Anti-resonance hollow core optical fiber, preform for such optical fiber and manufacturing method

Abstract

The present invention discloses a forming tube (50, 51) for use as a component in manufacturing an anti-resonance hollow core optical fiber, the forming tube having a side wall of a transverse cross-sectional shape, the side wall including a plurality of major curved portions (52) alternating with an equal number of minor substantially straight portions (54), each of the curved portions (52) having an inwardly curved shape, and each straight portion (54) being equidistant from a central longitudinal axis of the forming tube (50, 51).

Description

Technical Field

[0001] The present invention relates to an anti-resonance hollow core optical fiber, a preform for such an optical fiber and a method for manufacturing the preform. Background Art

[0002] The category of optical fibers includes hollow-core fibers, in which light is guided along the longitudinal hollow space forming the fiber core by an optical guiding mechanism achieved by a structured arrangement of longitudinal voids or capillaries forming a cladding surrounding the core void. Various configurations of the cladding are known, resulting in different guiding effects.

[0003] One type of hollow-core fiber is an anti-resonance hollow-core fiber (ARF). This type of fiber has a relatively simple cladding structure consisting of a typically relatively small number of glass tubes or capillaries arranged in a ring around a central core void and fastened to the inner surface of a jacket tube to maintain the desired geometry. This arrangement does not provide any high degree of periodicity, and therefore guidance cannot operate via the photonic bandgap effect, such as in hollow-core photonic bandgap (crystal) fibers. Instead, anti-resonance is provided for propagating wavelengths that do not resonate with the wall thickness of the cladding capillaries; in other words, for wavelengths in the anti-resonance window defined by the wall thickness of the cladding capillaries. Anti-resonance acts to suppress coupling between air-guided optical modes supported by the core and any optical modes that may be supported by the cladding, so that light is confined in the core and can propagate with low loss via the anti-resonance light guiding effect.

[0004] In the simplest case, an ARF can consist of a single cladding capillary ring, but several modifications and variations of this arrangement have been proposed to improve performance in areas such as bandwidth and loss. Many known applications for conventional solid-core optical fibers have been demonstrated using hollow-core fibers, including telecommunications, optical power transmission, and optical sensing. Especially for telecommunications applications, low optical loss (i.e., the percentage of light lost per unit length of propagation (typically per kilometer)) is crucial.

[0005] The lowest loss reported to date for hollow-core fibers is 1.3 dB / km, achieved in an ARF with a nested antiresonant nodeless fiber (NANF) construction [1]. A NANF consists of a ring of spaced (non-contacting) nested capillaries (one or more smaller capillaries fixed within a larger capillary) secured within an outer sheath and surrounding a central hollow-core region. As with conventional ARFs, the primary optical guidance mechanism is a combination of antiresonance from the uniform thickness of the cladding capillary glass walls or films and suppressed coupling to modes in the cladding. NANFs are expected to offer significantly better loss performance than the already impressive results mentioned above and may one day even overcome the fundamental loss limitations of all-solid silica fibers [2].

[0006] Under these circumstances, large-scale production of NANF is needed to meet demand. Current manufacturing methods are expensive and slow, making them unsuitable for large-scale production. Therefore, improved methods for NANF manufacturing have received great attention. Summary of the Invention

[0007] Various aspects and embodiments are set out in the accompanying claims.

[0008] According to a first aspect of certain embodiments described herein, there is provided a forming tube for use as a component in manufacturing an anti-resonant hollow-core optical fiber, the forming tube having a sidewall with a transverse cross-sectional shape, the sidewall comprising a plurality of primary curved portions alternating with an equal number of secondary substantially straight portions, each curved portion having an inwardly curved shape, and each substantially straight portion being equidistant from a central longitudinal axis of the forming tube.

[0009] According to a second aspect of certain embodiments described herein, there is provided a preform for an anti-resonance hollow-core optical fiber, the preform comprising a forming tube according to the first aspect, the forming tube being fastened within an outer jacket tube of a glass material or a polymer material such that a substantially straight portion of the forming tube is in contact or nearly in contact with an inner surface of the outer jacket tube.

[0010] According to a third aspect of some embodiments described herein, there is provided a rod for drawing an anti-resonant hollow core optical fiber from a preform according to the second aspect.

[0011] According to a fourth aspect of some embodiments described herein, there is provided an anti-resonance hollow core optical fiber drawn from the preform according to the first aspect or the rod according to the second aspect.

[0012] According to a fifth aspect of certain embodiments described herein, there is provided a method of manufacturing a preform for an anti-resonance hollow-core optical fiber, the method comprising: providing a forming tube according to the first aspect; inserting the forming tube into an outer jacket tube of a glass material or a polymer material such that there is contact or near contact between a substantially straight portion of the forming tube and an inner surface of the outer jacket tube; and fastening the forming tube within the outer jacket tube.

[0013] According to a sixth aspect of some embodiments described herein, there is provided a method of manufacturing a rod for an anti-resonance hollow core optical fiber, the method comprising: manufacturing a preform according to the method of the fifth aspect; and drawing the preform into a rod.

[0014] According to a seventh aspect of some embodiments described herein, there is provided a method of manufacturing an anti-resonant hollow core optical fiber, the method comprising: manufacturing a preform according to the fifth aspect or manufacturing a rod according to the sixth aspect; and drawing the preform or rod into an optical fiber.

[0015] According to an eighth aspect of certain embodiments described herein, a nested anti-resonance node-free hollow core optical fiber or a preform or rod for drawing into a nested anti-resonance node-free hollow core optical fiber is provided, comprising: a cylindrical, elongated outer sheath of glass or polymer material, a cladding structure comprising a plurality of cladding elements of glass or polymer material, the cladding elements spaced about an inner surface of the outer sheath and defining an elongated cladding void, each cladding element comprising a first inwardly curved wall extending from the inner surface toward a central longitudinal axis of the outer sheath and returning to the inner surface along a first curved shape, and a second inwardly curved wall extending from the inner surface toward the central longitudinal axis and returning to the inner surface along a second curved shape, the second curved shape having a different shape and / or a different length than the first curved shape; and an elongated hollow core region along the central longitudinal axis and defined by a negative curvature of the first inwardly curved wall of the cladding element.

