阅读说明：本技术 制造空芯光纤和空芯光纤预制件的方法 (Method for manufacturing hollow-core optical fiber and hollow-core optical fiber preform ) 是由 M·罗森伯格 J·维德拉 K·H·昌 Q·马 于 2020-07-15 设计创作，主要内容包括：本发明公开了一种制造反谐振空芯光纤的方法,反谐振空芯光纤具有沿光纤纵轴线延伸的空芯和包围所述空芯的包层区域,所述包层区域包括几个反谐振元件。本公开的方法包括以下步骤：提供包层管,所述包层管具有包层管内孔和包层管纵轴线,由内表面和外表面限定的包层管壁沿所述包层管纵轴线延伸；在所述包层管壁的规定位置处形成若干个用于反谐振元件的前驱体；以及将所述初级预制件拉伸成所述空芯光纤或将所述初级预制件再加工成次级预制件,由所述次级预制件拉制成所述空芯光纤。为了基于此以足够稳定和可重复的方式实现反谐振元件的高精度及其精确定位,根据本发明提出,所述形成所述反谐振元件前驱体包括形成伸长的压力室,所述伸长的压力室各自在所述反谐振元件的所述规定位置的区域中与受压力和热影响可变形的壁相邻,并且在根据方法步骤(c)执行过程时,由于压力和热的作用,所述伸长的压力室使得所述可变形的壁部分在所述包层管内孔的方向上突起,以形成反谐振元件或前驱。(The invention discloses a method of manufacturing an antiresonant hollow-core fiber having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding said hollow core, said cladding region comprising several antiresonant elements. The method of the present disclosure comprises the steps of: providing a cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall defined by an inner surface and an outer surface extends; forming a plurality of precursors for anti-resonance elements at specified positions of the wall of the cladding tube; and drawing the primary preform into the hollow-core optical fiber or reworking the primary preform into a secondary preform from which the hollow-core optical fiber is drawn. In order to achieve a high precision of the antiresonant element and its precise positioning in a sufficiently stable and reproducible manner on the basis thereof, it is proposed according to the invention that the forming of the antiresonant element precursor comprises forming elongated pressure chambers which are each adjacent to a pressure-and heat-affected deformable wall in the region of the defined position of the antiresonant element, and that, when the process is carried out according to method step (c), the elongated pressure chambers cause the deformable wall portions to protrude in the direction of the cladding tube bore, due to the effect of pressure and heat, to form the antiresonant element or precursor.)

1. A method of manufacturing an antiresonant hollow-core fiber (29; 129) having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding said hollow core, said cladding region comprising a plurality of antiresonant elements, said method comprising the steps of:

(a) providing a primary preform (26; 126) of the hollow-core optical fiber, the primary preform having at least one cladding tube with a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (24; 124) defined by an inner surface and an outer surface extends,

(b) forming a number of precursors (25 a; 25 b; 125; 213) for anti-resonant elements (28 a; 28 b; 128) at defined positions of the cladding tube wall (24; 124), and

(c) drawing the primary preform (26; 126) into the hollow-core optical fibre (29; 129) or pre-fabricating the primary preform

The piece (26; 126) is further processed into a secondary preform from which the hollow-core optical fibre is drawn

Wherein the reprocessing comprises performing one or more of the following thermoforming processes once or repeatedly:

(i) stretching the mixture to obtain a stretched mixture,

(ii) the material is collapsed and then the material is compressed,

(iii) the collapsing and the simultaneous stretching are carried out,

(iv) the additional cladding material is collapsed and the additional cladding material,

(v) the additional cladding material is collapsed and subsequently stretched,

(vi) the additional cladding material is collapsed and simultaneously stretched,

characterized in that the forming of the anti-resonance element precursor (25 a; 25 b; 125; 213) comprises forming elongated pressure chambers (25 a; 25 b; 125; 213) which are each adjacent to the deformable wall (21; 22) under pressure and heat in the region of the defined position of the anti-resonance element (28 a; 28 b; 128) and which, when the process is carried out according to method step (c), due to the effect of pressure and heat, cause a portion of the deformable wall (21; 22) to protrude in the direction of the cladding tube bore (16) to form an anti-resonance element (29; 129) or precursor.

2. The method according to claim 1, characterized in that the pressure chamber is designed as a hollow channel (13; 25 a; 25 b; 125; 213), adjacent to which is a wall portion of the glass tube (21; 221).

3. The method according to claim 2, wherein the hollow channel (213) is formed in the wall of the glass tube (221) and extends parallel to the longitudinal axis of the glass tube.

4. A method according to claim 3, characterized in that a coaxial tube assembly is formed, which comprises an inner row of hollow channels in the wall of the inner glass tube and an outer row of hollow channels in the wall of the outer glass tube, wherein the hollow channels of the inner and outer rows, seen in the radial direction, lie on a common connecting line and are spatially separated from each other by at least one circumferentially surrounding and inwardly deformable glass wall.

5. The method according to claim 2, characterized in that the forming of the hollow channel (13; 25 a; 25 b; 125) comprises a treatment measure in which an intermediate tube (10; 20) is arranged between the glass tube (21) and the outer tube (22; 23), which intermediate tube has an intermediate tube longitudinal axis along which an intermediate tube wall defined by an inner surface and an outer surface extends and in which a longitudinal opening (13) opens, wherein the hollow channel (25 a; 25 b; 125) is formed by the longitudinal opening (13) when the process is carried out according to method step (c).

6. A method according to claim 5, characterized in that an intermediate tube (10; 20) with a circular inner cross-section is provided and machined.

7. Method according to claim 5 or 6, characterized in that the intermediate tube (10; 20) has a frontal end and in that the longitudinal opening (13) ends before the frontal end.

8. Method according to one of claims 5 to 7, characterized in that a coaxial tube assembly (19) is formed, which comprises an inner glass tube (21), an inner intermediate tube (10), an inner and an outer tube (22) forming simultaneously an outer glass tube (22), an outer intermediate tube (20) and an outer tube (23), wherein the longitudinal openings (13) of the inner and outer intermediate tubes (10; 20) lie on a common connecting line in the radial direction and are spatially separated from one another by at least one circumferentially surrounding and inwardly deformable glass wall (22).

