Method for manufacturing a preform for an anti-resonant hollow-core optical fiber

Abstract

The present invention relates to a method (700) for manufacturing a preform for an anti-resonant hollow fiber, the anti-resonant hollow fiber having a hollow extending along a longitudinal axis of the fiber and a cladding region surrounding the hollow and including at least one anti-resonant element, the method having at least the following method steps: (a) providing (710) a cladding tube (200) including an inner surface of the cladding tube and an outer surface (210) of the cladding tube, wherein at least one anti-resonant element preform (400) is arranged at the inner surface of the cladding tube, (b) providing (720) a covering tube (300) including an inner surface of the covering tube (310), wherein an inner diameter of the covering tube (300) is larger than an outer diameter of the cladding tube (200), (c) arranging (730) the cladding tube (200) in the covering tube (300) such that the inner surface of the covering tube (310) surrounds the outer surface of the cladding tube (210), and (d) adding (740) the covering tube (300) to the cladding tube (200) such that the inner surface of the covering tube (310) is connected to the outer surface of the cladding tube (210).

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a preform for an anti-resonant hollow fiber optic. Background Art

[0002] Conventional single-mode optical fibers of solid materials have a glass core region surrounded by a glass cladding region with a lower refractive index. Optical guidance is thereby based on total reflection between the core region and the cladding region. However, the interaction of the guided light with the solid material is associated with increased latency during data transmission and a relatively low damage threshold (compared to high-energy radiation).

[0003] "Hollow fiber optics" avoid or reduce these drawbacks, in which case the core includes a evacuated cavity filled with gas or liquid. The interaction of light in a hollow fiber optic with the glass is less than that in a solid-core fiber optic. The refractive index of the core is less than that of the cladding, such that optical guidance by means of total reflection is not possible, and light will generally escape from the core into the cladding. Depending on the physical mechanism of optical guidance, hollow fiber optics are divided into "photonic bandgap fibers" and "anti-resonant hollow fiber optics".

[0004] In the case of "photonic bandgap fibers", the hollow region is surrounded by a cladding in which small hollow ducts are periodically arranged. The periodic structure of the hollow ducts in the cladding causes an effect referred to in reference to semiconductor technology as a "photonic bandgap", according to which light in a specific wavelength range scattered at the cladding structure undergoes constructive interference due to Bragg reflection in the central cavity and cannot propagate laterally in the cladding.

[0005] In the case of an embodiment of a hollow fiber optic known as an "anti-resonant hollow fiber optic" (ARHCF), the hollow region is surrounded by an inner cladding region in which so-called "anti-resonant elements" (or "anti-resonant elements"; abbreviated as "ARE") are arranged. The walls of the anti-resonant elements uniformly distributed around the hollow can act as Fabry-Perot cavities, which operate in anti-resonance and reflect the incident light and guide it through the fiber core.

[0006] This fiber optic technology ensures low optical attenuation, a very wide transmission spectrum (also in the UV or IR wavelength range), and small latency during data transmission.

[0007] Potential applications of hollow fiber optics are in the following fields: data transmission, high-performance beam guidance (e.g., for material processing), modal filtering, non-linear optical devices (especially for supercontinuum generation, from the ultraviolet to the infrared wavelength range).

[0008] One disadvantage of anti-resonant hollow-core fibers is that higher-order modes are not necessarily suppressed, such that they are generally not purely single-mode over large transmission lengths and the quality of the output beam deteriorates.

[0009] In the paper “Nested anti-resonant nodeless hollow core fiber” by Francesco Poletti; Optics Express, Vol. 22, No. 20 (2014); DOI: 10.1364 / OE 22.023807, a fiber design is proposed in which the anti-resonant elements are not formed as simple single structural elements, but rather consist of several nested structural elements. The nested anti-resonant elements are designed in such a way that the higher-order core modes are phase-matched to the cladding modes and suppressed, rather than the fundamental core mode. Thus, the propagation of the fundamental core mode is always ensured and the hollow-core fiber can be effectively made single-mode over a limited wavelength range.

[0010] Effective mode suppression is a function of the central wavelength of the transmitted light and the structural parameters of the fiber design, such as the radius of the hollow core and the diameter difference of the nested ring structures in the anti-resonant elements.

[0011] An anti-resonant hollow-core fiber (referred to herein as a “hollow-core fiber without a bandgap”) is known from EP 3136143 A1, in which, in addition to the fundamental mode, the core can also guide additional modes. For this purpose, the core is surrounded by an inner cladding including “anti-resonant elements” which provide phase matching of the anti-resonant modes to the higher modes. The hollow-core fiber is manufactured according to the so-called “stack-and-draw” technique, in which the output elements are arranged to form an axially parallel assembly and are fixed to form a preform, which is subsequently elongated. Thereby, a cladding tube with 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 tube. The preform is stretched in two stages to form the hollow-core fiber.

[0012] The precise and reproducible manufacture of the preform has proven to be difficult, since already small dimensional deviations adversely affect the effectiveness of light guidance. Specifically, with increasing wall thickness of the cladding tube and the associated need for heat supply, uncontrolled deformations occur during the connection of the cladding tube and the anti-resonant element preforms. It may therefore be advantageous to use a relatively thin-walled cladding tube for connection to the anti-resonant element preforms and to surround the cladding tube in a separate method step by means of a covering tube made of bulk glass in order to set the desired wall thickness.

[0013] The connection of the overlay tube to the cladding tube (so-called "addition") and the drawing into the final glass fiber can be carried out in a coordinated step or in two steps separate from each other. Even without direct drawing into the final glass fiber, the addition can be combined with partial drawing in order to reduce the diameter of the preform and thus prepare it for the final drawing.

[0014] A method for manufacturing a microstructured optical fiber is known from US2010 / 030429 A1, in which case a preform consisting of a core including a plurality of microtubes and an overlay tube made of bulk glass enclosing the core is drawn into an optical fiber. In order to connect the core and the overlay tube during drawing, a negative pressure is applied between the overlay tube and the core, while a positive pressure is applied inside the core, so that the microtubes do not collapse during drawing into the final optical fiber.

