Rotational alignment of an optical fibre

Abstract

Provided is a method of determining a rotational alignment of an optical fibre having a structure that varies around a longitudinal axis. The method comprises capturing test measurements of the optical fibre, and determining the rotational alignment based on the test measurements and stored data. The stored data is obtained based on a reference fibre having a known rotational alignment and a structure corresponding to the structure of the optical fibre. Also provided are a method of obtaining stored data for use in a method of determining a rotational alignment, methods of preparing an optical fibre for machining by a machining apparatus, and a method of machining an optical fibre.

Description

[0001] ROTATIONAL ALIGNMENT OF AN OPTICAL FIBRE

[0002] The present disclosure relates to methods for the rotational alignment of an optical fibre, in particular to improve the accuracy of machining of the optical fibre.

[0003] Optical fibres are used for a wide range of applications including communications and various types of environmental sensing. The propagation of light in optical fibres is heavily affected by the properties of the material of the optical fibre. Much research has been conducted into optimising the materials and combination of materials used in optical fibres intended for different applications. However, it can nonetheless be difficult to find suitable materials or material combinations that have the desired properties of particular applications. In particular, the properties of optical fibre materials may change with temperature and pressure, thereby causing unwanted effects on the propagation of light in the fibres.

[0004] To address these shortcomings, it is known to provide optical fibres with holes along their length in order to alter the properties of the optical fibre. For example, hollowcore anti-resonant fibres (ARFs) provide low loss light confinement for long distance transmission.

[0005] ARFs belong to the class of hollow core fibres including photonic crystal fibres (PCFs) and “Kagome" fibre. The internal structure of anti-resonant fibres includes several hollow capillaries which surround the core. The light confinement within the ARF is provided by the negative curvature created by these capillaries, providing low loss transmission where the light guidance mechanism follows the anti-resonant reflecting optical waveguide (ARROW) model. The cylindrical surface created by the capillaries can be considered to be an azimuthal diffraction grating and these fibres can also be considered single mode after a few metres.

[0006] Specific transmission properties can be targeted by the design of the internal structure of the ARF. To reduce confinement loss, the core should be enlarged with thin capillary walls where each capillary is spaced apart from its neighbours to avoid creating additional optical resonators. Although, the latter is a balance between light confinement and single mode operation.

[0007] The internal structure of ARFs allows microchannels to be fabricated in the gap between the capillaries for gaseous access to the core. The entire fibre is therefore used as the active medium and the microchannels allow for gas sensing with much greater sensitivity and close to real-time response. The use of optical fibre for gas detection has been demonstrated for the detection of nitrous oxide, ethane and dissolved methane gas in liquid, for example.

[0008] In particular, the capillary gap in ARF allows for the fabrication of microchannels to be used for gaseous access to the core without affecting the light guidance structure, unlike would be the case with photonic crystal fibre, for example. A series of these microchannels allows the entire length of fibre to be used as the active medium for gas detection applications. From the Beer-Lambert law, it can be seen that the sensitivity in gas sensing experiments is heavily dictated by the interaction length and previously this has been increased by repeated reflections of the light. Hence, increasing the absorbance for a given molecular concentration and greater sensitivity is therefore achievable. This has been done using multipass cells such as the Herriott cell and the White cell and even larger pathlengths can be achieved using a resonant optical cavity. Using metres of optical fibre instead is advantageous, allowing for remote sensing and smaller, more compact devices than a resonant cavity.

[0009] The benefit of the microchannels on the response time of gas sensing can be seen by considering the free diffusion time and the Einstein- Smoluchowski equation. If the gas is inserted in one end of the fibre, the fibre filling time is heavily limited by the free diffusion of gases which occurs at a rate of several hours per metre. For the example of carbon dioxide detection with a diffusion coefficient in air of 0.16 cm2 / s and microchannels at a separation of 1 cm, then the diffusion time greatly decreases to 1 s. The reduction in the diffusion time via the introduction of microchannels allows for close to real-time monitoring of gas composition while lengths of fibre of the order of 10 m or higher could be used for increased sensitivity without a compromise in response time and the need for a pressure differential.

[0010] Such fibre systems can be used for remote sensing of gas concentration including atmospheric mapping and combustion diagnostics. Many medical uses revolve around isotopic ratio measurements. . The environmental applications can be extended to the use of drones for atmospheric mapping, and methane production has been monitored in real time via an absorption cavity present in a moving vehicle. Isotopic ratio measurements can also determine the mixture of organic compounds in combustion products.

[0011] Further applications include detection of isotopes of carbon in the geochemical surveillance of volcanoes. Anti-resonant fibre, specifically, has been used for the detection of various physical parameters including temperature, pressure and mechanical force by exploiting the anti-resonant properties of the fibre, monitoring the shift in its resonant wavelengths. The fibres can also be employed to detect liquid where the hollow core allows for injection into the fibre for optofluidic applications such as gas leakage and tissue fluid detection.

[0012] While the applications and advantages of ARFs including microchannels are numerous, the fabrication of the microchannels is challenging. In particular, even a small rotational misalignment during the machining of the microchannels could cause damage to the structure of the hollow capillaries. This will significantly reduce the optical performance of the ARF. In long fibres, current rotational alignment methods that rely on imaging of the end face of the fibre may be ineffective because the end face may be a significant distance from the machining point. This means that the rotational alignment at the end face cannot be relied on as a reference for the alignment at the machining point because even a small twist in the fibre will cause a significant difference in rotational alignment between the end face and the alignment at the machining point.

[0013] It is therefore desirable to provide improved methods of aligning optical fibres such as ARFs that have a structure that varies around the longitudinal axis of the optical fibre.

[0014] According to a first aspect of the invention, there is provided a method of determining a rotational alignment of an optical fibre, wherein a structure of the optical fibre varies around a longitudinal axis of the optical fibre, the method comprising: capturing one or more test measurements of the optical fibre; and determining the rotational alignment of the optical fibre based on the one or more test measurements and stored data, wherein the stored data is obtained based on a reference fibre having a known rotational alignment, a structure of the reference fibre corresponding to the structure of the optical fibre.

[0015] Determining the alignment of the optical fibre based on stored data obtained using a reference fibre improves accuracy because the reference fibre corresponds to the optical fibre. This provides a direct comparison to a known reference, unlike methods such as maximising the reflection from the hollow capillaries as the optical fibre is rotated. In addition, the method can be more responsive by reducing the computational burden of determining the alignment.

[0016] Optionally, determining the rotational alignment comprises: calculating a comparative metric between the one or more test measurements and the stored data; and determining the rotational alignment of the optical fibre based on the comparative metric, optionally wherein the comparative metric is a cross-correlation. Calculating a comparative metric simplifies the alignment procedure by providing a parameter for optimisation to determine the correct alignment.

[0017] Optionally, the one or more test measurements comprise one test measurement; the stored data comprise a plurality of reference measurements; calculating the comparative metric comprises calculating a comparative metric between the test measurement and each of the plurality of reference measurements; and determining the rotational alignment of the optical fibre based on the comparative metric comprises: identifying, based on the comparative metric, a first reference measurement of the plurality of reference measurements having a greatest similarity to the test measurement; and determining the rotational alignment of the optical fibre corresponding to the test measurement using the known rotational alignment of the reference fibre corresponding to the first reference measurement.. This removes the need to rotate the optical fibre to determine its alignment, which may be preferable depending on the setup in which the optical fibre is being used.

[0018] Optionally, the stored data comprise one reference measurement; the one or more test measurements comprise a plurality of test measurements captured at different rotational alignments of the optical fibre; and calculating the comparative metric comprises calculating a comparative metric between the reference measurement and each of the plurality of test measurements; determining the rotational alignment of the optical fibre based on the comparative metric comprises: identifying, based on the comparative metric, a first test measurement of the plurality of test measurements having a greatest similarity to the reference measurement; and determining the rotational alignment of the optical fibre corresponding to the first test measurement using the known rotational alignment of the reference fibre corresponding to the reference measurement. This removes the need to have a larger number of reference measurements by instead rotating the optical fibre. This may be preferable if there is a lack of available reference measurements.

