Antiresonant hollow core fiber without rotational symmetry

Abstract

An antiresonant hollow core fiber (700) is described. The fiber comprises an outer cladding (704) and an inner cladding comprising a plurality of primary cladding capillaries (710), each primary cladding capillary bonded to an inner wall (706) of the outer cladding (704) at a peripheral location around the circumference of the inner wall. The ring of primary capillaries defines a hollow core (106) formed by a central void within the ring of primary cladding capillaries (710). One of the outer cladding and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said one of the outer cladding and inner cladding.

Description

ANTIRESONANT HOLLOW CORE FIBER WITHOUT ROTATIONAL SYMMETRYBACKGROUND

[0001] Splicing is typically used to join optical fibers and there are various splicing methods that may be used, including fusion splicing and mechanical splicing. Splicing involves aligning the fibers end-to-end and fixing them in the aligned position. Fusion splicing, for example joins the two fibers by heating the end region in order to soften the glass from which the fibers are made. By pressing the ends together, the softened glass is made to fuse so that the fibers are permanently connected when the glass cools and hardens. Mechanical splicing, in contrast, does not permanently join the fibers together but instead uses a mechanical arrangement to maintain their aligned position and hold the fiber ends together.

[0002] The joint formed by splicing two fibers together is referred to as a splice. The quality of the splice is an important factor in enabling low loss optical propagation for light travelling from one fiber to the other. Accurate alignment of structural features within the two fibers so as to reduce structural discontinuities at the splice contributes to low loss.

[0003] Conventional solid core optical fibers, comprising an annular cladding surrounding a single circular core, are relatively simple to align for splicing. The structures have continuous rotational symmetry in transverse cross-section so that transverse alignment of the fiber ends to match the positions of the longitudinal axes of the fibers necessarily aligns the cores and the cladding. However, antiresonant hollow core fibers have a complex internal structure that lacks continuous rotational symmetry, similar to multi-core solid core fibers which also lack continuous rotational symmetry’. Misalignment of this internal structure and / or of the individual cores increases the optical loss that occurs when light travels from one fiber to the other across the splice.

[0004] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known antiresonant hollow core fibers and methods of manufacturing such fibers.SUMMARY

[0005] The following presents a simplified summary' of the disclosure in order to provide a basic understanding to the reader. This summary is not intended to identify key features or essential features of the claimed subject matter nor is it intended to be used to limit the scope of the claimed subject matter. Its sole purpose is to present a selection ofconcepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0006] An antiresonant hollow core fiber is described. The fiber comprises an outer cladding and an inner cladding comprising a plurality of primary cladding capillaries, each primary cladding capillary bonded to an inner wall of the outer cladding at a peripheral location around the circumference of the inner wall. The ring of primary capillaries defines a hollow core formed by a central void within the ring of primary cladding capillaries. One of the outer cladding and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said one of the outer cladding and inner cladding.

[0007] Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings.DESCRIPTION OF THE DRAWINGS

[0008] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein:FIGs. 1-3 show transverse cross-sectional views of three different examples of antiresonant hollow' core fibers;FIG. 4 shows a plot of the splice loss (on the y-axis) as a function of rotational misalignment (on the x-axis) for an ARE with a slightly imperfect five-fold rotational symmetry which has been spliced to itself;FIG. 5 show's an arrangement for end-view observation in a splicer;FIG. 6 show s a plot of the cross-talk or intermodal interference (IMI) into the first higher order mode group (on the y-axis), as a function of rotational misalignment of the two fibers (on the x-axis) for an ARF;FIGs. 7-12 show transverse cross-sectional view s of six different examples of ARFs that include a marker that is formed integrally with the structure of the fiber;FIGs. 13 and 14 are flow diagrams of two example methods of fabricating a jacket tube with an embedded marker; andFIGs. 15, 16, 17 and 18 are schematic diagrams showing the fabrication of ajacket tube with an embedded marker according to the method of FIG. 14.Like reference numerals are used to designate like parts in the accompanying drawings.DETAILED DESCRIPTION

[0009] The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present examples are constructed or utilized. The description sets forth the functions of the examples and the sequence of operations for constructing and operating the examples. However, the same or equivalent functions and sequences may be accomplished by different examples.

