preform in order to form a secondary preform, including an elongation process; and drawing the secondary preform in order to form the hollow-core fiber. 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 after the primary preform is elongated, at least some of the formerly tubular anti-resonant element preforms of the primary preform have an oval outer cross-sectional shape with a longest cross-sectional axis A.sub.L and a shortest cross-sectional axis A.sub.K, wherein the ratio A.sub.L/A.sub.K of the length of the axes ranges from 1.01 to 1.27, and the shortest cross-sectional axis A.sub.K runs in a radial direction when viewed from the cladding tube longitudinal axis.

Claims

1. Method for producing an anti-resonant hollow-core fiber (33) 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 (2) delimited by an inner side and an outer side extends, (b) providing a number of tubular anti-resonance element preforms (4; 34), (c) arranging the anti-resonance element preforms (4; 34) at desired positions of the inner side of the cladding tube wall (2) to form a primary preform (3; 31), which comprises a hollow core region and an inner sheath region, (d) further processing of the primary preform (3; 31) to form a secondary preform (32) from which the hollow-core fiber (33) is drawn, wherein the further processing comprises an elongation and, optionally, a single or repeated performance of one or more of the following hot-forming processes: (i) collapse, (ii) collapse and simultaneous elongation, (iii) collapse of additional sheath material, (iv) collapse of additional sheath material and subsequent elongation, (v) collapse of additional sheath material and simultaneous elongation, and (e) drawing the secondary preform (32) to form the hollow-core fiber (33), characterized in that, after elongation in accordance with method step (d), at least a portion of the former tubular anti-resonance element preforms (4; 34) of the primary preform (3; 31) has an oval outer cross-sectional shape, with a longest cross-sectional axis A.sub.L and a shortest cross-sectional axis A.sub.K, wherein the axis length ratio A.sub.L/A.sub.K is in the range between 1.01 and 1.27, and wherein the shortest cross-sectional axis A.sub.K extends in the radial direction when viewed from the longitudinal axis of the cladding tube.

2. Method according to claim 1, characterized in that at least a portion of the tubular anti-resonance element preforms (4; 34) is composed of several nested structural elements (4a; 4b) and comprises at least one ARE outer tube (4a) and an ARE inner tube (4b) running in the ARE outer tube (4a) and in parallel to the longitudinal axis of the ARE outer tube, wherein the ARE outer tube (4a) has an oval, preferably elliptical, outer cross-sectional shape with the axis length ratio A.sub.L/A.sub.K in the range between 1.07 and 1.27, and the ARE inner tube (4b) has an oval, preferably elliptical, outer cross-sectional shape with the axis length ratio A.sub.L/A.sub.K in the range between 1.01 and 1.05.

3. Method according to any of the preceding claims, characterized in that the primary preform (3; 31) has an outer diameter in the range of 20 to 70 mm.

4. Method according to any of the preceding claims, characterized in that, during elongation, the primary preform (3; 31) is continuously supplied to a heating zone at a feed rate, softened zone by zone in the heating zone, and removed from the heating zone at a removal rate, wherein the feed rate is set so as to result in a throughput of at least 0.8 g/min, preferably a throughput in the range from 0.8 g/min to 85 g/min, and particularly preferably a throughput in the range of 3.3 g/min to 85 g/min, and an average dwell time in the heating zone of less than 25 min, preferably an average dwell time in the range of 5 to 25 min.

5. Method according to any of the preceding claims, characterized in that the draw-down ratio during elongation is set to a value in the range of 1.05 to 10, preferably 1.05 to 5.

6. Method according to any of the preceding claims, characterized in that a temperature-controlled heating zone is used for elongating the primary preform (3; 31), the desired temperature of which is kept with an accuracy of +/−0.1° C.

7. Method according to any of the preceding claims, characterized in that the fixing of the anti-resonance element preforms (4) takes place using a sealing or bonding compound (5) containing amorphous SiO.sub.2 particles.

8. Method according to any of the preceding claims, characterized in that open ends of the anti-resonance element preforms (4) and/or individual structural elements (4a; 4b) of the anti-resonance element preforms (4) and/or any annular gap between tube elements are sealed by means of a sealing or bonding compound (5) when the primary preform (3; 31) is elongated.

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

10. Method according to any of the preceding claims, characterized in that the anti-resonance element preforms (4; 34) are positioned at the desired position by means of a positioning template.

