METHOD FOR PRODUCING A PREFORM FOR A HOLLOW-CORE FIBER

Abstract

A method for producing a preform for an anti-resonant hollow-core fiber which comprises a hollow core extending along a longitudinal axis of the fiber and a sheath that surrounds the hollow core and through which hollow channels pass.

Claims

1. A method for producing a preform for an anti-resonant hollow-core fiber which comprises a hollow core extending along a longitudinal axis of the fiber and a sheath that surrounds the hollow core and through which hollow channels pass, comprising the following method steps: (a) providing a first cylindrical intermediate product VP1 that has a first outer diameter OD1, (b) thermally stretching the first intermediate product VP1 with a first draw ratio AV1 to form a second cylindrical intermediate product VP2 that has a second outer diameter OD2, (c) thermally stretching the second intermediate product VP2 with a second draw ratio AV2 to form the preform or a third cylindrical intermediate product VP3 which is further processed to form the preform, wherein the preform, or the third intermediate product, has a third outer diameter OD3, wherein the first draw ratio AV1 is less than 1.4.

2. The method according to claim 1, wherein the second draw ratio AV2 is greater than 1.2, preferably greater than 1.25, and particularly preferably greater than 1.3.

3. The method according to claim 1, wherein the first draw ratio AV1 is less than 1.28 and in particular lies between 1.002 and 1.25.

4. The method according to claim 1, wherein during the thermal stretching of the first intermediate product VP1 according to method step (b), a drawing bulb with a drawing bulb length L.sub.Z and a central constriction angle is formed, wherein the central constriction angle is less than 5 degrees, preferably less than 4 degrees, and particularly preferably less than 3 degrees.

5. The method according to claim 1, wherein the first outer diameter OD1 is in a range of 30 to 230 mm, preferably in a range of 35 to 160 mm, and particularly preferably in a range of 38 to 120 mm.

6. The method according to claim 1, wherein the second outer diameter OD2 is in a range of 25 to 200 mm, preferably in a range of 35 to 120 mm.

7. The method according to claim 1, wherein the third outer diameter OD3 is in a range of 5 to 100 mm, preferably in a range of 8 to 60 mm.

8. The method according to claim 1, wherein the first cylindrical intermediate product VP1 is an ensemble that comprises a cladding tube with a cladding tube longitudinal axis and a wall inner side, as well as a plurality of cylindrical ARE preform blanks arranged on the wall inner side.

9. The method according to claim 8, wherein the first cylindrical intermediate product VP1 comprises a cladding tube with a large wall thickness in a range of 20 to 75 mm and a large outer diameter in a range of 60 to 200 mm.

10. The method according to claim 8, wherein the ARE preform blanks are not fused, or are only fused at points, to the wall inner side of the cladding tube.

11. The method according to claim 1, wherein the second cylindrical intermediate product VP2 is a first core preform.

12. The method according to claim 10, wherein the third cylindrical intermediate product VP3 is a second core preform.

13. The method according to claim 1, wherein the further processing of the third cylindrical intermediate product VP3 comprises thermal stretching of the third intermediate product and simultaneous overlaying using an overlay cylinder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. In detail, in a schematic representation,

[0064] FIG. 1 shows a diagram for determining geometric data of a drawing bulb;

[0065] FIG. 2 shows method steps for producing a preform for a hollow-core fiber by means of a first procedure;

[0066] FIG. 3 shows method steps for producing a preform for a hollow-core fiber by means of a second procedure;

[0067] FIG. 4 shows method steps for producing a preform for a hollow-core fiber by means of a third procedure;

[0068] FIG. 5a shows a simple thermal stretching process for producing an intermediate product;

[0069] FIG. 5b shows a cross-section of the intermediate product after the simple thermal stretching process;

[0070] FIG. 6a shows a twofold thermal stretching process for producing an intermediate product, and,

[0071] FIG. 6b shows a cross-section of the intermediate product after the simple thermal stretching process.

