Multicore hollow core optical fibre, and preform and method of fabrication therefor

Abstract

An antiresonant hollow core optical fibre 30 has an outer jacket 12. Primary capillaries 14 are each bonded to a surface of a cavity within the outer jacket. At least two hollow cores 18a-b are forme

Description

BACKGROUND OF INVENTION The present invention relates to antiresonant hollow core optical fibres having multiple cores. Optical fibres having more than one waveguiding core for guiding light, commonly referred to as multicore optical fibres or more simply, multicore fibres, are now a commercially available product, following a decade of research development. Conventional single core optical fibres comprise one waveguiding core surrounded by a cladding layer. In contrast, a multicore fibre has two or more cores (i.e. multiple cores) disposed within a single cladding layer, so that the cladding is shared among the different cores. This configuration allows a more efficient use of space, since more optical waves can be propagated simultaneously by fewer fibres, enabling so-called space division multiplexing. The denser waveguiding provision (more cores per volume of cladding, giving denser optical fibre cables) offers higher overall optical signal transmission capacity and improved routing of signals in fibre-dense environments such as optical communications data centres. Also, multicore fibres offer interesting and valuable opportunities for laser beam delivery, since a beam can be divided into multiple channels which are each transmitted through a different core in one multicore fibre and recombined at the fibre output, and also for optical fibre sensing applications, in which different stresses experienced by the different cores can be detected to sense the curvature of a multi core fibre. However, to date, commercial availability of multicore fibres has been limited to solid core optical fibres, in which the core and cladding are formed from solid material such as glass. Attempts to implement multiple cores in a hollow core optical fibre format have not been similarly successful, meaning that the many advantages of hollow core optical fibres are not yet combined with the additional advantages of multiple cores. A hollow core optical fibre is a fibre in which light is guided along a longitudinal hollow void forming the core of the fibre by an optical guidance mechanism enabled by the presence of a structured arrangement of longitudinal voids or capillaries forming a cladding surrounding the core void. Various configurations for the cladding are known, producing different guidance effects. The absence of a solid glass core offers various benefits including low optical propagation loss, low latency and higher optical power handling (enabled by the absence of nonlinear optical effects that arise for light propagating in glass). Accordingly, multicore optical fibres having multiple hollow cores are of interest. SUMMARY OF THE INVENTION Aspects and embodiments are set out in the appended claims. According to a first aspect of certain embodiments described herein, there is provided an antiresonant hollow core optical fibre comprising: an outer jacket; cladding capillaries comprising a plurality of primary capillaries which are each bonded to a surface of a cavity within the outer jacket; and at least two hollow cores, each hollow core formed by a void defined within a ring comprising a maximum of four primary capillaries, each primary capillary of the ring being spaced apart from adjacent primary capillaries around the ring. According to a second aspect of certain embodiments described herein, there is provided a preform for fabricating a multicore antiresonant hollow core optical fibre, the preform comprising: a glass tubular major outer jacket; two or more canes stacked inside the major outer jacket, each cane comprising: a glass minor outer jacket having a tubular void defining a cane cavity; glass cladding primary capillary tubes comprising a maximum of four primary capillary tubes arranged in a ring and bonded to a surface of the cane cavity in spaced apart locations around the perimeter of the minor outer jacket; and a hollow core void defined within the ring of primary capillary tubes; the two or more canes positioned inside the major outer jacket at required relative positions for hollow cores of the multicore antiresonant hollow core optical fibre. According to a third aspect of certain embodiments described herein, there is provided a method of fabricating a multicore antiresonant hollow core optical fibre, the method comprising drawing a preform according to the second aspect into the multicore antiresonant hollow core optical fibre or into a cane for subsequent drawing into the multicore antiresonant hollow core optical fibre while applying a vacuum to the one or more interstitial spaces to cause the one or more interstitial spaces to collapse. According to a fourth aspect of certain embodiment described herein, there is provided a method of fabricating a multicore antiresonant hollow core optical fibre, the method comprising: obtaining, for each hollow core required in the multicore hollow core optical fibre, a cane comprising: a glass minor outer jacket having a tubular void defining a cane cavity; glass cladding primary capillary tubes comprising a maximum of four primary capillary tubes arranged in a ring and bonded to a surface of the cane cavity in spaced apart locations around the perimeter of the minor outer jacket; and a hollow core void defined within the ring of primary capillary tubes; stacking the canes inside a glass tubular major outer jacket at required relative positions for hollow cores of the multicore antiresonant hollow core fibre to create a preform; optionally stacking glass packing elements inside the major outer jacket in interstitial spaces bounded by outer surfaces of the canes and an inner surface of the major jacket; heating the preform to soften the glass; and drawing the preform into the multicore antiresonant hollow core optical fibre while applying a vacuum to interstitial spaces of the preform bounded only by outer surfaces of the canes to cause the interstitial spaces to collapse, such that the major outer jacket, the minor inner jackets and any stacking elements fuse to create an outer jacket of the antiresonant hollow core optical fibre. These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, devices and methods may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: Figures 1, 2 and 3 show schematic transverse cross-sectional views of antiresonant hollow core optical fibres, including cladding features from known fibre designs; Figure 4 shows an image of the end of a known antiresonant hollow core optical fibre having two cores configured for waveguide coupling; Figure 5 shows an image of the end of a four cell double nested nodeless antiresonant hollow core optical fibre having a structure that can be utilised according to the present disclosure; Figure 6 shows a schematic transverse cross-sectional view of a first example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having two uncoupled cores located within a single cavity within the fibre; Figure 7 shows a schematic transverse cross-sectional view of a second example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having three uncoupled cores located within a single cavity within the fibre; Figure 7A shows a schematic transverse cross-sectional view of an example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having three uncoupled cores located within a single cavity within the fibre, and modified compared to the Figure 7 example by a different external shape; Figure 8 shows a schematic transverse cross-sectional view of a third example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having two coupled cores located within a single cavity within the fibre; Figure 9 shows a schematic transverse cross-sectional view of a fourth example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having four cores located within four separate cavities within the fibre; Figure 10 shows a schematic transverse cross-sectional view of an example preform suitable for fabricating the multicore antiresonant hollow core optical fibre of Figure 9; Figure 11 shows a flow chart of steps in an example method of fabricating a multicore antiresonant hollow core optical fibre which may use a preform such as the Figure 10 example; Figure 12 shows a schematic transverse cross-sectional view of another example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having four cores located within two cavities within the fibre; Figure 13 shows a schematic transverse cross-sectional view of another example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having five cores located with a single cavity within the fibre, each core defined by three primary capillaries; and Figure 14 shows a schematic transverse cross-sectional view of a further example multicore antiresonant hollow core optical fibre configured according to the present disclosure, having two cores located with a single cavity within the fibre, each core defined by three primary capillaries. DETAILED DESCRIPTION Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed / described in detail in the interests of brevity. It will thus be appreciated that aspects and features of devices, methods and apparatus discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features. One type of hollow core fibre is the antiresonant hollow core fibre (ARF). Fibres of this type have a relatively simple cladding structure, comprising a typically small number of glass tubes or capillaries arranged in a ring around a central void, and secured or bonded to the inner surface of a tubular outer jacket to maintain the required geometry. The central void within the ring of capillaries forms a hollow core along which one or more optical modes can be guided by an antiresonant waveguiding effect. The cladding arrangement does not offer any high degree of periodicity so guidance cannot operate via photonic bandgap effects such in hollow core photonic bandgap (crystal) fibres. Instead, antiresonance is provided for propagating wavelengths that are not resonant with a wall thickness of the cladding capillaries; in other words, for wavelengths in an antiresonance window which is defined by the cladding capillary wall thickness. The antiresonance acts to inhibit coupling between air-guided optical modes supported by the core and any optical modes which the cladding may support, so light is confined to the core and can propagate along the fibre at low loss by an antiresonant optical guidance effect. Figure 1 shows a transverse cross-sectional view of a known antiresonant hollow core fibre, of a very simple design. The view shows a full transverse cross section through a fibre with a circular cross-section. The fibre 10 has an outer tubular jacket 12. A plurality of tubular or hollow cladding capillaries or cells 14, in this example six capillaries of the same cross-sectional size and shape (circular), are arranged inside the jacket 12 in a ring, so that the longitudinal axes of each cladding capillary 14 and of the jacket 12 are substantially parallel. The cladding capillaries define elongate holes, lumen or cavities which run continuously along the length of the optical fibre. The number of capillaries allows this structure of this example to be labelled as a 6-cell ARF. The cladding capillaries or tubes 14 are each in contact with (bonded to) the inner surface of the jacket 12 at an azimuthal or peripheral location 16, such that the cladding capillaries 14 (and hence also the locations 16) are evenly spaced around the inner circumference of the jacket 12. The cladding capillaries are also spaced apart from each other (there is no contact between neighbouring capillaries). The cladding structure is limited to these cladding capillaries only [1], In some designs of ARF, the cladding tubes 14 may be positioned around the ring so that adjacent tubes are in contact with each other (in other words, not spaced apart as in Figure 1), but spacing to eliminate this contact can improve the fibre’s optical performance. The spacing removes optical nodes that arise at contact points between touching adjacent tubes and which tend to cause undesirable resonances that result in high losses. Accordingly, fibres with spaced-apart cladding capillaries as in Figure 1 may be referred to as “nodeless” antiresonant hollow core fibres. The arrangement of the cladding capillaries 14 in a single ring around the inside of the jacket 12 creates a central space, cavity or void within the fibre 10, also with its longitudinal axis parallel to those of the jacket 12 and capillaries 14, which is the fibre’s hollow core 18, also extending continuously along the fibre’s length. The core 18 is bounded by the inwardly facing parts of the outer surfaces of the cladding capillaries 14. This is the core boundary, and the material (glass or polymer, for example) of the capillary walls that make up this boundary provides the required antiresonance optical guidance effect or mechanism. The core boundary has a shape comprising a series of adjacent inwardly curving surfaces (that is, convex from the point of view of the core). This contrasts with the usual outward curvature of the corecladding interface in a solid-core fibre, and the substantially circular core boundary of a hollow core photonic bandgap optical fibre. Accordingly, antiresonant hollow core fibres can be described as negative curvature fibres. Mathematically, this can be defined as the surface normal vector of the core boundary being oppositely directed to a radial unit vector (vector along a radius of the transverse cross-section of the fibre). The negative curvature (convex shape) of the core boundary also inhibits coupling between the fundamental core mode and any cladding modes. A negative curvature antiresonant hollow core fibre has a core boundary formed by a convex membrane or wall with a thickness which is matched to be in antiresonance with the guided optical wavelength. Some geometrical or dimensional parameters of interest are indicated in Figure 1. The capillaries 14 have a wall thickness t. Each capillary 14 is spaced apart from its neighbour by a gap, separation or spacing d, which is the smallest distance between the outer surfaces of two adjacent capillaries. Typically, the capillaries 14 are evenly spaced around the inner surface of the jacket 12, so each gap has the same value of d. The central core 18 has a radius R, which is the smallest distance from the centre of the fibre 10 (location of the fibre’s longitudinal axis, which is orthogonal to the plane of the page showing Figure 1) to the outer surface of each cladding capillary 14. In this example, the capillaries are all the same size, so this distance is the same for each capillary 14 and is the radius of the largest circle which can be fitted into the cross-section of the core 18. The core 18 has a diameter D equal to 2R, being the diameter of this largest circle. Figure 2 shows a transverse cross-sectional view of a second known antiresonant hollow core fibre [2, 3], The fibre 10 includes all the features of the Figure 1 example, but the cladding has a more complex structure. Each cladding capillary 14 is a primary capillary, still spaced from its neighbour by a gap d, and has a secondary, smaller capillary 20 nested inside it, bonded to the inner surface of the primary cladding capillary 14 at the same azimuthal position 16 around the jacket 12 as the point of bonding between the primary capillary 14 and the jacket 12. These additional smaller capillaries 20 are included with the aim of reducing the optical loss in the fibre 10. Each provides another pair of glass / air interfaces along the radial direction which can act like an additional corrugation or period of refractive index change in a Bragg grating, which reduces light leakage and improves optical confinement in the core, thereby reducing loss of optical power [4], ARF designs of this type, with secondary capillaries nested within primary capillaries, may be referred to as “nested antiresonant nodeless fibres”, or NANFs. The six primary capillaries of this example allow this structure to be labelled as a 6-cell NANF. Figure 3 shows a transverse cross-sectional view of third known antiresonant hollow core optical fibre [2, 3, 5]. The fibre 10 includes all the features of the Figure 2 example, but with a still more complex cladding structure. Each of the secondary cladding capillaries 20 has a still smaller, tertiary, cladding capillary 22 inside it, bonded to the inner surface of the secondary cladding capillary 20 at the same azimuthal position around the jacket 12 as the point of bonding 16 between the primary capillary 14 and the jacket 12. These additional tertiary capillaries provide more pairs of glass / air interfaces, and hence act to further reduce optical loss. ARF designs of this type, having two additional capillaries inside one another and within each primary capillary to provide two layers of nesting, or double nesting, may be referred to as “double nested antiresonant nodeless fibres”, or DNANFs. Note also that in this example, the fibre 10 has only five primary capillaries, so can be labelled as a 5-cell DNANF. An ARF of this 5-cell DNANF design has the current known lowest optical loss for a hollow core fibre, namely 0.174 dB / km. ARFs with other numbers of primary capillaries forming the cladding boundary around the hollow core are also known. A feature common across the various known designs of hollow core fibre is the requirement for a large core diameter surrounded by a large cladding thickness, compared to solid core fibres. A solid core optical fibre configured to guide light at the standard optical telecommunications wavelength of 1550 nm is able to tightly confine the propagating light around a high refractive core using a relatively confined cross-sectional area, such that a cross-sectional circular region of about 30 to 40 pm diameter is sufficient for light guiding if a trench-assisted core is used. Accordingly, it is possible to accommodate multiple cores (such as two, four or even seven cores) within what is the optical telecommunications industry standard fibre outer glass diameter of 125 pm. In contrast, hollow core fibre designs require about a region of about 80 to 100 pm diameter to accommodate just one core, since a large core diameter is required for low loss operation. It is therefore clearly impossible to replicate the same multicore density of hollow cores within the same or similar outer fibre diameter as is achievable for solid core fibres. Hence, to date, the only way to incorporate multiple hollow cores that can function as uncoupled waveguides (so that generally undesirable coupling of propagating waves between different cores is avoided) would be to use a very large diameter of cladding, such as in excess of 250 to 300 pm. In addition to poor compatibility and inefficient coupling with conventional 125 pm diameter fibres as typically used in data centres, such larger glass outer diameters are problematic as regards fibre reliability. Large diameters produce bigger stresses when fibres are bent or coiled, which accelerate the growth of microcracks (stress corrosion) and eventually lead to fibre failure. Hence, the currently accepted and successful configurations of hollow core optical fibres, requiring large core and cladding diameters, present a significant challenge to the implementation of multicore hollow core optical fibres. A known proposal for a multicore hollow core optical fibre uses a photonic bandgap structure for the cladding, in which optical guidance operates via photonic bandgap effects provided by a cladding with a high degree of periodicity arising from a large plurality of very narrow capillaries packed into a hexagonal array [6], Seven hollow cores, each surrounded by its own separate cladding, are arranged within a solid jacket that separately surrounds each cladding. In order to achieve a relatively small outer diameter for the fibre of 151 pm (within which the individual core are spaced apart by about 30 pm), operation is targeted at a guided wavelength