Modified drawdown hollow-core optical fiber drawing method.

Abstract

A method for manufacturing a hollow core optical fiber includes the steps of: providing an initial preform formed from glass having a cross-sectional structure configured to form, in an optical fiber drawn from the preform, a cross-sectional structure including a hollow core surrounded by a plurality of voids that define a microstructured cladding; heating an end portion of the preform; drawing a length of intermediate cane from the softened glass of the preform through a first neck-down, the intermediate cane having a glass cross-sectional area smaller than the glass cross-sectional area of the preform; heating a portion of the intermediate cane spaced from the first neck-down, the intermediate cane remaining integral with the preform; and drawing a length of hollow core optical fiber from the softened glass of the intermediate cane through a second neck-down, the hollow core optical fiber having a glass cross-sectional area smaller than the cross-sectional area of the intermediate cane.

Description

[Technical Field]

[0001] The present invention relates to a method for drawing a hollow-core optical fiber, the method having a modified drawdown. [Background technology]

[0002] Optical fibers are traditionally solid-core designs, 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-drawing process, in which a short 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, a narrower cane is first drawn from the preform and separated from the preform for later drawing into fiber. The process of drawing a cane from the preform to narrow the width of the glass structure is called drawdown.

[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 geometry 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 typically 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 void collapse and destruction of the intended structure.

[0004] The fiber drawing can be measured by a parameter known as the draw-down ratio. This is the ratio of the glass cross-sectional area of the preform to the glass cross-sectional area of the fiber, and the cross-section in question is transverse to the longitudinal axis of the glass structure (i.e., the preform, the fiber, and, as will be discussed later, the cane). Since it is equal to the number of meters of fiber that can be drawn per meter of the preform, a high draw-down ratio is desirable. As the preform gets larger, the volume of glass contained increases, so it should be expected to produce more fiber. For a given volume of glass, the preform size can be increased by increasing the preform width (more glass is available per meter of the preform), which suggests a higher draw-down ratio, or by increasing the preform length (less glass is available per meter of the preform), which reduces the draw-down ratio [1]. However, when drawing a wide preform into a narrow fiber, the dynamics of pressurization for hollow-core fibers are more complex and difficult to control. This is because the changes in the outer diameter and diameter of the hole become more extreme. Therefore, a high draw-down ratio is not only usually not suitable for hollow-core fiber manufacturing but is even more unattainable. Thus, a narrower preform and accordingly a lower draw-down ratio, which results in a smaller fiber yield and the corresponding inefficiency of fiber production, are generally required for successful hollow-core fiber manufacturing. However, pressurization using a narrower preform can be more difficult to implement because the voids are smaller and more difficult to access, which can reduce the structural quality of the finished fiber. Therefore, there are several factors that inhibit the efficient high-volume production of high-quality hollow-core optical fibers. Thus, the cost of hollow-core fibers is high, and the maximum achievable length of the fiber is limited compared to solid-core fibers. These factors are particularly undesirable in light of their excellent optical properties, which make hollow-core fibers very attractive for many applications, including long-distance communication where very long and inexpensive fibers are desirable.

[0005] Communication methods typically propagate light at infrared wavelengths, although attenuation in glass is low. Optical fibers configured to propagate visible and ultraviolet light are also of interest for applications such as quantum computing, Raman spectroscopy, photochemistry, and wastewater pollutant detection. In solid-core fibers made of silica glass, exposure to ultraviolet light creates permanent defects in the glass structure, leading to increased attenuation. Hollow-core fibers are less susceptible to this damage due to low light-glass interaction, making them excellent candidates for propagating light at shorter wavelengths. However, the wavelengths that hollow-core fibers can propagate with low loss depend on the precise geometry of the core and cladding. Shorter wavelengths can be most effectively propagated over a wide bandwidth by using thinner glass films around the cladding voids. Thinner films are more difficult to fabricate because the pressure regimes that can effectively counter surface tension effects during drawing are more difficult to achieve for thinner glasses. For shorter wavelengths, the overall fiber structure also becomes smaller, implying the need for larger drawdown ratios, which, as noted above, is problematic for hollow-core fiber fabrication.

[0006] Thus, there are various challenges in hollow-core optical fiber production that tend to limit achievable fiber length and wavelength performance, reduce efficiency, and increase complexity and cost. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, techniques for improving the manufacturing of hollow-core optical fibers are directed. [Means for solving the problem]

[0008] 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 an initial preform formed from glass having a cross-sectional structure configured to form, in an optical fiber drawn from the initial preform, a cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding, the initial preform having a first glass cross-sectional area; heating an end portion of the initial preform to soften the glass in the end portion; and drawing the softened glass of the initial preform through a first neck-down. the intermediate cane having a second glass cross-sectional area smaller than the first glass cross-sectional area; heating a portion of the intermediate cane spaced from the first neck-down to soften the glass in the portion of the intermediate cane spaced from the first neck-down, the intermediate cane remaining integral with the initial preform; and drawing a length of hollow-core optical fiber from the softened glass of the intermediate cane through the second neck-down, the hollow-core optical fiber having a third glass cross-sectional area smaller than the second glass cross-sectional area.

