Improved antiresonant hollow core optical fibre

Abstract

An antiresonant hollow core optical fibre (10) comprising: a tubular outer jacket (12); a cladding comprising a plurality of primary capillaries (14) arranged in a ring and each bonded to an inner surface of the outer jacket (12) at a peripheral location around the outer jacket (12), and three or more additional capillaries (20,22,24) nested within each primary capillary (14) such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary (14); and a hollow core formed by a central void within the ring of primary capillaries (14), the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of λ µm in a fundamental core mode, and the hollow core has a diameter of D µm.

Description

[0001] TITLE OF INVENTION

[0002] IMPROVED ANTIRESONANT HOLLOW CORE OPTICAL FIBRE

[0003] BACKGROUND OF INVENTION

[0004] The present invention relates to antiresonant hollow core optical fibres having an improved configuration.

[0005] Classes of optical fibre include hollow core optical fibres, 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.

[0006] 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 core void, and secured to the inner surface of a jacket tube to maintain the required geometry. This 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.

[0007] At its simplest, an ARF can comprise a single ring of cladding capillaries, but several modifications and variations of this arrangement have been proposed in order to enhance performance in areas such as bandwidth and loss. Many applications known for conventional solid core optical fibres have been demonstrated with hollow core fibres, including telecommunications, optical power delivery and optical sensing. For telecommunications uses in particular, low optical loss (being the fraction of propagating light lost per unit length of propagation, typically per kilometre) and low latency (the time delay that occurs when transmitting a light signal along a length of optical fibre) are important.

[0008] An example of low loss reported to date for a hollow core fibre is 1.3 dB / km, achieved in an ARF with a nested antiresonant nodeless fibre (NANF) configuration [1], A NANF comprises a ring of spaced-apart (non-contacting) nested capillaries (one or more smaller capillaries fixed inside a large capillary) secured within an outer jacket and surrounding a central hollow core region. As with ARFs in general, the main optical guidance mechanism is a combination of antiresonance from uniform thickness of the glass walls or membranes of the cladding capillaries, and inhibited coupling to modes in the cladding. NANFs are proven to offer excellent low loss performance, and may one day overcome the fundamental loss limit of all-solid silica fibres [2],

[0009] Accordingly, alternative and improved configurations for the NANF type of ARFs are of significant interest.

[0010] SUMMARY OF THE INVENTION

[0011] Aspects and embodiments are set out in the appended claims.

[0012] According to a first aspect of certain embodiments described herein, there is provided an antiresonant hollow core optical fibre comprising: a tubular outer jacket; a cladding comprising a plurality of primary capillaries arranged in a ring and each bonded to an inner surface of the outer jacket at a peripheral location around the outer jacket, and three or more additional capillaries nested within each primary capillary such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary; and a hollow core formed by a central void within the ring of primary capillaries, the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of A pm in a fundamental core mode, and the hollow core has a diameter of D pm.

[0013] 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 may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] 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;

[0016] Figure 4 shows a graph of computer simulated total optical loss against fibre bend diameter for a prior art 5-cell double nested nodeless antiresonant hollow core fibre and a prior art 4-cell double nested nodeless antiresonant hollow core optical fibre; Figure 4A shows a graph of computer simulated total optical loss against fibre bend diameter for the prior art 4-cell nested nodeless antiresonant hollow core optical fibre modelled in Figure 4, for a range of bend orientations and light polarisations;

[0017] Figure 5 shows the graph of Figure 4, plus computer simulated total optical loss for a 4-cell double nested nodeless antiresonant hollow core optical fibre according to an example of the present disclosure;

[0018] Figure 5A shows a graph of computer simulated total optical loss against fibre bend diameter for the 4-cell double nested nodeless antiresonant hollow core optical fibre modelled in Figure 5, for a range of bend orientations and light polarisations;

[0019] Figure 6 shows the graph of Figure 5, plus computer simulated total optical loss for a 4-cell triple nested nodeless antiresonant hollow core optical fibre according to an example of the present disclosure;

[0020] Figure 7 shows a schematic transverse cross-sectional view of a first example antiresonant hollow core optical fibre configured according to the present disclosure, with a cladding comprising four primary capillaries and three additional capillaries nested within each primary capillary;

[0021] Figure 8 shows a schematic transverse cross-sectional view of a second example antiresonant hollow core optical fibre configured according to the present disclosure, with a cladding comprising three primary capillaries and three additional capillaries nested within each primary capillary;

[0022] Figure 9 shows a schematic transverse cross-sectional view of a third example antiresonant hollow core optical fibre configured according to the present disclosure, with a cladding comprising four primary capillaries and four additional capillaries nested within each primary capillary;

[0023] Figure 10 shows a schematic transverse cross-sectional view of a fourth example antiresonant hollow core optical fibre configured according to the present disclosure, with a cladding comprising three primary capillaries and four additional capillaries nested within each primary capillary;

[0024] Figures 11A-11C show schematic transverse cross-sectional views of further example antiresonant hollow core optical fibres configured according to the present disclosure, with a cladding comprising four primary capillaries and three additional capillaries nested within each primary capillary, and a reduced volume of an interstitial space achieved via a range of techniques;

[0025] Figures 12A-12C show schematic transverse cross-sectional views of still further example antiresonant hollow core optical fibres configured according to the present disclosure, with a cladding comprising three primary capillaries and three additional capillaries nested within each primary capillary, and a reduced volume of an interstitial space achieved via a range of techniques; and

[0026] Figures 13, 14 and 15 show schematic transverse cross-sectional views of other example antiresonant hollow core optical fibres configured according to the present disclosure, with, respectively, a cladding comprising five, six and eight primary capillaries, and in each case, three additional capillaries nested within each primary capillary.

[0027] DETAILED DESCRIPTION

[0028] Aspects and features of certain examples and embodiments are discussed I described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed I 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.

[0029] The type of hollow core optical fibre that can be described as antiresonant hollow core fibre (ARF) at its simplest comprises a tubular outer jacket, and number of cladding capillaries arranged in a ring inside the jacket, and secured or bonded to the inner surface of the jacket. A 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.

[0030] Figure 1 shows a transverse cross-sectional view of a first previously-proposed antiresonant hollow core fibre. 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 [3], 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 otherwords, 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.

[0031] 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.

[0032] 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.

[0033] Figure 2 shows a transverse cross-sectional view of a second previously-proposed antiresonant hollow core fibre [2, 4], 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 [5], 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.

[0034] Figure 3 shows a transverse cross-sectional view of third previously-proposed antiresonant hollow core optical fibre [2, 4, 6], 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.

[0035] In general, it is commonly accepted in the technical field of hollow core optical fibres that the inclusion of secondary and optionally tertiary capillaries nested inside a primary capillary, with all capillaries within a nested group bonded at the same azimuthal position on the circumference of the outer jacket, provides additional reflective elements along the radial direction that acts to reduce optical loss. The gap or spacing between the adjacent primary capillaries, while improving performance by removing unwanted resonances at contact between adjacent capillaries, also contributes to the leakage of optical power. Therefore, the gap is generally made small to reduce leakage and hence loss, while being maintained above a zero spacing so as to avoid unwanted resonances. Nested groups of two and three capillaries are commonly used in ARFs.

[0036] From this, it might be supposed that further levels of nesting of the cladding capillaries (i.e. each cladding capillary group would comprise more than three capillaries of decreasing size arranged inside one another) would be desirable, since it would appear that the mechanism providing reduced optical loss would be enhanced by additional glass walls along the radial direction. However, NANF ARFs with more than three capillaries in each nested group have not been proposed or considered useful or achievable to date.