of 800 nm, making the fibre unsuitable for many applications, and in particular telecommunications where 1550 nm operation is commonly used. Many other existing proposals for multicore hollow core fibres are theoretical only, with published fibre operational data limited to simulations, and suggested fibre structures for which the rheology used for hollow core fibre fabrication would be very difficult or impossible to achieve. Alternative existing proposals have circumvented the difficulty of achieving uncoupled operation by exploring possibilities of dual or multiple hollow fibres expressly designed as optical couplers, in which propagating optical power is intended to couple between the cores. One example is a dual core design based on a Kagome hollow core fibre structure with inhibited coupling cladding [7], Figure 4 shows an image of a cross-section through another example of a dual core coupler design, this time using an antiresonant hollow core fibre structure as discussed with regard to Figure 1 [8], The two cores are each defined within an associated ring of six capillaries, the capillaries being located within a shared void within an outer jacket formed from solid glass rods stacked inside a glass tube. Each ring comprises five capillaries bonded to the inner surface of the outer jacket. The sixth capillary for each ring is provided by a single further capillary shared between the two rings; this capillary divides the two cores from one another by being located between the cores. To achieve this it is necessary to bond the single capillary to a capillary from each of the two rings. This necessarily creates a node (adjacent capillaries in contact) in each cladding, so that the low loss achievable from nodeless designs of ARFs is unattainable. Also, a relatively large spacing between capillaries is created on the opposite, noncontacting side of the single capillary. This allows optical coupling between the two cores, but is clearly undesirable if uncoupled operation for a multicore fibre is desired. Further, in addition to the nodes the capillaries are otherwise unevenly spaced around their respective cores, which is known to contribute to poor optical performance in antiresonant hollow core fibres. In order to address these difficulties and to provide an effective design for a multicore hollow core optical fibre, it is proposed herein to use antiresonant wave guidance implemented via cladding designs that comprise only four primary cladding capillaries around each hollow core. Conventionally, antiresonant hollow core fibres (ARFs) configured with a cladding comprising a ring of four capillaries, rather than the more usual five, six or seven cladding capillaries, have been dismissed as unviable. This is because it is known that such fibres suffer from a large bend loss, making them impractical and inefficient for many applications, including use in telecommunications data centres where fibre bending and curvature at small bend radii is common to accommodate the large number of optical fibre cables. As noted above, ARFs have a large core diameter, which is needed to achieve low optical propagation loss (attenuation). However, a large core diameter causes multimode operation, in which the core can support multiple optical modes at low loss, which is undesirable for many applications, including telecommunications. Effectively single moded operation can be achieved by introducing gaps or spaces between adjacent primary cladding capillaries, in a nodeless configuration as shown in Figures 1, 2, and 3. The gaps remove unwanted resonances at contact point (nodes) between adjacent capillaries and introduce loss for higher order modes, which couple out of the core. However, the gaps contribute more generally to a leakage of optical power (loss) and therefore must not be made too large, while remaining non-zero to avoid nodes. Also as noted above, loss can be reduced by nesting, being the inclusion of additional smaller capillaries nested within the primary capillaries, with all capillaries within a nested group bonded at the same azimuthal position on the circumference of the outer jacket. The nested capillaries provide additional reflective elements along the radial direction that counter optical loss. Nested groups of two and three capillaries as shown in Figure 2 (nested design) and Figure 3 (double nested design) are commonly used in ARFs. It may be supposed from this that additional levels of nesting of the cladding capillaries would be desirable, since the mechanism for countering optical loss would be enhanced by additional glass walls along the radial direction. However, geometry prohibits this; larger primary capillaries are needed to accommodate additional levels of nesting, which in turn makes the required gaps between the primary capillaries too small. This difficulty has recently been overcome [9], The use of only three or four primary capillaries instead of the usual five or more provides more space within the outer jacket so that the primary capillary diameter can be enlarged to accommodate additional nested capillaries, while maintaining an adequate gap size between the adjacent primary capillaries. Such an arrangement appears to return us to the problem noted above of high bend loss associated with four primary capillaries, which is also unpredictable and varies significantly with bend direction. However, it has been found possible to address this by reducing the core diameter in an ARF with only three or four primary capillaries or cells, for example from a typical core diameter of about 30 pm for a five cell ARF to about 15 pm for a four cell ARF. The smaller core size reduces the mode field diameter of the fundamental optical mode propagating in the fibre core. In turn this reduces the centrifugal force that acts to push the fundamental mode towards the outside of a bend, which otherwise causes high bend loss in a four cell ARF owing to the comparatively large interstitial spaces between the inner surface of the outer jacket and the outer surfaces of the capillaries which can support parasitic modes. For a same level of nesting, a smaller core size in a four cell ARF removes unpredictable peaks and resonances in the bend loss. The ability to then add further nesting levels, such as in a triple NANF structure having three levels of capillary nesting (three smaller capillaries of decreasing width nested within each primary capillary), enabled by a four cell or a three cell design, can then be used to reduce the overall loss and compensate the bend loss. The result is a small core hollow core ARF that has loss characteristics comparable to large core hollow core five cell ARF at large bend radii, and actually with lower end loss at tight bend radii. Figure 5 shows a scanning electron microscope image end view of an example fabricated hollow core ARF having four primary capillaries or cells, showing the cross-sectional structure of the fibre. As can be seen, the primary capillaries are equally spaced apart in a ring around the void defining the central hollow core. Each primary capillary is bonded to the inner surface of an outer jacket, as described above in more detail for the five cell and six cell ARFs in Figures 1-3. In this example, a secondary and a tertiary capillary are nested within each primary capillary, each bonded to the inner surface of the next larger capillary at the same azimuthal location around the outer jacket as the corresponding primary capillary. As noted above, this level of nesting, with two smaller capillaries inside each primary capillary, can be referred to as double nesting, so that this fibre, with gaps between adjacent primary capillaries to eliminate nodes, is a double nested four cell ARF, or four cell DNANF. Higher levels of nesting, such as triple nesting or quadruple nesting (not shown), are enabled by the use of only four primary capillaries. The outer diameter of the fibre (being the outside diameter of the outer jacket) is 125 pm, giving compatibility with standard solid core fibres, and the diameter of the cladding structure (corresponding to the inner diameter of the outer jacket) is of the order of only 50 to 55 pm. This fibre has a propagation loss of less than 2 dB / km, so is compatible with short distance data transmission applications. Hence, a hollow core ARF with four cells or primary capillaries is realisable. Optical loss, previously perceived to be a problem for a four cell configurations, can be improved and / or reduced by moving to a smaller core size, and the use of four cells opens up the option to use higher levels of cladding capillary nesting to enable a further reduction in loss. In particular, the enablement of smaller core diameters, which are unrealisable with larger numbers of primary capillaries due to space constraints, makes a four or three primary capillary configuration of ARF relevant for use in practical designs of multicore hollow core optical fibres. The problem of the large core diameter of five and higher cell ARFs that necessarily renders any multicore format fibre bulky, incompatible with standard fibre sizes and prone to stress damage, is overcome. It is therefore proposed herein that a multicore hollow core fibre be implemented as a antiresonant hollow core optical fibre, in which each core is defined by a ring of four primary capillaries (which may or may not be nested). Such a fibre may comprise, as with the known example ARFs described above, an outer jacket, and cladding capillaries which comprise a plurality of primary capillaries each bonded or secured to a surface of a cavity within the outer jacket. For a multicore fibre, however, it is proposed that the fibre comprises at least two hollow cores. Each hollow core is formed by a void defined within and surrounded by a ring comprising a maximum of four primary capillaries, with each primary capillary of a ring being spaced apart from adjacent primary capillaries around the ring, to give a nodeless configuration. Hence, each hollow core and its associated primary capillaries may have or approximately have a four-fold symmetry. The primary capillaries