[0016] These and further aspects of certain embodiments are set forth in the accompanying independent and dependent claims. It will be appreciated that the features of the dependent claims may be combined with each other and with the features of the independent claims in combinations different from those explicitly set forth in the claims. Furthermore, the methods described herein are not limited to the specific embodiments such as set forth below, but rather include and contemplate any suitable combination of the features set forth herein. For example, methods, apparatus, and devices may be provided according to the methods described herein that include any one or more of the various features described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to better understand the present invention and to show how it may be put into effect, reference will now be made by way of example to the accompanying drawings, in which:

[0018] 1(A), 1(B), and 2 show schematic transverse cross-sectional views of an anti-resonant hollow-core fiber including cladding features from three known fiber designs;

[0019] Figure 3 A transverse cross-sectional view of an anti-resonant hollow core fiber is shown according to one embodiment.

[0020] FIG4(A) shows a comparison of a partial transverse cross-sectional view of a known anti-resonance hollow core fiber design and an anti-resonance hollow core fiber design according to one embodiment;

[0021] FIG4(B) shows a diagram of optical loss simulation results of the optical fiber design of FIG4(A);

[0022] Figure 5 (A) and Figure 5(B) shows a transverse cross-sectional view of a first shaped glass tube and a second shaped glass tube for making an optical fiber preform according to one embodiment;

[0023] Figure 6 (A) and Figure 6 (B) shows Figure 5 (A) and Figure 5 (B) a perspective view of a formed glass tube;

[0024] Figure 7 shows a transverse cross-sectional view of a preform for making an anti-resonance hollow core optical fiber according to one embodiment;

[0025] Figure 8 A flow chart illustrating an exemplary method of manufacturing an anti-resonance hollow core optical fiber according to one embodiment is shown;

[0026] Figure 9 A flow chart illustrating an exemplary method for manufacturing a preform for manufacturing an anti-resonance hollow core optical fiber according to another embodiment;

[0027] Figure 10 (A) Figure 10 (B) and Figure 10 (C) shows a transverse cross-sectional view of a preform for manufacturing an anti-resonance hollow-core optical fiber according to another embodiment;

[0028] Figure 11 shows a simplified schematic diagram of an apparatus for making a shaped glass tube according to one embodiment;

[0029] Figure 12 shows a simplified schematic diagram of an apparatus for making a shaped glass tube according to another embodiment; and

[0030] Figure 13 Transverse cross-sectional views of portions of two shaped glass tubes defining cladding elements are shown according to various embodiments. DETAILED DESCRIPTION

[0031] Aspects and features of certain examples and embodiments are discussed / described herein. Certain aspects and features of certain examples and embodiments may be implemented conventionally and, for the sake of brevity, are not discussed / described in detail. Therefore, it will be appreciated that aspects and features of the apparatus, methods, and devices discussed herein but not described in detail may be implemented according to any conventional techniques for implementing such aspects and features.

[0032] In its simplest form, this type of hollow-core fiber, which can be described as an antiresonant hollow-core fiber (ARF), consists of a tubular outer sheath and a number of cladding capillaries arranged in a ring within the outer sheath and fastened or bonded to the inner surface of the outer sheath. The central void within the capillary ring forms a hollow core, along which one or more optical modes can be guided through the hollow core via an antiresonant waveguiding effect.

[0033] FIG1(A) illustrates a transverse cross-sectional view of a first previously proposed antiresonant hollow-core fiber. This view shows a transverse cross-sectional view through an optical fiber having a circular cross-section. The optical fiber 10 has an outer tubular jacket 12. Multiple tubular or hollow cladding capillaries or cells 14 (in this example, six capillaries having the same circular cross-sectional size and shape) are arranged in a ring within the jacket 12, such that the longitudinal axis of each cladding capillary 14 is substantially parallel to the longitudinal axis of the jacket 12. The cladding capillaries define an elongated pore, lumen, or chamber that extends continuously along the length of the optical fiber. The number of capillaries allows this structure to be labeled a 6-cell ARF. Each cladding capillary or tube 14 contacts (is bonded to) the inner surface of the jacket 12 at position 16, such that the cladding capillaries 14 are evenly spaced around the inner circumference of the jacket 12 and are also separated from each other by a gap or spacing d (no contact between adjacent capillaries). The cladding structure is limited to these cladding capillaries. In some ARF designs, the cladding tubes 14 may be positioned around the ring so that adjacent tubes touch each other (in other words, not spaced apart as in FIG. 1(A)), but the spacing used to eliminate such contact can improve the optical performance of the fiber. This spacing eliminates optical nodes that occur at the contact points between adjacent tubes, which tend to induce undesirable resonances that lead to high losses. Therefore, an optical fiber with spaced cladding tubes as in FIG. 1(A) can be referred to as a "node-free" anti-resonance hollow-core fiber.

[0034] The cladding capillary 14 is arranged in a ring around the inside of the jacket 12, creating a central space, cavity, or void within the optical fiber 10. Its longitudinal axis is also parallel to the longitudinal axes of the jacket 12 and capillary 14. This is the fiber's hollow core 18, which also extends continuously along the length of the fiber. The core 18 is bounded by the inward-facing portion of the outer surface of the cladding capillary 14. This is the core boundary, and the material (e.g., glass or polymer) that constitutes the capillary wall of this boundary provides the desired anti-resonant light guiding effect or mechanism. The shape of the core boundary comprises a series of adjacent inwardly curved surfaces (i.e., convex from the perspective of the core). This contrasts with the typically outward curvature of the core-cladding interface in solid-core fibers and the essentially circular core boundaries of photonic bandgap-type hollow-core fibers. Therefore, anti-resonant hollow-core fibers can be described as having negative curvature. Mathematically, this can be defined as the surface normal vector of the core boundary pointing oppositely to the radial unit vector (a vector along the transverse cross-sectional radius of the fiber). The negative curvature (convex shape) of the core boundary also suppresses coupling between the fundamental core mode and any cladding modes. Negative curvature antiresonant hollow-core fibers have a core boundary formed by a convex film or wall (usually glass) having a thickness that is matched to antiresonantly match the wavelength of the guided light.