[0010] FIGs. 1-3 show transverse cross-sectional views of three different examples of antiresonant hollow core fibers (ARFs). Light is guided in these fibers by an antiresonant optical effect. Each of the fibers 100, 200, 300 comprises a tubular outer cladding (or jacket) 102, a structured, inner, cladding comprising a plurality of tubular cladding capillaries 104, 204, 304 and a hollow core 106. The outer cladding 102 has a glass thickness that is typically much larger than that of the cladding capillaries 104, 204, 304. In the first example, shown in FIG. 1, the structured, inner, cladding comprises five capillaries 104 of the same cross- sectional size and shape, which are arranged inside the outer cladding 102 in a single ring so that the longitudinal axes of each cladding capillary 104 and of the outer cladding 102 are substantially parallel. Each cladding capillary 104 is in contact with (e g. bonded to) the inner surface of the outer cladding 102 at an azimuthal location 108, such that the cladding capillaries 104 are evenly spaced around the inner circumference of the outer cladding 102, and are also spaced apart from each other by gaps 110 (i.e. such that there is no contact between neighbouring capillaries). In some designs of ARF, the cladding capillaries 104 may be positioned in contact with each other (in other words, not spaced apart as in FIG. 1), but spacing to eliminate this contact can improve the fiber’s optical performance. The gaps 110 remove nodes that arise at the contact points between adjacent tubes and which tend to cause undesirable resonances that result in high propagation losses. Accordingly, fibers with spaced-apart cladding capillaries may be referred to as ‘modeless antiresonant hollow core fibers”.

[0011] The arrangement of the cladding capillaries 104 in a ring around the inside of the tubular outer cladding 102 creates a central space, cavity, or void within the fiber, also with its longitudinal axis parallel to those of the outer cladding 102 and the cladding capillaries 104, which is the fiber’s hollow core 106. The hollow core 106 is bounded by the inwardly facing parts of the outer surfaces of the cladding capillaries 104. 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 cladding capillaries 104 have a thickness, t, at the core boundary which defines the wavelength for which antiresonant optical guiding occurs in the ARF.

[0012] In the second example, shown in FIG. 2, each primary cladding capillary 104 has a secondary, smaller capillary 204 nested inside it, bonded to the inner surface of the primary cladding capillary 104, in this example at the same azimuthal location 108 as the point of bonding between the primary cladding capillary’ 104 and the outer cladding 102 (although in other examples, they may be bonded at different azimuthal locations). These additional smaller capillaries 204 can reduce the optical loss. ARF designs of this type, with secondary’ capillaries, may be referred to as "nested antiresonant nodeless fibers’' (NANFs) (TM).

[0013] The third example, shown in FIG. 3. has two smaller cladding capillaries 204, 304 nested inside each cladding capillary 104. As with the example shown in FIG. 2, each of the smaller capillaries 204, 304 is bonded to the inner surface of the immediately larger capillary at the same azimuthal location as the point of bonding between the primary cladding capillary 104 and the outer cladding 102. In this example, the smaller capillary 304 may be referred to as the secondary cladding capillary and the smallest capillary 204 may be referred to as the tertiary cladding capillary’. The tertiary cladding capillary 204 is bonded to the inner surface of the secondary’ cladding capillary’ 304 and the secondary’ cladding capillary’ 304 is bonded to the inner surface of the primary cladding capillary 104. ARF designs of this type, with secondary and tertiary cladding capillaries may be referred to as “double-nested antiresonant nodeless fibers” (DNANFs). In yet further examples (not shown in the drawings) there may be a different configuration of cladding capillaries. For example, there may be smaller further capillaries, within the tertiary’ capillary 204 to provide further levels of nesting and / or there may be a plurality of secondary cladding capillaries within each primary cladding capillary, each secondary cladding capillary being bonded to the inner surface of the primary cladding capillary’ at a different azimuthal location and / or each primary cladding capillary may have an internal structure (e.g. one or more dividing walls).

[0014] All of the examples shown in FIGs. 1-3 comprise five primary cladding capillaries 104 and hence have five-fold rotational symmetry. Furthermore, all the cladding capillaries are circular in cross-section. In other examples, there may be a different number of primary’ cladding capillaries surrounding the core (e.g. four, six, seven, eight, nine or ten) and / or the cladding capillaries may not be of circular cross-section. Additionally, whilst in the examples of FIGs. 1-3. all the primary cladding capillaries 104 are of the same size andshape, in other examples, the primary cladding capillaries within the outer cladding 102 may not all be the same size and / or shape.

[0015] Regardless of the precise details of the structure, it will be apparent from FIGs. 1-3 and the description above that an ARF lacks continuous circular symmetry. When splicing two lengths of ARF together, any misalignment between the cladding capillaries (which may be referred to collectively as the microstructure of the fiber) will increase the optical propagation loss of the splice, as shown in FIG. 4.

[0016] FIG. 4 shows a plot of the splice loss (on the y-axis) as a function of rotational misalignment (on the x-axis) for an ARF with five-fold rotational symmetry (e.g. as shown in FIGs. 1-3) which has been spliced to itself (e.g. a length of the ARF has been broken into two lengths and then spliced back together). The five-fold symmetry of the ARF shown in FIG. 4 is slightly imperfect (e.g. due to manufacturing variations) as otherwise the loss curve would be sinusoidal. The loss attains local minima 401-405 when the internal microstructure (e.g. the capillaries 104, 204, 304) of the tw o fibers is aligned and is maximized (and i.e. worst) when the capillaries 104, 204, 304 in one of the two fibers is aligned with the gaps 110 between the capillaries in the other of the tw o fibers. A maximum splice loss in excess of 0.15dB exceeds the propagation loss incurred over lengths of greater than a kilometer of high-performance fiber and so typically cannot be tolerated in a system. Alignment when splicing two lengths of ARF together therefore requires careful rotational adj ustment about the longitudinal axis of the fiber in order to bring the structural features into the same orientation, and this can be challenging and time-consuming to achieve.

