Azimuthal alignment of antiresonant hollow core fibers

Abstract

A method of azimuthal alignment of an antiresonant hollow core fiber (ARF) in a splicer is described. The method comprises for each antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into the splicer, side-illuminating the antiresonant hollow core fiber, capturing intensity data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data (e.g. a maximum intensity value), the pixel position corresponding to an initial azimuthal orientation. The method further comprises rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature.

Description

AZIMUTHAL ALIGNMENT OF ANTIRESONANT HOLLOW CORE FIBERSBACKGROUND

[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 join 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 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. Misalignment of this internal structure 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 know n fiber positioning or fiber splicing apparatus.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] A method of azimuthal alignment of an antiresonant hollow core fiber (ARF) in a splicer is described. The method comprises for each antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into the splicer, side-illuminating the antiresonant hollow core fiber, capturing intensity data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data (e.g. a maximum intensity value), the pixel position corresponding to an initial azimuthal orientation. The method further comprises rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature.

[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 an arrangement for end-view observation in a splicer;FIG. 5 show s side view images of two example double-nested antiresonant nodeless fibers;FIGs. 6A and 6B show sinograms obtained from an antiresonant hollow- core fiber using two different cameras on a splicer;FIG. 7 show s a first example method of azimuthal alignment of an antiresonant hollow core fiber for splicing;FIG. 8 shows a schematic diagram of a first example apparatus for performing the method of FIG. 7;FIG. 9 show s a graph of data obtained using the method of FIG. 7;FIG. 10 shows a schematic diagram of a second example apparatus for performing the method of FIG. 7 ;FIG. 11 shows a graph of data obtained using the method of FIG. 7 and the apparatus of FIG. 10;FIG. 12 shows a second example method of azimuthal alignment of an antiresonant hollow core fiber for splicing;FIG. 13 shows a graph of data obtained using the method of FIG. 12;FIG. 14 shows a third example method of azimuthal alignment of an antiresonant hollow core fiber for splicing;FIG. 15 shows a graph of data obtained using the method of FIG. 14;FIG. 16 shows a fourth example method of azimuthal alignment of an antiresonant hollow core fiber for splicing; andFIG. 17 is a block diagram of a splicer configured to perform any of the methods described herein.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 somedesigns 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 1 10 removes nodes that arise at the contact points between adjacent tubes and which tend to cause undesirable resonances that result in high losses. Accordingly, fibers with spaced-apart cladding capillaries may be referred to as “nodeless 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 fiberfiber'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. 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 ty pe, 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 claddingcapillaries. 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 crosssection. Additionally, whilst in the examples of FIGs. 1-3, all the primary cladding capillaries 104 are of the same size and shape, 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. Alignment requires careful rotational adjustment about the longitudinal axis of the fiber in order to bring the structural features into the same orientation, and this can be awkward and time-consuming to achieve.

[0016] 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- loss splice 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. 4. To perform end-view observation, a mirror 402 is temporarily inserted between the end-facets 404, 406 that are to be spliced. A light source 408 couples light into the fiber 410 to illuminate the end-facet 404 and it is imaged by a camera 412 via the mirror 402 and a lens 414. The fiber 410 can then be rotated into a pre-defined orientation and the process is then repeated for the second fiber 416 using a second light source 418 (to couple light into the second fiber 416) and either a second mirror (not shown in FIG. 4) or by rotating the mirror 402 through 90°. Such apparatus is slow, costly, delicate and bulky (e.g. because of the moving mirror) and therefore largely suitable only for interior laboratoryand 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).

[0017] Described herein are methods and apparatus for rotationally aligning ARF using side-viewing of the fiber only. By using side-viewing (and not viewing the endfacet), it avoids the need to insert a mirror to view an end-facet and this results in equipment which is simpler, smaller, lower cost, more robust (e.g. as there are fewer moving parts) and quicker to operate. Where the alignment is performed in preparation for splicing two fibers, side-viewing enables the same camera to view both fibers at the same time using the same camera arrangement (which is not possible with end-viewing) and this also results in equipment that is simpler, smaller, lower cost and quicker to operate. The methods described below may be used for any ty pe of ARF (e.g. any ARF with greater than two-fold rotational symmetry) and in particular they may be used for NANF (e.g. as shown in FIG. 2) and DNANF (e.g. as shown in FIG. 3). The alignment methods described herein may be used to rotationally align two lengths of ARF prior to splicing (e.g. prior to fusion splicing) or for other applications where precise rotational alignment of ARF is required.

[0018] The methods and apparatus described herein measure the intensity of a line of pixels along the fiber cross-section in a captured side-view image of the ARF as a function of azimuthal angle. This intensity data for a plurality of azimuthal angles may be referred to as a sinogram. Reflection and refraction of light beams passing through the microstructure of the ARF give rise to intensity patterns on the camera that depend upon the azimuthal orientation of the fiber. This pattern that appears as intensity variations along the fiber cross-section is used to detect the azimuthal orientation of the fiber.

[0019] The terms ‘azimuthal orientation’ and ‘azimuthal rotation' are used herein to refer to the angular (or rotational) orientation and rotation of the ARF about its long axis.

[0020] FIG. 5 shows side view images 502 of two example DNANFs at a random azimuthal angle (i.e. the left and right fibers have not been aligned azimuthally). FIG. 5 also shows a graph 504 of the relative pixel intensity of a line of pixels along the fiber cross-section (marked with the dotted white line 506) in the captured side-view image. The intensity graph 504 shows peaks 508-514 at the outer edges and inner edges of theouter cladding 102 and between these peaks, a more complex intensity pattern 516 resulting from the microstructure.

