Hollow-core optical fiber stretching method with improved stretching

Abstract

A method of manufacturing a hollow-core optical fiber includes: providing an initial preform formed of glass and having a cross-sectional structure configured to form the cross-sectional structure in an optical fiber stretched from the preform, the cross-sectional structure including a hollow core surrounded by a plurality of voids defining a microstructured cladding; heating an end portion of the preform; stretching a length of an intermediate billet from the softened glass of the preform via a first necking, the billet having a glass cross-sectional area that is less than the glass cross-sectional area of the preform; heating a portion of the rod spaced from the first necking, the rod being held integrally with the preform; and stretching a length of a hollow-core optical fiber from the softened glass of the billet via a second necking, the hollow-core optical fiber having a glass cross-sectional area that is less than the glass cross-sectional area of the billet.

Description

Background Art

[0001] The present invention relates to a method for stretching a hollow core optical fiber, the method having a modified stretch.

[0002] Optical fibers typically have a solid core design that includes a solid glass core surrounded by a solid glass cladding of 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 having a cross-sectional structure matching the intended refractive index profile of the finished fiber but with a width many times the width of the finished fiber is lowered axially into an annular furnace that heats and softens the glass. The softened glass is then pulled or stretched from one end of the preform under gravity or under tension to a reduced width for the finished fiber, which is wound onto a spool. In some processes, a preform of narrower width is first stretched from the preform and separated from the preform to be stretched into a fiber at a later time. The process of stretching from the preform to the preform to narrow the width of the glass structure is called drawing.

[0003] In recent years, hollow core optical fibers have been developed. These hollow core optical fibers have a central longitudinal hole or void defining a hollow core, which is surrounded by a cladding that typically includes a plurality of smaller longitudinal holes or voids in a specific geometry, which may be referred to as a microstructure. Depending on the geometry of the microstructured cladding, light is guided along the hollow core by one of several mechanisms. Hollow core fibers are attractive because they have superior light propagation properties 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 amount of air and correspondingly reduced amount of glass inside the fiber. Light propagates primarily in air, thereby avoiding deleterious effects due to propagation in glass. Hollow core fibers can also be made by a fiber drawing process, starting with a preform (optionally drawn into a preform) that forms a desired void cross-sectional profile. In order to achieve the desired geometry of the core and cladding in the finished fiber (which requires precise location, void size, and thickness of the glass film between the voids) for obtaining the optical properties expected for a particular fiber design, pressure is typically applied to the voids during drawing of the fiber from a preform or billet. The pressurization counteracts the surface tension in the softened glass, which would otherwise tend to cause collapse of the voids and destruction of the intended structure.

[0004] Fiber draw can be measured by a parameter known as the draw ratio. This is the ratio of the cross-sectional area of ​​the glass in the preform to the cross-sectional area of ​​the glass in the fiber, where the relevant cross section is transverse to the longitudinal axis of the glass structure (i.e., the preform, the fiber, and, as discussed later, the blank). It is equal to the number of meters of fiber that can be drawn per meter of preform, so that high draw ratios are desirable. Larger preforms contain a greater volume of glass and therefore would be expected to produce more fiber. For a given volume of glass, preform size can be increased by increasing the preform width [1], which indicates a higher draw ratio (more glass available per meter of preform) or by increasing the preform length, which reduces the draw ratio (less glass available per meter of preform). However, for hollow-core fibers, the dynamics of pressurization are more complex and difficult to control when drawing a wider preform into a narrower fiber because the changes in the outer diameter and diameter of the hole are more extreme. Therefore, high draw ratios are generally not applicable or even achievable for hollow-core fiber manufacturing. Therefore, for successful hollow core optical fiber manufacturing, narrower preforms and corresponding lower draw ratios are generally required, giving smaller fiber yields and corresponding inefficiencies in fiber production. However, pressurization can be more difficult to achieve with narrower preforms because the voids are smaller and more difficult to access; this can reduce the structural quality of the finished fiber. Therefore, there are several factors that hinder the efficient, high-volume manufacturing of high-quality hollow core optical fibers. As a result, the cost of hollow core fibers is high and the maximum achievable length of the fibers is limited compared to solid fibers. These factors are particularly detrimental in view of the excellent optical properties of hollow core fibers, which make them very attractive for many applications, including long-distance telecommunications, for which very long lengths of inexpensive fibers are desirable.

[0005] Telecommunication methods typically propagate light at infrared wavelengths, where attenuation in glass is lower. Optical fibers configured to propagate visible and ultraviolet light are also of interest for applications including quantum computing, Raman spectroscopy, photochemistry, and wastewater pollutant detection. In solid core fibers of silica glass, exposure to ultraviolet light creates permanent defects in the glass structure, resulting in increased attenuation. Hollow core fibers are less susceptible to such damage due to reduced light-glass interactions, and are therefore good candidates for light propagation at short optical wavelengths. However, the wavelengths at which hollow core fibers can propagate with low loss depend on the precise geometry of the core and cladding. By using thinner glass films around the voids in the cladding, shorter wavelengths can be propagated most efficiently with a wider bandwidth. Thinner films are more difficult to manufacture because the pressurization methods that can effectively counteract the surface tension effects during stretching are more difficult to achieve for thinner glass. For shorter wavelengths, the entire fiber structure is also smaller, which means that a larger stretch ratio is required, which is problematic for hollow core fiber manufacturing as mentioned above.

[0006] Thus, there are a series of difficulties in hollow core optical fiber production that tend to limit achievable fiber length and wavelength performance, reduce efficiency, and increase complexity and cost.

[0007] Therefore, methods to improve hollow core optical fiber fabrication are of interest. Summary of the invention

[0008] Various aspects and embodiments are set out in the accompanying claims.

[0009] According to a first aspect of certain embodiments described herein, there is provided a method for manufacturing a hollow-core optical fiber, the method comprising: providing an initial preform formed of glass and having a cross-sectional structure, the cross-sectional structure being configured to form a cross-sectional structure in an optical fiber drawn from the initial preform, the cross-sectional structure comprising 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 of the end portion; drawing an intermediate billet of a certain length from the softened glass of the initial preform via a first necking, the intermediate billet having a second glass cross-sectional area smaller than the first glass cross-sectional area; heating a portion of the intermediate billet spaced apart from the first necking to soften the glass of the portion, the intermediate billet being maintained integral with the initial preform; and drawing a hollow-core optical fiber of a certain length from the softened glass of the intermediate billet via a second necking, the hollow-core optical fiber having a third glass cross-sectional area smaller than the second glass cross-sectional area.