[0017] To achieve the lowest loss (e.g. local minimum 401), particularly when splicing together two lengths of fiber that were formed from the same preform or cane, the internal microstructure needs to be rotationally aligned such that a particular set of one or more capillaries 306 A in the first fiber is aligned with the corresponding set of one or more capillaries 306A in the second fiber. If instead, the first set of one or more capillaries 306A in the first fiber is aligned with one of the other, nominally identical, sets of one or more capillaries 306B-E in the second fiber, the loss is slightly higher (local minima 402-405). This may be a consequence of slight differences in the local microstructure caused by tiny differences in the original tubes used to fabricate the preform or cane and / or tiny differences as a result of the manufacturing process (e.g. in the precise cross-sectional shape and / or position of the capillaries).

[0018] Most existing splicers (or splicing apparatus) are not designed for use with ARF and as a result cannot perform the necessary azimuthal alignment to produce a low losssplice when splicing ARF. The rare examples of splicers that can perform azimuthal alignment, and hence are more suitable for splicing ARF, involve observation or detection of the end facets with a camera and an example arrangement is shown in FIG. 5. To perform end-view observation, a mirror 502 is temporarily inserted between the end-facets 504, 506 that are to be spliced. A light source 508 couples light into the fiber 510 to illuminate the endfacet 504 and it is imaged by a camera 512 via the mirror 502 and a lens 514. The fiber 510 can then be rotated into a pre-defined orientation and the process is then repeated for the second fiber 516 using a second light source 518 (to couple light into the second fiber 516) and either a second mirror (not shown in FIG. 5) or by rotating the mirror 502 through 90°. Such apparatus is slow, costly, delicate and bulky (e.g. because of the moving mirror) and therefore largely suitable only for interior laboratory and clean-room use rather than use in the field. Often, however, fiber splicing needs to be performed in the field such as during the installation of optical fiber communications networks. Applications of this type require equipment which is preferably robust, portable, simple, and quick to operate (e.g. since there may be 60 or more optical fibers in any cable). This implies that the fibers are only viewed from the side, as then there is no need for a moving mirror 502. Where side-viewing is used, a camera images the side of the fiber (e.g. at an angle perpendicular to the longitudinal axis of the fiber) and there is typically a light source on the opposite side of the fiber to the camera. Even where end-viewing is used, it is not possible to visually distinguish between the different sets of one or more capillaries 306A-E since the differences are very small and the available magnification on a splicer and / or resolution of the camera is too low.

[0019] Described herein is an antiresonant hollow core fiber (ARF) that includes one or more markers formed integrally with the structure of the fiber. Also described herein are methods for manufacturing an ARF that includes one or more markers. The markers are formed from a rod or tube (or part thereof of a different refractive index and / or thickness than surrounding or proximate material. The markers formed from a rod or tube of a different refractive index from surrounding or proximate material or formed from a tube filled with a gas of a different refractive index from surrounding or proximate material are visible when the fiber is illuminated from the side (e.g. as a consequence of total internal reflection of light within the marker). This enables the ARF with the marker to be rotationally aligned (e.g. on a splicer w ith a second ARF with a marker) using side illumination only and w ithout requiring end-viewing or complex image processing techniques. Where the marker is formed from a tube having a different thickness from surrounding or proximate tubes but having the same refractive index as those surrounding or proximate tubes (and not being filled with a gashaving a different refractive index), the marker may be visible when illuminated from the side and the marker is also visible when viewed from the end (e.g. without requiring the same very high levels of magnification or camera resolution that is required to detect manufacturing differences).

[0020] Including the one or more markers within the ARF also disrupts the rotational symmetry’ of the ARF (which as described above is not continuous but n-fold in the absence of a marker). This lack of rotational symmetry means that there is only one azimuthal orientation in which two lengths of ARF can be aligned (i.e. when using the markers). Referring back to FIG. 4, this ensures that the fibers are aligned at the smallest of all the local minima, 401. As well as reducing the splice loss, this also reduces the excitation of the first higher order mode group. FIG. 6 shows a plot of the cross-talk or cross-coupling into the first higher order mode group (on the y-axis), as a function of rotational misalignment of two fibers (on the x-axis). The first higher order mode group is the most detrimental as it contributes the most to intermodal interference (IMI) at the end of a length of fiber (because it has the lowest loss of all higher order modes and receives the most power from the fundamental mode as light propagates down the fiber). The IMI is essentially noise, which if too high, results in the fiber not being able to transmit data. As shown in FIGs. 4 and 6, if a uniquely identifiable capillary’ (or set of capillaries) is aligned with itself (i.e. the rotational misalignment between the two fibers is zero), the loss is kept low and the higher order mode excitation is simultaneously minimized.

