Method for producing a preform for an anti-resonant hollow-core fiber having nested capillaries; preform and intermediate product

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 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 sleeve tube comprising an inner bore of the sleeve tube and a longitudinal axis of the sleeve tube, along which a sleeve tube wall delimited by an inner side and an outer side extends,
• (b) providing tubular anti-resonance element preforms, at least a portion of which is present as a capillary blank comprising at least one anti-resonant element (ARE) external capillary and at least one nested element (NE) internal capillary connected to an inner lateral surface of the ARE external capillary, wherein the production of the capillary blank comprises the following method steps:
• (b1) fixing an NE inner tube, which has an external diameter ODNE and an internal diameter IDNE, on the inner lateral surface of an ARE outer tube having an external diameter ODARE and an internal diameter IDARE, to form a capillary blank ensemble,
• (b2) thermal stretching of the capillary blank ensemble to form the capillary blank, which has a maximum external diameter ODARE_cap and a maximum wall thickness WTARE_cap,
• (c) mounting the capillary blank at target positions on the inner side of the sleeve tube wall to form a primary preform comprising a hollow core region and a sheath region, and
• (d) further processing the primary preform to form 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 geometric dimensions ODNE, IDNE, ODARE, IDARE as well as ODARE_cap and WTARE_cap are aligned in relation to one another in such a way that the ARE external capillary of the capillary blank has a degree of ovality of less than 1.025.