[0019] Optionally, the stored data comprise a plurality of reference measurements; determining the rotational alignment comprises using a machine learning algorithm; and the machine learning algorithm is trained using the plurality of reference measurements and the known rotational alignments of the reference fibre corresponding to the plurality of reference measurements. This may improve the ability of the method to extrapolate between reference measurements to provide a more accurate determination of the alignment of the optical fibre.

[0020] Optionally, the machine learning algorithm comprises a neural network. This is a convenient type of machine learning algorithm that is suitable for adaptation to this specific application.

[0021] Optionally, one or more of the plurality of reference measurements corresponds to one or more different illumination conditions. This will improve the ability of the machine learning algorithm to provide accurate determinations of alignment under different operating conditions.

[0022] Optionally, capturing the one or more test measurements comprises capturing a plurality of test measurements; and the optical fibre is rotated about the longitudinal axis of the optical fibre between the capturing of each test measurement. This provides plural measurements to compare to the stored data so that the rotational alignment can be discerned.

[0023] Optionally, a relative change in the rotational alignment of the optical fibre between the capturing of each test measurement is at most 30 degrees, optionally at most 20 degrees, optionally at most 10 degrees. This ensures sufficient resolution of the test measurements that an accurate determination of the rotational alignment can be determined.

[0024] Optionally, the structure of the optical fibre has rotational symmetry of at least order two about the longitudinal axis of the optical fibre; and a relative change in the rotational alignment of the optical fibre between the capturing of each test measurement is at most 50% of an angle between rotationally symmetric positions of the optical fibre about the longitudinal axis of the optical fibre, optionally at most 25%, optionally at most 10%. Lower resolution is required for an adequate resolution if the rotational symmetry of the fibre is of lower order. This therefore provides adequate resolution while avoiding the need for a number of measurements that is inappropriate for the symmetry of the optical fibre.

[0025] Optionally, the stored data comprise one or more reference images that are captured image data or generated image data, optionally wherein the generated image data is based on modification of data relating to one or more images of the reference fibre. Image data is a convenient format for comparison with the test measurements, which may themselves be images. Generated image data may be used to post-process captured images to make them more suitable for subsequent comparison. Alternatively or additionally, some image data may be entirely generated, for example based on a simulation using the known structure of the reference fibre. This can provide additional training or reference material.

[0026] Optionally, the one or more reference images correspond to a view of the reference fibre based on a spatial distribution of one or more data points in one or more dimensions. A full image of the reference fibre may not be necessary. For example, a smaller number of sampled data points may be sufficient to align the optical fibre. This simplifies the process of collecting reference data and reduces memory requirements on the stored data.

[0027] Optionally, the one or more reference images correspond to a view from a side of the reference fibre. This allows the images to be taken at any point along the length of the reference fibre for comparison with corresponding images of the side of the optical fibre, thereby avoiding the need to cleave the optical fibre when collecting test measurements.

[0028] Optionally, the one or more test measurements comprise one or more test images of the optical fibre, optionally wherein the one or more test images correspond to a view from a side of the optical fibre; the spatial distribution of one or more data points in one or more dimensions comprises an intensity profile along a line, the line at an angle to a length of the reference fibre; and determining the rotational alignment comprises using an intensity profile along a corresponding line in the one or more test images, optionally wherein the line is perpendicular to the length of the optical fibre and reference fibre. This greatly simplifies the process of collecting the test measurements.

[0029] According to a second aspect of the invention, there is provided a method of obtaining stored data for use in a method of determining a rotational alignment of an optical fibre, the method of obtaining stored data comprising: capturing, generating and / or modifying a reference image of a reference fibre, a structure of the reference fibre corresponding to a structure of the optical fibre; capturing, generating and / or modifying an alignment image corresponding to a view from a longitudinal end face of the reference fibre, optionally wherein the longitudinal end face is cleaved; and determining a rotational alignment of the reference fibre when capturing, generating and / or modifying the reference image based on the alignment image.

[0030] As discussed above, the stored data can be used for more accurately and efficiently determining the rotational alignment. The view from a longitudinal end face provides a definite and easily interpreted measure of the rotational alignment because the internal structure of the reference fibre is visible. The alignment image can therefore be used to accurately label the reference images with the corresponding rotational alignment of the reference fibre.

[0031] Optionally capturing, generating and / or modifying the reference image comprises capturing a plurality of reference images; the reference fibre is rotated about the longitudinal axis of the reference fibre between the capturing, generating and / or modifying of each reference image; capturing, generating and / or modifying the alignment image comprises capturing, generating and / or modifying a plurality of alignment images corresponding to the plurality of reference images; and determining the rotational alignment comprises determining a rotational alignment of the reference fibre during the capturing, generating and / or modifying of each reference image based on the corresponding alignment image. This provides a series of reference images and alignment images at varying rotational positions for accurate determination of the rotational alignment in each reference image.

[0032] Optionally, a relative change in the rotational alignment of the reference fibre between the capturing, generating and / or modifying of each reference image is at most 30 degrees, optionally at most 20 degrees, optionally at most 10 degrees. This ensures sufficient resolution of the reference images that an accurate determination of the rotational alignment can be determined during test measurements of the optical fibre.

[0033] Optionally, the structure of the reference fibre has rotational symmetry of at least order two about the longitudinal axis of the reference fibre; and a relative change in the rotational alignment of the reference fibre between the capturing, generating and / or modifying of each reference image is at most 50% of an angle between rotationally symmetric positions of the reference fibre about the longitudinal axis of the reference fibre, optionally at most 25%, optionally at most 10%. Lower resolution is required for an adequate resolution if the rotational symmetry of the fibre is of lower order. This therefore provides adequate resolution while avoiding the need for a number of measurements that is inappropriate for the symmetry of the optical fibre.

[0034] According to a third aspect of the invention, there is provided a method of preparing an optical fibre for machining by a machining apparatus comprising: determining a rotational alignment of the optical fibre using the method of the first aspect; and rotating the optical fibre to a target rotational alignment relative to the machining apparatus based on the determined rotational alignment. This allows the machining to be carried out accurately at a desired point around the circumference of the optical fibre.

[0035] According to a fourth aspect of the invention, there is provided a method of preparing an optical fibre for machining by a machining apparatus comprising: determining a rotational alignment of the optical fibre using the method of the first aspect; and adjusting a configuration of the machining apparatus based on the determined rotational alignment. The necessary configuration of the machining apparatus may depend on the rotational alignment of the fibre. In addition, the position at which the optical fibre is machined around the circumference of the optical fibre can be varied by changing the configuration of the machining apparatus rather than rotating the optical fibre. This can reduce the complexity of the apparatus and its setup by removing the need to rotate the optical fibre.

[0036] Optionally, adjusting the configuration of the machining apparatus comprises adjusting the configuration such that the machining apparatus is configured to machine the optical fibre at a target position within a cross-section of the optical fibre. As well as the holes discussed above, other advantageous properties can be imparted to the optical fibre by machining structures within the cross-section.

[0037] Optionally, adjusting the configuration comprises one or more of a) modifying a wavefront of an optical beam, optionally wherein modifying the wavefront comprises using an adaptive optical element, optionally wherein the adaptive optical element is a spatial light modulator; b) adjusting an aberration compensation of the machining apparatus; and c) adjusting a coma of the machining apparatus. All of these factors may need adjusting because focussing the beam within the optical fibre will cause distortion of the optical beam that is highly dependent on the specific material and structure of the optical fibre.

[0038] Optionally, the machining apparatus is configured to machine the optical fibre using a laser focussed on the optical fibre through an objective lens; and the one or more test images are captured by imaging through the objective lens. Using the same lens for machining and imaging makes the apparatus more compact and increases accuracy by ensuring that the view used for alignment is exactly that which will be seen by the machining apparatus.