[0015] A method for manufacturing a hollow-core optical fiber is known from US2020 / 0024178 A1, in which case a preform consisting of a hollow core and an overlay tube made of bulk glass enclosing the core is drawn into a hollow-core optical fiber. In order to connect the core and the overlay tube during drawing, a negative pressure is applied between the overlay tube and the core, while a gas is introduced into the hollow core to prevent collapse of the core. Summary of the Invention

[0016] Anti-resonant hollow-core optical fibers, especially those including nested structural elements, have a complex internal geometry, which makes their precise and reproducible manufacturing more difficult. This applies in particular because in order to respectively follow the resonant or anti-resonant conditions, small dimensional deviations of the magnitude of the operating wavelength of the light to be guided can no longer be tolerated. The configuration of the optical fiber preform can be the cause of deviations from the target geometry, and they can also occur due to disproportionate and uncontrolled deformations during the addition.

[0017] During the addition of the overlay tube to the cladding tube provided with an anti-resonant element preform, each structural deviation from the target geometry reduces the quality of the final anti-resonant hollow-core optical fiber.

[0018] The object of the present invention is to provide a method for cost-effectively manufacturing an anti-resonant hollow-core optical fiber, which method avoids the limitations of conventional manufacturing methods.

[0019] Another object of the present invention is to provide a method which enables mass production of anti-resonant hollow-core optical fibers.

[0020] Specifically, the object of the present invention is to provide a method for manufacturing a preform for an anti-resonant hollow-core optical fiber, by means of which a high precision of the structural elements can be obtained in a reproducible manner, if possible, when adding the overlay tube to the cladding tube.

[0021] The present invention provides a method for manufacturing a preform for an anti-resonant hollow fiber, comprising:

[0022] / 1 / A method for manufacturing a preform for an anti-resonant hollow fiber, the anti-resonant hollow

[0023] fiber having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding the hollow core and including at least one anti-resonant element, the method having at least the following method steps:

[0024] (a) Providing a cladding tube including an inner surface and an outer surface of the cladding tube, wherein at least one anti-resonant element preform is arranged at the inner surface of the cladding tube,

[0025] (b) Providing a covering tube including an inner surface of the covering tube, wherein the inner diameter of the covering tube is larger than the outer diameter of the cladding tube,

[0026] (c) Arranging the cladding tube inside the covering tube such that the inner surface of the covering tube surrounds the outer surface of the cladding tube,

[0027] (d) Adding the covering tube to the cladding tube such that the inner surface of the covering tube is connected to the outer surface of the cladding tube,

[0028] Characterized in that,

[0029] During the addition in method step (d), a first negative pressure P1 is applied between the inner surface of the covering tube and the outer

[0030] surface of the cladding tube, and a second negative pressure P2 is applied inside the cladding tube. / 2 / The method according to embodiment 1, characterized in that, compared with the ambient pressure, the

[0031] first negative pressure P1 is in the range of -100 mbar to -10 mbar, and the second negative pressure

[0032] P2 is in the range of -50 mbar to -1 mbar.

[0033] / 3 / The method according to embodiment 1 or 2, characterized in that the first negative pressure P1 has

[0034] a greater magnitude than the second negative pressure P2.

[0035] / 4 / The method according to any one of the foregoing embodiments, characterized in that during the addition in method step

[0036] (d), a third negative pressure P3 is applied inside the at least one anti-resonant element preform.

[0037] / 5 / The method according to embodiment 4, characterized in that the third negative pressure P3 has a magnitude

[0038] The same amount as the second negative pressure P2.

[0039] / 6 / The method according to embodiment 4, characterized in that the third negative pressure P3 has an amount different from

[0040] that of the first negative pressure P1 and the second negative pressure P2, in particular, the third negative pressure P3 has an amount smaller than that of the second negative pressure P2.

[0041] / 7 / The method according to any one of the foregoing embodiments, characterized in that, before method step

[0042] (d), the cladding tube is closed in an airtight manner at the first cladding tube end, and in some regions, the inner surface of the covering tube is connected to the outer surface of the cladding tube in an airtight manner at the second cladding tube end.

[0043] / 8 / The method according to embodiment 7, characterized in that the cladding tube has a holding ball at the second cladding tube end for arranging the cladding tube inside the covering tube in method step (c).

[0044] / 9 / The method according to any one of embodiments 7 or 8, characterized in that the cladding tube is closed in an airtight manner at the first cladding tube end by forming a tip.

[0045] / 10 / The method according to any one of embodiments 7 to 9, characterized in that the first negative pressure P1 is applied in the region of the first cladding tube end, and the second negative pressure P2 is applied in the region of the second cladding tube end.

[0046] / 11 / The method according to embodiment 10, characterized in that the first negative pressure P1 is applied by means of a first holding pipe attached to the first covering tube end facing the first cladding tube end, and the second negative pressure P2 is applied by means of a second holding pipe attached to the second covering tube end facing the second cladding tube end.

[0047] / 12 / The method according to any one of embodiments 9 to 11, characterized in that, before method step (d), the tip is inserted into a perforated plate.

[0048] / 13 / The method according to any one of the foregoing embodiments, characterized in that the first negative pressure P1 is applied first, and then the second negative pressure P2 is applied.

[0049] / 14 / The method according to any one of the foregoing embodiments, characterized in that the preform is stretched during the addition in method step (d).

[0050] In this specification, range specifications also include the values mentioned as limits. The designation "in the range of X to Y" with respect to the type of variable A thus means that A can take the value X, the value Y, and the values between X and Y. Thus, a range restricted on one side of the type of variable A "up to Y" thus means Y and values less than Y.