[0009] According to a second aspect of certain embodiments described herein, there is provided a hollow-core optical fiber manufactured using a method according to the first aspect. 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 techniques 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, a method according to the techniques described herein may be provided that includes any one or more of the various features described below, as appropriate.

[0010] 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]

[0011] [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. 1 shows a graph of fiber capillary diameter versus applied differential pressure for a modeled fiber drawing process according to a conventional method in which the fiber is drawn directly from a preform. [Figure 5] FIG. 5 shows a graph of fiber capillary diameter versus applied differential pressure for a modeled example fiber draw process according to the present disclosure for the same fiber and preform as the model in FIG. 4. [Figure 6] 6(a)-(c) show schematic diagrams of the steps of fiber drawing in a draw tower according to an exemplary method of the present disclosure. [Figure 7] FIG. 1 shows a flowchart of steps of an exemplary fiber drawing method according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] 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 provide improved fiber yield and enhanced fiber structure.

[0014] 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 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. For example, the longest reported single span of hollow-core fiber with reasonable propagation loss values (4-5 dB / km) known to the inventors is only 12 km [2], while other reports indicate that 14 km can be made with 10 dB / km loss [3]. Operation at shorter wavelengths requires thinner membrane structures to define the cladding holes, so only much shorter lengths, such as 33.6 m, have been achieved [4].

[0015] 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).

[0016] FIG. 1 shows a schematic cross-sectional view of an exemplary HCPBF 10. In this fiber type, a structured inner cladding 1 contains a 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.

[0017] 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.

[0018] 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."

[0019] 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.

[0020] 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.

[0021] 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 a coupled tube structure [5] and as a hemispherical tube structure [6].

[0022] 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.

[0023] The hollow-core fiber fabrication method proposed herein is applicable to all and any type of hollow-core optical fiber, as discussed above. As a specific example, the fabrication of an anti-resonant hollow-core optical fiber including a single ring of non-nested cladding capillaries, such as the example of FIG. 2, will be considered in detail. While the proposed method can improve hollow-core fiber fabrication of all fiber types, a particular benefit is the enhanced fabrication of anti-resonant fibers configured to guide light at shorter optical wavelengths, meaning visible and especially ultraviolet wavelengths, which are not easily produced using conventional optical fiber drawing techniques. In addition to providing improved yields and longer lengths for hollow-core fibers in general, the proposed method also makes it possible to produce very thin cladding films required for short wavelength anti-resonance. For example, kilometer-scale yields of anti-resonant hollow-core fibers configured for broadband propagation of ultraviolet and visible wavelengths of light are achievable, and similar orders of magnitude improvements in yield are offered for obtaining fibers configured for near-infrared light propagation required for communications applications.

[0024] The proposed method utilizes modifications to the fiber draw process that enable hollow-core fiber manufacturing at higher overall drawdown ratios than previously feasible. In other words, hollow-core fiber can be successfully produced from relatively wider preforms or canes than previously possible. In some instances, hollow-core fiber configured for infrared wavelength propagation can be drawn from wider preforms or canes than typically used for such fiber, where wider preforms or canes have traditionally been precluded by high drawdown ratios. This provides increased yield of fiber per draw, leading to higher production efficiency and providing much longer continuous fiber lengths. In other instances, hollow-core fiber configured for visible or ultraviolet propagation can be drawn from preforms or canes designed for infrared fiber production, where high drawdown ratios would also typically be precluded. Under conventional drawing, it is laborious to prepare and use narrower preforms to reduce the drawdown ratio for these finer and more sophisticated fibers. Therefore, production efficiency can also be improved and longer fiber lengths can be drawn.

[0025] The proposed method is modified compared to conventional drawing processes by the introduction of one (or more) additional draw stages to introduce at least one intermediate preform or cane width between the initial preform or cane (which is conventionally prepared by drawing a preform down to a narrower width and separating it from the preform) and the finished fiber. Unlike conventional fiber manufacturing processes in which a preform is drawn into a narrower cane, which is separated from the preform at a convenient length and then drawn separately to produce fiber from the cane, the proposed method does not separate the intermediate cane from the initial preform or cane before drawing it into fiber. Thus, the total drawdown ratio from the initial cane to the finished fiber spans two (or more) successive stages, each of which is easier to manage in terms of pressure than a single large drawdown. This approach can be described as staged drawdown, where the total drawdown ratio from the initial preform or cane to the finished fiber is achieved in two or more stages or phases within the same overall drawing procedure, with each stage or phase contributing a portion of the desired total drawdown. Holding the initial preform or cane at the non-fiber end of the entire glass structure facilitates the application of pressure to the air gap, making it easier to handle compared to narrower canes. Once the intermediate cane is consumed by fiber drawing, additional portions of the intermediate cane are drawn from the initial preform or cane, and the process continues. The reduced drawdown ratio provided by the intermediate cane reaching the fiber results in more manageable mechanical surface tension of the softened glass, allowing fiber geometries to be drawn that would otherwise be unachievable if attempting to draw directly from the same initial preform. This is augmented by repeated replenishment of the intermediate cane from the initial preform or cane.This allows the entire initial preform or cane (minus normal wastage) to be drawn into hollow-core optical fiber, thereby greatly increasing fiber yield for a given volume of initial preform or cane.