which define one hollow core can be arranged effectively at the four corners of a square. Alternative configurations for implementing the hollow cores within the outer jacket are possible. Some configurations place all the capillaries and all the cores within a single cavity inside the outer jacket. In other configurations, each core with four associated primary capillaries is located within a separate cavity inside the outer jacket. Figure 6 shows a simplified schematic transverse cross-sectional view of a first example multicore hollow core ARF. This example has two cores, and locates both cores and all primary capillaries in a single cavity. The fibre 30 comprises an outer jacket 12, with a circular outer cross section. A cavity 34 is defined centrally within the outer jacket, and extends along the length of the optical fibre. The cavity 34 has an elongate cross-sectional shape (in this example, a rectangular shape with substantially parallel long sides). Six primary capillaries 14 are disposed within the cavity 34, each primary capillary 14 being bonded to the surface or wall of the cavity 34. The fibre 30 is a double NANF fibre, in that each primary capillary 14 has a smaller secondary capillary 20 within it, and a still smaller tertiary capillary 22 within the secondary capillary 20. Each smaller capillary 20,22 is bonded to the inner surface of the next largest capillary 14, 20 in line with the location 16 at which the primary capillary 14 is bonded to the cavity surface. This structure provides a double nested configuration. This is purely for the purposes of example, however, and single, triple, quadruple and higher levels of nesting might be used instead, or indeed, no nesting at all so that the primary capillaries are the only capillaries. Since both cores are located in the same cavity within the outer jacket, both cores are defined by primary capillaries which are located within that same cavity. Two cores are achieved using six primary capillaries by arranging the primary capillaries 14 in two rows 32a, 32b. A first row 32a comprises three primary capillaries 14 bonded to the surface of the cavity 34 along one of the long sides of the cavity 34, and a second row 32b comprises three primary capillaries 14 bonded to the surface of the cavity 34 along the other long side of the cavity 34. Hence the primary capillaries 14 in the first row 32a face the primary capillaries 14 in the second row 32b. The primary capillaries 14 are equally spaced within each row 32a, 32b, and the two rows 32a, 32b have the same spacing. Hence, along a row the primary capillaries 14 are spaced apart by equally sized gaps d. Also, in this example, because the cavity 34 has a rectangular shape with parallel long sides, opposite capillaries 14 facing each other across the width of the cavity 34 are all spaced apart from one another by the same distance, to give a gap size, in this example, which is again d. Hence, all the primary capillaries 14 are spaced apart from their adjacent capillaries both within a row and between the rows by the same gap size d. The primary capillaries 14 at the ends of each row are bonded into the corners of the rectangular cavity 34. This minimises the interstitial spaces between the outwardly facing parts of the primary capillary and the surface of the cavity 14. Each hollow core is defined by two adjacent primary capillaries 14 from the first row 32a, and the oppositely facing two adjacent primary capillaries 14 from the second row 32b. This allows two hollow cores 18a, 18b to defined by the six primary capillaries 14. A first hollow core 18a is formed by the void defined within a ring of four primary capillaries 14 being the capillary at a first end of the first row 32a, the facing capillary at the first end of the second row 32b, the middle capillary of the first row 32a and the middle capillary of the second row 32b (the two middle capillaries facing one another). A second hollow core 18b is formed by the void defined within a ring of four primary capillaries 14 being the capillary a second end of the first row 32a, the facing capillary at the second end of the second row 32b, the middle capillary of the first row 32a and the middle capillary of the second row 32b. Hence, the middle capillaries define and are shared by both hollow cores 18a, 18b. Each hollow core occupies the space between four capillaries which are adjacent around a ring, which because there are four capillaries corresponds to adjacent capillaries that occupy the four corners of a square. This is a very simple way to achieve two hollow cores within the same ARF. The capillaries are used efficiently since they are shared between cores. The use of a four cell cladding structure is particularly beneficial here since a nodeless and regularly spaced ring of non-contacting primary capillaries can be achieved by bonding all capillaries in the ring to the surface of simply shaped cavity; there are no capillaries that are required to be interposed between adjacent hollow cores as would arise with higher cell number geometries. Also, compact multicore geometries are made possible by the four cell concept, owing to ability to share primary capillaries between adjacent cores. It is envisaged that the outer diameter of the outer jacket need not be increased above a single hollow core arrangement, effectively doubling the density of data that can be transmitted along the fibre. An additional benefit is that the configuration can be very readily extended to provide more than two hollow cores, allowing further increases in data density. Figure 7 shows a simplified schematic transverse cross-sectional view of a second example multicore hollow core ARF. The fibre 30 is very similar to that example of Figure 6, and differs in that the rectangular cross-section of the cavity 34 is extended to be more elongate (the parallel long sides are longer). This allows the cavity 34 to accommodate more primary capillaries 14 (which are double nested in this example also). In this example, a total of eight primary capillaries 14 are located in the cavity 34, arranged along the long sides of the cavity 34 in two rows of four capillaries 14 each, the capillaries 14 in each row facing one another across the cavity 14, and being equally spaced apart by same-sized gaps d between adjacent capillaries 14 (both within and between the two rows 32a, 32b) as before. The inclusion of an extra pair of capillaries 14, one in each row 32a, 32b, creates a third hollow core 18c, since a third ring of four adjacent primary capillaries 14 is defined. A central hollow core 18c shares two of its capillaries 14 with a first adjacent hollow core 18a on one side, which is otherwise defined by the two capillaries at the first end of the first and second rows 32a, 32b, and shares its other two capillaries 14 with a second adjacent hollow core 18b on the other side, which is otherwise defined by the two capillaries at the second end of the first and second rows 32a, 32b. It will be appreciated that this concept can be readily extended by increasing the length of the elongate cavity 34 in the outer jacket 12 so that a larger number of primary capillaries can be provided within each row. We can consider the generic case in which the primary capillaries comprise a total of 2n primary capillaries, which are arranged in two rows on opposite sides of the cavity, with each row comprising n primary capillaries. Within each row, the primary capillaries are spaced apart from one another, and also spaced apart between the rows, such that they face the primary capillaries in the other row. Hence, an extensive row of hollow cores might be provided in a single hollow core fibre. In Figures 6 and 7, the outer surface of the outer jacket has a circular cross-sectional shape. This is not essential, however. Optionally, the outer jacket might instead have an elongated cross section for its outer surface, i.e. the outer surface mirrors or corresponds more closely with the cross-sectional shape of the cavity within. If a row of many hollow cores is present, an elongate or flat outer jacket shape can provide a ribbon format optical fibre. Figure 7A shows a simplified schematic transverse cross-sectional view of another example multicore hollow core ARF, modified compared to the Figure 7 example by having a non-circular outer jacket. The cladding comprises two rows 32a, 32b of three primary capillaries 14, defining three hollow cores 18a, 18b, 18c within a rectangular cavity, as in the Figure 7 example. The outer jacket 12 has a rectangular external cross-sectional shape, that matches the rectangular shape of the cavity 34. It will be appreciated that other cross-sectional outer shapes for the outer jacket can be used as desired or appropriate, which may or may not match the shape of the cavity. If the spacing within each row is kept constant or substantially or approximately constant, it is possible to share capillaries between all adjacent pairs of hollow cores, as in the Figures 6 and 7 examples. This maximises the number of hollow cores that can be provided for a given amount of primary capillaries. As noted above, gaps between adjacent capillaries are provided to achieve a nodeless configuration to remove higher order modes, but should not be too large, otherwise loss becomes too high and waveguiding is not possible. Hence, if a same and appropriate gap size is used all along the rows, any pair of adjacent primary capillaries in a row can form a ring of four capillaries with the facing pair of adjacent primary capillaries in the opposite row, so that any capillary not at the end of a row can be part of two different rings when paired with the different adjacent capillaries on its two sides. In the above generic example of 2n capillaries in two rows of n capillaries each, this arrangement allows that the 2n capillaries will define a total of n-1 hollow cores, each hollow core defined by two adjacent primary capillaries from each of the two rows. In the Figure 6 example, n is three, such that six primary capillaries create two hollow cores. In the Figure 7 example, n is four, such that eight