Hollow-core optical fiber drawing method using modified preforms.

Abstract

A method of manufacturing a hollow-core optical fiber includes the steps of providing a thin preform formed from glass having a cross-sectional structure configured to form, in a hollow-core optical fiber drawn from the thin preform, a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the thin preform having a width in the range of 0.5 to 5 mm and a total length available for drawing into a hollow-core optical fiber; heating end portions of the thin preform to soften the glass therein; and drawing a length of hollow-core optical fiber from the softened glass of the thin preform, the hollow-core optical fiber having a width smaller than the width of the thin preform.

Description

[Technical Field]

[0001] The present invention relates to a method for drawing hollow-core optical fiber, which includes modifications compared to conventional methods. [Background technology]

[0002] Optical fibers traditionally have a solid-core design, including a solid glass core surrounded by a solid glass cladding with a lower refractive index than the core. Light is guided along the core by total internal reflection at the core-cladding boundary. Solid-core fibers are manufactured by a fiber-draw process, in which a short, rigid glass preform, with a cross-sectional structure matching the desired refractive index profile of the finished fiber but many times wider than the finished fiber, is lowered axially into a circular furnace, which heats and softens the glass. The softened glass is then pulled or drawn from the end of the preform under gravity or tension until it is narrowed to the width required for the finished fiber and wound onto a spool. In some processes, narrower canes several meters long are first drawn from the preform and separated from the preform for later drawing into fiber. The process of drawing the glass structure from preform to cane or fiber and from preform or cane to fiber to narrow the width is called drawdown. Fiber drawability can be described by a parameter known as the drawdown ratio, which is the ratio of the width or cross-sectional area of the preform to the width or cross-sectional area of the finished fiber. Another related parameter is known as the fiber yield ratio, which is the number of meters of fiber that can be drawn per meter of preform. A high value is desirable for both obtaining long lengths of fiber and improving fiber manufacturing efficiency. Commercially, the total continuous length of fiber that can be produced in a single draw has increased over the decades from tens of kilometers to as much as 10,000 km. This increase has been achieved in large part by increasing the size of the preform, so that a larger initial volume of glass is available for conversion into fiber. Preform cross-sectional area has increased approximately 30-fold, modestly increasing preform length to approximately 3 meters and correspondingly increasing preform width. Thus, both the drawdown ratio and the fiber yield ratio have increased significantly, which is achieved by significantly increasing preform width while keeping preform length relatively short.

[0003] Hollow-core optical fibers have been developed in recent years. These have a central longitudinal hole or void that defines the hollow core, surrounded by a cladding that typically contains multiple smaller longitudinal holes or voids of a specific geometry, sometimes 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. Hollow-core fibers are attractive due to their superior light propagation characteristics compared to solid-core fibers, including reduced light propagation losses (attenuation), increased propagation velocity, greater optical bandwidth, and reduced parasitic nonlinear optical effects, resulting from the large proportion of air inside the fiber and the corresponding reduced amount of glass. Light propagates primarily through air, thereby avoiding the deleterious effects that result from propagation through glass. Hollow-core fibers can also be made by a fiber-drawing method starting with a preform (optionally drawn into a cane) formed with the desired void cross-sectional profile. To achieve the desired geometric configuration of the finished fiber's core and cladding (which must be precise in terms of location, void size, and void glass film thickness) for the purpose of obtaining the desired optical properties of a particular fiber design, pressure is applied to the voids during drawing of the fiber from the preform or cane. The applied pressure counteracts the surface tension of the softened glass, which would otherwise lead to collapse of the voids and destruction of the intended structure.

[0004] Fiber yield ratio and drawdown ratio are also important for drawing hollow-core fiber. Similar to solid-core fiber, a larger hollow-core preform overall contains a larger volume of glass and should be expected to yield more fiber. For example, the preform width may be increased [1], thereby requiring a higher drawdown ratio to produce fiber of the same width. However, when drawing a wide preform into a narrow fiber with a large drawdown ratio, the dynamics of pressure application to the hollow-core fiber are more complex and difficult to control because the variations in the outer diameter and diameter of the hole become more extreme. Therefore, high drawdown ratios are generally not suitable, or even unachievable, for hollow-core fiber production. Therefore, narrower preforms and correspondingly lower drawdown ratios, which result in lower yield ratios and corresponding inefficiencies in fiber production, are generally required for successful hollow-core fiber production. However, pressing with narrower preforms can be more difficult to implement because the air gaps become smaller and more difficult to access, which can reduce the structural quality of the finished fiber. Therefore, several factors hinder efficient, high-volume manufacturing of high-quality hollow-core optical fibers. While reported single spans of hollow-core fiber with reasonable propagation loss values recognized by the inventors are relatively short, at 12 km (4–5 dB / km loss) [2] and 14 km (10 dB / km loss) [3], shorter lengths of 10 km or less are more common with current hollow-core fiber drawing methods. Therefore, the cost of hollow-core fiber is high, and the maximum achievable fiber length is significantly limited compared to solid-core fiber. These factors are particularly unfavorable in light of the excellent optical properties that make hollow-core fiber so attractive for many applications, including long-distance communications, where very long, inexpensive fiber is desirable. Thus, there are a range of difficulties in producing hollow-core optical fibers that tend to limit achievable fiber length, reduce efficiency, and increase complexity and cost.

[0005] Therefore, approaches to improve the manufacturing of hollow-core optical fibers are of interest. Summary of the Invention

[0006] Aspects and embodiments are set out in the accompanying claims. According to a first aspect of certain embodiments described herein, there is provided a method of manufacturing a hollow-core optical fiber, the method comprising the steps of: providing a thin preform formed from glass having a cross-sectional structure configured to form, in a hollow-core optical fiber drawn from the thin preform, a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the thin preform having a width in the range of 0.5 mm to 5 mm and a total length available for drawing into a hollow-core optical fiber; heating end portions of the thin preform to soften the glass therein; and drawing a length of hollow-core optical fiber from the softened glass of the thin preform, the hollow-core optical fiber having a width smaller than the width of the thin preform.

[0007] According to a second aspect of certain embodiments described herein, there is provided a method of manufacturing a preform for a hollow-core optical fiber, the method comprising the steps of: providing an initial preform formed from glass having a cross-sectional structure configured to form, when drawn from the initial preform, a cross-sectional structure capable of being drawn into a hollow-core optical fiber having a cross-sectional structure including a hollow core surrounded by a plurality of voids that define a microstructured cladding, the initial preform having an initial preform width; applying pressure to the voids within the initial preform; heating end portions of the initial preform to soften the glass therein; and drawing a thin preform from the softened glass of the initial preform while under pressure, the thin preform having a width smaller than the width of the initial preform and in the range of 0.5 mm to 5 mm.

[0008] According to a third aspect of certain embodiments described herein, there is provided a thin preform formed from glass having a cross-sectional structure configured to form in a hollow-core optical fiber drawn from the thin preform a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the thin preform having a width in the range of 0.5 mm to 5 mm and a length of at least 30 m.

[0009] According to a fourth aspect of certain embodiments described herein, there is provided 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.

[0010] These and further aspects of certain embodiments are set forth in the accompanying independent and dependent claims. It will be understood that features of the dependent claims may be combined with each other and with features of the independent claims in combinations other than those explicitly set forth in the claims. Furthermore, the approaches described herein are not limited to the specific embodiments as set forth below, but include and contemplate any suitable combination of features presented herein. For example, methods and apparatus according to the approaches described herein may be provided that include any one or more of the various features described below, as appropriate.

[0011] For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: [Brief explanation of the drawings]

[0012] [Figure 1] 1A and 1B show cross-sectional views of a first example hollow-core optical fiber that may be manufactured according to exemplary methods of the present disclosure. [Figure 2] FIG. 2 shows a cross-sectional view of a second example hollow-core optical fiber that may be manufactured according to an exemplary method of the present disclosure. [Figure 3] FIG. 2 shows a cross-sectional view of a third example hollow-core optical fiber that may be manufactured according to an exemplary method of the present disclosure. [Figure 4] FIG. 2 illustrates a flowchart of steps in an exemplary hollow-core optical fiber drawing method, according to aspects of the present disclosure. [Figure 5] FIG. 1 illustrates a flowchart of steps in an exemplary thin preform drawing method, according to aspects of the present disclosure. [Figure 6]Figure 6A shows a schematic diagram of an exemplary apparatus suitable for carrying out an exemplary method of manufacturing a hollow core optical fiber according to the following disclosure, in which a narrow preform is provided, according to a first alternative. Figure 6B shows a schematic diagram of an exemplary apparatus suitable for carrying out an exemplary method of manufacturing a hollow core optical fiber according to the following disclosure, in which a narrow preform is provided, according to a first alternative. [Figure 7] FIG. 1 shows a schematic diagram of an exemplary apparatus suitable for carrying out an exemplary method for manufacturing a hollow-core optical fiber according to the following disclosure in which a narrow preform is provided according to a second alternative embodiment. [Figure 8] FIG. 10 shows a schematic diagram of an exemplary apparatus suitable for carrying out an exemplary method for manufacturing a hollow-core optical fiber according to the following disclosure in which a narrow preform is provided according to a third alternative. DETAILED DESCRIPTION OF THE INVENTION

[0013] Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented in a conventional manner, and for purposes of brevity, they are not discussed / described in detail. Thus, it will be understood that aspects and features of the devices and methods discussed herein that are not described in detail can be implemented by conventional techniques for implementing such aspects and features.

[0014] The present disclosure proposes a method for the manufacture of hollow-core optical fibers that aims to address several problems with known methods in order to increase production and improve fiber yield.