[0037] There are significant reasons why additional nested capillaries or tubes are considered an impractical, unachievable and undesirable development to the known single and double NANF cladding structures such as the examples of Figures 2 and 3.

[0038] Firstly, adding more tubes necessarily increases the complexity of the fibre fabrication process, and is therefore only worthwhile if an improvement in fibre performance will result. It is appreciated that additional layers of nested tubes are theoretically effective in reducing leakage loss (confinement loss). This is shown in Figure 13 of [2] which compares losses for single and double NANF structures with that for a basic un-nested ARF, and shows the leakage loss reducing with an increasing number of nested tubes. However, additional tubes do not modify the surface scattering loss of the fibre, which at some wavelengths becomes the dominant loss mechanism. As indicated in [2], the additional nested tube in a DNANF structure brings the leakage loss below the surface scattering loss, so there is no perceivable benefit regarding loss to be attained by adding still further nested tubes; the surface scattering will determine the overall loss. Hence, it appears not to be worthwhile to undertake the increased fabrication complexity required to increase the level of nesting of the capillary tubes, because optical loss will not be lowered any further.

[0039] A second reason is considered to be equally significant. Many of the applications for optical fibres (and certainly those in the largest sector of data communication at a high data rate and over distances in excess of 100 metres) require the fibre to guide a single spatial optical mode only, known as single mode operation. This is applicable to both solid optical fibres and hollow core optical fibres. However, in the specific context of hollow core optical fibres, it is known to configure the fibre with a core diameter D that is many times larger than the intended operating wavelength A (typically D > 18-22A, corresponding to D in a range of about 28 to 35 pm when the wavelength is 1 .55 pm, which is the literature standard wavelength for low loss hollow core fibres operating at telecommunications wavelengths) in order to achieve acceptably low loss for these applications. A large core diameter causes the fibre to guide multiple optical core modes.

[0040] The conventional way to address this contradiction for ARF hollow core fibres is to configure them to operate in effect as single mode fibres by providing the gaps or spaces between adjacent primary cladding capillaries, providing a nodeless configuration as described above for Figures 1-3. When suitably sized, the gaps allow all higher order core modes to couple efficiently into modes which are guided in these gaps and are lossier. Meanwhile, the fundamental core mode cannot couple into the cladding and its loss remains low. The fundamental mode is efficiently guided, the higher order modes are lost, and single mode operation is emulated. Hence, gaps between cladding capillaries are essential in ARFs intended for single mode applications, and the gaps must be of a particular size (with regard to the intended operating wavelength).

[0041] For ARFs with five or six primary cladding capillaries, as in the examples of Figures 1- 3, an appropriately sized gap is achievable when the total number of capillaries per primary capillary group or cell is kept to three or fewer, in other words, limited to a DNANF with three tubes per cladding capillary group (Figure 3), a NANF with two tubes per cladding capillary group (Figure 2). or a simple ARF without nested capillaries (Figure 1). There is inadequate capacity inside the outer jacket to accommodate additional levels of nesting while maintaining the capillary spacing or gap at the required size. The spacing tends towards being insufficiently large to make the high order core modes lossy, and the single mode operation required for many applications cannot be achieved. Hence, nested designs with more than two levels of nesting (three tubes in total per cladding capillary group) have not been considered viable.

[0042] From this, it may appear that a possible solution to enable the provision of extra nesting levels in a NANF could be to reduce the number of primary cladding capillaries, say from five (recalling that a 5-cell NANF design currently offers the lowest loss capability for hollow core fibres and is therefore perceived as advantageous) to four. The lower number of primary capillary tubes frees up space within the outer jacket. This allows each primary capillary to be made larger, offering more space inside for smaller tubes to be nested, and therefore having the potential to accommodate more smaller tubes for more levels of nesting, while at the same time permitting a sufficiently large gap for higher order core mode loss to achieve effectively single mode operation.

[0043] However, 4-cell ARFs with various configurations have been studied, but have features and disadvantages which can compromise their practical application. In particular, such fibres operate well when kept straight, but their optical properties degrade very significantly when bent. This arises because a bend acts as a centrifugal force on the propagating optical mode, pushing it towards the outside of the bend and outwardly from the core. In a 4-cell ARF, the interstitial spaces behind the primary capillary tubes (the regions bounded by the inner surface of outer jacket and the rearward facing parts of the outer surface of the primary capillary tubes, i.e. the parts of the primary capillary tubes not forming the core boundary, indicated as “17” in Figure 3) are sufficiently large that when the fundamental core mode is pushed towards it in a bent fibre, undesirable coupling and a very large loss can occur. Hence, 4-cell ARFs have a high bend loss, which is not seen in fibres with five or more primary capillary tubes because the interstitial spaces are smaller (relative to the overall dimensions of the fibre).

[0044] Figure 4 shows a graph of simulated total loss (in dB / km; y-axis) determined by computer modelling in the lowest loss known 5-cell DNANF mentioned above, and in an example 4-cell DNANF, as a function of bend diameter (diameter of a coil of fibre, in cm; x- axis). Transverse cross-sectional views of the two fibres are also shown to indicate the difference in cladding structure. The core diameter of the known 5-cell DNANF is 30 pm for waveguiding of light at a wavelength of 1 .55 pm, in line with the conventional arrangement of a large core diameter compared to the operational wavelength used to achieve low loss, as noted above. The wall thickness of the capillaries is 0.5 pm. These dimensions are typical for conventionally available ARFs, and the example 4-cell DNANF is similarly sized, with a 30 pm core diameter and 0.5 pm capillary wall thickness for operation at 1 .55 pm. The loss for the 5- cell DNANF 10a is shown as a solid line, and it can be seen that the loss remains low and relatively constant at larger bend diameters, and only changes at small bend diameters, increasing exponentially for bend diameters below 8 cm, as is usual in most optical fibres of any type. The loss for the 4-cell DNANF 10b is shown as a dashed line, and in contrast, is generally higher, much less predictable as a function of radius, and shows resonance peaks of very high loss at smaller bend diameters. The simulation shows the loss behaviour for one polarisation of propagating light and for a bend constantly oriented along one direction. In reality, fibres have a degree of twist and a bend typically scans many possible orientation angles, so that the range of loss values for a real 4-cell NANF will change even more dramatically than the depicted simulation. Hence, 4-cell ARFs are not bend-tolerant. For this reason, it is commonly accepted that ARFs with only four (or fewer) primary capillary tubes (nested and un-nested) are not viable for use.

[0045] To explain this further, Figure 4A shows a graph of simulated loss for the 4-cell DNANF 10b for other bend orientations of the fibre and polarisations of the propagating light. Depicted are loss profiles for bends oriented towards a cladding capillary or towards a gap between adjacent cladding capillaries, for both fast axis and slow axis polarisations, as indicated in the key. From this is can be seen that each bend orientation induces resonance peaks at different bend diameters, and that different polarisations experience different levels of loss. In practice, this means that such a fibre is practically useful only a straight configuration or at large bend diameters no smaller than 100 cm. At tighter bends the loss becomes completely unpredictable, hence the non-viability of this fibre structure.

