Hollow-core optical fibers with yield-resistant microstructures

Abstract

A hollow-core optical fiber (HCF) configured for guiding optical waves by antiresonance comprises a tubular glass jacket, a hollow core defined in a central lumen of the jacket and having a polygonal cross-section, and a cladding including flat glass membranes disposed in the central lumen and each having two opposing edges extending along the length of the HCF and a membrane thickness less than the jacket wall thickness, the membranes being arranged in a plurality of groups, each group including a core boundary membrane defining one side of the polygon and at least one cladding membrane disposed between the core boundary membrane and the jacket, all membranes in the group including at least one spaced apart cladding membrane, all sides of the polygon being formed by the core boundary membranes, and all membranes being secured to the jacket only along two opposing edges, with no contact between any of the membranes.

Description

[Technical Field]

[0001] Background of the Invention The present invention relates to hollow-core optical fibers having microstructure designs that facilitate high-yield manufacturing, and methods of manufacturing. [Background technology]

[0002] Research into optical fibers has led to the development of "holey" optical fibers, which contain longitudinal voids, lumens, or capillaries filled with air (or other gas) within the internal structure of each individual fiber. These fibers comprise a hollow-core optical fiber, with a central longitudinal void, which serves as a waveguide core, surrounded by a microstructured cladding formed from a specified arrangement of longitudinal voids contained within an outer tubular jacket. The void parameters, including number, size, shape, relative position, and thickness of the dividing membrane or wall, are critical to ensuring the optical fiber's superior waveguide performance. The membrane is typically curved. Similar to conventional solid-core optical fibers, hollow-core optical fibers are manufactured using the fiber drawing process. In the fiber drawing process, a preform or strut formed from glass and having a cross-sectional geometry suitable for forming the intended optical fiber with a defined structure is heated on a large scale to soften the glass, and then drawn or pulled to produce long lengths of optical fiber, with the required cross-sectional structure formed from the preform's cross-section. To achieve the desired microstructure at the end of the drawing process, increased internal pressure must be applied to the voids in the preform to prevent them from collapsing as the glass softens. Realizing the parameters necessary to achieve the intended microstructure design requires applying different pressures to different voids to counteract surface tension effects that would otherwise function to collapse or deform the various voids. In particular, differential pressures tend to be required to produce curved membranes in the microstructure from the tubular elements often used in preforms.

[0003] Selecting the appropriate pressure is a complex procedure because of an interplay between applied pressure and surface tension, which requires that some of the fiber shape must be able to overshoot its intended dimensions in the initial stages of drawing before shrinking toward the target structure at the end of the draw. Applying the correct pressure becomes more difficult as the draw yield—i.e., the fiber length that can be successfully fabricated as a continuous length or range—increases. At long lengths, the shrinking dynamics eventually become so aggressive that the initial overshoot required to preempt and suppress the shrinking phase becomes so large that unacceptable geometric deformations occur, such as contact between structural elements that should be spaced apart or gross asymmetry of symmetrical elements. This difficulty imposes an upper limit on the length of hollow-core optical fiber that can be successfully fabricated while maintaining quality. Currently, the longest single stretch of hollow-core fiber known to the inventors with an acceptable level of optical propagation loss (approximately 5 dB / km) is 11 km long [1]. Lengths of approximately 13 km have also been demonstrated with unspecified loss [2].

[0004] This limited length is a significant drawback. Hollow-core optical fibers have superior optical propagation properties compared to solid-core fibers, including reduced and uniform chromatic dispersion, increased propagation velocity, wider optical bandwidth, and parasitic nonlinear optical effects resulting from high air friction within the fiber and the corresponding reduced volume of glass, with light propagating primarily through air and avoiding the deleterious effects resulting from propagation in glass, while attenuation has recently been significantly reduced to levels comparable to that of solid-core fibers. These properties make hollow-core optical fibers particularly attractive for use in telecommunications applications in which optical signals convey data between two distant transceiver stations. Enabling telecommunications between widely separated transceiver stations (e.g., transoceanic telecommunications) requires very long lengths of optical fiber. The low loss afforded by hollow-core optical fibers is well suited to such configurations. While existing solid-core optical fiber manufacturing techniques can produce continuous spans of several hundred kilometers in length, as noted above, the achievable yield lengths for hollow-core fibers are currently very short. Therefore, to take advantage of the superior properties of hollow-core optical fibers for long-distance telecommunications, it is currently necessary to splice multiple lengths of fiber end-to-end, introducing undesirable losses at each splice that mitigate the inherent low loss of the fiber.

[0005] Therefore, techniques that can increase the yield of hollow core optical manufacturing are of interest. Summary of the Invention [Means for solving the problem]

[0006] Summary of the Invention Aspects and embodiments are set out in the accompanying claims.

[0007] According to a first aspect of certain embodiments described herein, there is provided a hollow core optical fiber configured for guiding optical waves by anti-resonance, the hollow core comprising a tubular glass jacket having a central lumen and a wall having a wall thickness; a hollow core defined in said central lumen, said hollow core having a polygonal cross section; and a cladding disposed in said central lumen, said cladding including flat glass films each having two opposing edges extending along a length of said hollow core optical fiber and a thickness less than said wall thickness, said flat glass films being arranged in a plurality of groups of glass films, each group being spaced apart from said central lumen. The present invention provides a hollow-core optical fiber comprising a core boundary glass film defining one side of the polygon of the hollow core, and at least one cladding glass film disposed between the core boundary glass film and the tubular glass jacket, wherein all of the core boundary glass films and cladding glass films in each group are spaced apart from one another, all sides of the polygon of the hollow core are formed by core boundary glass films, and all of the flat glass films are fixed to the tubular glass jacket only along the two opposing edges, and there is no contact between the flat glass films.

[0008] According to a second aspect of certain embodiments described herein, there is provided a preform for producing a hollow-core optical fiber configured for guiding light waves by anti-resonance as set forth in claim 1, comprising: a plurality of hollow glass tubes, each having a wall with a thickness, to provide a flat glass membrane of the hollow-core optical fiber, the plurality of hollow glass tubes of reduced diameter being concentrically arranged inside one another with spaces between the hollow glass tubes; an outer hollow glass tube, arranged concentrically around and spaced from the plurality of hollow glass tubes, to provide a tubular glass jacket of the hollow-core fiber, the outer hollow glass tube having a wall with a thickness greater than the thickness of each of the walls of the hollow glass tubes; and three or more groups of glass spacer elements, the groups spaced circumferentially around the preform corresponding to corners of the polygonal hollow core of the hollow-core fiber, the spacer elements in each group including at least one spacer element in each space between the hollow glass tubes and the outer hollow glass tube.