[0010] According to a second aspect of some embodiments described herein, there is provided a hollow core optical fiber manufactured using the method according to the first aspect.

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

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

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

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

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

[0016] Figure 4 shows a graph of fiber capillary diameter versus applied pressure for an example fiber drawing process modeled according to a conventional approach in which the fiber is drawn directly from a preform;

[0017] Figure 5 Shows the Figure 4 Model identical fibers and preforms, plot of fiber capillary diameter versus applied pressure for an example fiber drawing process modeled in accordance with the present disclosure.

[0018] Figure 6 (a) to Figure 6 (c) shows a schematic diagram showing the steps of drawing a fiber in a drawing tower according to an exemplary method of the present disclosure; and

[0019] Figure 7 A flow chart showing steps in an example fiber drawing method according to the present disclosure is shown. DETAILED DESCRIPTION

[0020] Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally, and for the sake of brevity, these aspects and features are not discussed / described in detail. It will therefore be understood that aspects and features of the devices and methods discussed herein that are not described in detail may be implemented according to any conventional techniques for implementing such aspects and features.

[0021] The present disclosure proposes a method for manufacturing hollow core optical fibers which aims to solve some of the problems with known methods to provide improved fiber yield and enhanced fiber structure.

[0022] A hollow core optical fiber has a core in which light is guided, the core comprising a central longitudinal hole or void (usually filled with air, but alternatively filled with another gas or gas mixture, or a vacuum), the central longitudinal hole or void being surrounded by a cladding comprising a structured arrangement of longitudinal holes, voids, or capillaries (microstructure) extending along the length of the fiber. The absence of a solid glass core reduces the proportion of guided light waves propagating in glass compared to that in a solid core fiber, thereby providing benefits such as increased propagation speed, reduced losses from absorption and scattering, and reduced nonlinear interactions. These properties make hollow core optical fibers very attractive for use in many applications. These include optical telecommunications systems. However, it is currently difficult to manufacture longer lengths of hollow core optical fiber, which is desirable for telecommunications systems, where data centers between which optical data is transmitted may be tens or hundreds of kilometers apart. For example, the longest reported single span of a hollow core optical fiber known to the inventors with reasonable propagation loss numbers (4 to 5 dB / km) is only 12 km [2], while other reports show that 14 km can be performed with 10 dB / km losses [3]. At shorter wavelength operation, thinner membrane structures are required to define the cladding holes, which has only been achieved at much shorter lengths, such as 33.6 m [4].

[0023] Hollow core optical fibers can be classified into two main categories or types according to their optical guiding mechanism: hollow core photonic bandgap fibers (HCPBFs, often alternatively referred to as hollow core photonic crystal fibers, HCPCFs) and antiresonant hollow core fibers (AR-HCFs or ARFs). There are various subclasses of ARFs characterized by their geometric structures, including Kagome fibers, nested antiresonant knotless fibers (NANFs), and tubular fibers. The present disclosure is applicable to all types of hollow core fibers, including these two main categories and their associated subtypes plus other hollow core designs. Note that in the art, there is some overlapping use of terms for various categories of fibers. For the purposes of the present 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 a hollow core as described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow core fibers (described in more detail below) having a structure that provides waveguiding through the photonic bandgap effect. The terms "ARF" and "antiresonant hollow core fiber" are used to refer to a hollow core fiber having a structure that provides waveguiding by the antiresonance effect (also described in more detail below).

[0024] Figure 1A schematic cross-sectional view of an example HCPBF 10 is shown. In this fiber type, the structured internal cladding 1 comprises a regular, close-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 provides a periodic structure of the refractive index and therefore a photonic bandgap effect which confines light waves propagating toward the core. This is the fundamental optical mode or fundamental mode (FM). One or more higher order optical modes or higher order modes (HOMs) may be supported in the core 2 and / or the cladding 1. These fibers can be described in terms of the number of cladding capillaries or "cells" that are excluded to make the core 2. Figure 1 In the example, 19 elements from the center of the array are absent in the core region, making it a 19-element core HCPBF. The structured cladding 1 is formed by six rings of elements around the core 2, with some elements in the seventh ring added to improve the roundness of the outer surface of the cladding. An outer cladding or sheath 3 surrounds the structured cladding 1.

[0025] In contrast to HCPBF, antiresonant hollow core fiber guides light through the antiresonant light guiding effect. The structured cladding of ARF has a simpler configuration, including much fewer larger glass capillaries or tubes than HCPBF to give a structure that lacks any highly periodicity, so that the photonic bandgap effect is not significant. Instead, antiresonance is provided for propagating wavelengths that do not resonate with the wall thickness of the cladding capillary, in other words, for wavelengths in the antiresonance window defined by the wall thickness of the cladding capillary. The cladding capillary surrounds a central void or cavity that provides the hollow core of the fiber, and the central void or cavity can support antiresonant guided light modes, including fundamental modes and one or more higher-order modes. The structured cladding can also support cladding modes that can propagate mainly inside the capillary, in the glass of the capillary wall, or in the space or gap between the cladding capillary and the outer cladding of the fiber. The losses of these additional high-order non-core guided modes are usually much higher than those of the core guided modes. The fundamental core guided mode usually has the lowest loss among the core guided modes. Anti-resonance is provided by the capillary wall thickness being anti-resonant with the wavelength of the propagating light to suppress coupling between the fundamental core mode and any cladding modes so that the light is confined to the core and can propagate with very low loss.

[0026] Figure 2A schematic cross-sectional view of an example simple antiresonant hollow core fiber is shown. The fiber 10 has an outer tubular cladding or jacket 3. The structured inner cladding 1 includes a plurality of tubular cladding capillaries 14, in this example seven capillaries of the same cross-sectional size and shape arranged in a single ring inside the outer cladding 3 such that the longitudinal axes of each cladding capillary 14 and the outer cladding 3 are substantially parallel. Each cladding capillary 14 contacts (bonds) the inner surface of the outer cladding 3 at an azimuthal position 16 such that the cladding capillaries 14 are evenly spaced around the inner circumference of the outer cladding 3 and are also spaced from each other by gaps 5 (with no contact between adjacent capillaries). In some designs of the ARF, the cladding tubes 14 may be positioned so as to contact each other (in other words, not as Figure 2 The spacing of the cladding capillaries is not as spaced as in the example above, but spacing that eliminates such contact improves the optical performance of the fiber and is generally preferred. Spacing 5 eliminates nodes that occur at the contact points between adjacent tubes and that tend to cause undesirable resonances that result in higher losses. Therefore, fibers with spaced cladding capillaries can be referred to as "knotless antiresonant hollow core fibers."