[0021] FIGs. 7-12 show transverse cross-sectional views of six different examples of ARFs that include a marker that is formed integrally with the structure of the fiber. In FIGs. 7-12 the outer cladding (or jacket) 704 is shown with a more representative thickness than the examples shown in FIGs. 1-3. In all these examples, the internal microstructure, excluding the marker, is the same as that shown in FIG. 3 and described above (i.e. a DNANF with fivefold symmetry); however, it will be appreciated that the markers may be included in fibers with different internal microstructure, e.g. with or without nesting, and with different numbers of sets of capillaries (i.e. different degrees of rotational symmetry were it not for the presence of the marker). Furthermore, whilst the examples shown each include only a single marker, in other examples, more than one marker may be included provided that the combination and position of the markers means that there is no rotational symmetry’ of the resulting ARF.

[0022] FIG. 7 shows a transverse cross-sectional view of a first example ARF 700 including a marker 702. In this example, the marker 702 is a solid rod that is fused to theinner wall 706 of the outer cladding 704 between two sets of nested capillaries. The marker 702 may have a refractive index that different from that of the outer cladding 704, i.e. the marker 702 may have a refractive index that is higher or lower than that of the outer cladding 704.

[0023] FIG. 8 shows a transverse cross-sectional view of a second example ARF 800 including a marker 802. This example has the marker 802 in the same place as the marker 702 in FIG. 7; however, in this example, the marker 802 is a tube that is fused to the inner wall 706 of the outer cladding 704 between two sets of nested capillaries. The marker 802 (i.e. the material forming the tube wall) may have a refractive index that is different from (i.e. higher or lower than) that of the outer cladding 704.

[0024] The first and second example ARFs 700. 800 shown in FIGs. 7 and 8 can be easily fabricated, with the rod / tube forming the marker 702, 802 being fused to the inner wall 706 of the outer cladding 704 when forming the preform from which the fiber is drawn, in the same way that the largest of the cladding capillaries 710 (the primary cladding capillary) is fused to the inner wall 706 of the outer cladding 704. Furthermore, as the marker 702, 802 is physically separated from the cladding capillaries 708-710. the marker does not affect the optical properties of the ARF.

[0025] FIG. 9 shows a transverse cross-sectional view of a third example ARF 900 including a marker 902. In this example, the marker 902 is a solid rod that is embedded in the outer cladding 704. The marker 902 has a refractive index that is different from (i.e. higher or lower than) that of the rest of the outer cladding 704.

[0026] FIG. 10 shows a transverse cross-sectional view of a fourth example ARF 1000 including a marker 1002. This example has the marker 1002 in the same place as the marker 902 in FIG. 9; however, in this example, the marker 1002 is a cylindrical hole, with its long axis parallel to the long axis of the ARF (and perpendicular to the plane of the diagram in FIG. 10) in the outer cladding 704. The hole in the marker 1002 may be filled with air or another gas.

[0027] The third and fourth example ARFs 900. 1000 shown in FIGs. 9 and 10 can be fabricated by embedding the marker in the jacket tube that is then assembled to form the preform from which the fiber is drawn. Two example methods of assembly of such a jacket tube that forms an outer cladding with an embedded marker are shown in FIGs. 13-18 and described below. As the marker 902, 1002 is physically separated from the cladding capillaries 708-710, the marker does not affect the optical properties of the ARF.

[0028] FIGs. 11 and 12 show a transverse cross-sectional view of a fifth and a sixth example ARF 1100, 1200 including a marker 1102, 1202. In these examples, the marker 1 102, 1202 is one of the cladding capillaries: however, the refractive index and / or thickness of the marker capillary is different from the other cladding capillaries. In some examples, both the refractive index and the thickness may be different so that the optical thickness of the marker capillary is the same as the other cladding capillaries whilst the physical thickness and refractive index are both different from the other cladding capillaries. In the example shown in FIG. 11, the marker capillary is the secondary cladding capillary in a nested set of cladding capillaries (i.e. the middle-sized capillary' of the three capillaries) and in the example shown in FIG. 12, the marker capillary' is the tertiary' capillary' (i.e. the smallest of the cladding capillaries) in a nested set of cladding capillaries. In other examples, where there is a greater or lesser degree of nesting, the marker capillary may be any cladding capillary in a nested set of cladding capillaries except for the primary' (i.e. largest and hence outer) cladding capillary 710. This is because if the marker capillary' was the primary cladding capillary' in a nested set of cladding capillaries, the optical properties of the ARF would potentially be degraded (e.g. the loss would be higher). In yet further examples, there may be more than one marker capillary w ithin the same nested set of cladding capillaries (e.g. both the secondary' and tertiary' capillaries may be marker capillaries); however, again the primary' capillary' in the nested set of capillaries is not a preferable marker capillary. Having multiple marker capillaries within the same nested set of capillaries may increase the visibility of the marker when viewed from the side.