11. Method according to claim 10, 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.

12. Method according to any of the preceding claims, characterized in that, when the primary preform (3; 31) is elongated in accordance with method step (d) and/or when the hollow-core fiber (33) is drawn in accordance with method step (e), several components of the preform 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.

13. Method according to claim 12, characterized in that additional sheath material is collapsed in accordance with method step (d), and in that the quartz glass of the cladding tube 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).

14. Method according to any of the preceding claims, characterized in that the arrangement of the anti-resonance element preforms (4; 34) at desired positions of the inner side of the cladding tube wall (2) and/or the drawing of the hollow-core fiber (33) in accordance with method step (d) comprises a fixing measure and/or a sealing measure using a sealing or bonding compound (5) containing amorphous SiO.sub.2 particles.

15. Method for producing a preform (32) 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 (2) delimited by an inner side and an outer side extends, (b) providing a number of tubular anti-resonance element preforms (4), (c) arranging the anti-resonance element preforms (4; 34) at desired positions of the inner side of the cladding tube wall (2) to form a primary preform (3; 31), which comprises a hollow core region and an inner sheath region, (d) further processing the primary preform (3; 31) to form a secondary preform (32) for the hollow-core fiber, wherein the further processing comprises an elongation and optionally a single or repeated performance of one or more of the following hot-forming processes: (i) collapse, (ii) collapse and simultaneous elongation, (iii) collapse of additional sheath material, (iv) collapse of additional sheath material and subsequent elongation, (v) collapse of additional sheath material and simultaneous elongation, and characterized in that, after elongation in accordance with method step (d), at least a portion of the former tubular anti-resonance element preforms (4; 34) of the primary preform (3; 31) has an oval outer cross-sectional shape, with a longest cross-sectional axis A.sub.L and a shortest cross-sectional axis A.sub.K, wherein the axis length ratio A.sub.L/A.sub.K is in the range between 1.01 and 1.27, and wherein the shortest cross-sectional axis A.sub.K extends in the radial direction when viewed from the longitudinal axis of the cladding tube.

Description

EXEMPLARY EMBODIMENT

[0110] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in detail:

[0111] FIG. 1 a primary preform with a cladding tube and anti-resonance element preforms positioned and fixed therein for producing a secondary preform for a hollow-core fiber based on a view of the cross-section in a schematic representation,

[0112] FIG. 2 a photograph of a secondary preform with preformed anti-resonance element preforms in a view of the cross-section,

[0113] FIG. 3 processing stages from the primary preform to the anti-resonant hollow-core fiber, and

[0114] FIG. 4 a sketch for explaining force conditions in the hot processing of nested anti-resonance elements.

[0115] In the production of the hollow-core fiber or the preform for the hollow-core fiber, a plurality of components is to be connected to one another. 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 quartz 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.

[0116] FIG. 1 schematically shows a primary preform 3 with a cladding tube 1 having a cladding tube wall 2, to 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.

[0117] The 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 thicknesses of the two structural members (4a; 4b) are the same and are 0.3 mm. The lengths of ARE outer tube 4a and ARE inner tube 4b correspond to the cladding tube length.

[0118] The cladding tube 1 is produced in a vertical drawing process without a molding tool with a two-stage elongation process. In the first stage, a hollow starting cylinder made of glass is machined for setting the final dimensions of the hollow starting cylinder. Per the final dimension, the outer diameter is 90 mm and the diameter ratio of outer and inner diameters is 2.5. In a first elongation process, the starting cylinder with a vertically oriented longitudinal axis is continuously supplied to a heating zone having a heating zone length of 200 mm, softened in regions therein, and an intermediate cylinder is withdrawn from the softened region. In a second elongation process, the intermediate cylinder with a vertically oriented longitudinal axis is continuously supplied to a different heating zone having a heating zone length of 100 mm, softened in regions therein, and a tube section is withdrawn from the softened region. The cladding tube is obtained from the tube section by cutting it to length.

[0119] The anti-resonance element preforms 4 are fixed to the inner side of the cladding tube 1 by means of the bonding compound 5 based on SiO.sub.2.

[0120] 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.

[0121] 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 the anti-resonance element preforms 4 is avoided.