[0039] According to a fifth aspect of the invention, there is provided a method of machining an optical fibre using a machining apparatus comprising: preparing the optical fibre for machining using the method of the third or fourth aspect; and machining the optical fibre using the machining apparatus. As discussed above, the alignment methods allow for more accurate alignment, resulting in more accurate machining. Optionally, the structure of the optical fibre and the reference fibre is a hollow core structure; and the machining of the optical fibre comprises machining a channel fluidly connecting the hollow core with an exterior of the optical fibre. The holes allow for penetration of fluids into the hollow cores for applications such as sensing the composition of a gas.

[0040] Optionally, the method further comprises repeating the steps of preparing the optical fibre and machining the optical fibre for each of a plurality of target rotational alignments or target positions around the circumference of the optical fibre. This can produce more complex structures, for example rotationally symmetric structures to match a rotational symmetry of the internal structure of the optical fibre.

[0041] Optionally, the method further comprises: adjusting a position of the optical fibre and / or a configuration of the machining apparatus such that the machining apparatus is configured to machine the optical fibre at a different longitudinal position along a length of the optical fibre; and repeating the steps of preparing the optical fibre and machining the optical fibre at the new longitudinal position. Machining at several positions along the length allows for longer lengths of optical fibre to be processed, for example to increase sensitivity in sensing applications by making a greater length of optical fibre more sensitive and responsive to ingress of fluid.

[0042] In any of the above aspects, the optical fibre and the reference fibre may be hollow core fibres, photonic crystal fibres, anti-resonant fibres, polarisation maintaining fibres, or side-hole fibres. These are common types of fibres with rotationally-varying internal structure that can advantageously be more accurately aligned and machined using the above methods.

[0043] In any of the above aspects, the optical fibre and the reference fibre may comprise one or more hollow capillaries surrounding the longitudinal axis of the optical fibre and the reference fibre. The hollow capillaries can be used to vary the properties of the optical fibre for particular applications.

[0044] In any of the above aspects, each of the optical fibre and the reference fibre may comprise a plurality of cores. This is a common type of rotationally-varying internal structure that can advantageously be more accurately aligned and machined using the above methods.

[0045] Optionally, the cores are twisted around the longitudinal axis of the optical fibre and the reference fibre along a length of the optical fibre and the reference fibre. This type of structure is advantageous in certain applications, but is particularly difficult to align using existing methods.

[0046] Optionally, a core of the plurality of cores that is located away from the longitudinal axis of the optical fibre and the reference fibre is laser modified using its known rotational position, optionally where the laser modification comprises the inscribing of a Bragg grating. Selective modification of cores can be useful in certain applications. For example, Bragg gratings provide a clear peak in reflectivity which can be shifted by external conditions to permit sensing applications.

[0047] According to a sixth aspect of the invention, there is provided an apparatus for determining a rotational alignment of an optical fibre, wherein a structure of the optical fibre varies around a longitudinal axis of the optical fibre, the system comprising: a sensor configured to capture one or more test measurements of the optical fibre; and a processing unit configured to determine the rotational alignment of the optical fibre based on the one or more test measurements and stored data, wherein the stored data is obtained based on a reference fibre having a known rotational alignment, a structure of the reference fibre corresponding to the structure of the optical fibre. The apparatus of the sixth aspect can be used to carry out the method of the first aspect of the invention.

[0048] According to a seventh aspect of the invention, there is provided an apparatus for obtaining stored data for use in determining a rotational alignment of an optical fibre, the apparatus comprising: a first sensor configured to capture a reference image of a reference fibre, a structure of the reference fibre corresponding to a structure of the optical fibre; a second sensor configured to capture an alignment image corresponding to a view from a longitudinal end face of the reference fibre; and a processing unit configured to determine a rotational alignment of the reference fibre when capture the reference image based on the alignment image. The apparatus of the seventh aspect can be used to carry out the method of the second aspect of the invention.

[0049] According to an eighth aspect of the invention, there is provided a system for machining an optical fibre, comprising the apparatus for determining a rotational alignment of the optical fibre of the sixth aspect; and an apparatus configured to machine the optical fibre based on the rotational alignment of the optical fibre determined by the apparatus for determining the rotational alignment of the optical fibre.

[0050] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols represent corresponding parts, and in which

[0051] Fig. 1 A and Fig. IB show a cross-section and side view of an anti-resonant optical fibre;

[0052] Fig. 2A and Fig. 2B show the optical fibre of Fig. 1 A and Fig. IB after a series of microchannels have been fabricated along the length of the optical fibre;

[0053] Fig. 3 A and Fig. 3B show a photonic crystal fibre;

[0054] Fig. 4A and Fig. 4B show the photonic crystal fibre of Fig. 3 A and Fig. 3B after microchannels have been fabricated to provide fluid access to the hollow core;

[0055] Fig. 5 shows a side view of an embodiment in which the position of the microchannels around the circumference of the optical fibre changes along the length of the optical fibre;

[0056] Fig. 6 shows the mode loss of the fundamental mode of an optical fibre plotted against the rotation of the microchannel relative to the centre of the core;

[0057] Fig. 7 shows the microchannel positions corresponding to the labelled positions in Fig. 6;

[0058] Fig. 8A to Fig. 8C show examples of optical fibres with a structure that varies around the longitudinal axis;

[0059] Fig. 9A to Fig. 9C show the optical fibres of Fig. 8 A to Fig. 8C after they have been rotationally aligned such that a machined structure is formed symmetrically with respect to the internal structure;

[0060] Fig. 10 shows a multicore optical fibre with cores twisted around the longitudinal axis;

[0061] Fig. 11 is a flowchart of a method of determining the rotational alignment of an optical fibre;

[0062] Fig. 12 shows an apparatus for carrying out the method of Fig. 11;

[0063] Fig. 13A and Fig. 13B show view of the platen of the apparatus of Fig. 12;

[0064] Fig. 14 shows a graph of cross-correlation against optical fibre rotational alignment;

[0065] Fig. 15 shows a flowchart of a method of obtaining stored data for use in the method of Fig. 11;

[0066] Fig. 16 shows an apparatus for capturing reference images;

[0067] Fig. 17A to Fig. 17C show reference images captured at 10-degree intervals of rotation of an ARF having hollow internal capillaries; Fig. 18A to Fig. 18C show alignment images corresponding to the images in Fig.

[0068] 17Ato Fig. 17C;

[0069] Fig. 19A and Fig. 19B illustrates a step of rotating the optical fibre to a target rotational alignment based on the determined rotational alignment;

[0070] Fig. 20A and Fig. 20B illustrate adjusting a configuration of a machining apparatus based on the determined rotational alignment;

[0071] Fig. 21 A and Fig. 21B illustrate steps in the machining of microchannels;

[0072] Fig. 22 shows an optical fibre having microchannels machined therein;

[0073] Fig. 23 A and Fig. 23B show the dimensions of the core of a commercial iXBlue ARF;

[0074] Fig. 24A and Fig. 24B show the dimensions of the core of an ARF provided by the University of Bath;

[0075] Fig. 25 is a diagram of a laser system used for microchannel fabrication;

[0076] Fig. 26A and Fig. 26B show examples of reference images;

[0077] Fig. 27 shows the cross-correlation between test and reference images;

[0078] Fig. 28 shows multiple maxima in cross-correlation corresponding to different capillaries aligned with the reference image;

[0079] Fig. 29A and Fig. 29B show cross-sectional SEM images of microchannels fabricated into the iXBlue;

[0080] Fig. 30 shows an in-situ loss measurement system;

[0081] Fig. 31 shows the increase in loss in an optical fibre with increasing microchannel length;

[0082] Fig. 32 is an SEM image showing debris present in the core of an optical fibre after machining;

[0083] Fig. 33 illustrates a process for fabricating microchannels in a coated optical fibre; Fig. 34 shows images of a coated optical fibre in which microchannels have been fabricated using the method illustrated in Fig. 33; and

[0084] Fig. 35 shows an apparatus in which a multicore optical fibre is used as a 3D shape sensor.