9. The method of claim 2, wherein said forming said hollow channel comprises a process step wherein a glass tube and an intermediate tube are used, said intermediate tube coaxially surrounding said glass tube, wherein said glass tube has a glass tube outer envelope surface with longitudinal grooves cut therein, said longitudinal grooves extending parallel to a glass tube longitudinal axis; and/or the intermediate tube has an intermediate tube inner envelope surface in which longitudinal grooves are cut, which extend parallel to the intermediate tube longitudinal axis, wherein the hollow channel is formed by the longitudinal grooves when the process is carried out according to method step (c), and wherein the hollow channel is deformed into an elongated protrusion as a result of the action of pressure and heat.

10. Method according to one of claims 5 to 9, characterized in that the longitudinal opening (13) or the longitudinal groove is made by means of a cutting process, in particular by means of cutting, drilling, sawing, milling or grinding.

11. Method according to one of claims 5 to 10, characterized in that the longitudinal opening (13) and/or the longitudinal groove has longitudinal edges and the longitudinal edges are fused with the surrounding glass material by softening, preferably with simultaneous elongation.

12. Method according to one of claims 2 to 11, characterized in that the glass tube (21; 22) is made of glass, which contains a viscosity-reducing dopant.

13. A method of manufacturing an antiresonant hollow-core fiber preform having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding said hollow core, said cladding region comprising several antiresonant elements, said method comprising the steps of:

(a) providing a primary preform (26; 126) of the hollow-core optical fiber, the primary preform having at least one cladding tube with a cladding tube inner bore and a cladding tube longitudinal axis along which a cladding tube wall (24; 124) defined by an inner surface and an outer surface extends,

(b) forming a number of precursors (25 a; 25 b; 125; 213) for anti-resonant elements (28 a; 28 b; 128) at defined positions of the cladding tube wall (24; 124),

(c) optionally, the primary preform (26; 126) is reworked into a secondary preform of the hollow-core optical fiber, wherein the reworking comprises one or more of the following thermoforming processes carried out once or repeatedly:

(i) stretching the mixture to obtain a stretched mixture,

(ii) the material is collapsed and then the material is compressed,

(iii) the collapsing and the simultaneous stretching are carried out,

(iv) the additional cladding material is collapsed and the additional cladding material,

(v) the additional cladding material is collapsed and subsequently stretched,

(vi) the additional cladding material is collapsed and simultaneously stretched,

characterized in that the forming of the anti-resonance element precursor (25 a; 25 b; 125; 213) comprises forming elongated pressure chambers (25 a; 25 b; 125; 213) which are each adjacent to the deformable wall (21; 22) under pressure and heat in the region of the defined position of the anti-resonance element (28 a; 28 b; 128) and which, when the process is carried out according to method step (c), due to the effect of pressure and heat, cause a portion of the deformable wall (21; 22) to protrude in the direction of the cladding tube bore (16) to form an anti-resonance element (29; 129) or precursor.

Technical Field

The present invention relates to a method of manufacturing an antiresonant hollow-core fiber having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding said hollow core, said cladding region comprising a number of antiresonant elements, said method comprising the steps of:

(a) providing a primary preform of said hollow-core optical fiber, said primary preform having at least one cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis, a cladding tube wall defined by an inner surface and an outer surface extending along said cladding tube longitudinal axis,

(b) forming a plurality of precursors for anti-resonance elements at prescribed positions of the wall of the cladding tube, an

(c) Drawing the primary preform into the hollow-core optical fiber or reworking the primary preform into a secondary preform from which the hollow-core optical fiber is drawn, wherein the reworking comprises one or more of the following thermoforming processes being performed once or repeatedly:

(i) stretching the mixture to obtain a stretched mixture,

(ii) the material is collapsed and then the material is compressed,

(iii) the collapsing and the simultaneous stretching are carried out,

(iv) the additional cladding material is collapsed and the additional cladding material,

(v) the additional cladding material is collapsed and subsequently stretched,

(vi) collapsing the additional cladding material and simultaneously stretching.

Furthermore, the present invention relates to a method of manufacturing an antiresonant hollow-core fiber preform having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding said hollow core, said cladding region comprising several antiresonant elements, said method comprising the steps of:

(a) providing a primary preform of said hollow-core optical fiber, said primary preform having at least one cladding tube having a cladding tube inner bore and a cladding tube longitudinal axis, a cladding tube wall defined by an inner surface and an outer surface extending along said cladding tube longitudinal axis,

(b) forming a plurality of precursors for anti-resonance elements at specified positions of the wall of the cladding tube,

(c) optionally, reworking the primary preform into a secondary preform for the hollow-core optical fiber, wherein said reworking comprises performing one or more of the following thermoforming processes once or repeatedly:

(i) stretching the mixture to obtain a stretched mixture,

(ii) the material is collapsed and then the material is compressed,

(iii) the collapsing and the simultaneous stretching are carried out,

(iv) the additional cladding material is collapsed and the additional cladding material,

(v) the additional cladding material is collapsed and subsequently stretched,

(vi) collapsing the additional cladding material and simultaneously stretching.

Conventional single mode optical fibers made of solid materials have a core region made of glass surrounded by a cladding region made of lower index glass. Wherein the light conduction is based on total reflection between the core and the cladding region. However, the interaction of the introduced light with the solid material is associated with an increase in data transmission delay and a relatively low damage threshold compared to high-energy radiation.

These disadvantages are avoided or reduced by a "hollow core fiber" in which the core comprises a cavity filled with a gas or liquid. The interaction of light with glass is smaller in hollow core fibers than in solid core fibers. The refractive index of the core is less than that of the cladding, so light cannot be conducted by total reflection and will generally escape from the core into the cladding. Hollow-core fibers are classified into "photonic band-gap fibers" and "antiresonant reflecting fibers" according to the physical mechanism of light conduction.