[0051] Some of the described features are related to the term "substantially". The term "substantially" should be understood in such a way that, under actual conditions and manufacturing techniques, a mathematically exact interpretation of terms such as "overlap", "perpendicular", "diameter", or "parallelism" may never be exactly provided, but can only be applied within certain manufacturing-related error tolerances. For example, "substantially perpendicular axes" draw an angle of 85 degrees to 95 degrees with each other, and "substantially equal volumes" include a deviation of at most 5% by volume. An "apparatus substantially composed of fused silica glass" includes, for example, ≥95 wt% to ≤100 wt% of fused silica glass parts. "Substantially completely filling volume B" includes, for example, filling ≥95% by volume to ≤100% of the total volume B. Description of the Drawings

[0052] Figure 1 shows a longitudinal section of a first intermediate product of a method for manufacturing a preform for an anti-resonant hollow fiber,

[0053] Figure 2 shows a longitudinal section of a second intermediate product of a method for manufacturing a preform for an anti-resonant hollow fiber,

[0054] Figure 3 shows a longitudinal section of a second intermediate product of a method for manufacturing a preform for an anti-resonant hollow fiber when a first negative pressure P1 and a second negative pressure P2 are applied, and

[0055] Figure 4 shows a flow chart of a method for manufacturing a preform. Detailed Description of the Invention

[0056] The present invention relates to a method for manufacturing a preform for an anti-resonant hollow fiber. As part of method step (a), a cladding tube including an inner side of the cladding tube and an outer surface of the cladding tube is provided, wherein at least one anti-resonant element preform is arranged at the inner surface of the cladding tube.

[0057] Method step (b) includes providing a covering tube including an inner surface of the covering tube, wherein the inner diameter of the covering tube is greater than the outer diameter of the cladding tube.

[0058] Method step (c) includes arranging the cladding tube inside the covering tube such that the inner surface of the covering tube surrounds the outer surface of the cladding tube, in particular substantially concentrically.

[0059] Method step (d) includes adding a covering tube to the cladding tube such that the inner surface of the covering tube is connected to the outer surface of the cladding tube, in particular by a material-to-material bond.

[0060] A target solution is obtained, wherein at least during the addition of the covering tube to the cladding tube in method step (d), a first negative pressure P1 is applied between the inner surface of the covering tube and the outer surface of the cladding tube, and a second negative pressure P2 is applied inside the cladding tube (and optionally also inside the anti-resonator preform), in order to provide the addition while highly maintaining the accuracy of the structural elements (in particular the cladding tube and the anti-resonator preform) in a reproducible manner.

[0061] A preform is a component from which an anti-resonant hollow fiber can be drawn. In an alternative, the preform can be further processed into a secondary preform from which an anti-resonant hollow fiber is drawn. This further processing can include one or repeated executions of a thermoforming process (such as stretching, collapsing, or adding additional cladding material).

[0062] The cladding tube is a tubular element made substantially of silica glass, which is used to arrange the anti-resonator preform at the inner surface of the cladding tube. When drawing the final preform, the cladding tube surrounds the hollow of the anti-resonant hollow fiber. In one embodiment, the inner diameter of the cladding tube is in the range of 10 mm to 60 mm. In one embodiment, the outer diameter of the cladding tube is in the range of 25 mm to 250 mm, preferably in the range of 30 mm to 200 mm. In one embodiment, the length of the cladding tube is in the range of 400 mm to 1200 mm.

[0063] The anti-resonator elements can be simple or nested structural elements of a hollow fiber. They have at least two walls which, when viewed from the direction of the hollow, have a negative curvature (convex) or no curvature (flat, straight). They are typically composed of a material transparent to the working light (such as glass, in particular doped or undoped silica glass (SiO2), plastics, in particular polymers, composites, or crystalline materials).

[0064] The component part of a tubular component or preform is called an anti-resonator preform, which essentially becomes an anti-resonator in a hollow-core optical fiber by means of simple elongation during the optical fiber drawing process. In one embodiment, the anti-resonator preform has a wall thickness at least partially in the range of 0.1 mm to 3 mm, preferably in the range of 0.1 mm to 2 mm, more preferably in the range of 0.2 mm to 1.5 mm. The anti-resonator preform can be a simple or nested component to which a positioning aid can be additionally fixed. The anti-resonator preform has at least two walls, which, when viewed from the hollow-core direction, have a negative curvature (convex) or no curvature (flat, straight). By means of further processing of the preform, in particular by means of a thermoforming step, an intermediate product can be produced, in which the initial anti-resonator preform is present in a shape changed compared to the initial shape.

[0065] The covering tube is a tubular element, which is essentially made of silica glass and, as part of the method according to the invention, is arranged around the cladding tube and is connected to the cladding tube (so-called "addition") using a first negative pressure P1, in particular by means of a substance-to-substance bond, which substance-to-substance bond is effected by heat input, for example via a flame process (e.g., a hydrogen torch) or a flameless process (e.g., by means of an electric furnace). When using a relatively thin cladding tube, the covering tube ensures additional cladding material during the formation of the final anti-resonator hollow-core optical fiber and thus enables the arrangement of the anti-resonator preform using a relatively thin cladding tube.

[0066] The component of the preform essentially made of silica glass can have a doping. The doping provides an adaptation of the thermal expansion coefficient in the vicinity of the preform in order to avoid or reduce tension. Fluorine, chlorine, and / or hydroxyl groups are preferably used as dopants, which reduce the viscosity of the silica glass. Doping can also be used to reduce the thermal stability of the component part, thus favoring the stability of the adjacent component part. For example, it has proven advantageous during the addition that when, at a measurement temperature of 1250 °C, the silica glass of the cladding tube has a viscosity at least 0.5 dPa·s higher, preferably at least 0.6 dPa·s higher (when the viscosity is specified as the logarithm expressed in dPa·s), than the silica glass of the additionally applied cladding material.