[0085] As discussed above, it is desirable to be able to accurately determine the rotational alignment of optical fibres. In particular, an important application in which it is desirable to determine the rotational alignment of an optical fibre is in the fabrication of microchannels in optical fibres. For example, microchannels may be fabricated in hollow- core optical fibres such as anti-resonant fibres (ARFs) to allow fluid access for sensing applications.

[0086] Fig. 1 A shows a cross-section of an anti-resonant optical fibre 1 having an outer cladding 3 surrounding a hollow core 5. A plurality of capillaries 7 are present surrounding the hollow core 5 within the outer cladding 3. Fig. IB shows a corresponding side view of the optical fibre 1 of Fig. 1A.

[0087] Fig. 2A and Fig. 2B show the optical fibre 1 of Fig. 1A and Fig. IB after a series of microchannels 9 have been fabricated along the length of the optical fibre 1. As shown in the cross-section view of Fig. 2A, these microchannels provide fluid access to the hollow core 5 of the optical fibre 1 from the outside environment.

[0088] Another example of an optical fibre 1 to which the method may be applied is a photonic crystal fibre, such as shown in Fig. 3 A and Fig. 3B. The photonic crystal fibre comprises a large plurality of holes 12 within its outer cladding 3 that are arranged in a regular pattern around the hollow core 5. Fig. 4 A and Fig. 4B show the photonic crystal fibre of Fig. 3 A and Fig. 3B after microchannels 9 have been fabricated to provide fluid access to the hollow core 5.

[0089] Microchannels may be fabricated in a variety of different configurations depending on the application. The spacing and geometry of the microchannels can be used to change the sensitivity and response time of the fibre system. Fig. 5 shows a side view of an embodiment in which the position of the microchannels around the circumference of the optical fibre 1 changes along the length of the optical fibre 1. This may be desirable in some circumstances, for example if the internal structure of the optical fibre 1 varies along its length. Fig. 5 also shows the cross-section at various positions along the length of the optical fibre 1.

[0090] The fabrication of microchannels can introduce transmission loss mostly resulting from damage to the fibre structure or the presence of debris in the core. The effects of these can be reduced by ensuring correct fibre rotation with the capillary gap underneath the objective lens.

[0091] To illustrate the adverse effect of poor rotational alignment, the maximum increase in the mode loss caused by the introduction of the microchannels was found by modelling a slot down the length of an optical fibre present in the capillary gap. The fibre modelled was a commercial fibre from iXBlue photonics designed for light guidance at 1550 nm and having seven anti-resonant capillaries. By modelling the additional loss incurred to the fundamental mode by fabricating the microchannel at different rotations of the fibre, it can be seen how it is essential to have an estimate for the location of the capillaries to avoid damaging the light guiding structure. The alignment is also required for non-destructive gaseous access to the light-localised core.

[0092] Fig. 6 shows the mode loss of the fundamental mode plotted against the rotation of the microchannel relative to the centre of the core. Fig. 7 shows the microchannel positions corresponding to the labelled positions in Fig. 6. The modelling is done using an FEM solver on FIMMWAVE made by Photon Design. The refractive index of the pure silica cladding was taken to be around 1.445 at 1550 nm where the iXBlue fibre is designed for transmission of this wavelength of light. The effective refractive index of the mode for which the loss was recorded, varied slightly around 0.999722.

[0093] It is found that the mode loss increases to 0.2 dB / m from around the quoted value of 0.034 dB / m when the slot is introduced. Maxima can be seen in the mode loss where the light confinement is greatly reduced by damage to the inside capillary structure, causing the mode loss to increase to 45 dB / m. Additionally, a small increase in mode loss is found when the microchannel destroys the sides of the capillary structure. The angles of lowest mode loss correspond to non-destructive gaseous access solely to the core as well as no structural disturbances to the capillaries. The features of the recorded mode loss in Fig.

[0094] 6 are mirrored on the other side of the maxima where the microchannel reaches the other edge of the capillary.

[0095] These results demonstrate the desirability of accurate determination of the rotational alignment of the optical fibre. In the example of microchannels in an ARF, a robust and repeatable method for finding the capillary gap provides long path lengths and hence higher sensitivity as well as short response time. This will benefit many of the applications of gas sensing and also may provide the higher sensitivity required for telecommunications wavelengths.

[0096] The local cross section of the fibre will be inaccessible for long lengths of fibre where twisting may be present. A method therefore needs to be developed where internal structural features such as the local capillary position can be determined by imaging the optical fibre from the top or side, rather than from an end where the cross-section is visible. Previous demonstrations of non-destructive side imaging of fibre involve monitoring of fibre diameter and capillary size during fibre fabrication [3-6],

[0097] A method for determining capillary position has recently been demonstrated by Koziol et al. [1], A series of 25030 pm microchannels were fabricated using a femtosecond laser in the outer cladding of an ARF, directly above the gap between the cladding capillaries. The gap was found by focusing on the surface of the inner capillaries and rotating the fibre until these points showed the same light intensity on the CCD camera.

[0098] Similarly, Xiao et al. [2] moved a multicore fibre using a translation stage underneath a fixed femtosecond laser and a CCD was used to capture images of the core. An image recognition algorithm was then used to position the centre of the fibre in realtime and a micro-displacement compensation technique used for auto-aligning the core centre with the focus of the femtosecond laser.

[0099] All of these existing methods have drawbacks in speed of processing and the complexity of setup. To address some of these drawbacks, the present disclosure provides a method of determining a rotational alignment of an optical fibre 1, wherein a structure of the optical fibre varies around a longitudinal axis of the optical fibre 1. The present method allows the determining of the rotational alignment of an optical fibre without access to the local cross section. This enables the local alignment of sections of a long, continuous length of optical fibre (e.g. several metres) where the local cross-section cannot be accessed.

[0100] The structure of the optical fibre 1 that varies around the longitudinal axis may be an internal structure of the optical fibre 1. The optical fibre 1 may comprise one or more hollow capillaries 7 surrounding the longitudinal axis. Examples of possible optical fibre types that can be used with the present method include hollow core fibres, photonic crystal fibres, anti-resonant fibres, polarisation maintaining fibres, or side-hole fibres. An example of an ARF with capillaries 7 arranged around its longitudinal axis is shown in Fig.

[0101] 1 A and Fig. 2A. An example of a photonic crystal fibre is shown in Fig. 3A and Fig. 4A.

[0102] Other examples exist of optical fibres 1 with a structure that varies around the longitudinal axis are shown in Fig. 8 A to Fig. 8C. Fig. 8 A shows an optical fibre 100 with a series of voids 101 around the core 11. The voids 101 may be filled with air or another fluid to affect the transmissive properties of the optical fibre 100.

[0103] The optical fibre 1 may comprise a plurality of cores 11. Fig. 8B shows a multicore fibre 110 with plural cores 11 arranged in a regular pattern through the interior of the cladding 3. The cores 11 may be twisted around the longitudinal axis of the optical fibre 110 along a length of the optical fibre 110, such as shown in Fig. 10. The cores of the reference fibre may also be twisted or, optionally, a sufficiently short section of fibre may be used as the reference fibre such that the degree of twisting along the length of the reference fibre is below a predetermined threshold.

[0104] Fig. 8C shows an air-hole fibre with a pair of large air-filled holes 121 running either side of the core 11. All of these optical fibres could be machined with microchannels or other structures, and so it may be desirable to align them rotationally such that the machined structure is formed symmetrically with respect to the internal structure, as shown in Fig. 9A to Fig. 9C.