[0046] From the above discussion it can be appreciated that increasing the level of nested capillaries in the cladding of a NANF above a total of three capillaries per nested group or cell appears unfeasible. A minimum of five cells seems necessary for acceptable loss characteristics and single mode operation, but a five cell design offers little space to accommodate additional nested capillaries. Hence, the reduction in loss potentially offered by higher levels of nesting seems inaccessible.

[0047] Surprisingly, however, the inventors have determined that an ARF with four cells can be made operable, and robust to the expected high bend loss. It has been ascertained that the bend sensitivity observed in 4-cell ARFs is strongly dependent on the ratio between the core size (radius or diameter) and the size (radius or diameter) of a bend in the fibre. Decreasing the core size of the fibre has the effect of decreasing the mode field diameter of the fundamental optical mode propagating in the core. In turn, the centrifugal force that acts to push the fundamental mode towards the outside of a bend is also reduced, for the same bend size. Therefore, the coupling of the fundamental mode into parasitic modes supported in the interstitial gaps between and behind the cladding capillaries is also reduced. The fibre is made able to withstand much tighter bends without the resonant peaks seen in larger core fibres, shown in Figure 4.

[0048] Figure 5 shows the graph of simulated loss of Figure 4, with the addition of calculated loss data for a modelled 4-cell DNANF with a small core diameter, as compared to the 30 pm core diameter of the previously discussed 4-cell DNANF corresponding to the dashed line. A transverse cross-sectional view of the smaller fibre is also shown, the views being approximately to scale. The smaller 4-cell NANF 10c has a core diameter of 15 pm. The calculated loss for this smaller fibre is shown as a dotted line. Although it can be seen that reducing the core size produces an overall increase in loss (as would be expected, since it is known that a large core diameter compared to the wavelength is needed for low loss in an ARF), the resonances at particular bend diameters are eliminated and the bend loss behaviour is regularised. The shape of the loss profile is similar to that for the 5-cell DNANF, being substantially constant for larger bends and increasing rapidly for bend diameters below 8 cm, but is overall higher. Nevertheless, the loss value at larger bend diameters, even though increased, is around 0.5 dB / km which is low enough to be useful and acceptable for many applications.

[0049] Figure 5A shows a graph of simulated loss for the small core fibre 10c for different bend orientations and polarisations (the same as those presented in Figure 4A, and indicated in the key). From this it can be seen that reducing the core size eliminates resonant peaks in the loss for multiple bend orientations and polarisations, and the total loss variation as a function of bend orientation and polarisation is considerably reduced. Compare with the multiple peaks and large variations shown in Figure 4A. Hence the reduced core size in combination with a lower number of primary capillaries provides a fibre which is usable with a predictable performance in both straight and bent configurations.

[0050] Accordingly, it is proposed herein that an ARF may have a cladding structure with fewer than five primary capillaries or cells, in combination with a small core diameter (suitable dimensions for the core are discussed below). If a sufficiently small core size is employed, ARFs with, in particular, four primary capillaries, which may each have a nested configuration comprising smaller additional capillaries nested within the primary capillaries, can be bent with an acceptably low bend loss. This property makes such fibres widely useful. The concept can be further extended to ARFs with fewer than four primary capillaries, in particular three primary capillaries, which again, may each have a nested configuration. Furthermore, as noted above, a 4-cell ARF offers the possibility of adding further levels of capillary nesting, owing to the additional space available inside the outer jacket compared to a 5-cell ARF. Therefore, it is additionally proposed herein, in some examples, that triple or even higher levels of nesting are utilised. It has been determined that additional nesting in conjunction with a small core diameter offers further benefits and improved optical performance.

[0051] Figure 6 shows the graph of simulated loss of Figure 5, with the addition of calculated loss data for a modelled small core 4-cell ARF with three levels of capillary nesting in each cell. In other words, each primary capillary contains within it three smaller capillaries of decreasing width, nested inside one another and all bonded at the same azimuthal position around the outer jacket. We may term a fibre with this structure a triple NANF, or TNANF. A transverse cross-sectional view of the 4-cell TNANF 10d is included in Figure 6. Its core diameter is the same as that of the 4-cell DNANF discussed with regard to Figure 5, namely 15 pm. The two 4-cell fibres differ only in the number of nested capillaries in each cell; the primary capillaries are the same diameter and the capillary wall thicknesses (0.5 pm) are the same. Hence the two fibres are designed to propagate light of the same wavelength, namely 1.55 pm. The calculated loss for the 4-cell TNANF is shown as a dash-dot line in Figure 6. It can be seen that the additional nested capillary, which provides an extra pair of glass-air interfaces along the radial direction in the fibre, significantly reduces the loss, by an order of magnitude. Unpredictable resonance peaks in the bend loss are absent, as with the small core 4-cell DNANF, but the loss is reduced substantially below that of the small core 4-cell DNANF (dotted line), and is almost as low as the loss for the standard core low loss 5-cell DNANF (solid line) at larger bend diameters. At the larger bend diameters, the loss is constant and flat, as for the 5-cell DNANF. Most significantly, though, the bend loss is actually lower than that of the 5-cell DNANF at small bend diameters, and remains substantially constant below 8 cm bend diameter, lacking the usual exponential increase in bend loss at tight bends observed generally in optical fibres. Hence, the bend performance of the small core 4-cell TNANF may be characterised as better than that of the standard core 5-cell DNANF. The combination of small core size and an extra cladding capillary interface enables this.

[0052] Overall, therefore, the use of a small core size enables a move to a lower number of primary capillaries or cells in an ARF, so that ARFs with four or fewer primary capillaries become viable. This reduction in the number of primary capillaries in turn enables the inclusion of more than two smaller capillaries nested within each primary capillary, so that triple or higher nested cladding structures become viable. Fibres configured in this way offer improved optical loss characteristics.

[0053] An additional benefit is offered by a smaller core size, since ARFs configured with a small core diameter have a correspondingly smaller outer diameter imposed by the geometry of the cladding structure when the primary capillaries have a circular shape. Hollow core optical fibres offer a range of advantages that makes them attractive for carrying optical signals and optical data in telecommunications applications, including low latency and high optical power capability. However, the requirement to propagate the optical signals exists both within and between data centres comprised in optical telecommunications networks, which house a range of optical apparatus for generating, transmitting, receiving and processing the optical signals. This apparatus is typically configured for compatibility with conventional solid core single mode telecommunications optical fibre, which is manufactured with a standard outer diameter of 125 pm. The apparatus, and existing optical cabling, is designed for connection with optical fibre cables containing fibres of this size. Hence, hollow core optical fibres currently having a larger outer diameter cannot be directly connected with existing apparatus and solid core cabling, and adapters are required. The integration of hollow core fibres into existing networks of optical fibres is therefore restricted, particularly for intra data centre connections, and the advantages offered by hollow core fibres have limited accessibility. The new ARF designs proposed herein address this drawback, since the reduction in core size allows the outer diameter of the hollow core fibres to be reduced to 125 pm. Optical fibre cabling containing these fibres can thereby become directly compatible with existing apparatus and cabling. In some examples, therefore, antiresonant hollow core optical fibres configured as proposed herein have an outer diameter that matches or is similar to the outer diameter of conventional solid core single mode telecommunications fibre, namely an outer diameter of 125 pm. This is not a limiting feature, however, fibres with outer diameters small or larger than this value are not excluded.

[0054] Note that in this 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 Figures 1-3). 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.