[0027] The arrangement of the cladding capillaries 14 in a ring around the interior of the tubular outer cladding 3 creates a central space, cavity or void within the fiber 10, whose longitudinal axis is also parallel to the longitudinal axis of the outer cladding 3 and the capillaries 14, which is the hollow core 2 of the fiber. The core 2 is bounded by the inwardly facing portion of the outer surface of the cladding capillaries 14. This is the core boundary, and the material of the capillary walls (e.g., glass or polymer) that constitutes this boundary provides the desired anti-resonant optical guiding effect or mechanism. The capillaries 14 have a thickness t at the core boundary, which defines the wavelength bandwidth over which anti-resonant optical guiding occurs in the ARF.

[0028] Figure 2 Only one example of an ARF is shown, and many other ARF structures are known.

[0029] Figure 3A schematic transverse cross-sectional view of a second example ARF is shown. 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. In this example, at the same azimuthal position 16 as the junction point between the primary capillary 14 and the outer cladding 3, each cladding capillary 14 has a secondary smaller capillary 18 nested inside it, which is bonded to the inner surface of the cladding capillary 14. These additional smaller capillaries 18 can reduce optical losses. Additional smaller tertiary capillaries can be nested inside the secondary capillaries 18. This type of ARF design with secondary and optionally smaller further capillaries can be referred to as a "nested antiresonant nodeless fiber" or NANF, where a "double nested antiresonant nodeless fiber" or DNANF refers to a fiber including tertiary nested capillaries.

[0030] Many other capillary configurations for the structured cladding of an ARF are possible, and the present disclosure is not limited to the examples described above. For example, the capillaries need not have a circular cross-section, and / or may or may not all be of the same size and / or shape. The number of capillaries surrounding the core may be, for example, four, five, six, seven, eight, nine, or ten, but other numbers are not excluded. The cladding capillary ring in an ARF produces a core boundary having a shape comprising a series of adjacent inwardly curved surfaces (i.e., convex from the perspective of the core). This is in contrast to the typical outward curvature of the core-cladding interface in conventional solid core fibers and HCPBF (see Figure 1 ) in contrast to the substantially circular core boundary of the HCPBF. Thus, antiresonant hollow core fibers can be described as negative curvature fibers. Kagome-class ARFs can also be configured as negative curvature fibers and have a structured cladding of multiple small capillaries in an array, similar to a HCPBF, but are not configured to provide a photonic bandgap. In contrast to the HCPBF, the guiding mechanism operates through the antiresonance effect. Other examples of ARFs include designs having a cladding formed as a combined tube structure[5] and a hemispherical tube structure[6].

[0031] As used herein, the terms hollow core fiber, hollow core fiber, hollow core waveguide, hollow core optical waveguide and similar terms are intended to cover optical waveguide structures configured according to any of the above examples and similar structures, wherein light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonance guiding and / or forbidden coupling guiding) in an empty elongated void or core surrounded by a structured cladding comprising a plurality of longitudinal capillaries. The capillaries include or define an elongated hole, void, cavity, cell or cavity extending continuously along the length or longitudinal extent of the optical fiber, substantially parallel to the elongated core also extending continuously along the length of the optical fiber. These various terms may be used interchangeably in the present disclosure.

[0032] As described above, the hollow core fiber manufacturing method proposed in this article is applicable to all and any types of hollow core fibers. As a specific example, the manufacture of an antiresonant hollow core fiber with a single ring of non-nested cladding capillaries, such as Figure 2 While the proposed method can improve hollow core fiber manufacturing for all fiber types, a particular benefit is the enhanced manufacturing of antiresonant fibers that are configured to guide light at shorter wavelengths, meaning visible wavelengths, and particularly ultraviolet wavelengths, that are not easily produced using conventional fiber drawing techniques. The proposed method, in addition to providing improved yields and longer lengths for hollow core fibers generally, enables the production of very thin cladding films required for short wavelength antiresonance. For example, kilometer-scale yields of antiresonant hollow core fibers configured for broadband propagation of ultraviolet and visible wavelengths of light are achievable, with similar orders of magnitude or orders of magnitude improvements in yields provided for fibers configured for near-infrared light propagation required for telecommunications applications.

[0033] The proposed method utilizes modifications to the fiber drawing process that enable hollow core fibers to be made at higher total draw ratios than conventionally available, resulting in increased fiber yields. In other words, hollow core fibers can be successfully produced from relatively wider preforms or preforms than previously possible. In some examples, hollow core fibers configured for infrared wavelength propagation can be drawn from wider preforms or preforms than are typically used for such fibers; wider preforms or preforms are typically prohibited by high draw ratios. This provides increased yield per drawn fiber, gives higher production efficiency and provides much longer continuous fiber lengths. In other examples, hollow core fibers configured for visible or ultraviolet propagation can be drawn from preforms or preforms designed for infrared fiber production, which are also typically prohibited by high draw ratios. Providing and using narrower preforms in the context of conventional drawing to reduce the draw ratio for these thinner and more delicate fibers is complicated, thus again improving production efficiency and allowing greater fiber lengths to be drawn.

[0034] The proposed method is modified by introducing one (or more) additional stretching stages to introduce at least one intermediate preform or preform width between the initial preform or preform (where the preform is conventionally prepared by stretching the preform to a narrower width and separating it from the preform) and the finished fiber compared to conventional stretching processes. Unlike conventional fiber manufacturing processes, in which the preform is stretched into a narrower preform, which is separated from the preform at a convenient length, and then stretched itself separately to produce fibers from the preform, the proposed method does not separate the intermediate preform from the initial preform or preform before the intermediate preform is stretched into fibers. Therefore, the total stretch ratio from the initial preform to the finished fiber is distributed over two (or more) consecutive stages, each of which is easier to manage in terms of pressurization than a single larger stretch. The method can be described as a staged stretch, in which the total stretch ratio from the initial preform or preform to the finished fiber is achieved in two or more stages or periods within the same total stretching program, each of which produces a portion of the desired total stretch. Compared to narrower preforms, the retention of the initial preform or preform at the non-fiber end of the entire glass structure promotes the application of pressure to the void and facilitates handling. When the fiber drawing has consumed the intermediate preform, another portion of the intermediate preform is drawn from the initial preform or preform, and the process continues. The reduced draw ratio provided by the intermediate preform entering the fiber provides a more manageable surface tension dynamic in the softened glass, enabling fiber geometries to be drawn that would otherwise be unattainable if they were attempted to be drawn directly from the same initial preform. This is enhanced by repeatedly replenishing the intermediate preform from the initial preform or preform. This allows the entire initial preform or preform (minus the usual waste) to be drawn into a hollow-core optical fiber, greatly increasing the fiber yield for a given volume of initial preform or preform.