[0015] A hollow-core optical fiber has a core in which light is guided, the core comprising a central longitudinal hole or void (typically filled with air, but alternatively filled with another gas or mixture of gases or vacuum) surrounded by a cladding comprising a structured arrangement of longitudinal holes or voids or capillaries extending along the fiber length (microstructure). The absence of a solid glass core reduces the proportion of guided optical waves propagating within the glass compared to that in solid-core fibers, resulting in benefits such as increased propagation velocity, 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, including optical communication systems. However, it is currently very difficult to fabricate the long hollow-core optical fibers desired for communication systems where data centers between which optical data is transmitted are separated by tens or hundreds of kilometers. Lengths of only about 10 km are typically achievable.

[0016] Hollow-core optical fibers are sometimes classified into two main classes or types depending on their optical guidance mechanism: hollow-core photonic bandgap fiber (HCPBF, often alternatively called hollow-core photonic crystal fiber (HCPCF)), and antiresonant hollow-core fiber (AR-HCF, or ARF). There are various subcategories of ARFs characterized by their geometry, including kagome fiber, nested antiresonant nodeless fiber (NANF), and ring fiber. This disclosure is applicable to all types of hollow-core fiber, including these two main classes and their associated subtypes, as well as other hollow-core designs. It should be noted that there is some overlapping use of terminology for various classes of fiber in the art. In this disclosure, the terms "hollow-core optical fiber" and "hollow-core fiber" are used interchangeably and are intended to cover all types of these fibers having hollow cores as described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow-core fibers having a structure that achieves waveguiding through the photonic bandgap effect (described in more detail below). The terms "ARF" and "anti-resonant hollow-core fiber" are used to refer to hollow-core fibers having a structure that achieves waveguiding through the anti-resonance effect (described in more detail below).

[0017] FIG. 1 shows a schematic cross-sectional view of an exemplary HCPBF 10. In this fiber type, a structured inner cladding 1 includes a substantially regular, closely packed array of many small glass capillaries, from which a central group is excluded to define a substantially circular hollow core 2. The periodicity of the cladding structure results in a periodically structured refractive index, and therefore a photonic bandgap effect, that confines optical waves propagating toward the core. This is the fundamental optical mode (FM). One or more higher-order optical modes (HOMs) may be supported in the core 2 and / or cladding 1. These fibers can be described by the number of cladding capillaries or "cells" excluded to create the core 2. In the example of FIG. 1, the central 19 cells from the array are absent in the core region, making this a 19-cell core HCPBF. The structured cladding 1 is formed from six cell rings surrounding the core 2, with a seventh ring of cells added to improve the roundness of the cladding's outer surface. An outer cladding or jacket 3 surrounds the structured cladding 1.

[0018] In contrast to HCPBFs, antiresonant hollow-core fibers guide light via antiresonant optical guidance. The structured cladding of ARFs has a simpler configuration, including a much smaller number of larger glass capillaries or tubes than HCPBFs, providing a structure without a high degree of periodicity so that photonic bandgap effects are not significant. Rather, antiresonance is achieved for propagation wavelengths that are not resonant with the cladding capillary wall thickness, i.e., within the antiresonance window defined by the cladding capillary wall thickness. The cladding capillaries surround a central void or cavity that provides the fiber's hollow core, which can accommodate antiresonant guided optical modes, including the fundamental mode and one or more higher-order modes. The structured cladding can also accommodate cladding modes that can propagate primarily within the capillary, within the glass of the capillary wall, or within the space or gap between the cladding capillary and the fiber's outer cladding. The loss of these additional, higher-order, non-core-guided modes is generally much higher than the loss of the core-guided modes. The core-guided fundamental mode typically has by far the lowest loss of all core-guided modes: the anti-resonance achieved by the capillary wall thickness being anti-resonant with the wavelength of the propagating light acts to prevent coupling between the fundamental core mode and any of the cladding modes, so that the light is confined to the core and can propagate with extremely low loss.

[0019] FIG. 2 shows a schematic cross-sectional view of an exemplary simple anti-resonant hollow-core fiber. The fiber 10 has an outer tubular cladding or jacket 3. The structured inner cladding 1 includes multiple tubular cladding capillaries 14—in this example, seven capillaries of the same cross-sectional size and shape—disposed within the single-annular 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 (is bonded to) the inner surface of the outer cladding 3 at an azimuthal location 16 so that the cladding capillaries 14 are evenly spaced around the inner circumference of the outer cladding 3 and are spaced apart from each other by a gap 5 (so that adjacent capillaries do not touch). In some designs of ARF, the cladding tubes 14 may be positioned in contact with one another (i.e., rather than spaced apart as in FIG. 2), but spacing them out to eliminate this contact improves the optical performance of the fiber and is generally preferred. Spacing 5 eliminates nodes that occur at contact points between adjacent tubes and tend to cause undesirable resonances that result in high loss. Thus, fibers including spaced-apart cladding capillaries are sometimes referred to as "nodeless anti-resonant hollow-core fibers."

[0020] The formation of the cladding capillaries 14 within the annulus around the inside of the tubular outer cladding 3 creates a central space, cavity, or void within the fiber 10 that is the hollow core 2 of the fiber, further having a longitudinal axis parallel to the longitudinal axes of the outer cladding 3 and the capillaries 14. The core 2 is bounded by the inward-facing portion of the outer surface of the cladding capillaries 14. This is the core boundary, and the capillary wall material (e.g., glass or polymer) that makes up this boundary provides the required anti-resonant optical guiding effect or mechanism. The capillaries 14 have a thickness t at the core boundary that defines the wavelength bandwidth over which anti-resonant optical guiding occurs within the ARF.

[0021] FIG. 2 shows just one example of an ARF; many other ARF structures are known. FIG. 3 shows a schematic cross-sectional view of a second exemplary ARF. The ARF has a structured inner cladding 1 including six cladding capillaries 14 evenly spaced around the inner surface of a tubular outer cladding 3 and surrounding a hollow core 2. Each cladding capillary 14 has a secondary, smaller capillary 18 nested within it and bonded to the inner surface of the cladding capillary 14, in this example at the same azimuthal location 16 as the point of bond between the primary capillary 14 and the outer cladding 3. These additional smaller capillaries 18 can reduce optical losses. Additionally, even smaller tertiary capillaries may be nested within the secondary capillaries 18. This type of ARF design, which includes secondary capillaries and optionally further smaller capillaries, is sometimes referred to as a "nested antiresonant nodeless fiber," or NANF, where "double nested antiresonant nodeless fiber," or DNANF, refers to a fiber that includes tertiary nested capillaries.

[0022] Many other capillary configurations for the structured cladding of an ARF are feasible, and the present disclosure is not limited to the above examples. For example, the capillaries need not be of 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 can 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 having a shape with a series of adjacent (i.e., convex from the perspective of the core) inner curved surfaces. This contrasts with the typical outward curvature of the core-cladding interface of a conventional solid-core fiber and the substantially circular core boundary of an HCPBF (see Figure 1). Thus, antiresonant hollow-core fibers may be described as negative curvature fibers. The kagome category of ARFs may also be configured as negative curvature fibers, having a structured cladding of multiple small capillaries in an array, similar to HCPBFs but not configured to achieve a photonic bandgap. In contrast to HCPBFs, the induction mechanism works via the anti-resonance effect. Other examples of ARFs include designs with cladding formed as coupled tube structures [4] and hemispherical tube structures [5].

[0023] As used herein, the terms hollow-core optical fiber, hollow-core fiber, hollow-core waveguide, hollow-core optical waveguide, and similar terms are intended to cover optical waveguiding structures constructed according to any of the above examples and similar structures, where light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonance guiding, and / or blocked-coupling guiding) within a hollow, elongated void or core surrounded by a structured cladding containing a plurality of longitudinal capillaries. The capillaries include or define elongated holes, voids, lumens, cells, or cavities running continuously along the length or longitudinal extent of the optical fiber, substantially parallel to the elongated core, which also extends continuously along the length of the fiber. These various terms may be used interchangeably in this disclosure.

[0024] The hollow-core fiber manufacturing method proposed herein is applicable to all and any type of hollow-core optical fiber, as discussed above. The proposed method can improve hollow-core fiber manufacturing yields for all fiber types, enabling orders of magnitude improvements in yield and providing continuous lengths of hollow-core fiber in excess of 100 km. This will enable the commercial adoption of hollow-core fiber for new applications, particularly improved optical fiber communication links, enabling, for example, transoceanic hollow-core optical fiber links. Currently, 100 km long links are considered desirable, but must be assembled from multiple shorter lengths of hollow-core fiber, which requires significant resources.

[0025] The proposed method utilizes modifications to the fiber drawing process that enable hollow-core fiber fabrication with significantly improved production yields for continuous lengths of fiber than can be achieved using conventional methods.

[0026] The proposed method is modified compared to conventional drawing processes by the introduction of a modified narrower preform, having a width intermediate between that of a conventional hollow-core preform and that of a finished fiber. The narrow or narrow preform is smaller in width than a conventional cane and, unlike a cane, is not separated by a short length from the preform from which it is drawn. Also unlike conventional canes, the narrow preform can be drawn into fiber without applying pressure to its cavity during drawing. The narrow width of the narrow preform results in a relatively small drawdown ratio from preform to fiber, thereby avoiding the need for dedicated pressurization. Furthermore, because the narrow preform has a very long total length and therefore a large total glass volume, significantly increased continuous lengths of fiber can be drawn therefrom. Thus, a high overall yield of fiber is possible despite a low yield ratio and drawdown ratio from the narrow preform to fiber (due to the long length and small width of the preform). The long length of the slim preform may be generated entirely from an initial preform before fiber drawing, or the long length of the slim preform may be generated simultaneously with fiber drawing such that the slim preform has a transitional length that is replenished at a first end (which in some configurations may be the top end) by drawing from the initial preform at the same time that it is consumed at a second end (which in some configurations may be the bottom end) by being drawn into fiber, with the transitional length accumulating over time resulting in a long total length.