[0026] Consider the example of an anti-resonant hollow-core optical fiber in which the cladding comprises spaced-apart single annulus and, optionally, nested capillaries or tubes disposed around the hollow core, as in Figures 2 and 3. The capillaries have wall thicknesses (film thicknesses) selected based on the wavelength of light that the fiber is intended to guide, according to relationships that define a low-loss transmission window between the resonant wavelengths λ.

[0027]

number

[0028] where t is the wall thickness, n is the refractive index of the glass, and m is the resonant order. m = 1 is generally preferred because it provides the widest bandwidth transmission window. High-performance hollow-core fibers suitable for communications applications, propagating in the near-infrared region around 1550 nm, have been developed. These fibers have a film thickness of around 500 nm, and production is currently limited to single-span lengths of around 10 km due to the various challenges of drawing hollow-core optical fibers discussed above. We can define a film thickness of around 300 nm as the boundary between infrared fibers and shorter-wavelength fibers, with shorter-wavelength fibers being more difficult to fabricate. For shorter-wavelength guidance across useful transmission bandwidths in the visible and ultraviolet regions of the spectrum, much thinner films, around 200 nm or less, are required. In addition to this reduced film thickness, the overall cross-sectional structure of the fiber is smaller / narrower. Therefore, fabrication of such fibers by conventional drawing from standard preforms and canes designed for near-infrared hollow-core fibers requires significantly increased drawdown ratios, which are difficult or impossible to achieve. The staged drawdown approach proposed herein addresses these issues.

[0029] As discussed above, current hollow-core optical fiber designs, including HCPBF and ARF, require the application of pressure to voids within their preforms or canes during drawing. Pressure application is often based on a differential pressure, where two or more different pressures are applied to different voids. The pressure is necessary to counteract the surface tension of the softened glass during drawing. This surface tension acts to shrink or collapse the voids, potentially altering or destroying the intended fiber structure if not countered by the applied pressure. Adjusting 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 a narrowing of the glass structure as it softens and becomes more pliable in the draw furnace, decreasing its width continuously over the length of the structure, from that of the preform or cane to that of the fiber. When pressure is applied, the geometry at the wide end of the neckdown (the open, undrawn, end of the preform or cane closest to the pressure input) (typically the top end, since fiber drawing is usually vertical) is dictated by the 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 interplay between the forces of applied pressure and surface tension, and to achieve a 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.

[0030] Attempting to improve the fiber yield 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 fiber yield for hollow-core fiber production. For shorter wavelength fibers, the thinner membranes and smaller capillaries required are more susceptible to pressure and surface tension, further amplifying the effects of MDC, thus necessitating the use of smaller drawdown ratios and corresponding reductions in yield. If the proposed drawdown ratio from preform / cane to fiber is greater than the limit imposed by MDC, the desired fiber cannot be produced.

[0031] Figure 4 shows a graph of the results of computer modeling of fiber drawing to form an ARF using conventional fiber drawing directly from a preform or cane with a single drawdown. The graph plots the variation in the outer diameter of the cladding capillary of the finished ARF fiber with the differential pressure applied to the capillary during drawing. A higher differential pressure is required to expand to larger capillary sizes and thinner capillary membranes. The thick vertical line indicates the target capillary diameter between 11 and 12 μm, corresponding to a capillary membrane thickness of 150 nm for optical guidance in the UV-visible wavelength range for the specific fiber modeled. To achieve this target, a differential pressure of 2.5 kPa must be applied to the capillary. The shaded area indicates where MDC occurs, which begins at an applied pressure of 2.497 kPa. Therefore, the target fiber structure is above the upper pressure limit to avoid MDC, and the desired fiber cannot be produced from the specified cane.

[0032] To address this and overcome the limitations imposed by MDC, the proposed method introduces an intermediate stage in the draw where an initial cane or preform is first partially drawn into an intermediate narrower cane or preform, and then the drawn intermediate narrower cane or preform is further drawn down to form a fiber, and while still integral with the initial cane or preform, is subjected to pressure during at least the second draw phase, and optionally during the first draw phase.