primary capillaries create three hollow cores. This configuration can be modified to provide fewer hollow cores if desired. If the gaps between adjacent primary capillaries in a ring are too large, the optical loss will be too high and waveguiding will not be efficiently supported along the void within the ring which would otherwise form the hollow core. If it is desired that hollow cores be significantly separated across the elongate cavity, separate groups of four primary capillaries can be provided for each core, with a gap size in excess or greatly in excess of d between adjacent primary capillaries in a row which belong to separate groups. Capillaries are not shared between adjacent hollow cores. In the example of Figure 7, the central gaps in the first row 32a and the second row 32b might be significantly increased, dividing the eight primary capillaries 14 into two separate groups of four primary capillaries 14, thereby providing two hollow cores 18a, 18b rather than three. The central hollow core 18c would be lost. More generically in this configuration, for even values of n, 2n primary capillaries arranged in two rows of n primary capillaries will define n / 2 hollow cores. The capillaries are arranged such that the gap size between facing capillaries in the two rows is d. Within a row, pairs of adjacent capillaries are spaced apart by the same gap d, while the pairs themselves are spaced apart from adjacent pairs by a larger gap size, bigger than d. In such a configuration, each group of four primary capillaries and any nested smaller capillaries might have different capillary diameters and capillary wall thicknesses from those of other groups so that different wavelengths of light can be guided by the different hollow cores. As noted above, the size of the gaps d between primary capillaries around a ring is chosen to minimise loss from coupling through the gaps while avoiding resonant nodes produced by contacting capillaries. The size is chosen with reference to the desired wavelength or wavelength range of light to be guided along the hollow core, as is understood. Hence, an arrangement as described above, in which the two facing rows of capillaries have the same or similar gap size between the rows as the gap size within the rows, provides equal values of d all around any ring of four adjacent primary capillaries. Propagating light is thereby well confined within each of the hollow cores, and there is no leakage of light between adjacent hollow cores. This provides uncoupled waveguides so that different light beams can be propagated independently along adjacent hollow cores without cross-talk or interference. This is beneficial for some applications, in particular telecommunications where it is important that optical signals are isolated to avoid errors in the transmitted data. The example fibres of Figures 6 and 7 therefore support uncoupled propagation of light along the hollow cores. However, in other situations, coupling of light between two adjacent waveguides is useful, and the fibre configurations of Figures 6 and 7 can be readily adapted to provide an optical fibre coupling device. Coupling can be achieved by increasing the size of the gap between two facing primary capillaries (one capillary from each row) which are shared between the primary capillary rings defining two adjacent hollow cores. The larger gap size reduces confinement of propagating light within the two cores, and allows outward coupling of light from each core to the adjacent core so that light energy can be transferred from one core to the other. The level of coupling can be controlled by changing the size of the gap. Figure 8 shows a simplified schematic transverse cross-sectional view of an example multicore hollow core ARF configured as an optical coupler, in which coupling of light can occur between two adjacent hollow cores. The fibre 30 is similar to that of the Figure 8 example, comprising an outer jacket 12 of circular outer cross section, a single cavity 34 within the outer jacket 12, and six primary capillaries 14 (again with double nesting of smaller capillaries 20, 22 within the primary capillaries 14) bonded to the surface of the cavity 34 as a first row 32a of three primary capillaries 14 facing a second row 32a of three primary capillaries 14 across the cavity 13. In this example, however, the cavity 34, while again being of elongate cross-section to accommodate two facing rows of primary capillaries, has an oval or elliptical cross-sectional shape instead of being rectangular. This means that the two long sides are no longer parallel, and instead have a greater separation at their centres than at their ends. Hence, the two facing rows of capillaries 14 are curved, and the gap size between the facing capillaries is not equal along the rows. The curvature allows the gap size between facing the capillaries at the ends of the rows 32a, 32b to be maintained at d to match the gap size within the rows, while the gap size D between the two central capillaries which are shared between the two hollow cores 18a, 18b is increased so that D >d. This larger gap size prevents good optical confinement within either of the hollow cores 18a, 18b, so that propagating light can couple between the hollow cores 18a, 18b, and the optical fibre 30 operates as an optical coupler. The outer jacket 12 in this example has a circular cross-sectional outer shape for the outer jacket 12, but a different shape may be used instead, such an ellipse to mirror the shape of the cavity 34. In summary, the boundary between enabling uncoupled or coupled optical transmission along two adjacent hollow cores can be dependent on the size of the gap between two facing capillaries shared between the hollow cores. For a gap size d between the other adjacent capillaries in one or both rings of primary capillaries, a gap size greater than d between the facing shared capillaries will allow coupled operation. The gap size between the facing shared capillaries should be set to be d or less than d to support uncoupled operation. It will be appreciated that with an appropriately shaped cavity, this concept can be extended to larger numbers of capillaries to provide a row of coupled waveguides. Alternatively, the gap size between pairs of facing capillaries might be varied along the rows of primary capillaries to enable coupling between some adjacent hollow core and prevent or inhibit coupling between other adjacent cores, or to provide different levels of coupling between different adjacent hollow cores. Accordingly, the cross-sectional shape of the cavity is not limited to the rectangular and elliptical shapes illustrated, but can be chosen as appropriate to enable desired relative positions of primary capillaries for providing particular gap sizes or spacings between adjacent capillaries. This is enabled by all primary capillaries being secured to the wall or surface of the cavity, so that the cavity shape defines the relative positions of the capillaries. Fabrication of the above examples and other geometries using a single cavity to accommodate all the capillaries and multiple cores can be achieved using appropriate modifications of existing hollow core fabrication techniques. Such fibres are manufactured by securing hollow glass tubes for the capillaries at appropriate positions within a cavity or void inside a larger hollow glass tube for the outer jacket to create a preform. The preform is drawn down into the desired dimensions for the finished optical fibre in a fibre drawing tower by heating the glass preform to soften the glass and drawing the soften glass down to smaller cross-sectional dimensions while applying pressures to the various voids within the tubes to avoid collapse and achieve desired glass wall thicknesses and relative cross-sectional areas for the capillaries and the hollow core. The softened glass of the capillary tubes fuses with the softened glass at the cavity surface in the outer jacket tube to bond the capillaries into place in the finished fibre. Accordingly, use of a hollow tube for the outer jacket having a central cavity of the appropriate cross-sectional shape, securing the hollow tubes for the capillaries at the appropriate positions, and applying appropriate pressurisation during the draw can achieve the required cross-sectional structure for the proposed multicore hollow core ARFs. It is also possible to draw the preform down into a cane of intermediate dimensions, which is in turn drawn down into the finished fibre, as is known. Figure 9 shows a transverse simulated end view of another example multicore hollow core ARF 30, having a configuration in which each of the hollow cores with their four associated primary capillaries is located within a separate cavity inside the outer jacket. In effect, the four cell single hollow core ARF structure shown in Figure 5 is replicated multiple time, in this example four time, within a single outer jacket. The outer jacket 12 has four separate cavities 34 within it, in this example arranged in a 2x2 square array. Each cavity has a roughly circular cross-section, and has four primary capillaries 14 (again each nested with secondary and tertiary capillaries to give a double nested cladding structure) bonded to the surface of the cavity in a ring. The four primary capillaries within each cavity are equally spaced apart around the ring by gaps to give a nodeless cladding structure. The central void within each ring of capillaries defines a hollow core, as before. Owing to the use of a separate cavity for each hollow core and associated group of four capillaries, there is no optical coupling between the individual hollow cores. The illustrated example shows all the cavities, primary capillaries and hollow cores as having the same dimensions, so that each hollow core will guide the same wavelength or wavelengths of light. In other examples, the dimensions may differ between cavities and capillary groups so that different hollow cores are configured to guiding light of different wavelengths. The example of Figure 9 can be extended to provide more than four cores, by providing