Hollow-core optical fiber drawing method using improved preform

Abstract

A method of manufacturing a hollow fiber includes providing a fine preform formed of glass and having a cross-sectional structure configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow fiber drawn from the fine preform, the width of the fine prefabricated body is in a range of 0.5-5mm, and the total length of the fine prefabricated body can be drawn into a hollow-core optical fiber; heating the end part of the fine preform so as to soften the glass at the end part; and drawing a length of a hollow fiber from the softened glass of the fine preform, the width of the hollow fiber being less than the width of the fine preform.

Description

Background Art

[0001] The present invention relates to a method for drawing a hollow core optical fibre, including improvements compared to conventional methods.

[0002] Conventional optical fibers have a solid core design that includes a solid glass core surrounded by a solid glass cladding having a lower refractive index than the core. Light is guided along the core by total internal reflection at the core-cladding boundary. Solid core optical fibers are manufactured by a fiber drawing process in which a short rigid glass preform is axially lowered into an annular furnace that heats and softens the glass, wherein the cross-sectional structure of the preform matches the desired refractive index profile of the finished optical fiber, but the width of the preform is many times that of the finished optical fiber. The softened glass is then stretched or drawn from one end of the preform, either under the action of gravity or under tensioning, to the reduced width of the finished optical fiber, which is wound on a spool. In some processes, a preform of relatively narrow width and several meters in length is first drawn from the preform, and then the preform is separated from the preform for later drawing into optical fiber. The process of narrowing the width of a glass structure from a preform to a blank or optical fiber and from a preform or blank to an optical fiber is called drawing. Fiber drawing can be described by a parameter called the drawdown ratio, where the drawdown ratio is the ratio of the width or cross-sectional area of ​​the preform to the width or cross-sectional area of ​​the finished optical fiber. Another related parameter is called the fiber yield ratio, where the fiber yield ratio is the number of meters of optical fiber that can be drawn per meter of preform. Ideally, this is of great value for obtaining long fiber lengths and improving fiber manufacturing efficiency. Commercially, the total continuous length of optical fiber that can be produced in a single draw has increased from tens of kilometers to lengths of the order of 10,000 km in a few decades. This increase is largely achieved by increasing the size of the preform so that a larger initial volume of glass is available for conversion into optical fiber. The cross-sectional area of ​​the preform has increased by about 30 times, resulting in a corresponding increase in the width of the preform with a moderate increase in the length of the preform to about 3 meters. As a result, both the draw ratio and the fiber yield ratio have been significantly increased, which is achieved by greatly increasing the preform width while the preform length remains relatively short.

[0003] In recent years, hollow core optical fibers have been developed. Hollow core optical fibers have a central longitudinal hole or void defining a hollow core surrounded by a cladding, wherein the cladding typically includes a plurality of smaller longitudinal holes or voids of a specified geometry, which may be referred to as a microstructure. Light is guided along the hollow core by one of several mechanisms depending on the geometry of the microstructured cladding. The appeal of hollow core optical fibers is due to superior optical propagation properties compared to solid core optical fibers, including reduced optical propagation losses (attenuation), increased propagation velocity, greater optical bandwidth, and reduced parasitic nonlinear optical effects, which are derived from the large fraction of air and the corresponding reduction in the amount of glass in the fiber. Light propagates primarily in air, thereby avoiding the deleterious effects caused by propagation in glass. Hollow core optical fibers can also be made by optical fiber drawing methods starting from a preform (optionally drawn into a preform) formed with a void of the desired cross-sectional profile. In order to achieve the desired geometry of the core and cladding in the finished optical fiber (precision is required for positioning, void size and glass film thickness between voids) in order to obtain the expected optical properties of a particular optical fiber design, pressure is applied to the voids during the process of drawing the optical fiber from the preform or billet. The pressurization counteracts the surface tension in the softened glass, which would otherwise tend to cause the voids to collapse and destroy the intended structure.

[0004] Fiber yield and draw ratio are also relevant to the drawing of hollow-core fibers. Compared to solid-core fibers, an overall larger hollow-core preform contains a larger volume of glass and is therefore expected to produce more fiber. For example, the preform width can be increased [1], which requires a higher draw ratio to produce a fiber of the same width. However, for hollow-core fibers, the dynamics of pressurization are more complex and difficult to control when drawing a wide preform into a narrow fiber via a high draw ratio because the outer diameter and diameter of the holes vary more extreme. Accordingly, high draw ratios are typically unsuitable or even impossible to achieve for hollow-core fiber manufacturing. Therefore, for successful hollow-core fiber manufacturing, narrower preforms and correspondingly lower draw ratios are generally required, resulting in lower yields and correspondingly inefficient fiber production. However, pressurization can be more difficult to implement with narrower preforms because the voids are smaller and more difficult to access; this can reduce the structural quality of the finished fiber. Thus, there are many factors that impede the efficient, high-volume manufacturing of high-quality hollow-core fibers. The single span hollow core fibers for which the inventors are aware of reported reasonable values ​​of propagation loss are relatively short, at 12 km (4-5 dB / km loss) [2] and 14 km (10 dB / km loss) [3], with shorter lengths of 10 km or less being more common with current hollow core fiber drawing methods. As a result, the cost of hollow core fibers is high compared to solid core fibers, and the maximum achievable length of the fiber is greatly limited. These factors are particularly detrimental in light of the excellent optical properties of hollow core fibers, which make them extremely attractive for many applications, including long-haul telecommunications, where very long lengths of inexpensive fiber are desirable. Consequently, there are a number of difficulties in hollow core fiber production that tend to limit achievable fiber lengths, reduce efficiency, and increase complexity and cost.

[0005] Accordingly, methods to improve hollow-core optical fiber fabrication have attracted great interest. Summary of the invention

[0006] Various aspects and embodiments are set out with particularity in the appended claims.

[0007] According to a first aspect of certain embodiments described herein, there is provided a method for manufacturing a hollow-core optical fiber, the method comprising: providing a thin preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the thin preform being configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in a range of 0.5 mm to 5 mm and a total length that can be used to draw the hollow-core optical fiber; heating an end of the thin preform to soften the glass at the end; and drawing a certain length of hollow-core optical fiber from the softened glass of the thin preform, the width of the hollow-core optical fiber being smaller than the width of the thin preform.

[0008] According to a second aspect of certain embodiments described herein, there is provided a method for manufacturing a preform for a hollow-core optical fiber, the method comprising: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the initial preform being configured to form, in a thin preform drawn from the initial preform, a cross-sectional structure capable of being drawn from the thin preform into a hollow-core optical fiber, the cross-sectional structure of the hollow-core optical fiber comprising a hollow core surrounded by a plurality of voids defining a microstructured cladding, the initial preform having an initial preform width; applying an increased pressure to the voids in the initial preform; heating an end of the initial preform so as to soften the glass at the end of the initial preform; and drawing a thin preform from the softened glass of the initial preform while applying the increased pressure, the width of the thin preform being smaller than the width of the initial preform and being in the range of 0.5 mm to 5 mm.

[0009] According to a third aspect of certain embodiments described herein, there is provided a thin preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the thin preform being configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in the range of 0.5 mm to 5 mm and a length of at least 30 m.

[0010] According to a fourth aspect of some embodiments described herein, a hollow core optical fiber is provided, wherein the cross-sectional structure of the hollow core optical fiber comprises a hollow core surrounded by a plurality of voids defining a microstructured cladding, and the length of the hollow core optical fiber is at least 20 km.

[0011] These and other aspects of certain embodiments are set out in detail in the accompanying independent and dependent claims. It will be understood that the features of the dependent claims may be combined with each other, and that the features of the independent claims may be combined with the features explicitly set out in detail in the claims. Further, the methods described herein are not limited to specific embodiments such as those set out in detail below, but include and contemplate any appropriate combination of the features presented herein. For example, methods and apparatus may be provided according to the methods described herein that appropriately include any one or more of the various features described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

[0013] Figure 1 shows a cross-sectional view of a first example hollow core optical fiber that can be fabricated according to an example method of the present disclosure;

[0014] Figure 2 shows a cross-sectional view of a second example hollow core optical fiber that can be fabricated according to an example method of the present disclosure;

[0015] Figure 3 shows a cross-sectional view of a third example hollow core optical fiber that may be fabricated according to an example method of the present disclosure;

[0016] Figure 4 A flow chart illustrating steps in an example hollow core optical fiber drawing method according to one aspect of the present disclosure;

[0017] Figure 5 A flow chart showing steps in an example thin preform drawing method according to one aspect of the present disclosure;

[0018] Fig. 6A and Figure 6B A schematic diagram showing an example apparatus suitable for carrying out an example method of manufacturing a hollow core optical fiber according to the following disclosure, wherein a thin preform is provided according to a first alternative;

[0019] Figure 7 A schematic diagram showing an example apparatus suitable for carrying out an example method of manufacturing a hollow core optical fiber according to the following disclosure, wherein a thin preform is provided according to a second alternative; and

[0020] Figure 8 A schematic diagram of an example apparatus suitable for performing an example method of manufacturing a hollow core optical fiber according to the following disclosure is shown, wherein a thin preform is provided according to a third alternative. DETAILED DESCRIPTION

[0021] Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be conventionally implemented, and for the sake of brevity, these are not discussed / described in detail. Therefore, it will be understood that aspects and features of the apparatus and methods discussed herein that are not described in detail may be implemented according to any conventional techniques for implementing these aspects and features.

[0022] The present disclosure proposes a method for manufacturing hollow core optical fiber, aiming to solve some problems of the known methods, with a view to improving output and increasing optical fiber yield.

[0023] A hollow core optical fiber has a core in which light is guided, wherein the core comprises a central longitudinal hole or void (usually filled with air, but alternatively filled with another gas or gas mixture, or a vacuum) surrounded by a cladding comprising a structured arrangement of longitudinal holes, voids, or capillaries (microstructures) extending along the length of the fiber. The absence of a solid glass core reduces the proportion of guided light waves that propagate in the glass compared to a solid core optical fiber, thereby providing benefits such as increased propagation speed, reduced losses from both absorption and scattering, and reduced nonlinear interactions. These properties make hollow core optical fibers very attractive for use in many applications. These include optical telecommunication systems. However, it is currently very difficult to manufacture hollow core optical fibers of the lengths desired for telecommunication systems, where data centers between which optical data is transmitted may be tens or hundreds of kilometers apart. Typically, lengths of only about 10 km are achievable.