[0035] Consider the example of an antiresonant hollow core fiber in which the cladding comprises a single ring of spaced and optionally nested capillaries or tubes arranged around the hollow core, such as Figure 2 and 3 According to the limited resonant wavelength λ m The capillary has a wall thickness (film thickness) that is selected based on the wavelength of light the fiber is intended to guide:

[0036]

[0037] Where t is the wall thickness, n is the refractive index of the glass, and m is the resonance order, with m=1 generally preferred to provide the widest bandwidth transmission window. High-performance hollow-core fibers have been developed for near-infrared propagation around 1550 nm, which are suitable for telecommunication applications. These fibers have a film thickness of about 500 nm, and due to the various difficulties in stretching hollow-core fibers discussed above, current production is limited to a single span length of about 10 km. We can define a film thickness of about 300 nm as the boundary between infrared fibers and shorter wavelength fibers, the latter of which are more difficult to manufacture. For guiding shorter wavelengths over a useful transmission bandwidth in the visible and ultraviolet regions of the spectrum, much thinner films of about 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, the manufacture of such fibers by conventional stretching from standard preforms and blanks designed for near-infrared hollow-core fibers requires a greatly increased draw ratio, which is difficult or impossible to achieve. The stage drawing method proposed in this article solves these problems.

[0038] As discussed above, current hollow core fiber designs (including HCPBF and ARF) require that pressure be applied to the voids in the preform or billet during stretching. The pressurization is usually different because two or more different pressures are applied to different voids. Pressure is needed to counteract the surface tension in the softened glass during stretching. If not checked by the applied pressurization, the surface tension is used to shrink or collapse the voids and can change or destroy the intended fiber structure. (Multiple) The adjustment of pressures can accurately control the final geometry of the fiber. During stretching, the glass structure undergoes a geometric change called necking. This is the narrowing of the glass structure, where the glass structure is softened and becomes flexible in the stretching furnace, and the width is continuously reduced from the width of the preform or billet to the width of the fiber over a certain length of the structure. When pressurization is achieved, the geometry is dominated by pressure at the wider end (usually the upper end, because fiber stretching is usually implemented vertically) of the necking (closest to the pressure input at the open, unstretched end of the preform or billet). At the narrower end of the constriction, where the fiber forms, the geometry is dictated by the surface tension in the softened glass, which causes the capillary to contract. Thus, there is an interaction between the force of the applied pressure and the surface tension, and to achieve the desired capillary size and film thickness, sufficient pressure must be applied to initially expand the capillary beyond the desired size so that subsequent contraction under the surface tension at the lower end of the constriction pulls the capillary back to the target size.

[0039] Attempts to increase the fiber yield of stretching by using a wider preform or billet so that more fibers can be produced from the preform or billet increase the complexity of the pressure-surface tension relationship and make it more difficult to identify the correct parameters for stretching. Eventually, the contraction phase of the stretching dynamics becomes very intense and requires increased capillary expansion under higher applied pressure to offset it. However, in glass structures, there is limited space for capillary expansion, and there is a point at which the expansion overshoot required to pre-eliminate contraction causes unacceptable deformation in the fiber structure. In particular, in ARF designs where the capillaries should be spaced, contact may occur between adjacent capillaries, causing the capillaries to fuse together. In HCPBF designs, overall asymmetry of the hollow core may occur. This defect is very unfavorable to the optical properties and performance of the finished fiber, so it needs to be avoided. For convenience, this phenomenon can be referred to as intermediate drawing contact (MDC), and is an important limiting factor in increasing the fiber yield of hollow core fiber production. For shorter wavelength fibers, the required thinner membranes and smaller capillaries are more sensitive to pressure and surface tension, and the effects of MDC are further amplified, forcing the use of smaller draw ratios and corresponding reductions in throughput. If the proposed draw ratio from preform / preg to fiber is greater than the limit imposed by MDC, the desired fiber cannot be made.

[0040] Figure 4 A graph showing the results of a computer simulation of fiber drawing to form an ARF using conventional drawing from a preform or billet directly to a fiber with a single draw. The graph plots the outer diameter of the cladding capillaries in the finished ARF fiber as a function of the pressure differential applied to the capillaries during drawing; a higher pressure differential is required to expand to a larger capillary size and a thinner capillary film. The thick vertical line shows that the target diameter for the capillaries is between 11 and 12 μm, corresponding to a capillary film thickness of 150 nm for guiding light in the UV-visible wavelength range, and the particular fiber is modeled. A pressure differential of 2.5 kPa needs to be applied to the capillaries to achieve this goal. The shaded area indicates where the MDC occurs, starting at an applied pressure of 2.497 kPa. Therefore, the target fiber structure is above the upper pressure limit for avoiding MDC, and the intended fiber cannot be made from the specified billet.

[0041] To address this problem and overcome the limitations imposed by MDC, the proposed method introduces intermediate stages in the stretching, such that an initial billet or preform is first partially stretched into an intermediate narrower billet or preform, which is subsequently further stretched to form a fiber while still being integral with the initial billet or preform, with an increased pressure being applied during at least the second stretching stage and optionally during the first stretching stage.

[0042] Figure 5A graph showing computer simulation results of fiber stretching with Figure 4 The same initial billet or preform in the simulation forms the same ARF, but is modified by an additional intermediate stretching stage. Figure 4 The modeling is the same, so the overall draw ratio is the same, but the draw is divided into two stages, which are implemented in a single draw process, during which the initial preform / billet can be manipulated in the draw tower while maintaining the pressurization connection. Again, the target capillary diameter between 11μm and 12μm is indicated by the thick vertical line. However, the target fiber structure is now well below the pressure differential marking the onset of MDC, as shown by the shaded area. The target fiber is now obtainable at a pressure differential below the MDC threshold, and the intended fiber can be successfully produced from the specified billet. Note that the applied pressure differential required to produce the fiber is now 26.56kPa, which is 26.56kPa higher than the MDC threshold. Figure 4 The pressure difference required to achieve the same fiber structure is an order of magnitude higher in the model, but despite this, MDC contact is avoided. The higher pressure difference required to achieve the same fiber structure originates from the lower draw ratio of the initial preform, which enables drawing at higher viscosity and correspondingly higher drawing tension. The increased viscosity requires more pressure to expand the glass structure, but this is acceptable because it is desirable to draw at the highest possible tension while avoiding glass structure fracture that occurs when the tension is too high. The high tension helps to maintain the desired internal glass structure. Therefore, the limitations imposed by MDC under conventional drawing are overcome, and the expected fiber can be successfully manufactured.