In "photonic band gap fibers", the hollow core region is surrounded by a cladding in which small hollow channels are periodically arranged. The periodic structure of the hollow channels of the cladding layer gives rise to an effect, known as "photonic band gap" based on semiconductor technology, according to which light of a specific wavelength range scattered at the cladding structure constructively interferes due to bragg reflection in the central cavity and cannot propagate laterally in the cladding layer.

In the embodiment of hollow-core fibers known as "antiresonant hollow-core fibers"; ARHCF), the hollow-core region is surrounded by an inner cladding region, in which a so-called "antiresonant element" (or "antiresonant element"; shortly, "ARE") is arranged. The walls of the anti-resonant element, which are uniformly distributed around the hollow core, may act as a fabry-perot resonator operating in anti-resonance, reflecting incident light and directing it through the core.

This fiber technology ensures low optical attenuation, extremely wide transmission spectral range (also in the ultraviolet or infrared wavelength range), and low delay of data transmission.

Potential application areas for hollow core optical fibers are data transmission, high performance beam guiding, e.g. for material processing, modal filtering, nonlinear optics, in particular for supercontinuum production in the ultraviolet to infrared wavelength range.

Background

One disadvantage of anti-resonant hollow core fibers is that the higher order modes are not necessarily suppressed, so they are generally not pure single mode over long distances and the quality of the output beam is degraded.

The paper "Nested anti-resonant hollow-core fiber" by Francesco Poletti; optics Express, vol.22, No.20(2014) [ optical flash, vol.22, No.20(2014) ]; DOI 10.1364/OE 22.023807, which proposes an optical fiber design in which the anti-resonant element is not designed as a simple single structural element, but consists of several nested (english: nested) structural elements. Such nested anti-resonant elements are constructed such that the high order core mode is phase matched to the cladding mode and the high order core mode is suppressed instead of the core fundamental mode. This always ensures transmission of the fundamental core mode, and the hollow core fiber can be made effectively single-mode in a limited wavelength range.

Effective mode suppression depends on the center wavelength of the transmitted light and structural parameters of the fiber design, such as the radius of the hollow core and the difference in the diameters of the nested ring structures in the anti-resonant element.

EP 3136143 a1 discloses an antiresonant hollow-core fiber (referred to herein as a "bandgap-free hollow-core fiber") in which the core can conduct other modes in addition to the fundamental mode. For this purpose, it is surrounded by an inner cladding with "non-resonant elements" that match the anti-resonant mode to the phase of the higher modes. Hollow core optical fibers are manufactured using the so-called "stack-and-draw technique" in which the starting elements are arranged in one axially parallel entity and fixed to a preform, which is then drawn. Here, a cladding pipe having a hexagonal inner cross section is used, and six so-called "ARE preforms" (anti-resonant element preforms) ARE fixed in the inner edge of the cladding pipe. The preform is drawn into a hollow core optical fiber in two steps.

WO 2018/169487 a1 discloses a method of manufacturing an antiresonant hollow-core optical fiber preform, wherein a first cladding region comprises a plurality of rods and a second cladding region comprises a plurality of tubes surrounded by an outer cladding tube. The rod, tube and clad tube are joined together by a "stack-and-draw" technique to form a preform. The ends of the preform are sealed by applying a sealant before stretching the preform. For example, a UV adhesive is used as the sealant.

Technical problem

Antiresonant hollow-core fibers, particularly those having nested structural elements, have complex internal geometries that make it difficult to accurately and reproducibly produce them. It is particularly difficult to tolerate even small range deviations in the order of magnitude of the operating wavelength of the light-conducting light in order to maintain the resonant or anti-resonant condition. Deviations from the target geometry may be caused by the configuration of the optical fiber preform or by unwanted, out-of-proportion deformations occurring during fiber drawing.

In the known "stack-and-wire" technique, a number of elements must be joined together positionally accurately. For example, in order to manufacture a hollow core optical fiber of the "NANF" design disclosed in the above-mentioned article, six antiresonant element preforms each consisting of an antiresonant preform outer tube (hereinafter, ARE outer tube) and an antiresonant preform inner tube (hereinafter, ARE inner tube) which is unilaterally welded at an inner cladding surface of the ARE outer tube must be attached at an inner surface of the cladding tube.

In order to achieve low attenuation values and a wide transmission spectral range, in addition to uniform wall thickness of the anti-resonant element, the azimuthal position of the anti-resonant element within the cladding tube is important. This cannot be easily achieved using the "stack-and-draw" technique. It is an object of the present invention to provide a low cost method of manufacturing an antiresonant hollow-core fiber that avoids the limitations of conventional manufacturing methods.

In particular, it is an object of the present invention to provide a method of manufacturing an anti-resonant hollow-core optical fiber and an anti-resonant hollow-core optical fiber preform, with which a high accuracy of the structural elements and a precise positioning of the anti-resonant elements in the optical fiber can be reproducibly achieved in a sufficiently stable and reproducible manner.

Furthermore, the disadvantage of the conventional "stack-and-wire" technique, i.e. the inability to easily achieve the required structural accuracy, in particular the uniform wall thickness of the anti-resonant element and the precise positioning of the predetermined azimuthal position, should be avoided as much as possible.

Disclosure of Invention

With respect to the manufacture of anti-resonant hollow-core fibers, this problem is solved by a method of the above-mentioned kind according to the invention: said forming of said antiresonant element precursor comprises forming elongated pressure chambers, which are each adjacent to a wall deformable under pressure and heat in the region of said prescribed location of said antiresonant element, and which, when the process is carried out according to method step (c), cause said deformable wall portion to protrude in the direction of the cladding tube inner bore due to the effect of pressure and heat, so as to form an antiresonant element or precursor.

