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 in that the step of providing and/or arranging the anti-resonant element preforms and/or the process of carrying out a hot-forming process includes a fixation measure and/or a sealing measure using an amorphous SiO.sub.2 particle-containing sealing or joining compound.

Claims

1. Method for producing an anti-resonant hollow-core fiber comprising a hollow core extending along a longitudinal axis of the fiber and a sheath region surrounding the hollow core and comprising a plurality of anti-resonance elements, comprising the method steps of: (a) providing a cladding tube (2) comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube, along which a cladding tube wall (2a) delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (4), (c) arranging the anti-resonance element preforms (4) at setpoint positions of the inner side of the cladding tube wall (2a) to form a primary preform (1) for the hollow-core fiber, which comprises a hollow core region and a sheath region, and (d) elongating the primary preform (1) to form the hollow-core fiber or further processing the primary preform (1) 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 the provision and/or the arrangement of the anti-resonance element preforms (4) and/or the execution of a process according to 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.

2. Method according to claim 1, characterized in that the anti-resonance element preforms (4) are fixed to the inner side of the cladding tube (2) using the sealing or bonding compound.

3. Method according to claim 2, characterized in that the anti-resonance element preforms (4) are cylindrical and have two opposite end regions, and that fixing using the sealing or bonding compound (5) takes place exclusively at one of the end regions or exclusively at both end regions.

4. Method according to any one of the preceding claims, characterized in that a cladding tube (2) having a circular inner cross-section is provided, and in that a longitudinal structure, preferably a longitudinal groove, is produced on the inner side of the cladding tube wall (2a) and has a recess to which or in which the anti-resonance element preforms (4) are fixed.

5. Method according to claim 4, characterized in that the sealing or bonding compound (5) is introduced into the recess of the longitudinal structure (3) and that the anti-resonance element preforms (4), when arranged at the setpoint position according to method step (c), contact the sealing or bonding compound (5).

6. Method according to claim 4 or 5, characterized in that the longitudinal structure (3) of the inner side of the cladding tube is produced by means of drilling, sawing, milling, cutting or grinding.

7. Method according to any one of claims 2 through 6, characterized in that the arranging of the anti-resonance element preforms (4) on the inner side of the cladding tube comprises fixing by means of a positioning template to be introduced into the inner bore of the cladding tube, which positioning template has multiple radially outward-facing holding elements for positioning the anti-resonance element preforms at the setpoint positions.

8. Method according to claim 7, characterized in that the positioning template is used exclusively in the region of the cladding tube end faces, preferably in the region of both cladding tube end faces.

9. Method according to any one of the preceding claims, characterized in that the anti-resonance element preforms (4) are composed of a plurality of nested structural elements (4a; 4b) and that structural elements (4a; 4b) are fixed with respect to one another using the sealing or bonding compound (5), wherein the fixing of cylindrical structural elements (4a; 4b) using the sealing or bonding compound (5) preferably takes place only at one face end or only at both face ends of the structural elements (4a; 4b).

10. Method according to any one of the preceding claims, characterized in that, in order to carry out a process according to method step (d), the preform (1) is bonded to a holder, and that the bond between the preform (1) and the holder is produced by means of the sealing or bonding compound (5).

11. Method according to any one of the preceding claims, characterized in that, in order to carry out a process according to method step (d), the preform (1) is bonded to a gas connection, and that the bond between the preform (1) and the gas connection is produced by means of the sealing or bonding compound (5).

12. Method according to any one 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 can be closed by means of the sealing or bonding compound (5) to carry out a process according to method step (d).

13. Method according to any one of the preceding claims, characterized in that the sealing or bonding compound (5) is compacted by heating and preferably vitrified by heating when carrying out a process according to method step (d).

14. Method according to any one of the preceding claims, characterized in that the anti-resonance elements (4) are arranged about the hollow core with an odd symmetry.

15. 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 a sheath region surrounding the hollow core, said sheath region comprising a plurality of anti-resonance elements, with the method steps of: (a) providing a cladding tube (2) comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube, along which a cladding tube wall (2a) delimited by an inner side and an outer side extends, (b) providing a number of anti-resonance element preforms (4), (c) arranging the anti-resonance element preforms (4) in each case at setpoint positions of the inner side of the cladding tube wall (2) to form a primary preform (1) for the hollow-core fiber, which comprises a hollow core region and a sheath region, and (d) optionally further processing the primary preform (1) 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 the provision and/or the arrangement of the anti-resonance element preforms (4) and/or the execution of a process according to 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.

Description

[0093] The following are shown in detail in schematic representation and in a plan view of the cross-section:

[0094] FIG. 1 a first embodiment of a primary preform having anti-resonance element preforms positioned therein and fixed using a sealing or bonding compound containing SiO.sub.2 to produce a preform for a hollow core fiber,

[0095] FIG. 2 a second embodiment of a primary preform having anti-resonance element preforms positioned therein and fixed using a sealing or bonding compound containing SiO.sub.2 to produce a preform for a hollow core fiber,

[0096] FIG. 3 an anti-resonance element preform in a magnified representation composed of a plurality of nested structural elements connected to one another using a sealing or bonding compound containing SiO.sub.2,

[0097] FIG. 4 a primary preform having anti-resonance element preforms positioned and fixed therein, a part of which is closed using a sealing or bonding compound containing SiO.sub.2,

[0098] FIG. 5 a primary preform having anti-resonance element preforms positioned and fixed therein, which are closed using a sealing or bonding compound containing SiO.sub.2, and

[0099] FIG. 6 a primary preform having a hollow core closed using a sealing or bonding compound containing SiO.sub.2 and open anti-resonance element preforms positioned and fixed about the hollow core.