[0043] Figure 6 (a) to Figure 6(c) shows a simplified schematic diagram of an optical fiber drawing apparatus at various stages in a hollow core fiber drawing method according to an example of the present disclosure. As a preliminary, the terms preform and preform will be discussed and defined. In optical fiber manufacturing, the first step is to form a relatively large cylindrical glass structure, referred to as a preform. For hollow core optical fibers, the preform has a cross-sectional structure containing glass elements that, once drawn, will form the desired structural elements in the finished optical fiber, wherein the fusion, expansion and contraction of the glass elements that occur during drawing are taken into account in the preform structure so that the final fiber structure is as desired. The preform is manufactured by assembling the glass elements that make up the preform into an appropriate arrangement. For hollow core fibers, the preform is typically manufactured by stacking a plurality of capillaries that will define the fiber cladding inside an outer glass tube that supports the cladding tube, possibly with spacer elements to hold the capillaries in place. The bonding of the elements can be implemented to fix the capillaries in position relative to the outer tube, or the bonding can be performed as part of the drawing process. The preform can be directly drawn into the desired fiber. Alternatively, in conventional methods, the preform can be stretched into one or more individual blanks, which have an intermediate width between the preform and the desired fiber, and a length of, for example, about one meter. Each blank can then be stretched into a separate length of the desired fiber. Blanks can be easier to handle than preforms, and also reduce the draw ratio for stretching into fibers. However, the terms preform and blank tend to be used interchangeably to some extent, and therefore some clarification is needed in this context. A preform can be considered to be any glass structure that can be stretched into a narrower structure (it is a preform in the form of a narrower structure). Under this definition, a preform made by stacking or other assembly of glass elements constitutes a preform that will be stretched into a blank or fiber, but similarly a blank can be considered a preform because it can be stretched into a narrower blank or fiber. Similarly, a blank is a glass structure that can be stretched into a narrower structure (a narrower blank or fiber), but it is formed by stretching from a wider glass structure, rather than by assembling separate elements. For simplicity, in the following description, the term "initial preform" will be used to refer to the initial glass structure at the beginning of the drawing process, and it should be understood that it can include a preform made by assembling glass elements into a desired structure, or a blank that has been drawn from such a preform. The term "intermediate blank" will be used to refer to a glass structure of intermediate width drawn from the initial preform, and the descriptor "intermediate" indicates the difference from a conventional blank that is separated from its initial preform before being drawn into a fiber. In this article, the intermediate blank remains integral with the initial preform during the process of drawing the fiber from the intermediate blank. The term "fiber" is generally used to refer to the optical fiber drawn from the intermediate blank.

[0044] The initial preform has a preform width or diameter (first width), the intermediate billet has a billet width or diameter (second width) less than the preform width, and the fiber has a fiber width or diameter (third width) less than the billet width. The total draw 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 draw ratio is achieved in stages or periods, including the billet draw ratio as the ratio of the preform glass cross-sectional area to the billet glass cross-sectional area and the fiber draw ratio as the ratio of the billet glass cross-sectional area to the fiber glass cross-sectional area width.

[0045] Figure 6 (a) shows an apparatus as an optical fiber drawing tower during the first stage of the method, in which an intermediate billet is drawn from an initial preform. As is known, the initial preform 20 has a cross-sectional structure including, the glass elements and voids are configured and arranged to transform into the desired hollow core and microstructured cladding under the action of drawing and applying pressure in the voids (thus, the cross-sectional structure includes a hollow core surrounded by a plurality of voids defining a microstructured cladding). Typically, it will have a length in the range of about 1m to 3m, although the invention is by no means limited in this respect. In this example, the drawing will be performed in a conventional vertical fiber drawing tower, so the upper end of the initial preform 20 is mounted on a support structure or rig 22, generally referred to as a preform assembly block, which is configured to vertically suspend the initial preform 20, hanging freely from its upper end. The rig 22 is configured so that a gas supply line from a pressurized gas source 26 can be connected to the void in the initial preform 20 during at least part of the drawing process so that one or more pressures P are applied to the void. The gas source 26 is under the control of a controller (not shown) so that the pressure differential applied to the gap can be turned on, off or adjusted during stretching.

[0046] The lower end of the initial preform 20 is placed in a fiber drawing furnace 28, which is generally a vertically mounted annular furnace, so that the initial preform 22 is vertically inserted into the central hole of the furnace 28 so that the lower end portion of the initial preform 20 is within the furnace 28. The initial preform 20 has an initial preform width and a glass cross-sectional area Ap.

[0047] The first stage of the actual stretching can then be carried out. The furnace 28 is operated to heat the portion of the initial preform 20 therein and soften the glass of that portion. In the usual manner, the softened glass is stretched from the lower end of the initial preform 20 to form a structure of narrower width and smaller glass cross-sectional area Ac via the first necking 30, in this case an intermediate billet 32. The stretching can be carried out under gravity, or more generally, a tension is applied to the lower end of the intermediate billet 32. When the softened lower portion of the initial preform 20 is transformed into the intermediate billet 30, the initial preform 20 is lowered by the downward movement of the drill 22, so that another portion of it is located in the furnace 28 and softened for stretching into the intermediate billet 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 thereof has been stretched into the intermediate billet 32 ​​having a length Lc. During the stretching of the intermediate billet 32, pressure or no pressure may be applied to the void of the initial preform 20. If the billet draw ratio Ap:AC is not very large, pressurization may not be required to maintain the proper shape and size of the voids. In other cases, pressurization from the gas source 26 may be required, for example, if the billet draw ratio is relatively large, or if a specific size of voids is desired in the initial billet 32 ​​in order to achieve a specific void size or wall thickness in the finished fiber.