DETAILED DESCRIPTION OF THE INVENTION

[0072] FIG. 1 schematically shows two wide drawing bulbs A, B with a comparatively small constriction angle, as typically formed in a first thermal stretching process in which a first intermediate product VP1 is elongated to form a second intermediate product VP2. In the diagram, a height or length unit h (in mm) is plotted on the Y-axis against the one intermediate product diameter D (in mm).

[0073] The first intermediate product VP1 is a component ensemble having an outer diameter of 41 mm. Drawing bulb A belongs to a drawing process in which the second intermediate product VP2 (sample 10 in Table 1) is a preform blank having an outer diameter of 28 mm, and drawing bulb B belongs to a drawing process in which the second intermediate product VP2 (sample 8 in Table 1) is a preform blank having an outer diameter of 35 mm. For drawing bulb A, a drawing bulb length L.sub.Z of 78 mm can be read off from the diagram using the method mentioned in the definitions and a central constriction angle of 3.8 degrees can be calculated therefrom. For drawing bulb B, on the basis of a drawing bulb length L.sub.Z of 78 mm a central constriction angle of 1.8 degrees can be calculated.

Production of a First Intermediate ProductMethod Variant 1

[0074] FIG. 2 schematically shows starting components 1 for producing a hollow-core fiber with the DNANF design. They include a cladding tube 1a, tertiary tubes 1b, secondary tubes 1c and primary tubes 1d. The cladding tube 1a has an outer diameter of 39 mm and an inner diameter of 22.5 mm.

[0075] A tertiary tube 1b, a secondary tube 1c and a primary tube 1d are each combined to form an ARE preform blank 3 and mounted on the inner side of the cladding tube 1a and combined to form a component assembly 2 using an alignment template 2a.

[0076] In the region of the front ends of the cladding tube 1a, the ARE preform blanks 3 are fused to the inner side of the cladding tube at points by means of melting points 4a. The alignment template (2a) is then removed. The thus produced more or less loose ensemble of cladding tube 1a and ARE preform blanks 3 has an outer diameter OD1 which is determined by the outer diameter of the cladding tube 1a. It is subjected to a multi-stage thermal stretching process and thus forms the first intermediate product 4 (VP1) within the meaning of the invention (sample 5 in Table 1).

Multi-Stage Thermal StretchingMethod Variant 1

[0077] The first intermediate product 4 (VP1) is elongated by a first thermal stretching into a second intermediate product 5 (VP2) and this is elongated by a second thermal stretching into a third intermediate product 6 (VP3).

[0078] A small draw ratio AV1 1.26 is set during thermal stretching of the first intermediate product 4 (VP1). As a result, the material transport processes for forming are slower and therefore gentler. In addition, the feed rate can be increased so that the heating time during which the ARE preform blanks 3 are exposed to particularly high drawing temperatures is comparatively short. The ARE preform blanks 3 are deformed only slightly and largely retain their predetermined position and structural integrity, and they are fused to the inner side of the cladding tube 1a.

[0079] Slow and gentle forming of the first intermediate product 4 is evidenced by the fact that the drawing bulb has a slight constriction, as shown in FIG. 1 with the aid of two other exemplary embodiments. The decisive factors are feed rate, heating zone length and maximum temperature during thermal stretching. In the exemplary embodiment, the feed rate is 20 mm/min, the heating zone length is 78 mm and the maximum temperature is 1880 C.

[0080] In the first stage of the multi-stage thermal stretching process, the second intermediate product 5 (VP2) with the outer diameter OD2 of 31 mm is thus produced by thermal stretching from the first intermediate product 4 (VP1) with the outer diameter OD1 of 39 mm.

[0081] The second intermediate product 5 (VP2) is a solid so-called preform blank in which the hollow-core region is surrounded by a sheath region through which hollow channels pass.

[0082] In the second stage of the multi-stage thermal stretching process, the second intermediate product 5 (VP2) with the outer diameter OD2 is thermally stretched with a second draw ratio AV2 to produce a third cylindrical intermediate product 7 (VP3) with the outer diameter OD3 of 20 mm. The draw ratio AV2 is 1.55, which is greater than AV1.