[0029] The fifth and sixth example ARFs 1100, 1200 can be more easily fabricated than the third and fourth examples described above as no change is required to the jacket tube. The fifth and sixth example ARFs can be fabricated in the same way as an equivalent ARF without a marker; however, the tubes that are selected to form the preform for the ARF will be different. Unlike the first to fourth examples, in the fifth and sixth examples, the marker 1102, 1202 is not physically separated from the cladding capillaries; however as described above, to reduce the chance of the marker affecting the optical properties of the ARF. the marker is not the primary cladding capillary in a nested set of capillaries. If, therefore, the technique shown in the fifth and sixth examples is used where there is no nesting (e.g. as show n in FIG. 1 and described above), it may result in a degradation of the optical properties of the fiber, as in this implementation the marker capillary will be a primary cladding capillary.

[0030] An ARF with an embedded marker, as described herein, can be fabricated using known methods for making ARFs. ARFs can be drawn in a conventional manner from a preform, optionally via an intermediate cane, configured with the transverse cross-sectional structure desired for the finished fiber (e.g. as shown in FIGs. 7-12) but on a larger scale. The known reduction in cross-sectional area from a preform to a finished fiber can be used to appropriately scale up the dimensions required for the resultant ARF with an embedded marker in order to construct, from suitably sized tubes or rods, preforms from which the ARFs described herein can be fabricated.

[0031] The fibers may be made from materials known for the fabrication of existing designs of ARF, such as glass materials (e.g. silica), and polymer materials. The various tubes or capillaries (outer jacket and primary and secondary capillaries) in a single preform or fiber may be made from the same material or from different materials. Types of glass include "silicate glasses" or "silica-based glasses”, based on the chemical compound silica (silicon dioxide, or quartz), of which there are many examples. Other glasses suitable for optical applications include, but are not limited to, chalcogenide, tellurite glasses, fluoride glasses, and doped silica glasses. The materials may include one or more dopants for the purpose of tailoring the optical properties, such as modifying absorption / transmission or enabling optical pumping.

[0032] FIG. 13 is a flow diagram of a first example method of fabricating a jacket tube with an embedded marker. This jacket tube may then be used to fabricate a preform from which an ARF 900, 1000, as shown in FIG. 9 or 10, can be drawn. The method comprises selecting a jacket tube (block 1302) and then forming a cylindrical hole in the jacket tube (block 1304). The cylindrical hole is formed parallel to the longitudinal axis of the jacket tube (e.g. as shown in the cross-section in FIGs. 9 and 10) and may, for example, be formed by drilling. The resultant jacket tube is then ready to assemble into a preform for the ARF 1000 shown in FIG. 10. To fabricate ajacket tube for a preform for the ARF 900 shown in FIG. 9, an additional step is required that comprises filling the cylindrical hole with the material that forms the marker (block 1306), e.g. by inserting a cylindrical rod of material having a different refractive index from the jacket tube into the hole (e.g. a rod having a higher or low er refractive index than the jacket tube).

[0033] FIG. 14 is a flow diagram of a second example method of fabricating ajacket tube with an embedded marker and this method can be described with reference to the schematic diagrams shown in FIGs. 15 and 16. The jacket tube fabricated using the method of FIG. 14 may be used to fabricate a preform from which an ARF 900, 1000, as shown inFIG. 9 or 10, can be drawn. The method of FIG. 14 comprises selecting both an inner jacket tube 1502 and an outer jacket tube 1504, inserting the inner jacket tube 1502 into the outer jacket tube 1504 (block 1402) and then filling the gap between the two tubes with rods 1506, 1508 (block 1404) or a combination of rods 1506 and tubes 1608 (block 1405). As show n in FIG. 15, most of the rods 1506 are formed from the same material as both the inner and outer jacket tubes 1502, 1504 and there is either one (or at least one) rod 1508 that is formed from a material that has a higher refractive index than the other rods and the jacket tubes or one (or at least one) tube 1608. It is this rod or tube 1508, 1608 that will form the embedded marker. Having assembled the rods and the tubes (in block 1404 or 1405), the assembly 1500, 1600 is heated to fuse the rods and tubes together and form the jacket tube with an integral marker (block 1406). A vacuum may be applied when the heat is applied so that the voids 1510 between the rods are filled. The resultant jacket tube is then ready to assemble into a preform for the ARF 900, 1000 shown in FIG. 9 or 10.

