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 by providing anti-resonant element preforms which have at least one respective ARE outer tube and/or at least one respective ARE inner tube, wherein the ARE outer tube and/or the ARE inner tube is produced using a vertical drawing method without molding tools.

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 a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (21) comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube, along which a cladding tube wall (22) delimited by an inner side and an outer side extends, (b) providing a number of tubular anti-resonance element preforms (24), (c) arranging the anti-resonance element preforms (24) at desired positions on the inner side of the cladding tube wall (22) to form a primary preform (23), which comprises a hollow core region and an inner sheath region, and (d) elongating the primary preform (23) to form the hollow-core fiber or further processing the primary preform (23) into 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 anti-resonance element preforms (24) are provided, each of which has at least one ARE outer tube (24a) and optionally at least one ARE inner tube (24b), wherein the ARE outer tube (24a) and/or the ARE inner tube (24b) is produced by means of a vertical drawing process without using a molding tool.

2. Method according to claim 1, characterized in that the vertical drawing method comprises the following method steps: (aa) providing a hollow starting cylinder (4) made of glass, which has a longitudinal cylinder axis (2) and an outer cylinder surface and an inner cylinder surface, (bb) a first elongation process, with which the hollow starting cylinder (4) with a vertically oriented longitudinal axis (2) is continuously fed into a first heating zone (3) having a first heating zone length L.sub.H1, softens therein in some regions, and an intermediate cylinder (12) is pulled from the softened region without using a molding tool, (cc) a second elongation process, with which the intermediate cylinder (12) or an elongated intermediate cylinder obtained from the intermediate cylinder (12) by elongation is continuously fed into a second heating zone having a second heating zone length L.sub.H2, softens therein in some regions, and a drawn tube with an outer diameter T.sub.a and an inner diameter T.sub.i is pulled from the softened region without using a molding tool, wherein the following applies: L.sub.H2<L.sub.H1 and T.sub.a/T.sub.i<1.5, and (dd) cutting to length of the drawn tube to make ARE outer tubes (24a) or ARE inner tubes (24b).

3. Method according to claim 2, characterized in that the provision of the hollow starting cylinder (4) according to method step (aa) comprises mechanically machining the cylinder surfaces in order to set the final dimensions of the hollow starting cylinder, comprising an outer diameter C.sub.a of at least 90 mm, an inner diameter C.sub.i and a diameter ratio C.sub.a/C.sub.i of less than 2.8.

4. Method according to claim 3, characterized in that the mechanical machining of the cylinder surfaces of the hollow starting cylinder (4) takes place by cutting, milling, drilling, grinding, honing and/or polishing.

5. Method according to claim 3 or 4, characterized in that the outer diameter C.sub.a is set to at least 150 mm, preferably to at least 180 mm.

6. Method according to any one of claims 2 to 5, characterized in that the drawn tube is drawn with an outer diameter T.sub.a in the range from 7 to 35 mm.

7. Method according to any one of claims 2 to 6, characterized in that the first heating zone length L.sub.H1 is at least 200 mm, and preferably between 150 and 400 mm, and that the second heating zone length L.sub.H2 is at most 140 mm, and preferably between 50 and 140 mm.

8. Method according to any one of claims 2 to 7, characterized in that the wall thickness of the drawn tube is set to a value between 0.2 and 2 mm, preferably to a value between 0.22 and 1.2 mm, and that the diameter ratio T.sub.a/T.sub.i is set to a value in the range from 1.02 and 1.14, preferably to a value in the range from 1.04 to 1.08.

9. Method according to any one of claims 2 to 7, characterized in that the wall thickness of the drawn tube is set to a value between 0.2 and 2 mm, preferably to a value between 0.22 and 1.2 mm, and that the diameter ratio T.sub.a/T.sub.i is set to a value in the range from 1.05 and 1.5, preferably to a value in the range from 1.14 to 1.35.

10. Method according to any one of claims 2 to 8, characterized in that the draw-down ratio in the totality of elongation processes is set to a value in the range from 38 to 78.

11. Method according to any one of the preceding claims, characterized in that the ARE outer tube (24a) or the ARE inner tube (24b) has an outer surface, and that, after completion of the vertical drawing process, the outer surface is free of particles larger than 0.005 mm, and that the ARE outer tube (24a) or the ARE inner tube (24b) consists of quartz glass containing a tungsten concentration of less than 2 ppb by weight.