[0024] Hollow core optical fibres can be classified into two main categories or types according to their light guiding mechanism: hollow core photonic bandgap fibre (HCPBF) (alternatively, often referred to as hollow core photonic crystal fibre (HCPCF)) and antiresonant hollow core fibre (AR-HCF or ARF). There are various ARF sub-classifications characterized by their geometric structures, including Kagome fibres, nested antiresonant nodeless fibres (NANF) and tubular fibres. The present disclosure applies to all types of hollow core optical fibres, including these two main categories and their associated sub-types plus other hollow core designs. Note that in the art, there is some overlapping use of terms for various types of optical fibres. For the purposes of the present disclosure, the terms "hollow core fibre" and "hollow core fiber" are used interchangeably and are intended to cover all types of these optical fibres with a hollow core as described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow core fibers having a structure that provides waveguiding through the photonic bandgap effect (described in more detail below). The terms "ARF" and "antiresonant hollow core fiber" are used to refer to hollow core fibers having a structure that provides waveguiding through the antiresonance effect (also described in more detail below).

[0025] Figure 1 A schematic cross-sectional view of an example HCPBF 10 is shown. In this type of optical fiber, the structured inner cladding 1 comprises a substantially regular, closely packed array of many small glass capillaries, with a central group excluded from the array to define a substantially circular hollow core 2. The periodicity of the cladding structure provides a periodically structured refractive index, thereby providing a photonic bandgap effect that restricts the propagation of light waves toward the core. This is the fundamental optical mode or fundamental mode (FM). One or more higher-order optical modes or higher-order modes (HOM) may be supported in the core 2 and / or the cladding 1. These optical fibers can be described according to the number of cladding capillaries or "units" that are excluded in order to make the core 2. In Figure 1 In the example, the 19 elements in the center of the array are not present in the core region, making it a 19-element core HCPBF. The structured cladding 1 is formed by 6 turns of elements surrounding the core 2 plus some elements in the 7th turn to improve the roundness of the outer surface of the cladding. The outer cladding or sheath 3 surrounds the structured cladding 1.

[0026] In contrast to HCPBF, antiresonant hollow core fibers guide light by antiresonant photoconductive effects. The structured cladding of an ARF has a simpler construction comprising much fewer larger glass capillaries or tubes than an HCPBF, thereby giving a structure lacking any high degree of periodicity, making the photonic bandgap effect insignificant. More precisely, antiresonance is provided in order to propagate wavelengths that do not resonate with the wall thickness of the cladding capillaries, in other words, for wavelengths in the antiresonance window defined by the wall thickness of the cladding capillaries. The cladding capillaries surround a central void or cavity, wherein the central void or cavity provides the hollow core of the optical fiber and can support antiresonantly guided optical modes, including a fundamental mode and one or more higher order modes. The structured cladding can also support cladding modes, wherein the cladding modes can propagate primarily within the capillaries, in the glass of the capillary walls, or in the space or gap between the cladding capillaries and the outer cladding of the optical fiber. The losses of these additional higher order non-core guided modes are generally much higher than the losses of the core guided modes. The fundamental core guided mode typically has by far the lowest loss among the core guided modes. The antiresonance provided by the capillary wall thickness, which is antiresonant with the wavelength of the propagating light, acts to hinder coupling between the fundamental core mode and any cladding modes, so that the light is confined to the core and can propagate with very low loss.

[0027] Figure 2 A schematic cross-sectional view of an example simple antiresonant hollow core optical fiber is shown. The optical fiber 10 has an outer tubular cladding or jacket 3. The structured inner cladding 1 includes a plurality of tubular cladding capillaries 14, in this example 7 capillaries having the same cross-sectional size and shape, wherein the plurality of tubular cladding capillaries 14 are arranged in a single circle within the outer cladding 3 such that the longitudinal axis of each cladding capillary 14 is substantially parallel to the longitudinal axis of the outer cladding 3. Each cladding capillary 14 contacts (bonds) the inner surface of the outer cladding 3 at an azimuthal position 16 such that the cladding capillaries 14 are evenly spaced around the inner circumference of the outer cladding 3 and are also spaced apart from each other by gaps 5 (no contact between adjacent capillaries). In some designs of the ARF, the cladding tubes 14 may be positioned so as to contact each other (in other words, not so as to contact each other). Figure 2 The spacing of the cladding capillaries is not as spaced as in the example above, but the spacing that removes this contact improves the optical performance of the fiber and is generally preferred. The spacing 5 removes nodes that appear at the contact points between adjacent tubes and tend to cause undesirable resonances resulting in high losses. Accordingly, the fiber with spaced cladding capillaries can be referred to as a "node-free antiresonant hollow core fiber."

[0028] The cladding capillaries 14 are arranged in a circle around the interior of the tubular outer cladding 3 to create a central space, cavity or void within the optical fiber 10, i.e., the hollow core 2 of the optical fiber, wherein the longitudinal axis of the central space, cavity or void is also parallel to the longitudinal axis of the outer cladding 3 and the capillaries 14. The core 2 is bounded by the inwardly facing portion of the outer surface of the cladding capillaries 14. This is the core boundary, and the material (e.g., glass or polymer) of the capillary walls that constitute this boundary provides the desired anti-resonant photoconductive effect or mechanism. The thickness of the capillaries 14 at the core boundary is t, wherein the thickness t defines the wavelength bandwidth over which anti-resonant photoconductive action occurs in the ARF.

[0029] Figure 2 Just one example of an ARF is shown, and many other ARF structures are known.

[0030] Figure 3 A schematic cross-sectional view of a second example ARF is shown. The ARF has a structured inner cladding 1 including six cladding capillaries 14, wherein these cladding capillaries 14 are evenly spaced around the inner surface of a tubular outer cladding 3 and surround a hollow core 2. Each cladding capillary 14 has a smaller second-order capillary 18 nested therein, wherein the second-order capillary 18 is bonded to the inner surface of the cladding capillary 14, in this example at the same azimuthal position 16 as the bonding point between the main capillary 14 and the outer cladding 3. These additional smaller capillaries 18 can reduce optical losses. Additional, still smaller, third-order capillaries can be nested within the second-order capillaries 18. This ARF design with second-order capillaries and optionally additional smaller capillaries can be referred to as a "nested antiresonant nodeless fiber" or NANF, wherein a "double-nested antiresonant nodeless fiber" or DNANF refers to an optical fiber that includes a third-order nested capillary.

[0031] Many other capillary configurations for the structured cladding of an ARF are possible, and the disclosure is not limited to the examples described above. For example, the capillaries need not have a circular cross-section, and / or may or may not all be of the same size and / or shape. The number of capillaries surrounding the core may be, for example, 4, 5, 6, 7, 8, 9, or 10, although other numbers are not excluded. The ring of cladding capillaries in an ARF creates a core boundary, wherein the shape of the core boundary comprises a series of adjacent inwardly curved surfaces (i.e., convex from the perspective of the core). This contrasts with the typically outward curvature of the core-cladding interface of conventional solid core optical fibers and the essentially circular core boundary of HCPBFs (see Figure 1). Accordingly, antiresonant hollow core fibers can be described as negative curvature fibers. Kagome-class ARFs can also be constructed as negative curvature fibers and have a structured cladding of multiple small capillaries in an array, similar to HCPBFs, but are not constructed to provide a photonic bandgap. In contrast to HCPBFs, the guiding mechanism works through the antiresonance effect. Other examples of ARFs include designs with claddings formed as coupled tube structures [4] and hemispherical tube structures [5].

[0032] As used herein, the terms hollow core fiber, hollow core fiber, hollow core waveguide, hollow core optical waveguide and similar terms are intended to cover optical waveguide structures constructed according to any of the above examples and similar structures, wherein light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonant guiding and / or hindered coupling guiding) in a hollow elongated void or core surrounded by a structured cladding comprising a plurality of longitudinal capillaries. The capillaries include or define elongated holes, voids, cavities, cells or cavities extending continuously along the length or longitudinal extent of the optical fiber, which are also substantially parallel to the elongated core extending continuously along the length of the optical fiber. These various terms may be used interchangeably in the present disclosure.

[0033] The hollow core fiber manufacturing method proposed in this article is applicable to all types and any type of hollow core fiber, as described above. The proposed method can improve the hollow core fiber manufacturing output of all fiber types, and the output can be improved by one or more orders of magnitude, thereby providing hollow core fibers with continuous lengths exceeding 100km. This will allow the commercialization of hollow core fibers for new applications, and in particular will improve optical fiber telecommunication links, thereby realizing, for example, transoceanic hollow core fiber links. Currently, 100km long links are considered challenging and must be assembled from multiple short lengths of hollow core fibers, which requires a lot of resources.

[0034] The proposed method exploits improvements to the fiber drawing process to enable hollow core fiber manufacturing with greatly enhanced output in continuous lengths of optical fiber than is conventionally achievable.

[0035] Compared to conventional drawing processes, the proposed method improves upon conventional drawing processes by introducing an improved narrower preform having a width intermediate between that of conventional hollow core preforms and that of finished optical fibers. The narrow or thin preform has a smaller width than a conventional preform, and unlike a preform, a short length of thin preform is not separated from the original preform from which the thin preform is drawn. The thin preform can be drawn into optical fiber without applying pressure to its interstices during the drawing process, also unlike conventional preforms. The narrow width of the thin preform gives a relatively small preform-to-fiber draw ratio, which avoids the need for specialized pressurization. Moreover, the overall length of the thin preform is very long, and thus its overall glass volume is large, so that greatly increased continuous lengths of optical fiber can be drawn therefrom. Thus, even though the yield and draw ratios from the thin preform to optical fiber are low (due to the long length and small width of the preform), a high total optical fiber yield can be achieved. A thin preform with a long length can be produced as a whole from an initial preform before optical fiber drawing, or can be produced simultaneously with optical fiber drawing, so that the thin preform has a transient length, wherein the transient length is supplemented at a first end (which may be an upper end in some configurations) by being drawn from the initial preform, and is consumed at a second end (which may be a lower end in some configurations) by being drawn into an optical fiber, and the total length given by the transient length accumulated over time is long.