[0021] FIG. 6A shows sinograms obtained from an ARF using two different cameras on a splicer. These two different cameras are positioned orthogonally to each other and so capture different side views (labelled X and Y) of the right fiber of FIG. 5. In these sinograms, the horizontal axis shows the pixel position in the line of pixels along the fiber (with the pixel position being labelled 0-N, with pixel 0 being the first pixel in the line and pixel N being the last pixel in the line), the vertical axis shows the relative azimuthal angle (where the term ‘relative’ is used because there is no marker in the fiber that defines an angular reference position) and the shades of grey represent the relative pixel intensity, such that each horizontal line in the sinogram corresponds to the intensity graph 504 for a different azimuthal angle.

[0022] The sinograms shown in FIG. 6A comprise the raw pixel intensity data. FIG. 6B shows corresponding sinograms generated using processed pixel intensity data. The processed pixel intensity data is generated by processing the raw pixel data and in the example shown the processing comprises low-pass filtering. In the example shown in FIG. 6B, the intensity profile is substantially sinusoidal. Other processing techniques may be used instead of low-pass filtering (or in addition to low -pass filtering) in order to generate the processed pixel intensity data and further examples of processing techniques that maybe used include: averaging, spatial filtering, segmentation, Fourier transformation, etc.

[0023] Whilst the sinograms in FIGs. 6A and 6B are shown for a complete rotation of the fiber, using the methods described herein, the left and right fibers can be azimuthally aligned for splicing without having to perform a complete rotation of either or both fibers and this reduces the time taken to align the fibers and hence perform the splicing operation. It may also reduce the mechanical complexity of the splicer because it is not necessary- to be able to rotate either or both of the fibers through a full 360°.

[0024] The methods described herein comprise aligning two ARF either at the same time or separately. Once aligned, these two ARF are clamped in place and can be spliced together, e.g. using fusion or mechanical splicing. The alignment process for each fiber involves side-illuminating the ARF and capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated ARF. The intensity data is processed and then the processed intensity data is analysed to determine a pixel position in the line that corresponds to a pre-defined feature, such as a maximum value, a minimum value, or a series of peak and / or troughs in the processed intensity data. This determined pixelposition corresponds to an initial azimuthal orientation and depending upon the particular method used, the method may be repeated to determine pixel positions for the feature at different azimuthal orientations. To bring the two ARF into alignment, one or both of them are rotated based on the determined pixel positions of the pre-defined feature (e.g. at the initial azimuthal orientation and in some examples at other azimuthal orientations). Various different methods are described below of determining the angle through which one or both of the ARF should be rotated, using the determined pixel positions.

[0025] FIG. 7 shows a first example method of azimuthal alignment of an ARF for splicing. The method is show n for a single fiber, but may be repeated for a second fiber (as indicated by the dotted arrow from block 714 to block 702) or performed substantially in parallel for the second fiber. The method may be implemented on a splicer as shown in FIG. 8.

[0026] As shown in FIG. 7, an ARF 802 with a cleaved end-facet 804 is inserted into the splicer (block 702). The fiber may be placed into a removable fiber holder before being inserted into the splicer (in block 702) or alternatively the fiber may be placed into a fiber holder that is part of the splicer. The fiber is then illuminated from the side (block 704) using a light source 806. Intensity data for each pixel in a line of pixels in a sideview image captured by a camera 808 is captured and processed (block 705). The line of pixels crosses the fiber and is substantially perpendicular to the longitudinal axis of the fiber (e.g. as shown by dotted line 506 in FIG. 5). The position of a feature or characteristic in the processed intensity data (e.g. a peak, trough, combination of peaks and / or troughs, or other feature) in the line of pixels is determined (block 706). The feature that is used to determine a position (in block 706) excludes any features (e.g. peaks) that are from the outer cladding (e.g. any peaks in the processed data that correspond to peaks 508-514 in the graph 504 of raw intensity’ data in FIG. 5 are excluded). This pixel position (as determined in block 706) corresponds to the initial azimuthal orientation of the fiber and FIG. 6B shows an initial azimuthal orientation, ao, and an initial pixel position, Po, of the feature.

[0027] For this first iteration, where a single measurement has been made at the initial azimuthal orientation, the fiber is then rotated to a new azimuthal orientation (block 708) using a rotatable element 810 in the splicer. The rotatable element 810 may rotate the fiber and the fiber holder together (with the fiber remaining in a fixed orientation with respect to the fiber holder) or the rotatable element 810 may rotate the fiber within the fiber holder. Any suitable mechanical arrangement may be used for the rotatable element 810which enables the fiber to be rotated azimuthally in a controlled manner about its longitudinal axis (e.g. such that it can be rotated from a first angle to a second angle and then back to the first angle) whilst minimising (or preventing) any lateral or longitudinal movement (i.e. movement along or perpendicular to the longitudinal axis of the fiber).

[0028] The method steps are then repeated (blocks 704-706) at the new azimuthal orientation to determine the updated position of the same feature or characteristic (i.e. the same feature is used each time in block 706). This position that is determined in the second iteration corresponds to the new azimuthal orientation of the fiber (following the rotation in block 708). The process of rotating the fiber (in block 708) and determining a new pixel position having maximum intensity (in block 710) is repeated until a peak pixel position of the particular feature or characteristic is identified (block 712). Once the peak position is identified (‘Yes’ in block 712), then the fiber is set to the azimuthal angle corresponding to the identified peak position (which may require further rotation of the fiber in block 713, e.g. back to an earlier azimuthal position) and clamped in place (block 714). FIG. 6B shows a peak pixel position, Pp. of the feature and the corresponding azimuthal orientation, ai>.