[0105] Fig. 11 is a flowchart of the present method. The method comprises capturing S10 one or more test measurements 10 of the optical fibre 1. The one or more test measurements 10 may comprise one or more test images of the optical fibre 1. The one or more test images may correspond to a view of the optical fibre based on a spatial distribution of one or more data points in one or more dimensions. For example, the data points may comprise a set of pixel values or another set of measurement values such as pixel intensities or brightness values that are spatially distributed over a region in which the optical fibre 1 is located.

[0106] The test images may be two-dimensional images of a region containing the optical fibre 1. Alternatively, the test images may be one-dimensional images, each representing a set of points along a line at an angle to a length of the optical fibre 1. The line may be perpendicular to the length of the optical fibre 1. The internal structure of the optical fibre 1 will not change significantly over a short length of the optical fibre 1, and so a onedimensional line across the optical fibre 1 may be sufficient to determine the rotational alignment of the optical fibre 1.

[0107] An example apparatus for determining a rotational alignment of an optical fibre, which can be used for carrying out the method, is shown in Fig. 12. The apparatus comprises a rotatable platen 25 on which the optical fibre 1 is suspended. The rotatable platen is supported by a pair of supports 24 fixed to a baseplate 22. Each support 24 contains a cylindrical hole into which the platen 25 is rotatably secured. The optical fibre 1 is secured in place by a pair of clamps 29. Two reels 21 located either side of the platen allow a long length of optical fibre 1 to be moved through section by section and rotationally aligned. A rotational adjustment device 27 such as a gear or thumbwheel allows the rotational alignment of the section of optical fibre 1 held in the platen 25 to be adjusted. An air table 23 allows the position of the baseplate 22, and thereby the optical fibre 1, to be adjusted in an imaging plane.

[0108] Fig. 13 A shows a top-down view of the platen 25. A hole 26 is provided in a central region of the platen 25 to allow illumination of the optical fibre 1. Fig. 13B shows a cross-section view of the optical fibre 1 secured in the platen 25. A v-shaped notch 28 is provided in the platen 25 to help localise the optical fibre 1.

[0109] The apparatus comprises a sensor configured to capture the one or more test measurements 10. For example, where the test measurements are test images, the test images may be captured using a camera 35, optionally with an objective lens 31 to properly focus the image. The camera 35 may be any suitable type of camera such as a CCD camera. One or more other lenses 33 may be used to focus the image of the optical fibre 1 at the camera 35. The optical fibre 1 may be imaged with reflection illumination of the camera 35, or by transmission illumination.

[0110] The apparatus of Fig. 12 comprises further features including a beam splitter 41, mirror 43, spatial light modulator 45, laser 47, and further lenses 49. These can be used for machining of the optical fibre 1 in combination with determining the rotational alignment of the optical fibre 1. This will be discussed in further detail below. However, these additional elements are not necessary merely for determining the rotational alignment of the optical fibre 1.

[0111] The one or more test images may correspond to a view from a side of the optical fibre 1. In other words, an imaging axis along which the test image is captured may be more perpendicular to the longitudinal axis of the optical fibre 1 than parallel to the longitudinal axis of the optical fibre 1. A smallest angle between the imaging axis and the longitudinal axis of the optical fibre 1 may be at least 45 degrees, optionally at least 60 degrees, optionally at least 80 degrees. Optionally, the imaging axis is substantially perpendicular to the longitudinal axis of the optical fibre 1.

[0112] The test images may be of an outer surface of the optical fibre 1, or the test images may be focussed in an interior of the optical fibre 1. For example, where the optical fibre 1 comprises a plurality of capillaries 7, the test images may be focussed on a surface of one of the capillaries 7 or at a position within the optical fibre 1 where a surface of one of the capillaries 7 is expected to be when the optical fibre 1 has a target rotational alignment. The method may comprise a step of adjusting a depth within the optical fibre 1 at which the test image is focussed.

[0113] Where the method comprises capturing a plurality of test measurements 10, the optical fibre 1 is rotated about the longitudinal axis of the optical fibre 1 between the capturing of each test measurement 10. In order to ensure sufficient resolution to correctly align the optical fibre 1, a relative change in the rotational alignment of the optical fibre 1 between the capturing of each test measurement may be at most 30 degrees. Optionally, the relative change may be at most 20 degrees, optionally at most 10 degrees. The relative change here refers to the difference between the angular position before the rotation and after the rotation. A rotation of 365 degrees would only cause a relative change in the rotational alignment of 5 degrees.

[0114] The relative change in rotational alignment may also be expressed relative to a symmetry of the optical fibre 1, since fibres with a higher degree of rotational symmetry would require smaller angular rotations to accurately resolve their rotational alignment. Where the structure of the optical fibre 1 has rotational symmetry of at least order two about the longitudinal axis of the optical fibre 1, a relative change in the rotational alignment of the optical fibre 1 between the capturing of each test measurement may be at most 50% of an angle between rotationally symmetric positions of the optical fibre 1 about the longitudinal axis of the optical fibre 1. Optionally, the relative change may be at most 25%, optionally at most 10%.

[0115] The method further comprises determining S30 the rotational alignment of the optical fibre based on the one or more test measurements 10 and stored data 20. The apparatus of Fig. 12 comprises a processing unit (not shown) that is configured to perform the determining S30 of the rotational alignment.

[0116] The stored data 20 is obtained based on a reference fibre having a known rotational alignment. A structure of the reference fibre corresponds to the structure of the optical fibre 1. The structure of the reference fibre may be identical to the structure of the optical fibre 1. For example, the reference fibre and the optical fibre may be obtained from the same length of fibre. However, this is not essential and there may be minor variations in the dimensions of the reference fibre relative to the optical fibre 1 as long as the internal structure of the reference fibre, e.g. a cross-section of the reference fibre perpendicular to its length, is substantially the same as that of the optical fibre 1. In particular, the reference fibre should be of the same type as the optical fibre 1, having the same number and distribution through the cross-section of voids, capillaries, cores etc. as the optical fibre 1 as appropriate according to the type of the optical fibre 1.

[0117] The stored data 20 may comprise one or more reference measurements, which may be reference images. The reference images may be captured image data that are obtained by taking images of the reference fibre using a camera, similar as for the taking of the test images. Alternatively, the reference images may be generated image data. The generated image data could be created, for example computer generated, based on a simulation of the known structure of the optical fibre 1. The stored data 20 further comprise a known rotational alignment corresponding to each of the reference images.

[0118] The generated image data could be entirely generated or based on modification of data relating to one or more images of the reference fibre. For example, an image of a portion of the reference fibre could be duplicated or repeated around an axis as appropriate for the symmetry of the structure of the reference fibre and optical fibre. The modifications of the data relating to one or more images of the reference fibre may also comprise image processing steps such as altering the brightness or lighting conditions in the images. This could be used, for example, to standardise or normalise the lighting conditions or intensities in the reference images so that the reference images can be more consistently compared to each other and to the test measurements 10.

[0119] As for the test images, the one or more reference images may correspond to a view of the reference fibre based on a spatial distribution of one or more data points in one or more dimensions. For example, the data points may comprise a set of pixel values or another set of measurement values such as pixel intensities or brightness values that are spatially distributed over a region in which the reference fibre is located. Similarly as discussed for the test images, the one or more reference images may correspond to a view from a side of the reference fibre.

[0120] The reference images may be two-dimensional images of a region containing the reference fibre. Alternatively, the reference images may be 1 -dimensional images, i.e. for a short length of fibre. Since the structure does not change significantly over a short length of fibre, only a single pixel slice across the cross-section may be needed. The spatial distribution of one or more data points in one or more dimensions may comprise an intensity profile along a line at an angle to the length of the reference fibre in the one or more reference images. The line may be perpendicular to the length of the optical fibre and reference fibre. Determining the rotational alignment may comprise using an intensity profile along a corresponding line in the one or more test images.

[0121] In general, the formats of the test measurements and stored data will correspond to one another so that direct comparisons can be made between the test measurements 10 and stored data 20 for the purpose of determining the rotational alignment.