[0036] The absence of pressurization is not only convenient, but also solves a particular problem that limits the ability to draw hollow optical fibers from conventional preforms via relatively large drawing ratios. As described above, current hollow core fiber designs, including HCPBF and ARF, require pressure to be applied to the voids in the preform or billet during the process of drawing the optical fiber from the preform or billet. Pressurization is often differential because two or more different pressures are applied to different voids. Pressure is required during the drawing process to counteract surface tension in the softened glass, where the surface tension acts to collapse the voids, thereby changing or destroying the intended optical fiber structure if not contained. Regulation of (multiple) pressures enables precise control of the final geometry of the optical fiber. During the drawing process, the glass structure undergoes a geometric change known as necking. This is a narrowing of the glass structure, where the glass structure is softened and becomes plastic (making it pliable or ductile) in the drawing furnace, where the width is continuously reduced from the width of the preform or billet to the intended width of the optical fiber over the length of the structure. When pressurization is applied, at the wider end of the constriction (which is closest to the pressure input at the undrawn open end of the preform or billet, and is typically the upper end, since fiber drawing is usually done vertically), the geometry is determined by the applied pressure. At the narrower end of the constriction, where the fiber is formed, the geometry is determined by the surface tension in the softened glass that causes the capillary to contract. Thus, there is an interaction between the forces of applied pressure and surface tension, and in order to achieve the desired capillary size and film thickness, it is necessary to apply enough pressure to initially expand the capillary beyond the desired size so that subsequent contraction under surface tension at the lower end of the constriction stretches the capillary back to the target size.

[0037] Attempts to increase the fiber yield of drawing by increasing the drawing ratio using a wider preform or billet to enable more fiber to be produced from the preform or billet increase the complexity of the pressure-surface tension relationship and make it more difficult to identify the correct parameters for drawing. Eventually, the contraction phase of the drawing dynamics becomes very intense and requires increased capillary expansion at higher applied pressures to offset it. However, within the glass structure, there is limited space for capillary expansion and there will be situations where the expansion overshoot required to pre-stop contraction causes unacceptable deformation in the fiber structure. In particular, in the ARF design where the capillaries should be spaced apart, contact may occur between adjacent capillaries, causing the capillaries to fuse together. In the HCPBF design, severe asymmetry of the hollow core may occur. Such defects are extremely detrimental to the optical properties and performance of the finished optical fiber and therefore need to be avoided. For convenience, this phenomenon can be referred to as mid-draw contact (MDC) and is an important limiting factor in improving the fiber yield of hollow core fiber production. If the proposed draw ratio from preform / preg to fiber is greater than the limit imposed by the MDC, the intended fiber cannot be made.

[0038] As a prerequisite for a detailed discussion of the present invention, some terms related to optical fiber drawing will be discussed and defined. In optical fiber manufacturing, a preliminary step is to form a relatively large cylindrical glass structure (called a preform). For hollow core optical fibers, the preform has a cross-sectional structure containing glass elements, which will form the desired structural elements in the finished optical fiber once drawn, wherein the melting, expansion and contraction of the glass elements occurring during the drawing process are taken into account in the preform structure so that the final optical fiber structure meets expectations. The preform is made by assembling its component glass elements into an appropriate arrangement. For hollow core optical fibers, the preform is typically made by stacking a plurality of capillaries, wherein a plurality of capillaries (possibly with spacer elements to hold the capillaries in place) confine the optical fiber cladding within an outer glass tube supporting the cladding tube. Bonding of the elements can be performed to fix the capillaries in place relative to the outer tube, or bonding can be performed as part of the drawing process. The preform can be directly drawn into the desired optical fiber. Alternatively, in conventional methods, the preform may be drawn into one or more individual preforms, for example, the width of the preform is between the preform and the intended optical fiber, and its length is about one meter. Then, the preforms can be drawn into individual lengths of the intended optical fiber. The preforms may be easier to handle than the preforms, and also reduce the drawing ratio for drawing into optical fibers. However, the terms "preform" and "preform" tend to be used interchangeably to some extent, so some clarification is needed in this article. The preform can be considered to be any glass structure that can be drawn into a narrower structure (it is a preform of a narrower structural morphology). Under this definition, the preform made by stacking or otherwise assembling glass elements to be drawn into a preform or optical fiber constitutes a preform, but similarly, the preform can be considered a preform because it can be drawn into a narrower preform or optical fiber. Similarly, the preform is a glass structure that can be drawn into a narrower structure (a narrower preform or optical fiber), but it is drawn from a wider glass structure, rather than assembled from separate elements. In the presently proposed method, an intermediate glass structure is used that has a width intermediate between that of a preform and a finished optical fiber, but that has a number of characteristics that distinguish it from a conventional blank. Accordingly, the term "blanket" will not be used in the following description to refer to this intermediate structure. The term "initial preform" will be used to refer to the initial glass structure from which the intermediate glass structure is made, and it should be understood that it may include a preform made by assembling glass elements into a desired structure, or a blank drawn from such a preform. The term "slim preform" will be used to refer to a glass structure of intermediate width that is drawn from an initial preform and that is then drawn into an optical fiber, wherein the descriptor "slim" indicates that the width is reduced compared to a conventional preform or blank that is directly drawn into an optical fiber. A slim preform is also long compared to a conventional preform or blank that is directly drawn into an optical fiber.In different examples of the proposed method, the long length can be manufactured in a single stage before drawing the optical fiber from the thin preform, or can be accumulated or replenished "on the fly" while drawing the optical fiber from the thin preform. The term "optical fiber" is generally used to refer to an optical fiber drawn from a thin preform. The method includes drawing an optical fiber from a thin preform, and may also include drawing a thin preform from an initial preform.

[0039] The initial preform has an initial preform width or diameter (first width) and an initial preform cross-sectional area (first cross-sectional area), the thin preform has a thin preform width or diameter (second width) smaller than the initial preform width and a thin preform cross-sectional area (second cross-sectional area) smaller than the initial preform cross-sectional area, and the optical fiber has an optical fiber width or diameter (third width) smaller than the thin preform width and an optical fiber cross-sectional area (third cross-sectional area) smaller than the thin preform cross-sectional area. The drawing ratio of the optical fiber manufactured by drawing from the thin preform can be defined as the ratio of the thin preform width to the optical fiber width, or the ratio of the thin preform cross-sectional area to the optical fiber cross-sectional area. The drawing ratio of manufacturing a thin preform from the initial preform is the ratio of the initial preform width to the thin preform width, or the ratio of the initial preform cross-sectional area to the initial preform cross-sectional area. The total drawing ratio can be the ratio of the initial preform width to the optical fiber width, or the ratio of the initial preform cross-sectional area to the optical fiber cross-sectional area. When calculating the draw ratio, the relevant width and the relevant cross-sectional area are the outer diameter and area of ​​the cladding in the optical fiber, or the outer diameter and area of ​​the glass elements in the initial preform and the fine preform that will form the cladding in the optical fiber, and do not include any supporting layers and / or protective layers around the cladding.

[0040] Figure 4A flow chart showing steps in an exemplary method for manufacturing a hollow core optical fiber according to the present disclosure is shown. In a first step S10, a thin preform made of glass is provided, wherein a hollow core optical fiber can be drawn from the thin preform. As described above, after the thin preform is drawn into an optical fiber of a desired width, the cross-sectional structure of the thin preform is configured to form a desired cross-sectional structure of a desired hollow core optical fiber type and design. The thin preform structure is configured so that a hollow core optical fiber can be drawn therefrom without applying a pressurization to the void of the thin preform during the process of drawing the optical fiber. This is achievable in the case of using a relatively small drawing ratio to draw the optical fiber; therefore, the thin preform is required to be thin, where thin can be understood as being relative to the typical width of a preform or blank from which a hollow core optical fiber is directly drawn in a conventional process, where pressurization is required to offset the surface tension contraction of the softened glass as described above. When the thin preform is drawn into an optical fiber using a low drawing ratio, the surface tension contraction is less severe, and pressurization may not be required to balance it. Therefore, there is no expansion of the elements in the glass structure caused by the applied pressure, and the risk of MDC is avoided. However, in the thin preform, surface tension will still exist in the molten glass, causing some shrinkage to occur during the drawing of the optical fiber. In order to achieve the target size of the internal structure of the optical fiber, this can be solved by appropriate construction of the internal structure of the thin preform. In the absence of any forces from applied pressure (causing expansion) and surface tension (causing shrinkage), the drawn hollow-core fiber will maintain the proportions and geometric ratios of the preform structure from which the hollow-core fiber is drawn; only the scale will be reduced, where the amount of reduction can be controlled by the parameters of the fiber drawing (such as speed and tension). However, in this case, there will be some surface tension. To solve this, the thin preform can be constructed accordingly, so that the size of the part that will shrink under the surface tension is proportionally enlarged to compensate for the shrinkage. Thus, the geometric ratios of the various elements of the cross-sectional structure that the thin preform can have correspond to the expected geometric ratios of the hollow core fiber plus one or more compensations (where different elements may require different compensations if they experience different surface tensions (such as due to different diameters or film thicknesses), where the compensations are sized to balance the surface tension contraction of the softened glass of the thin preform during the drawing of the hollow core fiber. Since the draw ratio used to draw the optical fiber is small, the compensation may only require a size of a few percent. Therefore, some or all of the internal elements (such as hollow tubes or capillaries) may exceed the size required to match the geometric ratios of the hollow core fiber by 10% or less, or 5% or less, or 3% or less.