[0027] The absence of pressurized air is not only advantageous, but also addresses a specific problem that limits the ability to draw hollow optical fiber from conventional preforms through relatively large drawdown ratios. As discussed above, current hollow-core optical fiber designs, including HCPBF and ARF, require the application of pressure to voids within the preform or cane during fiber drawing. Pressurization is often based on a pressure differential, where two or more different pressures are applied to different voids. Pressure is necessary to counteract the surface tension of the softened glass during drawing, which acts to collapse the voids and, if unchecked, alter or destroy the intended fiber structure. Adjustment of the pressure(s) allows for precise control of the final fiber geometry. During drawing, the glass structure undergoes a shape change called neckdown. This is the narrowing of a glass structure, in which the glass structure is softened and plasticized in a draw furnace (to make it pliable or stretchable) and its width continuously decreases along the length of the structure, from the width of the preform or cane to the intended width of the fiber. When pressure is applied at the wider end of the neckdown (the opening, undrawn end of the preform or cane, and typically closest to the pressure input at the top, since fiber drawing is usually vertical), the geometry is dictated by the applied pressure. At the narrower end of the neckdown, where the fiber forms, the geometry is dictated by the surface tension of the softened glass, which causes capillary contraction. Thus, there is an interaction between the forces of applied pressure and surface tension, and to achieve the desired capillary size and film thickness, sufficient pressure must be applied so that the capillary initially expands beyond the required size, so that subsequent contraction under surface tension at the bottom of the neckdown will pull the capillary back to the target size.

[0028] Attempting to improve the fiber yield ratio of a draw by increasing the drawdown ratio using wider preforms or canes so that more fiber can be produced from them increases the complexity of the pressure-surface tension relationship, making it more difficult to identify corrective parameters for drawing. Eventually, the contraction phase of the draw dynamics becomes so intense that increased capillary expansion under higher applied pressures is required to counter it. However, there is limited room within the glass structure for capillary expansion, and the expansion overshoot required to avoid contraction reaches a point where unacceptable deformation of the fiber structure occurs. Particularly in ARF designs, where capillaries are required to be spaced apart, contact between adjacent capillaries can occur, causing them to fuse together. In HCPBF designs, significant hollow core asymmetry can occur. Such defects are highly detrimental to the optical properties and performance of the finished fiber and must be avoided. For convenience, this phenomenon is referred to as mid-draw contact (MDC) and is a significant limiting factor when trying to improve the fiber yield ratio for hollow-core fiber production. If the proposed drawdown ratio from preform / cane to fiber is greater than the limit imposed by MDC, the desired fiber cannot be produced.

[0029] As a prelude to discussing the present invention in detail, several terms related to fiber drawing will be discussed and defined. In optical fiber manufacturing, a preceding step is the formation of a relatively large cylindrical glass structure called a preform. In the case of hollow-core optical fiber, the preform has a cross-sectional structure containing glass elements that, once drawn, will form the structural elements required for the finished fiber, where the melting, expansion, and contraction of the glass elements that occur during drawing are taken into account in the preform construction so that the final fiber structure is as intended. A preform is made by assembling its constituent glass elements in the appropriate arrangement. In the case of hollow-core fiber, a preform is typically made by stacking multiple capillary tubes that will define the fiber cladding inside an outer glass tube that supports the cladding tube, sometimes using spacer elements to hold the capillary tubes in place. Bonding elements may be used to secure the capillary tubes in place relative to the outer tube, or bonding may be done as part of the drawing process. The preform may be drawn directly into the desired fiber. Alternatively, in conventional methods, a preform may be drawn into one or more separate canes, for example, having a width intermediate between the preform and the intended fiber and a length of approximately one meter. Each cane may then be drawn into a separate length of the intended fiber. A cane may be easier to handle than a preform and may also reduce the drawdown ratio for drawing into fiber. However, the terms preform and cane tend to be used somewhat interchangeably, so some clarification is necessary in this context. A preform may be considered any glass structure that can be drawn into a narrower structure (it is a precursor to the form of the narrower structure). Under this definition, a preform made by stacking or other assembly of glass elements to be drawn into cane or fiber constitutes a preform, but similarly, a cane may be considered a preform in that it can be drawn into a narrower cane or into fiber.A cane is similarly a glass structure that can be drawn into a narrower structure (narrower cane or fiber), but is formed by drawing from a wider glass structure rather than by assembling separate elements. The presently proposed method uses an intermediate glass structure of width intermediate between the width of a preform and the width of a finished fiber, but it has several attributes that distinguish it from a conventional cane. Therefore, the term "cane" 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, with the understanding that the initial preform can include a preform made by assembling glass elements into the required structure, or a cane drawn from such a preform. The term "narrow preform" will be used to refer to an intermediate-width glass structure that is drawn from an initial preform and further drawn into fiber, with the descriptor "narrow" indicating a reduced width compared to a conventional preform or cane drawn directly into fiber. Narrow preforms are also longer compared to a conventional preform or cane drawn directly into fiber. In various examples of the proposed method, long lengths may be produced in a single step prior to drawing fiber from a narrow preform, or may be accumulated or replenished "in situ" at the same time as fiber is drawn from the narrow preform. The term "fiber" is conventionally used to refer to optical fiber drawn from a narrow preform. The method includes drawing fiber from a narrow preform and may additionally include drawing a narrow preform from an initial preform.

[0030] The initial preform has an initial preform width or diameter (first width) and an initial preform cross-sectional area (first cross-sectional area), the narrow preform has a narrow preform width or diameter (second width) smaller than the initial preform width and a narrow preform cross-sectional area (second cross-sectional area) smaller than the initial preform cross-sectional area, and the fiber has a fiber width or diameter (third width) smaller than the narrow preform width and a fiber cross-sectional area (third cross-sectional area) smaller than the narrow preform cross-sectional area. The production of fiber by drawing from the narrow preform has a draw-down ratio that can be defined as the ratio of the narrow preform width to the fiber width or the ratio of the narrow preform cross-sectional area to the fiber cross-sectional area. The production of narrow preforms from the initial preform has a draw-down ratio that is the ratio of the initial preform width to the narrow preform width or the ratio of the initial preform cross-sectional area to the initial preform cross-sectional area. The total drawdown ratio can be the ratio of the initial preform width to the fiber width, or the ratio of the initial preform cross-sectional area to the fiber cross-sectional area. When calculating the drawdown ratio, the width and cross-sectional area in question are the outer diameter and area of the fiber cladding, or the outer diameter and area of the glass elements of the initial preform and narrow preform that will form the fiber cladding, excluding any supporting and / or protective layers around the cladding.

[0031] 4 shows a flowchart of the steps of an exemplary method for manufacturing a hollow-core optical fiber according to the present disclosure. In a first step S10, a narrow preform made of glass is provided from which a hollow-core optical fiber is to be drawn. As described above, the narrow preform has a cross-sectional structure that is configured to produce the required cross-sectional structure of a desired hollow-core fiber type and design after the narrow preform is drawn into a fiber of a desired width. The narrow preform structure is configured so that a hollow-core fiber can be drawn therefrom without the need to apply pressure to the narrow preform's voids during fiber drawing. This can be achieved if a relatively small drawdown ratio is used to draw the fiber, and therefore the narrow preform is required to be thin, such that its thinness can be understood relative to the typical width of a preform or cane from which a hollow-core fiber is directly drawn in conventional processes; however, as described above, pressure is required to compensate for the surface tension contraction of the softened glass. When drawing a thin preform into fiber at a low drawdown ratio, the surface tension contraction may not be as severe and no pressure may be needed to balance it. Therefore, there is no expansion of the glass structure elements caused by applied pressure, and the risk of MDC is avoided. However, in a thin preform, some shrinkage occurs during fiber drawing because surface tension will still be present in the molten glass. This can be addressed by appropriate configuration of the thin preform's internal structure to reach the target dimensions of the fiber's internal structure. In the complete absence of forces from applied pressure (which causes expansion) and surface tension (which causes shrinkage), the drawn hollow-core fiber will only reduce in size by an amount that can be controlled by fiber-draw parameters such as speed and tension, and should then maintain the proportions and geometry of the drawn preform's structure. However, in this case, some surface tension will be present. To address this, the thin preform can be appropriately structured so that the dimensions of the portions that would shrink under surface tension are proportionately larger to offset the shrinkage.Thus, the narrow preform can have a cross-sectional structure with geometric ratios of the various elements corresponding to the intended geometric ratio of the hollow-core fiber, plus a compensation amount or amounts sized to balance the surface tension contraction of the softened glass of the narrow preform during drawing of the hollow-core fiber (where different elements may require different compensation amounts when experiencing different surface tensions due to different diameters or film thicknesses, etc.). Compensation may be required by only a few percent in size due to the small draw-down ratio used to draw the fiber. Thus, some or all internal elements (such as hollow tubes or capillaries) may exceed the size required to match the geometric ratio of the hollow-core fiber by, for example, 10% or less, 5% or less, or 3% or less.

[0032] The thin preforms have widths in the range of 0.5 mm to 5 mm. In some situations, a larger minimum width may be more suitable, such as easier handling of the thin preforms, such as alternatively ranging from 1 mm to 5 mm. Similarly, a smaller maximum width may be more suitable in other situations, such as avoiding mid-draw contact or the requirement to bend or roll the thin preforms, such as alternatively ranging from 0.5 mm to 3.5 mm or 0.5 mm to 2.5 mm. Overall, therefore, a smaller range of thin preform widths, approximately 1 mm to 2.5 mm, may be defined according to the requirements of a particular manufacturing arrangement. These sizes are significantly narrower than conventional preforms, which typically have widths on the order of tens of millimeters, for drawing into optical fiber. The narrow width of the thin preforms allows for the elimination of pressure when drawing fiber from the thin preforms and also allows for different handling of the thin preforms before they are drawn into fiber, as will be discussed below. When drawn into conventional-width fiber, this range of narrow preform widths can result in fiber lengths of several meters to hundreds of meters, e.g., between 4 m and 500 m from a 1-meter narrow preform. Total lengths of the narrow preform on the order of 10 km are envisioned, with the potential to reach total fiber lengths of hundreds or thousands of kilometers. However, the invention is not limited in this manner; shorter fibers may be useful and required in some cases, and longer lengths may be achievable depending on the parameters of the initial preform, narrow preform, and hollow-core fiber, as well as the operating parameters of the drawing equipment used. Indeed, it is expected that minimum hollow-core fiber lengths of at least 20 km are routinely available, with lengths of 30 km or more, 50 km or more, and 100 km or more readily achievable.