[0033] Figure 5 shows a graph of the results of computer modeling of a fiber draw, forming the same ARF from the same initial cane or preform as in the modeling of Figure 4, but modified by an additional intermediate draw stage. Because the preform / cane and fiber are the same as in the modeling of Figure 4, the overall drawdown ratio is the same, but the drawdown is divided into two phases, performed in a single draw operation during which the initial preform / cane can remain set in the draw tower with the pressurized connection maintained. Again, the target capillary diameter between 11 and 12 μm is indicated by the thick vertical line. However, here the target fiber structure is well below the differential pressure that marks the onset of MDC, indicated by the shaded area. Here, the target fiber is obtainable at a differential pressure below the MDC threshold, and the desired fiber can be successfully produced from the specified cane. Note that the applied differential pressure required to produce the fiber is now 26.56 kPa, an order of magnitude higher than in the model of Figure 4, yet still avoids MDC contact. The higher differential pressure required to achieve the same fiber structure results from a lower drawdown ratio from the initial preform, which allows for drawing at a higher viscosity and correspondingly higher draw tension. While the increased viscosity requires higher pressure to expand the glass structure, this is acceptable because it is desirable to draw at the highest possible tension while avoiding breakage of the glass structure, which occurs when the tension is too high. The high tension helps maintain the required internal glass structure. Thus, the limitations imposed by MDC under conventional drawing regimes are overcome, and the desired fiber can be successfully produced.

[0034] 6(a)-6(c) show simplified schematic diagrams of an optical fiber draw apparatus at several stages of a hollow-core fiber drawing method according to one example of the present disclosure. As a prelude, the terms preform and cane will be discussed and defined. In optical fiber manufacturing, the first 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 of the elements may be performed to secure the capillary tube in place relative to the outer tube, or bonding may be performed as part of the drawing process. The preform may be drawn directly into the desired fiber. Alternatively, in a conventional manner, the preform may be drawn into one or more separate canes, for example, having a width intermediate between the preform and the desired fiber and a length of approximately one meter. The canes may then each be drawn into a separate length of the desired 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 the present 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 a cane or fiber constitutes a preform, but a cane may similarly be considered a preform in that it may be drawn into a narrower cane or into fiber. A cane is a glass structure that can similarly be drawn into a narrower configuration (narrower cane or fiber), but is formed by drawing from a wider glass structure rather than by assembling separate elements. For simplicity of the following description, the term "initial preform" will be used to refer to the initial glass structure from which the drawing process begins, with the understanding that it can include a preform made by assembling glass elements into a required configuration or a cane that has been drawn from such a preform. The term "intermediate cane" will be used to refer to an intermediate-width glass structure drawn from an initial preform, the "intermediate" descriptor indicating that it differs from a conventional cane that is separated from the original preform before being drawn into fiber. In the present context, the intermediate cane remains integral with the initial preform during the drawing of fiber from the intermediate cane. The term "fiber" is conventionally used to describe optical fiber drawn from an intermediate cane.

[0035] The initial preform has a preform width or diameter (first width), the intermediate cane has a cane width or diameter (second width) that is smaller than the preform width, and the fiber has a fiber width or diameter (third width) that is smaller than the cane width. The total drawdown ratio of the fiber drawing process is the ratio of the preform glass cross-sectional area to the fiber glass cross-sectional area. The total drawdown ratio is carried out in multiple stages or phases, including the cane drawdown ratio, which is the ratio of the preform glass cross-sectional area to the cane glass cross-sectional area, and the fiber drawdown ratio, which is the ratio of the cane glass cross-sectional area to the fiber glass cross-sectional width.

[0036] FIG. 6(a) illustrates an optical fiber draw tower during the first stage of the process, in which an intermediate cane is drawn from the initial preform. The initial preform 20, as is well known, has a cross-sectional structure including glass elements and voids configured and arranged to be converted into the required hollow core and microstructured cladding under the action of drawing and the applied pressure within the voids (the cross-sectional structure thus includes a hollow core surrounded by a plurality of voids that define a microstructured cladding). Typically, the initial preform 20 will have a length ranging from about 1 m to 3 m, although the invention is by no means limited in this respect. In this example, because drawing will be performed in a conventional vertical fiber draw tower, the upper end of the initial preform 20 is mounted on a support structure or apparatus 22, commonly referred to as a preform chuck, configured to vertically suspend the initial preform 20, which hangs freely from its upper end. Apparatus 22 is configured to allow connection of a gas supply line from a pressurized gas source 26 to the void in initial preform 20 during at least a portion of the drawing process to apply one or more pressures P to the void. Gas source 26 is under the control of a controller (not shown) so that the differential pressure applied to the void can be turned on, off, or adjusted during the draw.

[0037] The lower end of initial preform 20 is positioned within fiber draw furnace 28 such that initial preform 22 is inserted vertically into the central aperture of furnace 28, which is conventionally a vertically mounted tube furnace, so that the lower end portion of initial preform 20 is within furnace 28. Initial preform 20 has an initial preform width and a glass cross-sectional area Ap.