[0029] The determination of the peak position (in block 712) may comprise identifying an azimuthal orientation where the particular feature or characteristic is located at a maximum pixel position. As described above the pixels in the row of pixels are numbered from 0 to N and so the maximum pixel position is the pixel position with the highest number. Determination of the peak position may involve additional processing of the pixel position data (from block 706), e.g. fitting a curve to the pixel position data (as described below with reference to the example in FIG. 9).

[0030] In another example, the peak position may be defined relative to a predefined threshold and the peak position (as determined in block 712) may be a position that exceeds (or equals or exceeds) the threshold. Referring back to the example shown in FIG. 6B, the threshold may be set to a pixel position of 250. Where such a threshold is used, the number of iterations that are performed before the peak position is identified may be reduced (and hence the method of alignment may be faster) because it is not necessary to rotate past the peak position in order to identify that the peak position has been reached. It also may eliminate the step of repositioning the fiber (in block 713) after the peak position has been identified (in block 712).

[0031] The method of FIG. 7 may then be repeated for the second ARF 812, as indicated by the dotted arrow from block 714 to block 702, or the alignment of the secondARF 812 may be performed in parallel with the alignment of the first ARF 802 (e.g. substantially or partially in parallel). The same feature or characteristic as used for the first ARF 802 is also used for the second ARF 812. In the example shown in FIG. 8, a separate light source 816, camera 818 and rotatable element 820 is provided and used to azimuthally align the second ARF 812. In other examples, there may be a single light source and camera that is used for aligning both the first and second ARFs 802. 812. Once both of the ARF 802. 812 have been aligned, the two ARF can be spliced together.

[0032] Although not shown in FIG. 8, the splicer may additionally comprise a control unit that is configured to perform various steps of the method of FIG. 7. The control unit is connected to the light sources 806, 816, cameras 808, 818 and rotatable elements 810, 820 and is configured to control when each light source 806, 816 is illuminated, when images are captured by the cameras 808, 818 and how the rotatable elements 810, 820 move to rotate the ARFs 802, 812 (in blocks 710 and 714). The control unit also comprises image processing software that is configured to capture and process the intensity data (in block 705), determine the pixel position of the feature or characteristic (in blocks 706) and identify the peak position (in block 712). An example control unit is described in more detail with reference to FIG. 17. In various examples the control unit may select the feature or characteristic that is used in the method. In other examples, the feature or characteristic may be predefined and the same for all ARF (e g. the highest intensity peak) or may be selected based on user input.

[0033] FIG. 9 shows a graph of data obtained using the method of FIG. 7 (i.e. in multiple iterations of block 706), with the horizontal axis showing the relative azimuthal angle and the vertical axis showing the pixel position of the feature or characteristic. Two lines are shown in FIG. 9 - the feature position in the processed intensity data 902 (as determined in the various iterations of block 706) and a fitted sine curve 904. In the example shown in FIG. 9, in the initial azimuthal orientation, ao, the feature (e.g. the maximum intensity) is located at the pixel at position 175 (as determined in a first iteration block 706). The fiber is then rotated (in block 708) and measurements obtained (in subsequent iterations of block 706) at various relative angles between the initial azimuthal orientation, ao, and a final measured azimuthal orientation, an. The iterations of the method (blocks 704-708) do not continue beyond the final measured azimuthal orientation, an, because it is clear, from the processed data 902, that a peak pixel position has been identified ("Yes' in block 712). e.g. since as the fiber has been rotated, the feature position has increased from the initial pixel position of 175 to close to 180 and hasnow fallen to a pixel position below the initial pixel position of 175. In this example, in order to identify the peak position which is used to finally position the fiber (in blocks 713-714) given the noise in the processed intensity data 902, a fitted sine curve 904 is fitted to the processed intensify data 902. Analysis of the fitted sine curve 904 identifies the relative angle, aP, at which the peak pixel position occurs and so the fiber is moved back to that relative angle (in block 713) and clamped in place (in block 714). In other examples, the processed intensity data 902 may be used without fitting a curve to it or the processed intensity data 902 may be further processed in another way.

[0034] As shown in FIG. 9, it is not necessary7to rotate the fiber through a full 360° in order to perform the alignment using the method of FIG. 7. For an ARF w ith n-fold rotational symmetry (where n>2), the maximum rotation that is required is a little over 360° / n, since there is guaranteed to be a peak within this angular range. By reducing the angular movement that is required, it reduces the number of measurements that are taken and so reduces the time taken to perform the alignment. Additionally, if the amount of rotation that is needed to be performed by the rotatable elements 810, 820 is less than 360°. it reduces the mechanical complexity of the splicer. Even if the splicer is designed to accommodate any ARF with n-fold rotational symmetry where n>2, the rotatable elements 810, 820 need only be able to rotate the fiber by a maximum of a little over 120° (e.g. ±60° from a central position, with some additional small margin) and this reduces the mechanical complexity of the rotatable elements 810, 820.