[0009] According to a third aspect of certain embodiments described herein, there is provided a method of forming a preform according to the second aspect for producing a hollow-core optical fiber configured for guiding light waves by anti-resonance, the method comprising: a first step including forming a subassembly by arranging a plurality of hollow glass tubes of decreasing diameter concentrically inside one another with spaces between the hollow glass tubes, the plurality of hollow glass tubes having walls each having a thickness; and disposing glass spacer elements in the spaces between the hollow glass tubes, the spacer elements being arranged in at least three groups spaced circumferentially around the hollow glass tubes, the spacer elements in each group including at least one spacer element in each space between the hollow glass tubes; and drawing the subassembly until a diameter of the subassembly is reduced and the hollow glass tubes are integrated with the spacer elements. and optionally, if more hollow glass tubes are required in the preform, a third step including arranging additional hollow glass tubes and additional glass spacer elements around the subassembly, aligning the additional glass spacer elements in groups in the subassembly. A fourth step including arranging any additional hollow glass tubes required in the preform concentrically around the subassembly, and arranging outer hollow glass tubes having walls thicker than the thickness of each of the walls of the hollow glass tubes concentrically around the outermost hollow glass tubes with spaces therebetween, and arranging further additional glass spacer elements in each space between the hollow glass tubes and the outer hollow glass tube, aligning the further additional glass spacer elements in groups in the subassembly.

[0010] According to a fourth aspect of certain embodiments described herein, there is provided a method of manufacturing a hollow-core optical fiber configured for anti-resonant guiding of light waves, the method comprising heating and drawing a preform according to the second aspect to form an optical fiber.

[0011] These and further aspects of particular embodiments are set out in the accompanying independent and dependent claims. It is understood that features of the dependent claims may be combined with each other and with features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approaches described herein are not limited to the particular embodiments as described below, but rather encompass and contemplate any suitable combination of the features presented herein. For example, devices and methods may be provided in accordance with the approaches described herein, including any one or more of the various features described below, as appropriate.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows a cross-sectional view of a first known anti-resonant hollow-core optical fiber. [Figure 2] 1 shows a cross-sectional view of a second known anti-resonant hollow-core optical fiber. [Figure 3A] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3B] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3C] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3D] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3E] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3F] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3G] 1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 3H]1A-1D show cross-sectional views of known hollow-core optical fibers having various internal structures. [Figure 4] 1 shows a cross-sectional view of a first example anti-resonant hollow-core optical fiber according to the present disclosure, having a triangular hollow core. [Figure 5] 1 shows a cross-sectional view of a second example anti-resonant hollow-core optical fiber according to the present disclosure, having a square hollow core. [Figure 6] 10 shows a cross-sectional view of a third example anti-resonant hollow-core optical fiber according to the present disclosure, having a protrusion and a sink cavity for higher-order optical modes. [Figure 7] 10 shows a cross-sectional view of a fourth example anti-resonant hollow-core optical fiber according to the present disclosure, having a triangular core and a protrusion. [Figure 8] FIG. 10 shows a cross-sectional view of a fifth example anti-resonant hollow-core optical fiber according to the present disclosure, having a sink cavity for higher-order optical modes. [Figure 9] 10 shows a graph of computer-modeled variation of optical loss with increasing number of anti-resonant films for an example anti-resonant hollow-core optical fiber according to the present disclosure. [Figure 10] FIG. 1 shows a cross-sectional view of a first example preform for producing an anti-resonant hollow-core optical fiber according to the present disclosure. [Figure 11] FIG. 1 shows a cross-sectional view of a second example preform for producing an anti-resonant hollow-core optical fiber according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0014] Detailed Description Aspects and features of particular examples and embodiments are explained / described herein. Some aspects and features of particular examples and embodiments may be conventionally implemented, and these aspects and features will not be explained / described in detail for the sake of brevity. Accordingly, it will be understood that aspects and features of the devices and methods described herein that are not described in detail can be implemented in accordance with any conventional techniques for implementing such aspects and features.

[0015] A hollow-core optical fiber has a cross-sectional structure (transverse to the length of the fiber) that includes a central hollow void or lumen providing a core that guides or propagates a fundamental optical mode, surrounded by a microstructured cladding that includes multiple smaller voids or lumens arranged or configured to support the waveguide of the fundamental optical mode through one or more physical phenomena. The cladding voids are separated or divided from each other and from the core by thin glass membranes, walls, or struts. A thicker bulk glass tubular outer jacket surrounds and supports the cladding. Hollow-core fibers can be classified into two main classes or types according to the mechanism of light guiding: hollow-core photonic bandgap fibers (HCPBFs, alternatively called hollow-core photonic crystal fibers (HCPCFs)) and antiresonant hollow-core fibers (AR-HCFs or ARFs). In HCPBFs, the structured cladding includes a regular, close-packed array of lumens formed from many small glass capillaries that, excluding a central group, define a substantially circular hollow core. The periodicity of the cladding structure provides a substantially periodic structured refractive index and, therefore, a photonic bandgap effect that confines propagating light waves toward the core. In ARFs, the structured cladding does not have a high degree of periodicity so that the photonic bandgap effect is not significant, but it contains a very small number of large glass films with an overall structure that has some periodicity on a large scale due to the regular placement of the lumen. The cladding structure provides antiresonance for propagation wavelengths that are not resonant with the wall thickness of the cladding lumen—in other words, wavelengths in the antiresonance window defined by the thickness of the wall or film defining the cladding lumen. The cladding lumen provides the hollow core of the fiber and surrounds a central void or cavity that can support antiresonant guided optical modes. The structured cladding can also support cladding modes that can propagate primarily within the lumen, either in the glass of the lumen wall or in the space or gap between the cladding lumen and the outer jacket of the fiber. The loss of these additional non-core guided modes is generally much higher than the loss of the core guided modes. The fundamental core guided mode typically has by far the lowest loss of the core guided modes.The anti-resonance given by the wavelength of the propagating light and the lumen wall thickness in the anti-resonance state serves 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.

[0016] In recent years, various anti-resonance designs have been proposed. A particularly interesting design, which is amenable to manufacture by drawing from a preform over at least a medium length, consists of a series of tubes or capillaries, typically of circular cross section, arranged around the inside surface of a hollow circular tubular outer jacket, leaving a space in the center of the outer jacket to act as a core.

[0017] FIG. 1 shows a cross-sectional view of an example of a known simple anti-resonant hollow-core fiber structured in this manner. The fiber 10 has an outer tubular cladding or jacket 3 including a wall that defines a central lumen. 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 within the jacket 3 in a single ring, such that the longitudinal axes of each cladding capillary 14 and the jacket 3 are substantially parallel. Each cladding capillary 14 contacts (bonds to) the inner surface of the jacket 3 at location 16, such that the cladding capillaries 14 are equally spaced around the inner circumference of the jacket 3 and are further separated from one another by gaps 5 (no contact between adjacent capillaries). In some designs of ARFs, the cladding capillaries 14 are positioned in contact with one another (i.e., not spaced apart, as in FIG. 1). The contact points between the walls or films defining the cladding lumen are called nodes and are prone to undesirable resonances that result in high loss; consequently, elimination of the nodes (e.g., by spacing them as in FIG. 1) can improve the optical performance of the fiber. Thus, fibers with spaced cladding capillaries are sometimes called "nodeless anti-resonant hollow-core fibers."