[0035] FIG1(B) shows a transverse cross-sectional view of a second previously proposed antiresonant hollow-core fiber [2, 3]. The fiber includes all the features of the example in FIG1(A), but the cladding has a more complex structure. Each cladding capillary 14 is a first capillary, still separated from its adjacent capillary by a gap d. A second, imaginary, smaller capillary 20 is nested within the first capillary and bonded to the inner surface of the cladding capillary 14 around the jacket 12 at the same azimuthal position as the junction 16 between the first capillary 14 and the jacket 12. The purpose of including these additional smaller capillaries 20 is to reduce optical losses in the fiber. Providing a second air-glass interface in the radial direction reduces losses due to light leakage. An additional, smaller third capillary can be nested within the second capillary, also bonded coincidentally with the azimuthal contact location 16. This type of ARF design with a second capillary and optional further smaller capillaries can be referred to as a "nested antiresonant nodeless fiber," or NANF. The six first capillaries of this example allow this structure to be labeled as a 6-unit NANF.

[0036] The exemplary cladding structure shown in Figures 1(A) and 1(B) includes six first cladding capillaries arranged in a ring around the fiber core. However, the ARF is not limited thereto, but may include five or fewer capillaries or seven or more capillaries to form a boundary around the fiber core.

[0037] The circular cross-section of the cladding capillary in the ARF is derived from readily available cylindrical glass tubes, which are available from commercial sources in a variety of sizes, types, and grades of glass. Consequently, these tubes form the basis of the manufacturing technology used for all currently existing silica hollow-core optical fibers.

[0038] In the case of ARF, production requires the manufacture of a preform that is manually assembled by highly skilled operators (as with typical optical fiber production). Separate tubes are prepared for the outer jacket and for the cladding capillaries, and the cladding capillaries are stacked within the jacket tube, possibly along with additional tubes or rods to hold the capillaries in place and define the stack of tubes. This assembly is then fused to secure the capillaries in place against the inner surface of the jacket, creating a preform that can be pulled or drawn into optical fiber. Considerable expertise is required to correctly position the various tubes and maintain their relative positions during the production phase to produce the finished optical fiber. Consequently, large-scale, low-cost commercial production of ARF, including NANF with its desirable low-loss properties, presents considerable difficulties.

[0039] While the circular-based structure of the ARF is conventional, it is not essential for the high-quality optical performance of these fibers. Instead, the optical conductivity arises from the negative curvature of the core boundary described above, and similar performance can be obtained from non-circular cladding elements that still define a core boundary of appropriate shape.

[0040] FIG2 shows a transverse cross-sectional view of a third previously proposed anti-resonant hollow core fiber [4]. The longitudinal voids of the cladding are defined by a plurality (in this case eight) of semi-elliptical cladding elements 22, which are secured to the inner surface of the outer jacket 12 in a spaced arrangement as in the previous examples. This structure is one of several proposed in a study of the effect of core boundary curvature, in which models with varying degrees of ellipticity and varying numbers of cladding elements were constructed. Good optical performance in terms of light-conducting capacity and low loss was found, indicating that circular cladding capillaries are by no means essential for ARF. In addition, the semi-elliptical elements may have some benefits in that each semi-elliptical element has two anchoring points to the outer jacket, compared to one anchoring point for circular elements. This will provide increased structural stability during manufacturing, making it easier to achieve precise positioning of the cladding elements. However, having two anchoring points per cladding element obviously increases the amount of element fusion that needs to be performed when making the preform.

[0041] The present disclosure proposes that the use of non-circular cladding elements is also applicable to NANF-like ARFs, where two or more negatively curvature glass cladding walls are provided in the radial direction.

[0042] Figure 3A transverse cross-sectional view of an exemplary NANF-like ARF is shown. If the nested capillaries of the example of FIG1(B) are cut longitudinally at the point where they touch each other and fixed to the sheath, the tubes can be opened by moving the two cut edges away from each other. This will produce a double-walled curved element 30 comprising a first curved element 32 and a second curved element 34 having co-located end points 36, and each curved element having a curved shape and a different degree of curvature. The elements 32, 34 can then be attached to the interior of the outer sheath 12 in a spaced-apart arrangement along each of the cut edges as in a regular NANF. This will produce Figure 3 However, the characteristics of the double air-glass boundary with negative curvature and the gaps between adjacent cladding elements present in regular NANF are retained, so one can predict that the optical fiber will be able to maintain the special properties of ultra-low optical loss, wide optical bandwidth, and ultra-low overlap of optical modes with the glass walls.

[0043] FIG4(A) shows a transverse cross-sectional view of an exemplary NANF fiber structure, wherein the lower half of the figure shows a regular 6-unit NANF arrangement with first and second nested capillaries 14, 20, as shown in FIG1(B). The upper half shows a NANF having a structure similar to FIG1(B). Figure 3 The double-walled curved cladding element 30 in this example has a NANF-like arrangement. In this case, the first curved element 32 has a semi-elliptical shape, positioned adjacent to the fiber core 18 to define the core boundary and corresponding to the first capillary in a regular NANF. The semi-elliptical shape of the second curved element 34, corresponding to the second capillary in a regular NANF, has a lower aspect ratio / ellipticity than the first curved element. Consequently, a capillary void, lumen, or chamber is defined between the first and second curved elements 32, 34, and another capillary void, lumen, or chamber is defined between the second curved element 34 and the jacket 12. However, these curved shapes are merely examples, and other curved shapes may be used. An optical fiber constructed in this manner was modeled using computer simulation. For comparison, the two simulated optical fibers had the same core radius R of 30 μm (as shown in FIG. 4(A)) and the same glass wall or membrane thickness of the capillary element, which was 0.55 μm.