The starting point for the fabrication of antiresonant hollow-core fibers is the preform, also referred to herein as the "primary preform". The preform includes a cladding tube in or at which a precursor or preform of an element for forming anti-resonance in the hollow-core optical fiber (herein simply referred to as "anti-resonance element") is contained. The primary preform may be drawn into a hollow core optical fibre; however, it is common to add additional cladding material to the primary preform to thereby produce a preform referred to herein as a "secondary preform". If necessary, a hollow-core optical fiber is produced by drawing the secondary preform. Alternatively, the primary or secondary preform is surrounded by a cladding cylinder or cladding cylinders to form a coaxial whole of the component and the coaxial whole is directly drawn into a hollow core optical fibre. The generic term "preform" is understood herein to mean a component or a coaxial whole of a component from which a hollow-core optical fiber is ultimately drawn.

The positioning accuracy of the anti-resonant element is improved by implementing the precursor for the anti-resonant element in the form of an elongated pressure chamber formed in the region of the prescribed position of the anti-resonant element. These pressure chambers are designed such that, when the inner cladding tube wall sections adjacent thereto soften and gas pressure is applied in the pressure chambers, these wall sections project in the direction of the longitudinal axis of the cladding tube.

When performing the process according to method step (c), the elongated pressure chamber causes the elongated portion of the deformable wall to form an elongated protrusion in the direction of the inner hole of the cladding tube due to the action of pressure and heat to form an elongated anti-resonant element or precursor.

The respective wall portions of the deformable wall to be projected are elongated and extend along the pressure chamber and the anti-resonant element precursor at defined locations in the preform. In the simplest and in the following considered in more detail, the wall part of the deformable wall to be projected belongs to a glass tube.

In a first preferred method variant, the pressure chamber is formed in the wall of the glass tube; the pressure chamber in this case forms a hollow channel which extends parallel to the longitudinal axis of the glass tube from one end of the glass tube wall to the other and which is completely delimited by the glass of the glass tube.

In a further preferred method variant, the pressure chamber is provided by a separate component which is adjacent to the outer envelope surface of the deformable glass tube wall. The pressure chamber forms in this case a hollow channel which extends along the glass tube wall parallel to the longitudinal axis of the glass tube from one end to the other and which is bounded on one side by the glass of the glass tube.

In a further preferred method variant, the pressure chamber is formed in a recess of the outer envelope surface of the glass tube wall; the pressure chamber also forms in this case a hollow channel which extends along the glass tube wall from one end to the other parallel to the longitudinal axis of the glass tube and which is delimited by the glass of the glass tube in the region of the groove.

These process variants can also be combined with one another. In order to be able to introduce the pressure gas into the hollow channel, the hollow channel is open at one end, advantageously also from a manufacturing point of view.

In the manufacturing step, the antiresonant element precursor is made at these positions in such a way that the respective wall portions are made to protrude in the direction of the inner bore of the glass tube by applying pressure acting from the outer surface of the glass tube. This step can be performed, for example, when drawing the preform into a hollow-core optical fiber or into a semifinished product.

The positioning and fixing of the prefabricated anti-resonance element preform at the corresponding position of the inner wall of the cladding tube, as is known in the stack-and-wire technique, can thus be completely eliminated, or at least the number of anti-resonance element preforms to be so positioned can be reduced.

By using the invention, the anti-resonance hollow-core optical fiber and the prefabricated member thereof can be accurately and repeatedly manufactured.

Advantageously, the pressure chamber is designed as a hollow channel, adjacent to which is a wall portion of the glass tube to be deformed.

These hollow channels form pressure chambers into which a pressure gas can be introduced in the manufacturing step, so that the wall portion of the glass tube that is in contact with the pressure gas is deformed by the gas pressure.

Hollow channel made of holes in the wall of deformable glass tube

In some of the above preferred methods, a hollow channel is formed in the wall of the glass tube, the hollow channel extending parallel to the longitudinal axis of the glass tube.

The cross-section of the hollow channel may be circular or polygonal, in particular triangular or rectangular. In a hollow channel having a rectangular shape, the long rectangular sides are tangent to the wall portion to be deformed (to be bulged). In a hollow channel having a triangular shape, one of the triangular sides is tangent to the wall portion to be deformed (to be bulged). Thus, the gas pressure acts more strongly on the wall portion than in the other direction.

In particular for the production of complexly shaped anti-resonant element preforms with nested structural elements, a method variant has proved advantageous in which a coaxial glass tube assembly is formed which comprises an inner circumferential row of hollow channels in the tube wall of the inner glass tube and an outer circumferential row of hollow channels in the tube wall of the outer glass tube, wherein the hollow channels of the inner and outer circumferential rows lie radially on a common connecting line and are spatially separated from one another by at least one circumferentially surrounding and inwardly deformable glass wall.

The surrounding glass walls separate the pressure spaces of the inner and outer rows of hollow channels from each other and are caused to bulge inwardly during the thermal deformation process through the outer rows of hollow channels. If the surrounding glass wall belongs to the inner glass tube, the outer glass tube may be deformed in the inwardly protruding part of the inner glass tube, thereby producing an anti-resonance element preform for nesting the anti-resonance elements.

A hollow channel made of a longitudinal opening in the intermediate tube wall adjacent to the deformable glass tube

Another particularly elegant method for forming a hollow channel comprises a measure in which an intermediate tube is arranged between the glass tube and the outer tube, the intermediate tube having an intermediate tube longitudinal axis along which an intermediate tube wall defined by an inner surface and an outer surface extends, wherein a longitudinal opening opens out in the intermediate tube wall, wherein a hollow channel is formed by the longitudinal opening when the process is carried out according to method step (c).

The longitudinal opening penetrates the intermediate tube wall (preferably excluding the two frontal end regions). These longitudinal openings have parallel longitudinal edges. The intermediate tube may abut and be fused to the outer wall of the glass tube and may abut and be fused to the inner wall of the outer tube. The longitudinal opening is located between the glass tube and the outer tube at the location of the wall portion of the glass tube to be bulged. These longitudinal openings here form hollow channels or precursors of hollow channels, via which pressure can be applied to the outer surface of the glass tube in a later manufacturing step, to thereby cause the softened material of the glass tube to protrude in the direction of its inner bore. Wherein the hollow channel is deformed into an elongated protrusion due to the action of pressure and heat.