[0035] Whilst the description above and FIG. 7 refers to identifying a peak position of the feature (in block 712), in other examples, a trough (i.e. a minimum position of the feature) may alternatively be identified, or either a peak or a trough may be identified (in block 712). If a trough (i.e. minimum position) is used when aligning the first ARF 802, then when setting the azimuthal alignment of the second ARF 812 (i.e. the other ARF in the pair of ARF being spliced together), a trough must also be used. The term ‘extreme pixel position’ is used herein to refer to either a peak or a trough (i.e. a maximum or minimum position). Where a trough position is used (in block 712), a threshold may be used to identify this position (in a similar manner to where a peak position is used): however, the trough position is considered to be any position with a value that is less than the predefined threshold.

[0036] If the method of FIG. 7 identifies either a peak position or a trough position (in block 712) when aligning a first fiber of a pair of fibers to be spliced together, then the amount of rotation that is needed, and the number of measurements performed for the firstfiber is reduced; however, the rotation and number of measurements for the second fiber is not affected. Consequently, this reduces the time taken to perform the alignment but may not change the mechanical arrangement of the rotatable elements 810, 820 (e.g. since this would require that the same fiber, with the restricted rotational movement, was always orientated first). Furthermore, where either a peak or a trough is identified for the first fiber, the final positioning of the second fiber (in blocks 713-714) cannot be performed totally independently of the alignment of the first fiber (e.g. because the alignment of the second fiber needs to know whether to identify a peak or a trough in block 712).

[0037] In the arrangement shown in FIG. 8, there is a light source - camera pair (where the pair comprises a light source and a camera, arranged on opposite sides of the fiber) for each fiber; however, in other arrangements there may be a single light source - camera pair that is used for both fibers or two or more light source - camera pairs for each fiber. FIG. 10 shows an arrangement 1000, shown in cross-section, in which there are two light source - camera pairs (the first comprising light source 1002 and camera 1006 and the second comprising light source 1004 and camera 1008). The camera-light source pairs (1002 & 1006, 1004 & 1008) are arranged perpendicular to each other and may be referred to as capturing an X view (camera 1006) and Y view (camera 1008). It will be appreciated that there may be more than two light source - camera pairs at different orientations and even where there is more than one light source - camera pair, it is not necessary that all of the light source - camera pairs are used to perform the azimuthal alignment using the methods described herein (e.g. only one or a subset of the light source - camera pairs may be used in some examples).

[0038] FIG. 11 shows a graph of data obtained using the method of FIG. 7 and two camera-light source pairs as shown in FIG. 10. In the same way as the graph in FIG. 9, the horizontal axis shows the relative azimuthal angle and the vertical axis shows the pixel position of the feature or characteristic. Four lines are show n in FIG. 11 - a first line 1102 shows the feature position in the processed intensity data from the X-view camera 1006 (as determined in the various iterations of block 706), a second line 1106 shows the feature position in the processed intensity data from the Y-view camera 1008, and the third and fourth lines 1104, 1108 are sine curves that are fitted to the first and second lines respectively. Whilst the graph in FIG. 11 shows data for the full 360° and so show s the 5 peaks that are caused by the 5-fold symmetry' of the ARF, it is not necessary' to explore this full range of angles when performing the method of FIG. 7.

[0039] If two light source - camera pairs are used, the maximum angular rotation of the first fiber in order to be able to identify a peak in the feature position plot for one of the cameras is reduced; however, the rotation and number of measurements for the second fiber is not affected. Consequently, this reduces the time taken to perform the alignment but may not change the mechanical arrangement of the rotatable elements 810, 820 (e.g. since this would require that the same fiber, with the restricted rotational movement, was always orientated first). Furthermore, whichever light source - camera pair (X or Y) is used to orient the first fiber (in block 712) must also be used for the second fiber (i.e. they must either both use the X-view light source - camera pair or both use the Y-view light source - camera pair). Furthermore, where either the X-view light source - camera pair or the Y-view light source - camera pair is used to identify a peak for the first fiber, the final positioning of the second fiber (in blocks 713-714) cannot be performed totally independently of the alignment of the first fiber (e.g. because the alignment of the second fiber needs to know which light source - camera pair to use). As described above, the time taken to align the first fiber can be reduced further by looking for either a peak or a trough, as this halves the maximum amount of rotational movement that is required to locate a peak (or trough) for the first fiber but again limits the ability to perform the alignment of the second fiber totally independently of the alignment of the first fiber.

[0040] Where there are multiple light source - camera pairs, irrespective of whether one or more light source - camera pairs are used in performing the azimuthal alignment, the light sources may be selectively switched on and off so that when capturing pixel intensify data (in block 705) only the light source in the light source - camera pair that is being used to capture the intensity data is switched on. By switching off all other light sources during the measurement, the confounding effects (as caused by refraction or reflection of the extra light sources in the internal microstructure of the ARF) is reduced.

[0041] FIG. 12 shows a second example method of azimuthal alignment of an ARF for splicing. This method involves an initial, coarse, alignment of each of the two fibers using the method of FIG. 7 (block 1202) and then a subsequent fine alignment process (blocks 1204-1214). Whilst the coarse alignment (in block 1202) can be performed for one fiber followed by the second fiber or both fibers at the same time (e.g. where the light source - camera pair used and whether a peak / trough is sought is fixed), the fine alignment involves a direct comparison of the side view intensify graphs for the two fibers.