[0055] Importantly, also, the proposed smaller core size for ARFs can 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 two portions of optical fibre are coupled or connected together. Hence, the reduced hollow core size proposed herein facilitates coupling to ARFs to standard solid core fibres. More generally, the proposed smaller core size ARFs 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 proposed core diameters up to about 25 pm. The mode field diameter is typically about 70% of the core diameter. Small mode field diameters, in fibres with smaller core diameters, are not excluded, however. 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 ARF can be configured for propagation of light of any desired wavelength by suitable selection of the cladding capillary wall thickness. For nodeless designs, 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. The skilled person understands how to configure the various dimensions within an ARF appropriately for a wavelength of interest.

[0056] Figure 7 shows a transverse cross-sectional view of a first example hollow core optical fibre according to the present disclosure. As previously described, the hollow core fibre 10 comprises a hollow tubular outer jacket 12 and a cladding comprising a plurality of primary capillaries 14 (being hollow tubes extending along the length of the outer jacket 12) which are arranged in a ring around the inner surface of the outer jacket 12, and each is bonded to the inner surface of the outer jacket 12 at a peripheral location 16 around the outer jacket 12. In this example, the primary capillaries 14 are regularly arranged so that the peripheral locations 16 are spaced apart from adjacent peripheral locations 16 by a constant distance. Note that although Figure 7 and other depicted examples show a narrow region of contact between each primary capillary and the inner surface of the outer jacket at each peripheral location, this is illustrative only, and not limiting. In reality, the fluid dynamics of the softened glass during fabrication of the optical fibre can often cause a wider region of contact around the perimeter of the outer jacket where the primary capillary is bonded to the inner surface of the outer jacket. Hence the peripheral location may correspond to a single point of contact, or a region of contact, which may be relatively extensive. Around the perimeter of the outer jacket the width of the contact region may be equal to up to about one half of the diameter of the primary capillary, or up to about three quarters of the diameter of the primary capillary, for example.

[0057] A cladding configured in this manner to comprise a single ring of hollow capillaries gives a hollow core fibre that guides by antiresonance. As proposed herein an antiresonant hollow core fibre comprises a maximum of four primary capillaries, and in this example, there are four primary capillaries 14 (in other words, the hollow core fibre of this example comprises no fewer than four primary capillaries). The central void within the ring of primary capillaries 14 forms the hollow core of the fibre 10. As described above, 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 fibre to the outer surface of one or more of the primary capillaries 14 (in this example, to the outer surface of all the primary capillaries 14 because the primary capillaries 14 are all of the same size). The diameter D of the hollow core is twice the radius.

[0058] In accordance with the principles of antiresonant waveguiding in hollow core optical fibres, the hollow core fibre 10 is configured to guide light of a wavelength A which is determined by the wall thickness of the primary capillaries 14, which is specified to provide antiresonance at the chosen wavelength. The hollow core fibre 10 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. The hollow core has a small size compared to conventional standard antiresonant hollow core fibres. The diameter D of the core can be defined by reference to the wavelength A of the fundamental core mode, such that for a core diameter D in pm and a wavelength A in pm, the ratio D / A has a value of 16 or less. In some examples, this upper limit may be lower, such as D / A is less than or equal to 14, or less than or equal to 12, or less than or equal to 10.. In absolute terms, the diameter D of the core can be defined as having a maximum value of 25 pm, or a value of 22 pm or less, or a 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 set out herein, 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 D 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 4, so the antiresonant hollow core fibres proposed herein can be considered to be small core fibres.

[0059] In the Figure 7 example, the hollow core fibre 10 is a NANF, in other words its structure is nodeless and includes additional capillaries nested inside the primary capillaries 14. The nodeless aspect arises from the primary capillaries 14 being spaced apart by gaps (see Figures 1 and 2) in order to reduce resonances and hence decrease optical losses, as described above. The gaps may be sized to provide effectively single mode operation (in other words, propagation of only the fundamental core mode is supported without high loss) at the wavelength of the guided fundamental mode, by promoting coupling of higher order modes out of the core and into lossy modes guided in and behind the gaps, also as described above. Alternatively, the gaps between the primary capillaries 14 may be sized to retain at least some higher order modes above the fundamental mode, so that the hollow core fibre provides multimode operation at the wavelength of the guided fundamental mode. Multimode operation is intended to include “few” mode operation, by which is meant that only a few additional modes are effectively supported and guided, such as up to five groups of higher order spatial modes. In some other examples (not shown), the gaps may omitted if the loss reduction and / or mode tailoring effect provided by spacing the primary capillaries apart is not required. In other words, the fibre may not be nodeless.

[0060] Regarding the nested characteristics of this example hollow core fibre 10, the cladding comprises three additional capillaries nested within each primary capillary 14, so that the fibre can be described as a triple NANF or TNANF, such as that discussed with regard to Figure 6. A first additional capillary 20 (secondary capillary) has a smaller width or diameter that the primary capillary 14 and is positioned inside the primary capillary 14 and bonded to the inner surface of the primary capillary 14, a second additional capillary 22 (tertiary capillary) has a smaller width or diameter than the secondary capillary 20 and is positioned inside the secondary capillary 20 and bonded to the inner surface of the secondary capillary 22, and a third additional capillary 24 (quaternary capillary) has a smaller width or diameter than the tertiary capillary 22 and is positioned inside the tertiary capillary 22 and bonded to the inner surface of the tertiary capillary 22. Within each primary capillary 14, all the additional capillaries are bonded to the inner surface of the larger capillary within which they are located or nested at a location which is aligned with the peripheral location 16 of that primary capillary 14 around the outer jacket 12.

[0061] While three nested capillaries per primary capillary is advantageous for reducing optical loss as described with regard to Figure 6, other numbers of nested capillaries are not excluded. As described with regard to Figure 5, the reduction in the number of primary capillaries below five in conjunction with a small core size is beneficial in reducing bend sensitivity in hollow core fibre, so in some examples, each primary capillary may have only two additional capillaries (secondary capillary and tertiary capillary) nested within it (such as the double NANF structure of the example fibre 10c in Figure 5), or just one additional capillary (secondary capillary) nested within it (in line with the single NANF structure shown in Figure 2). In still other examples, additional capillaries may be omitted altogether so that only primary capillaries are present (in line with the simple ARF structure shown in Figure 1). Also, in some examples, when additional capillaries are included, the number of additional capillaries within each primary capillary need not be the same.

[0062] Conversely, higher numbers of additional capillaries may be nested inside each primary capillary, so that each primary capillary includes more than three additional capillaries, such as four, five or more additional capillaries per primary capillary. Each additional capillary provides a further pair of glass-air interfaces for improved confinement of the fundamental optical mode within the core and reduced loss. The proposed structure is analogous with Figure 7, with each further additional capillary bonded to the inner surface of the immediately larger additional capillary in line with the peripheral location of the primary capillary.