[0122] Determining the rotational alignment of the optical fibre based on the one or more test measurements 10 and stored data 20 may comprise calculating S20 a comparative metric between the one or more test measurements 10 and the stored data 20, and determining S30 the rotational alignment of the optical fibre based on the comparative metric. The comparative metric may be any suitable measure that can be used to compare the similarity of the test measurements 10 and the stored data 20. In particular, the comparative metric may be a cross-correlation. Cross-correlation coefficients have previously been used by Shen at al. [7] for precise alignment of special optical fibres required for splicing.

[0123] Various methods may be used to compare the test measurements 10 and the stored data 20 depending on the configuration of the apparatus that is being used.

[0124] In a first example, the one or more test measurements 10 comprise one test measurement 10, optionally exactly one test measurement 10, and the stored data 20 comprise a plurality of reference measurements. This is a one-to-many relationship between the test measurements 10 and reference measurements.

[0125] In this first example, determining the rotational alignment of the optical fibre 1 comprises calculating a comparative metric between the test measurement 10 and each of the plurality of reference measurements, and identifying, based on the comparative metric, a first reference measurement of the plurality of reference measurements having a greatest similarity to the test measurement 10.

[0126] The rotational alignment of the optical fibre 1 corresponding to the test measurement is then determined using the known rotational alignment of the reference fibre corresponding to the first reference measurement. This could be by determining the rotational alignment of the optical fibre 1 corresponding to the test measurement to be the known rotational alignment of the reference fibre corresponding to the first reference measurement. Alternatively, interpolation may be performed using the first reference measurement and one or more further reference measurements of the plurality of reference measurements 10 to arrive at the rotational alignment of the optical fibre 1. The interpolation may depend on the comparative metric between the test measurement 10 and the first reference measurement.

[0127] An example of results from a method of this type is shown in Fig. 14. Each point in the graph corresponds to the cross-correlation coefficient calculated between a test image and one of a plurality of reference images. The points A, B, and C of highest crosscorrelation correspond to angles at which the rotational alignment of the optical fibre 1 in the test image matches the rotational alignment of the reference fibre 2 in the reference image.

[0128] This example may be advantageous in a circumstance where the apparatus used to carry out the method does not comprise a rotational adjustment device 27 for adjusting the rotational position of the optical fibre 1. In this case, it may be preferable to determine the rotational alignment without having to rotate the optical fibre.

[0129] In a second example, the opposite relationship is used. The stored data 20 comprise one reference measurement, optionally exactly one reference measurement, and the one or more test measurements 10 comprise a plurality of test measurements 10 captured at different rotational alignments of the optical fibre 1. This is a many-to-one relationship between the test measurements 10 and reference measurements.

[0130] In this second example, determining the rotational alignment of the optical fibre 1 comprises calculating a comparative metric between the reference measurement and each of the plurality of test measurements 10, and identifying, based on the comparative metric, a first test measurement of the plurality of test measurements having a greatest similarity to the reference measurement.

[0131] The rotational alignment of the optical fibre 1 corresponding to the first test measurement is then determined using the known rotational alignment of the reference fibre corresponding to the reference measurement. As for the first example, this could be by determining the rotational alignment of the optical fibre 1 corresponding to the first test measurement to be the known rotational alignment of the reference fibre corresponding to the reference measurement. Alternatively, interpolation may be performed using the first test measurement and one or more further test measurements of the plurality of test measurements 10 to arrive at the rotational alignment of the optical fibre 1. The interpolation may depend on the comparative metric between the first test measurement 10 and the reference measurements.

[0132] As a specific example, the stored data 20 may comprise a one-dimensional intensity profile measured across the reference fibre for a known alignment. A cross-correlation can be calculated between a test measurement comprising a corresponding intensity profile across the optical fibre 1 and intensity profile of the reference fibre in the stored data 20. In the case where the optical fibre 1 (and the reference fibre) comprise a plurality of capillaries 7 surrounding a hollow core 5, the light reflection from the top two capillaries should be approximately equal when the imaging axis is aligned between the two capillaries. The cross-correlation between the test measurement 10 and the stored data 20 can be used to confirm this and give a measure of the correct rotational alignment, where the highest cross-correlation is predicted to give the desired capillary positioning.

[0133] Where the stored data comprise a plurality of reference measurements, determining the rotational alignment may comprise using a machine learning algorithm. The machine learning algorithm may be any suitable algorithm, for example a neural network. The machine learning algorithm is trained using the plurality of reference measurements and the known rotational alignments of the reference fibre corresponding to the plurality of reference measurements.

[0134] Machine learning algorithms can allow greater levels of extrapolation from training data to test data and so can provide more robust determinations of the rotational alignment. For example, one or more of the plurality of reference measurements may correspond to one or more different illumination conditions. This can improve the robustness of the determination of the rotational alignments in cases where the conditions under which the reference measurements were obtained differs from the condition under which the test measurements are obtained.

[0135] As discussed above, the stored data 20 may comprise one or more reference images that are captured image data or generated image data based on modification of data relating to one or more images of the reference fibre. The captured or generated image data must be obtained before they can be used in the above-described method to determine the rotational alignment of the optical fibre 1.

[0136] To this end, the present disclosure also provides a method of obtaining stored data for use in a method of determining a rotational alignment of an optical fibre 1. Fig. 15 shows a flowchart of the method of obtaining stored data.

[0137] The method of obtaining stored data comprises capturing, generating and / or modifying S50 a reference image of a reference fibre 2. As discussed above, a structure of the reference fibre corresponds to a structure of the optical fibre 1. As for the test images, the method may comprise a step of adjusting a depth within the optical fibre 1 at which the reference image is focussed, for example such that the image is focussed on a surface of the capillaries 7.

[0138] An apparatus that can be used for obtaining the stored data by capturing the reference image is shown in Fig. 16. The apparatus is similar to that shown in Fig. 12, except that no reels are provided because the reference fibre 2 is generally a shorter length of fibre. The apparatus comprises a first sensor configured to capture the reference image of the reference fibre. Similar to the apparatus of Fig. 12, the first sensor may be a camera 35. The reference fibre 2 is suspended in the rotatable platen 25 by clamps 29. Similarly as for the optical fibre 1, the reference fibre 2 is imaged from a side of the reference fibre 2 by the camera 35. This allows the reference images to be obtained that can subsequently be compared to the test measurements 10.

[0139] To determine the known rotational alignment of the reference fibre 2 corresponding to each of the reference image, the method further comprises capturing, generating and / or modifying S60 an alignment image.

[0140] The apparatus of Fig. 16 comprises a second sensor configured to capture the alignment image. In the example of Fig. 16, the second sensor comprises a camera 36 configured to take the alignment image. The cross-sectional structure of the reference fibre 2 is visible in the alignment image. The longitudinal end face of the reference fibre 2 may be cleaved. The alignment image corresponds to a view from a longitudinal end face of the reference fibre 2. In other words, an imaging axis along which the alignment image is captured may be more parallel to the longitudinal axis of the reference fibre 2 than perpendicular to the longitudinal axis of the reference fibre 2. A smallest angle between the alignment axis and the longitudinal axis of the reference fibre 2 may be at most 45 degrees, optionally at most 30 degrees, optionally at most 10 degrees. Optionally, the imaging axis is substantially parallel to the longitudinal axis of the reference fibre 2.

[0141] The method comprises determining S70 a rotational alignment of the reference fibre 2 when capturing, generating and / or modifying the reference image based on the alignment image. The apparatus of Fig. 16 comprises a processing unit (not shown) configured to determine the rotational alignment. The rotational alignment of the reference fibre 2 can be determined from the alignment image because the cross-section of the reference fibre 2 is visible in the alignment image. Thereby, the reference image can be associated with the known rotational alignment determined from the alignment image.

[0142] Capturing, generating and / or modifying the reference image may comprise capturing a plurality of reference images. In this case, the reference fibre 2 is rotated about the longitudinal axis of the reference fibre 2 between the capturing, generating and / or modifying of each reference image so that different rotational alignments are provided for comparison to one or more test measurement.