[0033] The thin preform has a length, including the total length available for drawing. The total length is very long compared to the lengths of conventional preforms and canes. When combined with the narrow width of the thin preform, a large total volume of glass is available in the thin preform, which allows for extremely long lengths of fiber to be produced from the thin preform in a single draw, despite a small drawdown ratio and a low fiber yield ratio. The length of the thin preform is defined as the total length available for drawing into a hollow-core fiber and can be provided in various ways. In some examples, the total length can be provided as a single span of the thin preform, such that the actual length of the thin preform equals the total length available for drawing into fiber before the fiber draw begins. In other examples, fiber drawing begins at the distal end of the thin preform, while the thin preform continues to be drawn from the initial preform at the proximal end before the thin preform is produced to the total length available for drawing (proximal and distal are defined relative to the draw direction). In this case, the thin preform has a transitional length between its proximal end, where the glass is being replenished from the initial preform, and its distal end, where the glass is being consumed by the fiber. The transitional length may be much smaller than the total length available for draw, but over the duration of the draw process, the glass available for fiber draw at the transitional length accumulates to correspond to the total length. The total length of the thin preform available for drawing into a hollow-core fiber can be as long as convenient, depending on factors such as the required total length of the hollow-core fiber, the available run time of the draw equipment, and the size of the initial preform from which the thin preform is drawn. However, to realize useful improvements beyond the current limits of hollow-core fiber draw yields, the thin preform can have a total length available for drawing that is 30 m or more. However, shorter lengths between about 10 m and 30 m are not excluded, but more importantly, much greater extensions may be implemented to realize significant expansions in hollow-core fiber production volumes.For high-yield commercial fiber production, for example, the slender preform may have a length of at least 30 m, at least 100 m, at least 1000 m (1 km), at least 5000 m (5 km), or at least 50,000 m (50 km). The upper length limit will be governed largely by practical considerations of handling (such as draw furnace size and speed of feeding and winding equipment) and production time, rather than by technological limitations. It is envisioned that readily producible and manageable lengths of slender preforms may range, for example, from 30 to 50 km. However, the invention is not limited in this manner, and other total lengths are not excluded.

[0034] In step S10, the narrow preform may be provided by being obtained from elsewhere, by being separately manufactured to its full usable length earlier in the manufacturing process, or via a transitional length, these alternatives being described in more detail below.

[0035] The method proceeds to step S11, where the end portion of the narrow preform is heated to soften the glass, for example, in a fiber draw tower furnace in a conventional manner. In the next step S12, a hollow-core fiber is drawn from the initial preform, the hollow-core fiber having a width smaller than that of the narrow preform. Typical widths of hollow-core fibers range from 100 to 500 μm. This results in a drawdown ratio for drawing hollow-core fiber from the narrow preform, which is defined by the ratio of the narrow preform cross-sectional area to the hollow-core fiber cross-sectional area and is smaller than traditionally used for drawing hollow-core fiber, ranging from about 4 to 500 μm, or about 2 to 150 μm. As discussed above, the low drawdown ratio allows the fiber to be drawn without the need to apply pressure to the void in the narrow preform. This contrasts with the typical pressure differential applied when drawing hollow-core fiber from a preform or cane. Thus, the fiber draw can be performed without pressure being applied to the voids of the narrow preform. However, the method is not limited to this approach, and pressure may be applied to the voids of the narrow preform if preferred in some circumstances. This may be during the draw process. Alternatively, a pre-pressurization step may be performed in which one or more gases at various pressures are flowed into the voids of the narrow preform, thereby pressurizing the entire narrow preform with a predefined set of pressures for the various voids before heating and fiber draw. Thus, while it is not possible to adjust the pressure or vary any differentials during the draw, modifications of temperature and tension during the draw can be performed to generally change the impact of the pre-applied pressure and thus control the internal structure of the fiber.

[0036] Any required length of hollow-core fiber can be drawn from the thin preform within the maximum limit of the glass available from the thin preform. However, for maximum efficiency, it is useful to draw successive lengths of hollow-core fiber until the available glass in the thin preform is exhausted and the thin preform is completely consumed. Thus, the method can proceed to decision step S13, where it is determined whether the total length of the thin preform available for drawing has been drawn into hollow-core fiber. If yes, the thin preform has been used up and there is nothing left of the total length, and the method proceeds to step S14, where fiber drawing is terminated and the fiber can be separated from the remaining waste portion of the thin preform that cannot be made into fiber and is therefore not part of the total available length (as is common in fiber manufacturing, a certain amount of waste glass is unavoidable, such as neckdown between two widths of a glass structure). If no, and the thin preform is not yet completely consumed, the method can return to step S11, where the thin preform is gradually fed into the furnace for heating as usual, while the end portion of the thin preform continues to be heated and drawn into fiber.

[0037] FIG. 5 shows a flowchart of exemplary method steps for providing a thin preform of step S10 of the method of FIG. 4. As noted above, the thin preform can be provided in several ways. In situations where the thin preform is obtained other than passively from another source, the method steps of FIG. 5 can precede the method steps of FIG. 4. In a first step S1, an initial preform made of glass and having a cross-sectional structure configured to be drawn into a thin preform is provided, thereby providing a thin preform having a cross-sectional structure for forming a final hollow-core fiber, as described above. The initial preform may be a conventional preform, such as a glass structure formed by stacking a tube and, optionally, other glass elements, such as spacers inside an outer glass tube, as previously described, or the initial preform may be a cane previously drawn from a conventional preform. The initial preform has a width greater than the intended width of the thin preform, which, as previously described, may range from 0.5 mm to 5 mm. Higher drawdown ratios, e.g., in the range of 200 to 16,000, may be used to make the narrow preform than to make the hollow-core fiber, although it is contemplated that the drawdown ratio at this stage is the ratio of the initial preform cross-sectional area to the narrow preform cross-sectional area. Thus, the initial preform may have a width in the range of about 20 mm to 200 mm. However, the invention is not limited in this respect, and other drawdown ratios and initial preform widths may be used, including lower drawdown ratios for the fiber draw stage and larger initial preform widths to yield longer length narrow preforms and subsequent hollow-core fiber.

[0038] The use of higher drawdown ratios for thin preform production allows standard hollow core preforms to be used as initial preforms, e.g., they are conventionally drawn directly into fiber or first drawn into cane. Traditionally, relatively small drawdown ratios are used to draw preforms into cane, and no pressure is required. Instead, pressure is reserved for the drawdown from the cane to fiber. However, in the method proposed herein, pressure is applied to the initial preform as it is drawn from the initial preform, as discussed above, to obtain a cross-sectional structure of the thin preform suitable for drawing into fiber without the need to apply pressure to the thin preform. Suitable application of pressure, including a differential pressure or multiple differential pressures, when various pressures are applied to various voids within the glass structure of the preform or cane, is a known technique for drawing optical fiber and can be implemented in the same or similar manner when drawing a thin preform from an initial preform, and therefore will not be further described herein. Those skilled in the art know how to apply the appropriate pressure to obtain the desired target glass structure from the initial glass structure under the dynamics of pressure and surface tension contraction under drawing.

[0039] In the next step S2, an end portion of the initial preform is heated to soften the glass, for example, using a furnace in a fiber draw tower. In step S3, a thin preform begins to be drawn from the softened glass of the initial preform while pressure is applied to the cavity of the initial preform. To enable drawing of additional thin preforms, the initial preform is continually fed into the furnace for heating and drawing, so that the glass of the initial preform is gradually consumed and converted into thin preforms, increasing their length. After step S3, the method can follow one of two alternative paths depending on how the total length of thin preforms available for drawing into fiber is derived.

[0040] In a first pass, the method proceeds to step S4a. In this pass, the entire length of the thin preform available for drawing into a hollow-core optical fiber is produced in a single draw, so that the actual length of the finished thin preform is equal to the total length available for fiber drawing. The total length can be very long, as noted above, and therefore requires proper handling, which differs from the handling of a cane, which is substantially rigid and much shorter. Thus, in step S4a, the thin preform is collected by winding it onto a spool, drum, or bobbin as it is drawn from the initial preform. This is consistent with collecting optical fiber in a conventional drawing process. The ability to collect the thin preform in a wound format, which is not possible with a cane, is made possible by the thin preform's narrow width, which makes it flexible enough to be wound onto a spool without damage. The spool should have a diameter such that the curvature of the winding surface around which the thin preform is wrapped or coiled is equal to or greater than the bend radius of the thin preform, which is determined in part by the width of the thin preform. The smaller width allows for a tighter bend radius around the narrow preform, which can be bent before cracking or breaking.

[0041] In the next step S5a, thin preforms continue to be drawn from the initial preforms while the initial preforms continue to be fed into the furnace for heating. This continues until the length of the thin preforms is the desired total length available for drawing into hollow-core fiber. This can be accomplished before all of the initial preforms are used, but for maximum efficiency, the thin preforms continue to be drawn until all of the initial preforms (minus consumables) are converted into thin preforms. Once the total length of thin preforms has been produced, the method proceeds to step S6a, where the thin preforms are separated from the remainder of the initial preforms. The thin preforms are now stored on a spool and available for drawing into hollow-core fiber according to the method of FIG. 4. This may occur immediately or at a later time, in the same draw tower or in a different draw tower. Finally, when needed, the thin preforms are provided for drawing hollow-core optical fiber, as in the first step S10 of the method of FIG. 4.