[0038] Next, the first stage of the actual draw can be performed. The furnace 28 is operated to heat a portion of the initial preform 20 therein and soften the glass therein. The softened glass is drawn from the lower end of the initial preform 20 to form a structure of narrower width and smaller glass cross-sectional area Ac, in this case a conventional intermediate cane 32, via a first neckdown 30. The drawing can be performed under gravity or, more commonly, by applying tension to the lower end of the intermediate cane 32. As the softened lower end of the initial preform 20 is transformed into the intermediate cane 30, the initial preform 20 is lowered by the downward movement of the apparatus 22, so that a further portion of it is positioned inside the furnace 28 and softened for drawing into the intermediate cane 30. The initial preform 20 is gradually fed into the furnace 28 (by moving the initial preform axially toward the furnace) until a predetermined portion of it has been drawn into the intermediate cane 32 having the length Lc. Pressure may or may not be applied to the voids of the initial preform 20 during drawing of the intermediate cane 32. If the cane drawdown ratio Ap:Ac is not too large, pressurized air may not be necessary to maintain the proper shape and size of the voids. In other cases, for example, if the cane drawdown ratio is relatively large or if specific void sizes are required in the initial cane 32 to achieve a specified void size or wall thickness in the finished fiber, pressurized air from the gas source 26 may be required.

[0039] FIG. 6(b) shows the next step in the method. Once the length Lc of the intermediate cane 32 has been drawn, the temperature of the furnace 28 is reduced by idling the furnace 28, where cooler operation there would not deform the glass (thus, the furnace is inoperable as long as it allows drawing). The motion of the apparatus 22 is directed in the opposite direction, so that the suspended initial preform 20, along with the intermediate cane 32 extending from its lower end, is retracted upward (the initial preform 20 is moved axially away from the furnace), and the first neck-down 30 is withdrawn from the furnace 28, so that the initial preform 20 and the intermediate cane 32 form a unitary structure. Retraction continues until a portion of the intermediate cane 30 spaced from the first neck-down 30 is within the furnace 28. Preferably, this is the lower or lowest portion of the intermediate cane 30, because the glass extending below the furnace 28 cannot be heated and drawn into fiber and is therefore discarded. The length Lc of the intermediate cane 32 formed in the first draw stage may be determined by the dimensions of the draw tower and the range of vertical movement of the apparatus 22. Advantageously, the length Lc is set so that the apparatus 20 can sufficiently lift the initial preform 20 so that the lower end of the intermediate cane 32 is positioned within the furnace 28 to minimize waste of the lower portion of the intermediate cane 32. On the other hand, a longer length of the intermediate cane 32 increases the amount of fiber that can be produced at one time in the second draw stage (described below) and minimizes the number of draw cycles required to consume the initial preform 20 in the finished fiber. Therefore, the maximum length of the intermediate cane 32 that the lifting apparatus 22 can accommodate can be advantageously made. However, if very long lengths could be accommodated, this could alternatively impose an upper limit on length Lc, as the ability of the pressure system in apparatus 22 to properly apply the differential pressure required for the second draw stage to the lower end of intermediate cane 32 away from apparatus 22 could be hindered because the applied gas would still have to travel along a narrow gap that would impose a pressure drop over the distance.However, it is generally expected that the actual length of the intermediate cane 32 will range from about 1 m to 10 m.

[0040] FIG. 6(c) shows the second stage of the actual draw. Once the portion of intermediate cane 30 of interest is positioned inside furnace 28, a differential pressure is applied from the pressure system in apparatus 22, spreading along the voids in initial preform 20 and reaching the voids in intermediate cane 32. Furnace 28 is activated so that heat is applied to intermediate cane 32, softening the glass in its lower portion. The required hollow-core optical fiber 34 is then drawn from the softened glass, which forms a second neck-down 36 at the bottom of intermediate cane 32. Fiber 32 can be drawn under gravity or tension, as desired, in the usual manner and collected by winding onto a spool or bobbin (not shown). Since fiber 34 has a fiber width and a glass cross-sectional area Af, the fiber draw-down ratio achieved in the second stage draw is Ac:Af. As the intermediate cane 32 is converted into fiber 34, the apparatus 22 is operated to move the intermediate cane 32 downward and gradually feed it into the furnace 28 so that the fiber 34 can continue to be drawn.