[0143] Where a plurality of reference images are captured, capturing, generating and / or modifying the alignment image comprises capturing, generating and / or modifying a plurality of alignment images corresponding to the plurality of reference images, and determining the rotational alignment comprises determining a rotational alignment of the reference fibre 2 during the capturing, generating and / or modifying of each reference image based on the corresponding alignment image.

[0144] Where a plurality of reference images and corresponding alignment images are captured, a relative change in the rotational alignment of the reference fibre 2 between the capturing, generating and / or modifying of each reference image may be at most 30 degrees, optionally at most 20 degrees, optionally at most 10 degrees. Where the structure of the reference fibre 2 has rotational symmetry of at least order two about the longitudinal axis of the reference fibre 2, a relative change in the rotational alignment of the reference fibre 2 between the capturing, generating and / or modifying of each reference image may be at most 50% of an angle between rotationally symmetric positions of the reference fibre 2 about the longitudinal axis of the reference fibre 2, optionally at most 25%, optionally at most 10%.

[0145] Fig. 17A to Fig. 17C show examples of reference images captured at 10-degree intervals of rotation of an ARF having hollow internal capillaries 7. Reflections are visible from the capillaries 7. Fig. 18A to Fig. 18C show corresponding alignment images in which the cross-section of the reference fibre, including the hollow capillaries is visible.

[0146] The above methods of determining the rotational alignment may be used in a method of preparing an optical fibre 1 for machining by a machining apparatus. The method of preparing the optical fibre comprises a first step of determining a rotational alignment of the optical fibre 1 using the method described above.

[0147] As discussed above, it is desirable to machine the optical fibre 1 at a particular position relative to the internal structure of the optical fibre 1 around the circumference of the optical fibre 1. In general terms, the correct relative positioning of the machining apparatus and optical fibre 1 can be achieved in the method in one of two ways.

[0148] In a first example, the method comprises a step of rotating the optical fibre 1 to a target rotational alignment relative to the machining apparatus based on the determined rotational alignment. This is illustrated in Fig. 19A and Fig. 19B for an ARF with seven capillaries 7. The line 150 indicates a line along which the machining apparatus will act on the optical fibre 1. In Fig. 19A, the method discussed above is used to determine the rotational alignment of the initially misaligned optical fibre 1. Once the rotational alignment is determined, the optical fibre 1 is rotated to the target rotational alignment such that the machining line 150 falls between two of the capillaries 7.

[0149] This may be appropriate in a situation where the machining apparatus is only configured to machine the optical fibre 1 at a fixed position relative to the machining apparatus. The optical fibre 1 may be rotated automatically or manually using a rotational adjustment device 27.

[0150] In particular in this first example, the determining of the rotational alignment may simply involve a determination that the optical fibre 1 is not at the target rotational alignment, rather than a determination of a specific measure of misalignment of the optical fibre 1 from the target rotational alignment. If the determining indicates that the optical fibre 1 is not at the target rotational alignment, the rotating of the optical fibre 1 to a target rotational alignment may comprise rotating the optical fibre 1 until the comparative metric indicates a maximum similarity between the optical fibre 1 and the reference fibre at the target rotational alignment.

[0151] In a second example, the method comprises a step of adjusting a configuration of the machining apparatus based on the determined rotational alignment. Adjusting the configuration of the machining apparatus may comprise adjusting the configuration such that the machining apparatus is configured to machine the optical fibre 1 at a target position within a cross-section of the optical fibre 1.

[0152] The apparatus of Fig. 12 comprises further features including a beam splitter 41, mirror 43, spatial light modulator 45, laser 47, and further lenses 49. These can be used for machining of the optical fibre 1 in combination with determining the rotational alignment of the optical fibre 1. Where a laser machining apparatus such as that of Fig. 12 is used for machining the optical fibre 1, the adjusting of the configuration of the machining apparatus may comprise moving the focal spot of the laser apparatus. The laser may be an ultrafast femtosecond laser, i.e. a pulsed laser with pulse durations on the order of femtoseconds.

[0153] This is illustrated in Fig. 20A and Fig. 20B for a multicore optical fibre 110. In Fig. 20A, the method discussed above is used to determine the rotational alignment 9 of the initially misaligned optical fibre 110. Once the rotational alignment is determined, the configuration of the laser machining apparatus is adjusted, for example using the spatial light modulator 45. The laser beam is then focussed onto a target core of the plurality of cores 11 when the laser beam enters the optical fibre 110, as shown in Fig. 20B.

[0154] Depending on the relative alignment of the laser apparatus and the optical fibre 1, further adjustments may be required to compensate for distortion of the focal spot caused by the material and / or structure of the optical fibre 1. For example, the curved air / glass interface of the outer surface of the optical fibre can introduce significant aberration to the laser beam. As the machining proceeds, adjustments may also be needed depending on the point within the optical fibre cross section that is being machined.

[0155] Adjusting the configuration may comprise one or more of a) modifying a wavefront of an optical beam, b) adjusting an aberration compensation of the machining apparatus, and c) adjusting a coma of the machining apparatus. Modifying the wavefront may be achieved using an adaptive optical element such as the spatial light modulator 45 shown in Fig. 12.

[0156] Advantageously, as shown in Fig. 12, the laser beam used for machining the optical fibre 1 may be focussed through the same optical arrangement as is used for capturing the test images. This reduces the complexity of the system, but also means that the accuracy is improved by machining the optical fibre 1 in the same regions as is being used to determine the rotational alignment. In particular, the machining apparatus may be configured to machine the optical fibre 1 using a laser focussed on the optical fibre 1 through an objective lens 31, and the one or more test images are captured by imaging through the objective lens 31.

[0157] The method of preparing an optical fibre 1 for machining by a machining apparatus may be automated. The rotation of the fibre can be scanned through automatically to detect a maximum in the correlation coefficient and the expected intensity pattern and microchannels subsequently fabricated.

[0158] A method of machining an optical fibre 1 using the machining apparatus is also provided. The method of machining the optical fibre 1 comprises preparing the optical fibre 1 for machining using the method described above, and machining the optical fibre 1 using the machining apparatus.

[0159] Where the structure of the optical fibre 1 and the reference fibre 2 is a hollow core structure, the machining of the optical fibre 1 may comprise machining a channel, also referred to as a microchannel 9, fluidly connecting the hollow core 5 with an exterior of the optical fibre 1. As discussed above, this can be particularly useful for sensing applications.

[0160] In order to fabricate microchannels 9 into the core 5 of optical fibres such as an ARF, a section of cladding 3 is first removed, as illustrated in Fig. 21 A. Once the desired fibre rotation is found, part of the cladding structure is ablated layer by layer to avoid any damage to the capillaries. This can be achieved by ablation using the laser beam at a first, lower power. The lowest pulse energy possible is used to minimise the amount of material ablated in each pass over the fibre. High aspect-ratio microchannels 9 with smooth sidewalls can then be fabricated using a second laser power, higher than the first laser power, as illustrated in Fig. 2 IB.

[0161] Fig. 22 shows the finished result in perspective view. A notch in the cladding is created by ablating the cladding before the microchannels 9 are fabricated. Two types of microchannel 9 that may be produced are illustrated in Fig. 22. The first are substantially circular viewed in cross-section perpendicular to their length between the exterior and interior of the optical fibre, although as shown in Fig. 2 IB, the cross-section may change along the length of the microchannel 9. The second type of microchannel 9 is a slot, where the cross-section perpendicular to the length between the exterior and interior of the optical fibre is wider in a first dimension than in a second dimension perpendicular to the first dimension, optionally at least twice as wide, optionally at least three times as wide.

[0162] Other machining operations are also possible such as laser modification. The laser modification may comprise inscribing a Bragg grating. In a multicore optical fibre 110 such as that shown in Fig. 10, a core 11 of the plurality of cores that is located away from the longitudinal axis of the optical fibre 110 may be laser modified using its known rotational position, for example to inscribe a Bragg grating.