[0048] Figure 6(b) shows the next step in the method. Once the intermediate billet 32 ​​of length Lc has been stretched, the temperature of the furnace 28 is lowered by turning the furnace 28 idle for a cooling operation in which the glass is not deformed (thus, the furnace is inoperable as far as stretching is concerned). The movement of the drill 22 is reversed, so that the suspended initial preform 20 is retracted upward (the initial preform 20 moves axially away from the furnace), and the first constriction 30 is withdrawn from the furnace 28, wherein the intermediate billet 32 ​​extends from its lower end so that the initial preform 20 and the intermediate billet 32 ​​form a unitary structure. The retraction continues until a portion of the intermediate billet 30 spaced from the first constriction 30 is located within the furnace 28. Preferably, this is the lower or lowest portion of the intermediate billet 30, because the glass extending below the furnace 28 cannot be heated and stretched into fibers and is therefore wasted. The intermediate billet 32 ​​of length Lc formed in the first stretching stage can be determined by the size of the stretching tower and the vertical range of motion of the drill 22. Conveniently, the length Lc is set so that the equipment 20 can sufficiently lift the initial preform 20 so that the lower end of the intermediate billet 32 ​​is positioned within the furnace 28, so as to minimize the waste of the lower portion of the intermediate billet 32. On the other hand, a longer length of the intermediate billet 32 ​​increases the number of fibers that can be produced at one time in the second stage of stretching (described below) and minimizes the number of stretching cycles required to consume the initial preform 20 in the finished fiber. Therefore, it is useful that the maximum length of the intermediate billet 32 ​​that can be accommodated by the elevated rig 22 can be advantageously manufactured. However, in the case where very long lengths can be accommodated, since the applied gas must travel further along the narrow gap that applies the pressure drop over a certain distance, the ability of the pressure system at the rig 22 to properly apply the pressure difference required for the second stretching stage to the lower end of the intermediate billet 32 ​​away from the rig 22 may be inhibited, so this may alternatively impose an upper limit on the length Lc. However, it is generally expected that the actual length range for the intermediate billet 32 ​​will be in the range of about 1m to 10m.

[0049] Figure 6(c) shows the second stage of the actual drawing. Once the relevant portion of the intermediate preform 30 is located in the furnace 28, a pressure differential is applied from the pressure system at the drill 22 to extend along the void in the initial preform 20 and into the void of the intermediate preform 32. The furnace 28 is activated so that heat is applied to the intermediate preform 32 and the glass at its lower portion is softened. The desired hollow core optical fiber 34 is then drawn from the softened glass, which forms a second neck 36 at the bottom of the intermediate preform 32. The fiber 32 can be drawn in the usual manner under gravity or under tension as required, and can be collected by winding onto a reel or bobbin (not shown). The fiber 34 has a fiber width and a glass cross-sectional area Af, so that the fiber draw ratio obtained in the second stage of drawing is Ac:Af. When the intermediate preform 32 is converted into a fiber 34, the drill 22 is operated to move downward and gradually feed the intermediate preform 32 into the furnace 28 so that the fiber 34 can continue to be drawn.

[0050] Once the intermediate billet 32 ​​has been consumed in whole or in part and the first constriction 30 is close to the second constriction 36, the second stretching stage is stopped. A length of hollow core optical fiber 34 has been stretched and can be separated from the intermediate billet. A complete stretching cycle has been completed, including the formation of the intermediate billet 32 ​​from the initial preform 20 and the subsequent formation of the fiber 34 from the intermediate billet 32. A second cycle can now be started, in which more intermediate billets 32 are produced. If any, the pressurization mode of the second stretching stage is turned off and replaced by the pressurization mode of the first stretching stage. Figure 6As shown in (a), the furnace 28 is switched to idle to allow repositioning of the drill, which moves downward to position the first constriction 30 (more generally, the end portion of the initial preform 20 near the first constriction 30) within the furnace 28. The furnace 28 is reactivated to soften the glass at or near the first constriction, and the first stretching stage is repeated to stretch more intermediate billet 32 ​​from the initial preform 20, replenishing or recovering the length previously consumed by stretching the fiber 34. Once another length Lc of the intermediate billet 32 ​​has been produced, the furnace is again switched to idle, the drill is retracted upward to position the second constriction 36 (more generally, a portion of the intermediate billet near the second constriction 36) in the furnace, the pressurization mode for the second stretching stage is reapplied, the furnace 28 is activated, and the stretching of the fiber 34 is restarted. If the previously stretched length of fiber 34 is separated from the intermediate billet, the fiber stretching produces a new length of fiber. If the previously stretched length of fiber 34 is not separated from the intermediate billet 32, the new fiber stretching increases the fiber length. This completes the second cycle, and the continuous cycle can be repeated until the initial preform is completely consumed (minus the usual draw waste, in other words, the accessible or usable glass of the initial preform is exhausted) and has been converted into multiple lengths or continuous lengths of fiber 34, which are collected on one or more reels. In this way, a large overall draw ratio Ap:AF can be achieved, resulting in a high fiber yield. The fiber production efficiency is thereby improved. Additionally, as described above, dividing the draw ratio into smaller steps overcomes surface tension and pressure dynamic problems, which can prevent the production of fibers at a single large draw ratio, enabling the production of narrower fibers with thinner films capable of transmitting visible and ultraviolet wavelengths.

[0051] Although not mentioned above, it is conventional that the initial preform may include an outer glass sheathing layer around the preform structure in order to produce a finished fiber with an outer glass sheath. Typically, the preform structure used to define the cross-sectional structure of the fiber will be inserted into a larger outer glass tube before drawing, which provides the sheathing layer. The heating of the drawing process causes the glass of the sheathing surrounding the inner structure to fuse to form an integral sheathed fiber. This is conventional in conventional fiber drawing, where the preform or blank is directly drawn into a fiber. In this case, the outer glass tube is added around the preform or blank before drawing and only becomes a fused outer layer during drawing of the fiber. In contrast, in the currently proposed method, the outer glass tube is placed around the initial preform, so that the outer glass tube exists when the intermediate blank is drawn from the initial preform. Therefore, during the first drawing stage, the glass used for the sheathing layer becomes a fused outer layer, which is fused around the intermediate blank. Therefore, before drawing the fiber, the sheath is already a fused layer. Thus, the intermediate preform differs from a conventional preform in that it includes a sheath as a fusion layer, whereas a conventional preform receives an outer glass tube as a sheath, and the two are not fused together.

[0052] Note that at the initial preform stage, intermediate billet stage, or fiber stage, the thickness of the jacket may or may not contribute to the glass cross-sectional area from which the draw ratio is calculated; the mathematical results are the same for both approaches.

[0053] As a practical example, the inventors have used the modified fiber drawing process described above to manufacture fibers with the goal of achieving hollow core ARFs with a film thickness of less than 200 nm, and therefore suitable for visible and ultraviolet wavelength propagation. An initial preform of about 3.8 mm in diameter (actually a preform previously drawn from a preform and located in an outer sleeve layer of 28 mm outer diameter) was used, which was originally designed to produce fibers with thicker films and had a structure with a single ring of capillaries. The initial preform was drawn into an intermediate preform of about 1 mm in diameter, which was then drawn into a fiber using a fused outer sleeve as described above with an outer diameter of 6.6 mm, applying appropriate boost. A fiber yield of more than 3 km was achieved, with a film thickness of about 170 nm inferred from transmission measurements.