[0063] In some examples, and regardless of the quantity of additional capillaries, all the primary capillaries within the fibre may have nominally the same capillary wall thickness, where as noted above the thickness of the capillary walls is chosen with reference to the intended wavelength of light to be guided by the fibre. The wall thickness is the distance along the radial direction from the central longitudinal axis of the capillary between the inner surface and the outer surface of the capillary wall. Any additional capillaries may have this wall thickness also, so that all capillaries with the fibre have the same nominal wall thickness. In other examples, additional capillaries may have wall thicknesses deliberately different from the primary capillary wall thickness, and / or additional capillaries of different orders or levels of nesting may have deliberately different capillary wall thicknesses. By “same wall thickness” it is meant that the capillaries are intended to have a same or a substantially same wall thickness, chosen for provide waveguiding at a desired wavelength or wavelengths. In reality, capillaries intended to have the same wall thickness will have some variation in wall thickness in the finished fibre, arising from manufacturing tolerances of the fibre drawing process. A variation in wall thickness of up to about 20%, often in the range of about 10% to 20% is usual and gives acceptable optical performance. Hence, capillaries described as having a same wall thickness as each other includes capillaries having wall thicknesses that vary between capillaries (and / or within a single capillary) by up to about 20%. Where wall thicknesses of the additionally capillaries are deliberately different from the primary capillary wall thickness, a variety of configurations are possible. For example, within a nested group the capillary wall thickness may decrease from the primary capillary such that the wall thickness becomes less as the additional capillary diameter decreases. Hence the capillary wall thickness decreases with distance from the centre of the core. Conversely, within a nested group the capillary wall thickness may increase from the primary capillary such that the wall thickness becomes greater as the additional capillary diameter decreases. Hence the capillary wall thickness increases with distance from the centre of the core. In still other examples, the wall thicknesses within a nested group may alternate between thicker and thinner with distance from the core, or have some other variation. These various alternatives give scope for tailoring the performance of the optical fibre.

[0064] The term “cell” has been used herein for convenience as referring to a single primary capillary, so that an ARF with N primary capillaries can be labelled as a N-cell ARF (additionally nodeless and / or nested as appropriate). For completeness, herein “cell” is also intended to encompass any additional capillaries nested within a primary capillary. So, in the Figure 7 example, a cell 15 comprises a primary capillary 14 plus a secondary capillary 20, a tertiary capillary 22 and a quaternary capillary 24 successively nested within the primary capillary 14. Hence, each cell comprise a total of four capillaries in this example. More generally, a cell comprises a total of M capillaries where M = A+1 , A being the number of additional capillaries, and the “+1” accounting for the primary capillary. Where M > 1 , the additional capillary or capillaries are successively nested inside the primary capillary.

[0065] Figure 8 shows a transverse cross-sectional view of a second example hollow core optical fibre according to the present disclosure. The fibre 10 comprises the same features (outer jacket 12, hollow core, and cladding capillary cells 15) as the first example of Figure 7, except that only three primary capillaries 14 (hence three cells 15) are included. In other words, the hollow core fibre 10 of this example comprises no fewer than three primary capillaries. A comparison of Figures 7 and 8 shows that the use of three primary capillaries enables a smaller diameter for the hollow core to be achieved for an otherwise similar inner diameter of the outer jacket 12, as a consequence of the geometry. The additional space afforded by fewer primary capillaries allows each primary capillary to be made larger (up to a maximum diameter that still gives an appropriate spacing between adjacent primary capillaries for a nodeless cladding structure), which in turn can make the core smaller. Hence, the mode field diameter of the fundamental core mode can be reduced compared to a four cell structure, and the bend tolerance might be enhanced.

[0066] Otherwise, the cladding of the second example fibre is the same as that of the first example fibre, in that a triple nested structure is provided with each primary capillary 14 so that each primary capillary 14 contains three additional capillaries, being a secondary capillary 20, a tertiary capillary 22 and a quaternary capillary 24, nested one inside the other as before. Again, however, fewer or more additional capillaries may be included in each primary capillary

[0067] 14, as desired. Also, the cladding structure may be nodeless or non-nodeless, where gaps provided between the adjacent primary capillaries 14 in a nodeless structure can be sized for effectively single moded or multimoded waveguiding at the wavelength of the fundamental core mode, again as before.

[0068] Figures 9 and 10 show transverse cross-sectional views of third and fourth example hollow core optical fibres according to the present disclosure, and having higher numbers of additional capillaries. Figure 9 shows a fibre 10 with four cells 15, and differing from the first example of Figure 7 in that each cell 15 comprises a primary capillary with four additional capillaries nested within it. In line with the nomenclature introduced earlier, this fibre 10 can be labelled as a 4-cell quadruple NANF (QNANF). Figure 10 shows a fibre 10 with three cells

[0069] 15, and differing from the second example of Figure 8 in that each cell 15 comprises a primary capillary with four additional capillaries nested within in. This fibre 10 can be labelled as a 3- cell QNANF.

[0070] As noted above, the interstitial spaces between the primary capillaries and the inner surface of the outer jacket of an ARF can, if sufficiently large, allow coupling from the fundamental core mode into the cladding when the ARF is bent, leading to loss. As proposed herein, this can be addressed by a reduced core size and corresponding smaller mode field diameter for the fundamental mode. For further improvement, it is additionally proposed that, optionally, the size of the interstitial spaces may be reduced in any of the small core ARF structures proposed herein. Smaller interstitial spaces reduce coupling of optical power from the core into these regions when a fibre is bent, so can reduce the loss still further. The interstitial spaces can be reduced in volume in a variety of ways, but in general it is proposed that the interstitial spaces may have a reduced volume compared to the volume they would have in an otherwise same configuration in which the inner surface of the outer jacket has a circular transverse cross-section (such as shown in Figures 7-10).

[0071] Figures 11A-11C show three transverse cross-sectional views of example hollow core optical fibres configured with a reduced interstitial space volume. Aside from the reduced volume of the interstitial spaces, the fibres are otherwise the same as that of the first example in Figure 7, having four cells and a triple-nested structure of three additional capillaries within each primary capillary. Figure 11A shows an example fibre in which the interstitial spaces are made smaller by shaping of the outer jacket 12 such that the inner surface 12a of the outer jacket has a transverse cross-sectional shape which is non-circular. In particular, assuming that the outer surface of the outer jacket 12 remains of circular transverse cross-section, the inside volume at the location of the interstitial spaces can be reduced if the wall thickness of the outer jacket 12 (distance between the outer surface and inner surface) is made larger in the regions between the peripheral locations at which the cells are bonded to the inner surface, compared to the thickness at the peripheral locations, while leaving adequate space around the primary capillaries such that they do not contact the inner surface other than at the peripheral locations. In the example of Figure 11 A, this is achieved by making the inner surface of outer jacket have substantially straight portions 12b (in transverse cross-section) between the peripheral locations 16. Since this example fibre has four cells, the inner surface 12a effectively has a cross-sectional shape which is a square with rounded corners, where the peripheral locations 16 are located in the corners. It will be appreciated that other cross- sectional shapes for the inner surface 12a can be used to achieve the same effect; any shape that brings at least part of the inner surface 12a between the peripheral locations 16 closer to the centre of the core of the fibre 10 could be used. This covers various convex or other protruding shapes, and shapes which are less curved or less concave than a circular crosssection.

[0072] Reducing the interstitial space volume by reshaping of the outer jacket requires fabrication of a specifically shaped outer jacket (or of a tubular element in a preform which is drawn into the finished fibre, that forms the outer jacket). Depending on the shape and the fabrication process used, this may be more or less convenient. An alternative approach to reducing the interstitial space volume is to include one or more elements located within the interstitial spaces that occupy some of the volume, and therefore reduce the interstitial space volume. Such elements can be bonded to the inner surface of the outer jacket to retain them in the intended location.