12. 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 a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (21) comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube, along which a cladding tube wall (22) delimited by an inner side and an outer side extends, (b) providing a number of tubular anti-resonance element preforms (24), (c) arranging the anti-resonance element preforms (24) in desired positions on the inner side of the cladding tube wall (22) to form a primary preform (23), which comprises a hollow core region and an inner sheath region, and (d) optionally further processing the primary preform (23) into 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 anti-resonance element preforms (24) are provided, each of which has at least one ARE outer tube (24a) and optionally at least one ARE inner tube (24b), wherein the ARE outer tube (24a) and/or the ARE inner tube (24b) is produced by means of a vertical drawing process without using a molding tool.

Description

EXEMPLARY EMBODIMENT

[0122] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in schematic representation

[0123] FIG. 1 a primary preform with a cladding tube and anti-resonance element preforms positioned and fastened therein for producing a preform for a hollow-core fiber, based on a view of the radial cross-section, and

[0124] FIG. 2 a device for use in the tool-free production of ARE outer tubes and ARE inner tubes by means of a vertical drawing process.

[0125] 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.

[0126] FIG. 1 schematically shows a primary preform 23 with a cladding tube 21 having a cladding tube wall 22, on the inner surface of which are fastened equidistantly spaced anti-resonance element preforms 24 at previously defined azimuthal positions; in the exemplary embodiment, there are six preforms 24; in another preferred embodiment (not shown), there is an odd number of preforms.

[0127] The inner cladding tube 21 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 24 are present as an ensemble of nested structural elements consisting of an ARE outer tube 24a and an ARE inner tube 24b. The ARE outer tube 24a has an outer diameter of 6.2 mm and the ARE inner tube 24b has an outer diameter of 2.5 mm. The wall thickness of both structural elements (24a; 24b) is the same and is 0.3 mm. The diameter ratio in the ARE outer tube is thus 1.107 and in the ARE inner tube it is 1.315. The lengths of ARE outer tube 24a and ARE inner tube 24b correspond to the length of the cladding tube.

[0128] The anti-resonance element preforms 24 are fastened to the inner wall of the cladding tube 21 by means of the bonding compound 25 based on SiO.sub.2.

[0129] The bonding compound 25 is applied locally to the inner surface of the cladding tube in the region of the end-face ends, and the anti-resonance element preforms 24 are placed thereon using a positioning template with a structurally predetermined star-shaped arrangement of holding arms for the individual anti-resonance element preforms 24. In this case, the positioning template is limited to the region around the two end-face ends of the cladding tube.

[0130] This method creates a precise and reproducible connection between the cladding tube 21 and the anti-resonance element preforms 24. Solidification of the bonding compound 25 at a low temperature is sufficient for fastening, so that an intense heating of the surrounding regions and thus a deformation of anti-resonance element preforms 24 is avoided.

[0131] The temperature required for drying is below 300° C., which facilitates compliance with the dimensional stability of the preform and avoids thermal impairments. Heating to higher temperatures around 800° C., for example during elongation of the preform to form the hollow-core fiber, results in further thermal solidification of the sealing or bonding compound 25, which is also suitable for forming opaque or transparent glass. This is done by sintering or vitrifying, wherein sintering to form opaque glass requires comparatively lower temperatures and/or short heating durations than vitrifying up until complete transparency. The sealing or bonding compound 25 can thus be completely compacted by heating and vitrified by heating in the hot-forming process. The sealing or bonding compound behaves like quartz glass; it becomes viscous and deformable.

[0132] The primary preform 23 is covered with a buffer tube made of quartz glass, wherein the buffer tube collapses onto the cladding tube 1, and at the same time, the tube ensemble is elongated to form a secondary preform. The buffer tube has an outer diameter of 63.4 mm and a wall thickness of 17 mm.

[0133] In the collapse and elongation process, the coaxial arrangement of the cladding tube 1 and the buffer tube coming from below in a vertically oriented longitudinal axis is fed into a temperature-controlled heating zone and softens therein zone-by-zone starting with the upper end of the arrangement.

[0134] 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 +/−0.5° C.

[0135] 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.

[0136] The following table lists the drawing parameters for different outer diameters before (BEFORE) and after (AFTER) the forming process (collapsing and elongation).