[0041] Once the intermediate cane 32 has been completely or partially consumed, the first neck-down 30 approaches the second neck-down 36, and the second draw stage is stopped. A length of hollow-core optical fiber 34 has been drawn and can be separated from the intermediate cane. The entire draw cycle, including the formation of the intermediate cane 32 from the initial preform 20 and the subsequent formation of fiber 34 from the intermediate cane 32, is now complete. A second cycle can then begin, producing more intermediate canes 32. The second draw stage pressure regime is switched off and replaced with the first draw stage pressure regime, if any. The furnace 28 is switched to idle to allow for repositioning of the equipment, and the equipment is moved downward to position the first neck-down 30 (or more generally, the end portion of the initial preform 20 around the first neck-down 30) within the furnace 28, as in FIG. 6(a). The furnace 28 is restarted to soften the glass at or near the first neckdown, and the first draw stage is repeated to draw an additional intermediate cane 32 from the initial preform 20, replenishing or restoring the previously consumed length by drawing fiber 34. Once the additional length Lc of intermediate cane 32 has been produced, the furnace is again switched to idle, the apparatus is retracted upward to position the second neckdown 36 (or more generally, a portion of the intermediate cane around the second neckdown 36) within the furnace, the second draw stage pressure regime is reactivated, the furnace 28 is started, and drawing of fiber 34 resumes. If the previously drawn length of fiber 34 was separated from the intermediate cane, this fiber draw produces a new length of fiber. If the previously drawn length of fiber 34 was not separated from the intermediate cane 32, a new fiber draw increases the fiber length. This completes the second cycle, and subsequent cycles can be repeated until the initial preform is completely consumed (excluding normal draw wear, in other words, all of the available or usable glass in the initial preform has been used) and converted into multiple or continuous lengths of fiber 34, which are collected on one or more spools.In this way, large overall drawdown ratios Ap:Af can be achieved, thereby enabling high fiber yields, thereby increasing fiber production efficiency. Furthermore, as noted above, dividing the drawdown ratio into smaller steps overcomes surface tension and pressure dynamics that can prevent fiber production at a single large drawdown ratio, thereby enabling the production of narrower fibers with thinner films capable of transmitting visible and ultraviolet wavelengths.

[0042] Although not mentioned above, the initial preform can include an outer glass jacket layer surrounding the preform structure to produce a finished fiber with an outer glass jacket, as is conventional. Typically, the preform structure defining the fiber cross-section is inserted into a larger outer glass tube before drawing, with the outer glass tube providing the jacket layer. Heating during the draw process melts the jacket glass around the inner structure, forming an integral, jacketed fiber. This is conventional in regular fiber drawing, where a preform or cane is directly drawn into fiber. In this situation, the outer glass tube is added around the preform or cane before drawing and only becomes a molten outer layer during fiber drawing. In contrast, under the currently proposed method, the outer glass tube is placed around the initial preform so that it is present when the intermediate cane is drawn from the initial preform. Thus, the glass for the jacket layer is melted around the intermediate cane during the first draw stage, becoming a molten outer layer. Thus, the jacket is already a molten layer before the fiber is drawn. Thus, an intermediate cane differs from a regular cane in that it includes a jacket as a fused layer, whereas a regular cane receives an outer glass tube as a jacket; the two are not fused together.

[0043] It should be noted that the thickness of the jacket layer at the initial preform stage, intermediate cane stage, or fiber stage may or may not contribute to the glass cross-sectional area from which the drawdown ratio is calculated, and the mathematical results are the same for both approaches.

[0044] As a practical example, the inventors have used the modified fiber drawing process described above to produce a hollow-core ARF with a film thickness of less than 200 nm, and thus suitable for visible and ultraviolet wavelength propagation. An initial preform (actually a cane previously drawn from a preform and positioned within an outer jacket tubular layer with an outer diameter of 28 mm) with a diameter of approximately 3.8 mm, originally designed to produce a fiber with a thicker film and a structure with a single capillary ring, was used. The initial preform was drawn into an intermediate cane with a diameter of approximately 1 mm, with a fused jacket as described above with an outer diameter of 6.6 mm, which was then drawn into fiber under appropriate pressure. Fiber yields of over 3 km were achieved with a film thickness of approximately 170 nm, as estimated from transmission measurements.

[0045] The above detailed description includes a single intermediate stage forming an intermediate cane between the initial preform and the fiber. However, if desired, additional intermediate stages may be included to provide a series of additional intermediate canes, each having a width smaller than that of the preceding intermediate cane. This divides the total drawdown ratio into more fractions of smaller drawdown ratios, which may be beneficial when larger ratios are not preferred, such as when pressing at any stage is easier to achieve with a smaller ratio. To perform additional stages, the intermediate cane may be a first intermediate cane with a cross-sectional area Ac1, which is drawn into a narrower second intermediate cane with a glass cross-sectional area Ac2 smaller than Ac1, which in turn is drawn into a still narrower third intermediate cane with a glass cross-sectional area Ac3 smaller than Ac2, and so on, with n being 2 or greater, to an nth intermediate cane with a glass cross-sectional area Acn greater than the fiber glass cross-sectional area Af, and so on, where n is 2 or greater. Each of these intermediate cane draws may or may not be pressed. The last or final intermediate cane is then drawn into fiber before being replenished by drawing from the preceding intermediate cane, and so on back to replenish the first intermediate cane from the initial preform.