[0073] Figure 11 B shows a first example of a fibre in which the interstitial space volume is reduced by additional elements. In this example, a plurality of further tubular capillaries 26 are used, one located within each interstitial space and bonded to the inner surface 12a of the outer jacket 12 at locations between the peripheral locations 16 of the primary capillaries I cells. Each further capillary 26 extends along the hollow core optical fibre 10 substantially parallel to the primary capillaries and to the longitudinal axis of the fibre. Figure 11C shows a second example of a fibre in which the interstitial space volume is reduced by additional elements. In this example a plurality of solid rods 28 (of glass or polymer) are used, again with one located in each interstitial space and bonded to the inner surface 12a of the outer jacket 12 at locations between the peripheral locations 16 of the primary capillaries I cells, so as to extend along the hollow core optical fibre 10. The rods 26 are shown as having a substantially semi-circular transverse cross-sectional shape, with their curved sides facing towards to the core of the fibre 10, but other shapes may be used as desired or convenient. In either example, more than one capillary or rod, or a combination of capillaries and rods, could be included within each interstitial space to further reduce the volume. Using capillaries as additional elements allows control of the additional element size and hence control of volume of the interstitial space, by the application of pressure to the interior of the capillaries during drawing of the fibre from a preform. The use of solid rods precludes this flexibility but avoids the complexity of pressure control additional to the pressurisation which is usually applied to the cladding capillaries during the draw to control their relative size in the finished fibre.

[0074] Figures 12A-12C show three transverse cross-sectional views of further example hollow core optical fibres configured with a reduced interstitial space volume. Aside from the reduced volume of the interstitial spaces, the fibres are otherwise the same as that of the second example in Figure 8, having three cells and a triple-nested structure of three additional capillaries within each primary capillary. Figure 12A is analogous with Figure 11 A, in that the interstitial volume is reduced by shaping of the inner surface 12a of the outer jacket 12. Again, this is achieved by making the inner surface of outer jacket have substantially straight portions 12b (in transverse cross-section) between the peripheral locations 16. Since this example fibre has three cells, the inner surface 12a effectively has a cross-sectional shape which is a triangle with rounded corners, where the peripheral locations 16 are located in the corners. In addition, a protruding portion 12c extends inwardly (towards the core) from the centre of each straight portion 12b, in order to further reduce the volume. Such protruding portions can be shaped integrally with the outer jacket, or might be added by bonding solid rods to the inner surface as in the Figure 11C example. Again, it will be appreciated that other cross-sectional shapes for the inner surface 12a can be used to achieve the same effect in a 3-cell design.

[0075] Figures 12B and 12C show example fibres that are analogous with the Figures 11 B and 11C examples. In Figure 12B, further capillaries 26 are bonded to the inner surface 12a at locations between the peripheral locations 16, and in Figure 12C, solid rods 28 are bonded to the inner surface 12a at locations between the peripheral locations 16. Again, differently shaped rods, more than one rod or capillary per interstitial space, or a combination of further capillaries and rods may be used.

[0076] It has been mentioned above that some configurations of 4-cell ARFs have been studied and reported. These include large core designs, with core diameters of 35 pm and unnested, single and double nested cladding capillaries [7], and a core diameter of 30 pm and double nested cladding capillaries [8], Smaller cores have been considered for some very specific applications, with configurations using primary capillaries of different thicknesses and with nesting to achieve high birefringence and polarising effects and core diameters of 14 pm and 20 pm [9, 10], and an un-nested configuration with a core diameter of 14.6 pm, and primary capillaries designed to guide ultraviolet light at 532 nm, giving a large D / A ratio of 27.5

[0011] , These fibres are specialist and entirely unsuitable for general use, and in particular not intended for or suitable for the important and widespread application of optical telecommunications.

[0077] While the main examples of hollow core fibres described above utilise the combination of a maximum of four primary capillaries, triple nesting, and a small core diameter in order to access a range of beneficial features and performance advantages, the present disclosure also proposes other examples with fewer limitations.

[0078] In particular, the triple (and higher) NANF concept per se is proposed for use in ARF designs, regardless of core size and primary capillary quantity. As discussed above, the addition of further nesting levels within each primary capillary reduces loss via light leakage and improved optical confinement, by providing additional air / glass interfaces along the radial direction, so is a useful hollow core fibre design. While 4-cell arrangements can assist in enabling the accommodation of additional capillaries to increase the nesting level, designs with more than four primary capillaries can also be configured with triple and higher nesting of additional capillaries that still provide useful optical performance. In cases where it is found that five or more primary capillaries with triple nesting offer insufficient spacing between primary capillaries for good single mode operation, a useful fibre for few-mode or multimode operation, with low loss, can still be achieved. In other cases, spacing for effectively single moded operation may be achieved together with triple nesting, for example if capillary walls are relatively thin so that more space is made available for additional capillaries to be nested. In other examples, the feature of a small core may be omitted, in other words, the core diameter may exceed 25 pm. The mode field diameter may exceed 17. 5 pm. The core diameter to wavelength ratio D / A may exceed 17. While, as explained above, a smaller than conventional core diameter can assist in reducing bend loss in fibre designs with only three or four primary capillaries, and provide benefits in coupling to existing solid core optical fibres, in some situations these characteristics may not be important, and a larger core may be tolerable or preferable. The advantage of higher capillary nesting levels (triple and higher) can be utilised in larger core formats when bend loss is not a concern (for fibres deployed in configurations without small bends), and / or where a small core and corresponding mode field diameter is not of interest (for coupling to existing larger core hollow core fibres, for example).

[0079] In summary, therefore, examples of the present disclosure are directed to antiresonant hollow core optical fibres in which the cladding comprises a plurality of primary capillaries, each having three or more additional capillaries nested within, and bonded to an inner surface of a larger capillary at a location aligned with a peripheral location at which the primary capillary is bonded to an inner surface of the outer jacket of the optical fibres. In some examples, the ratio D / A of the hollow core diameter D and the guided wavelength A is less than or equal to 16, and / or the hollow core diameter D is 25 pm or less; this is a small core regime. In other examples, the optical fibre has a larger, more conventional core size, such that D / A is greater than 17, and / or D is greater than 25 pm. For either a small core or a large core, in some examples the plurality of primary capillaries in the cladding comprises a maximum of four primary capillaries (such as no fewer than four primary capillaries or no fewer than three primary capillaries), and in other examples, the plurality of primary capillaries in the cladding may comprise five or more primary capillaries.

[0080] Figure 13 shows a transverse cross-sectional view of a hollow core optical fibre according to the present disclosure, as a first example having more than four primary capillaries. The fibre 10 comprises the same features (outer jacket 12, hollow core, and cladding capillary cells 15) as the examples of Figures 7 and 8, except that five primary capillaries 14 (hence five cells 15) are included. As with these examples, the fibre has a triple NANF structure, so each primary capillary 14 contains three additional capillaries, nested within it, namely a smallest quaternary capillary 24 inside a larger tertiary capillary 22 inside a larger secondary capillary 20, which is inside the primary capillary 14. Each additional capillary 20, 22, 24 is bonded to the inner surface of the immediately larger capillary at a position which is in line with the peripheral location 16 at which the primary capillary 14 is bonded to the inner surface of the outer jacket 12. As with the 3-cell and 4-cell TNANF examples, further still smaller additional capillaries might be included (not shown), and / or elements in the form of solid rods or capillaries might be included in the interstitial spaces 17 to reduce the volume of the interstitial spaces 17, and / or the inner surface of the outer jacket 12 might be shaped to reduce the volume of the interstitial spaces 17.