[0041] The width of the thin preform is in the range of 0.5 mm to 5 mm. In some cases, a larger minimum width may be more appropriate, such as to facilitate handling of the thin preform, so that the range may alternatively be, for example, 1 mm to 5 mm. Similarly, in other cases, a smaller maximum width may be more appropriate, such as to avoid intermediate drawing contact, or to require bending or winding the thin preform, so that the range may alternatively be, for example, 0.5 mm to 3.5 mm or 0.5 mm to 2.5 mm. In general, a thin preform width of a smaller range of about 1 mm to 2.5 mm can be defined according to the requirements of a particular manufacturing arrangement. These dimensions are much narrower than conventional preforms used to draw optical fibers, where conventional preforms typically have widths on the order of tens of millimeters. The narrow width of the thin preform enables no pressurization when drawing optical fiber from the thin preform, and also enables different handling of the thin preform before it is drawn into optical fiber, as will be discussed further below. When drawn into conventional width optical fiber, thin preform widths within this range can produce fiber lengths of several meters to several hundred meters, for example between 4m and 500m of optical fiber, from a one meter thin preform. Conceived of a total thin preform length of about 10 km, this can yield a total fiber length of several hundred or several thousand kilometers. However, the invention is not limited in this manner, and in some cases, shorter lengths of optical fiber are useful and may be desired, and longer lengths can be achieved depending on the parameters of the initial preform, the thin preform and the hollow core fiber, as well as the operating parameters of the drawing apparatus used. In practice, it is expected that minimum hollow core fiber lengths of at least 20 km will be routinely available, while lengths of 30 km or more, 50 km or more, and 100 km or more are readily achievable.

[0042] The length of the thin preform includes the total length available for drawing. The total length is very long compared to the length of conventional preforms and blanks. Combined with the narrow width of the thin preform, a large total volume of glass is available in the thin preform, which enables the production of extremely long lengths of optical fiber from the thin preform in a single draw, despite a small draw ratio and low fiber yield. The thin preform length is defined as the total length available for drawing into a hollow core fiber, and can be provided in different ways. In some examples, the total length can be provided as a single span thin preform so that the actual length of the thin preform is equal to the total length available for drawing into optical fiber before optical fiber drawing begins. In other examples, optical fiber drawing begins at the distal end of the thin preform before the thin preform has become the total length available for drawing, while the thin preform continues to be drawn from the initial preform at the proximal end (the proximal and distal ends are defined relative to the drawing direction). In this case, the thin preform has a transient length between its proximal end where glass is replenished from the initial preform and its distal end where glass is consumed by the optical fiber. The transient length can be much smaller than the total length available for drawing, but over the duration of the drawing process, glass that has become available for optical fiber drawing in the transient length accumulates until it corresponds to the total length. The total length of the thin preform available for drawing into the hollow core fiber can be a convenient length, depending on factors such as the total length of the hollow core fiber required, the available operating time of the drawing device, and the size of the initial preform from which the thin preform is drawn. However, in order to provide a useful improvement on the current limitations of hollow core fiber drawing output, the total length of the thin preform available for drawing can be 30m or more. However, shorter lengths between about 10m and 30m are not excluded, but more importantly, much larger lengths can be implemented to provide a significant improvement in hollow core fiber production. For high throughput commercial fiber production, for example, the length of the thin preform may be at least 30 m, or at least 100 m, or at least 1000 m (1 km), or at least 5000 m (5 km), or at least 50000 m (50 km). The upper limit on length is largely determined by practical reasons of processing (such as drawing furnace size and speed of feed and winding equipment) and manufacturing time, rather than technical limitations. It is envisioned that thin preforms of length that are easy to produce and manage may be in the range of, for example, 30-50 km. However, the invention is not limited in this manner, and other total lengths are not excluded.

[0043] For step S10, the thin preform may be provided by being obtained from elsewhere, by being separately manufactured to its total usable length at a previous stage of the manufacturing process, or via a transient length. These alternatives will be described in more detail later.

[0044] The method proceeds to step S11, in which the end of the thin preform is heated to soften the glass, for example in a conventional manner in a furnace of an optical fiber drawing tower. In the next step S12, a hollow core fiber is drawn from the initial preform, the width of the hollow core fiber being less than the width of the thin preform. A typical width of a hollow core fiber is in the range of 100 to 500 μm. This gives a draw ratio for drawing a hollow core fiber from a thin preform, wherein the draw ratio is defined by the ratio of the cross-sectional area of ​​the thin preform to the cross-sectional area of ​​the hollow core fiber, in the range of about 4-500 or about 2-150, and is less than the draw ratio conventionally used for hollow core fiber drawing. As described above, the low draw ratio allows the fiber to be drawn without the need to apply a pressurization to the void of the thin preform. This is in contrast to the pressure differential that is typically applied when drawing a hollow core fiber from a preform or billet. Accordingly, the fiber drawing can be performed without applying any pressurization to the void of the thin preform. However, the method is not limited in this manner, and in some cases pressurization of the thin preform interstices may be applied, if preferred. This may be during the drawing process. Alternatively, a pre-pressurization stage may be implemented, in which one or more gases at different pressures are flowed into the interstices of the thin preform, so that the entire thin preform is pre-pressurized with a predefined set of pressures for the various interstices before heating and fiber drawing. Although the pressure may not then be adjusted or any differences altered during the drawing process, the temperature and tension may be modified during the drawing process to generally change the effects of the pre-applied pressures, and thus control the internal structure of the fiber.

[0045] Within the limits of the volume of glass available from the thin preform, any desired length of hollow core fiber can be drawn from the thin preform. However, for maximum efficiency, it is useful to draw the hollow core fiber in continuous lengths until the available glass of the thin preform is exhausted and the thin preform is completely exhausted. Therefore, the method can proceed to a determination step S13 in which it is determined whether the total length of the thin preform available for drawing has been drawn into hollow core fiber. If so, and the thin preform has been exhausted and no total length remains, the method proceeds to step S14 in which the fiber drawing is stopped and the fiber can be separated from the remaining waste portion of the thin preform, wherein the remaining waste portion cannot be made into fiber and is therefore not part of the total available length (as is common in optical fiber manufacturing, where a certain amount of waste glass is unavoidable, such as necking between two widths of the glass structure). If not, and the thin preform has not been completely consumed, the method can loop back to step S11, in which the end of the thin preform continues to be heated and the thin preform is drawn into an optical fiber as the thin preform is gradually fed into the furnace for heating, as in the conventional method.

[0046] Figure 5 Shown for Figure 4 A flow chart of steps in an example method of providing a thin preform is provided in step S10 of the method. As described above, the thin preform can be provided in several ways. In the case where the thin preform is not provided by passively obtaining it from another source, Figure 5 The steps of the method can be found in Figure 4 Prior to the steps of the method. In a first step S1, an initial preform formed of glass and having a cross-sectional structure is provided, the cross-sectional structure of the initial preform being configured to be drawn into a thin preform so as to provide the thin preform with a cross-sectional structure for forming a final hollow core optical fiber, as described above. The initial preform may be a conventional preform, such as a glass structure formed by stacking tubes and optionally stacking other glass elements (such as spacers inside an outer glass tube, as already described), or the initial preform may be a blank previously drawn from a conventional preform. The width of the initial preform is greater than the expected width of the thin preform, wherein, as already mentioned, the expected width of the thin preform may be in the range of 0.5 mm to 5 mm. It is expected that a higher drawing ratio may be used for making a thin preform than for making a hollow core optical fiber, for example in the range of 200 to 16000, wherein the drawing ratio used for this stage is the ratio of the cross-sectional area of ​​the initial preform to the cross-sectional area of ​​the thin preform. Accordingly, the width of the initial preform may be in the range of about 20 mm to 200 mm. However, the invention is not limited in this regard and other draw ratios and initial preform widths may be used, including lower draw ratios and larger initial preform widths than the fiber drawing stage to produce longer lengths of thin preforms and subsequently hollow core fibers.

[0047] The use of a higher draw ratio for thin preform manufacturing allows the use of a standard hollow core preform as an initial preform, such as an initial preform that is conventionally drawn directly into an optical fiber, or an initial preform that is first drawn into a preform. Conventionally, the drawing of a preform into a preform uses a relatively small draw ratio and does not require pressurization. Instead, pressurization is reserved for drawing from a preform into an optical fiber. However, in the method proposed herein, pressurization is applied to the initial preform when the thin preform is drawn from the initial preform so as to obtain a cross-sectional structure of the thin preform suitable for drawing into an optical fiber without the need to apply pressurization to the thin preform, as described above. Applying a suitable pressurization including one or more pressure differentials (where different pressures are applied to different voids in the glass structure of the preform or preform) is a known technique for drawing optical fiber and will not be further described herein because it can be implemented in the same or similar manner when drawing a thin preform from an initial preform. The skilled person knows how to apply the appropriate pressure to obtain the desired target glass structure from the initial glass structure during the dynamics of surface tension contraction under pressurization and drawing.

[0048] In the next step S2, the end of the initial preform is heated to soften the glass, for example using a furnace in an optical fiber drawing tower. In step S3, a thin preform is drawn from the softened glass of the initial preform while applying a pressurization to the void of the initial preform. In order to achieve the drawing of a thinner preform, the initial preform is steadily fed to the furnace for heating and drawing, so that the glass of the initial preform is gradually consumed and converted into a thin preform of increasing length. After step S3, the method can follow one of two optional paths, depending on the way in which the total length of the thin preform available for drawing into optical fiber is provided.

[0049] In a first path, the method proceeds to step S4a. In this path, the entire length of the thin preform available for drawing into a hollow core optical fiber is made in a single draw, so that the actual length of the finished thin preform is equal to the total length available for optical fiber drawing. As mentioned above, the total length can be very long and therefore requires appropriate handling; this is different from the handling of a preform, where the preform is essentially rigid and very short. Accordingly, in step S4a, as the thin preform is drawn from the initial preform, it is collected by winding the thin preform on a winding wheel, drum or spool. This is consistent with the collection of optical fiber in traditional drawing processes. The ability to collect the thin preform in a wound form (which is not possible with a preform) is achieved by the narrow width of the thin preform, where the narrow width of the thin preform makes the thin preform soft enough to be wound on a winding wheel without damage. The diameter of the winding wheel should be such that the curvature of its winding surface around which the thin preform is wound or coiled is equal to or greater than the bending radius of the thin preform, wherein the bending radius of the thin preform is in turn partially determined by the width of the thin preform. A smaller width provides a tighter bending radius, i.e., the radius to which the thin preform can bend before cracking or breaking.