[0018] The arrangement of the cladding capillaries 14 in a ring around the inside of the tubular jacket 3 creates a central space, cavity, or void within the fiber 10, which is the fiber's hollow core 2, further having a longitudinal axis of the fiber 10 that is parallel to the longitudinal axes of the jacket 3 and the cladding capillaries 14. The core 2 is bounded by the inward-facing outer surface of the circular cladding capillary 14. This is the core boundary, and the capillary wall or film material that makes up this boundary provides the necessary antiresonant light guiding effect or mechanism. The capillary 14 has a thickness t at the core boundary that defines the wavelength at which antiresonant light guiding occurs in the antiresonant fiber. This thickness t is significantly less than the wall thickness of the jacket 3. The curvature of the cladding capillary is convex, or inward, from the perspective of the core 2; as a result, the core boundary is considered to have a negative curvature. Negative curvature structures are generally considered important for achieving low loss in hollow-core fibers.

[0019] Figure 2 shows a cross-sectional view of a second known example of a negative curvature anti-resonant hollow-core fiber formed from a circular tubular capillary. This fiber 10 includes six cladding capillaries 14 evenly spaced around the inner surface of a tubular outer jacket 3 and has a structured inner cladding 1 surrounding a hollow core 2. The difference from the example of Figure 1 is that each cladding capillary 14 has smaller secondary capillaries 18 nested within it, in this example, bonded to its inner surface at the same azimuthal position 16 as the contact point between the primary capillary 14 and the jacket 3. These additional smaller capillaries 18 can reduce optical losses. Further smaller tertiary capillaries may be nested within the secondary capillaries 18. This type of anti-resonant fiber design, with secondary capillaries and optionally additional smaller capillaries, is sometimes referred to as a "nested anti-resonant nodeless fiber" or NANF [3].

[0020] The nested configuration introduces additional membranes between the core and jacket. These additional layers increase the antiresonance effect, thus reducing the optical losses incurred by the fundamental mode propagating in the core and improving the performance of the optical fiber. Therefore, multiple separating membranes arranged consecutively between the core and jacket are considered a beneficial feature in hollow-core optical fibers. However, implementing this feature with circular capillaries, such as the designs of Figures 1 and 2, complicates fabrication because different pressures must be applied to the various capillaries during fiber drawing to maintain the circular capillary shape. Ultimately, this makes fabrication quite difficult because the forces between the pressure differential and the surface tension of the softened glass in the fiber preform can cause overexpansion of some lumens and subsequent distortion of the finished fiber structure. Therefore, the achievable range of drawable hollow-core fibers is limited, and the fiber's special performance characteristics cannot be optimally utilized for long-length applications (e.g., telecommunications).

[0021] Various other designs of hollow-core optical fibers have been proposed elsewhere, but the design objective is often to provide a fiber with very low optical propagation loss, with little regard for manufacturing feasibility.

[0022] Figure 3 shows a selection of known hollow-core optical fiber designs (cross-sections) with many multiple layers of cladding films intended to provide low loss, but all pose various challenges in terms of manufacturing or optical performance. Figure 3A shows a design in which the cladding includes multiple concentric circular films of increasing size positioned between the core and a circular jacket [4]. The fiber is actually designed as a Bragg grating fiber, but the configuration shown, in which the concentric films are not attached and are free-floating, cannot be fabricated; in fact, the cladding has a solid structure of concentric dielectric layers. Figures 3B and 3C show similar designs, in this case with concentric circular cladding films supported by radial struts between the films [5, 6]. Such designs for terahertz applications can be fabricated using three-dimensional printing or extrusion, but are significantly deformed when drawn from a preform. Therefore, fiber length is limited. Figure 3D shows an early design for an antiresonant hollow-core optical fiber [7]. Antiresonant hollow-core optical fibers can be drawn from preforms assembled from circular capillaries using a single pressure, causing the curved walls of the capillary to flatten under surface tension, creating a hexagonal core, but without multiple film layers in the cladding. The cladding has nodes (circled in the drawing) that form when the films are touching, resulting in high optical loss. Figure 3E shows a design with multiple non-concentric circular cladding film layers contacting at various offset locations [8]. The contact provides support to the films, eliminating the difficulties of the floating design of Figure 3A. However, the contact points create nodes and introduce resonant loss. Furthermore, complex curved structures require multiple different pressure zones during fiber drawing, possibly resulting in significant deformation. Figure 3F shows a design in which a roughly triangular hollow core is bounded by azimuthally spaced parallel pairs of inwardly curved films [9]. Because curvature requires differential pressure for fiber drawing, the achievable range is limited, just as with the designs of Figures 1 and 2. Figure 3G shows a design with a square hollow core and a square jacket, cladding formed from layered parallel non-curved membranes

[10] .Some membranes contain 90-degree angles that deform into curved shapes during fiber drawing, even under differential pressure; as a result, successful fabrication is limited to three-dimensional printing, and achievable lengths are therefore severely limited. Figure 3H shows another square design in which a concentric square cladding membrane is supported by support membranes at the corners of the membrane

[11] . The support membranes create nodes at the junctions with the cladding membrane, causing resonant losses and consequently impairing fiber performance.

[0023] The present disclosure proposes an anti-resonant hollow-core fiber structure that eliminates loss-inducing nodes and can be drawn using a single pressure across the entire void or lumen within the fiber, if desired, thereby upgrading fiber drawing compared to currently achievable processes and achieving much longer lengths of high-quality fiber with consistent structure than is possible with existing designs.