[0100] In the method according to the invention for producing a hollow core fiber or for producing a preform for a hollow core fiber, a sealing or bonding compound based on SiO.sub.2 is used for fixing components of the preform or for sealing hollow channels in the preform. The production of said compound is described, for example, in DE 10 2004 054 392 A1. Amorphous SiO.sub.2 particles are obtained by wet milling grains of high-purity silica glass. An aqueous base slip is produced 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. 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 approximately 90%, which consists of at least 99.9 wt. % SiO.sub.2. Small proportions of doping agents may be present in order to adapt the thermal expansion coefficient.

[0101] The slip compound is applied to one or both of the contact surfaces to be bonded. It is also possible to form a slip compound between the contact surfaces fixed to one another beforehand. This is subsequently solidified by drying and heating. The amorphous SiO.sub.2 particles used to form the slip consist of synthetically prepared SiO.sub.2 or are prepared based on purified naturally occurring raw material.

[0102] FIG. 1 schematically shows a primary preform 1 having a cladding tube 2 with a wall 2a, on the inner side of which are fixed anti-resonance element preforms 4 at a uniform distance at previously defined azimuthal positions; in the exemplary embodiment, there are six preforms 4.

[0103] The cladding tube 2 consists of silica glass and has a length of 500 mm, an external diameter of 30 mm and an internal diameter of 24 mm. The anti-resonance element preforms 4 are present as an ensemble of nested structural elements (4a; 4b) made from an ARE outer tube 4a and an ARE inner tube 4b. The ARE outer tube 4a has an external diameter of 7.4 mm and the ARE inner tube 4b has an external diameter of 3.0 mm. The wall thickness of both structural elements (4a; 4b) is the same and is 0.35 mm. The lengths of ARE outer tube 4a and ARE inner tube 4b correspond to the cladding tube length 1.

[0104] The anti-resonance element preforms 4 are fixed to the inner side of the wall of the cladding tube 2 by means of the bonding compound 5 based on SiO.sub.2.

[0105] The bonding compound 5 is applied to the inner side of the cladding tube wall in strips, and the anti-resonance element preforms 4 are placed thereon using a positioning template having a structurally predetermined star-shaped arrangement of holding elements for the individual anti-resonance element preforms 4.

[0106] This method creates a precise and reproducible bond between the cladding tube 2 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 strong heating of the surrounding regions, and thus a deformation of the anti-resonance element preforms 4, is avoided.

[0107] In the embodiment shown in FIG. 2, longitudinal grooves 3 are created beforehand in each case on the inner side of the wall 2a of the cladding tube 2 at the setpoint positions of the anti-resonance element preforms 4 by means of milling. The longitudinal grooves 3 are distributed evenly in hexagonal symmetry about the inner circumference of the cladding tube 2. The bonding compound 5 is introduced into the longitudinal axis grooves 3 in the region of the two face ends, and the anti-resonance element preforms 4 are pressed thereon so as to have two contact lines axially parallel to the longitudinal edges of the longitudinal grooves 3. This ensures an exact alignment of anti-resonance element preforms 4.

[0108] In order to realize the fixing between anti-resonance element preforms 4 and the longitudinal edges of the longitudinal grooves 3, a small clearance (a few μm) or contact is necessary. The anti-resonance element preforms 4 are therefore preferably pressed against the longitudinal grooves 3 during the bonding process by means of the aforementioned positioning template.

[0109] FIG. 3 schematically shows an embodiment of the anti-resonance element preform 4 in the form of an ensemble of nested structural elements (4a; 4b) consisting of an ARE outer tube 4a and an ARE inner tube 4b. The two structural elements (4a; 4b) are connected to one another by means of the bonding compound 5 based on SiO.sub.2.

[0110] The primary preform 1 (which is the ensemble consisting of cladding tube 2 and structural elements (4a; 4b) inserted therein) is subsequently further processed to form a larger, secondary preform for the hollow core fiber. For this purpose, it is overlaid with a collecting cylinder made from silica glass and the coaxial ensemble consisting of primary preform and collecting cylinder is simultaneously elongated to form the secondary preform. In this collapse and elongation process, which takes place in the vertical orientation of the preform longitudinal axis, the upper ends of the primary preform and the collecting cylinder are bonded to a holder made of silica glass, wherein the bond between the preform and the holder is produced by means of the bonding compound described.

[0111] The secondary preform obtained according to the collapse and elongation process is also referred to in the English technical literature as “cane.” It is drawn to form the hollow core fibers. For this purpose, the preform is additionally bonded to a gas connection made from silica glass, wherein the bond between the preform and the gas connection is also produced by means of the bonding compound. In order to prevent pressurization of the fine structural elements, such as the ARE inner tube 4b, their open ends are sealed beforehand by means of the SiO.sub.2-based sealing and bonding compound 5, as shown schematically in FIG. 4. The sealing and bonding compound 5 can be seen as a gray-shaded area. It is sufficient for the sealing and bonding compound 5 to seal a short length of the inner bore of the ARE inner tube 4b, preferably at that end that corresponds to the upper end when carrying out a hot-forming process with a vertically oriented preform longitudinal axis.

[0112] FIG. 5 shows an embodiment of the preform 1, wherein both structural elements (4a; 4b) of the anti-resonance element preforms 4 are closed with the sealing and bonding compound.

[0113] It is also possible to introduce a pressure supply line into the structural elements (4a and/or 4b) and to subsequently seal the remaining opening with the sealing compound containing SiO.sub.2, so that no pressure can escape upward.

[0114] FIG. 6 shows an embodiment with which both structural elements (4a; 4b) of the anti-resonance element preforms 4 are open, but the remaining cross-section of the inner bore of the cladding tube is closed with the sealing and bonding compound 5.