[0067] In method step (c), the cladding tube is arranged inside the overtube such that the outer surface of the cladding tube faces the inner surface of the overtube. For this purpose, the inner diameter of the overtube is larger than the outer diameter of the cladding tube. In one embodiment, the inner diameter of the overtube is 1 mm to 15 mm larger than the outer diameter of the cladding tube. This allows for an easily performable arrangement of the cladding tube inside the overtube and, during addition, by means of the heat input in method step (d), a material-to-material bonding between the inner surface of the overtube and the outer surface of the cladding tube can be achieved simultaneously. If the diameter difference is larger than 15 mm, a material-to-material bonding of the overtube and the cladding tube will only be possible under an uncontrolled plastic deformation of the preform, which will adversely affect the photoconductivity of the final antiresonant hollow core fiber.

[0068] The purpose of the addition in method step (d) is a material-to-material bonding between the inner surface of the overtube and the outer surface of the cladding tube by means of a heat input. The heat input must be carried out in such a way that a material-to-material bonding between the materials of the two elements is possible. This effect can be achieved where the inner surface of the overtube and the outer surface of the cladding tube at least partially change from the solid aggregate state to the liquid (especially viscous) aggregate state.

[0069] The heat input can be achieved in different ways, for example by means of:

[0070] - Flame-based processes: based on the oxidation of an exothermic reaction gas. An example is the use of hydrogen (also referred to as "H2") as the combustion gas (flame hydrolysis). It reacts with the oxygen in the air (also referred to as "O2"); or

[0071] - Flame-free processes: using other systems that are heated and do not require an open flame. An example is the use of a resistor that can convert electrical energy into heat (thermal energy).

[0072] To provide the addition, at least as part of method step (d), a first negative pressure P1 is applied between the inner surface of the overtube and the outer surface of the cladding tube. The first negative pressure P1 contributes to better reproducibility because it generates an additional inward-acting force during the addition, such that random fluctuations in other process parameters that could lead to an uncontrolled addition process are compensated for. The gap resulting from the difference between the inner diameter of the overtube and the outer diameter of the cladding tube can thus be closed in a reproducible manner without striations and / or bubbles in the preform.

[0073] The heat input required during the addition in method step (d) not only causes at least partial softening of the inner surface of the covering tube and the outer surface of the cladding tube, but also the antiresonator element preform at least partially changes from a solid aggregate state to a liquid (in particular viscous) aggregate state. Due to the softening, there is furthermore a risk of uncontrolled and undesired structural deformations of the individual components, which adversely affects the quality of the final antiresonator hollow fiber. Lower structural dimensions compared to the covering tube and the associated smaller thermal mass can in particular lead to uncontrolled structural deformations of the cladding tube and at least one antiresonator element preform.

[0074] During the manufacture of the preform of the antiresonator hollow fiber it becomes apparent that a negative pressure between the inner surface of the covering tube and the outer surface of the cladding tube at least partially causes uncontrolled expansion of the cladding tube. In order to prevent this and thus ensure precise and reproducible shaping of the preform of the antiresonator hollow fiber, the invention discloses that at least during the addition in method step (d), a first negative pressure P1 is applied between the inner surface of the covering tube and the outer surface of the cladding tube, and a second negative pressure P2 is applied inside the cladding tube.

[0075] This concept allows for a reproducible and precise manufacturing method for the preform of the antiresonator hollow fiber on an industrial scale.

[0076] According to the invention, a negative pressure is understood as a relative pressure below the ambient pressure surrounding the preform. For example, a negative pressure of -50 mbar corresponds to a negative pressure difference of 50 mbar compared to the ambient pressure, which would correspond to a pressure of 950 mbar, for example at an ambient pressure of 1000 mbar. When applying a complete vacuum, the maximum possible negative pressure is reached, and it will correspond to the ambient pressure.

[0077] During the addition in method step (d), the first negative pressure P1 and the second negative pressure P2 can assume values different from the ambient pressure.

[0078] One embodiment of the method is characterized in that, compared to the ambient pressure, the first negative pressure P1 is in the range of -150 mbar to -5 mbar, preferably in the range of -120 mbar to -5 mbar, more preferably in the range of -100 mbar to -10 mbar, and the second negative pressure P2 is in the range of -100 mbar to -1 mbar, preferably in the range of -70 mbar to -1 mbar, more preferably in the range of -50 mbar to -1 mbar. In this way, the addition in method step (d) takes place in a reproducible and precise manner with only slight and controllable deformation of the components of the preform, in particular the cladding tube and the anti-resonator element preform. If the first negative pressure P1 has a magnitude greater than 150, unwanted deformation of the components of the preform, in particular the cladding tube and at least one anti-resonator element preform, occurs. In contrast, if the magnitude of the first negative pressure P1 is less than 5, no reproducible and precise material-to-material bonding of the covering tube and the cladding tube takes place. If the second negative pressure P2 has a magnitude higher than 100, an uncontrolled collapse of parts of the cladding tube occurs, which results in a negative impact on the quality of the final anti-resonant hollow fiber. In contrast, if the second negative pressure P2 has a magnitude less than 1, or if even a positive pressure is applied, an uncontrolled deformation (in particular expansion) of the cladding tube and the anti-resonator element preform occurs, which also results in a negative impact on the quality of the final anti-resonant hollow fiber.

[0079] One embodiment of the method is characterized in that the first negative pressure P1 has a greater magnitude than the second negative pressure P2. Thus, the pressure applied between the inner surface of the covering tube and the outer surface of the cladding tube is smaller than the pressure applied inside the cladding tube. This allows material-to-material bonding of the inner surface of the covering tube and the outer surface of the cladding tube without uncontrolled deformation (in particular expansion) of the cladding tube and / or the anti-resonator preform.

[0080] It has proven advantageous that there is a pressure difference in the range of 3 mbar to 60 mbar, preferably between 5 mbar and 40 mbar, more preferably between 10 mbar and 35 mbar between the first negative pressure P1 and the second negative pressure P2.

[0081] The method can be designed in such a way that at least during the addition in method step (d), the second negative pressure P2 is applied throughout the cladding tube and thus also inside the anti-resonator element preform.

[0082] One embodiment of the method is characterized in that at least during the addition in method step (d), a third negative pressure P³ is applied inside at least one anti-resonator element preform. This allows for a finer adjustment of the different pressure ratios according to the selected material and / or wall thickness of the individual components of the preform and thus allows for an even more precise manufacturing method.