[0042] To protect the thin preform while it is being collected on the spool, stored on the spool, and later unwound from the spool, it may be coated as it is produced and before it reaches the spool. This can help protect against breakage and surface damage. Note that a separate protective coating can be applied to the hollow-core fiber after it is drawn, which is conventional fiber manufacturing practice, but the coating should be removable from the thin preform before it is drawn into the hollow-core fiber. Any coating that can be easily applied and removed from the thin preform may be used. A first example is a UV-curable acrylate coating, of the type also used to coat optical fiber. This will provide sufficient protection for the thin preform, but it adheres tightly to the glass, so removal may be too time-consuming to be practical. An alternative is to extrude a polymer tube around the thin preform, for example, by using a crosshead extruder. The polymer tube may be separated, peeled, or torn off from the thin preform as it is advanced for fiber drawing. The tube may have an inner diameter the same as the outer diameter of the thin preform, so that it contacts but does not tightly adhere to the outer diameter of the thin preform, or the tube may have a larger inner diameter so that the thin preform is loose within the tube. The latter arrangement may facilitate separating the tube from the thin preform without damaging its surface. A further alternative is to apply a coating that is susceptible to pyrolysis (chemical decomposition by heating), as described, for example, in U.S. Pat. No. 5,596,669 [6]. This allows the coating to be removed when the thin preform is passed through a furnace or otherwise heated before fiber drawing.

[0043] In a second pass of the method of Figure 5, the process proceeds from step S3 to step S4b. In this pass, the narrow preform is formed as a transitional length, which is much smaller than the intended total length available for drawing into a hollow-core fiber at any given time. The transitional length will depend on the configuration of the drawing apparatus used to produce the hollow-core fiber, and examples are described in more detail below. In step S4b, the length of the narrow preform as drawn from the intended preform reaches the transitional length, and without separating the narrow preform from the initial preform, the narrow preform is provided for drawing into a hollow-core fiber, as in step S10 of the method of Figure 4. In step S5b, drawing of the narrow preform from the initial preform continues while simultaneously drawing a hollow-core fiber from the end of the narrow preform (the end distal to the neckdown between the initial preform and the narrow preform). Thus, the thin preform is replenished at its proximal end and simultaneously depleted from its distal end, so that the transition length between these ends remains limited and substantially constant, but the total glass contained in the transition length between being added and then removed accumulates over time. This continues until the transition length accumulates (in terms of the glass it contains) to the total length of the thin preform available for drawing into a hollow-core fiber. In this way, a thin preform containing the total length available for drawing required by step S10 of FIG. 4 is provided.

[0044] Figures 6A and 6B show simplified schematic diagrams of example apparatus suitable for performing an example hollow-core optical fiber manufacturing method according to the present disclosure. Figure 6A shows the apparatus performing a first portion of the example method, i.e., steps S1 through S7a, corresponding to the first pass of Figure 5. Figure 6B shows the apparatus performing a second portion of the example method, corresponding to steps S10 through S14 of Figure 4.

[0045] 6A shows an optical fiber draw tower 20a in which an initial preform is being drawn into a narrow preform. The initial preform 22 has an initial preform width and, in this example, includes a ring of hollow capillary tubes 24 for forming a hollow-core optical fiber microstructured cladding within an outer glass support tube 26 arranged to define a gap for forming the hollow core of the hollow-core optical fiber within the cladding. The initial preform 22 is surrounded by an outer glass layer 28 intended to form the glass jacket of the finished hollow-core fiber, as described above. The initial preform, with its longitudinal axis vertical, is suspended at a first upper end from a support structure or apparatus (schematically shown as element 30), which is configured to feed the initial preform 22 and outer glass layer 28 axially downward at a first feed rate F1. A pressurization system 32, also as previously described, has connections with the core and cladding capillary cavities of the initial preform for supplying pressurized gas to the cavities at one or more differential pressures ΔP. Pressurization is applied during drawing of a thin preform from the initial preform 22.

[0046] The draw tower further includes a vertically positioned tube furnace 34 in which the initial preform 22 is positioned such that its lower end portion is heated by the furnace to soften the glass in the end portion. The softened glass is drawn through a neckdown 38 formed by the softened glass into a narrow preform 36 having a narrow preform width smaller than the initial preform width, and the width of the glass structure across the neckdown 38 narrows from the initial preform width to the narrow preform width. The softened glass is pulled downward by the action of a belt puller 40, which includes a pair of opposing, vertically arranged, circulating belts. The glass is gripped between the pair of circulating belts and pulled downward by the rotation of the belts to form the narrow preform. After the belt puller 40, the thin preforms 36 pass around a capstan 42 rotating in a vertical plane, which also acts to pull the thin preforms downward so that they are continuously drawn from the softened glass at neckdown 38. The thin preforms are replenished from their upper ends by the initial preforms 28 gradually being fed into the furnace. The belt puller 40 and capstan 42 act to apply tension to the thin preforms 36 so that they are pulled from the initial preforms 22 at a constant speed, which is the first draw speed D1 specified to form the thin preforms 36 at the required width. The thin preforms 36 are narrow enough to be flexible enough to be wrapped around the capstan for tensioning; it should be noted that this would not be possible when drawing cane from the preforms, as the cane would be thicker and too rigid. To achieve this, other tensioning devices may additionally or alternatively be used in accordance with known techniques for drawing optical fiber. The first draw speed D1 is faster than the first feed speed F1 so as to match the supply of freshly softened glass from the relatively wide initial preform 22 with the uptake of softened glass by the relatively narrow thin preform to achieve the drawdown ratio required in the neckdown 38.Achieving the required narrow preform width may be aided by a first diameter meter 44, in this example located between the belt puller 40 and the capstan 42, which includes an aperture through which the drawn narrow preform passes for diameter measurement by a laser beam, and from which results may be fed back to control the feed of the initial preform 22. Diameter meters are also known in the fiber drawing art. Additionally, an optional coating device 46 is included below the diameter meter 44 and configured to apply an optional removable polymer coating onto the narrow preform 36 according to any of the exemplary coating methods previously described.

[0047] The thin preform 36 passes around the capstan 42 and is taken up by a rotating spool, bobbin, or drum 48, which winds the thin preform 36 away from the capstan 42, causing the thin preform 36 to be wound onto the rotating spool, bobbin, or drum 48 by the action of rotation. Drawing of the thin preform 36 can continue for as long as needed until the initial preform 22 is completely consumed, in that as much of the glass in the initial preform 22 as possible (excluding unavoidable wastage) has been converted into thin preform 36. The thin preform 36 collected on the spool 48 has an actual length that is the total length available for drawing into hollow-core optical fiber. The wound thin preform 36 can be stored on the spool 48 for as long as needed and does not need to be immediately drawn down into fiber. Thus, multiple spools of thin preforms 36 can be successfully drawn from one or more initial preforms, while the draw tower 20a is specifically configured for this process to improve efficiency.

[0048] Figure 6B shows optical fiber draw tower 20b, in which the previously produced thin preform 36 according to Figure 6A is being drawn into a hollow-core optical fiber. Draw tower 20b may be the same draw tower 20a in which thin preform 36 was drawn, but reconfigured for fiber drawing, or draw tower 20b may be a different draw tower. Thin preform 36 is provided as wound onto spool 48 during drawing of the thin preform (or optionally rewinding onto a different spool), and is placed above furnace 34 of draw tower 20b. Furnace 34 of draw tower 20b may have the same specifications as furnace 34 of Figure 6A, or may differ, for example, in temperature, length, and / or inner diameter. A feeding mechanism (not shown, including an arrangement of pulleys, dancers, accumulators, and / or belt pullers to allow direct feeding or to more precisely control the feed rate) is provided to pull the thin preforms 36 from the spool 48 and feed the end portions of the thin preforms 36 into the top of the furnace 34 at a feed rate F2. This differs from conventional draw tower feeding arrangements designed to handle short, stiff preforms and canes that must be vertically oriented and fed; instead, the long, thin, flexible form of the thin preforms 36 allows them to be fed by being unwound from the spool 48. One or more capstans or similar arrangements (not shown) may be included between the spool 48 and the furnace 34 to hold the thin preforms 36 in an optimal position within the furnace's heating zone as they are unwound from the spool 48. Because the drawdown ratio from thin preform to hollow-core fiber is small and it is generally desirable to produce optical fiber as quickly as possible, feed rate F2 may be faster than feed rate F1 used to feed initial preform 22 into the furnace. Additionally, or alternatively, a lower furnace temperature may be used. Furnace 34 softens the glass at the end portions of thin preform 36 so that hollow-core optical fiber 50 can be drawn from the glass at those end portions.The fiber 50 is drawn under tension by passing through the furnace 34 and down to a capstan 42 that is positioned vertically below the furnace 34 and rotates in a vertical plane, the fiber 50 passing around the capstan 42 via a tension meter 52 positioned between the furnace 34 and the capstan 42 through which the fiber 50 passes, and tension is applied by the capstan 42. This tensioning arrangement pulls the fiber 50 from the softened glass of the thin preform 36 in the furnace 34 at a second draw speed D2 through a second neckdown 54 where the width of the glass structure narrows from the thin preform width to the fiber width. The second draw speed D2 selected to achieve the required fiber width is faster than the second feed speed F2 to match the supply of freshly softened glass from the wider narrow preforms 36 as they enter the furnace with the uptake of softened glass by the narrower fiber 50 to achieve the required drawdown ratio in the second neckdown 54. Achieving the required fiber width may be aided by a second diameter meter 56, which in this example is located below the furnace 34, defining an aperture through which the drawn fiber 50 passes, before the fiber 50 reaches the tensioning element 52 and capstan 42.