[0050] In the next step S5a, the thin preforms continue to be drawn from the initial preform while the initial preform continues to be fed to the furnace for heating. This continues until the length of the thin preform is the desired total length available for drawing into hollow core optical fiber. This can be achieved before the initial preform is exhausted, but for maximum efficiency, the thin preforms are continued to be drawn until all of the initial preform (minus losses) has been converted into thin preforms. Once the total length of thin preform has been produced, the method proceeds to step S6a in which the thin preforms are separated from the remainder of the initial preform. The thin preforms are now stored on the reel and can be used according to the method. Figure 4 This can be done immediately or later, and in the same draw tower or in a different draw tower. Finally, when necessary, a thin preform is provided to draw a hollow core fiber, such as Figure 4 The first step in the method is S10.

[0051] In order to protect the thin preform while it is collected on the winding wheel, stored on the winding wheel and later unwound from the winding wheel, a coating can be applied to the thin preform when it is manufactured and before the thin preform reaches the winding wheel. This can help prevent breakage and surface damage. It should be possible to remove the coating from the thin preform before it is drawn into a hollow core fiber, but it should be noted that a separate protective coating can be applied to the hollow core fiber after the hollow core fiber has been drawn, which is a traditional practice in optical fiber manufacturing. Any coating that can be easily applied to and removed from the thin preform can be used. The first example is a UV-cured acrylate coating, which is also the type used to coat optical fibers. This will provide good protection for the thin preform, but it adheres well to glass, so removal may be too time-consuming and impractical. An alternative is to extrude a polymer tube around the thin preform by using, for example, a crosshead extruder. The polymer tube can be cut, stripped or torn off from the thin preform when the polymer tube is transported for drawing the optical fiber. The inner diameter of the polymer tube can be the same as the outer diameter of the thin preform, so that it contacts the outer surface of the thin preform but is not firmly adhered to it, or its inner diameter can be larger, so that the thin preform is loose inside the tube. The latter arrangement can facilitate cutting the polymer tube from the thin preform without damaging the surface of the thin preform. Another alternative is to apply a coating that is easily eliminated upon pyrolysis (chemical decomposition by heating), for example, as described in US5596669[6]. This allows the coating to be removed when the thin preform passes through a furnace or is otherwise heated before the optical fiber.

[0052] exist Figure 5 In a second path of the method, the process proceeds from step S3 to step S4b. In this path, the thin preform is formed to a transient length, wherein the transient length is much less than the expected total length available for drawing into the hollow core fiber at any given time. The transient length will depend on the configuration of the drawing apparatus used to produce the hollow core fiber; examples are described in more detail below. In step S4b, the length of the thin preform drawn from the initial preform reaches the transient length, and the thin preform is provided for drawing into the hollow core fiber without separating the thin preform from the initial preform, such as Figure 4Step S10 of the method. In step S5b, the thin preform is continued to be drawn from the initial preform while the hollow core fiber is being drawn from one end of the thin preform (i.e., the distal end away from the necking between the initial preform and the thin preform). Thus, the thin preform is replenished at its proximal end while being consumed from its distal end, so that the transient length between these ends remains finite and substantially constant, but the total glass that has been contained in the transient length between being added and then removed accumulates over time. This continues until the transient length (in terms of the glass it has contained) has accumulated to the total length of the thin preform that can be drawn into the hollow core fiber. In this way, a method is provided. Figure 4 The thin preform with a total length available for drawing required in step S10.

[0053] Fig. 6A and Figure 6B A simplified schematic diagram of an example apparatus suitable for performing an example hollow core optical fiber manufacturing method according to the present disclosure is shown. Fig. 6A shows where the Figure 5 The first path corresponds to the means of the first part of the example method (ie steps S1 to S7a). Figure 6B shows where the Figure 4 Steps S10 to S14 correspond to the apparatus of the second part of the exemplary method.

[0054] Fig. 6A An optical fiber drawing tower 20a is shown, wherein an initial preform is drawn into a thin preform. The initial preform 22 has an initial preform width and includes a ring of hollow capillaries 24 in this example to form a hollow core fiber microstructured cladding within an outer glass support tube 26, wherein the hollow core fiber microstructured cladding is arranged to define a void for forming a hollow core of the hollow core fiber within the cladding. The initial preform 22 is surrounded by a glass outer layer 28 intended to form a glass jacket of the finished hollow core fiber, as described above. The initial preform is suspended at a first upper end with its longitudinal axis perpendicular to a support structure or support (schematically shown as element 30), wherein the support structure or support is configured to feed the initial preform 22 and the glass outer layer 28 downwardly along the axial direction at a first feed speed F1. A pressurization system 32 is connected to the core and cladding capillary voids of the initial preform to supply pressurized gas to the voids at one or more pressure differences ΔP, as also already described. During the process of drawing the thin preform from the initial preform 22, an increased pressure is applied.

[0055] The drawing tower also includes a vertically positioned annular furnace 34, in which the initial preform 22 is arranged so that a portion at its lower end is heated by the furnace to soften the glass at the end. The softened glass is drawn into a thin preform 36 having a thin preform width less than the initial preform width via a necking 38 formed in the softened glass, wherein the width of the glass structure is narrowed from the initial preform width to the thin preform width by the necking 38. The softened glass is pulled downward by the action of a belt puller 40 including a pair of facing vertically arranged annular belts, between which the glass for forming the thin preform is clamped and pulled downward by the rotation of the belts. After the tape drawer 40, the thin preform 36 passes around a capstan 42 that rotates in a vertical plane, wherein the capstan 42 also serves to stretch the thin preform downward so that the thin preform is continuously drawn from the softened glass at the neck 38, wherein the neck 38 is supplemented from its upper end by gradually feeding the initial preform 28 into the furnace. The tape drawer 40 and the capstan 42 serve to tension the thin preform 36 so that the thin preform 36 is drawn from the initial preform 22 at a constant rate, wherein the constant rate is a first drawing speed D1, which is specified to form the thin preform 36 at a desired width. The thin preform 36 is narrow enough so that it is flexible enough to be wrapped around the capstan for tensioning; note that this is not possible when drawing a blank rod from the preform because the blank rod is thicker and too hard. Other tensioning devices can be used additionally or alternatively in accordance with known techniques for drawing optical fiber. The first drawing speed D1 is faster than the first feed speed F1 so as to match the supply of newly softened glass from the relatively wide initial preform 22 with the collection of softened glass by the relatively narrow thin preform, thereby providing the desired drawing ratio at the necking 38. The desired thin preform width can be assisted by a first diameter gauge 44 located between the belt puller 40 and the driving wheel 42 in this example, wherein the first diameter gauge 44 includes an aperture through which the drawn thin preform passes for diameter measurement by a laser beam and from which the results can be fed back to control the feeding of the initial preform 22. Diameter gauges are also known in the art of optical fiber drawing. Moreover, according to any of the example coating methods described, an optional coating device 46 is included below the diameter gauge 44 and is configured to provide an optional removable polymer coating on the thin preform 36.

[0056] The thin preform 36 passes around the driving wheel 42 and is collected from the driving wheel 42 by the rotating winding wheel, spool or reel 48, where the thin preform 36 is wound on the winding wheel, spool or reel 48 by the action of rotation. The drawing of the thin preform 36 can continue as long as necessary until the initial preform 22 is completely consumed, because the glass of the initial preform 22 (minus inevitable losses) has been converted into the thin preform 36 as much as possible. The actual length of the thin preform 36 collected on the winding wheel 48 is the total length available for drawing into hollow core optical fiber. The wound thin preform 36 can be stored on the winding wheel 48 as long as needed, and does not need to be immediately drawn into optical fiber. Accordingly, multiple rolls of thin preform 36 can be continuously drawn from one or more initial preforms while the drawing tower 20a is specially constructed for this process to improve efficiency.

[0057] Figure 6B Fiber drawing tower 20b is shown, where Fig. 6A The previously manufactured thin preform 36 is drawn into a hollow core optical fiber. The drawing tower 20b may be the same as the drawing tower 20a in which the thin preform 36 is drawn, but is reconfigured for drawing optical fiber, or it may be a different drawing tower. The thin preform 36 is provided during the thin preform drawing process while being wound on a winding wheel 48 (or optionally rewound on a different winding wheel) and is mounted above the furnace 34 of the drawing tower 20b. The furnace 34 of the drawing tower 20b may have the same Fig. 6AThe furnaces 34 of the drawing tower may be of the same specification, or may be different (e.g., in terms of temperature, length, and / or inner diameter). A feeding mechanism (not shown, achieving direct feeding, or an arrangement including pulleys, floating wheels, hoppers, and / or belt pullers to more accurately control the feed speed) is provided to draw the thin preform 36 from the winding wheel 48, and feed the end of the thin preform 36 into the upper end of the furnace 34 at a feed speed F2. This is different from a conventional drawing tower feeding arrangement configured to handle short rigid preforms and blanks, in which the short rigid preforms and bars must be oriented and fed vertically; the long, thin, and flexible form of the thin preform 36 enables it to be fed by unwinding from the winding wheel 48 instead. One or more drive wheels or similar arrangements (not shown) may be included between the winding wheel 48 and the furnace 34 to maintain the thin preform 36 in an optimal position within the heating zone of the furnace as the thin preform 36 is unwound from the winding wheel 48. The feed rate F2 may be faster than the feed rate F1 used to feed the initial preform 22 into the furnace because the draw ratio from the thin preform to the hollow core fiber is smaller and it is generally desirable to produce the fiber at the highest rate possible. Additionally or alternatively, a lower furnace temperature may be used. The furnace 34 softens the glass at the end of the thin preform 36 so that the hollow core fiber 50 can be drawn from the thin preform 36. The optical fiber 50 is drawn under tension by exiting the furnace 34 downward to a drive wheel 42 located vertically below the furnace 34 and rotating in a vertical plane, wherein the optical fiber 50 passes around the drive wheel 42 via a tension meter 52 located between the furnace 34 and the drive wheel 42 through which the optical fiber 50 passes, and the tension is applied by the drive wheel 42. The tensioning arrangement draws the optical fiber 50 from the softened glass of the thin preform 36 in the furnace 34 via the second constriction 54 at a second draw speed D2, where the width of the glass structure narrows from the thin preform width to the fiber width at the second constriction 54. The second draw speed D2 selected to achieve the desired fiber width is faster than the second feed speed F2 so as to match the supply of newly softened glass from the thin preform 36 with the take-up of softened glass by the narrower optical fiber 50 as the wider thin preform 36 is fed into the furnace, thereby providing the desired draw ratio at the second constriction 54. The desired fiber width can be aided by a second diameter gauge 56 that defines the aperture through which the drawn optical fiber 50 passes, which in this example is located below the furnace 34 before the optical fiber 50 reaches the tensioning element 52 and the drive wheel 42.