Further Processing to Form the PreformMethod Variant 1

[0083] The third intermediate product 6 (VP3) is further processed to form a preform 8. For this purpose, it is overlaid using an overlay cylinder 7, which provides additional sheath material sufficient to adjust the predetermined core-sheath-cross-sectional structure of the hollow-core fiber. During overlaying using the overlay cylinder 7 with vertically oriented longitudinal axis, a holding rod 7a is welded to the upper end of the intermediate product 6 (VP3) and a drawing tip 7b to the lower end. Preform 8 is obtained by simultaneous collapsing and elongation. From this, the hollow-core fiber is drawn by means of a standard fiber drawing process.

[0084] Table 1 summarizes the dimensions and method parameters of the multi-stage thermal stretching process for sample 5 described in detail above and for other samples.

TABLE-US-00001 TABLE 1 OD1 OD2 Lz suit- OD3 suit- No [mm] [mm] AV1 [mm] degrees able [mm] AV2 able 1 30 27 1.11 78 0.9 yes 15 1.80 yes 2 30 25 1.20 78 1.5 yes 20 1.25 yes 3 34 30 1.13 78 1.2 yes 23 1.30 yes 4 34 27 1.26 78 2.1 yes 8 3.38 yes 5 39 31 1.26 78 2.4 yes 20 1.55 yes 6 41 38 1.08 78 0.9 yes 24 1.58 yes 7 41 38 1.08 78 0.9 yes 10 3.80 yes 8 41 35 1.17 78 1.8 yes 24 1.46 yes 9 41 29 1.41 82 3.4 no 10 41 28 1.46 78 3.8 no 11 41 24 1.71 90 4.3 no 12 56 50 1.12 78 1.8 yes 30 1.67 yes 13 56 48 1.17 78 2.4 yes 14 90 80 1.13 78 2.9 yes 45 1.78 yes 15 90 45 2.00 100 10.4 no 16 110 90 1.22 140 3.3 yes 40 2.25 yes 17 110 40 2.75 140 11.5 no 18 140 120 1.17 140 3.3 yes 60 2.00 yes 19 200 180 1.11 150 3.3 yes 100 1.80 yes Legend: OD1 Outer diameter of the first intermediate product OD2 Outer diameter of the second intermediate product OD3 Outer diameter of a third intermediate product or preform AV1 Draw ratio in the first thermal stretching process AV2 Draw ratio in the second thermal stretching process Lz Length of the drawing bulb (empirically determined) Central constriction angle of the drawing bulb (calculated) Suitable - no: The dimensional accuracy or cross-sectional geometry of the intermediate product after the thermal stretching process is such that further processing is not advisable.

[0085] Samples 9, 10, 11, 15 and 17 are comparative examples. They have proven to be unsuitable for further processing into preforms already after the first thermal stretching, which is attributed to the comparatively large draw ratio AV1 of these samples. FIG. 5a schematically shows the one-stage thermal stretching from the first intermediate product 4 (VP1) directly into a cylindrical intermediate product 9 with the outer diameter OD3.

[0086] FIG. 5b schematically shows the cross-section of the intermediate product 9 and the reason for its unsuitability for further processing into the preform for the hollow-core fiber. This is because the five previously nested ARE preform blanks 3 are evenly distributed within the former cladding tube 1a. However, in particular some of the nested former tertiary tubes 1b show significant deviations from their target position.

[0087] In comparison, FIG. 6a schematically shows a two-stage thermal stretching of the first intermediate product 4 (sample 5 of Table 1) with the outer diameter OD1, first with a small draw ratio relative to the second intermediate product 5 (VP2) with the outer diameter OD2 and only then with a higher draw ratio relative to the third intermediate product 6 (VP3) with the outer diameter OD3. As shown in FIG. 6b on the basis of the cross-section of said intermediate product 6 (VP3), the five former nested ARE preform blanks 3 are evenly distributed within the former cladding tube 1a and fused to the inner side of the cladding tube. The nested former tertiary tubes 1b, secondary tubes 1c and primary tubes 1d also show no deviations from their target position. For this reason, the intermediate product 6 of sample 5 is suitable for further processing into a preform.