An advantage of this embodiment is that the distance of the hollow channels to the inner bore becomes particularly uniform along their length and with respect to each other.

Advantageously, when the hot forming process is carried out according to method step (c), an internal pressure is generated in the hollow channel by introducing a pressure gas and thereby deforming the glass tube wall portion which is contacted by the pressure gas through the longitudinal opening. In which elongated projections are formed at the glass tube, directed inwards, i.e. in the direction of the inner bore of the glass tube and in the direction of the hollow core, which projections serve as anti-resonant element preforms or anti-resonant elements.

The longitudinal opening preferably terminates before the frontal end of the intermediate tube to ensure that the remaining longitudinal ligaments remain together.

In particular for the production of a complexly shaped preform of antiresonant elements with nested structural elements, a method variant has proved advantageous in which a coaxial tube assembly is formed which comprises an inner glass tube, an inner intermediate tube, an inner and an outer tube which simultaneously form the outer glass tube, an outer intermediate tube and an outer tube, wherein the longitudinal openings of the inner intermediate tube and the outer intermediate tube lie on a common connecting line in the radial direction and are spatially separated from one another by at least one circumferentially surrounding and inwardly deformable glass wall.

For this purpose, such coaxial pipe assemblies are used to make at least two hollow channels or pressure chambers, which are arranged in pairs one behind the other, seen in the radial direction. The surrounding glass walls separate the pressure spaces of the inner and outer rows of hollow channels from each other and are caused to bulge inwardly during the thermal deformation process through the outer rows of hollow channels. If the surrounding glass wall belongs to the inner glass tube, the outer glass tube may be deformed in the inwardly protruding part of the inner glass tube, thereby producing an anti-resonance element preform for nesting the anti-resonance elements.

Preferably, an intermediate tube having a circular inner cross-section is provided and machined. The longitudinal opening is embodied to be continuous in the radial direction and can be produced easily and precisely therein; for example by milling, drilling or cutting. The inner geometry of the longitudinal opening or the longitudinal groove is, for example, rectangular or V-shaped.

The longitudinal opening is preferably made by machining the intermediate pipe wall, in particular by cutting, drilling, sawing, milling and grinding.

Machining refers to a mechanical machining technique that removes material, such as turning, cutting, drilling, sawing, milling, or grinding. These machining techniques provide a more precise and finer structure than other known forming techniques using heat and pressure, and they avoid contamination of the surfaces by forming tools such as nozzles, presses or melt formers.

It is useful that the longitudinal opening has longitudinal edges and the glass tube and the outer tube are joined to the longitudinal edges by softening.

For this purpose, the coaxial tube unit consisting of the outer tube, the longitudinally open intermediate tube and the glass tube is heated, and the cutting edge of the longitudinal opening is bonded over its entire length to the outer wall of the glass tube and to the inner wall of the outer tube. Unwanted deformation in the radial direction is suppressed by simultaneous elongation. Alternatively, the tubes are joined to one another in pairs one after the other in two process steps.

In this way a preform is obtained in which the original longitudinal opening is closed off as a hollow channel. These hollow channels may be exposed on one or both sides by removing the closed front end region of the preform.

Hollow channels made of longitudinal grooves in the deformable glass tube and/or in the intermediate tube adjacent to the glass tube Become into

Alternatively or additionally to the above-described method of forming the hollow channel, it is also useful that the forming the hollow channel comprises a treatment in which a glass tube and an intermediate tube are used, the intermediate tube coaxially surrounding the glass tube, wherein the glass tube has a glass tube outer envelope surface in which longitudinal grooves are cut, the longitudinal grooves extending parallel to the glass tube longitudinal axis; and/or the intermediate tube has an intermediate tube inner envelope surface in which the longitudinal grooves are cut, which extend parallel to the intermediate tube longitudinal axis, wherein during the execution of process step (c) hollow channels are formed from the longitudinal grooves, and wherein the hollow channels are deformed into elongated projections as a result of the action of pressure and heat.

The longitudinal grooves in the outer envelope surface of the glass tube also form channels and thus pressure chambers in cooperation with the tube wall surrounding the glass tube for deforming the wall portion in which the longitudinal grooves extend. As explained in detail above, the longitudinal grooves at the inner envelope surface of the intermediate tube serve to form the hollow channel in a similar manner to the longitudinal opening of the intermediate tube.

It is advantageous for the application of pressure that the longitudinal grooves are continuous, that is to say that they preferably extend from one end of the respective tube to the opposite end.

With respect to the fabrication of preforms for hollow-core optical fibres, the above technical problem is solved by a method of the above-mentioned type according to the present invention: said forming of said anti-resonant element precursor comprises forming elongated pressure chambers, which are each adjacent to a pressure-and heat-affected deformable wall in the region of said prescribed location of said anti-resonant element, and which, when the process is carried out according to method step (c), cause said deformable wall portion to bulge in the direction of the inner bore of said cladding tube due to the effect of pressure and heat, so as to form an anti-resonant element or precursor.

The preform is the starting point for the fabrication of an anti-resonant hollow-core fiber. The anti-resonance hollow-core optical fiber is directly drawn by drawing the prefabricated member, or a semi-finished product is firstly made, and then the semi-finished product is drawn into the anti-resonance hollow-core optical fiber. The manufacturing of the preform comprises bulging of the wall portion of the glass tube in the region of the prescribed position of the anti-resonant element by applying a pressure in the pressure chamber.

These portions of the glass tube wall to be embossed are elongated and extend along the prescribed locations of the precursor anti-resonant elements in the preform. In the manufacturing step, anti-resonant element precursors are made at these locations in such a way that the respective wall portions are caused to protrude in the direction of the inner hole of the cladding tube by applying pressure acting from the opposed wall surfaces. The positioning and fixing of the prefabricated anti-resonance element preform at the corresponding position of the inner wall of the cladding tube, as is known in the stack-and-wire technique, can thus be completely eliminated, or at least the number of anti-resonance element preforms to be so positioned can be reduced. The fabrication scheme of the preform has been described above in connection with the fabrication of hollow core optical fibres and all such descriptions are included in the present invention.