[0024] These objectives are addressed through the use of a single flat glass film to define the cladding. Flatness eliminates voids or lumens bounded by curved films, allowing for the application of a single pressure during fiber drawing. In the originating preform, the film can be flat or curved, and application of the same pressure across the entire fiber allows the softened glass film to remain flat or flatten under the action of surface tension, resulting in the desired flat film being present in the finished drawn fiber. The single pressure allows for simple dynamics during the drawing process, where the entire microstructure expands and contracts together, reducing or avoiding the complex interactions between pressure and surface tension that arise when multiple pressures are required to maintain a curved shape. This avoids the limitations imposed by the need for differential pressures to avoid over-expansion and structural distortion that occurs in extended fiber draws, and allows for the production of very long lengths of fiber from a single draw. The preform film has an appropriate thickness or thicknesses so that, when thinned by drawing, the final film thickness in the fiber provides anti-resonance at the intended wavelength of light to be transmitted by the optical fiber. In contrast, the outer glass jacket surrounding and supporting the cladding structure has a significant thickness that allows it to withstand deformation from surface tension during drawing, and as a result, it is curved and can still maintain its shape despite the use of no differential pressure. Thus, while usefully, the outer jacket can have a circular cross-sectional hollow tubular shape consistent with conventional optical fiber design, this is not essential. The jacket wall (in both the preform and the finished fiber) that defines the hollow central lumen in which the cladding is located has a thickness that is greater than (typically significantly greater than) the thickness of the film and that does not impart any anti-resonant effects to the propagating wavelength. Due to its thickness greater than that of the film, the jacket can be considered to be formed from bulk glass. While the ability to manufacture the proposed fiber without differential pressure is a useful advantage, it should be noted that the fiber can also be drawn using differential pressure if desired.

[0025] Furthermore, the planar glass membranes in the optical fiber are supported within the jacket by being secured (clamped, spliced, or otherwise in contact) with the jacket only along two opposing edges parallel to the longitudinal extent or length of the fiber, with no contact points between any of the glass membranes. All membranes are in contact (anchored) with the inner surface of the jacket wall and are not in contact with any other membranes. This configuration eliminates nodes, which are a source of propagation loss in hollow-core antiresonant microstructured optical fibers, as described above. Therefore, the proposed structure can be manufactured into extended lengths of low-loss, high-performance optical fiber. Currently, hollow-core fiber production is limited to only about 10 km per preform. The proposed structure is highly tolerant to increased yields due to the lack of microstructure deformation when upgrading production. It is expected that production rates can be increased by more than tenfold. This not only provides a much more useful extension range of the fiber, but also reduces costs. Current hollow-core fiber is very expensive due to manufacturing limitations. Applications such as transoceanic fiber optic links between telecommunications data centers, which require thousands of kilometers of fiber in a cable, become very feasible.

[0026] The flat glass film necessarily prevents the hollow core of the proposed fiber from having a negatively curvature core boundary. However, contrary to popular understanding in the field that negative curvature is important for low-loss operation, it has been found that comparable operation can be achieved by appropriate configuration of the film. Instead of a negatively curvature core boundary, the flat film defines straight sides to the core boundary in cross-section through the fiber, such that the core has a polygon formed from multiple straight sides. The polygon may or may not be a regular polygon, although regular polygonal symmetry can improve waveguide performance. In the proposed design, the core is defined and bounded exclusively by the flat film, such that all sides of the polygon are formed by the glass film, with no intermediate sides imparted by portions of the inner surface of the jacket wall. (Note that some slight deviations from this may occur in actual fibers due to surface tension effects and manufacturing errors at the vertices of the polygon, but this is of no practical importance and is not an intended feature of the fiber design.)

[0027] FIG. 4 shows a cross-sectional view of a first example hollow-core optical fiber as proposed herein. The hollow-core optical fiber 20 includes a tubular glass jacket 22 having a circular cross-sectional shape formed from a bulk (solid) glass outer wall 24 that encloses and seals a central space or void (lumen). The wall has a thickness T. Located within the central lumen of the jacket 22 is the cladding of the fiber 20, which is composed of a plurality of flat, sheet-glass films 26. Each film 26 has a thickness t that is thinner than the thickness T of the jacket wall 24 and is selected for antiresonance at the wavelength at which the fiber 20 is intended to propagate. The films 26 may or may not all have the same thickness. The films 26 extend along the length of the fiber 20 perpendicular to the cross-section (i.e., the plane of the paper, as shown) and have two opposing edges 26 a, 26 b that are parallel to this perpendicular direction, with the flat-plate-shaped surface of each film 26 extending between the two opposing edges 26 a, 26 b. For ease of understanding and explanation, the membranes 26 can be thought of as being arranged in multiple groups 28 of membranes. In this example, there are three groups 28 of membranes 26. Each group 28 includes at least two membranes 26; in this particular example, each group 28 includes seven membranes 26. The groups 28 are arranged consecutively around the circumference of the jacket 22 within the central lumen. The membranes 26 comprising each group 28 are arranged in a layered stack, with the individual membranes spaced apart (perpendicular to the plane of the membranes 26) by a spacing or gap 30 of depth s, and the membranes 26 in a group 28 are substantially parallel to one another. Within each group 28, the membranes 26 include a core boundary membrane 32 closest to the center of the central lumen within the jacket 22, and one or more (six in this example) cladding membranes 34 arranged at spaced locations in a radial row between the core boundary membrane 32 and the inner surface of the jacket 22. In this example, the spacing cladding membranes 34 occupy substantially all of the space behind the corresponding core boundary membranes 32. All membranes 26 in all groups 28 (i.e., both core boundary membranes 32 and cladding membranes 34) contact (are secured, clamped, or joined) the inner surface of the wall 24 of the jacket 22 along both of two opposing edges 26 a, 26 b.This contact between the edges 26a, 26b of the membranes 26 only and the jacket 22, and the spaced apart arrangement of the membranes 26, means that the membranes 26 do not touch each other (there is no contact between the membranes), and therefore there are no lossy nodes in the cladding structure.

[0028] The fiber 20 has a hollow core 36 defined in the central lumen of the jacket 22. The grouping of membranes 26 around the jacket means that the core 36 is bounded by core boundary membranes 32, which are planar (defining straight lines in the cross-section of the fiber 20), so that the hollow core 36 has a polygonal cross-sectional shape, with each side of the polygon formed by one of the core boundary membranes 32. The core boundary membranes 32 have widths, and are positioned within the jacket 22 such that adjacent edges of adjacent core boundary membranes 32 are secured to the jacket 22 adjacently, with no intervening portion of the inner surface of the jacket 22 exposed to the area forming the core 36. Thus, all sides of the polygon of the hollow core 36 are formed by one core boundary membrane 32. This example has three groups 28 of membranes, and therefore three core boundary membranes 32, and the polygon of the hollow core 36 is triangular.

[0029] In the example of FIG. 4 , each group 28 of films includes the same number of films 26. This is not required, as equality results in a symmetrical design that supports waveguides, but different groups 28 may have different numbers of films 26. While seven films 26 are shown per group 28, fewer or more may be included, for example, ranging from two to twenty films per group, although higher numbers are not excluded. The number of films 26 used depends on the film thickness t and the available space behind each core boundary film 32 that is available to accommodate cladding films. A larger number of films 26 per group 28 improves optical performance because each additional layer enhances the antiresonant effect and reduces the propagation loss of the fundamental optical mode. This is discussed further below.