[0081] Figures 14 and 15 show transverse cross-sectional views of hollow core optical fibres according to the present disclosure, as further examples having more that four primary capillaries. Figure 14 shows an example fibre 10 with six primary capillaries 14 (and therefore six cells 15), and Figure 15 shows an example fibre 10 with eight primary capillaries 14 (and therefore eight cells 15). Examples with seven primary capillaries are not excluded, although not depicted. As with the Figure 13 example, these examples have a TNANF structure, and the remarks made above for Figure 13 apply also here.

[0082] 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.

[0083] Further particular and preferred aspects of the invention are set out in the accompanying independent and dependent clauses. Features of the dependent clauses may be combined with those of the independent clauses and independent claims as appropriate and in combinations other than those explicitly set out in the clauses and claims.

[0084] A1 . An antiresonant hollow core optical fibre comprising: a tubular outer jacket; a cladding comprising a maximum of four primary capillaries arranged in a ring and each bonded to an inner surface of the outer jacket at a peripheral location around the outer jacket, and three or more additional capillaries nested within each primary capillary such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary; and a hollow core formed by a central void within the ring of primary capillaries, the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the hollow core has a diameter D of 25 pm or less. A2. An antiresonant hollow core optical fibre according to clause A1 , wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of A pm in a fundamental core mode, and the diameter D of the hollow core is such that D / A is less than or equal to 16.

[0085] A3. An antiresonant hollow core optical fibre according to clause A1 or clause A2, comprising no fewer than four primary capillaries.

[0086] A4. An antiresonant hollow core optical fibre according to clause A1 or clause A2, comprising no fewer than three primary capillaries.

[0087] A5. An antiresonant hollow core optical fibre according to any one of clauses A1 to A4 claim, wherein each primary capillary is spaced apart from adjacent primary capillaries around the ring of primary capillaries.

[0088] A6. An antiresonant hollow core optical fibre according to clause A5, wherein the primary capillaries are spaced apart by gaps sized to provide effectively single mode operation for light of the wavelength A pm.

[0089] A7. An antiresonant hollow core optical fibre according to clause A6, wherein the primary capillaries are spaced apart by gaps sized to provide multimode operation for light of the wavelength A pm.

[0090] A8. An antiresonant hollow core optical fibre according to any one of clauses A1 to A7, configured such that the fundamental core mode has a mode field diameter in the range of 7 pm to 17.5 pm.

[0091] A9. An antiresonant hollow core optical fibre according to any one of clauses A1 to A8, wherein the outer jacket has an outside diameter of substantially 125 pm.

[0092] A10. An antiresonant hollow core optical fibre according to any one of clauses A1 to A9, wherein interstitial spaces between the outer jacket and the primary capillaries have a reduced volume compared to a volume of the interstitial spaces in a configuration where the inner surface of the outer jacket has a circular transverse cross-section.

[0093] A11. An antiresonant hollow core optical fibre according to clause A10, wherein the inner surface of the outer jacket has a non-circular transverse cross-section.

[0094] A12. An antiresonant hollow core optical fibre according to clause A10 or clause A11 , further comprising elements located within the interstitial spaces to reduce the volume of the interstitial spaces, the elements being bonded to the inner surface of the outer jacket.

[0095] A13. An antiresonant hollow core optical fibre according to clause A12, wherein the elements comprise capillaries extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0096] A14. An antiresonant hollow core optical fibre according to clause A13, wherein the elements comprise solid rods extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0097] B1 . An antiresonant hollow core optical fibre comprising: a tubular outer jacket; a cladding comprising a maximum of four primary capillaries arranged in a ring and each bonded to an inner surface of the outer jacket at a peripheral location around the outer jacket; and a hollow core formed by a central void within the ring of primary capillaries, the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of A pm in a fundamental core mode, each primary capillary has a wall thickness and all the primary capillaries have the same wall thickness, and the hollow core has a diameter of D pm such that D / A is less than or equal to 16.

[0098] B2. An antiresonant hollow core optical fibre according to clause B1 , wherein the hollow core has a diameter of 25 pm or less.

[0099] B3. An antiresonant hollow core optical fibre according to clause B1 or clause B2, comprising no fewer than four primary capillaries.

[0100] B4. An antiresonant hollow core optical fibre according to clause B1 or clause B2, comprising no fewer than three primary capillaries.

[0101] B5. An antiresonant hollow core fibre according to any one of clauses B1 to B4, further comprising one or more additional capillaries nested within each primary capillary such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary.

[0102] B6. An antiresonant hollow core optical fibre according to clause B5, comprising one or two additional capillaries nested within each primary capillary.

[0103] B7. An antiresonant hollow core optical fibre according to clause B5, comprising three or more additional capillaries nested within each primary capillary.

[0104] B8. An antiresonant hollow core optical fibre according to any one of clauses B5 to B7, wherein each of the one or more additional capillaries has a wall thickness the same as the wall thickness of the corresponding additional capillaries nested in the other primary capillaries.

[0105] B9. An antiresonant hollow core optical fibre according to any one of clauses B5 to B7, wherein each of the one or more additional capillaries has a thickness which is the same as the wall thickness of the primary capillaries. B10. An antiresonant hollow core optical fibre according to any one of clauses B1 to B9, wherein each primary capillary is spaced apart from adjacent primary capillaries around the ring of primary capillaries.

[0106] B11 . An antiresonant hollow core optical fibre according to clause B10, wherein the primary capillaries are spaced apart by gaps sized to provide effectively single mode operation for light of the wavelength A pm.

[0107] B12. An antiresonant hollow core optical fibre according to clause B10, wherein the primary capillaries are spaced apart by gaps sized to provide multimode operation for light of the wavelength A pm.

[0108] B13. An antiresonant hollow core optical fibre according to any one of clauses B1 to B12, configured such that the fundamental core mode has a mode field diameter in the range of 7 pm to 17.5 pm.

[0109] B14. An antiresonant hollow core optical fibre according to any one of clauses B1 to B13, wherein the outer jacket has an outside diameter of substantially 125 pm.

[0110] B15. An antiresonant hollow core optical fibre according to any one of claims B1 to B14, wherein interstitial spaces between the outer jacket and the primary capillaries have a reduced volume compared to a volume of the interstitial spaces in a configuration where the inner surface of the outer jacket has a circular transverse cross-section.

[0111] B16. An antiresonant hollow core optical fibre according to clause B15, wherein the inner surface of the outer jacket has a non-circular transverse cross-section.

[0112] B17. An antiresonant hollow core optical fibre according to clause B15 or clause B16, further comprising elements located within the interstitial spaces to reduce the volume of the interstitial spaces, the elements being bonded to the inner surface of the outer jacket.

[0113] B18. An antiresonant hollow core optical fibre according to clause B17, wherein the elements comprise capillaries extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0114] B19. An antiresonant hollow core optical fibre according to clause B17, wherein the elements comprise solid rods extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0115] C1 . An antiresonant hollow core optical fibre comprising: a tubular outer jacket; a cladding comprising a maximum of four primary capillaries arranged in a ring and each bonded to an inner surface of the outer jacket at a peripheral location around the outer jacket; and a hollow core formed by a central void within the ring of primary capillaries, the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the hollow core has a diameter D of 10 pm or less.

[0116] C2. An antiresonant hollow core optical fibre according to clause C1 , wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of A pm in a fundamental core mode, and the diameter D of the hollow core is such that D / A is less than or equal to 16.

[0117] C3. An antiresonant hollow core optical fibre according to clause C1 or clause C2, comprising no fewer than four primary capillaries.