[0058] Note (in this example) that no pressurization is applied to the void of the thin preform 36 while it is being drawn into the hollow core fiber 50. As described above, the small draw ratio used for this drawing avoids the need for pressurization. Not requiring pressurization enables the processing of extremely long lengths of thin preforms, where any pressurization applied at one end of the thin preform will not be effective at the other end where it is formed into a necking of the fiber. The small diameter of the void in the thin preform prevents the pressurized gas from propagating rapidly along the void, so if the length is too large, the pressurization cannot effectively counteract the surface tension during the drawing process. This is in contrast to conventional hollow core fiber drawing from a preform, where the fiber structure can be monitored during the drawing process and the pressure adjusted to achieve the desired geometry. In the case of longer and narrower thin preforms, changes in applied pressure are not expected to reach the necking region, making it impossible to control the geometry by pressurization.

[0059] If the thin preform 36 has been provided with a protective polymer coating during the drawing process of the thin preform 36, as shown in FIG. Fig. 6A As described, it is necessary to remove the protective polymer coating from the thin preform 36 before the thin preform 36 is drawn into the optical fiber 50. Therefore, the drawing tower 20b can optionally include a coating removal device or apparatus 58 located above the furnace 34 and configured to remove the coating by a process depending on the nature of the coating. For example, the coating removal device 58 can include one or more blades for engaging with the tubular coating and cutting the tubular coating as the thin preform passes through the device 58, and a component for collecting the cut coating and pulling it from the thin preform 36, or a heat source for heating the passing optical fiber and pyrolyzing the fugitive coating. Alternatively, the heating provided by the furnace can be suitable for removing the fugitive coating before the glass of the thin preform softens to form the second neck 54.

[0060] The drawing tower 20b may also optionally include an optical fiber coating device 60, wherein the optical fiber 50 passes through the optical fiber coating device 60 after the optical fiber 50 is drawn, and in this case before reaching the tensiometer 52. Conventional optical fibers are often provided with an outer coating after drawing in order to protect them from damage during subsequent storage and use, wherein the outer coating is a layer of material such as a resin or polymer. Accordingly, any known optical fiber coating device can be used and any known optical fiber coating can be applied. In this example, a UV-curable coating, such as an acrylate material, is applied, so that the device 60 includes a coating cup 60a containing a bath or reservoir of coating material in liquid form, wherein the optical fiber 50 passes through the coating cup 60a so as to collect a layer of coating material on the outer surface of the coating cup 60a above a UV light source 60b, and the UV light source 60b directs UV radiation onto the coating layer so as to cure and solidify the coating layer.

[0061] Finally, after passing around the drive wheel 42, the finished optical fiber 50 leaves the drive wheel 42 and is collected and collected by winding around a rotating optical fiber take-up reel, drum or spool in the same manner as the thin preform 36 from the first drawing stage. The drawing of the optical fiber 50 can continue as long as necessary until the thin preform 36 has been completely consumed because the glass of the thin preform 36 (minus avoidable losses) has been converted to hollow core optical fiber 50 as much as possible. This enables the longest possible continuous length of optical fiber to be manufactured (if necessary) and maximizes production efficiency because a single fiber draw can produce the longest length of optical fiber from a single thin preform. In general, for maximum efficiency and maximum fiber span, all of the initial preform is drawn into a thin preform, and then all of the thin preform is drawn into a hollow core optical fiber.

[0062] Other example methods utilize the concept of the transient length of the thin preform described above, wherein the total usable length of the thin preform is achieved by constantly adding glass material at one end of the transient length while constantly removing glass material at the other end of the transient length. This avoids the need to collect and store the entire usable length of the thin preform. However, in order to form the necking from the initial preform to the thin preform separately, but to form the necking from the thin preform to the hollow core fiber at the same time, two heat sources are required to soften the glass structure both during the process of drawing from the thin preform and drawing to the thin preform. Examples of optical fiber drawing devices suitable for implementing this arrangement will now be described. In these devices, the same methods as described above can be performed. Figure 5 The hollow core optical fiber manufacturing method corresponding to the second path, i.e. steps S1 to S5b, is followed by Figure 4 Steps S10 to S14.

[0063] Figure 7A schematic diagram of a first example apparatus for processing thin preforms in transient length form is shown. The apparatus includes two optical fiber drawing towers 20a and 20b, wherein the drawing towers 20a and 20b are laterally offset from each other relative to the vertical optical fiber drawing direction of the drawing towers 20a, 20b. The lateral offset is partly for convenience, as the combined height of two vertically stacked drawing towers is prohibitive in many environments, and also helps to isolate tensioning and pressurization between two simultaneous draws. The addition of a driving wheel and a floating wheel or similar feed control device to transfer the preform from one drawing tower to the other separates the two draws and prevents the overall dynamic process from appearing as a single draw in two stages. The first drawing tower 20a on the left hand side is used to draw a thin preform from an initial preform and is constructed in substantially the same manner as the drawing tower 20a of Figure 6a, wherein the same reference numerals are used to indicate the same components. Therefore, the detailed description will not be repeated. However, in summary, the initial preform 22 constructed as described above is vertically fed into the annular drawing furnace 34a of the first drawing tower 20a at a first feed speed F1, while a pressure difference is applied to the gap of the initial preform 22 by the booster system 32. The first furnace plays a role in softening the glass at the end of the initial preform 22, so that the initial preform 22 can be drawn into a thin preform 36 through necking 38 under the action of tension and stretching of the belt puller 40, wherein the drawn thin preform passes through the thin preform diameter gauge 44 before being taken by the vertically rotating driving wheel 42 (in this example, the first driving wheel for handling thin preforms) below the first furnace 34a, as described above. The belt puller 40 and the first driving wheel 42 provide a first drawing speed D1 for drawing the thin preform that is faster than the first feed speed F1, also as described above. Note that in this example, the thin preform coating device 46 is not included in the first drawing tower 20a. This is because the thin preform will not be collected and stored on a winding wheel, but will be immediately drawn into a hollow core optical fiber. Therefore, there is no need to provide a protective coating for the thin preform 36.

[0064] However, in this example, after passing around the first driving wheel 42, the thin preform 36 is not collected on the winding wheel for later processing into a hollow core optical fiber at some time. Instead, once the thin preform has acquired a length sufficient to extend from the outlet of the first drawing tower 20a (at its bottom) to the inlet of the second drawing tower 20b (at its top), the thin preform is transferred to the second driving wheel 43 above the furnace 34b of the second drawing tower 20b. An arrangement of feed control devices 47 (in the middle) that can include driving wheels, floating wheels, hoppers or the like between the driving wheels 42, 43 is provided. Figure 7), wherein the thin preform 36 passes through or around the driving wheels 42, 43 to handle any slack in the thin preform 36 caused by the disturbance of the driving wheel rotation, which may cause a change in the transient length of the thin preform 36 between the driving wheels 42, 43. Then, instead of Figure 6B The second driving wheel 43 serves as a feeding arrangement for the second drawing tower 20b. Therefore, the thin preform 36 passes over the second driving wheel 43 (possibly after one or more turns) and is guided to the furnace 34b of the second drawing tower 20b to heat the end of the thin preform 36. The rest of the second drawing tower 20b is constructed as follows: Figure 6B The fiber drawing tower 20a, 20b is provided with a first driving wheel 42b, so that the softened thin preform 36 is drawn into a hollow core optical fiber 50 in the furnace 34b via the second necking 54 and via the fiber diameter gauge 56 and the tension gauge 52 under tension from the fiber driving wheel 42b providing the drawing speed D2. After optionally receiving a protective coating at the coating device 60, the optical fiber 50 is finally collected on the winding wheel 62. The first driving wheel 42 and the second driving wheel rotate at appropriate speeds to maintain the first drawing speed D1 and the second feed speed F2 substantially equal to each other so that the thin preform 36 is smoothly and continuously transferred between the towers 20a, 20b (with the help of the optional feed control device 47). Note that in this example, the coating removal device 58 is not included above the furnace 34b because the thin preform has not been coated after drawing. However, in an alternative, a coating device for coating the thin preform 36 and a subsequent coating removal device may be included if preferred.

[0065] The length of the thin preform 36 extending from the first drawing tower 20a to the second drawing tower 20b (e.g., the length measured from the exit of the first furnace 34a to the entrance of the second furnace 34b) is the transient length of the thin preform as previously discussed. Figure 7 As can be readily appreciated, the proximal end of the thin preform 36 is continually replenished with new glass from the drawing of the initial preform 22, and the distal end of the thin preform 36 is continually consumed as its glass is drawn into the optical fiber 54. As the glass material constituting the thin preform travels along the transient length, the transient length remains substantially constant (except for transient changes caused by disturbances in the feed arrangement to be handled by the feed control device 47). Thus, in terms of glass, the transient length accumulates over time to provide the total length of the thin preform 36 available for drawing into the hollow core optical fiber 50. In this configuration, the entire initial preform 22 can be converted into a single continuous length of hollow core optical fiber 50 via the thin preform 36 in a single manufacturing process including two drawing stages.

[0066] and Figure 6BAs an example, during the process of drawing the hollow core optical fiber 50 from the thin preform 36, the pressurization is not directly applied to the void of the thin preform 36. There is no free end of the thin preform 36 at the second drawing tower 20b where the pressurization is applied (if necessary), but this is not a difficulty because the free end is not necessary. The pressurization applied to the void of the initial preform 22 in the first drawing tower has no measurable effect at the second constriction 54 because the length of the void in the thin preform is too long and the width of the void is too narrow so that the applied gas cannot travel to the second constriction in time to produce any significant or appreciable pressure at the location where the optical fiber is drawn.