[0030] Similarly, symmetry of the design can be enhanced by arranging the membranes 26 within each group 28 so that they are parallel to one another, as shown in Figure 4, but this is not essential. Additionally, the membranes 26 in each group 28 may be separated by spaces 30 that are all the same depth, and / or the spaces 30 within a group 28 may have the same depth as the spaces 30 in each of the other groups 28. Again, equal spacing is not essential, and spacings of different depths may be used within or across groups 28.

[0031] Regarding the shape of the hollow core, it may be a regular polygon if all core boundary membranes 32 have the same width (the distance between two opposing edges 26 a, 26 b), so that all sides of the polygon have the same length. Alternatively, the core boundary membranes 32 may have different widths, giving the core 36 an irregular polygonal shape.

[0032] The triangular shape of the hollow core 36 in the example of Figure 4 can be beneficial in that it allows the corners or vertices of the core 36 to be as far away as possible from the nominally circular area at the center of the core 36 extending between the core boundary films 32, shown by the dotted line in Figure 4. For the purposes of characterizing the optical fiber and comparing it to other fiber designs, the radius of this circle can be referred to as the core radius. This is the region through which the majority of the fundamental optical mode propagates, and moving the corners of the hollow core space away from this region can also contribute to reducing optical losses. However, the proposed hollow core design is not limited in this respect.

[0033] FIG. 5 shows a cross-section through a second example hollow-core optical fiber 20. In this example, there are four groups 28 of membranes 26, so that the hollow core 36 has a four-sided polygonal cross section. Each core boundary membrane 32 has the same edge-to-edge width, so that the polygon is a square. As mentioned above, other numbers of membranes can be used, but in this example, each group 28 includes four membranes 26. However, a greater number of groups 28, and therefore a greater number of sides to the core, tends to reduce the available space between each core boundary membrane 26 and the jacket 22, so that fewer cladding membranes 34 can be accommodated in each group 28. For this reason, a triangular core is preferred because it allows for more anti-resonant layers in the cladding, thereby reducing propagation loss.

[0034] The examples of Figures 4 and 5 each show the membrane 26 connected at its edge to the inner surface of the jacket, the jacket having a smooth inner surface and a constant wall thickness. In other configurations, the jacket can be configured differently in that it has multiple inward protrusions that the membrane edge meets.

[0035] FIG. 6 shows a cross-sectional view of a third example fiber. In this example, the jacket 22 of the fiber 10 has three protrusions or projections 40 projecting inward into the central lumen of the jacket 22. The projections are integral with the glass of the jacket wall 24, and thus are also bulk solid glass, and have an overall dimension thicker than the thickness of the membranes 26. This avoids interference with antiresonant induction due to resonances (such as nodes) or antiresonances. Three groups 28 of membranes 26 include cladding, so that the hollow core 36 has a triangular shape, as in the example of FIG. 4, with the core boundary membrane 32 of each group forming one side of the hollow core 36. The edges of each membrane 26 are fixed to the jacket 22 as described above, but in this example, the fixed contact is made by the projections 40, and each group 28 is positioned between a pair of adjacent projections 40, with the edges 26a, 26b of each membrane contacting the projections 40. Thus, the protrusions 40 are aligned with the corners (in this case, the vertices of a triangle) of the polygon of the hollow core 36. For other polygons, a corresponding number of protrusions for each vertex and membrane groups for each side can be used instead.

[0036] It can be seen from Figure 6 that all of the membranes 26 are connected to the edges of the protrusions 40 along the entire height of the protrusions. Alternatively, to provide more membrane layers, additional cladding membranes can be included behind the membranes shown in Figure 6, with the edges of the membranes secured to the curved inner surface of the jacket wall 24 between the protrusions 40, as in Figures 4 and 5, completely occupying the space between the core boundary membrane 32 and the wall 24, as described above. The height of the protrusions 40 may be increased to accommodate the additional membranes, so that they extend further into the central lumen of the jacket 22, which provides more room along the height of the protrusions 40 for securing the membrane edges.

[0037] However, a space can otherwise be left behind each group 28 of films 26 (with or without the use of protrusions), as shown in FIG. 6 . This creates a cavity 42 between each group 28 and the wall 24 of the jacket 22 behind the last or outermost (radially) cladding film 34 in each group 28. The cavity has a radial depth z greater (typically several or many times greater) than the spacing s between the films 26 in the associated group 28. These cavities act as a “sink” for higher-order propagating optical modes, suppressing them by allowing the energy carried by these modes to be attenuated by significant propagation losses. This allows the majority of the optical power propagating in the fiber to be concentrated in the fundamental mode propagating in the core, improving the fiber’s optical performance. Suppression of higher-order modes can be important for some applications. For example, in telecommunications, power can be coupled from the fundamental mode to higher-order modes at joints and other interruptions along the fiber length. The modes may experience different propagation velocities, and if allowed to remain and propagate to the receiving station, some of the signal carried by the higher order modes can interfere with the fundamental mode and reduce the signal-to-noise ratio.

[0038] In an alternative design, a cavity that provides a sink for higher-order optical modes can be implemented by arranging the membranes in groups such that one of the spacings between adjacent membranes is significantly larger than the others. In this way, one of the spacings may be increased or widened to provide a sink cavity that includes the space between the core boundary membrane and the innermost cladding membrane.

[0039] The protrusions can have any convenient shape and can protrude into the lumen of the jacket as little or as much as is convenient for supporting the required number of membranes in each group. The shape of the protrusions can be the end result of surface tension effects on the fiber during drawing. The protrusions can be integrally formed with the jacket used in the generating preform for the fiber, can be intended to retain the same or similar shape during drawing and in the finished fiber, or can adopt a different shape under surface tension during drawing, or can be created by melting two or more smaller portions of glass during drawing.

[0040] FIG. 7 shows a cross-sectional view of another example hollow-core optical fiber. In this fiber 10, the protrusions 40 have a height greater than the base thickness of the jacket wall 24 and are of greater extent both radially and circumferentially than the example of FIG. 6 in that they extend around the circumference of the wall 24, meeting adjacent protrusions at their bases. The sides of the protrusions 40 are curved, thus defining concave spaces or recesses between each protrusion. Groups 28 of membranes 26 are located in each recess, and core boundary membranes 32 extend between the tips of the protrusions 40 to bound and define the hollow core 36. The hollow core includes three protrusions and three groups 28, resulting in a triangular shape. Each group 28, in this example, includes 12 membranes, which can be accommodated by the taller protrusions 40, allowing the membranes 26 to fill the recesses between the protrusions 40, resulting in no sink cavities. However, membrane 26 may be omitted at the rear of each stack to provide a sink cavity as in FIG. 6, if desired, or membrane 26 may be omitted within each stack to provide a sink cavity in the form of wider membrane spacing, as described above.