[0122] The primary preform 3 is overlaid with an overlay cylinder made of quartz glass, wherein the overlay cylinder collapses onto the cladding tube 1, and at the same time, the tube ensemble is elongated to form a secondary preform. The overlay cylinder has an outer diameter of 63.4 mm and a wall thickness of 17 mm.

[0123] In the collapse and elongation process, the coaxial arrangement of the cladding tube 1 and of the overlay cylinder with a vertically oriented longitudinal axis is supplied from below to a temperature-controlled heating zone and softens therein zone by zone starting with the upper end of the arrangement.

[0124] The heating zone is kept at a desired temperature of 1600° C. with a control accuracy of +1-0.1° C. Temperature fluctuations in the hot-forming process can thereby be limited to less than +1-0.5° C.

[0125] In the collapse and elongation step, an oval outer cross-sectional shape is impressed on the former tubular anti-resonance element preforms 4 of the primary preform 3 by generating and maintaining the same internal pressure in the hollow core region and in the tubular anti-resonance element preforms 4. FIG. 4 outlines the force conditions occurring in this case. During elongation, ambient pressure p.sub.a prevails both in the hollow core region 40 and in the inner bore of the ARE inner tube 4b. With an overpressure of 2.36 mbar in the cavity of the ARE inner tube 4b (in comparison to the pressure outside it), the ARE inner tube 4a would be stable during elongation. Without the internal pressure, forces act as indicated by the arrows F. The ratio of the normal force F.sub.N1 at the upper apex “S” to the resulting normal force F.sub.N2 on the side of the ARE inner tube wall is between 1.07 and 1.27 (1,07<F.sub.N1/F.sub.N2<1,27), As a result of this force ratio, the ARE inner tube 4b becomes oval and approximately elliptical during elongation, wherein the long axis of the ellipse runs tangentially to the wall of the ARE outer tube 4a and the shorter axis runs perpendicularly thereto.

[0126] Corresponding considerations for the ARE outer tube 4a result in an oval outer cross-sectional shape, which is characterized by an axis length ratio A.sub.L/A.sub.K being between 1.07 and 1.27 (1.07<A.sub.L/A.sub.K<1.27).

[0127] The secondary preform formed in this way during 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 photograph of FIG. 2 shows the secondary preform 32 thus produced with the deformed anti-resonance element preforms 35 and in particular the oval-shaped ARE outer tubes. The ARE inner tubes are also measurably deformed ovally but to such a small extent that the deformation is not visible in FIG. 2.

[0128] The secondary preform is elongated to form an anti-resonant hollow-core fiber. For this purpose, all structural elements of the former anti-resonance element preforms, i.e., ARE outer tube 4a and ARE inner tube 4b, are sealed with the aforementioned sealing or bonding compound. Here, the sealing compound is applied only to the end face of the anti-resonance element preforms that points upward during the fiber-drawing process.

[0129] The same end face is then connected to a holding tube made of quartz glass, which simultaneously serves as a gas connection. The holder is fixed to the overlay cylinder and to the cladding tube by means of the sealing or bonding compound 5. In the fiber-drawing process, the secondary preform with a vertically oriented longitudinal axis is supplied from above to a temperature-controlled heating zone and softens therein zone by zone starting at the lower end. The heating zone is kept at a desired temperature of approximately 2100° C. with a control accuracy of +/−0.1° C. Temperature fluctuations in the hot-forming process can thereby be limited to less than +/−0.5° C. At the same time, gas is supplied to the core region (hollow core) so that an internal pressure of 4 mbar is adjusted in the core region.

[0130] As a result of the fiber-drawing process conducted in this manner, the oval cross-sectional shape of the former structural elements of the anti-resonance element preforms becomes a round cross-sectional shape so that an anti-resonant hollow-core fiber with anti-resonance elements embedded therein, which have a round cross-sectional shape, is obtained.

[0131] FIG. 3 schematically shows the method stages of the process. As a result of an elongation and collapse process 37, the primary preform 31 with anti-resonance element preforms 34 having a round cross-section becomes a secondary preform 32 with elongated anti-resonance element preforms 35 having an oval cross-sectional shape (characterized by the short cross-sectional axis A.sub.K and the long cross-sectional axis A.sub.L), which is elongated in a fiber-drawing process 38 to form an anti-resonant hollow-core fiber 33 containing anti-resonance elements 36 with a round cross-sectional shape.