[0044] Figure 4(B) shows the results of modeling the optical loss caused by the leakage of optical power from the guided modes in the fiber core according to the guided wavelength. It can be seen that the bandwidth of the optical fibers is from about 1.3 μm to about 2.1 μm, which means that they can guide light with wavelengths in this range with low loss. Curve 40 shows the loss of a regular NANF, and curve 42 shows the loss of a NANF-like arrangement of semi-elliptical elements. It is easy to appreciate that the losses of these two structures are very similar and also very low (less than 2 dB / km in the central part of the bandwidth). Therefore, NANF-like fibers with semi-elliptical cladding elements instead of circular cladding elements are useful structures for ARF.

[0045] However, in the context of known ARF manufacturing techniques based on stacking cylindrical tubes, fusing the tubes together at contact points and drawing the resulting preform into an optical fiber, there seems to be no straightforward way to manufacture an ARF with a double-walled curved cladding element or even a single-walled curved cladding element with the structure of Figure 2.

[0046] For example, US 2016 / 0124144 [5] describes a photonic crystal hollow core fiber in which the core boundary is formed by a series of adjacent arcs to provide a shape similar to that obtainable from a curved cladding element, but it is formed by conventionally stacking and drawing circular cross-section capillaries.

[0047] The present disclosure proposes a method for fabricating ARFs having curved non-circular cladding elements, such as those shown in Figures 2, 3, and 4(A), which can be greatly simplified compared to conventional ARF fabrication based on cylindrical tubes, while still producing optical fibers with at least comparable optical performance.

[0048] The method proposes using elongated forming tubes, each of which defines a plurality of spaced-apart curved cladding elements in a circumferential configuration. One tube, or two or more tubes defining the various shaped cladding elements, can be directly inserted into an appropriately sized outer jacket tube to create a preform for an ARF. A single forming tube provides a complete ring of cladding voids surrounding the hollow fiber core.

[0049] Figure 5(A) shows a transverse cross-sectional view of a first exemplary formed tube 50. Tube 50 is shaped to provide six curved cladding elements for an ARF and is therefore intended for use in the manufacture of a six-cell ARF. Tube 50 has a sidewall comprising primary curved portions 52 alternating with secondary substantially straight portions 54. The terms "primary" and "secondary" are intended to convey that the length of the curved portions is significantly greater than the length of the straight portions. In this example, all straight portions 54 are of the same size and shape, and all curved portions 52 are of the same size and shape (however, this is not required, and differences in shape can be utilized to provide more complex cladding structures). Thus, tube 50 has rotational symmetry about its central longitudinal axis X, in this case sixfold (or six degrees) rotational symmetry, corresponding to the number of curved cladding elements and, therefore, the intended number of cells in the final ARF. Note that the tube has the same or substantially the same cross-section along its entire length.

[0050] While this example has six curved sections and six intervening straight sections, any number of sections may be included depending on the desired cladding structure for the optical fiber. Thus, the forming tube may have two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, or more curved sections and a corresponding number of straight sections.

[0051] The straight sections 54 are all equidistant from the central longitudinal axis X of the tube 50 and are actually positioned at equal intervals around the circumference of a circle centered at X and having a radius equal to the distance from X to each straight section 54 (hereinafter referred to as the "nominal circle"). The straight sections 54 are substantially perpendicular or nearly perpendicular to the radius at the relevant position, in other words, follow a tangent. Although the straight sections 54 can be truly straight and perpendicular, they can also have a certain degree of curvature, such as following the circumference of the aforementioned imaginary circle. This may be due to the manufacturing method of forming the shaped tube from a circular tube, some examples of which are described in more detail below. The straight sections can be considered to be substantially straight because they occupy a small part of the entire circumference. The straight sections can also have other shapes besides straight lines or shapes that are completely curved along the circumference. When the components of the preform are assembled, a certain curvature can allow for better fit against the inner surface of the sheath tube.

[0052] As described above, the straight sections 54 are relatively short, thus occupying a small portion of the entire circumference of the imaginary circle. The purpose of the straight sections 54 is to separate each curved section 52 from its adjacent adjacent curved section 52 to define a desired spacing between cladding elements, corresponding to the gap d in FIG. 1(A) , which is crucial for reducing losses at optical nodes in the final optical fiber. For example, in a tube for a six-element ARF, each straight section may be in the range of 0.5% to 20% or 0.5% to 25% of the total circumference of the imaginary circle, such as between 1% and 20%, or between 1% and 10%, or between 10% and 20%, or between 5% and 20%. These values ​​apply to tubes formed for other numbers of cells, although a larger number of cells may correspond to a smaller spacing between cladding elements and, therefore, shorter straight sections. In the case of two or more nested shaped tubes (described in more detail below), a larger straight section length may be applied to the outer shaped tube, while a smaller straight section length may be applied to the inner shaped tube. However, other values ​​are not excluded. The actual diameter of the preform can range from about 2 cm to 25 cm or 30 cm (although other diameters can be used). For example, current production preforms used for mass production of standard optical fibers typically have a diameter of about 20 cm. In this case, the length of each straight portion in the circumferential direction can be between about 0.2 cm and 15 cm. Again, these are examples only, and other values ​​are not excluded.

[0053] Each curved portion 52 comprises an inwardly curved portion of the sidewall. "Inwardly curved" means that the curved portion is concave when viewed from outside the tube, but convex when viewed from inside the tube. The curved portion 52 extends between two ends, each end being at the intersection of the ends of two adjacent straight portions 54. Typically, the curved shape is symmetrical about a line that follows the radius of an imaginary circle located at the midpoint between the two adjacent straight portions 54 (although this is not required, and other curved shapes can be utilized to provide more complex cladding structures). The curve of the curved portion 52 can be any smoothly varying shape that exhibits negative curvature from the perspective of the axis X to provide the desired negatively curved core boundary in the final ARF. The curved portion 52 extends inward to a point closest to the axis X that is separated from the axis X by a distance less than the nominal circular radius. In the case of a symmetrical curved portion, the point closest to the axis X lies on the radius of the imaginary circle located at the midpoint between the two adjacent straight portions, in other words, on the axis of mirror symmetry of the curved shape.

[0054] For example, the curve can be a semicircle or a semi-ellipse. Alternatively, the curve can be a portion of the circumference or perimeter of a circle or ellipse, in other words, an arc of a circle or ellipse that is smaller or larger than a semicircle or a semi-ellipse. Other curved or smoothly varying shapes or contours can be used in other examples.