[0041] Figure 8 shows a cross-sectional view of yet another example optical fiber. Similar to the example of Figure 6, this fiber 10 has three lobes 40 and three groups 28 of membranes 26 connected to the wall 24 of the jacket 22 at the sides of the lobes 40 so as to define a triangle with respect to the hollow core 36, which represents the core radius R. Each group 28, in this example, includes only two membranes: one core boundary membrane 32 and one cladding membrane 34. Thus, a significant space remains between each group 28 and the jacket 22, which provides a higher-order mode sink cavity 42 of radial depth z behind the cladding membrane 34, as described above in Figure 6.

[0042] Additionally, the fiber 10 of FIG. 8 includes a secondary, flat glass membrane 44 associated with the sink cavity 42. The secondary membrane 44 is located behind the sink cavity 42, between the sink cavity 42 and the inner surface of the jacket wall 24. Each cavity 42 has a secondary membrane 44 associated with it, which forms the outermost boundary (in the radial direction) of the sink cavity 42, instead of the jacket wall 24 as in the example of FIG. 6. Generally, at least two secondary membranes can be provided in each sink cavity 42, spaced apart by gaps in the manner previously described for the glass membrane groups 28. The secondary membranes 44 may, for example, be positioned substantially parallel to the glass membranes in the groups 28, but spaced apart from the outermost cladding membrane 34 by the sink cavity, thereby providing a sink cavity in the form of a wider spacing between parallel membranes as described above. However, FIG. 8 illustrates an alternative arrangement in which the secondary membranes 44 associated with each cavity 42 are themselves located in the groups 46 of secondary membranes 44. The example of Figure 8 shows two groups 46 of secondary films 44, although more groups may be included if desired. Within each group 46, the secondary films are spaced apart from one another. In Figure 8, each group 46 includes four secondary films 44, but fewer or more secondary films may be included, perhaps depending on the available space within the cavity, a desire to limit the complexity of fiber manufacturing, or a desire to tailor the optical properties of the fiber 10. The grouping and positioning of the groups 46 of secondary films 44 is such that the innermost (radially) of the secondary films 44 in each group 46 forms one side of the cross-sectional shape of the cavity 42. The cavities 42 are bounded only by the glass films, so that the shape of the cavities 42 has all sides defined by the outermost cladding films 34 in the associated group 28 of films and the innermost secondary films in each group 46. (Subject to any misalignment that occurs during fiber manufacturing, the jacket wall 24 and / or protrusions 40 may form only a small portion of the cavity boundary; this may occur similarly for the shape of the core 36.) In this example, each cavity 42 has two groups 46 of secondary films 44 associated with it, so that the cavities 42 have three sides and a triangular cross-sectional shape.8, each secondary membrane is secured to the jacket wall 24 by having one edge of the secondary membrane secured to a protrusion 40 and the other edge of the secondary membrane secured to the inner surface of the intermediate portion of the jacket wall 24 between adjacent protrusions 40. For each cavity 42, one group 46 of secondary membranes has an edge secured to one of the two protrusions 40 that establish the cavity 42, and the other group 46 of secondary membranes 44 has an edge secured to the other of the two protrusions 40. However, the secondary membranes 44 may also be implemented in a configuration without protrusions.

[0043] The secondary film functions to reduce light leakage from the core. The leakage path includes the protrusion route, which can be reduced by the closely spaced film. Furthermore, flexibility in the positioning of the secondary film allows for tuning of the sink cavity size, thereby providing control over which higher-order core modes are most strongly coupled out of the others.

[0044] It has been described above that superior optical performance can be achieved from the proposed hollow-core antiresonant optical fiber, despite the absence of a negative curvature antiresonant hollow core. Furthermore, performance improvements have also been described that result from increasing the number of separating glass films in each group. This adds an additional layer to the antiresonant structure, increasing the antiresonant effect that enables light guiding, thereby increasing the confinement of the fundamental mode to the hollow core and correspondingly reducing the amount of optical loss experienced by the propagating light. The amount of spacing between the films can also contribute to optical performance.

[0045] Figure 9 shows a graph of the results of computer modeling performed to investigate the effect of the number of films on optical loss. The horizontal axis shows the total number N of glass films included in each group (i.e., associated with each side of the polygonal hollow core) for a range of two films up to a maximum of 16 films. Partial depictions of examples of fibers with two and 16 films are included in the figure, and it can be seen that the modeled fiber has a triangular hollow core, configured with protrusions, and without a higher-order mode sink cavity, as in the example of Figure 7. The vertical axis shows the optical loss (referred to as confinement loss) in dB / km. The graph represents two lines. Line 48 shows the loss for the fundamental optical mode, and it can be seen that adding more films significantly reduces the optical loss, with a 100,000-fold reduction achieved by increasing from two films to 16 films. Line 50 shows the loss for higher-order optical modes, and it can be seen that going from two films to 16 films reduces the loss to a similar level. Recall that the modeled fiber lacks a higher-order mode sink cavity, so there is no suppression of higher-order modes, and the loss levels for higher-order modes are expected to be much larger than for a fiber with a sink cavity. The absence of a sink cavity means that in each instance, a group of films occupies the same space between the core and the wall, and as a result, the spacing between films decreases with the number of films. Therefore, it will be further appreciated that the results shown in Figure 9 demonstrate that decreasing film spacing reduces optical loss.

[0046] The number of films can be selected depending on the required loss level, where one needs to balance the increased complexity of the fiber structure, and therefore fiber manufacturing, due to the greater number of films against the loss tolerance. For applications where low loss is critical, the number of films may be selected to be, for example, eight or more. Eight films will provide a loss of about 1 dB / km, while 14 films can provide a loss of about 0.2 dB / km, comparable to that of conventional telecommunications optical fiber.

[0047] It has also been found that membranes placed closer to the core are more important for loss reduction: if the same narrow membrane spacing is used as in the N=16 example in Figure 9, the same performance can be achieved with just the six innermost membranes closest to the core.

[0048] As described, a key aspect of the proposed hollow-core fiber design is that it can withstand fiber draw using only a single pressure applied across the entire cavity in the preform, without the need for a pressure differential. This simplifies the drawing process and allows for significantly increased fiber lengths to be drawn compared to current hollow-core fiber designs that require a pressure differential. Optical fibers are typically produced by drawing fiber from a heated preform in a fiber drawing tower. Therefore, a suitable preform is needed that can produce the intended fiber structure during the drawing process. One way to achieve this is to use a preform that replicates the intended fiber structure, so that the drawing process simply reduces the cross-sectional dimensions of the preform down to the required dimensions of the finished fiber. Alternatively, preforms of different cross-sectional shapes can be used because, in the absence of a competing pressure differential, the surface tension of the softened glass during drawing acts to flatten the thin sections of glass, thereby producing the desired flat glass film. In this way, curved glass elements can be included in the preform and flattened during drawing. The benefit of this is that preform manufacturing can be based on existing techniques for making hollow-core optical fiber preforms, in which cylindrical glass tubes or capillary tubes are assembled together to form preforms for fiber designs with circular elements, such as those in Figures 1 and 2. The term "stack and draw" is used for these techniques because multiple tubes, or in some cases solid rods, are stacked in the appropriate configuration within a tube for the outer jacket and then drawn into a fiber, or through intermediate supports.