additional cavities housing additional groups of four capillaries within the outer jacket. A square array can be extended in one or other dimensions to provide six, eight, nine or more hollow cored, for example. Triangular or hexagonal arrays may also be implemented to provide closer packing of the hollow cores and an increased data density for the fibre. It is further proposed that fabrication of multicore hollow core ARFs in which each hollow core is defined within a separate cavity can be achieved by adapting and extending existing ARF fabrication methods. As described above, ARFs are commonly fabricated by securing hollow glass tubes for the capillaries at appropriate positions within a cavity or void inside a larger hollow glass tube for the outer jacket to create a preform, which is drawn down into the desired dimensions for the finished optical fibre while applying different pressures to the various voids. Alternatively a cane of intermediate dimensions can be fabricated for later drawing into the finished fibre. It is proposed to utilise a cane stage to make a preform for the multicore ARFs described herein. Figure 10 shows a simplified schematic cross-sectional view of an example preform for fabricating multicore ARFs having separate cavities for separate hollow cores. This example preform is configured for making a square array four core ARF of the structure shown in Figure 9, but it will be understood that the approach can readily modified for other numbers of hollow cores and other array configurations. In order to simplify fabrication of a fibre comprising many elements, to avoid difficulties in securing a large number of separate elements in the correct relative positions during the draw, the preform is assembled partly from canes. In particular, a cane is used to provide each cavity with its associated group of four primary capillaries (and any nested capillaries). Accordingly, the preform 40 comprises four canes 42. Each cane comprises a tubular minor outer jacket 12a, the tubular void inside the minor outer jacket defining a cane cavity 34a that will become a cavity within the outer jacket of the finished fibre. Four cladding primary capillary tubes 14a that will become primary capillaries in the finished fibre are bonded to the inner surface of the minor outer jacket 12a in spaced apart locations around the perimeter of the minor outer jacket 12a, to define the required capillary ring in the finished fibre. In this example, secondary capillary tubes 20a are nested inside the primary capillary tubes 14a to provide a single nested configuration in the finished fibre; these tubes may be omitted if no nesting is desired, or additional tubes may be included to provide double, triple or higher levels of nesting. A hollow core void 18d is present within the ring of primary capillary tubes 12a; this will become one of the multiple hollow cores of the finished fibre. The four canes 42 are stacked inside the central hollow void 36 of a tubular major outer jacket 38, in the desired relative positions for the hollow cores in the finished fibre. In this example the canes 42 are stacked in a 2x2 square array, to produce a fibre structured as the Figure 9 example. In order to maintain the canes 42 in position during subsequent drawing, solid (or hollow) glass packing rods 45 or other packing elements are also stacked inside the hollow void 36, to fill or partly fill the interstitial spaces 43 bounded by the inner surface of the major outer jacket 38 and the outer surfaces of the canes 42. In contrast, in this example, the central interstitial space 44 bounded only by the outer surfaces of the canes 42 is left empty. Packing elements may be omitted elsewhere if interstitial spaces are small or if it is desired that they collapse during drawing, however. In other examples, three primary capillaries per hollow core may be used instead of four, since this arrangement also enables hollow core ARFs with a reduced core diameter which are suitable for adaption to multicore fibres. To fabricate the fibre, the preform is then heated and drawn in a fibre drawing tower to reduce the transverse dimensions and fuse the various elements together in the required relative positions. As noted above, it is common to apply pressurisation to some or all of the voids within a preform for a ARF in order to avoid the collapse of hollow elements, and / or relatively inflate other elements, and to achieve the desired relative sizes and wall thicknesses for the capillaries in the finished fibre. Different pressures may be applied to different voids. Softening and fusion of the various glass elements causes the major outer jacket 38, the minor outer jackets 12a and any packing rods or elements 45 to coalesce into a single glass body that becomes the outer jacket of the finished fibre. In the present example, a vacuum may be applied to the central interstitial space 44 to cause it to collapse. This pulls the canes 42 inwardly to create a denser packing for the hollow cores in the finished fibre. Alternatively, packing rods might be included in the central interstitial space 44 to maintain the hollow cores at a larger separation. The preform 40 can be pulled directly into the finished fibre, or may be pulled into an intermediate cane which is then itself pulled into the finished fibre. Figure 11 shows a flow chart of steps in an example method for fabricating a multicore hollow core ARF such as that shown in Figure 9, having each hollow core defined in a separate cavity within the outer jacket. In a first step S1, a plurality of canes are obtained, either by direct assembly and drawing of preforms, or from an external source. A cane for each hollow core required in the finished fibre is necessary. Each cane comprises a tubular minor outer jacket, four (optionally three) primary capillary tubes (optionally with smaller nested capillary tubes as mentioned above) bonded to the inner surface of the minor outer jacket and spaced apart in a ring around the perimeter of the minor outer jacket, and a central void space defined inside the ring to create a hollow core in the finished fibre. In a second step S2, the canes are stacked inside a tubular major outer jacket, in positions corresponding to the desired transverse pattern of hollow cores in the finished fibre. This creates a preform. In an optional third step S3, glass packing elements such as rods or tubes can be stacked into the preform, to occupy interstitial spaces between the canes and major outer jacket. In a fourth step S4, the preform is drawn down (using pressurisation as necessary to produce the required dimensions and structure as noted above), either directly into the finished fibre or into a cane from which the fibre can later be drawn. During the draw, the major and minor outer jackets and any packing elements fuse to become the outer jacket of the fibre (or that part of a cane that will become the outer jacket). To aid this process, optionally in step S5 a vacuum can be applied to the interstitial spaces between the canes to aid collapse. A further example multicore hollow core ARF is also proposed. This is a hybrid design that combines the configuration in which all primary capillaries and cores are located in a single cavity in the outer jacket and the configuration in which each core and its associated primary capillaries are located in a separate cavity in the outer jacket. Figure 12 shows a simplified schematic transverse cross-sectional view of an example fibre 30 of this type. The outer jacket 12 houses two cavities 34, each of which contains more than four primary capillaries 14, in this case six primary capillaries 14 so that each of the two cavities 34 is essentially like the single cavity in the Figure 6 example, having primary capillaries 20 defining two hollow cores 18. A configuration like this could be used to implement two separate couplers in the same optical fibre, for example, since coupling can be provided between hollow cores in the same cavity but not between hollow cores in different cavities. The configuration of Figure 12 can be extended or modified with more cavities and / or each cavity having more than six primary capillaries. Each cavity may contain the same or different numbers of primary capillaries, and these may be configured to provide hollow cores with the same or different waveguiding properties (such as regards wavelength, coupling and loss for example). More generally, in some examples a multicore ARF can comprise two or more cavities, each cavity containing a plurality of primary capillaries that define two or more hollow cores. Figure 13 shows a simplified schematic transverse cross-sectional view of another example optical fibre, being a modified version of the Figure 7 example. As has been noted, configurations using three primary capillaries per hollow core instead of four also enable hollow core ARFs with a reduced core diameter which are suitable for adaption to multicore fibres. Where the Figure 7 example uses four primary capillaries to define each hollow core, with all primary capillaries located in a single cavity of the outer jacket, Figure 13 shows an example using three primary capillaries to define each hollow core. Again, the primary capillaries are arranged in two rows, bonded respectively to opposite long sides of a cavity of elongate cross-sectional shape, but instead of being arranged to face the capillaries in the opposite (in other words the capillaries of one row being aligned with the capillaries of the other row), in this example, the capillaries of one row are offset from the capillaries of the outer row, in a triangular array. This allows adjacent capillaries to be grouped into threes arranged in ring, such that a hollow core is defined by the void in the centre of each ring of three capillaries. For clarity, each hollow core is shown by an X in the Figure. In the example, a first row of capillaries comprises three capillaries, and the facing second row comprises four capillaries. Five hollow cores are defined. More generically, a triangular array arrangement of two rows of offset capillaries that comprises