[0055] Thus, in general, the tube wall comprises an alternating sequence of substantially straight secondary portions connected to inwardly curving concave curved portions, wherein the number of curved portions is the same as the number of straight portions.

[0056] An alternative way of describing the cross-section of a shaped tube is as a quasi-hypocycloid. A hypocycloid is a plane curve resulting from the locus of fixed points on a small circle rolling around the inside of the circumference of a larger circle. For suitably proportioned circles, this produces a closed shape consisting of points or cusps located on the circumference of the larger circle connected by smoothly symmetrical inward curvatures. The relative sizes of the circles determine the curvature of the curvatures and the number of cusps. In the present case, the descriptor "quasi" has been added to indicate that the relevant shape of the tube has the same general properties as a hypocycloid, so that a similar impression is given to the observer. However, each cusp of the hypocycloid is replaced by a short, secondary straight line portion (an arc or tangent to the larger circle), and the shape / curvature / profile of the curvature can be different from that determined by the rolling small circle.

[0057] A preform for an ARF similar to the example of FIG2 can be created using a single forming tube inside an outer jacket tube. The maximum outer dimension of the forming tube, determined by the line along which the straight sections lie (the circumference of a nominal circle) or the distance between two oppositely disposed straight sections (the diameter of a nominal circle), should correspond to or be comparable to the inner dimensions of a cylindrical jacket tube, allowing the forming tube to be accommodated inside the jacket tube, with the forming tube in contact or near contact between the outer side of the forming tube and the jacket tube interior at the straight sections. This allows the two tubes to be fused at these points during preform fabrication, before the preform is drawn into a rod or optical fiber. In other arrangements, fusion of the forming tube and jacket tube can occur during drawing. In this case, a tight fit and a small separation are less critical, as the vacuum applied to the lattice interstices in the cladding to eliminate potential air bubbles also serves to draw the tubes together. Therefore, a larger separation can be tolerated. Thus, in various cases, the separation between the jacket tube and forming tube can be approximately 10% or less of the jacket tube diameter, for example, approximately 5% or 2%. For contact, the separation is essentially zero. These arrangements can be described as contacting or near contacting.

[0058] Example values ​​for the wall thickness of the forming tube may be in the range of approximately 0.05 to 0.2 of the forming tube diameter. The outer jacket tube may be sized such that the ratio of its outer diameter to its inner diameter is in the range of 2:1 to 6:1. Other tube sizes are not excluded.

[0059] Alternatively, one or more additional forming tubes may be inserted into the jacket tube such that the forming tubes are located within each other in a nested arrangement with their straight sections at the same circumferential position around the outer jacket tube. Again, the maximum dimensions of the tubes should be selected to provide contact or near contact at the straight sections for fusion of the tubes.

[0060] To simulate the double-wall effect of the cladding in a NANF having first and second cladding capillaries (such as the example of FIG. 1(B) ), a second shaped tube can be used together with the first shaped tube, where the two tubes have curved portions with different curvatures.

[0061] Figure 5 (B) shows that Figure 5 (A) shows a transverse cross-sectional view of another exemplary forming tube 51 used in conjunction with the forming tubes of FIG. Forming tube 51 has the same number of straight sections 54 and curved sections 52 as first forming tube 50—in this case, six—so that both can be used together. The outer diameters / circumferences are substantially the same, with slight differences to allow insertion of first tube 50 into second tube 51. However, curved sections 52 have a shallower, less concave shape than those of first tube 50. In other words, the inward bend in second tube 51 is less than that in first tube 50. The distance from the central longitudinal axis X of second tube 51 to the nearest point of the curved section is greater than the corresponding distance in first tube 50. Therefore, when first tube 51 is inserted or nested into the second tube, two glass walls are radially disposed within each curved section 52, providing a double air / glass boundary for a NANF having two nested capillaries.

[0062] Figure 6 (A) and Figure 6 (B) shows a perspective view of the first tube 50 and the second tube 51 respectively, illustrating how the shape of the side walls extends in the same shape along the entire length of the tubes 50, 51.

[0063] Figure 7 Shown by using Figure 5 and Figure 6A transverse cross-sectional view of a preform fabricated from a shaped tube of the illustrated form. A first tube 50 having a significantly more inwardly concave bend is nested within a second tube 51 having a less inwardly concave bend, which in turn is positioned within a cylindrical glass outer sheath tube 55. The outer surface of the straight portion 54 of the first tube 50 contacts the inner surface of the straight portion 54 of the second tube 51, and the outer surface of the straight portion 54 of the second tube 51 contacts the inner surface of the sheath tube 55. These aligned straight portions 54 define equal spacing between the curved cladding element 30 formed by the curved portion 52 of the first tube 50, which is the most inwardly positioned curved portion and thus corresponds to the first capillary of a conventional NANF, and the curved portion 52 of the second tube 51, which is located between the first tube 50 and the sheath tube 55 and thus corresponds to the second (nested) capillary of a conventional NANF.

[0064] It should be noted that in the production of optical fibers made from a preform via an optional intermediate stage called a rod, the aim is to substantially maintain the cross-sectional configuration. The process of drawing the preform into an optical fiber of much smaller diameter merely reduces the cross-sectional dimensions, while the shape is maintained or altered or somewhat controlled by applying pressure to the voids in the structure during the drawing process. Thus, Figure 7 Any and all of preforms, rods, or ARFs according to embodiments and examples of the present disclosure are shown.

[0065] Figure 8 A flow chart is shown that lists the steps in a first exemplary method for manufacturing an optical fiber preform according to an embodiment of the present disclosure, as well as additional steps for forming the preform into a rod or optical fiber. In a first step S1, a shaped glass (or polymer) tube having a quasi-hypercycloidal cross-section as described above is provided. This providing step includes alternatives of obtaining the shaped tube from an external source or supplier and directly manufacturing the shaped tube. An exemplary technique for tube manufacturing is described below. In a second step S2, the shaped glass tube is inserted into a cylindrical outer sheath tube made of glass (or polymer), wherein the dimensions of the shaped tube and the sheath tube are suitable for providing the above-mentioned contact or near contact. The shaped tube and the sheath tube can have the same or similar length. For example, the sheath tube can have an outer diameter of approximately 20 cm, or between 2 cm and 25 cm, or between 2 cm and 30 cm. The length of the tube can range from approximately 20 cm to 100 cm. Other dimensions can be used depending on the desired size of the preform.