[0054] The above detailed description includes a single intermediate stage for forming an intermediate billet between the initial preform and the fiber. However, if preferred, additional intermediate stages may be included to provide a series of additional intermediate billets, each intermediate billet having a width less than the width of the preceding intermediate billet. This will divide the total draw ratio into more portions of smaller draw ratios, which may be beneficial if a larger draw ratio is not preferred, for example if pressurization at any stage is easier to achieve for a smaller draw ratio. To achieve the additional stages, the intermediate billet may be a first intermediate billet of cross-sectional area Ac1, which is drawn into a narrower second intermediate billet, the second intermediate billet glass cross-sectional area Ac2 is less than Ac1, the second intermediate billet when drawn into a narrower third intermediate billet, the third intermediate billet glass cross-sectional area Ac3 is less than Ac2, and so on to the nth intermediate billet glass cross-sectional area Acn greater than the fiber glass cross-sectional area Af, where n is 2 or greater. Each of these intermediate billet draws may be pressurized or not pressurized. The final or last of the intermediate bars is then drawn into fibers before the first intermediate bar is replenished from the initial preform by drawing from a preceding intermediate bar or the like.

[0055] Figure 7A flow chart showing steps in an exemplary fiber drawing method according to the present disclosure is shown. In a first step S1, a glass initial preform is provided, which is configured and structured for drawing into a hollow fiber. In a second step S2, an end portion of the initial preform is heated to soften the glass in that portion, making it ductile for drawing. In a third step S3, a first drawing stage is achieved at a first drawing ratio, wherein a certain length of an intermediate blank is drawn from the softened glass of the initial preform, the intermediate blank having a width and a glass cross-sectional area less than the width and glass cross-sectional area of ​​the initial preform. The method proceeds to a fourth step S4, wherein the intermediate blank is heated while remaining attached to the initial preform at its end portion (away from the initial preform) to soften the glass and make it ductile for drawing. This can be achieved by withdrawing the initial preform from a furnace in which the heating of step S2 is performed so as to position the end portion of the intermediate blank in the furnace, but other arrangements may be conveniently used depending on the available equipment. In a fifth step S5, a second drawing stage is implemented at a second drawing ratio, wherein a length of hollow core optical fiber is drawn from the softened glass of the intermediate preform, the fiber having a width and a glass cross-sectional area less than the width and glass cross-sectional area of ​​the intermediate preform. During the second drawing stage, or during both the first and second drawing stages, different boost pressures may be applied via the voids in the initial preform. The method proceeds to a decision step S6, wherein it is determined whether there is glass remaining in the initial preform for further drawing. If there is more glass available, the method proceeds to an optional step S7, wherein the drawn fiber is separated from the intermediate preform. If it is preferred to increase the fiber length already achieved by further drawing, the separation is omitted. The method then loops back to repeat steps S2 to S5 in order to draw more fiber into new individual lengths or extend existing lengths. If all available glass from the initial preform has been consumed (or alternatively, the desired number of fibers has been produced), the method terminates in a final step S8, wherein heating and drawing are terminated, and the length or portion of the total length of the most recently drawn fiber is separated from the intermediate preform. Thus, the total length of fiber is produced from the initial preform as a plurality of individual lengths from each fiber drawing stage, or as a continuous single length accumulated over multiple fiber drawing stages.

[0056] Although the methods have thus far been described as being performed in a conventional fiber drawing tower, the invention is not limited in this respect, and the heating and drawing of the glass structure may be implemented using other preferred apparatus. However, the use of a drawing tower is particularly convenient in view of the ability of the methods to increase fiber yields, such that larger continuous lengths of hollow core optical fiber may be readily produced. Indeed, a benefit of the proposed methods is that they may be readily implemented using conventional fiber drawing apparatus.

[0057] As explained above, the described method proposes to modify the drawing of hollow core optical fibers by dividing the drawing into two or more stages, which enables the achievement of a greater total draw ratio than previously achievable. This achieves several benefits, including increased fiber yield from a single initial preform, longer fiber continuous lengths, improved production efficiency, and fiber manufacturing with thinner membrane reinforcements. Typically, the achievable draw ratio for hollow core fiber drawing is in the range of about 500 to 20,000, where the draw ratio is the ratio of the total cross-sectional area of ​​the glass in the preform to the total cross-sectional area of ​​the glass in the fiber. This is based on a typical preform width in the range of 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 a typical fiber width is in the range of about 100 μm to 500 μm. In contrast, the proposed technology achieves the use of a greatly increased total draw ratio (glass cross-sectional area of ​​the initial preform to the glass cross-sectional area of ​​the finished fiber) in the range of 10,000 to 3,000,000, but divides it into two or more smaller draw ratios (note that the total draw ratio is the product of the two or more smaller draw ratios). The division can be between any portion of the total draw ratio. In the case of two draws, i.e., a first draw from the initial preform to an intermediate billet, followed by a second draw from the intermediate billet to the finished fiber, it is contemplated that the first draw ratio can be in the range of 10 to 150, and the second draw ratio can be in the range of 1,000 to 20,000, although this is purely exemplary and other ranges for each of the two draw ratios are not excluded. It is contemplated that a greater portion of the total draw ratio can be achieved in the second stage of the draw process while keeping the draw ratio low enough to avoid MDC. This enables the fiber from a given length of intermediate billet to have a longer span, where the achievable length of the intermediate billet may be limited by practical considerations, such as the height of the available fiber draw tower.

[0058] For the manufacture of infrared hollow core ARF optical fibers having membrane thicknesses in the range of about 300 nm to 2500 nm, total draw ratios from the initial preform to the finished fiber are expected to be achievable in the range of 200,000 to 1,000,000, with initial preform widths in the range of 20 mm to 200 mm. This is in contrast to the much smaller preform widths of about 10 mm to 20 mm and corresponding limited draw ratios of about 2,000 to 10,000 that are currently common. These types of hollow core fibers have widths in the range of about 150 μm to 500 μm.

[0059] Total draw ratios from the initial preform to the finished fiber are expected to be achievable in the range of 20,000 to 3,000,000 or more for the manufacture of ultraviolet and visible hollow core ARF optical fibers having membrane thicknesses in the range of 50 nm to 300 nm, with initial preform widths in the range of about 6 mm to 200 mm. These types of hollow core fibers have widths in the range of about 50 μm to 250 μm. Currently, it is impractical to manufacture these types of fibers using conventional fiber drawing due to the need for extremely high draw tensions and the attendant increase in fiber breakage frequency and consequent limitations on collectable fiber yield, or the use of very small initial preforms, which also limits fiber yield and makes pressurization during drawing more difficult.