n primary capillaries defines n-2 cores. Hence, this configuration enables a higher core to primary capillary ration, recalling that the four primary capillary square array configuration of Figures 6 and 7 provides n-1 cores for 2n primary capillaries. Figure 14 shows a simplified schematic transverse cross-sectional view of a modified example of the optical fibre of Figure 14. Hollow cores defined by only three primary capillaries are again provided, but in this example, only four primary capillaries 14 (with single nesting in this example) are included, defining two hollow cores 18a, 18b to give a dual core hollow core fibre. The primary capillaries 14 are again arranged in two rows 32a, 32b, with just two primary capillaries 14 in each row. The cavity 34 has a rhombus or parallelogram cross-sectional shape, so that the primary capillaries 14 can be bonded in the four corners of the cavity 34, but a rectangular shape as in Figure 13 might be used instead. Similarly, an elongated parallelogram could be used in the Figure 13 example, with the two rows of primary capillaries arranged along the long sides of the parallelogram. Compared with a rectangle, a rhombus reduces the size of the interstitial spaces outside the end cores. The empty spaces behind the primary capillaries 14 at the corners of the cavity 34, shown by shading in Figure 14, may alternatively be glass-filled. The depicted configuration has equal gap sizes between the primary capillaries of each ring of three, such that d is does not exceed D, where D is the gap size between the two cores 18a, 18b. Accordingly, the two cores 18a, 18b are not coupled, and can separately guide light. Alternatively, the cavity 34 could be resized to increase the length of the short diagonal, which will increase the relative size of the gap between the two cores 18a, 18b, such that D is greater than d. This enables coupling between the two cores 18a, 18b, as discussed above for the Figure 8 example. Similar rearrangement of the primary capillaries in the Figure 13 example (or examples having a three cell format with other numbers of cores) can be used to provide or inhibit coupling between any two adjacent cores. The outer jacket 12 has a oval cross-sectional outer shape to correspond more closely to the rhombus-shaped cavity 34 than, say, a circular outer shape, but as noted above, the choice of outer shape is not limited and can be selected as convenient. It has been mentioned above that a reason for using the three or four cell formats of ARF to implement the multicore hollow core fibres proposed herein is that hollow cores of smaller diameter can be achieved from three and four cell configurations compared with conventional configurations using five cells or more. This allows multiple cores to be accommodated within a single fibre that retains a compact and conventional outer diameter, which is more robust and more compatible with common commercially produced fibres. Note that in the current context the outer diameter of an optical fibre refers to the outside diameter of the outer cladding layer, so for the ARFs described here, the outer diameter is considered to be the outside diameter of the outer jacket (labelled 12 in the Figures). For comparison, in conventional solid core single mode fibre formed from a solid core embedded in a solid cladding layer of lower refractive index, the outer diameter is the outside diameter of the cladding layer. Outer diameters of 125 pm are common for solid core fibres, and the small core four cell ARF format allows multiple hollow cores to be accommodated in a 125 pm optical fibre. Hence, in some examples, the multicore hollow core fibre has an outer diameter of around 125 pm, although larger and smaller outer diameters are not excluded. Regarding possible sizes of the hollow core, recall that the central void within the ring of primary capillaries forms the hollow core of the fibre. The hollow core has a radius which is the radius of the largest circle that can be accommodated within the ring of primary capillaries. Hence, the radius of the core is the shortest distance from the central longitudinal axis of the core to the outer surface of one or more of the primary capillaries. The diameter C of the hollow core is twice the radius. In accordance with the principles of antiresonant waveguiding in hollow core optical fibres, a hollow core fibre is configured to guide light of a wavelength A which is determined by the wall thickness of the primary capillaries, which is specified to provide antiresonance at the chosen wavelength. The hollow core fibre supports a fundamental guided mode of propagating light at the wavelength A in the hollow core, being a fundamental core mode, which has a mode field diameter. In cases where the hollow core has a small size compared to conventional standard antiresonant hollow core fibres, we can, for example, define the diameter C of the core by reference to the wavelength A of the fundamental core mode such that for a core diameter C in pm and a wavelength A in pm, the ratio C / A has a value of 16 or less. In the present context, a hollow core of this size can be considered to be “small”, and or particular use in implementing multicore hollow core ARFs. In some examples, this ratio might so as C / A is less than or equal to 14, or less than or equal to 12, or less than or equal to 10. We can also consider the core diameter is absolute terms, and consider that for a small core the diameter C of the core can be defined as having a maximum value of 25 pm, or a value of 22 pm or less, ora value of 20 pm or less, in other examples. The core diameter may be made as small as is practically achievable in order to access the technical advantages thereof, so there is no particular lower limit, although practically for some applications, core diameters down to values of 5 pm or 3 pm might be useful. Hence in some examples, the core diameter may be in the range of 3 pm to 25 pm. In other examples, the hollow core diameter C may be defined to be 10 pm or less. These core sizes are smaller than the cores typically used in known hollow core fibres, such as the 5-cell ARF described with regard to Figure 3. Accordingly, in some examples, the multi-core hollow core fibres proposed herein have hollow cores with diameters of the values noted above. However, these values are not limiting, and larger core diameters may be used if the resulting outer diameter of the optical fibre is acceptable for some applications or otherwise desirable. Further regarding the hollow core size, a smaller core size for hollow core ARFs may in some cases usefully be selected to support a mode field diameter for the fundamental guided core mode that matches or is similar to the mode field diameter of solid core single mode telecommunications fibre, which is typically around 10.4 pm. Matching of the mode field diameter minimises coupling losses when the two cores of two portions of optical fibre are connected together. Hence, a small hollow core size can facilitate coupling of multicore hollow core ARFs to standard solid core fibres. More generally, a smaller hollow core may have a mode field diameter for the core mode that is in the range of about 7 pm to 17.5 pm, for the example small core diameters up to about 25 pm. The mode field diameter is typically about 70% of the core diameter. Commonly, telecommunications fibre is configured to guide light at or around 1550 nm, although several wavelength bands are used for optical telecommunications signals, including the E band (1360-1460 nm), the S band (1460-1530 nm), the C band (1530-1565 nm) and the L band (1565-1625 nm). As noted above, the guided wavelength which is supported for propagation in an ARF depends on the wall thickness of the cladding capillaries, so that the walls provide an antiresonance at that wavelength, and the guided light is confined to the core. Hence, an ARF can be configured such that the cladding capillary wall thickness is appropriate for guiding light of a particular wavelength of interest. For telecommunications applications, this may be 1550 nm, or light with wavelengths in the other bands noted above, but this is not a limitation, and it will be appreciated that the multicore hollow cores ARFs proposed herein can be configured for propagation of light of any desired wavelength by suitable selection of the cladding capillary wall thickness. As already mentioned, the size of the gap or spacing between adjacent primary cladding capillaries is also selected with reference to the wavelength, and the desire to provide effectively single moded operation, or multi-moded operation in some cases. The skilled person understands how to configure the various dimensions within an ARF appropriately for a wavelength of interest. The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future. REFERENCES [1] Anton N. Kolyadin, Alexey F. Kosolapov, Andrey D. Pryamikov, Alexander S. Biriukov, Victor G. Plotnichenko, and Evgeny M. Dianov, "Light transmission in negative curvature hollow core fiber in extremely high material loss region", Opt. Express 21, 9514-9519(2013) [2] Francesco Poletti, "Nested antiresonant nodeless hollow core fiber," Opt. Express 22, 23807-23828 (2014) [3] WO 2015 / 185761 A1 [4] David Bird, "Attenuation of model hollow-core, anti-resonant fibres," Opt. Express 25, 23215-23237(2017) [5] Gregory. T. Jasion et al, "0.174 dB / km Hollow Core Double Nested Antiresonant Nodeless Fiber (DNANF)," 2022 Optical Fiber Communications Conference (OFC), paper Th4C.7 [6] B.J. Mangan, A.C. Muir and J.C. Knight, “Photonic bandgap fiber with multiple hollow cores,” Journal of Lightwave Technology, vol. 28, no. 9, pp. 1287-1290, 2010. [7] N. V. Wheeler, T. D. Bradley, J. R. Hayes, G. T. Jasion, Y. Chen, S. R. Sandoghchi, P. Horak, F. Poletti, M. N. Petrovich and D. J. Richardson, "Dual hollow-core antiresonant fibres," Proc. SPIE 9886, Micro-Structured and Specialty Optical Fibres IV, 988617,27 April 2016. [8] X. Huang, J. Zang, and S. Yoo, “Multiple hollow-core anti-resonant fiber as a supermodal fiber interferometer” Sci Rep 9, 9342, 2019. [9] GB 2404063.6