[0066] Then, in step S3, the shaping glass tube is secured within the outer jacket tube to form the optical fiber preform. This securing can be simple or more complex, depending on preference and the anticipated amount of future processing of the preform. The shaping glass tube is secured to the inner surface of the jacket tube by fusing the tube material in the contact areas at the straight sections of the shaping tube. This fusing can be performed by any known technique, such as that used to manufacture ARF preforms from cylindrical tubes. The fusing can be performed at a single straight section, at some straight sections, or at all straight sections. The fusing can be performed along the entire length of the tube or at one or more spaced-apart locations to simply position the shaping tube in place. Alternatively, the securing can be simpler, requiring only some arrangement to retain the shaping tube within the jacket tube when the preform is held in a vertical orientation (as it will be used to draw into optical fiber). This is particularly relevant if the relative dimensions of the tubes result in a relatively tight fit within the jacket tube, preventing the shaping tube from easily rotating or sliding within the jacket tube once inserted. Furthermore, the tight fit allows the tubes to be properly bonded or fused to one another during the process of heating the preform and drawing it into optical fiber, without requiring any initial bonding stage. For example, a simple fastening step in manufacturing the preform can include a jacket tube having a restricted or closed hole at one end, through which the forming tube is inserted. For example, after insertion of the forming tube, the restricted or closed hole can be restricted by heating the glass of the jacket tube to soften it, then clamping its sidewalls or bending them inward. Alternatively, some form of separate closure or restraining element can be fixed to the end of the jacket tube. In these cases, the forming tube can remain free to move within the jacket tube. However, because the spacing and relative positions of the various cladding elements are set by the overall properties of the forming tube, any such movement (such as rotation relative to the jacket tube) does not alter the relative positions of the elements, and the intended configuration for the final fiber is not lost.

[0067] Thus, it can be appreciated that the fabrication of preforms for ARF production is simplified compared to assembling multiple round tubes according to known techniques. At its simplest, the proposed method requires only the assembly of two tubular elements. The relative positions of the cladding elements are determined by the structure of the forming tube, eliminating the labor required to correctly position the various tubes relative to one another. Furthermore, spacers and similar additional elements, which can be used to pack the stack of round tubes within the jacket tube to maintain the desired relative position, are not required.

[0068] The preform produced in step S3 can be made into an optical fiber using known optical fiber manufacturing techniques immediately after step S3 or at a later time. The preform can be drawn directly into a fiber in step S4a. Alternatively, in step S4b, the preform can be drawn into a rod having an intermediate diameter between the preform and the intended optical fiber, which can then be drawn into the final optical fiber in step S4c. In either case, the typical optical fiber drawing process (step 4a or step 4c) includes sheathing the preform or rod with an outer glass tube prior to drawing to provide sufficient glass material for the finished optical fiber.

[0069] Figure 9 FIG. 1 is a flow chart listing the steps in a second exemplary method of manufacturing an optical fiber preform according to an embodiment of the present disclosure. In step S10, as in Figure 8 A first shaped glass (or polymer) tube is provided (by purchase or manufacture) as in the method. In step S11, a second shaped glass (or polymer) tube having a curved portion with a different curvature is provided. In step S12, one or more additional shaped glass (or polymer) tubes having other different curvatures are optionally provided, depending on the number of nested NANF elements intended to be simulated in the structure of the finished optical fiber. Each additional shaped tube provides an additional film layer and thus an additional air / glass interface in the cladding in the radial direction. Each shaped tube has the same number of curved portions.

[0070] In step S13, the shaped glass tubes are inserted into one another to form a nested group or array, with the tubes with the curved portion having the greatest curvature (greatest concavity) being at the innermost position, and the amount of curvature gradually decreasing as one moves outward through the nested group. In step S14, the nested group is inserted into a cylindrical outer jacket tube. As an alternative to steps S13 and S14, the shaped tubes can be inserted into the outer jacket one at a time, rather than first assembling the tubes into a group and then inserting them into the outer jacket. The processes are equivalent.

[0071] In step S15, the nested glass tubes are fastened within the outer sheath to form the desired preform (which can then be Figure 8 The preform is drawn into an optical fiber in one of the alternative steps S4a and S4b / S4c). The fastening can be as for Figure 8 The fastening can be performed as described in step S3 of step S13. The fastening can be performed in a single step to hold each forming tube in position within each other and within the jacket tube. Alternatively, some fastening can be applied to the nested group produced in step S13 before the nested group is inserted into the jacket tube in step S14. In this case, the fastening in step S15 is performed to fasten the nested group as a whole to the jacket tube.

[0072] Any number of forming tubes may be used, depending on the complexity of the desired cladding structure. Typically, the optical fiber preform may be approximately 20 cm in diameter (other sizes may be used), so there is sufficient space available inside the outer jacket tube to accommodate multiple forming tubes, if desired. The preform may include one, two, three, four, five, or more than five forming tubes, although one, two, or three tubes may be the most useful configuration.

[0073] Figure 10 (A) shows a cross-sectional view of a preform having a forming tube defining the core region, Figure 10 (B) shows a cross-sectional view of a preform having three forming tubes defining the core region, and Figure 10 (C) shows a cross-sectional view of a preform having four forming tubes defining the core region.

[0074] Various methods have been proposed for producing shaped tubes.

[0075] A first exemplary method for shaped tube manufacturing is to use a shaped former to reform a cylindrical glass tube into the desired shape.

[0076] Figure 11 A simplified schematic diagram of an exemplary apparatus suitable for practicing this method is shown. Figure 11 The left side shows the Figure 5 (A) Example of forming a shaped tube, the right side shows the corresponding Figure 5 (B) Formation of a forming tube in accordance with an example. A former 60 is provided for each size and shape of forming tube desired. The outer surface of the former 60 is shaped to correspond to the desired cross-section of the forming tube. The former 60 is made of a suitable material capable of withstanding the high temperatures required for reforming the glass, such as graphite, glassy carbon, glassy carbon, boron nitride, or aluminum nitride. Furthermore, the dimensions of the former 60 are designed to account for expansion during heating and for contraction of the glass tube as it cools after reforming, so that the dimensions of the former during heating correspond to the desired dimensions of the forming tube.