[0046] FIG. 7 shows a flowchart of steps of an exemplary fiber drawing method according to the present disclosure. In a first step S1, a glass initial preform configured and structured for drawing into a hollow-core fiber is provided. In a second step S2, an end portion of the initial preform is heated to soften the glass in the end portion so that it is malleable for drawing. In a third step S3, a first draw stage is performed at a first drawdown ratio, in which a length of intermediate cane is drawn from the softened glass of the initial preform, the intermediate cane having a width and glass cross-sectional area smaller than that of the initial preform. The method moves to a fourth step S4, in which the intermediate cane remains attached to the initial preform and has its end (away from the initial preform) heated to soften the glass and make it malleable for drawing. This can be accomplished by retracting the initial preform from the furnace where the heating of step S2 occurred to position the end portion of the intermediate cane within the furnace, although other arrangements may be used as convenient depending on available equipment. In the fifth step S5, a second draw stage is performed at a second drawdown ratio, in which a length of hollow-core optical fiber is drawn from the softened glass of the intermediate cane, the fiber having a width and glass cross-sectional area smaller than the width and glass cross-sectional area of the intermediate cane. A differential pressure may be applied through the void in the initial preform during the second draw stage, or during both the first and second draw stages. The method proceeds to decision step S6, in which it is determined whether glass remains in the initial preform for further drawing. If more glass is available, the method proceeds to optional step S7, in which the drawn fiber is separated from the intermediate cane. This separation is omitted if the already achieved fiber length is preferably increased by further draws. The method then loops back and repeats steps S2 through S5 to draw additional fiber, either as a new, separate length or to extend the existing length.When all available glass from the initial preform has been consumed (or alternatively, the required amount of fiber has been produced), the method terminates in a final step S8, where heating and drawing ceases and the most recently drawn length of fiber, or a portion of the total length, is separated from the intermediate cane. Thus, a total length of fiber is produced from the initial preform, either as multiple individual lengths from each fiber draw stage, or as a single continuous length accumulated over multiple fiber draw stages.

[0047] 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 method's ability to increase fiber yield, and the ease with which large continuous lengths of hollow-core optical fiber can be 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.

[0048] As explained above, the described method proposes a modification to the drawing of hollow-core optical fiber by dividing the drawdown into two or more stages, thereby making it possible to reach greater overall drawdown ratios than previously available. This allows for benefits such as increased fiber yield from a single initial preform, longer continuous lengths of fiber, improved production efficiency, and improved production of fiber with thinner coatings. Previously, achievable drawdown ratios for hollow-core fiber drawing have been in the range of about 500 to 20,000, where drawdown ratio is the ratio of the total cross-sectional area of glass in the preform to the total cross-sectional area of glass in the fiber. This is based on typical preform widths ranging from about 6 mm to 30 mm, where this is the outer jacket width; the inner preform cladding width is about 20 to 40% of the outer width; and typical fiber widths are in the range of about 100 μm to 500 μm. In contrast, the proposed technique allows for the use of a significantly increased total drawdown ratio (initial preform glass cross-sectional area to finished fiber glass cross-sectional area) in the range of 10,000 to 3,000,000, but divided into two or more smaller drawdown ratios (note that the total drawdown ratio is the product of two or more smaller drawdown ratios). The division can be between any fraction of the total drawdown ratio. In the case of two drawdowns, in other words, a first drawdown from the initial preform to an intermediate cane, followed by a second drawdown from the intermediate cane to a finished fiber, it is expected that the first drawdown ratio could be in the range of 10 to 150 and the second drawdown ratio could be in the range of 1,000 to 20,000, although this is purely exemplary and other ranges for each of the two drawdown ratios are not excluded. It is envisioned that a larger fraction of the total drawdown ratio may be performed in the second stage of the draw process, while still keeping the drawdown ratio low enough to avoid MDC. This allows for longer spans of fiber from a given length of intermediate cane, although the achievable length of the intermediate cane may be limited by practical considerations such as the height of the available fiber draw tower.

[0049] For the fabrication of infrared hollow-core ARF optical fibers with membrane thicknesses ranging from approximately 300 to 2500 nm, it is expected that total drawdown ratios from initial preform to finished fiber can be achieved in the range of 200,000 to 1,000,000 for initial preform widths ranging from 20 to 200 mm. This contrasts with the much smaller preform widths currently typical, approximately 10 to 20 mm, and the correspondingly limited drawdown ratios of approximately 2,000 to 10,000. These types of hollow-core fibers have widths ranging from approximately 150 to 500 μm.

[0050] For the production of ultraviolet and visible hollow-core ARF optical fibers with membrane thicknesses ranging from 50 nm to 300 nm, it is expected that total drawdown ratios from initial preform to finished fiber can be achieved in the range of 20,000 to 3,000,000 when the width of the initial preform ranges from approximately 6 mm to 200 mm. These types of hollow-core fibers have widths ranging from approximately 50 μm to 250 μm. Currently, producing these types of fibers using conventional fiber drawing is impractical because it requires extremely high draw tensions with the attendant increased frequency of fiber breakage and consequently limited yield of salvageable fiber, or the use of very small initial preforms, which also limit fiber yield and make application of pressure during drawing more difficult.

[0051] However, these drawdown ratios are merely examples, and larger values may be available through careful control of the pressure regime and other operating parameters of the draw tower, such as furnace temperature, draw tension, and draw speed.

[0052] 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.