Claims

1. An antiresonant hollow core optical fibre comprising:an outer jacket;cladding capillaries comprising a plurality of primary capillaries which are each bonded to a surface of a cavity within the outer jacket; andat least two hollow cores, each hollow core formed by a void defined within a ring comprising a maximum of four primary capillaries, each primary capillary of the ring being spaced apart from adjacent primary capillaries around the ring.

2. An antiresonant hollow core optical fibre according to claim 1, wherein the at least two hollow cores comprise two or more hollow cores defined by primary capillaries bonded to the surface of one cavity within the outer jacket.

3. An antiresonant hollow core optical fibre according to claim 2, wherein the primary capillaries comprise 2n primary capillaries arranged in two rows along opposite sides of the cavity such that each row comprises n primary capillaries, the primary capillaries in each row being spaced apart from and facing the primary capillaries in the other row.

4. An antiresonant hollow core optical fibre according to claim 3, wherein each hollow core is defined by four primary capillaries comprising two adjacent capillaries from each of the two rows.

5. An antiresonant hollow core optical fibre according to claim 4, wherein the primary capillaries define n-1 hollow cores.

6. An antiresonant hollow core optical fibre according to claim 5, wherein adjacent primary capillaries within each row are spaced apart from one another by a gap of size d and the primary capillaries are spaced apart from the facing primary capillaries in the other row by a gap of size d or smaller, such that optical coupling between adjacent hollow cores is inhibited.

7. An antiresonant hollow core optical fibre according to claim 5, wherein adjacent primary capillaries within each row are spaced apart from one another by a gap of size d and at least one of the primary capillaries in each row is spaced apart from the facing primary capillary in the other row by a gap of size D where D is greater than d, such that opticalcoupling is enabled between the adjacent hollow cores which are partly defined by these facing primary capillaries.

8. An antiresonant hollow core optical fibre according to claim 4, wherein n is an even number and the primary capillaries define n / 2 hollow cores.

9. An antiresonant hollow core optical fibre according to claim 2 wherein the primary capillaries comprise n primary capillaries arranged into two rows along opposite sides of the cavity, the primary capillaries in each row being spaced apart from and offset from the primary capillaries in the other row, such that the primary capillaries define n-2 hollow cores, each hollow core is defined by three primary capillaries comprising two primary capillaries from one row and one primary capillary from the other row.

10. An antiresonant hollow core optical fibre according to claim 1, wherein each one of the at least two hollow cores is located in a separate cavity within the outer jacket, with three primary capillaries or four primary capillaries bonded to the surface of each separate cavity to define one hollow core.

11. An antiresonant hollow core optical fibre according to any preceding claim, further comprising at least one smaller capillary nested within each of the primary capillaries defining at least one of the hollow cores.

12. An antiresonant hollow core optical fibre according to claim 11, wherein the at least one smaller capillary comprises one smaller capillary to give a single nested cladding configuration for the at least one of the hollow cores, or comprises two smaller capillaries to give a double nested cladding configuration for the at least one of the hollow cores, or comprises three smaller capillaries to give a triple nested cladding configuration for the at least one of the hollow cores.

13. An antiresonant hollow core optical fibre according to any one of claims 1 to 12,wherein the at least two hollow cores have a core diameter of 25 pm or less.

14. An antiresonant hollow core optical fibre according to any one of claims 1 to 12,wherein the at least two hollow cores have a core diameter in the range of 3 pm to 25 pm.

15. An antiresonant hollow core optical fibre according to any one of claims 1 to 12,wherein the primary capillaries are configured for the at least two hollow cores to guide lightof a wavelength of A pm in a fundamental core mode, the at least two hollow cores have a core diameter of C pm, and C / A is less than or equal to 16.

16. An antiresonant hollow core optical fibre according to any one of claims 1 to 12, configured such that fundamental core modes of the at least two hollow cores have a mode field diameter in the range of 7 pm to 17.5 pm.

17. An antiresonant hollow core optical fibre according to any preceding claim, wherein the outer jacket has an outside diameter of substantially 125 pm.

18. A preform for fabricating a multicore antiresonant hollow core optical fibre, the preform comprising:a glass tubular major outer jacket;two or more canes stacked inside the major outer jacket, each cane comprising:a glass minor outer jacket having a tubular void defining a cane cavity;glass cladding primary capillary tubes comprising a maximum of four primary capillary tubes arranged in a ring and bonded to a surface of the cane cavity in spaced apart locations around the perimeter of the minor outer jacket; anda hollow core void defined within the ring of primary capillary tubes;the two or more canes positioned inside the major outer jacket at required relative positions for hollow cores of the multicore antiresonant hollow core optical fibre.

19. A preform according to claim 18, further comprising glass packing elements stacked inside the major outer jacket to maintain the two or more canes at the required relative positions.

20. A preform according to claim 18 or claim 19, comprising one or more interstitial spaces bounded only by outer surfaces of the two or more canes, the one or more interstitial spaces free from glass packing elements.

21. A method of fabricating a multicore antiresonant hollow core optical fibre according to claim 10, the method comprising drawing a preform according to claim 20 into the multicore antiresonant hollow core optical fibre or into a cane for subsequent drawing into the multicore antiresonant hollow core optical fibre while applying a vacuum to the one or more interstitial spaces to cause the one or more interstitial spaces to collapse.

22. A method of fabricating a multicore antiresonant hollow core optical fibre according to claim 10, the method comprising:obtaining, for each hollow core required in the multicore hollow core optical fibre, a cane comprising:a glass minor outer jacket having a tubular void defining a cane cavity;glass cladding primary capillary tubes comprising a maximum of four primary capillary tubes arranged in a ring and bonded to a surface of the cane cavity in spaced apart locations around the perimeter of the minor outer jacket; anda hollow core void defined within the ring of primary capillary tubes;stacking the canes inside a glass tubular major outer jacket at required relative positions for hollow cores of the multicore antiresonant hollow core fibre to create a preform;optionally stacking glass packing elements inside the major outer jacket in interstitial spaces bounded by outer surfaces of the canes and an inner surface of the major jacket;heating the preform to soften the glass; anddrawing the preform into the multicore antiresonant hollow core optical fibre while applying a vacuum to interstitial spaces of the preform bounded only by outer surfaces of the canes to cause the interstitial spaces to collapse, such that the major outer jacket, the minor inner jackets and any stacking elements fuse to create an outer jacket of the antiresonant hollow core optical fibre.

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