[0077] A cylindrical glass tube 62, which may be from a commercial source and has a diameter that closely matches the width of the former 60, is provided and placed over the former 60. The former 60 and tube 62 assembly is heated to soften the glass tube and enable it to be deformed. While the glass is in a softened state, a vacuum (or reduced pressure) is applied to one or more spaces between the former 60 and the glass tube 62. The reduced pressure allows the softened glass of the tube 62 to be pulled inward, as indicated by the arrows, until it rests on the surface of the former 60, thereby acquiring the outer shape of the former 60. The assembly is allowed to cool. If the material of the former is selected so that the coefficient of thermal expansion of the former 60 is higher than that of the glass, the former 60 will contract more than the reformed glass tube as it cools. Therefore, once cooling occurs, the former 60 can be removed from the interior of the now-formed tube.

[0078] A second exemplary method for forming tube fabrication is glass soot deposition. This is a technique used commercially for the high-volume manufacture of glass tubes, glass preforms, and optical fibers. External vapor deposition using flame hydrolysis can be used.

[0079] Figure 12 A simplified schematic diagram of an exemplary apparatus suitable for implementing this method is shown. A mandrel 64 is provided, the outer surface of which has a shape and dimensions corresponding to the desired shape of the forming tube. As the mandrel rotates relative to the flame position, glass soot is deposited onto the outer surface of the mandrel using flame hydrolysis using an oxyhydrogen flame 66, which carries the appropriate chemical reagents of the desired composition of the glass used to form the glass tube. After an appropriate deposition time, a white glass soot forming tube is formed on the mandrel. The tube is then heated in a furnace to sinter the soot, which causes the soot to solidify and become transparent, while also being purified as any impurities in the deposit are removed.

[0080] A third exemplary method for shaped tube manufacture is extrusion.

[0081] This is a known technique for making preforms from relatively soft, low-melting-temperature glass materials. The glass blank is heated above its glass transition temperature (melting temperature) and forced through a suitably shaped mold using high pressure to achieve the desired cross-sectional shape. This is currently challenging with fused silica because its glass transition temperature is very high and molds that can withstand these temperatures are difficult to come by. However, with future improvements in high-temperature composite materials, this may become possible. This method has certainly been adapted for glasses with lower glass transition temperatures, such as lead silicates, tellurites, germanates, and chalcogenides.

[0082] A fourth exemplary method for formed tube fabrication is three-dimensional (3D) printing.

[0083] The first type of 3D printing uses a filament of material that is pushed through a nozzle to "write" the first layer of the desired object onto a substrate that is moved in a plane perpendicular to the nozzle to trace the desired shape. The substrate is then moved further away from the nozzle and the process is repeated to build the object layer by layer.

[0084] The second type of 3D printing uses a dust bed of the material to be printed. The first layer of the object is formed by applying a laser beam to the dust, sintering it. Another layer of dust is then applied and sintered, building up the object layer by layer.

[0085] While materials such as plastics, metals, and ceramics are commonly used for 3D printing, these processes are beginning to be adapted for glass materials, and it is expected that in due course, silica will also be able to be formed using 3D printing.

[0086] The one or more forming tubes and outer jacket tubes used to make the preforms and subsequent optical fibers as described herein can be made of any material known to be used to make anti-resonant hollow core fibers of existing designs, including glass materials (such as silica) and polymeric materials. The various forming tubes and outer jacket tubes in a single preform or optical fiber can be made of the same material or of different materials. Types of glass include "silicate glasses" or "silica-based glasses" based on the compound silicon dioxide (silicon dioxide or quartz), of which there are many examples. Other glasses suitable for optical applications include, but are not limited to, chalcogenides, tellurite glasses, fluoride glasses, and doped silica glasses. The materials can include one or more dopants for tailoring optical properties, such as modifying absorptivity / transmittance or enabling optical pumping.

[0087] It is noted that the curvature imparted to the curved portion of the shaped glass tube during manufacture of the tube is not critical to the desired curvature of the cladding element in the final optical fiber. Known fiber drawing techniques for manufacturing hollow-core fibers involve applying one or more pressures to the interior of various capillaries or groups of capillaries within the preform during the drawing process to control and customize the cross-sectional size and shape of the voids or cavities defined by these capillaries in the finished fiber. In the present case, this can also be used to define the size and shape of the various hollow spaces in the curved cladding element. Therefore, the requirement for the shaped glass tube is simply that, if more than one shaped glass tube is included, the curved portions have different amounts of curvature to define the desired hollow space between the radially separated glass walls or membranes.

[0088] Alternatively, the hollow space between the two radially separated glass membranes can be defined by a shaped tube having curved sections with substantially identical curved shapes but different lengths, resulting in curved sections with different dimensions. The curved shapes are identical, but the radius of curvature at any point between the two curved sections differs. Thus, the curved sections have corresponding shapes but differ in proportion. In other words, the two curved sections are substantially parallel to each other, and their endpoints are located at different points of contact with the interior of the sheath tube.

[0089] Figure 13 A cross-sectional view of an example of a pair of bends from two nested forming tubes bent in this manner is shown. The inner bend 52a of the inner forming tube 50 forms part of the core boundary and is parallel to the outer bend 52b of the outer forming tube 51 located between the inner bend and the inner surface of the jacket tube 55. For example, Figure 7 For comparison, Figure 7 In one embodiment, the curved portions have different curved shapes and are therefore not parallel to each other, but meet with the sheath tube at the same engagement position. In other examples, the curved portions may have different curved shapes (therefore not parallel) and also have different engagement positions.

[0090] Although the forming tube has been described as having straight sections between curved sections to provide the required cladding element spacing in a nodeless ARF, the forming tube can also be constructed to make an ARF without such spacing. The forming tube may not have any straight sections in its sidewall, but instead include a series of curved sections connected together at points or tips around the circumference, such as a hypocycloid.