[0049] Note that (in this example) no pressure is applied to the void of the thin preform 36 while it is being drawn into the hollow-core fiber 50. As noted above, this small drawdown ratio to draw makes pressure unnecessary. The lack of a pressure requirement allows for extremely long lengths of thin preforms to be handled, where any pressure applied at one end of the long thin preform would be ineffective at the other end where the neckdown to the fiber forms. The small diameter of the void within the thin preform prevents the pressurized gas from propagating rapidly along the void; therefore, if the length is too great, pressure cannot be effective in opposing surface tension forces during drawing. This contrasts with conventional hollow-core fiber drawing from a preform, where the fiber structure can be monitored during drawing and the pressure adjusted to achieve the required geometry. As the thin preform becomes longer and narrower, control of the geometry by pressure becomes impossible, as changes in the applied pressure would be expected not to reach the neckdown region.

[0050] 6A , if a protective polymer coating is applied to the thin preform 36 during its draw, the protective polymer coating must be removed from the thin preform 36 before the thin preform 36 is drawn into fiber 50. Accordingly, the draw tower 20b may optionally include a coating removal device or apparatus 58 located above the furnace 34, configured to remove the coating by a process dependent on the nature of the coating. For example, the coating removal device 58 may include one or more blades for engaging and slicing the tubular coating as the thin preform passes through the apparatus 58, a means for reeling up the slicing coating and pulling it away from the thin preform 36, or a heat source for heating the passing fiber to pyrolyze the fugitive coating. Alternatively, the heating provided by the furnace may be adequate to remove the fugitive coating before softening the glass of the thin preform to form the second neck-down 54.

[0051] Draw tower 20b may further optionally include a fiber coating apparatus 60 through which fiber 50 passes after it has been drawn, and in this case before reaching tension meter 52. Optical fibers are often conventionally provided with an outer coating, which is a layer of material such as a resin or polymer, after drawing to protect the optical fiber from damage during subsequent storage and use. Accordingly, any known fiber coating apparatus may be used, and any known fiber coating may be applied. In this example, a UV-curable coating, such as an acrylate material, is applied, so apparatus 60 includes a coating cup 60a containing a bath or reservoir of coating material in liquid form above a UV light source 60b; fiber 50 passes through coating cup 60a to collect a layer of coating material on its outer surface; and UV light source 60b directs UV radiation onto the coating layer to cure and solidify the coating layer.

[0052] Finally, after passing around the capstan 42, the finished fiber 50 leaves the capstan 42 and is wound and collected by being wound around a rotating fiber take-up spool, drum, or bobbin in the same manner that the thin preform 36 was collected from the first draw stage. Drawing of the fiber 50 can continue for as long as necessary until the thin preform 36 is completely consumed, in that as much of its glass as possible (excluding unavoidable wastage) has been converted into hollow-core fiber 50. This allows the longest continuous length of fiber possible to be produced, if desired, while maximizing production efficiency in that a single fiber draw can produce the maximum length of fiber from a single thin preform. Overall, for maximum efficiency and maximum fiber span, the entire initial preform is drawn into a thin preform, and then the entire thin preform is drawn into hollow-core fiber.

[0053] Another exemplary method utilizes the transitional length concept of the thin preform described above, where the total usable length of the thin preform is achieved by the constant addition of glass material to one end of the transitional length and the simultaneous constant removal of glass material from the other end of the transitional length. This eliminates the need to collect and store the entire total usable length of the thin preform. However, the simultaneous drawing process both into and from the thin preform requires two heat sources to soften the glass structure for the purposes of forming the neckdown from the initial preform to the thin preform and the separate but simultaneous neckdown from the thin preform to the hollow-core fiber. Examples of fiber drawing equipment suitable for implementing this configuration will now be described. In these equipment, the hollow-core fiber manufacturing method corresponding to the second pass of FIG. 5, i.e., steps S1 through S5b, followed by steps S10 through S14 of FIG. 4, can be performed.

[0054] FIG. 7 shows a schematic diagram of a first exemplary apparatus for handling thin preforms in a transitional length format. The apparatus includes two optical fiber draw towers 20a and 20b, which are laterally offset relative to one another with respect to the vertical fiber draw direction of the draw towers 20a and 20b. The lateral offset is partly for convenience, as the combined height of two vertically stacked draw towers would be prohibitive in many environments, and to further aid in separating the tensions and pressures between the two simultaneous draws. The addition of a capstan and dancer wheel or similar feed control device to route preforms from one draw tower to the other separates the two draws, preventing the overall dynamics from behaving like a single draw in two stages. The first, left-hand draw tower 20a is for drawing thin preforms from initial preforms and is configured in substantially the same manner as the draw tower 20a of FIG. 6a, and like reference numerals are used to indicate like components. Therefore, a detailed description will not be repeated. In summary, however, an initial preform 22, configured as previously described, is fed vertically into the annular draw furnace 34a of the first draw tower 20a at a first feed rate F1 while a differential pressure is applied to the cavity thereof by the pressurization system 32. The first furnace serves to soften the glass at the end portions of the initial preform 22, which is then drawn through a neckdown 38 into a thin preform 36 under the tensioning and pulling action of a belt puller 40. As before, the drawn thin preform can pass through a thin preform diameter scale 44 before being taken up by a vertically rotating capstan 42 (in this example, the first capstan for handling the thin preform) below the first furnace 34a. The belt puller 40 and first capstan 42 again achieve a first draw speed D1, this time faster than the first feed speed F1, to draw the thin preform. Note that in this example, a thin preform coating device 46 is not included in the first draw tower 20a.This is because the thin preforms will not be collected and wound onto spools for storage, but instead will be immediately drawn into hollow-core fiber, and therefore, there is no need to provide a protective coating on the thin preforms 36.

[0055] However, in this example, after passing around the first capstan 42, the thin preforms 36 are not collected on a spool for processing into hollow-core fiber at some later point. Instead, once the thin preforms have gained a length sufficient to extend from the exit of the first draw tower 20a (at its base) to the entrance of the second draw tower 20b (at its top), the thin preforms are passed to a second capstan 43 above the furnace 34b of the second draw tower 20b. To accommodate any slack in the narrow preforms 36 resulting from perturbations in capstan rotation that could cause instantaneous length variations in the narrow preforms 36 between capstans 42 and 43, a capstan, dancer wheel, accumulator, or similar feed control device 47 (shown diagrammatically as a block in FIG. 7) through or around which the narrow preforms 36 pass may be included between capstans 42 and 43. The second capstan 43 then serves as the feed structure for the second draw tower 20b, instead of the starting point from the rotating spool 48 in FIG. 6B. Thus, the narrow preforms 36 pass over the second capstan 43 (possibly after wrapping around the second capstan 43 one or more times) and are guided into the furnace 34b of the second draw tower 20b for heating of their end portions. 6B, so that the softened thin preform 36 is drawn into hollow-core fiber 50 via second neckdown 54 in furnace 34b and fiber diameter meter 56, and via tension meter 52 at draw speed D2 under tension from fiber capstan 42b. Fiber 50 optionally receives a protective coating in coating device 60 and is finally collected on spool 62. First capstan 42 and second capstan 44 are rotated at appropriate speeds to maintain first draw speed D1 and second feed speed F2 substantially equal to one another for smooth and continuous movement of thin preform 36 between towers 20a and 20b (with assistance from optional feed control device 47).Note that in this example, no coating removal equipment 58 is included above furnace 34b because the slender preforms are not coated after drawing. However, alternatively, coating equipment for coating the slender preforms 36 and subsequent coating removal equipment may be included, if preferred.

[0056] The length of the thin preform 36 extending from the first draw tower 20a to the second draw tower 20b (e.g., measured from the exit of the first furnace 34a to the entrance of the second furnace 34b) is the previously discussed transitional length of the thin preform. From FIG. 7, it can be readily seen that the proximal end of the thin preform 36 is continuously replenished with new glass from the drawdown of the initial preform 22, and the distal end of the thin preform 36 is continuously consumed as that glass is drawn down into fiber 54. While the transitional length remains substantially constant (other than momentary fluctuations caused by perturbations in the feed arrangement, which are addressed by the feed control device 47), the glass material making up the thin preform advances along the transitional length. Thus, in terms of glass, the transitional length accumulates over time to result in the total length of the thin preform 36 available for drawing into hollow-core fiber 50. In this configuration, the entire initial preform 22 can be converted into a single continuous length of hollow-core fiber 50 via the narrow preform 36 in a single manufacturing process involving two drawdown stages.

[0057] As with the example of FIG. 6B , no pressure is applied directly to the void of the narrow preform 36 during drawing of the hollow-core fiber 50 from the narrow preform 36. The second draw tower 20b does not have a free end of the narrow preform 36 at which to apply pressure, if any, but this is not an issue because it is not needed. The pressure applied to the void of the initial preform 22 in the first draw tower has no measurable effect on the second neck-down 54 because the length of the narrow preform void is too long and the width of the void too narrow for the applied gas to travel up to the second neck-down in time to generate any significant or substantial pressure at the location of the draw-down into the fiber.

[0058] It will be appreciated from FIG. 7 that the transition length of the narrow preform will depend on the relative location and physical arrangement of the first and second draw towers 20a, 20b. However, considering the typical size and height of fiber draw towers and noting that the first draw tower may be a standard-sized or smaller tower, it is contemplated that a transition length ranging from about 1 m to 100 meters may typically be utilized, although this is purely an exemplary range and the invention is not limited thereto. For example, if the first and second draw towers are adjacent (transversely or laterally offset), the transition length may be on the order of 5 m to 100 m, taking into account tower height, distance from the first tower to the second tower, and the potential use of accumulators, as well as between draw towers. If there is little or no lateral offset and axially aligned towers are available, the transition length may be 10 m or less, such as about 1 m to 5 m.