Definition of

The individual process steps and terms described above are defined in supplementary terms as follows. These definitions are part of the present specification. The contents of the specification are authoritative if there is a substantial conflict between any of the following definitions and the rest of the specification.

Anti-resonance element

The anti-resonant element may be a simple or nested structural element of a hollow core optical fibre. These anti-resonant elements have at least two walls which have a negative curvature (convex) or no curvature (flat, straight) when seen in the direction of the hollow core. Generally, they consist of a material which is transparent to the operating light, for example glass, in particular doped or undoped SiO₂Synthetic materials, in particular polymers, composites or crystalline materials.

Antiresonant element preform/antiresonant element precursor

The components or elements of the preform, referred to as the antiresonant element preform, are essentially converted to antiresonant elements in hollow fibers by simple elongation during fiber draw. The components or parts of the preform, referred to as the precursor of the anti-resonant element, are first deformed into the preform of the anti-resonant element or directly into the anti-resonant element. The anti-resonant element preform may be a simple or nested component where additional positioning aids may also be secured. These antiresonant element preforms are initially present in the form of a primary preform. The nested anti-resonant element preform forms a nested anti-resonant element in the hollow core optical fiber. They consist of an outer tube and at least one individual structural element, which is arranged in the inner bore of the outer tube. The further structural element may be a respective tube abutting at the inner envelope surface of the outer tube. The outer tube is referred to as the "anti-resonant element outer tube" or simply as the "ARE outer tube", and the other tube is referred to as the "anti-resonant element inner tube" or simply as the "ARE inner tube" or "nested ARE inner tube".

In the case of multiple nested anti-resonant element preforms, at least one individual structural element may be disposed within the inner bore of the nested ARE inner tube, such as a third tube abutting the inner cladding surface of the nested ARE inner tube. In order to distinguish between several tubes arranged inside the ARE outer tube in the case of multiple nested antiresonant element preforms, it is necessary to distinguish between "outer nested ARE inner tube" and "inner nested ARE inner tube".

The term "section" in relation to the preform of the cylindrical anti-resonant element and its cylindrical structural element always refers to a cross section perpendicular to the respective cylindrical longitudinal axis, i.e. in the case of a tubular component, unless otherwise stated, to the cross section of the outer contour (and not the cross section of the inner contour).

By reworking the primary preform, in particular by a thermoforming step, an intermediate product can be produced in which the original preform of the anti-resonant element is present in a modified shape compared to the original shape. The altered shape is also referred to herein as an anti-resonant element preform or anti-resonant element precursor.

Preform/primary preform/secondary preform/core preform (Cane)

The preform is the component from which the anti-resonant hollow-core fiber is drawn. It is a primary preform or a secondary preform made by reworking the primary preform. The primary preform may be present as a whole consisting of at least one cladding tube and a preform or precursor of the anti-resonant element loosely received therein or firmly fixed therein. Reworking the primary preform into a secondary preform from which the hollow-core optical fiber is drawn, the reworking comprising performing one or more of the following thermoforming processes once or repeatedly:

(i) stretching the mixture to obtain a stretched mixture,

(ii) the material is collapsed and then the material is compressed,

(iii) the collapsing and the simultaneous stretching are carried out,

(iv) the additional cladding material is collapsed and the additional cladding material,

(v) the additional cladding material is collapsed and subsequently stretched,

(vi) collapsing the additional cladding material and simultaneously stretching.

In the literature, core preforms (Can) are preforms obtained by collapsing and/or drawing a primary preform. Typically, the hollow-core fiber is clad with additional cladding material prior to or during drawing thereof.

Stretching/collapsing

During drawing, the primary preform is elongated. Elongation can be performed simultaneously without collapsing. The drawing may be done in proportions such that, for example, the shape and arrangement of the components or constituents of the primary preform are reflected in the drawn final product. However, the primary preform may also not be drawn to scale and its geometry may be altered during drawing.

Upon collapse, the inner bore narrows or the annular gap between the tubular members closes or narrows. Typically, collapse occurs with stretching.

Core/inner/outer cladding region

The whole consisting of at least one cladding tube and a preform or precursor of the anti-resonant element loosely received therein or firmly fixed therein is also referred to herein as a "primary preform". The primary preform includes a core and a cladding region. When there is simultaneously an "outer cladding region" made in its entirety, for example by collapsing, and these cladding regions should be distinguished, this cladding region is also referred to as an "inner cladding region". The designations "inner cladding region" and "outer cladding region" are also used for the corresponding regions in the hollow-core fiber or in the intermediate product obtained by reworking the primary preform.

The term "tube inner surface" is also used as a synonym for "inner envelope of the tube", while the term "tube outer surface" is also used as a synonym for "outer envelope of the tube". The term "internal bore" in relation to the tube does not mean that the internal bore is created by a drilling process.

Cutting machining

Machining is understood to be a separate mechanical manufacturing method for separately machining a workpiece, in particular turning, cutting, drilling, sawing, milling and grinding. These processes create a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube, which serves as a positioning aid for the preform of the antiresonant element. The longitudinal structure is accessible from the inner surface of the cladding tube; the longitudinal structure may also extend through the entire cladding tube wall to the outer surface.

Particle size and particle size distribution

SiO₂The particle size and particle size distribution of the particles being determined by means of D₅₀And (4) value characterization. These values are taken from the particle size distribution curves which show the particle size dependent SiO₂The cumulative volume of the particles. The particle size distribution is generally determined by means of the corresponding D₁₀、D₅₀And D₉₀And (4) value characterization. Wherein D is₁₀Values indicating less than 10% SiO₂Particle size of cumulative volume of particles, correspondingly, D₅₀Value sum D₉₀Values representing SiO less than 50% and 90%₂Particle size of cumulative volume of particles. The particle size distribution was determined by scattered light and laser diffraction spectroscopy according to ISO 13320.