[0053] 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] F Yu et al., “Single-mode solarization-free hollow-core fiber for ultraviolet pulse delivery,” Opt. Express, vol. 26, 10879-10887 (2018) [5] S.F. Gao et al., "Hollow-core conjoined-tube negative-curvature fiber with ultralow loss," Nat. Commun, 9, p. 2828 (2018). [6]CN111474628

Claims

1. A method for manufacturing 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 an optical fiber drawn from the initial preform, comprising a hollow core surrounded by a plurality of voids defining a microstructured cladding, and the initial preform having a first glass cross-sectional area. The steps include heating the edge portion of the initial preform to soften the glass in the edge portion, A step of drawing an intermediate cane of a certain length from the softened glass of the initial preform via a first neck-down, wherein the intermediate cane has a second glass cross-sectional area smaller than the first glass cross-sectional area, A step of heating the portion of the intermediate cane spaced away from the first neck down in order to soften the glass in that portion, wherein the intermediate cane remains integrated with the initial preform, A step of drawing a hollow core optical fiber of a certain length from the softened glass of the intermediate cane via a second neck-down, wherein the hollow core optical fiber has a third glass cross-sectional area smaller than the second glass cross-sectional area. A method that includes this.

2. A method according to claim 1, wherein after drawing the hollow core optical fiber of a certain length from the softened glass of the intermediate cane, The steps include heating the end portion of the initial preform at or near the first neck-down to soften the glass at the end portion, To recover the intermediate cane of a certain length, the steps include drawing the intermediate cane from the softened glass of the initial preform, The steps include heating the intermediate cane at or near the second neck-down to soften the glass of the intermediate cane, The steps of drawing a further length of the hollow core optical fiber from the softened glass of the intermediate cane, A method that includes further steps to perform the task.

3. The method according to claim 2, wherein the desired total length of the hollow core optical fiber is drawn. And, on the other hand, all usable glass of the initial preform is consumed. A method further comprising repeating the steps of claim 2 until both of the above occur.

4. A method according to claim 1, further comprising the step of applying one or more pressures to the voids in the cross-sectional structure of the initial preform during the drawing of the hollow core optical fiber.

5. A method according to claim 4, further comprising the step of applying one or more pressures to the voids in the cross-sectional structure of the initial preform during the drawing of the intermediate cane.

6. A method according to claim 1, further comprising the step of separating a certain length of the hollow core optical fiber from the intermediate cane.

7. A method according to claim 1, wherein the method is performed in an optical fiber laying tower, and the furnace of the optical fiber laying tower provides the heating.

8. A method according to claim 7, wherein the steps of drawing the intermediate cane and drawing the hollow core optical fiber include axial motion of the initial preform toward the furnace to provide further heating and softening of the glass.

9. A method according to claim 8, further comprising, after drawing the intermediate cane, an axial movement of the initial preform away from the furnace in the axial direction in order to position the intermediate cane in the furnace for heating.

10. A method according to claim 1, wherein the intermediate cane of a certain length is in the range of 1 m to 10 m.

11. A method according to claim 1, wherein the first glass cross-sectional area and the third glass cross-sectional area are in the range of 10,000 to 3,000,000, for defining the total drawdown ratio of the first glass cross-sectional area to the third glass cross-sectional area.

12. A method according to claim 1, wherein the first glass cross-sectional area and the second glass cross-sectional area define a first-stage drawdown ratio of the first glass cross-sectional area to the second glass cross-sectional area, wherein the first glass cross-sectional area and the second glass cross-sectional area are in the range of 10 to 150, and the second-stage drawdown ratio of the second glass cross-sectional area to the third glass cross-sectional area defines a second-stage drawdown ratio of the second glass cross-sectional area to the third glass cross-sectional area, wherein the second glass cross-sectional area and the third glass cross-sectional area are in the range of 1,000 to 20,000.

13. A method according to claim 1, wherein the first glass cross-sectional area and the second glass cross-sectional area define a first-stage drawdown ratio of the first glass cross-sectional area to the second glass cross-sectional area, and the second glass cross-sectional area and the third glass cross-sectional area define a second-stage drawdown ratio of the second glass cross-sectional area to the third glass cross-sectional area, wherein the first-stage drawdown ratio is smaller than the second-stage drawdown ratio.

14. A method according to claim 1, further comprising the step of surrounding the initial preform with an outer glass layer before heating the end portion of the initial preform so that the intermediate cane is melted and drawn together with the outer glass layer, wherein the outer glass layer is intended to be a jacket for the hollow core optical fiber.

15. A method according to claim 1, further comprising the step of drawing one or more additional intermediate canes from the intermediate cane, before drawing the hollow core optical fiber from the end of the additional intermediate cane, wherein each additional intermediate cane has a glass cross-sectional area smaller than the glass cross-sectional area of ​​the preceding intermediate cane.

16. The method according to claim 1, wherein the hollow core optical fiber is drawn such that the thickness of the air gap in the hollow core optical fiber is greater than 200 nm. ,method.

17. A method according to claim 1, wherein the hollow core optical fiber is drawn such that the thickness of the air gap in the hollow core optical fiber is 200 nm or less.

18. A hollow core optical fiber manufactured using the method described in claim 1.

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