[0034] It will be appreciated that the relative sizes of the inner and outer jacket tubes 1502, 1504 and the diameters of the rods 1506, 1508 and tubes 1608 shown in FIGs. 15 and 16 are by way of example only. In other examples, the rods may have a much smaller diameter and / or the thickness of one or both of the inner and outer jacket tubes may be much larger. In addition, or instead, there may be more than one row of rods betw een the inner and outer jacket tubes. FIGs. 17 and 18 show two example assemblies 1700, 1800 in which there are smaller rods and tubes and two rows of rods / tubes between the inner and outer jacket tubes. As described above, having assembled the rods and the tubes (in block 1404 or 1405), the assembly 1700, 1800 is heated to fuse the rods and tubes together and form the jacket tube with an integral marker (block 1406). A vacuum may be applied w hen the heat is applied so that the voids between the rods are filled. The resultant jacket tube is then ready to assemble into a preform for the ARF 900, 1000 shown in FIG. 9 or 10.

[0035] In addition to, or instead of, using the markers to rotationally align fibers, by including more than one marker in a fiber, the markers may be used to uniquely identify a fiber or batch of fibers. This may be achieved by using a unique arrangement of markers for each preform or batch of preforms. This may be particularly suited to the example ARFs shown in FIGs. 9 and 10 where the markers are embedded within the outer cladding. In the example shown in FIG. 17, the jacket tube may be assembled using a unique arrangement of rods 1702 of higher refractive index, with the pattern of these rods uniquely both identifying the fiber preform and providing markers for use in rotationally aligning the fiber for splicing. In the example shown in FIG. 18, the jacket tube may be assembled using a uniquearrangement of tubes 1802, with the pattern of tubes uniquely both identifying the fiber preform and providing markers for use in rotationally aligning the fiber for splicing. Use of a unique marker may enable improved traceability or simplify identification of different types of fibers (e.g. where different internal microstructures correspond to different marker arrangements).

[0036] Although the examples shown in FIGs. 7-12 show nodeless antiresonant hollow core fibers, the techniques described above may also be applied to ARFs which are not nodeless, i.e. fibers where there are not gaps between the primary cladding capillaries that are positioned around the inner circumference of the outer cladding. This means that the primary cladding capillary in any set of cladding capillaries is in contact with the primary cladding capillary in each of the two adjacent sets of cladding capillaries.

[0037] Furthermore, whilst the examples shown in FIGs. 7-12 each comprise five sets of cladding capillaries, the techniques described above are also applicable to ARFs with different numbers of sets of cladding capillaries and there may be one or more cladding capillary in each set.

[0038] In the examples described above, the markers have a higher refractive index than the surrounding material (where the marker is embedded within the outer cladding) or than other cladding capillaries. In further examples, the markers may instead have a lower refractive index.

[0039] By including markers into ARFs as described herein, it is possible to rotationally align two lengths of ARF for splicing by side viewing only. This reduces the time taken for alignment and reduces the complexity of the equipment required to perform the alignment. Furthermore, as the presence of the marker destroys the rotational symmetry of the ARF. it ensures that both the loss and excitation of higher order modes are minimized when the fibers are rotationally aligned.

[0040] In the examples described above, the internal microstructure of the ARF that forms the inner cladding is formed from a plurality of capillaries which are circular in crosssection and are formed by the drawing down of tubes within the preform or cane. In other examples, these capillaries may not have a circular cross-section and instead may have an elliptical or other non-circular cross-section. The capillaries that form the inner cladding comprise elongate holes, voids, lumina, cells or cavities which may have any cross-sectional shape. The walls that form the capillaries may be formed from tubes having any cross- sectional shape and / or portions of tubes.

[0041] Alternatively or in addition to the other examples described herein, further examples are set out in the following clauses.

[0042] Clause A: An antiresonant hollow core fiber comprising: an outer cladding; an inner cladding comprising a plurality of primary cladding capillaries, each primary cladding capillary bonded to an inner wall of the outer cladding at a peripheral location around a circumference of the of the inner wall; and a hollow core formed by a central void within a ring of primary cladding capillaries, wherein one of the outer cladding and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said one of the outer cladding and inner cladding.

[0043] Clause B: An antiresonant hollow core fiber comprising: an outer cladding; an inner cladding comprising a plurality of primary cladding capillaries, each primary cladding capillary bonded to an inner wall of the outer cladding at a peripheral location around a circumference of the of the inner wall; and a hollow core formed by a central void within a ring of primary cladding capillaries, wherein one of the outer cladding and the inner cladding comprises a rod or tube of a different refractive index to the rest of said one of the outer cladding and inner cladding.

[0044] Clause C: The antiresonant hollow core fiber according to clause A or B, wherein the inner cladding comprises a rod or tube of a different refractive index from the plurality of primary cladding capillaries, said rod or tube fused to the inner wall of the outer cladding between two primary cladding capillaries.

[0045] Clause D: The antiresonant hollow core fiber according to clause A or B, wherein the outer cladding comprises an embedded rod or tube of a different refractive index from the rest of said outer cladding.