[0067] from Figure 7 It will be appreciated that the transient length of the slender preform will be determined by the relative positions and physical arrangement of the first drawing tower 20a and the second drawing tower 20b. However, given typical sizes and heights of optical fiber drawing towers, and noting that the first drawing tower can be a full-size tower or a shorter tower, it is contemplated that a transient length in the range of approximately 1m to 100m can typically be utilized, although this is a purely exemplary range and the invention is not limited thereto. For example, if the first drawing tower and the second drawing tower are adjacent (laterally or laterally offset), the transient length can be on the order of 5m to 100m, taking into account the tower height, the horizontal distance from the first tower to the second tower, and the storage tanks that may be used between the drawing towers. If axially aligned towers are available with little or no lateral offset, the transient length can be 10m or less, such as approximately 1m to 5m.

[0068] Figure 8 A schematic diagram of a second example apparatus for processing a thin preform in a transient length form is shown. In this example, the apparatus includes only one optical fiber drawing tower 20, so that the overall drawing from the initial preform to the hollow-core optical fiber is performed along a single vertical drawing direction. However, the drawing tower 20 includes two annular drawing furnaces, namely an upper first furnace 34a and a lower second furnace 34b, wherein the first furnace 34a and the second furnace 34b are arranged to be axially aligned with each other along the drawing direction with an axial spacing therebetween, but without lateral deviation. The first furnace 34a is used to draw a thin preform from the initial preform, and the second furnace 34b is used to draw a hollow-core optical fiber from the thin preform. Figure 7 Compared to the exemplary lateral spacing arrangement, a different configuration is utilized to provide isolation of tension and pressurization between the two simultaneous draws.

[0069] There are many Figure 7The first and second drawing towers of the example have the same components and use the same reference numerals; therefore, a detailed description of these will not be repeated. The initial preform 22 constructed as described above is fed vertically into the first furnace 34a at a feed speed F, while a pressure difference is applied to the gap of the initial preform 22 by the booster system 32. The first furnace 34a serves to soften the glass at the end of the initial preform 22 so that the initial preform 22 can be drawn into a thin preform 36 via necking 38 after the thin preform diameter gauge 44 under the tensioning and stretching action of the belt puller 40 located below the first furnace 34a and above the second furnace 34b. The belt puller 40 provides tension for drawing the thin preform 36 from the initial preform 22, and isolates the drawing tension from the optical fiber drawing in the second furnace 34b. Likewise, the thin preform coating device 46 is not included because the thin preform is immediately drawn into a hollow core optical fiber and does not require a protective coating for storage.

[0070] As in Figure 7 In the example, when the thin preform 36 has acquired a length sufficient to extend from the outlet of the first furnace to the entrance of the second furnace, the thin preform is directly fed vertically into the second furnace to heat the end of the thin preform without any orientation or turning. The rest of the drawing tower 20 is constructed as follows: Figure 7 The second drawing tower from the second furnace 34b downwardly is provided so that, with the tension meter 52 providing a drawing speed D, the softened thin preform 36 is drawn into a hollow core optical fiber 50 in the second furnace 34b via the second necking 54 and via the optical fiber diameter gauge 56 under tension from the optical fiber driving wheel 42, and is collected on the winding wheel 62 after optionally receiving a protective coating at the coating device 60. In the arrangement shown, the puller 40 and the driving wheel 42 are operated to provide an increase in the drawing speed along the drawing direction, thereby maintaining a first drawing ratio at the first necking 38 and a second drawing ratio at the second necking 54 to achieve a final drawing speed D of the optical fiber 50. In order to isolate the two drawing tensions, and possibly allow different drawing speeds, additional tensioning elements (such as pulleys, floating wheels and / or hoppers, all known for handling optical fiber during drawing) can be included above the second furnace 34b to manage any slack, although this is not the same as the embodiment of the present invention. Figure 7 This is possible due to the narrow width and thus flexibility of the thin preform. Also, in this example, no coating removal device is included above the second furnace 34b, since the thin preform has not yet been coated after drawing.

[0071] The length of the thin preform 36 extending between the two constrictions 38, 54 (e.g., the length measured from the exit of the first furnace 34a to the entrance of the second furnace 34b) is the transient length of the thin preform as previously discussed. Figure 7 In the example, from Figure 8As can be readily appreciated, the proximal end of the thin preform 36 is continually replenished with new glass from the drawing of the initial preform 22, and the distal end of the thin preform 36 is continually consumed as its glass is drawn into the optical fiber 54. The transient length remains constant while the glass material constituting the thin preform travels along the transient length. Thus, the transient length accumulates over time in terms of glass to provide the total length of the thin preform 36 available for drawing into the hollow core optical fiber 50. In this configuration, the integral initial preform 22 can be converted into a single continuous length of hollow core optical fiber 50 via the thin preform 36 in a single manufacturing process including two draws arranged along a single draw direction.

[0072] from Figure 8 It will be appreciated that the transient length of the thin preform will be determined by the relative axial offset of the first furnace 34a and the second furnace 34b, and can be made shorter or longer by spacing the two furnaces apart by a smaller or larger amount. This can be determined at least in part by the vertical space available within the drawing tower 20 and the overall height that the drawing tower 20 can occupy. Typically, however, unless the available vertical space is large, it is expected that the maximum feasible transient length in this example will be less than Figure 7 Example. Given a typical height of an optical fiber drawing tower, it is contemplated that a transient length in the range of about 1 to 5 meters may typically be utilized, although this is a purely exemplary range and the invention is not limited thereto. Large industrial drawing towers can have considerable heights, so in some cases a transient length range of about 1 to 10 meters or 1 to 15 meters or more may be feasible. It is useful to select a minimum transient length that allows cooling of the thin preform 36 sufficient so that the ribbon puller 40 is not damaged by the hot glass, but in general, a shorter transient length may more conveniently reduce the required overall height of the drawing tower.

[0073] and Figure 6B and Figure 7As in the example, during the drawing of the hollow core optical fiber 50 from the thin preform 36, no pressurization is applied directly to the void of the thin preform 36. There is no free end of the thin preform 36 at the second drawing tower 20b where pressurization is applied (if necessary), but this is not a difficulty because the free end is not necessary. However, since the transient length is relatively short in this example, there may be a concern that the pressurization applied to the void of the initial preform 22 may have a measurable effect at the second constriction 54, causing unintended expansion of the internal geometry of the glass structure, which will not be offset by surface tension in the optical fiber drawing. To address this, it is proposed to take measures to isolate the lower drawing in the second constriction 54 from the pressurization applied to the initial preform 22 for the higher drawing in the first constriction 38. The initial preform 22 can be fed into the first furnace 34a at a feed rate F, and the gas applied from the pressurization system 32 travels along the voids of the initial preform to the first constriction 38, thereby allowing control of the microstructure in the thin preform 36. At the exit of the first furnace 34a, the thin preform 36 travels rapidly, so that the pressure applied to the initial preform is insufficient to cause the gas to flow at a speed that matches the speed of the stretched voids in the first constriction 38, so that the glass structure can "run through" the gas and reach the second furnace with little or no gas in the voids, so that there is no pressurization in the second constriction 54, or the pressurization can be negligible.

[0074] Further in this regard, and applicable to all examples, for a given width of hollow core fiber, a thinner preform allows faster feeding and drawing of the fiber, thereby giving better production efficiency due to a smaller draw ratio. However, higher feed speeds require higher furnace temperatures due to reduced residence time, and therefore the maximum temperature achievable with existing furnaces may become a limiting factor when choosing the width of the thin preform to be made. Accordingly, difficulties may be encountered when making the thin preform thinner or thicker. Therefore, the use of existing fiber drawing apparatus may impose parameters on the implementation of the methods described herein, and in order to maximize drawing efficiency and fiber output, new fiber drawing apparatus may be designed.

[0075] Although the methods described so far are performed in a conventional optical fiber drawing tower, the invention is not limited in this respect, and the heating and drawing of the glass structure may be performed using other preferred apparatus. However, the use of a drawing tower is particularly convenient given that these methods can increase optical fiber output so that large continuous lengths of hollow core optical fiber can be easily produced. In fact, the benefits of the proposed methods are that they can be easily implemented using conventional optical fiber drawing equipment with only minor modifications.

[0076] A first additional example provides a method for manufacturing a hollow-core optical fiber, the method comprising: providing a thin preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the thin preform being configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in a range of 0.5 mm to 5 mm and a total length that can be drawn into the hollow-core optical fiber; heating an end of the thin preform to soften the glass at the end; and drawing a length of hollow-core optical fiber from the softened glass of the thin preform, the width of the hollow-core optical fiber being less than the width of the thin preform.

[0077] In a first additional example, drawing the length of hollow core optical fiber can be performed without pressurizing the void space in the capillary preform during drawing of the hollow core optical fiber.

[0078] In a first additional example, the method may further include flowing a gas at one or more different pressures into the void of the slender preform to pre-pressurize the slender preform before heating the end of the slender preform.

[0079] In a first additional example, drawing the length of hollow core optical fiber can include drawing the hollow core optical fiber until a total length of the drawable capillary preform has been drawn into the hollow core optical fiber.

[0080] In a first additional example, a drawing ratio of a cross-sectional area of ​​the thin preform to a cross-sectional area of ​​the hollow core optical fiber may be in a range of 2 to 150.

[0081] In a first further example, the total length available for drawing may be 30 m or more.

[0082] In a first additional example, the method may also include providing a thin preform by the following steps: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the initial preform being configured to form a cross-sectional structure of the thin preform when the thin preform is drawn from the initial preform while pressurizing a void in the initial preform, the width of the initial preform being greater than the width of the thin preform; applying pressurization to the void in the initial preform; heating an end of the initial preform so as to soften the glass at the end of the initial preform; and drawing the thin preform from the softened glass of the initial preform while applying the pressurization.

[0083] In a first additional example, a drawing ratio of the cross-sectional area of ​​the initial preform to the cross-sectional area of ​​the thin preform may be in the range of 200 to 16,000.

[0084] In a first additional example, the method may further include: collecting the thin preform while it is being drawn by winding it on a winding wheel until the length of the thin preform equals the total length available for drawing; and separating the thin preform from the initial preform.

[0085] In a first additional example, the method may further include applying a protective coating to the thin preform before winding the thin preform onto a winding reel; and removing the protective coating from the thin preform before drawing the hollow core optical fiber from the thin preform.