[0042] As shown in FIG. 12. having performed the coarse alignment of each fiber using the method of FIG. 7 (in block 1202), both fibers are illuminated from the side(block 1204). Where there are multiple camera-light source pairs, the same orientation of camera-light source pair (e.g. X-view or Y-view) is used for each of the fibers in the fine alignment operation. The relative pixel intensity for a line of pixels in an image along a fiber cross-section is captured for each fiber (block 1206), e.g. a graph similar to that shown in FIG. 5 is captured for each fiber. The peaks 508-514 from the outer cladding are used to laterally align the fibers (block 1208) and in performing this lateral alignment, both the X-view and Y-view camera-light source pairs may be used in order to perform lateral fine alignment in both orientations. One or both of the fibers are then rotated to improve alignment between the respective intensity patterns 516 resulting from the microstructure (block 1210). This can be described with reference to the example shown in FIG. 13. The amount of angular movement in the fine alignment operation (in block 1210) is much smaller than in the coarse alignment operation (of FIG. 7).

[0043] In the first graph 1300 in FIG. 13, the intensity' plots 1302, 1304 from the two fibers do not overlay exactly; however, by rotating and / or laterally moving (in XY plane) the fibers, they can be overlapped, as shown in the second graph 1306. Whilst FIG. 13 shows the intensity plots overlapping exactly, in other examples, the fine rotational adjustment (in block 1210) may reduce the differences between the two plots but they may not overlap exactly (e.g. due to manufacturing imperfections in one or other fiber) and any suitable measure may be used (e.g. aligning peaks, mean square error, crosscorrelation. FFT. etc.). Having performed the fine alignment, both fibers are clamped in position (block 1214) ready for splicing.

[0044] FIG. 14 show s a third example method of azimuthal alignment of an ARF for splicing. This method involves an initial calibration process 1400 for a fiber type (e.g. for a fiber with a particular internal microstructure) and then an alignment process 1401 in which the calibration data is used to reduce the time taken to perform the alignment prior to splicing compared to the methods described above. The calibration process 1400 needs only be performed periodically and may only be performed once for each fiber type to generate calibration data.

[0045] The calibration process 1400 involves generating at least a portion of a graph similar to that shown in FIG. 1 1. Whilst FIG. 11 show s the feature position data for two light source - camera pairs and the full 360° of rotation, the calibration process 1400 in FIG. 14 generates the feature position data for one or more light source - camera pairs and at least 360° / (m x n) of rotation, where m is the number of light source - camera pairs. As shown in FIG. 14, the method comprises inserting the fiber used for calibration into thesplicer (block 1402), and for each light source - camera pair and for a range of angular positions (spanning at least 360° / (m x n)), determining a feature position along the fiber cross-section in the processed intensity data (block 1406). The processed intensity data may be captured and processed as described above and the feature position may be identified as described above. This data is then stored (block 1408).

[0046] In use, following calibration, a first fiber to be spliced is inserted into the splicer (block 1410) and the first fiber is side illuminated using the same arrangement of one or more light source - camera pairs as used for calibration. For each light source - camera pair, the feature position along the first fiber cross-section is determined by capturing, processing and analysing intensity' data for a line of pixels (block 1412). Based on this feature position data, a look-up is performed using the calibration data to determine a current relative azimuthal orientation of the first fiber (block 1414). The method is also performed for the second fiber (blocks 1410-1414) to determine a current relative azimuthal orientation of the first fiber and this may be performed sequentially (as indicated by the arrow from block 1414 to block 1410) or in parallel. Having determined the relative azimuthal orientation of each of the two fibers to be aligned, one or both fibers are rotated so that they are at the same relative azimuthal orientation (block 1416) and the fibers are clamped in place (block 1418) so that they are ready for splicing.

[0047] FIG. 14 determines relative, rather than absolute azimuthal orientation because of the rotational symmetry of an ARF and the lack of predefined zero position (unlike fibers with markers in them). Furthermore as a consequence of the rotational symmetry there are multiple azimuthal orientations which give the same feature position. This can be described with reference to FIG. 15 which shows the graph of FIG. 11 and corresponds to the calibration data (as stored in block 1408). As described above, the ARF has greater than two-fold rotational symmetry and in the example shown, the ARF has 5-fold rotational symmetry (n=5). This means that there are five different azimuthal orientations that give the same feature position. The five positions with a feature position of pixel no. 178 are marked by arrows 1501-1505 in FIG. 15.

[0048] An example of the operation of the method of FIG. 14 can be described with reference to FIG. 15. If, for example, the feature position data (from block 1412) identifies a feature position from the X camera of 175 and a feature position from the Y camera of 165 for the first fiber, the initial azimuthal position, ao,i, of the first fiber can be identified. Similarly, if the feature position data (from block 1412) identifies a feature position from the X camera of 171 and a feature position from the Y camera of 147 for the second fiber.the initial azimuthal position, ao,2, of the second fiber can be identified. Based on these two positions, the amount of rotation to be applied to either or both of the fibers (in block 1414) can be determined. There are several different ways to achieve this.

[0049] In a first example, the amount of rotation that is required, Aai, to move the first fiber from its initial azimuthal position, ao.i, to a peak position, aP, can be determined (e.g. using Aai = aP- ao.i) and similarly, the amount of rotation that is required, Aa2. to move the first fiber from its initial azimuthal position, ao,2, to a peak position, 0.2. can be determined (e.g. using Aa2 = aP- 00,2). Each fiber is then rotated through the determined angle, Aai or Aa2 (block 1416) and clamped (block 1418) ready for splicing. In a second example, once the initial azimuthal position of both fibers has been determined (in block 1414), one of the fibers can be rotated by the amount of their initial angular difference, Aai, 2 (e.g. using Aai, 2 = ao,2 - ao,i) or Aa2,i where this second angular difference is smaller as it takes into consideration the rotational symmetry of the ARF (e.g. by considering that the initial azimuthal positions of the first fiber ao,i and a’o.l give the same feature position and so are equivalent).