[0083] Thus, the second negative pressure P2 and the third negative pressure P3 can have different amounts as well as the same amount.

[0084] One embodiment of the method is characterized in that the third negative pressure P3 has an amount different from that of the first negative pressure P1 and the second negative pressure P2. In particular, the third negative pressure P3 has an amount smaller than that of the second negative pressure P2, and preferably also has an amount smaller than that of the first negative pressure P1. In one embodiment, compared with the ambient pressure, the third negative pressure P3 is in the range of -100 mbar to -1 mbar, preferably in the range of -70 mbar to -1 mbar, more preferably in the range of -50 mbar to -1 mbar, and even more preferably in the range of -10 mbar to -1 mbar. If the third negative pressure P3 has an amount smaller than that of the second negative pressure P2, that is, if the pressure acting in the anti-resonant element preform is higher than the pressure acting in the surrounding cladding tube, the risk of the anti-resonant element preform being damaged by collapse is reduced. On the contrary, slight expansion of the anti-resonant element preform may occur, which has less negative impact on the quality of the final anti-resonant hollow-core optical fiber compared with collapse.

[0085] The different negative pressures P1, P2 and optionally P3 can be applied in various ways to manufacture the preform. One embodiment of the method is designed in such a way that at least the first negative pressure P1 and the second negative pressure P2 are applied at the same end of the preform to be manufactured.

[0086] In order to apply the different negative pressures P1, P2 and optionally P3 for manufacturing the preform, the components of the preform can be arranged and / or connected to each other in different ways.

[0087] One embodiment of the method is characterized in that, before method step (d), the cladding tube is closed in an airtight manner at the first cladding tube end, and in some regions, the inner surface of the covering tube is connected to the outer surface of the cladding tube in an airtight manner at the second cladding tube end, which is axially opposite to the first cladding tube end. An airtight closure of the first cladding tube end can be obtained, for example, by heat-fusing together by forming a tip (especially a tip of fused silica glass), or by means of an airtight closure attached to the first cladding tube end. In a preferred embodiment of the present invention, when viewed from the earth's surface, the first cladding tube end (especially the tip) is arranged at the bottom of the preform to be manufactured.

[0088] For example, an airtight connection covering the inner surface of the covering tube and the outer surface of the cladding tube can be obtained via local heat input, which, for example, establishes a material-to-material bond of the radial circumference of the inner surface of the covering tube and the outer surface of the cladding tube via an axial expansion of 20 mm to 60 mm. It may be advantageous to apply a negative pressure between the inner surface of the covering tube and the outer surface of the cladding tube in order to form an airtight connection.

[0089] The application of the corresponding negative pressure can be carried out at the same spatial end of the preform to be manufactured.

[0090] One embodiment of the method is characterized in that a first negative pressure P1 is applied in the region of the end of the first cladding tube and a second negative pressure P2 is applied in the region of the end of the second cladding tube. Due to spatial reasons, this provides a simplified process.

[0091] The arrangement of the cladding tube within the covering tube can be achieved in different ways.

[0092] One embodiment of the method is characterized in that the cladding tube has a retaining ball at the second end in order to arrange the cladding tube within the covering tube in method step (c). The retaining ball should be understood as a part or attachment of the cladding tube, in particular of spherical design, the diameter of which (specifically the outer diameter) is increased compared to the rest of the cladding tube and which is designed to be larger than the diameter of the covering tube (specifically the inner diameter). This allows the cladding tube (first the first end) to be inserted into the covering tube until the retaining ball is reached, which cannot be introduced into the covering tube due to its larger diameter. Since, as part of the method, the individual components of the preform are perpendicular to the earth's surface and, in particular, when viewed from the earth's surface, the fact that the second end of the cladding tube is thus vertically arranged above the first end of the cladding tube, the retaining ball allows the cladding tube to be hooked into the covering tube in an uncomplicated and safe manner. In one embodiment, the cladding tube is deformed at the second end by forming a retaining ball, specifically by means of heat input and pressurized air. In another preferred embodiment, the retaining ball is connected to the second end of the cladding tube as a separate component, for example by means of a material-to-material bond, before the cladding tube is arranged within the covering tube. It is thus preferred that the retaining ball as well as the cladding tube are formed from fused silica with the same or a similar composition in order not to generate any thermally induced tensions during the input. The interior of the cladding tube is connected in a gas-conducting manner via a fluid-conducting opening in the retaining ball to the surrounding area of the cladding tube. In order to provide gas exchange around the retaining ball, the retaining ball can have axially extending grooves, notches or other types of recesses.

[0093] The application of the corresponding negative pressure to the preform to be manufactured can be achieved in various ways.

[0094] One embodiment of the method is characterized in that a first negative pressure P1 is applied by means of a first holding duct, which is attached to a first covering duct end facing a first cladding duct end, and a second negative pressure P2 is applied by means of a second holding duct, which is attached to a second covering duct end facing a second cladding duct end.

[0095] The holding duct is to be understood as a component made specifically of fused silica or metal and shaped in the form of a hood, which includes a holding duct opening and a holding duct edge surrounding the holding duct opening. The holding duct edge is designed to be attached to the axial covering duct and connected to the axial covering duct in a gas-tight manner (e.g., by means of heat input) over the entire circumference of the holding duct edge. This establishes the mechanical fastening of the holding duct to the covering duct. In addition, the attached holding duct closes the covering duct end, such that no gas exchange occurs between the holding duct edge and the covering duct end with the surrounding area of the preform to be manufactured.

[0096] The first holding duct has a first gas connection, which is connected or can be connected in a fluid-conducting manner to the first holding duct opening. At least during the addition in method step (d), the first holding duct (in particular the first holding duct edge) is connected to the first covering duct end, such that the first gas connection is connected or can be connected in a fluid-conducting manner via the first holding duct opening and the first covering duct end to the gap between the inner surface of the covering duct and the outer surface of the cladding duct. If the inner surface of the covering duct is connected to the outer surface of the cladding duct in a gas-tight manner at the second end, the first negative pressure P1 can be applied via the first gas connection. A positive pressure can also be applied via the first gas connection, or the first gas connection can be closed in a fluid-conducting manner.