TABLE-US-00001 TABLE 1 Outer diameter Outer diameter Cladding tube Feed rate Drawing BEFORE [mm] AFTER [mm] length [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

[0137] 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 a feed rate of 15 mm/min, a throughput of 2.49 g/min results in the case of a tube with an outer diameter of 25 mm and 1 mm wall thickness.

[0138] 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 determined.

[0139] The device shown in FIG. 2 serves for the tool-free elongation of an starting cylinder 4 of undoped quartz glass to form an intermediate cylinder.

[0140] The outer wall of the starting cylinder 4 is coarsely ground by means of a peripheral grinder equipped with a #80 grinding stone, whereby the predetermined desired outer diameter is essentially obtained. The outer surface is then finely ground by means of an NC peripheral grinder. The inner surface of the tube thus obtained is honed as a whole by means of a honing machine equipped with a #80 honing stone, wherein the degree of smoothing is continuously refined, and final treatment is carried out with a #800 honing stone. The starting cylinder 4 is then briefly etched in a 30% hydrofluoric acid etching solution. In this way, a starting cylinder 4 with an outer diameter of 200 mm and an inner diameter of 70 mm is produced. This is then elongated in a device according to FIG. 2 to form an intermediate cylinder 12.

[0141] The device comprises a vertically oriented resistance heating tube 1 made of graphite, which encloses a heating chamber 3 that is circular in cross-section. The heating tube 1 consists of an annular element with an inner diameter of 240 mm, an outer diameter of 260 mm and a length of 200 mm. The heating tube 1 surrounds the actual heating zone. At each end it is extended by means of 55 mm wide extension pieces 5 made of graphite tubing, which have an inner diameter of 250 mm and an outer diameter of 280 mm. The internal volume of the heating zone Vc is approximately 8140 mm.sup.3.

[0142] pyrometer 6, which detects the surface temperature of the starting cylinder 1, is arranged at the level of an upper detection plane E1 (at the upper edge of the upper extension piece 5). A further pyrometer 7, which detects the surface temperature of the elongated drawn tube 12, is arranged at the level of an lower detection plane E2 (at the lower edge of the lower extension piece 5). The temperature measurement values of the pyrometers 6 and 7 and the temperature of the heating tube 1 measured by the pyrometer 16 are each fed to a computer 8.

[0143] The upper end of the starting cylinder 4 is connected via a welded connection 9 to a quartz-glass holding tube 10, by means of which it can be shifted in the horizontal and vertical directions.

[0144] The starting cylinder 4 is aligned such that its longitudinal axis runs as coaxially as possible with the center axis 2 of the heating tube 1. It is fed from above into the heating chamber 3 (starting with its lower end) at a constant feed rate and softened therein. An intermediate-cylinder tube 12 is drawn vertically downward from the softened region, whereby a drawing cone 11 forms. The intermediate-cylinder tube 12 is guided past a wall-thickness measuring device 14, which is also connected to the computer 8, so that during the drawing process, the wall thickness of the drawn tube 12 being drawn out can be recorded and evaluated with the aid of the computer 8. The continuous inner bore of the starting cylinder 4 and intermediate-cylinder tube 12 has reference number 13. The tube drawing rate is detected by means of a discharge 15 and adjusted via the computer 8.

[0145] In the vertically oriented heating tube 1, the quartz glass starting cylinder 4 with an outer diameter of 200 mm and an inner diameter of 75 mm is adjusted in such a way that its longitudinal axis runs coaxially with the center axis 2 of the heating tube 1. The starting cylinder 4 is heated in the heating zone 3 to a temperature above 2200° C. and discharged at a predetermined rate of advance. From the drawing cone 11 that forms, the quartz glass drawn tube 12 is drawn at a regulated drawing speed to a nominal outer diameter of 40 mm and an inner diameter of 30 mm (wall thickness: 5 mm) as an intermediate cylinder. This has a smooth, melted and particle-free surface.

[0146] In a second elongation step, it is used in a second drawing system as a starting cylinder for the production of ARE outer tubes or ARE inner tubes. The second drawing system used for this purpose is the same as the one in FIG. 2; differing essentially in the length and the inner diameter of its heating zone. The heating zone (the heating tube) has an inner diameter of 120 mm, an outer diameter of 140 mm and a length of 100 mm.