[0050] FIG. 16 shows a fourth example method of azimuthal alignment of an ARF for splicing which is a variation on that shown in FIG. 12. In the method of FIG. 16, the initial coarse alignment is performed using the alignment process 1401 of FIG. 14 (block 1602) and then the fine alignment is performed as described above with reference to FIG. 12.

[0051] Whilst the methods described above refer to capturing and processing intensity data for each pixel in a line of pixels in a side-view image captured by a camera (e.g. in block 705), in a variation of any of the methods above, intensity data may be captured for more than one line of pixels (e.g. 10-20 rows of pixels). Where intensity data is capture for more than one line of pixels, the raw intensity values for corresponding pixels in each of the lines are averaged (e.g. by, for each pixel position, i, where i=0-N, averaging the pixel intensity values for that pixel in each of the different lines) to create an average intensity data for each pixel and then the average intensity data is processed (e.g. low pass filtered) and used as described above. By averaging pixel intensity values from multiple lines of pixels, the methods described herein are less sensitive to defects and contamination (e.g. dirt on the fiber, or small pieces of coating that have not been fully stripped).

[0052] In all the methods described above, the determination of the feature position along a fiber cross-section (e.g. in any of blocks 706, 1206, 1406. 1412) excludes any features (e.g. peaks) in the processed intensity data that correspond to the outer cladding (e.g. features in the processed intensity data that correspond peaks 508-514 in raw intensity data shown in the graph 504 in FIG. 5 are excluded) and instead the feature used is one that results from the microstructure (e.g. a feature in the processed intensity data that corresponds to peak 518 in the raw intensity data of FIG. 5).

[0053] FIG. 17 is a block diagram of a splicer 1700 that is configured to perform the methods described herein. The splicer 1700 shown in FIG. 17 is a fusion splicer; however, as described above, the methods described herein may be used in relation to any type of splicing. The splicer 1700 comprises two fiber holders 1704, 1714 which may be removable or may be fixed in the splicer 1700. The splicer 1700 also comprises two cameras 808, 818, two light sources 806, 816, two rotatable elements 810, 820 and a fusion unit 1706 that performs the splice. For a mechanical splicer, the fusion unit 1706 is omitted. The cameras 808, 818 and light sources 806, 816 are arranged in light source - camera pairs, with the camera in a pair being positioned to capture light emitted from the light source in the pair after it has passed through the ARF. As described above, the splicer 1700 may a single light source - camera pair or may comprise additional light source - camera pairs (e.g. such that there are two light source - camera pairs for each fiber). The splicer also comprises a control unit 1702 that is configured to perform various steps of the methods described above. The control unit 1702 is connected to the light sources 806, 816, cameras 808, 818 and rotatable elements 810, 820 and is configured to control when each light source 806, 816 is illuminated and the intensity of each light source, when images are captured by the cameras 808, 818 and the gain of the cameras and how the rotatable elements 810, 820 move to rotate the ARFs 802, 812 (e.g. in blocks 710 and 713).

[0054] The control unit 1702 comprises one or more processors 1722, an input / output interface 1724 via which the control unit communicates with the other elements in the splicer and a memory 1726. The memory 1726 stores the software (processor-executable instructions) that is executed by the processor in order to implement the methods described above (e.g. image processing software 1728, fiber positioning software 1736 and splice control software 1734). The memory 1726 also stores the captured images (in image store 1730) and calibration graph data 1732, where used.

[0055] The image processing software 1728 is configured to capture pixel intensity data for one or more rows of pixels in the images captured by the cameras 808, 818, process the pixel intensity data (e.g. low pass filtering of the raw intensity data) and determine the pixel position of a feature in the processed intensity' data (in block 706) and identify the peak position (in block 712). The image processing software 1728 may also be configured to analyse the differences between relative pixel intensity graphs (in block 1210) and / or generate and / or use the calibration data (in the method of FIG. 14). The fiber positioning software 1736 is configured to control the rotatable elements 810, 820 to align the fibers (e.g. based on control signals received from the image processing software 1728) and the splice control software 1734 is configured to control the fusion unit 1706 to perform the splice once the fibers have been aligned. The image processing software 1728 may be further configured to analyse the quality of the splice after it has been performed, such as splice loss, amount of microstructure misalignment, fiber tilt angle, splice quality' pass or fail, etc.

[0056] Alternatively or in addition to the other examples described herein, examples include any combination of the following clauses:

[0057] Clause A: A method of azimuthal alignment of two antiresonant hollow core fibers comprising: for each antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into a splicer (702, 1410); side-illuminating the antiresonant hollow core fiber (704); capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data (705); and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data, the pixel position corresponding to an initial azimuthal orientation (706, 1412); and rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature (708-712, 1414-1416).

[0058] Clause B: The method according to clause A, wherein processing the intensity data comprises low pass filtering the intensity data.

[0059] Clause C: The method according to clause A or B. wherein the pre-defined feature comprises a peak in the processed the intensity' data.

[0060] Clause D: The method according to any of the preceding clauses, wherein rotating at least one of the two antiresonant hollow- core fibers based on the determined pixel positions of the pre-defined feature comprises, for each antiresonant hollow core fiber: rotating the antiresonant hollow core fiber to at least one new azimuthal orientation(708) and at each new azimuthal orientation; side-illuminating the antiresonant hollow core fiber (704); capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber at the new azimuthal orientation and processing the intensity data to generate processed intensity data (705); and determining, from the processed intensity data, a pixel position in the line corresponding to the pre-defined feature, the pixel position corresponding to the new azimuthal orientation (705-706); identifying, from the determined pixel positions, an extreme pixel position (712); and clamping the antiresonant hollow core fiber at an azimuthal orientation corresponding to the extreme pixel position (714).