[0046] Clause E: The antiresonant hollow core fiber according to clause A or B, wherein the inner cladding further comprises, within each primary cladding capillary, one or more smaller capillaries and wherein a smaller capillary inside one of the primary cladding capillaries comprises a tube of a different refractive index from the plurality' of primary cladding capillaries and others of the smaller capillaries.

[0047] Clause F: The antiresonant hollow core fiber according to clause E, wherein the one or more smaller capillaries comprises a secondary capillary and a tertiary capillary, wherein the secondary capillary is bonded to an inner surface of the primary cladding capillary and the tertiary capillary is bonded to an inner surface of the secondary capillary and wherein a secondary capillary inside one of the primary cladding capillaries comprisestube (1102) of a different refractive index to the plurality of primary cladding capillaries and others of the smaller capillaries.

[0048] Clause G: The antiresonant hollow core fiber according to clause E or F, wherein the one or more smaller capillaries comprises a secondary capillary and a tertiary capillary, wherein the secondary capillary is bonded to an inner surface of the primary cladding capillary and the tertiary capillary is bonded to an inner surface of the secondary capillary and wherein a tertiary capillary inside one of the primary cladding capillaries comprises tube (1102) of a different refractive index from the plurality of primary cladding capillaries and others of the smaller capillaries.

[0049] Clause H: The antiresonant hollow core fiber according to clause A or B, wherein one of the outer cladding and the inner cladding comprises a rod of a different refractive index from the rest of said one of the outer cladding and inner cladding.

[0050] Clause I: The antiresonant hollow core fiber according to clause A or B, wherein one of the outer cladding and the inner cladding comprises a tube of a different refractive index from the rest of said one of the outer cladding and inner cladding.

[0051] Clause J: The antiresonant hollow core fiber according to any of the preceding clauses, wherein the rod or tube of a different refractive index from the rest of said one of the outer cladding and inner cladding has a higher refractive index than the rest of said one of the outer cladding and inner cladding.

[0052] Clause K: The antiresonant hollow core fiber according to any of the preceding clauses, wherein the fiber lacks any rotational symmetry.

[0053] Clause L: A preform or cane configured to be drawn into an antiresonant hollow core fiber according any of the preceding clauses, said preform or cane comprising: a jacket tube for forming the outer cladding; and a plurality of primary tubes for forming primary cladding capillaries to define an inner cladding of the fiber, the primary tubes arranged in a ring around a central void to form a hollow core, each primary tube arranged against an inner surface of the j acket tube at a peripheral location around the circumference of the of the inner surface, wherein one of the jacket tube and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said jacket tube or said primary tubes.

[0054] Clause M: A method of manufacturing a jacket tube for use in the preform or cane according to clause L, the method comprising: selecting ajacket tube; and forming a cylindrical hole in the jacket tube, wherein the cylindrical hole is parallel to a longitudinal axis of the jacket tube.

[0055] Clause N: The method according to clause M, further comprising: inserting a rod into the cylindrical hole, wherein the rod has a different refractive index from the jacket tube.

[0056] Clause O: A method of manufacturing ajacket tube for use in the preform or cane according to clause L, the method comprising: forming an assembly by: inserting an inner jacket tube into an outer jacket tube; and fdling a gap between the inner jacket tube and the outer jacket tube with either: a plurality of rods, wherein at least one of the rods has a different refractive index from the inner and outer jacket tubes and others of the rods; or a plurality of rods and at least one tube; and heating the assembly to form the jacket tube.

[0057] Clause P: The method according to clause O, wherein the assembly comprises either a plurality of the rods having a different refractive index from the inner and outer jacket tubes and others of the rods or a plurality of tubes and wherein the plurality of rods with the different refractive index or the plurality of tubes are positioned within the assembly to uniquely identity' the jacket tube.

[0058] Some aspects of the methods described herein are performed, in some examples, by software in machine readable form on a tangible storage medium e.g. in the form of a computer program comprising computer program code means adapted to perform all the operations of one or more of the methods described herein when the program is run on a computer and where the computer program may be embodied on a computer readable medium. The software is suitable for execution on a parallel processor or a serial processor such that the method operations may be carried out in any suitable order, or simultaneously.

[0059] Those skilled in the art will realize that storage devices utilized to store program instructions are optionally distributed across a network. For example, a remote computer is able to store an example of the process described as software. A local or terminal computer is able to access the remote computer and download a part or all of the software to run the program. Alternatively, the local computer may download pieces of the software as needed, or execute some software instructions at the local terminal and some at the remote computer (or computer network). Those skilled in the art will also realize that by utilizing conventional techniques known to those skilled in the art that all. or a portion of the software instructions may be carried out by a dedicated circuit, such as a digital signal processor (DSP), programmable logic array, or the like.

[0060] Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

[0061] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

[0062] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[0063] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0064] The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[0065] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this specification.