[0049] FIG. 10 shows a cross-sectional view through a first example preform suitable for forming a hollow-core antiresonant optical fiber as disclosed herein. The preform 60 includes multiple concentric glass tubes. The outermost tube 62 has a relatively large wall thickness and forms the jacket of the finished fiber. Multiple tubes 64 with thinner walls and decreasing diameters, inserted inside each other in decreasing size, are positioned within the hollow lumen of the outermost tube 62. These tubes form the flat glass membrane of the finished fiber. These tubes 64 may all have the same wall thickness, or depending on the dynamics of subsequent fiber drawing, it may be necessary to use tubes 64 with different wall thicknesses to provide different membrane thicknesses in the finished fiber, or to form membranes with the same thickness. The thinner tubes 64 are selected to be spaced apart from each other and form the outermost tube by an appropriate amount to create gaps between membranes in the finished fiber, including any larger gaps that would create sink cavities, or to create different spacing between membranes in groups, and to avoid any contact points between individual membranes. The space within the smallest, innermost tube 64a forms the hollow core in the finished fiber. Thus, the innermost tube 64a forms the core boundary glass membrane, and the intermediate tube 64b between the innermost tube 64a and the outermost tube 62 forms the cladding glass membrane. In this example, there are a total of five thin glass tubes 64, so that each membrane group in the finished fiber includes five spaced apart flat glass membranes, and can include more or fewer thin glass tubes as desired.

[0050] To secure the membrane to the jacket in the finished fiber, the preform 60 further includes a plurality of bulk glass spacer elements 66, in this example in the form of solid rods. These bulk glass spacer elements 66 are interposed between all of the tubes 62, 64, spanning the spaces between adjacent tubes, and are grouped to correspond to each corner or vertex of the intended polygonal shape of the hollow core in the finished fiber. In this example, the intended shape is an equilateral triangle, resulting in three groups of spacer elements 66, evenly distributed around the circumference of the preform 60, one group at each core corner location. Each group includes at least one spacer element 66 in each space between adjacent tubes 62, 64 at the core location. In this example, six solid rods are positioned in each space, and the rods are in contact with each other. When the preform is heated and drawn, the spacer elements, along with the portions of the tube between the spacer elements in a group, fuse to each other, creating a protruding glass jacket in the finished fiber. Furthermore, during drawing, if the same pressure is applied to all gaps between the various glass portions in the preform, surface tension will flatten the portions of the thin glass tube between the spacer elements so as to extend in straight lines between adjacent protrusions, thereby creating glass films.

[0051] In another alternative, the preforms may be assembled in stages, from the inner tube outwards.

[0052] FIG. 11 shows a cross-sectional view of a second example preform suitable for forming a hollow-core antiresonant optical fiber as disclosed herein, assembled in stages. In the initial stage, multiple (in this example, three, as shown in solid lines) smaller thin tubes 64a, 64b are arranged concentrically as described above, separated by groups of spacer elements 66 at each corner core position. In the next stage, this subassembly is consolidated by small draws, reducing the diameter slightly (or to an intermediate diameter suitable for struts), and fusing the various sections together. In the next stage, more larger thin tubes 64 are arranged around the consolidated subassembly, again concentrically with additional spacer elements 66a. These additional spacer elements 66a are shown in dotted lines. When these additional tubes complete the preform 60, the outermost tube 62 is the thicker tube that forms the fiber jacket, and the others only add thinner tubes 64b. For the outermost tube 62, the preform is completed, and for the thinner tube 64b, additional consolidation is performed, and then additional tube and spacer elements are added to the thicker tube as needed. This process can provide a more stable method, enhanced by the large number of glass layers required for the preform.

[0053] Preforms may be formed from different stacking designs (e.g., including hollow tubes or other hollow elements that collapse during drawing to form protruding elements). Alternatively, the hollow elements for the protruding elements may be maintained hollow in the finished fiber by applying appropriate pressure during the drawing process, which reduces the amount of material required to form the fiber and reduces the weight of the finished fiber. Other techniques (e.g., forming from a solid glass slab by drilling or laser cutting) may also be used to produce suitable preforms. These processes are simplified because the ease of drawing under a single pressure eliminates the requirement for particularly long, thin preforms, allowing shorter and wider preform dimensions to be used, which are easier to shape by drilling or cutting.

[0054] Examples of dimensions for fibers such as those proposed herein include: A film thickness ("t" in Figure 4) in the range of approximately 350 nm to 600 nm, providing optical guidance for light with a wavelength of 1550 nm, often used in telecommunications applications. Jacket outer diameter in the range of 125 μm to 400 μm, where 125 μm is the industry standard for conventional solid fibers and facilitates integration of hollow-core and solid fiber types, while larger diameters are useful in other situations. An outer diameter in the range of 200 μm to 250 μm is common for known hollow-core fibers and, furthermore, when implemented for the presently described fibers, can facilitate integration and operation with existing hollow-core fiber equipment and devices. · Jacket inner diameter (diameter of the central lumen) ranging from 80 μm to 200 μm. The modeled fiber shown in FIG. 9, for example, has a jacket inner diameter of 150 μm. · Film spacing ("s" in Figure 4) ranging from 1 μm to 5 μm. The modeled fiber shown in Figure 9, for example, has a film spacing of 3 μm for the lowest loss design of 16 films. Although higher modal attenuation can be tuned using smaller and larger values ​​of z, a sink cavity size ("z" in Figure 6, which is the cavity depth along the radial direction of the fiber) of about 0.9 times the core radius ("R" in Figure 8) is useful to reduce the first higher-order mode [3]. · A core radius ("R" in Figure 8) in the range of 20µ to 50µm is suitable for guiding light at 1550nm, which, as mentioned above, is often used in telecommunications.

[0055] However, these values ​​are merely examples and are not limiting, and optical fibers according to the present disclosure may be implemented with other values, which can be selected as needed to tailor the optical properties and characteristics of the optical fiber for a particular application.

[0056] The various embodiments described herein are presented solely to aid in understanding and teaching the claimed features. These embodiments are provided merely as representative examples of embodiments and are not intended to be exhaustive and / or exclusive. The advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein should not be considered limitations on the scope of the invention as defined by the claims or equivalents thereof, and it is understood that other embodiments may be utilized and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the present invention may suitably include, consist of, or consist essentially of any suitable combination of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. Furthermore, this disclosure may include other inventions not currently claimed but which may be claimed in the future.