[0059] FIG. 8 is a schematic diagram of a second exemplary apparatus for handling transition-length-format thin preforms. In this example, the apparatus includes only one optical fiber draw tower 20, such that the entire draw from the initial preform to the hollow-core fiber is performed along a single vertical draw direction. However, the draw tower 20 includes two annular draw furnaces, a first upper furnace 34a and a second lower furnace 34b, which are axially aligned with each other along the draw direction with an axial space between them, but no lateral offset. The first furnace 34a is for performing the draw of the thin preform from the initial preform, and the second furnace 34b is for performing the draw of the hollow-core fiber from the thin preform. Compared to the transversely spaced configuration of the example of FIG. 7, a different configuration is utilized to achieve separation of tension and pressure between the two simultaneous draws.

[0060] Many of the same components as those of the first and second draw towers of FIG. 7 are present, and similar reference numerals are used; therefore, detailed descriptions thereof will not be repeated. An initial preform 22, configured as previously described, is vertically fed into the first furnace 34a at a feed rate F while a differential pressure is applied to the cavity by the pressurizing system 32. The first furnace 34a serves to soften the glass at the end portions of the initial preform 22 so that the glass at the end portions of the initial preform 22 can be drawn into thin preforms 36 via a neckdown 38 under the tensioning and pulling action of a belt puller 40 located below the first furnace 34a and above the second furnace 34b, following a thin preform diameter meter 44. The belt puller 40 provides tension for drawing the thin preforms 36 from the initial preforms 22 and separates this draw tension from the drawing of the fiber in the second furnace 34b. Again, a thin preform coating apparatus 46 is not included, since the thin preforms are immediately drawn into hollow-core fiber and do not require a protective coating for packaging.

[0061] As in the example of Figure 7, once the thin preform 36 has gained a sufficient length to extend from the exit of the first furnace to the entrance of the second furnace, the thin preform is fed vertically directly into the second furnace for heating of its end portion without any orientation or steering. The remainder of the draw tower 20 is configured like the second draw tower of Figure 7, downward from the second furnace 34b, so that the softened thin preform 36 is drawn under tension from the fiber capstan 42 through the second neckdown 54 in the second furnace 34b and the fiber diameter meter 56 into a hollow-core fiber 50, optionally receiving a protective coating in a coating device 60, with the tension meter 52 providing the draw speed D, before being collected on a spool 62. In the depicted configuration, the belt puller 40 and capstan 42 are operated to achieve an increasing draw rate along the draw direction, thus maintaining a first draw rate at the first neckdown 38 and a second draw rate at the second neckdown 54 to achieve a final draw rate D for the fiber 50. To separate the tensions of the two draws and potentially achieve different draw rates, additional tensioning elements (such as pulleys, dancers, and / or accumulators, all known for handling optical fiber during drawing) may be included above the second oven 34b to manage any slack, although this is less important than in the example of FIG. 7 . This may be due to the narrow width and resulting flexibility of the thin preforms. Again, because the thin preforms are not coated after drawing, no coating removal equipment is included above the second oven 34b in this example.

[0062] The length of the thin preform 36 extending between the two neckdowns 38, 54 (e.g., measured from the exit of the first furnace 34a to the entrance of the second furnace 34b) is the previously discussed transitional length of the thin preform. As in the example of FIG. 7, it can be readily seen from FIG. 8 that the proximal end of the thin preform 36 is continually replenished with new glass from the drawdown of the initial preform 22, and the distal end of the thin preform 36 is continually consumed as that glass is drawn down into fiber 54. While the transitional length remains constant, the glass material making up the thin preform progresses along the transitional length. Thus, in terms of glass, the transitional length accumulates over time to result in a total length of the thin preform 36 available for drawing into a hollow-core fiber 50. In this configuration, the entire initial preform 22 can be converted into a single continuous length of hollow-core fiber 50 via the narrow preform 36 in a single manufacturing step involving two drawdowns arranged along a single draw direction.

[0063] It will be appreciated from FIG. 8 that the transition length of the thin preform will depend on the relative axial offset of the first and second furnaces 34a, 34b and can be made shorter or longer by separating the two furnaces by a smaller or larger amount. This may be determined at least in part by the vertical space available within the draw tower 20 and the overall height the draw tower 20 can occupy. However, unless a large amount of vertical space is typically available, it is expected that the maximum feasible transition length will be smaller in this example than in the example of FIG. 7. Given the typical height of fiber draw towers, it is contemplated that transition lengths in the range of approximately 1 to 5 meters may typically be utilized, although this is purely an exemplary range and the invention is not limited thereto. Large industrial draw towers can have significant heights, and in some cases, transition length ranges of approximately 1 to 10 meters or 1 to 15 meters or more may be feasible. Beneficially, the minimum transition length may be selected to allow sufficient cooling of the narrow preform 36 so that the belt blur 40 is not damaged by the hot glass, but overall, a shorter transition length may be more convenient to reduce the overall height of the draw tower required.

[0064] 6B and 7, no pressure is applied directly to the void of the thin preform 36 during drawing of the hollow-core fiber 50 from the thin preform 36. The second draw tower 20b does not have a free end of the thin preform 36 at which to apply pressure, if any, but this is not an issue because it is not needed. However, in this example, because the transition length is relatively short, there may be a problem in that pressure applied to the void of the initial preform 22 could have a measurable effect on the second neck-down 54, causing unintended expansion of the internal geometry of the glass structure that is not counteracted by the surface tension of fiber drawing. To address this, it is proposed that measures could be taken to separate the lower drawdown of the second neck-down 54 from the pressure applied to the initial preform 22 for the upper drawdown of the first neck-down 38. The initial preform 22 is fed into the first furnace 34a at a feed rate F, and pressurized gas from the pressurizing system 32 travels along the initial preform's voids and into the first neck-down 38, which may allow for control of the microstructure of the thin preforms 36. The thin preforms 36 exit the first furnace 34a so fast that the pressure applied to the initial preforms is insufficient to force the gas to flow at a rate that matches the rate at which the voids are elongated in the first neck-down 38, allowing the glass structure to "outrun" the gas, and they arrive at the second furnace with little or no gas in the voids, resulting in no or negligible pressurization in the second neck-down 54.

[0065] Further in this regard, and applicable to all examples, for a given width of hollow-core fiber, thinner thin preforms allow for faster feeding and drawing into fiber due to smaller drawdown ratios, resulting in greater production efficiency. However, higher feed rates require higher furnace temperatures due to reduced residence times, and thus the maximum achievable temperature of existing furnaces may be a limiting factor when selecting the width of the thin preform to be produced. Thus, there may be a trade-off between finding thinner or thicker thin preforms. Thus, while using existing fiber draw equipment may require parameters when implementing the methods described herein, new fiber draw equipment may be designed to maximize draw efficiency and fiber production.

[0066] While the method has thus far been described as being performed in a conventional optical fiber draw tower, the invention is not limited in this respect, and the heating and drawing of the glass structure may be performed using other equipment as preferred. However, given the ability of the method to increase fiber yield, large continuous lengths of hollow-core optical fiber can be readily produced, the use of a draw tower is particularly convenient. Indeed, a benefit of the proposed method is that it can be readily implemented using conventional fiber draw equipment with only minor modifications.

[0067] A first further example provides a method of manufacturing a hollow-core optical fiber, the method comprising the steps of: providing a thin preform formed from glass having a cross-sectional structure configured to form, in a hollow-core optical fiber drawn from the thin preform, a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the thin preform having a width in the range of 0.5 mm to 5 mm and a total length available for drawing into a hollow-core optical fiber; heating end portions of the thin preform to soften the glass therein; and drawing a length of hollow-core optical fiber from the softened glass of the thin preform, the hollow-core optical fiber having a width smaller than the width of the thin preform.

[0068] In a first further example, the step of drawing the length of hollow-core fiber may be performed without applying pressure to the voids within the narrow preform during drawing of the hollow-core optical fiber.

[0069] In a first further example, the method may further comprise flowing one or more gases at different pressures into voids within the narrow preform to pre-pressurize the narrow preform before heating the end portion of the narrow preform.

[0070] In a first further example, drawing a length of hollow core optical fiber may include drawing the hollow core optical fiber until the total length of the narrow preform available for drawing has been drawn into hollow core fiber.

[0071] In a first further example, the drawdown ratio of the cross-sectional area of the narrow preform to the cross-sectional area of the hollow-core optical fiber may be in the range of 2 to 150. In a first further example, the total length available for drawing may be 30 m or more.

[0072] In a first further example, the method can further include providing an initial preform formed from glass having a cross-sectional structure configured to form the cross-sectional structure of the narrow preform when the narrow preform is drawn from the initial preform together with pressurizing voids in the initial preform, the initial preform having a width greater than the width of the narrow preform; applying pressure to the voids in the initial preform; heating end portions of the initial preform to soften the glass in the end portions; and drawing the narrow preform from the softened glass of the initial preform while the pressure is applied.

[0073] In a first further example, the drawdown ratio of the cross-sectional area of the initial preform to the cross-sectional area of the narrow preform may be in the range of 200 to 16,000. In a first further example, the method may further include collecting the thin preforms as they are drawn by winding them onto a spool and separating the thin preforms from the initial preforms until they have a length equal to the total length available for drawing.

[0074] In a first further example, the method may further include applying a protective coating to the slim preform before the slim preform is wound onto the spool, and removing the protective coating from the slim preform before drawing the hollow-core optical fiber from the slim preform.

[0075] In a first further example, drawing of a hollow-core fiber from the slim preform may be initiated while the slim preform is integral with the initial preform and when the slim preform reaches a transitional length that is less than the total length available for drawing, and drawing of the slim preform is continued during drawing of the hollow-core fiber, such that the transitional length is replenished from the initial preform until the transitional length accumulated over the duration of drawing equals the total length available for drawing.