[0091] The various embodiments described herein are presented only to aid understanding and teach the claimed features. These embodiments are provided only as representative examples of embodiments and are not exhaustive and / or exclusive. It should be understood that the advantages, embodiments, examples, functions, features, structures and / or other aspects described herein should not be construed as limiting the scope of the invention as defined by the claims or limiting the equivalents of the claims, and that other embodiments may be utilized and modifications may be made without departing from the scope of the claimed invention. In addition to those specifically described herein, various embodiments of the present invention may appropriately include, consist of, or essentially consist of appropriate combinations of the disclosed elements, components, features, parts, steps, devices, etc. In addition, the present disclosure may include other inventions that are not currently claimed but may be claimed in the future.

[0092] References

[0093] [1]TD Bradley,JR Hayes,Y Chen,GT Jasion,SR Sandoghchi,R Slavik,ENFokoua,S Bawn,H Sakr,IA Davidson,A Taranta,JP Thomas,MN Petrovich,DJRichardson,F Poletti,“Record Low-Loss 1.3dB / km Data Transmitting AntiresonantHollow Core Fibre”in Proc European Conference on Optical Communications(ECOC)2018,paper Th3F2

[0094] [2]Francesco Poletti,“Nested antiresonant nodeless hollow corefiber”,Opt.Express 22,23807-23828(2014)

[0095] [3]WO2015 / 185761

[0096] [4]L.D.van Putten et al.,“Exploring the Effect of the Core BoundaryCurvature in Hollow Antiresonant Fibers”,IEEE Photonics Technology Letters 29(2),263-266(2017)

[0097] [5]US 2016 / 0124144

Claims

1. A preform for an anti-resonant hollow core optical fiber, the preform comprising a plurality of forming tubes arranged in a nested manner, each forming tube having a sidewall with a transverse cross-sectional shape, the sidewall comprising a number of major curved portions alternating with an equal number of minor substantially straight portions, each of the substantially straight portions being equidistant from a central longitudinal axis of the forming tube, wherein: Each forming tube is fastened within an outer jacket tube of glass or polymer material such that the substantially straight portion of the forming tube is in contact or nearly in contact with the inner surface of the outer jacket tube, The plurality of forming tubes and the outer jacket tube have the same central longitudinal axis, the curved portion of each of the plurality of forming tubes has a different inward curvature shape and / or a different length than the curved portion of one or more other forming tubes, and the plurality of forming tubes have the same number of curved portions, and a cladding structure including non-circular cladding elements is formed between the curved portions of the plurality of forming tubes.

2. The preform according to claim 1, wherein The inwardly curved shape is symmetrical about a radius from the central longitudinal axis to a midpoint between two straight line portions on either side of the curved portion. The preform according to claim 2 , wherein the inwardly curved shape is a circular arc or an elliptical arc. The preform according to claim 2 , wherein the inwardly curved shape is a semicircle or a semi-ellipse.

5. The preform according to any one of claims 1 to 4, wherein Each of the curved portions has the same inwardly curved shape.

6. The preform according to any one of claims 1 to 4, wherein each of the curved portions has the same spacing from the central longitudinal axis.

7. The preform according to any one of claims 1 to 4, wherein The number of curved portions ranges from two curved portions to twelve curved portions.

8. The preform according to any one of claims 1 to 4, wherein The shaped tube is formed of a glass material or a polymer material.

9. A rod for an anti-resonance hollow-core optical fiber drawn from the preform according to claim 1. 10 . An anti-resonance hollow-core optical fiber, wherein the anti-resonance hollow-core optical fiber is drawn from the preform according to claim 1 or the rod according to claim 9.

11. A method for manufacturing a preform for an anti-resonance hollow-core optical fiber, the method comprising: providing a plurality of forming tubes, each forming tube having a sidewall with a transverse cross-sectional shape, the sidewall including a number of major curved portions alternating with an equal number of minor substantially straight portions, and each of the substantially straight portions being equidistant from a central longitudinal axis of the forming tube; inserting the plurality of forming tubes into an outer jacket tube of glass or polymer material such that there is contact or near contact between the substantially straight portions of the forming tubes and the inner surface of the outer jacket tube; and fastening the plurality of forming tubes within the outer sheath tube, wherein the plurality of forming tubes are inserted into each other to provide a set of nested forming tubes within the outer jacket tube, the plurality of forming tubes and the outer jacket tube have the same central longitudinal axis, the curved portion of each of the plurality of forming tubes has a different inward curvature shape and / or a different length than the curved portion of one or more other forming tubes, and the plurality of forming tubes have the same number of curved portions, and a cladding structure including non-circular cladding elements is formed between the curved portions of the plurality of forming tubes.

12. A method of manufacturing a rod for an anti-resonance hollow core optical fiber, the method comprising manufacturing a preform according to the method of claim 11, and drawing the preform into a rod.

13. A method for manufacturing an anti-resonance hollow-core optical fiber, the method comprising manufacturing a preform according to the method of claim 11 or manufacturing a rod according to the method of claim 12, and drawing the preform or rod into an optical fiber.

14. The method according to any one of claims 11 to 13, wherein Providing the plurality of shaped tubes includes manufacturing each shaped tube by heating and reforming a cylindrical glass tube on a former having an outer shape corresponding to a desired cross-sectional shape of each shaped tube.

15. The method according to any one of claims 11 to 13, wherein Providing the plurality of forming tubes includes: manufacturing each forming tube by depositing glass soot onto a rotating mandrel having an outer shape corresponding to a desired cross-sectional shape of each forming tube to form a forming tube of glass soot; and sintering the forming tube of glass soot to form a forming tube of consolidated glass material.

16. The method according to any one of claims 11 to 13, wherein Providing the plurality of shaped tubes includes manufacturing each shaped tube by extruding molten glass through a die shaped to correspond to a desired cross-sectional shape of each shaped tube.

17. The method according to any one of claims 11 to 13, wherein Providing the plurality of shaped tubes includes manufacturing each shaped tube using three-dimensional printing.

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