[0057] References [1] Chen, Yong, et al. “Multi-kilometer long, longitudinally uniform hollow core photonic bandgap fibers for broadband low latency data transmission.” Journal of Lightwave Technology 34.1, 104-113, 2016 [2] US 11203547 [3] Francesco Poletti, “Nested antiresonant nodeless hollow core fiber,” Opt. Express, vol. 22, 23807-23828, 2014 [4] Cruz, A.L.S. et al “3D-printed terahertz Bragg fiber”, In Proceedings of the 2015 40th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), Hong Kong, China, 23-28 August 2015; pp. 1-2. [5] Li, J. et al, “3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications”, Opt. Express 2017, 25, 4126-4144. [6] Hong, B. et al, “Low-Loss Asymptotically Single-Mode THz Bragg Fiber Fabricated by Digital Light Processing Rapid Prototyping” IEEE Trans. Terahertz Sci. Technol. 2018, 8, 90-99. [7] Hayes, John R. et al., “Anti-resonant hexagram hollow core fibers.” Optics Express 23.2 (2015): 1289-1299. [8] CN 108181685 [9] CN 110579836

[10] CN 109031517

[11] CN 101836143 (WO 2009044100)

Claims

1. 1. A hollow-core optical fiber configured for guiding light waves by anti-resonance, comprising: a tubular glass jacket having a central lumen and a wall having a wall thickness; a hollow core defined in the central lumen and having a polygonal cross section; a cladding disposed in the central lumen, the cladding including flat glass films each having two opposing edges extending along the length of the hollow-core optical fiber and a film thickness less than the wall thickness, the flat glass films arranged in a plurality of groups of glass films, each group comprising: a core boundary glass film defining one side of the polygon of the hollow core; at least one cladding glass film disposed between the core boundary glass film and the tubular glass jacket; all of the core boundary glass films and cladding glass films in each group are spaced apart from one another; All sides of the polygon of the hollow core are formed by core boundary glass films, and all of the flat glass films are fixed to the tubular glass jacket only along the two opposite edges, and there is no contact between the flat glass films. Hollow core optical fiber.

2. The hollow-core optical fiber of claim 1 , wherein the polygon of the hollow core is a regular polygon.

3. 3. The hollow core optical fiber of claim 1 or 2, wherein the polygon is a triangle, such that the plurality of groups of glass coatings consists of three groups of glass coatings.

4. A hollow core optical fibre according to any one of claims 1 to 3, wherein each group of glass coatings comprises more than two glass coatings.

5. 5. The hollow core optical fiber of claim 1, wherein each group of glass coatings comprises the same number of glass coatings as said other groups of glass coatings.

6. A hollow core optical fibre according to any one of claims 1 to 5, wherein the glass films in each group of glass films are parallel to each other.

7. 7. The hollow core optical fiber of claim 1, wherein the glass films in each group of glass films are separated from one another by spaces of equal depth, and optionally each said group of glass films has spaces of depth equal to the spaces in the other groups of glass films.

8. 8. The hollow core optical fiber of claim 1, further comprising a cavity between each group of glass films and the tubular glass jacket, each cavity having a depth greater than the depth of the space between which the glass films in said group are spaced.

9. 9. The hollow core optical fiber of claim 8, further comprising a secondary glass film disposed between each cavity and the tubular glass jacket, all secondary glass films being secured to the tubular glass jacket only along the two opposing edges, and optionally each cavity having at least two secondary glass films associated therewith, the at least two secondary glass films being spaced apart from one another.

10. 10. The hollow core optical fiber of claim 9, wherein the secondary glass films associated with each cavity include at least two groups of secondary glass films, each group of secondary glass films arranged so that the associated cavity has a cross-sectional shape with sides defined by an outermost cladding glass film in the group of glass films and an innermost secondary glass film in each group of secondary glass films, and optionally the secondary glass films associated with each cavity consist of two groups of secondary glass films and the associated cavity has a triangular cross-sectional shape.

11. 11. The hollow core optical fiber of claim 1, wherein the wall of the tubular glass jacket has a plurality of bulk glass protrusions protruding into the central lumen, each protrusion being aligned with a vertex of the polygon of the hollow core, and each glass film being secured to the tubular glass jacket by the two opposing edges that are respectively secured to adjacent protrusions.

12. The hollow core optical fiber according to any one of claims 1 to 11, wherein the central lumen of the tubular glass jacket has a circular cross-sectional shape.

13. 10. A preform for manufacturing a hollow-core optical fiber configured for guiding light waves by antiresonance according to claim 1, comprising: a plurality of hollow glass tubes of reduced diameter, each having a wall with a thickness, arranged concentrically inside one another with spaces between them, for providing a flat glass film of the hollow-core optical fiber; an outer hollow glass tube concentrically disposed around and spaced from the plurality of hollow glass tubes to provide a tubular glass jacket for the hollow-core optical fiber, the outer hollow glass tube having a wall thickness greater than the thickness of each of the walls of the hollow glass tubes; three or more groups of glass spacer elements, the groups being circumferentially spaced about the preform corresponding to corners of a polygon of a hollow core of the hollow-core optical fiber, each group including at least one spacer element in each space between the inner hollow glass tube and the outer hollow glass tube; A preform comprising:

14. 14. A method of forming a preform according to claim 13 for producing a hollow core optical fiber configured for anti-resonant guiding of light waves, comprising: a first step including forming a subassembly by arranging a plurality of hollow glass tubes of decreasing diameter concentrically inside one another with spaces between the hollow glass tubes, the hollow glass tubes each having a wall with a thickness, and disposing glass spacer elements in the spaces between the hollow glass tubes, the spacer elements being arranged in at least three groups spaced circumferentially around the hollow glass tubes, the spacer elements in each group including at least one spacer element in each space between the hollow glass tubes; a second step comprising reducing the diameter of the subassembly and withdrawing the subassembly until the spacer elements unite the hollow glass tube; Optionally, if more hollow glass tubes are required in the preform, a third step including placing additional hollow glass tubes and additional glass spacer elements around the subassembly, aligning the additional glass spacer elements with the groups in the subassembly; a fourth step including concentrically arranging any additional hollow glass tubes required in the preform around the subassembly, and concentrically arranging outer hollow glass tubes having walls thicker than the thickness of each of the walls of the hollow glass tubes around the outermost hollow glass tubes with spaces therebetween, and arranging additional glass spacer elements between the hollow glass tubes and the outer hollow glass tubes in each space, the fourth step aligning the additional glass spacer elements with the groups in the subassembly; A method comprising:

15. 1. A method of manufacturing a hollow-core optical fiber configured for guiding optical waves by antiresonance, comprising:

14. Heating and drawing the preform of claim 13 to form an optical fiber, optionally applying equal pressure to all voids in the preform during said drawing. method.

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