Examples

The invention is explained in more detail below with the aid of embodiments and the figures. Specifically, in the schematic diagram:

figure 1 shows in side view (a) and in cross-sectional top view (b) a first embodiment of an intermediate tube equipped with longitudinal openings for use in the process according to the invention,

FIG. 2 shows the method steps for manufacturing a preform (b) for a hollow-core optical fiber, using a tube ensemble (a) with several open intermediate tubes according to the first example,

fig. 3 shows, by an enlarged view of the cross-section of the preform in fig. 2(b), the drawing of the preform of fig. 3 into a hollow core optical fiber, to form an anti-resonant element,

figure 4 shows a second embodiment of an intermediate tube equipped with a longitudinal opening in side view (a) and in top view (b) in section,

FIG. 5 shows the method steps for manufacturing a preform (b) for a hollow-core optical fiber, using a tube ensemble (a) with one open intermediate tube according to a second example,

FIG. 6 shows, by an enlarged view of a cross-section of the preform in FIG. 5(b), the drawing of the preform of FIG. 5 into a hollow core optical fiber to form an anti-resonant element, an

Fig. 7 shows in top view a glass tube coaxial assembly with a heat deformable wall comprising a hollow channel and a cladding cylinder.

Fig. 1(a) shows an intermediate tube 10 in the wall of which elongated longitudinal openings 13 are cut at uniform intervals at predefined azimuthal positions, for example by means of a mechanical saw, water jet cutting, laser, etc. The longitudinal openings 13 are used to form anti-resonant elements in the manufactured hollow core optical fiber or anti-resonant element preforms in the optical fiber preform, and the number of longitudinal openings 13 corresponds to the number of anti-resonant element preforms or anti-resonant elements that can be manufactured together with the respective intermediate tube 10. In the example shown, six antiresonant elements are prefabricated or antiresonant elements. The longitudinal opening 13 ends before the tube end so that the front end region 12 continues to remain closed all around and to interconnect the remaining webs 14. The cut edge is then vitrified. The longitudinal openings 13 are cut to a uniform width of 2 mm.

As can be seen from the top view of the cross section of the intermediate tube 10 along the tangent a-a in fig. 1(b), six longitudinal openings 13 are evenly distributed around the tube wall and extend continuously from the outer wall of the intermediate tube to the inner wall of the intermediate tube up to the inner bore 16.

Fig. 2(a) schematically shows a top view of a coaxial assembly 19 of a total of five quartz glass tubes, including two intermediate tubes 10; 20 each having a longitudinal opening 13. The coaxial tube assembly 19 consists of two coaxial stacks, each consisting of a glass tube (21; 22) to be deformed, an intermediate tube (10; 20) and a sleeve (22; 23). The tube, reference numeral 22, has a dual function: a "sleeve" (Mantelrohr) is formed in the inner stack and the wall of the sleeve becomes an integral part of the wall of the hollow channel, and a "glass tube" with the wall to be deformed is formed in the outer stack.

Table 1 below summarizes the details of the dimensions and materials of these tubes:

TABLE 1

Reference numerals	Name/function	Inner diameter	Outer diameter	Material
					21	Glass tube to be deformed	21	24	Doped with fluorine; f320
10	Intermediate pipe	25	31.5	Is undoped;
					22	glass tubes or sleeves to be deformed	32.5	34	Doped with fluorine; f320
20	Intermediate pipe	35	39	Undoped
					23	Sleeve pipe	40	60	Undoped

Reference numbers: reference numerals in FIG. 2a

Doping with fluorine: f320: fluorine-doped quartz glass/low viscosity

Undoped: undoped quartz glass/high viscosity

The materials used in this case differ in viscosity. The raw pipes 21 and 22 are made of commercially available fluorine-doped quartz glass (trade name: F320) and have a lower viscosity than the open intermediate pipes 10, 20 and the outermost jacket pipe 23 (clad pipe).

Fig. 2(b) shows that the coaxial tube ensemble 19 is then collapsed into a primary preform 26 and therein elongated simultaneously. Wherein the annular gap between the tubes disappears and the tubes are tightly connected to each other and form a cladding tube with a common cladding tube wall 24. Wherein the open central tube 10; the longitudinal openings 13 of 20 create hollow channels that can be used as front and rear pressure chambers 25a during subsequent fiber draw; 25b, respectively. The two pressure chambers 25a, 25b are paired front and back as viewed in the radial direction. Prior to the fiber drawing process, at least one of the closed, longitudinally unopened end regions of the primary preform 26 is removed, so that pressure chambers 25a, 25b are obtained, which are open on the front side and into which a pressure gas can be introduced. The primary preform 26 has a hollow core region 27 surrounded by cladding (cladding tube wall 24). The pressure chamber 25 a; 25b form a precursor of the anti-resonant element of the hollow-core fiber to be drawn in the cladding tube wall 24.

FIG. 3(a) shows, in enlarged cross-section, a pressure chamber 25a made of an otherwise longitudinal opening in cladding tube wall 24; 25b, respectively. When the preform thus obtained is drawn into an optical fiber, a differential pressure is applied between the pressure chambers 25a, 25b and the hollow core region 27, so that the original glass tube 21; 22 and the pressure chamber 25 a; 25b adjacent and deformable wall regions expand inwardly along the pressure chambers 25a, 25 b.

Fig. 3(b) shows where a first protrusion 28a is created in the hollow-core fiber 29 at the inner surface 27 of the previously innermost glass tube, said first protrusion surrounding a second protrusion 28 b. First and second projections 28 a; 28b form a nested anti-resonant element having two glass films with negatively curved surfaces.

In the following description of fig. 4 to 7, the same or equivalent components or constituents are indicated if the same reference numerals are used as in fig. 1 to 3, as explained in more detail above with reference to these figures.