[0097] The second holding duct has a second gas connection, which is connected or can be connected in a fluid-conducting manner to the second holding duct opening. At least during the addition in method step (d), the second holding duct (in particular the second holding duct edge) is connected to the second covering duct end, such that the second gas connection is also connected or can be connected in a fluid-conducting manner via the second holding duct opening and, if there is a holding ball, also via the holding ball to the second cladding duct end. If the first cladding duct end is closed in a gas-tight manner, the second negative pressure P2 can be applied via the second gas connection. A positive pressure can also be applied via the second gas connection, or the second gas connection can be closed in a fluid-conducting manner.

[0098] One embodiment of the method is characterized in that the first cladding duct end (in particular the first cladding duct end shaped as a tip) is arranged in a perforated plate.

[0099] The perforated disk has at least one axially extending hole which is designed to receive the end of the first cladding tube (in particular the end of the first cladding tube formed as a tip) in such a way that the cladding tube (in particular the end of the first cladding tube) is fixed within the covering tube. In one embodiment, the perforated disk is formed as part of the first holding duct, in particular as part of the edge of the first holding duct. In order to effect gas exchange between the first gas connection and the gap between the inner surface of the covering tube and the outer surface of the cladding tube, the perforated disk has at least one axial through-passage extending through the perforated disk.

[0100] The perforated disk can be formed from different materials, where fused silica is preferred because it avoids thermal stresses.

[0101] During the method, different negative pressures can be applied simultaneously or in a different order.

[0102] One embodiment of the method is characterized in that a first negative pressure P1 is applied first and then a second negative pressure P2 is applied. The first negative pressure P1 applied is in particular conducive to an airtight connection between the inner surface of the covering tube and the outer surface of the cladding tube and a part of the end of the second cladding tube.

[0103] One embodiment of the method is characterized in that during the addition in method step (d), the preform is elongated.

[0104] During the elongation, the preform is extended. The elongation can be carried out proportionally such that for example the shape and arrangement of the components or parts of the components of the preform are reflected in the final elongated product. However, during the elongation, the primary preform can also be drawn non-proportionally and its geometry can be changed.

[0105] After the addition and optional elongation in method step (d) have ended, the preparation of the preform can be ended. In a further embodiment, a secondary preform is manufactured from the preform by means of further processing, where the further processing comprises carrying out one or several of the following thermoforming processes once or repeatedly:

[0106] i. Elongation,

[0107] ii. Collapse,

[0108] iii. Collapse and simultaneous elongation,

[0109] iv. Addition of additional cladding material,

[0110] v. Addition of additional cladding material and subsequent elongation,

[0111] vi. Addition of additional cladding material and simultaneous elongation.

[0112] During collapse, the inner bore narrows, or the gap between the tubular components closes or narrows. Collapse is typically associated with elongation. A secondary preform fabricated in this manner may have been designed and adapted for drawing a hollow core optical fiber. Optionally, the secondary preform may be further processed, where for example it is elongated or additional cladding material is added to it.

[0113] The invention will be further illustrated below in an exemplary manner. The invention is not limited to the examples.

[0114] Figure 1Shows a longitudinal section of an intermediate product 100' of a method for manufacturing a preform for an anti-resonant hollow-core optical fiber comprising a cladding tube 200 and a covering tube 300. The cladding tube 200 has a cladding tube outer surface 210 and a cladding tube inner surface, where a plurality of anti-resonant element preforms 400 are arranged in an axially aligned manner at the cladding tube inner surface and are connected to the cladding tube inner surface by means of a material-to-material bond. The cladding tube 200 is arranged within the covering tube 300 such that the cladding tube outer surface 210 is surrounded in a sleeve-like manner by the covering tube inner surface 310. The diameter of the cladding tube outer surface 210 is smaller than the diameter of the covering tube inner surface 310, such that an annular gap 450 is formed between the cladding tube 200 and the covering tube 300. The gap 450 allows the cladding tube 200 to be introduced into the covering tube 300 without contact and thus without damage. To arrange the cladding tube 200 within the covering tube 300, a second cladding tube end 235 is formed into a holding ball 240. In the shown embodiment, the cladding tube 200 and the holding ball 240 are formed as a single piece and made of the same material. In a further, not shown embodiment, the cladding tube 200 and the holding ball 240 are formed of different materials, for example, the holding ball 240 is made of glass, in particular of fused silica. The holding ball 240 has a holding ball outer diameter 245, which is larger than the inner diameter of the covering tube 300. In the case of the arrangement of the shown components (which is advantageous for the method and is designed to be perpendicular to the Earth's surface, where the holding ball is attached or formed to the cladding tube 200 at the top with respect to the Earth's surface), this allows the cladding tube 200 to be hooked into the covering tube 300. At a first holding ball end 241 facing the cladding tube 200, and at a second holding ball end 242 axially opposite the first holding ball end 241, the holding ball 240 is equipped in a fluid-conducting manner such that the interior of the cladding tube 200 is fluid-conductively connected via the holding ball 240 to the surrounding area of the cladding tube 200. To provide gas exchange around the holding ball 240 (and thus between the contact points of the holding ball 240 and the covering tube 300), the holding ball 240 may specifically have a holding ball outer surface, grooves, notches or other types of axially extending recesses (not shown). A first cladding tube end 230 facing the Earth's surface is formed into a tip, which fluid-conductively closes the first cladding tube end 230. In the shown embodiment, gas exchange between the interior of the cladding tube 200 and the surrounding area of the cladding tube 200 can only take place via the holding ball 240 at the second cladding tube end 235.