[0118] C4. An antiresonant hollow core optical fibre according to clause C1 or clause C2, comprising no fewer than three primary capillaries.

[0119] C5. An antiresonant hollow core fibre according to any one of claims C1 to C4, further comprising one or more additional capillaries nested within each primary capillary such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary.

[0120] C6. An antiresonant hollow core optical fibre according to clause C5, comprising one or two additional capillaries nested within each primary capillary.

[0121] C7. An antiresonant hollow core optical fibre according to clause C5, comprising three or more additional capillaries nested within each primary capillary.

[0122] C8. An antiresonant hollow core optical fibre according to any one of claims C1 to C7, wherein each primary capillary is spaced apart from adjacent primary capillaries around the ring of primary capillaries.

[0123] C9. An antiresonant hollow core optical fibre according to clause C8, wherein the primary capillaries are spaced apart by gaps sized to provide effectively single mode operation for light of the wavelength A pm.

[0124] C10. An antiresonant hollow core optical fibre according to clause C8, wherein the primary capillaries are spaced apart by gaps sized to provide multimode operation for light of the wavelength A pm.

[0125] C11. An antiresonant hollow core optical fibre according to any one of clauses C1 to C10, configured such that the fundamental core mode has a mode field diameter in the range of 7 pm to 17.5 pm.

[0126] C12. An antiresonant hollow core optical fibre according to any one of clauses C1 to C11 , wherein the outer jacket has an outside diameter of substantially 125 pm.

[0127] C13. An antiresonant hollow core optical fibre according to any one of clauses C1 to C12, wherein interstitial spaces between the outer jacket and the primary capillaries have a reduced volume compared to a volume of the interstitial spaces in a configuration where the inner surface of the outer jacket has a circular transverse cross-section.

[0128] C14. An antiresonant hollow core optical fibre according to clause C13, wherein the inner surface of the outer jacket has a non-circular transverse cross-section.

[0129] C15. An antiresonant hollow core optical fibre according to clause C13 or clause C14, further comprising elements located within the interstitial spaces to reduce the volume of the interstitial spaces, the elements being bonded to the inner surface of the outer jacket.

[0130] C16. An antiresonant hollow core optical fibre according to clause C15, wherein the elements comprise capillaries extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0131] C17. An antiresonant hollow core optical fibre according to clause C15, wherein the elements comprise solid rods extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

[0132] REFERENCES

[0133] [1] TD Bradley, JR Hayes, Y Chen, GT Jasion, SR Sandoghchi, R Slavik, EN Fokoua, S Bawn, H Sakr, IA Davidson, A Taranta, JP Thomas, MN Petrovich, DJ Richardson, F Poletti, “Record Low-Loss 1.3 dB / km Data Transmitting Antiresonant Hollow Core Fibre” in Proc European Conference on Optical Communications (ECOC) 2018, paper Th3F2

[0134] [2] Francesco Poletti, "Nested antiresonant nodeless hollow core fiber," Opt. Express 22, 23807-23828 (2014)

[0135] [3] 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)

[0136] [4] WO 2015 / 185761 A1

[0137] [5] David Bird, "Attenuation of model hollow-core, anti-resonant fibres," Opt. Express 25, 23215-23237 (2017)

[0138] [6] 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

[0139] [7] Xingtao Zhao et al, “Adjacent nested 4-tube hollow-core anti-resonant fiber”, Opt. Communications 522, 128631 (2022) [8] Yuxi Wang and Wonkeun Chang, “Multi-nested antiresonant hollow-core fiber with ultralow loss and single-mode guidance”, Opt. Express 31 , 18250-18264 (2023)

[0140] [9] Seyedmohammad Abokhamis Mousavi, Seyed Reza Sandoghchi, David J. Richardson, and Francesco Poletti, "Broadband high birefringence and polarizing hollow core antiresonant fibers", Opt. Express 24, 22943-22958 (2016)

[0141]

[0010] Yi-feng Hong et al, “Highly Birefrigent Anti-Resonant Hollow-Core Fiber with a BiThickness Fourfold Semi-Tube Structure”, Laser & Photonics Reviews https: / / doi.Org / 10.1002 / lpor.202100365 (2022)

[0142]

[0011] Shou-Fei Gao et al, “Hollow-core negative-curvature fiber for UV guidance”, Optics Letters 43, 1347-1350 (2018)

Claims

CLAIMS1 . An antiresonant hollow core optical fibre comprising: a tubular outer jacket; a cladding comprising a plurality of primary capillaries arranged in a ring and each bonded to an inner surface of the outer jacket at a peripheral location around the outer jacket, and three or more additional capillaries nested within each primary capillary such that each additional capillary is bonded to an inner surface of a larger capillary at a location aligned with the peripheral location of the primary capillary; and a hollow core formed by a central void within the ring of primary capillaries, the hollow core having a radius which is the shortest distance from a central longitudinal axis of the hollow core optical fibre to an outer surface of a primary capillary and a diameter which is twice the radius; wherein the primary capillaries are configured for the hollow core optical fibre to guide light of a wavelength of A pm in a fundamental core mode, and the hollow core has a diameter of D pm.

2. An antiresonant hollow core optical fibre according to claim 1 , wherein D / A is less than or equal to 16.

3. An antiresonant hollow core optical fibre according to claim 1 or claim 2, wherein the hollow core has a diameter of 25 pm or less.

4. An antiresonant hollow core optical fibre according to any one of claims 1 to 3, wherein the plurality of primary capillaries comprises a maximum of four primary capillaries.

5. An antiresonant hollow core optical fibre according to claim 4, comprising no fewer than four primary capillaries.

6. An antiresonant hollow core optical fibre according to claim 4, comprising no fewer than three primary capillaries.

7. An antiresonant hollow core optical fibre according to any one of claims 1 to 3, wherein the plurality of primary capillaries comprises five or more primary capillaries.

8. An antiresonant hollow core optical fibre according to any preceding claim, wherein each primary capillary is spaced apart from adjacent primary capillaries around the ring of primary capillaries.

9. An antiresonant hollow core optical fibre according to claim 8, wherein the primary capillaries are spaced apart by gaps sized to provide effectively single mode operation for light of the wavelength A pm.

10. An antiresonant hollow core optical fibre according to claim 8, wherein the primary capillaries are spaced apart by gaps sized to provide multimode operation for light of the wavelength A pm.

11. An antiresonant hollow core optical fibre according to any preceding claim, configured such that the fundamental core mode has a mode field diameter in the range of 7 pm to 17.5 pm.

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

13. An antiresonant hollow core optical fibre according to any preceding claim, wherein interstitial spaces between the outer jacket and the primary capillaries have a reduced volume compared to a volume of the interstitial spaces in a configuration where the inner surface of the outer jacket has a circular transverse cross-section.

14. An antiresonant hollow core optical fibre according to claim 13, wherein the inner surface of the outer jacket has a non-circular transverse cross-section.

15. An antiresonant hollow core optical fibre according to claim 13 or claim 14, further comprising elements located within the interstitial spaces to reduce the volume of the interstitial spaces, the elements being bonded to the inner surface of the outer jacket.

16. An antiresonant hollow core optical fibre according to claim 15, wherein the elements comprise capillaries extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

17. An antiresonant hollow core optical fibre according to claim 15 or claim 16, wherein the elements comprise solid rods extending along the hollow core optical fibre and bonded to the inner surface of the outer jacket at locations between the peripheral locations of the primary capillaries.

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