Fig. 4(a) shows another intermediate tube 110 in which elongated longitudinal openings 13 are cut at uniform intervals in the wall of the intermediate tube, for example by means of a mechanical saw, water jet cutting, laser, etc., at predefined azimuthal positions. The longitudinal openings 13 are used to form anti-resonant elements in the manufactured hollow core optical fiber or anti-resonant element preforms in the optical fiber preform, and the number of longitudinal openings 13 corresponds to the number of anti-resonant element preforms or anti-resonant elements that can be manufactured together with the respective intermediate tube 10. In the example shown, five antiresonant elements are prefabricated or antiresonant elements. The longitudinal opening 13 ends before the tube end so that the front end region 12 continues to remain closed all around and to interconnect the remaining webs 14. The cut edge is then vitrified. The longitudinal openings 13 are cut to a uniform width of 2 mm.

As can be seen from the top view of the cross section of the intermediate tube 110 along the tangent a-a of fig. 4(b), five longitudinal openings 13 are evenly distributed around the tube wall at a circumferential angle of 72 degrees and extend continuously from the outer wall of the intermediate tube to the inner wall of the intermediate tube up to the inner bore 16.

Fig. 5(a) schematically shows a top view of a coaxial ensemble of a total of three quartz glass tubes, including an open intermediate tube 110. The coaxial tube is composed overall of a glass tube 21 to be deformed, an intermediate tube 110 with a longitudinal opening 13 and a sleeve 22.

Table 2 below summarizes the details of the dimensions and materials of these tubes:

TABLE 2

Reference numerals	Name/function	Inner diameter [ mm ]]	Outer diameter [ mm ]]	Material
					21	Glass tube to be deformed	21	24	Doped with fluorine; f320
110	Intermediate pipe	25	31.5	Is undoped;
					22	sleeve pipe	32.5	60	Undoped

Reference numbers: reference numerals of FIG. 5a

Doped with fluorine; f320: fluorine-doped quartz glass/low viscosity

Undoped: undoped quartz glass/high viscosity

The materials used in this case differ in viscosity. The unmachined glass tube 21 is made of commercially available fluorine-doped quartz glass (trade name: F320) and has a lower viscosity than the open intermediate tube 110 and the jacket tube 22 (cladding tube).

Fig. 5(b) shows that the coaxial tube is then collapsed as a whole into a primary preform 126. Wherein the coaxial tubes are simultaneously elongated in their entirety and the annular gaps between the tubes 21, 22, 110 disappear, so that the tubes are tightly joined to each other, so that the tubes form a common cladding tube wall 124. Wherein hollow channels in the cladding tube wall 124 are created by the longitudinal openings 13 of the open intermediate tube 110, which hollow channels can be used as pressure chambers 125 in a subsequent fiber drawing process. The primary preform 126 has a hollow core region 127 surrounded by cladding (cladding tube wall 124). The pressure chamber 125 forms a precursor for the anti-resonant element in the cladding region of the hollow core fiber to be drawn.

Prior to the fiber drawing process, at least one of the closed, longitudinally unopened end regions of the preform 126 is removed, so that the pressure chamber 125 can be left open on the front and a pressure gas can be introduced.

Fig. 6(a) shows the pressure chamber 125 made of the original longitudinal opening 13 in an enlarged section. When the preform 126 thus obtained is drawn into an optical fiber, a differential pressure is applied between the pressure chamber 125 and the inner bore 16, causing the wall region adjacent to the pressure chamber 125 to expand inwardly along the pressure chamber 125.

Fig. 6(b) shows a case where a protrusion 128a is generated in the hollow-core optical fiber 129 at the inner surface 17 of the preceding glass tube, the protrusion forming an anti-resonance element having a glass film with a negative-bending surface.

Instead of the longitudinal opening 13, the intermediate tube 10, 20, 110 can also be provided with a longitudinal groove at its inner envelope surface. These longitudinal grooves are optionally produced by mechanical milling in the inner envelope surface of the intermediate tube.

Instead of or in addition to an intermediate tube equipped with longitudinal openings or longitudinal grooves, a glass tube 21 with a heat-deformable wall; 22 may also be provided with longitudinal grooves at its outer envelope surface. These longitudinal grooves are made by mechanical milling in the outer envelope surface of the glass tube, if necessary.

Instead of or in addition to the above-described embodiments of glass tubes and/or intermediate tubes with longitudinal openings or longitudinal grooves, a glass tube 21 with a heat-deformable wall; 22 may also be provided with a hollow channel. To this end fig. 7 schematically shows an embodiment. The coaxial assembly comprises a glass tube 221 with a heat deformable wall and a cladding cylinder 22 (sleeve). Within the wall of the glass tube 221 and in the vicinity of the inner envelope surface 221a, four hollow channels 213, which are distributed uniformly around the circumference, extend parallel to the glass tube longitudinal axis (which extends perpendicular to the plane of the paper). The hollow channels 213 are made by laser cutting and are continuous (i.e. they extend from one end of the wall to the other). In the cross-section shown, these hollow channels have a rectangular shape with long rectangular sides tangent to adjacent wall portions of the inner envelope surface 221 a.

The intermediate tube may be omitted in this embodiment. Table 3 below summarizes the details of the dimensions and materials of these tubes:

TABLE 3

Reference numerals	Name/function	Inner diameter [ mm ]]	Outer diameter [ mm ]]	Material
					221	Glass tube to be deformed	21	24	Doped with fluorine; f320
22	Sleeve pipe	25	60	Undoped

Reference numbers: reference numerals of FIG. 7

Doping with fluorine: f320: fluorine-doped quartz glass/low viscosity

Undoped: undoped quartz glass/high viscosity

When the thermoforming process is performed, an internal pressure may be generated in the hollow passage 213 by introducing a pressure gas, and thereby deforming the wall portion of the glass tube 221 that inwardly defines the hollow passage 213. In which inwardly directed, elongated projections are formed at the glass tube 221, i.e., in the direction of the inner bore 16 of the glass tube, and these projections serve as anti-resonant element preforms.