[0115] The first end 330 of the first covering tube facing the end 230 of the first cladding tube is axially connected to a first holding duct 500 designed in the form of a hood, in particular to a first holding duct edge 510 of the first holding duct 500. On the side facing the end 230 of the first cladding tube, the first holding duct 500 is shaped as a perforated disc 530, and the end 230 of the first cladding tube shaped as a tip is received in the perforated disc and is thus fixed against lateral movement. This prevents the covering tube 300 and / or the cladding tube 200 from being partially or completely damaged by an uncontrolled relative movement of these two components of the first intermediate product 100' relative to each other. In addition, the axial alignment of the cladding tube 200 within the covering tube 300 is facilitated by means of the perforated disc 530. In the illustrated embodiment, the first holding duct 500 and the perforated disc 530 are shaped as a single piece. In a further embodiment, the perforated disc 530 is shaped as a separate component, preferably made of quartz glass.

[0116] The first holding duct 500 has a first holding duct internal space 520 which is fluid-conductively connected via an axially extending through-passage 535 in the perforated disc 530 to the gap 450 between the inner surface 310 of the covering tube and the outer surface 210 of the cladding tube. At the end opposite the covering tube 300, the first holding duct 500 has a first gas connection 540. The first gas connection 540 can be reversibly opened in a fluid-conductive manner and can be closed in a fluid-conductive manner. Optionally, the first gas connection 540 can be connected to the surrounding area of the first intermediate product 100', a reservoir for a gas (such as nitrogen or argon) and a negative pressure source for applying a negative pressure, or the first gas connection 540 can be closed in an airtight manner

[0117] The second end 340 of the second covering tube facing the end 235 of the second cladding tube is axially connected to a second holding duct 550 designed in the form of a hood.

[0118] The second holding duct 550 has a second holding duct internal space 570 which is fluid-conductively connected via a holding ball 240 to the interior of the cladding tube 200. At the end opposite the covering tube 300, the second holding duct 550 has a second gas connection 580. The second gas connection 580 can be reversibly opened in a fluid-conductive manner and can be closed in a fluid-conductive manner. Optionally, the second gas connection 580 can be connected to the surrounding area of the first intermediate product 100', a reservoir for a gas (such as nitrogen or argon) and a negative pressure source for applying a negative pressure, or the second gas connection 580 can be closed in an airtight manner.

[0119] In the illustrated stage of the method for manufacturing a preform for an anti-resonant hollow-core optical fiber, the first gas connection 540 and the second gas connection 580 are opened in a fluid-conducting manner, wherein a gas (e.g., nitrogen or argon) is introduced through the first gas connection 540. The gas flows through the first holding pipe 500 and the first intermediate product 100' and is discharged again through the second gas connection 580, thereby removing as completely as possible the ambient air previously present in the first intermediate product 100', and preparing for the addition of the cladding tube 300 to the jacket tube 200, which is carried out as part of the method.

[0120] Figure 2 A longitudinal section through a second intermediate product 100'' of the method for manufacturing a preform for an anti-resonant hollow-core optical fiber is shown, which second intermediate product is manufactured from the Figure 1 first intermediate product 100'. The second intermediate product 100'' differs from the first intermediate product 100'' from Figure 1 in that the cladding tube 300 (in particular the outer surface 210 of the jacket tube) is connected in a gas-tight manner to the cladding tube 300 (in particular the inner surface 310 of the cladding tube) via an annular connection point 600 in the region of the second jacket tube end 235.

[0121] To connect the cladding tube 300 to the jacket tube 200, the second gas connection 580 is closed in a fluid-conducting manner, and the first gas connection 540 is connected in a fluid-conducting manner to a negative pressure source (e.g., a vacuum pump). In the region of the connection point 600 to be manufactured, a material-to-material bond between the inner surface 310 of the cladding tube and the outer surface 210 of the jacket tube takes place via the combination of the negative pressure generated in this way in the gap 450 and a locally restricted heat supply 650 applied in an annular manner around the cladding tube 300. For example, the material is heated in the region of the connection point 600 by means of an electric furnace (not shown) at an operating temperature in the range from 1950 °C to 2150 °C in order to provide a material-to-material bond between the inner surface 310 of the cladding tube and the outer surface 210 of the jacket tube.

[0122] The connection point 600 divides the gap 450 between the inner surface 310 of the cladding tube and the outer surface 210 of the jacket tube into a first gap portion 450' facing the first holding pipe 500 and being connected to it in a fluid-conducting manner and a second gap portion 450'' facing the second holding pipe and being connected to it in a fluid-conducting manner.

[0123] Figure 3 Shown is from Figure 2"second intermediate product 100". The connection point 600 has divided the second intermediate product 100" into two separate pressure regions from each other. The first pressure region includes a first holding pipe, in particular the inner space 520 of the first holding pipe and the first gap portion 450'. The second pressure region includes a second holding pipe 550, in particular the inner space 570 of the second holding pipe, the holding ball 240, and the interior of the cladding tube 200, together with the anti-resonator element preform 400. The first gas connection portion 540 and the second gas connection portion 580 are opened in a fluid-conducting manner and are each connected to a separate negative pressure source, for example, each connected to a separate vacuum pump. This provides for applying a first negative pressure P1 in the first pressure region and, independently thereof, applying a second negative pressure P2 in the second pressure region, and thus provides for adding the covering tube 300 to the cladding tube 200 while highly maintaining the accuracy of the cladding tube 200 and the anti-resonator element preform 400 in a reproducible manner (as part of another method).

[0124] Figure 4 An embodiment of a method 700 for manufacturing a preform for an anti-resonant hollow optical fiber is shown, which method has at least the following method steps:

[0125] (a) Step 710: Provide a cladding tube 200 including an inner surface of the cladding tube and an outer surface 210 of the cladding tube, wherein at least one anti-resonator element preform 400 is arranged at the inner surface of the cladding tube,

[0126] (b) Step 720: Provide a covering tube 300 including an inner surface 310 of the covering tube, wherein the covering tube 300

[0127] has an inner diameter greater than the outer diameter of the cladding tube 200,

[0128] (c) Step 730: Arrange the cladding tube 200 inside the covering tube 300 such that the inner surface 310 of the covering tube

[0129] surrounds the outer surface 210 of the cladding tube,

[0130] (d) Step 74 : Add the covering tube 300 to the cladding tube 200 such that the inner surface 310 of the covering tube is connected to the outer surface 210 of the cladding tube.