[0076] In a first further example, heating of the end portion of the initial preform may be performed in a first furnace, and heating of the end portion of the narrow preform may be performed in a second furnace that is laterally offset from the first furnace relative to the drawing direction.

[0077] In a first further example, the transition 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. In a first further example, the method may further include controlling the tension of the thin preform between the first furnace and the second furnace such that the tensioning member used to draw the hollow core optical fiber from the thin preform is separated from the tensioning member used to draw the thin preform from the initial preform.

[0078] In a first further example, the heating of the end portion of the initial preform may be performed in a first furnace, and the heating of the end portion of the narrow preform may be performed in a second furnace axially aligned with the first furnace relative to the drawing direction.

[0079] In a first further example, the transition 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. In a first further example, the method may further include controlling the tension of the thin preform between the first furnace and the second furnace such that the tensioning member used to draw the hollow core optical fiber from the thin preform is separated from the tensioning member used to draw the thin preform from the initial preform.

[0080] In a first further example, the method may further include the step of surrounding the initial preform with an outer glass layer before heating the end portion of the initial preform so that the thin preform is melted and drawn together with the outer glass layer, the outer glass layer being for forming a jacket for the hollow core optical fiber.

[0081] In a first further example, the cross-sectional structure of the narrow preform may be configured with a geometric ratio that matches the intended geometric ratio of the hollow-core optical fiber, plus a compensation amount to balance the surface tension contraction of the softened glass of the narrow preform during drawing of the hollow-core fiber.

[0082] A second further example provides a method for manufacturing a preform for a hollow-core optical fiber, the method comprising the steps of: providing an initial preform formed from glass having a cross-sectional structure configured to form, in a state where the thin preform is drawn from the initial preform, a cross-sectional structure capable of being drawn from the initial preform into a hollow-core optical fiber having a cross-sectional structure including a hollow core surrounded by a plurality of voids that define a microstructured cladding, the initial preform having an initial preform width; applying pressure to the voids in the initial preform; heating end portions of the initial preform to soften the glass in the end portions of the initial preform; and drawing a thin preform from the softened glass of the initial preform while the pressure is applied, the thin preform having a width in the range of 0.5 mm to 5 mm that is smaller than the width of the initial preform.

[0083] In a second further example, the method further comprises drawing the narrow preforms to a length of at least 30 m while collecting the narrow preforms on a spool, and separating the narrow preforms from the initial preforms.

[0084] A third further example provides a thin preform formed from glass having a cross-sectional structure configured to form in a hollow-core optical fiber drawn from the thin preform a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the thin preform having a width in the range of 0.5 mm to 5 mm and a length of at least 30 m.

[0085] In a third further example, the slender preform may be wound onto a spool. In a third further example, the slender preform may include an outer layer of glass fused with the slender preform to form a jacket for a hollow-core optical fiber.

[0086] In a third further example, the slim preform may include a protective coating that can be removed before the slim preform is drawn into a hollow-core optical fiber. A fourth further example provides a hollow core optical fiber having a cross-sectional structure including a hollow core surrounded by a plurality of voids that define a microstructured cladding, the hollow core optical fiber having a length of at least 20 km.

[0087] The various embodiments described herein are presented solely to aid in the understanding and teaching of the claimed features. These embodiments are provided as merely representative samples of embodiments and are not intended to be exhaustive and / or exclusive. The advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein should not be construed as limitations on the scope of the invention as defined by the claims or on the equivalents of the claims, and it should be understood that other embodiments may be utilized and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the present invention may suitably comprise, consist of, or consist essentially of any suitable combination of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. Additionally, the present disclosure may include other inventions not currently claimed but which may be claimed in the future.

[0088] References [1] G.T. Jasion et al., “Fabrication of tubular anti-resonant hollow core fibers: modeling, draw dynamics and process optimization.” Opt. Express, vol. 27, pp. 20567-20582 (2019). [2] Y. Chen et al., “Multi-kilometer long, longitudinally uniform hollow core photonic bandgap fibers for broadband low latency data transmission,” IEEE JLT, 34, pp. 104–113 (2016). [3] U.S. Patent No. 11203547 [4] S.F. Gao et al., “Hollow-core conjoined-tube negative-curvature fiber with ultralow loss,” Nat. Commun, 9, p. 2828 (2018). [5]CN111474628 [6] U.S. Patent No. 5,596,669

Claims

1. A method for manufacturing a hollow core optical fiber, A step of providing a thin preform formed from glass and having a cross-sectional structure, wherein the cross-sectional structure is configured to form a cross-sectional structure in a hollow core optical fiber drawn from the thin preform, the hollow core being surrounded by a plurality of voids defining a microstructured cladding, and the thin preform having a width in the range of 0.5 mm to 5 mm and a total length available for drawing into the hollow core optical fiber. The steps include heating the end portion of the narrow preform to soften the glass at the end portion, A step of drawing a hollow core optical fiber of a certain length from the softened glass of the thin preform, wherein the hollow core optical fiber has a width smaller than the width of the thin preform. A method that includes this.

2. A method according to claim 1, wherein the drawing of the hollow core fiber of a certain length is performed without applying pressure to the void in the thin preform during the drawing of the hollow core optical fiber.

3. A method according to claim 1 or 2, further comprising the step of injecting one or more gases of different pressures into the voids within the micro-preform to pre-pressurize the micro-preform before heating the end portion of the micro-preform.

4. A method according to claim 1, wherein the step of drawing a hollow core optical fiber of a certain length includes drawing the hollow core optical fiber until the total length of the thin preform available for drawing is drawn into the hollow core optical fiber.

5. The method according to claim 1, The drawdown 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 The total length available for drawing the lines must be 30m or more. A method that is one or both of the above.

6. A method according to claim 1, wherein the slim preform is A step of providing an initial preform formed from glass and having a cross-sectional structure, wherein the cross-sectional structure of the narrow preform is formed when the narrow preform is drawn from the initial preform along with the pressure of the voids within the initial preform, and the initial preform has a width greater than the width of the narrow preform, The steps include applying pressure to the voids within the initial preform, The steps include heating the end portion of the initial preform to soften the glass in the end portion of the initial preform, The steps include drawing the thin preform from the softened glass of the initial preform while applying the aforementioned pressure, and A method that further includes the steps provided by doing so.

7. The method according to claim 6, The steps include collecting the thin preform while it is being drawn by winding it onto a spool until it has a length equal to the total length available for drawing, The steps include separating the aforementioned slim preform from the initial preform. A method that further includes this.

8. The method according to claim 7, The steps include applying a protective coating to the thin preform before it is wound onto the spool, Before drawing the hollow core optical fiber from the thin preform, the step of removing the protective coating from the thin preform. A method that further includes this.

9. A method according to claim 6, wherein the drawing of the hollow core fibers from the thin preform is initiated while the thin preform is integral with the initial preform and when the thin preform has reached a transient length less than the total length available for drawing, the drawing of the thin preform is continued during the drawing of the hollow core fibers, thereby replenishing the transient length from the initial preform until the transient length accumulated over the duration of the drawing is equal to the total length available for drawing.

10. A method according to claim 9, wherein the heating of the end portion of the initial preform is performed in a first furnace, and the heating of the end portion of the narrow preform is performed in a second furnace positioned laterally from the first furnace with respect to the line drawing direction.

11. A method according to claim 10, further comprising the step of controlling the tension of the thin preform between the first furnace and the second furnace such that the tensioning unit used for drawing the hollow core optical fiber from the thin preform is separated from the tensioning unit used for drawing the thin preform from the initial preform.

12. A method according to claim 9, wherein the heating of the end portion of the initial preform is performed in a first furnace, and the heating of the end portion of the narrow preform is performed in a second furnace that is axially aligned with the first furnace with respect to the drawing direction.

13. A method according to claim 12, further comprising the step of controlling the tension of the thin preform between the first furnace and the second furnace such that the tensioning unit used for drawing the hollow core optical fiber from the thin preform is separated from the tensioning unit used for drawing the thin preform from the initial preform.

14. A method according to claim 6, further comprising the step of surrounding the initial preform with a glass outer layer so that the tapered preform is melted and drawn together with the glass outer layer before heating the end portion of the initial preform, wherein the outer glass layer is for forming a jacket for the hollow core optical fiber.

15. A method according to claim 1, wherein the cross-sectional structure of the thin preform is configured to have a geometric ratio that matches a predetermined geometric ratio of the hollow core optical fiber and a compensation amount for balancing the surface tension contraction of the softened glass of the thin preform during the drawing of the hollow core optical fiber.

16. A method for manufacturing a preform for a hollow core optical fiber, A step of providing an initial preform formed from glass and having a cross-sectional structure, wherein the cross-sectional structure is configured to form a cross-sectional structure that can be drawn from the thin preform to a hollow core optical fiber in a thin preform drawn from the initial preform, the hollow core optical fiber having a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, and the initial preform having the width of an initial preform. The steps include applying pressure to the voids within the initial preform, The steps include heating the end portion of the initial preform to soften the glass in the end portion of the initial preform, The steps include drawing the narrow preform from the softened glass of the initial preform while applying the aforementioned pressure, wherein the narrow preform is smaller than the width of the initial preform and has a width in the range of 0.5 mm to 5 mm, and A method that includes this.

17. A method according to claim 16, comprising the steps of drawing the thin preform to a length of at least 30 m while collecting the thin preform on a spool, and separating the thin preform from the initial preform.

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

19. A slim preform according to claim 18, A glass outer layer, fused together with the thin preform, to form a jacket for the hollow core optical fiber, The thin preform is a protective coating that can be removed before it is wired into the hollow core optical fiber. A slim preform containing one or both of the above.

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

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