[0088] Insofar as the same reference numerals are used in the method variants 2 and 3 explained below as in FIG. 1, they designate identical or equivalent components, constituents or process measures as explained above with reference to method variant 1.

Production of a First Intermediate ProductMethod Variant 2

[0089] Compared to the method variant 1, the starting components 1 in the method variant 2 schematically shown in FIG. 3 comprise a particularly thick-walled cladding tube 1a2 having an outer diameter of 90 mm and an inner diameter of 22.5 mm (sample 14 in Table 1).

[0090] The loose ensemble 2 of cladding tube 1a2 and ARE preform blanks 3 has an outer diameter OD1 which is determined by the outer diameter of the cladding tube 1a2. The ARE preform blanks 3 are bonded to the inner side of the cladding tube 1a2. The ensemble 2 consisting of cladding tube 1a2 and bonded ARE preform blanks 3 can be understood as the first intermediate product 4 (VP1) in terms of the invention; it is subjected to a multi-stage thermal stretching process.

Multi-Stage Thermal StretchingMethod Variant 2

[0091] The first intermediate product 4 (VP1) is further processed by first thermal stretching into a second intermediate product 5 (VP2) and the latter is further processed by second thermal stretching into a preform 8.

[0092] A small draw ratio AV1 of 1.13 is set during thermal stretching of the first intermediate product 4 (VP1). The ARE preform blanks 3 are deformed only slightly and largely retain their predetermined position and structural integrity, and they are fused to the inner side of the cladding tube 1a2.

[0093] In the first stage of the multi-stage thermal stretching process, the second intermediate product 5 (VP2) with the outer diameter OD2 of 80 mm is thus produced by thermal stretching from the first intermediate product 4 (VP1) with the outer diameter OD1 of 90 mm.

[0094] The second intermediate product 5 (VP2) is a solid so-called preform blank in which the hollow-core region is surrounded by a sheath region through which hollow channels pass.

[0095] In the second stage of the multi-stage thermal stretching process, the second intermediate product 5 (VP2) with the outer diameter OD2 is thermally stretched with a second draw ratio AV2 to produce preform 8 with the outer diameter OD3.

Further Processing to Form the PreformMethod Variant 2

[0096] The second intermediate product 5 (VP2) is further processed by thermal stretching into a preform 8 which has the predetermined core-sheath-cross-sectional structure of the hollow-core fiber. From this, the hollow-core fiber is drawn by means of a standard fiber drawing process.

Production of a First Intermediate ProductMethod Variant 3

[0097] In method variant 3, as schematically shown in FIG. 4, the method steps up to the production of the second intermediate product 5 (VP2) are almost the same as in method variant 1. Sample 7 in Table 1 shows the dimensions and draw ratios. Compared to sample 5, the first intermediate product is thermally stretched with an even smaller draw ratio of only 1.08 into a second intermediate product 5 (VP2) having an outer diameter of 38 mm.

[0098] The ARE preform blanks 3 are deformed only slightly and largely retain their predetermined position and structural integrity, and they are fused to the inner side of the cladding tube 1a.

[0099] The second intermediate product 5 (VP2) is a solid so-called preform blank in which the hollow-core region is surrounded by a sheath region through which hollow channels pass.

Multi-Stage Thermal StretchingMethod Variant 3

[0100] The second intermediate product 5 (VP2) is then elongated in a second thermal stretching process to form a particularly thin, third intermediate product 6 (VP3) having an outer diameter of 10 mm. The third intermediate product 6 (VP3) is also a solid preform blank.

Further Processing to Form the PreformMethod Variant 3

[0101] The third intermediate product 6 (VP3) is further processed to form a preform 8 by overlaying it with an overlay cylinder 7 so as to obtain the predetermined core-sheath cross-sectional structure of the hollow-core fiber. Overlaying using the overlay cylinder 7 takes place on-line during the fiber drawing process for the hollow-core fiber.