[0060] However, these stretch ratios are merely examples, and larger values ​​can be obtained with careful control of the pressurization regime and other operating parameters of the stretch tower, such as furnace temperature, stretch tension, and stretch speed.

[0061] The various embodiments described herein are presented only to help understand and teach the claimed features. These embodiments are provided only as representative examples of embodiments, rather than being exhaustive and / or exclusive. It will be understood that the advantages, embodiments, examples, functions, features, structures and / or other aspects described herein are not considered to be limitations on the scope of the invention defined by the claims or limitations on the equivalents of the claims, and other embodiments can be utilized and can be modified without departing from the scope of the claimed invention. Various embodiments of the present invention may appropriately include, consist of or consist essentially of the following: appropriate combinations of disclosed elements, components, features, parts, steps, devices, etc., rather than those specifically described herein. In addition, the present disclosure may include other inventions that are not currently claimed but may be claimed in the future.

[0062] References

[0063] [1]GT Jasion et al, "Fabrication of tubular anti-resonant hollow corefibers: modeling, draw dynamics and process optimization." Opt.Express vol.27,20567-20582(2019).

[0064] [2]Y Chen et al,“Multi-kilometer long,longitudinally uniform hollowcore photonic bandgap fibers for broadband low latency data transmission,”IEEE JLT,34,pp104-113,(2016).

[0065] [3]US11203547

[0066] [4]F Yu et al,“Single-mode solarization-free hollow-core fiber forultraviolet pulse delivery”,Opt.Express,vol.26,10879-10887(2018)

[0067] [5]S F Gao et al,“Hollow-core conjoined-tube negative-curvature fibrewith ultralow loss”,Nat.Commun,9,p2828(2018).

[0068] [6]CN 111474628

Claims

1. A method for manufacturing a hollow core optical fiber, the method comprising: providing an initial preform formed of glass and having a cross-sectional structure configured to form a cross-sectional structure in an optical fiber drawn from the initial preform, the cross-sectional structure comprising 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 said initial preform so as to soften said glass of said end portion; drawing a length of an intermediate rod from the softened glass of the initial preform via a first necking, the intermediate rod having a second glass cross-sectional area that is smaller than the first glass cross-sectional area; heating a portion of said intermediate blank spaced from said first constriction so as to soften said portion of said glass, said intermediate blank remaining integral with said initial preform; as well as A length of hollow core optical fiber is drawn from the softened glass of the intermediate billet via a second necking, the hollow core optical fiber having a third glass cross-sectional area that is smaller than the second glass cross-sectional area.

2. The method according to claim 1, further comprising: After drawing the length of hollow core optical fiber from the softened glass of the intermediate preform, performing the following steps: heating the end portion of the initial preform at or near the first constriction to soften the glass of the end portion; stretching said intermediate rod from said softened glass of said initial preform so as to restore said length of the intermediate rod; heating the intermediate ingot at or near the second constriction to soften the glass of the intermediate ingot; and Another length of the hollow core optical fiber is drawn from the softened glass of the intermediate preform.

3. The method according to claim 2, further comprising: The steps according to claim 2 are repeated until a desired total length of the hollow core optical fiber has been drawn and / or all available glass of the initial preform has been consumed.

4. The method according to any one of claims 1 to 3, further comprising: One or more pressures are applied to voids in the cross-sectional structure of the initial preform during the drawing of the hollow core optical fiber.

5. The method according to claim 4, further comprising: One or more pressures are applied to the voids in the cross-sectional structure of the initial preform during the stretching of the intermediate billet.

6. The method according to any one of claims 1 to 5, further comprising: The length of the hollow core optical fiber is separated from the intermediate preform.

7. The method according to any one of claims 1 to 6, wherein the method is performed in an optical fiber drawing tower, a furnace of the optical fiber drawing tower providing the heating.

8. The method according to claim 7, wherein drawing the intermediate billet and drawing the hollow core optical fiber comprises: The initial preform is moved in an axial direction towards the furnace so as to provide heating and softening of the additional glass.

9. The method according to claim 8, further comprising: After stretching the intermediate billet, the initial preform is moved in an axial direction away from the furnace in order to position the intermediate billet in the furnace for heating.

10. The method according to any of the preceding claims, wherein the length of the intermediate billet is in the range of 1 m to 10 m.

11. The method of any of the preceding claims, wherein the first glass cross-sectional area and the third glass cross-sectional area define an overall draw ratio of first glass cross-sectional area: third glass cross-sectional area in a range of 10,000 to 3,000,000.

12. The method according to any one of claims 1 to 11, wherein the first glass cross-sectional area and the second glass cross-sectional area limit a first stage stretching ratio of the first glass cross-sectional area: the second glass cross-sectional area in the range of 10 to 150, and the second glass cross-sectional area and the third glass cross-sectional area limit a second stage stretching ratio of the second glass cross-sectional area: the third glass cross-sectional area in the range of 1,000 to 20,000.

13. The method according to any one of claims 1 to 11, wherein the first glass cross-sectional area and the second glass cross-sectional area define a first stage stretching ratio of first glass cross-sectional area: second glass cross-sectional area, and the second glass cross-sectional area and the third glass cross-sectional area define a second stage stretching ratio of second glass cross-sectional area: third glass cross-sectional area, the first stage stretching ratio being less than the second stage stretching ratio.

14. The method according to any one of the preceding claims, further comprising: The initial preform is surrounded by an outer layer of glass before heating the end portion of the initial preform, so that the intermediate billet is fused with the outer layer of glass, the outer layer of glass being intended to act as a sheath for the hollow-core optical fiber.

15. The method according to any one of the preceding claims, further comprising: One or more additional intermediate preforms are drawn from the intermediate preform before drawing the hollow core optical fiber from a last one of the additional intermediate preforms, each additional intermediate preform having a glass cross-sectional area that is smaller than the glass cross-sectional area of ​​the preceding intermediate preform.

16. The method according to any one of claims 1 to 15, wherein the hollow core optical fiber is stretched to a film thickness between voids in the hollow core optical fiber of greater than 200 nm.

17. The method according to any one of claims 1 to 15, wherein the hollow core optical fiber is drawn to a film thickness between voids in the hollow core optical fiber of 200 nm or less.

18. A hollow core optical fiber manufactured using the method according to any one of the preceding claims.

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