Claims

CLAIMSWhat is claimed is:

1. An antiresonant hollow core fiber (700, 800, 900, 1000, 1 100, 1200) comprising: an outer cladding (704); an inner cladding comprising a plurality of primary7cladding capillaries (710), each primary7cladding capillary bonded to an inner wall (706) of the outer cladding at a peripheral location around a circumference of the of the inner wall; and a hollow core (106) formed by a central void within a ring of primary' cladding capillaries, wherein one of the outer cladding and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said one of the outer cladding and inner cladding.

2. The antiresonant hollow core fiber (700, 800) according to claim 1, wherein the inner cladding comprises a rod (702) or tube (802) of a different refractive index from the plurality7of primary cladding capillaries, said rod or tube fused to the inner wall (706) of the outer cladding between two primary cladding capillaries.

3. The antiresonant hollow core fiber (900, 1000) according to claim 1, wherein the outer cladding comprises an embedded rod (902) or tube (1002) of a different refractive index from the rest of said outer cladding.

4. The antiresonant hollow core fiber (1100, 1200) according to claim 1, wherein the inner cladding further comprises, within each primary' cladding capillary, one or more smaller capillaries and wherein a smaller capillary7inside one of the primary' cladding capillaries comprises a tube (1102) of a different refractive index from the plurality of primary7cladding capillaries and others of the smaller capillaries.

5. The antiresonant hollow core fiber (1100) according to claim 4, wherein the one or more smaller capillaries comprises a secondary capillary and a tertiary capillary, wherein the secondary7capillary7is bonded to an inner surface of the primary cladding capillary and the tertiary capillary is bonded to an inner surface of the secondary capillary and wherein a secondary capillary inside one of the primary cladding capillaries comprises tube (1102) of a different refractive index to the plurality' of primary cladding capillaries and others of the smaller capillaries.

6. The antiresonant hollow core fiber (1200) according to claim 4 or 5, wherein the one or more smaller capillaries comprises a secondary capillary and a tertiary capillary, wherein the secondary capillary is bonded to an inner surface of the primary' cladding capillary andthe tertiary capillary is bonded to an inner surface of the secondary capillary and wherein a tertiary capillary inside one of the primary cladding capillaries comprises tube (1102) of a different refractive index from the plurality of primary cladding capillaries and others of the smaller capillaries.

7. The antiresonant hollow core fiber (700, 900) according to claim 1, wherein one of the outer cladding and the inner cladding comprises a rod of a different refractive index from the rest of said one of the outer cladding and inner cladding.

8. The antiresonant hollow core fiber (700, 900) according to claim 1, wherein one of the outer cladding and the inner cladding comprises a tube of a different refractive index from the rest of said one of the outer cladding and inner cladding.

9. The antiresonant hollow core fiber according to any of the preceding claims, wherein the rod or tube of a different refractive index from the rest of said one of the outer cladding and inner cladding has a higher refractive index than the rest of said one of the outer cladding and inner cladding.

10. The antiresonant hollow core fiber according to any of the preceding claims, wherein the fiber lacks any rotational symmetry.1 1. A preform or cane configured to be drawn into an antiresonant hollow core fiber according to any of the preceding claims, said preform or cane comprising: a jacket tube for forming the outer cladding (704); and a plurality of primary tubes for forming primary cladding capillaries (710) to define an inner cladding of the fiber, the primary tubes arranged in a ring around a central void to form a hollow core, each primary tube arranged against an inner surface of the jacket tube at a peripheral location around the circumference of the of the inner surface, wherein one of the jacket tube and the inner cladding comprises a rod or tube of a different refractive index and / or thickness from the rest of said jacket tube or said primary tubes.

12. A method of manufacturing an antiresonant hollow core fiber according to any of claims 1-10, the method comprising forming a jacket tube for use in the preform or cane according to claim 11, by: selecting a jacket tube (1302); and forming a cylindrical hole in the jacket tube, wherein the cylindrical hole is parallel to a longitudinal axis of the jacket tube (1304).

13. The method according to claim 12, further comprising: inserting a rod into the cylindrical hole (1306), wherein the rod has a different refractive index from the jacket tube.

14. A method of manufacturing an antiresonant hollow core fiber according to any of claims 1-10, the method comprising forming a jacket tube for use in the preform or cane according to claim 11 by: forming an assembly by: inserting an inner jacket tube into an outer jacket tube (1402); and filling a gap between the inner jacket tube and the outer jacket tube with either: a plurality of rods (1404), wherein at least one of the rods has a different refractive index from the inner and outer jacket tubes and others of the rods; or a plurality of rods and at least one tube (1405); and heating the assembly to form the jacket tube (1406).

15. The method according to claim 14, wherein the assembly comprises either a plurality of the rods having a different refractive index from the inner and outer jacket tubes and others of the rods or a plurality of tubes and wherein the plurality7of rods with the different refractive index or the plurality of tubes are positioned within the assembly to uniquely identify the jacket tube.

Images