[0061] Clause E; The method according to clause D, wherein capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data comprises: for each of a first and second image of the side-illuminated antiresonant hollow core, capturing intensify data for each pixel in a line of pixels in the image and processing the intensify data to generate processed intensity data, wherein the first image is captured using a first light source - image pair and the second image is captured using a second light source - image pair, and wherein determining, from the processed intensify data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data comprises: for each of the first and second image of the side-illuminated antiresonant hollow core, determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensify data; and wherein identifying, from the determined pixel positions, an extreme pixel position comprises: identifying, from the determined pixel positions from images captured by either the first or second camera, an extreme pixel position.

[0062] Clause F: The method according to clause E, further comprising:

[0063] switching off the light source in the second light source - camera pair when capturing an image of the side-illuminated antiresonant hollow core fiber with the first light source - camera pair; and switching off the light source in the first light source - camera pair when capturing an image of the side-illuminated antiresonant hollow core fiber with the second light source - camera pair.

[0064] Clause G: The method according to any of clauses A-C, wherein rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature comprises: for each antiresonant hollow core fiber, using a calibration graph to determine a current relative azimuthal orientation of theantiresonant hollow core fiber (1414), wherein the calibration graph comprises calibration data generated using another antiresonant hollow core fiber of a same type as the antiresonant hollow core fiber; rotating one or both of the antiresonant hollow core fibers so that they are at the same relative azimuthal orientation (1416); and clamping both antiresonant hollow core fibers (1418).

[0065] Clause H: The method according to clause G, further comprising generating the calibration graph by: inserting the other antiresonant hollow core fiber into the splicer (1402); side-illuminating the other antiresonant hollow core fiber; for each of a plurality of azimuthal orientations of the other antiresonant hollow core fiber and each of one or more light source - camera pairs: capturing intensity data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity’ data; and determining, from the processed intensity data, a pixel position in the line corresponding to the pre-defined feature; and storing a graph of, for each of the plurality of azimuthal orientations and for each light source - camera pair, the pixel position corresponding to the pre-defined feature (1408).

[0066] Clause I: A method of azimuthal alignment of two antiresonant hollow core fibers comprising: performing azimuthal alignment of a first antiresonant hollow core fiber and a second antiresonant hollow core fiber using the method of any of the preceding clauses; side-illuminating both the first and second antiresonant hollow core fibers (1204); capturing an image of each side-illuminated antiresonant hollow core fiber and determining a relative pixel intensity along a fiber cross-section for each antiresonant hollow core fiber (1206); laterally aligning the first and second antiresonant hollow core fibers using peaks in the relative pixel intensity' caused by an outer cladding of the antiresonant hollow core fiber (1208); rotating one or both of the antiresonant hollow core fibers to improve matching between the relative pixel intensity along a portion of the fiber cross-section corresponding to microstructure of the antiresonant hollow core fiber (1210); and clamping the antiresonant hollow core fibers (1214).

[0067] Clause J: A method of splicing two antiresonant hollow core fibers together, the method comprising: performing azimuthal alignment of two antiresonant hollow core fibers using the method of any of the preceding claims; and splicing the aligned antiresonant hollow core fibers together.

[0068] Clause K: The method according to any of the preceding clauses, wherein the antiresonant hollow core fiber has greater than two-fold rotational symmetry.

[0069] Clause L: The method according to any of the preceding clauses, wherein the antiresonant hollow core fiber is a nested antiresonant nodeless fiber.

[0070] Clause M: The method according to any of clauses A-K, wherein the antiresonant hollow core fiber is a double-nested antiresonant nodeless fiber.

[0071] Clause N: A splicer (1700) for antiresonant hollow core fiber comprising: a first rotatable element (810) for rotating a first antiresonant hollow core fiber through a plurality of azimuthal orientations; a second rotatable element (820) for rotating a second antiresonant hollow core fiber through a plurality of azimuthal orientations; a first light source (806, 1004) for side-illuminating the first antiresonant hollow core fiber; a first camera (808, 1008) for capturing an image of the side-illuminated first antiresonant hollow core fiber; and a control unit (1702) configured to: for each of the first and second antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into a splicer (702, 1410); side-illuminating the antiresonant hollow core fiber (704); and capturing intensity7data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity7data to generate processed intensity data (705); and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data, the pixel position corresponding to an initial azimuthal orientation (706, 1412); and rotate at least one of the first and second antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature (708-712, 1414-1416).

[0072] Clause O: The splicer according to clause N, further comprising a fusion unit (1702), wherein the control unit is further configured to: splice the first and second antiresonant hollow7core fibers using the fusion unit.

[0073] 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.

[0074] 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 descnbed above are disclosed as example forms of implementing the claims.

[0075] 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 orall of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[0076] 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.

[0077] 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.

[0078] The term ’subset’ is used herein to refer to a proper subset such that a subset of a set does not comprise all the elements of the set (i.e. at least one of the elements of the set is missing from the subset).

[0079] 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. A method of azimuthal alignment of two antiresonant hollow core fibers comprising: for each antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into a splicer; side-illuminating the antiresonant hollow core fiber; capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity’ data, the pixel position corresponding to an initial azimuthal orientation; and rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature.