[0131] It is hereby stipulated that during the addition in method step (d), a first negative pressure P1 is applied between the inner surface 310 of the covering tube and the outer surface 210 of the cladding tube, and a second negative pressure P2 is applied inside the cladding tube 200 in order to provide an attachment while highly maintaining the accuracy of the structural elements (in particular the cladding tube 200 and the anti-resonator element preform 400) in a reproducible manner.

[0132] The features disclosed in the description may be important for the various designs of the claimed invention (individually and in any combination with each other). The features disclosed for the preforms, anti-resonant hollow fibers or intermediate products of the method are also disclosed for the method, and vice versa.

[0133] Reference numerals

[0134] 100' First intermediate product of the method for manufacturing a preform for an anti-resonant hollow fiber

[0135] 100” Second intermediate product of the method for manufacturing a preform for an anti-resonant hollow fiber

[0136] 200 Cladding tube

[0137] 210 Outer surface of the cladding tube

[0138] 230 First end of the cladding tube

[0139] 235 Second end of the cladding tube

[0140] 240 Holding ball

[0141] 241 First end of the holding ball

[0142] 242 Second end of the holding ball

[0143] 245 Outer diameter of the holding ball

[0144] 300 Cover tube

[0145] 310 Inner surface of the cover tube

[0146] 330 First end of the cover tube

[0147] 340 Second end of the cover tube

[0148] 400 Anti-resonant element preform

[0149] 450 Gap

[0150] 450' First gap portion

[0151] 450” Second gap portion

[0152] 500 First holding duct

[0153] 510 Edge of the first holding duct

[0154] 520 Inner space of the first holding duct

[0155] 530 Perforated disk

[0156] 535 Through-hole in the perforated disk

[0157] 540 First gas connection part

[0158] 550 Second holding pipe

[0159] 570 Inner space of the second holding pipe

[0160] 580 Second gas connection part

[0161] 600 Connection point

[0162] 650 Heat supply

[0163] 700 Method for manufacturing a preform for an anti-resonant hollow fiber

[0164] 710 Provide a cladding tube

[0165] 720 Provide a covering tube

[0166] 730 Arrange the cladding tube inside the covering tube

[0167] 740 Add the covering tube to the cladding tube

[0168] P1 First negative pressure

[0169] P2 Second negative pressure

Claims

1. A method for manufacturing a preform for an anti-resonant hollow fiber, the anti-resonant hollow fiber having a hollow core extending along the longitudinal axis of the fiber and a cladding region surrounding the hollow core and including at least one anti-resonant element, the method having at least the following method steps: (a) providing a cladding tube (200) including an inner surface of the cladding tube and an outer surface (210) of the cladding tube, wherein at least one anti-resonant element preform (400) is arranged at the inner surface of the cladding tube, (b) providing a covering tube (300) including an inner surface (310) of the covering tube, wherein the inner diameter of the covering tube (300) is larger than the outer diameter of the cladding tube (200), (c) arranging the cladding tube (200) inside the covering tube (300) such that the inner surface (310) of the covering tube surrounds the outer surface (210) of the cladding tube, (d) adding the covering tube (300) to the cladding tube (200) such that the inner surface (310) of the covering tube is connected to the outer surface (210) of the cladding tube, characterized in that, during the addition in method step (d), a first negative pressure P1 is applied between the inner surface (310) of the covering tube and the outer surface (210) of the cladding tube, and a second negative pressure P2 is applied inside the cladding tube (200).

2. The method according to claim 1, wherein Compared with the ambient pressure, the first negative pressure P1 is in the range of -100 mbar to -10 mbar, and the second negative pressure P2 is in the range of -50 mbar to -1 mbar.

3. The method according to claim 1 or 2, characterized in that, The first negative pressure P1 has a greater magnitude than the second negative pressure P2.

4. The method according to claim 1 or 2, characterized in that, during the addition in method step (d), a third negative pressure P3 is applied inside the at least one anti-resonant element preform (400).

5. The method according to claim 4, wherein The third negative pressure P3 has the same magnitude as the second negative pressure P2.

6. The method according to claim 4, wherein The third negative pressure P3 has a magnitude different from that of the first negative pressure P1 and the second negative pressure P2.

7. The method according to claim 1 or 2, characterized in that, Before method step (d), the cladding tube (200) is closed in an airtight manner at a first cladding tube end (230), and in some regions, the inner surface (310) of the covering tube is connected to the outer surface (210) of the cladding tube in an airtight manner at a second cladding tube end (235).

8. The method according to claim 7, wherein The cladding tube (200) has a retaining ball (240) at the second cladding tube end (235) for arranging the cladding tube (200) inside the covering tube (300) in method step (c).

9. The method according to claim 7, wherein The cladding tube (200) is closed in an airtight manner at the first cladding tube end (230) by forming a tip.

10. The method according to claim 7, wherein The first negative pressure P1 is applied in the region of the first cladding tube end (230), and the second negative pressure P2 is applied in the region of the second cladding tube end (235).

11. The method according to claim 10, wherein The first negative pressure P1 is applied by means of a first holding pipe (500) which is attached to a first covering pipe end (330) facing the first cladding pipe end (230), and the second negative pressure P2 is applied by means of a second holding pipe (550) which is attached to a second covering pipe end (340) facing the second cladding pipe end (235).

12. The method according to claim 9, wherein Before method step (d), the tip is inserted into the perforated disk (530).

13. The method according to claim 1 or 2, characterized in that, The first negative pressure P1 is applied first, and then the second negative pressure P2 is applied.

14. The method according to claim 1 or 2, characterized in that, During the addition in method step (d), the preform is elongated.

15. The method according to claim 4, wherein The third negative pressure P3 has an amount smaller than that of the second negative pressure P2.

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