[0086] In a first additional example, hollow-core optical fiber drawing from the thin preform may begin while the thin preform is integrated with the initial preform and when the thin preform has reached a transient length that is less than the total length available for drawing, and the thin preform may continue to be drawn during the hollow-core optical fiber drawing process such that the transient length is replenished from the initial preform until the transient length accumulated over the duration of the drawing is equal to the total length available for drawing.

[0087] In a first additional example, heating the end of the initial preform may be performed in a first furnace and heating the end of the thin preform may be performed in a second furnace that is laterally offset from the first furnace with respect to the drawing direction.

[0088] In a first further example, the transient length of the slender preform between the outlet of the first furnace and the inlet of the second furnace may be in the range of 1 meter to 1000 meters.

[0089] In a first additional example, the method may further include controlling tension of the thin preform between the first furnace and the second furnace so that tension for drawing the hollow core optical fiber from the thin preform is isolated from tension for drawing the thin preform from the initial preform.

[0090] In a first additional example, heating the end of the initial preform may be performed in a first furnace and heating the end of the thin preform may be performed in a second furnace that is axially aligned with the first furnace relative to the drawing direction.

[0091] In a first further example, the transient length of the slender preform between the outlet of the first furnace and the inlet of the second furnace may be in the range of 11 meters to 10 meters.

[0092] In a first additional example, the method may further include controlling tension of the thin preform between the first furnace and the second furnace so that tension for drawing the hollow core optical fiber from the thin preform is isolated from tension for drawing the thin preform from the initial preform.

[0093] In a first additional example, the method may further include surrounding the initial preform with a glass outer layer before heating the end of the initial preform so that the thin preform is drawn to fuse with the glass outer layer, the glass outer layer being used to form a jacket of the hollow core optical fiber.

[0094] In a first additional example, the cross-sectional structure of the thin preform can be constructed with geometric ratios that match those intended for hollow core fiber plus a compensation amount to balance the surface tension contraction of the softened glass of the thin preform during drawing of the hollow core fiber.

[0095] A second additional example provides a method for manufacturing a preform for a hollow-core optical fiber, the method comprising: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the initial preform being configured to form a cross-sectional structure capable of being drawn from the thin preform into a hollow-core optical fiber in a thin preform drawn from the initial preform, the cross-sectional structure of the hollow-core optical fiber comprising a hollow core surrounded by a plurality of voids defining a microstructured cladding, the initial preform having an initial preform width; applying an increased pressure to the voids in the initial preform; heating an end of the initial preform so as to soften the glass at the end of the initial preform; and drawing a thin preform from the softened glass of the initial preform while applying the increased pressure, the width of the thin preform being less than the width of the initial preform and being in the range of 0.5 mm to 5 mm.

[0096] In a second additional example, the method may further include drawing the thin preform to a length of at least 30 m while collecting the thin preform on a winding reel, and separating the thin preform from the initial preform.

[0097] A third additional example provides a thin preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the thin preform being configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in a range of 0.5 mm to 5 mm and a length of at least 30 m.

[0098] In a third additional example, a thin preform may be wound onto a winding reel.

[0099] In a third additional example, the slim preform may include an outer layer of glass that is fused to the slim preform to form a jacket of a hollow core optical fiber.

[0100] In a third additional example, the thin preform can include a protective coating that can be removed before the thin preform is drawn into a hollow core optical fiber.

[0101] A fourth additional example provides a hollow core optical fiber having a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the hollow core optical fiber having a length of at least 20 km.

[0102] The various embodiments described herein are presented only to help understand and teach the claimed features. These embodiments are provided only as representative samples of embodiments, rather than being exhaustive and / or exclusive. It should be understood that the advantages, embodiments, examples, functions, features, structures and / or other aspects described herein should not be considered as limitations on the scope of the invention defined by the claims or limitations on the equivalents of the claims, and other embodiments can be utilized and can be improved without departing from the scope of the claimed invention. In addition to what is specifically described herein, the various embodiments of the present invention may also suitably include appropriate combinations of disclosed elements, assemblies, features, parts, steps, means, etc., and may be composed of or substantially composed of appropriate combinations of disclosed elements, assemblies, features, parts, steps, means, etc. In addition, the present disclosure may include other inventions that are not currently claimed but may be claimed in the future.

[0103] References

[0104] [1]GT Jasion et al, "Fabrication of tubular anti-resonant hollow corefibers:modelling, draw dynamics and process optimization." Opt.Express vol.27,20567-20582(2019)

[0105] [2]Y Chen et al, "Multi-kilometer long, longitudinally uniform hollowcore photonic bandgap fibers for broadband low latency data transmission," IEEE JLT, 34, pp104-113, (2016)

[0106] [3]US11203547

[0107] [4]SF Gao et al, "Hollow-core conjoined-tube negative-curvature fiber with ultralow loss", Nat. Commun, 9, p2828 (2018)

[0108] [5]CN 111474628

[0109] [6]US 5596669

Claims

1. A method for manufacturing a hollow core optical fiber, the method comprising: providing a thin preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the thin preform being configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in the range of 0.5 mm to 5 mm and a total length that can be used to draw into the hollow-core optical fiber; heating an end portion of the slender preform to soften the glass at the end portion; and A certain length of hollow core optical fiber is drawn from the softened glass of the thin preform, wherein the width of the hollow core optical fiber is smaller than the width of the thin preform.

2. The method of claim 1, wherein drawing the length of hollow core optical fiber is performed without applying a pressurization to the void space in the thin preform during drawing the hollow core optical fiber.

3. The method according to claim 1 or 2, further comprising: Prior to heating the end of the slender preform, a gas is flowed into the void in the slender preform at one or more different pressures to pre-pressurize the slender preform.

4. The method of any one of claims 1 to 3, wherein drawing the length of hollow core optical fiber comprises: The hollow core optical fiber is drawn until the total length of the thin preform available for drawing has been drawn into the hollow core optical fiber.

5. The method according to any one of claims 1 to 4, wherein: The drawing ratio of the cross-sectional area of ​​the thin preform to the cross-sectional area of ​​the hollow core optical fiber is in the range of 2 to 150; and / or The total length available for drawing is 30 m or more.

6. The method according to any one of claims 1 to 5, further comprising providing the thin preform by: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the initial preform being configured to form a cross-sectional structure of the thin preform when the thin preform is drawn from the initial preform while pressurizing a void in the initial preform, the width of the initial preform being greater than the width of the thin preform; applying a pressurization to the void space in the initial preform; heating the end of the initial preform to soften the glass at the end of the initial preform; as well as The slender preform is drawn from the softened glass of the initial preform while the boost pressure is applied.

7. The method according to claim 6, further comprising: collecting the thin preform while drawing the thin preform by winding the thin preform on a reel until the length of the thin preform equals the total length available for drawing; and The thin preform is separated from the initial preform.

8. The method according to claim 7, further comprising: applying a protective coating to the thin preform before the thin preform is wound onto the winding wheel; as well as Before drawing the hollow core optical fiber from the thin preform, the protective coating is removed from the thin preform.

9. The method according to claim 6, wherein while the thin preform is integrated with the initial preform and when the thin preform has reached a transient length that is less than the total length available for drawing, the hollow-core optical fiber is started to be drawn from the thin preform, and the thin preform is continued to be drawn during the process of drawing the hollow-core optical fiber so that the transient length is replenished from the initial preform until the transient length accumulated during the duration of the drawing is equal to the total length available for drawing.

10. The method of claim 9, wherein heating the end of the initial preform is performed in a first furnace and heating the end of the slender preform is performed in a second furnace, the second furnace being laterally offset from the first furnace with respect to the drawing direction.

11. The method according to claim 10, further comprising: The tension of the thin preform between the first furnace and the second furnace is controlled so that the tension for drawing the hollow core optical fiber from the thin preform is isolated from the tension for drawing the thin preform from the initial preform.

12. The method of claim 9, wherein heating the end of the initial preform is performed in a first furnace and heating the end of the slender preform is performed in a second furnace, the second furnace being axially aligned with the first furnace with respect to the drawing direction.

13. The method according to claim 12, further comprising: The tension of the thin preform between the first furnace and the second furnace is controlled so that the tension for drawing the hollow core optical fiber from the thin preform is isolated from the tension for drawing the thin preform from the initial preform.

14. The method according to any one of claims 6 to 13, further comprising: The initial preform is surrounded by a glass outer layer before the end of the initial preform is heated, so that the thin preform is drawn to fuse with the glass outer layer, and the glass outer layer is used to form a jacket for the hollow core optical fiber.

15. A method according to any preceding claim, wherein the cross-sectional structure of the thin preform is configured with geometric ratios that match the geometric ratios intended for the hollow core fiber plus a compensation amount to balance the surface tension contraction of the softened glass of the thin preform during drawing of the hollow core fiber.

16. A method for manufacturing a preform for a hollow core optical fiber, the method comprising: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure of the initial preform being configured to form, in a thin preform drawn from the initial preform, a cross-sectional structure capable of being drawn from the thin preform into a hollow-core optical fiber, the cross-sectional structure of the hollow-core optical fiber comprising a hollow core surrounded by a plurality of voids defining a microstructured cladding, the initial preform having an initial preform width; applying a pressurization to the void space in the initial preform; heating the end of the initial preform to soften the glass at the end of the initial preform; as well as A thin preform is drawn from the softened glass of the initial preform while applying the increased pressure, the thin preform having a width smaller than that of the initial preform and in a range of 0.5 mm to 5 mm.

17. The method according to claim 16, comprising: The thin preform is drawn to a length of at least 30 m while being collected on a winding wheel, and the thin preform is separated from the initial preform.

18. A thin preform formed of glass and having a cross-sectional structure, wherein the cross-sectional structure of the thin preform is configured to form a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding in a hollow-core optical fiber drawn from the thin preform, the thin preform having a width in the range of 0.5 mm to 5 mm and a length of at least 30 m.

19. The slender preform according to claim 18, comprising: A glass outer layer, the glass outer layer is fused with the thin preform to form a sheath for the hollow core optical fiber; and / or A protective coating, which can be removed before drawing the thin preform into the hollow core optical fiber.

20. A hollow core optical fiber having a cross-sectional structure comprising a hollow core surrounded by a plurality of voids defining a microstructured cladding, the hollow core optical fiber having a length of at least 20 km.

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