2. The method according to claim 1, wherein processing the intensity data comprises low pass filtering the intensity data.

3. The method according to claim 1 or 2, wherein the pre-defined feature comprises a peak in the processed the intensity data.

4. The method according to any of the preceding claims, wherein rotating at least one of the two antiresonant hollow7core fibers based on the determined pixel positions of the pre-defined feature comprises, for each antiresonant hollow core fiber: rotating the antiresonant hollow core fiber to at least one new azimuthal orientation and at each new azimuthal orientation; side-illuminating the antiresonant hollow- core fiber; capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber at the new azimuthal orientation and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to the pre-defined feature, the pixel position corresponding to the new azimuthal orientation; identifying, from the determined pixel positions, an extreme pixel position; andclamping the antiresonant hollow core fiber at an azimuthal orientation corresponding to the extreme pixel position.

5. The method according to claim 4, wherein capturing intensity data for each pixel in a line of pixels in a respective image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data comprises: for each of a first and second image of the side-illuminated antiresonant hollow core, capturing intensity data for each pixel in a hne of pixels in the image and processing the intensity data to generate processed intensity data, wherein the first image is captured using a first light source - image pair and the second image is captured using a second light source - image pair, and wherein determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data comprises: for each of the first and second image of the side-illuminated antiresonant hollow core, determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity data; and wherein identifying, from the determined pixel positions, an extreme pixel position comprises: identifying, from the determined pixel positions from images captured by either the first or second camera, an extreme pixel position.

6. The method according to claim 5, further comprising: switching off the light source in the second light source - camera pair when capturing an image of the side-illuminated antiresonant hollow core fiber with the first light source - camera pair; and switching off the light source in the first light source - camera pair when capturing an image of the side-illuminated antiresonant hollow core fiber with the second light source - camera pair.

7. The method according to any of claims 1-3, wherein rotating at least one of the two antiresonant hollow core fibers based on the determined pixel positions of the predefined feature comprises: for each antiresonant hollow core fiber, using a calibration graph to determine a current relative azimuthal orientation of the antiresonant hollow core fiber, wherein the calibration graph comprises calibration data generated using another antiresonant hollow core fiber of a same type as the antiresonant hollow core fiber;rotating one or both of the antiresonant hollow core fibers so that they are at the same relative azimuthal orientation; and clamping both antiresonant hollow core fibers.

8. The method according to claim 7, further comprising generating the calibration graph by: inserting the other antiresonant hollow core fiber into the splicer; side-illuminating the other antiresonant hollow core fiber; for each of a plurality of azimuthal orientations of the other antiresonant hollow core fiber and each of one or more light source - camera pairs: capturing intensity data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to the pre-defined feature; and storing a graph of, for each of the plurality of azimuthal orientations and for each light source - camera pair, the pixel position corresponding to the pre-defined feature.

9. A method of azimuthal alignment of two antiresonant hollow core fibers comprising: performing azimuthal alignment of a first antiresonant hollow core fiber and a second antiresonant hollow core fiber using the method of any of the preceding claims; side-illuminating both the first and second antiresonant hollow core fibers; capturing an image of each side-illuminated antiresonant hollow core fiber and determining a relative pixel intensity along a fiber cross-section for each antiresonant hollow core fiber; laterally aligning the first and second antiresonant hollow core fibers using peaks in the relative pixel intensity caused by an outer cladding of the antiresonant hollow core fiber; rotating one or both of the antiresonant hollow core fibers to improve matching between the relative pixel intensity along a portion of the fiber cross-section corresponding to microstructure of the antiresonant hollow core fiber; and clamping the antiresonant hollow core fibers.

10. A method of splicing two antiresonant hollow core fibers together, the method comprising:performing azimuthal alignment of two antiresonant hollow core fibers using the method of any of the preceding claims; and splicing the aligned antiresonant hollow core fibers together.

11. The method according to any of the preceding claims, wherein the antiresonant hollow core fiber has greater than two-fold rotational symmetry.

12. The method according to any of the preceding claims, wherein the antiresonant hollow core fiber is a nested antiresonant nodeless fiber.

13. The method according to any of claims 1-11, wherein the antiresonant hollow core fiber is a double-nested antiresonant nodeless fiber.

14. A splicer for antiresonant hollow core fiber comprising: a first rotatable element for rotating a first antiresonant hollow core fiber through a plurality of azimuthal orientations; a second rotatable element for rotating a second antiresonant hollow core fiber through a plurality of azimuthal orientations; a first light source for side-illuminating the first antiresonant hollow core fiber; a first camera for captunng an image of the side-illuminated first antiresonant hollow core fiber; and a control unit configured to: for each of the first and second antiresonant hollow core fiber: inserting the antiresonant hollow core fiber into a splicer; side-illuminating the antiresonant hollow core fiber; and capturing intensity data for each pixel in a line of pixels in an image of the side-illuminated antiresonant hollow core fiber and processing the intensity data to generate processed intensity data; and determining, from the processed intensity data, a pixel position in the line corresponding to a pre-defined feature of the processed intensity' data, the pixel position corresponding to an initial azimuthal orientation; and rotate at least one of the first and second antiresonant hollow core fibers based on the determined pixel positions of the pre-defined feature.

15. The splicer according to claim 14, further comprising a fusion unit, wherein the control unit is further configured to: splice the first and second antiresonant hollow core fibers using the fusion unit.

Images