Methods for producing a hollow-core fiber and for producing a preform for a hollow-core fiber
A high degree of precision and an exact positioning of anti-resonant elements in a sufficiently stable and reproducible manner is achieved in that a step of providing a cladding tube includes a processing measure, in which the cladding tube wall is machined with a longitudinal structure extending in the direction of the cladding tube longitudinal axis in the region of the target positions.
1. A 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, the method comprising:
(a) providing a cladding tube comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall delimited by an inner side and an outer side extends,
(b) providing a number of 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 for the hollow-core fiber which comprises the hollow core region and the sheath region, and
(d) elongating the primary preform to form the hollow-core fiber or further processing the primary preform 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,
wherein the provision of the cladding tube according to method step (a) comprises a machining measure in which by machining in the region of the desired positions, the cladding tube wall is provided with a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube, wherein the longitudinal structure has longitudinal slots distributed about the circumference of the cladding tube wall, and the anti-resonance element preforms are arranged on one longitudinal slot in each case.
2. The method according to claim 1 , wherein the longitudinal structure of the cladding tube wall is produced by drilling, sawing, milling, cutting or grinding.
3. The method according to claim 1 , wherein the cladding tube with a circular inner cross-section is provided.
4. The method according to claim 1 , wherein the cladding tube comprises end-face ends, and that the longitudinal structure ends before the end-face ends.
5. The method according to claim 1 , wherein the anti-resonance element preforms, when arranged at the desired position according to method step (c), touch the longitudinal slot at two edges in each case.
6. The method according to claim 1 , wherein the longitudinal structure has a maximum width SB in the circumferential direction of the inner side of the cladding tube and that the anti-resonance element preforms have a diameter that is less than the maximum width SB of the longitudinal structure.
7. The method according to claim 1 , wherein the longitudinal slots have parallel longitudinal edges and a maximum slot width SB, and the anti-resonance element preforms are connected to the longitudinal edges.
8. The method according to claim 7 , wherein the anti-resonance element preforms are connected to the longitudinal edges by softening, accompanied by simultaneous lengthening.
9. The method according to claim 1 , wherein the anti-resonance element preforms each have at least one anti-resonance element and at least one capillary connected to the anti-resonance element, wherein the capillary is accommodated in a recess of the longitudinal structure, and when a process according to method step (d) is carried out, the capillary is deformed by applying an internal pressure in the inner bore of the capillary.
10. The method according to claim 1 , wherein the anti-resonance elements are arranged about the hollow core with an odd-numbered symmetry.
11. A 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, the method comprising:
(a) providing a cladding tube comprising an inner bore of the cladding tube and a longitudinal axis of the cladding tube along which a cladding tube wall delimited by an inner side and an outer side extends,
(b) providing a number of 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 for the hollow-core fiber which comprises the hollow core region and the sheath region, and
(d) optionally further processing the primary preform 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,
wherein the provision of the cladding tube according to method step (a) comprises a machining measure in which, by machining in the region of the desired positions, the cladding tube wall is provided with a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube, wherein the longitudinal structure has longitudinal slots distributed about the circumference of the cladding tube wall, and the anti-resonance element preforms are arranged on one longitudinal slot in each case.