[0163] It may be desirable to machine plural channels at different positions. The method may further comprise repeating the steps of preparing the optical fibre 1 and machining the optical fibre 1 for each of a plurality of target rotational alignments or target positions around the circumference of the optical fibre 1. The plural target rotational alignments may be equally spaced around the circumference of the optical fibre 1. One or more of the target rotational alignments may be such that the channel is machined equidistant between two of a plurality of capillaries 7 surrounding the hollow core 5.

[0164] The different positions may be positions along the length of the optical fibre 1. In this case, the method of machining may further comprise adjusting a position of the optical fibre 1 and / or a configuration of the machining apparatus such that the machining apparatus is configured to machine the optical fibre 1 at a different longitudinal position along a length of the optical fibre 1. Where the position of the optical fibre 1 is adjusted, this could be, for example, using the reels 21.

[0165] The optical fibre 1 may be twisted along its length, or the arrangement of capillaries or cores in the optical fibre may be intentionally twisted along the length such as shown in Fig. 10. Therefore, the rotational alignment needs to be determined at each longitudinal position. The method comprises repeating the steps of preparing the optical fibre 1 and machining the optical fibre 1 at the new longitudinal position.

[0166] Example

[0167] To show the adaptability of this method, microchannels are fabricated on two different ARFs. The first is a commercial fibre from iXBlue photonics designed for light guidance at 1550 nm and having seven anti-resonant capillaries. The second fibre is from the University of Bath and is designed for light guidance at 780 nm. The second fibre has six anti-resonant capillaries.

[0168] Fig. 23 A and Fig. 23B show the dimensions of the core of the commercial iXBlue ARF. Fig. 23A shows the diameter of the entire fibre and inner core. Fig. 23B shows the diameter of the inner hollow core and surrounding capillaries. The iXBlue fibre has an entire core diameter of 97.11 pm. Each anti -resonant capillary has a diameter of approximately 26 pm and a central core of 22.63 pm. The cladding thickness is 70 pm.

[0169] Fig. 24A and Fig. 24B show the dimensions of the core of the ARF provided by the University of Bath. Fig. 24A shows the diameter of the entire fibre and inner core. Fig.

[0170] 24B shows the diameter of the inner hollow core and surrounding capillaries. The fibre provided by University of Bath has a central core diameter of 24.69 pm with an entire core diameter of 55.05 pm. Each of the six capillaries has a diameter of approximately 16 pm. The cladding thickness is 35 pm. The iXBlue has a larger core and hence larger mode field diameter.

[0171] The process described above of cladding ablation and subsequent microchannel fabrication is used to process the two fibres. For the iXBlue fibre, a 30x50 pm section of cladding is ablated and a 5x50 pm microchannel slot is fabricated. For the Bath fibre, a 16x250 pm section of cladding is ablated and a series of 5x5 pm circular microchannels are fabricated down the fibre, separated approximately by 5 pm to preserve structural integrity.

[0172] Fig. 25 is a diagram of the femtosecond laser system used for cladding ablation and microchannel fabrication. The system includes adaptive optics and a system to change the laser power. The laser is a 1 kHz, 790 nm laser with a pulse duration of 140 fs and an 0.5 NA objective lens. The laser power is changed using the combination of the half wave plate and polariser and aberration is corrected using adaptive optics with an SLM. Light is sent to a CCD camera via a beam splitter for imaging.

[0173] To further reduce transmission loss, the cladding ablation procedure can be performed using a laser power and choice of the laser track over the fibre which reduces debris entry into the core. In the example procedure described herein, the initial method for debris removal from the created notch involves passing the laser over the same track, moving into the fibre by one less layer than the number which created the notch. This provides the power to remove the debris from the notch without ablating anymore cladding. Once a clean notch in the cladding 4 has been created, the microchannels 9 are fabricated. The process of cladding ablation and microchannel fabrication in relation to the laser track is as shown in Fig. 22.

[0174] The reference images for the iXBlue and Bath fibres are shown in Fig. 26A and Fig. 26B. Fig. 26A shows the commercial iXBlue fibre, and Fig. 26B shows the University of Bath fibre. The reference images are taken using the CCD camera of the fabrication laser. The images are of the top of the optical fibre with the desired capillary positions. The reflection off of the two top capillaries can be seen, providing conformation of the desired rotation.

[0175] The cross-correlation function, normxcorr2, provided by MATLAB, was used in conjunction with a Labview program used to control the femtosecond laser to calculate a live correlation coefficient based on the current image on the CCD camera.

[0176] The function, normxcorr2, calculates the cross correlation between two images in the spatial or frequency domain and uses calculated local sums to normalise the crosscorrelation to give a coefficient. Defining f as the image, t as the mean of the image template and fu vas the mean of (x, y) in the region under the template then the function closely follows the formula:

[0177] y(u, y) — -:- = -

[0178]

[0179] {Sx.y [f(x, y) - futv]2Zx,y J ~ v) “ f]2}0'5The variable y(u, v) will be labelled as C(0) to represent the correlation coefficient at a specific rotation of the optical fibre. Depth control between the fibre and the objective lens was carried out using the same method where the depth of the image through the fibre was changed until the capillaries could be seen and a high correlation coefficient was found. Images at different depths produced a lower value of C(0).

[0180] To demonstrate the rotation and cross-correlation system, a rotational alignment test was carried out on the commercial iXBlue fibre to see whether the method could correctly identify the desired orientation.

[0181] A short sample of the fibre was used to capture the reference intensity profile. Peaks in the value of C(0) around 0.98 were found and when the cross section of the fibre at the corresponding rotation was imaged, it was found that the fibre was close to the desired rotation.

[0182] The results are shown in Fig. 27. By plotting the intensity across the fibre, it can be seen how a higher C(0) correlated better with the reference intensity profile. Fig. 27 shows a comparison of the intensity profile across the width of the image between the reference and for the fibre rotations corresponding to the values of C(0) = 0.98 and C(0) = 0.95. The corresponding capillary position and CCD image is shown.

[0183] The method was tested on a sample, both with and without a standard protective polyacrylate coating present. By rotating the fibre through a range of angles, it can be seen in Fig. 28 how the cross-correlation method can correctly identify multiple maxima in the value of C(0) with the reference intensity as the fibre is rotated further. The maxima all correspond to the desired rotation, even with the external coating still present on the fibre. The variation of C(phi) has a periodic form. The uncoated fibre shows greater contrast, but the periodic variation is also clearly visible on the coated fibre. This is a promising result that will allow microchannels to be machined without mechanically stripping the fibre coating, thereby maintaining structural integrity. Instead, the external coating can be removed locally with laser ablation. When corresponding cross-sectional images were taken, all of the peaks corresponded to the desired rotation.

[0184] Once the cross-correlation method for determining rotational alignment was validated, the method was used to determine the correct rotation of the fibre and machining was carried out to fabricate microchannels.

[0185] The top of the fibre was first identified by thresholding until the etching effects could be seen. A low pulse energy of 0.5 pj with a 1 kHz repetition rate was used. Once this was located, a notch was created via cladding ablation at 5.5 pj. Microchannels are then fabricated using the same power and repetition rate.

[0186] For the iXBlue fibre a 5x50 pm microchannel slot was fabricated after a 30x50 pm section of cladding was ablated. For the Bath fibre, a series of 5x5 pm microchannels were fabricated along a 16x250 gm section after cladding ablation. Each microchannel was separated by 5 gm to preserve structural integrity. The notch removed cladding to a distance of 65 pm into the iXBlue fibre and 25 pm into the Bath fibre. Cross-sectional SEM images of the microchannels fabricated into the iXBlue fibre are shown below in Fig.

[0187] 29A and Fig. 29B. The channel is 50 gm deep into the fibre.

[0188] It is expected that each addition of a microchannel produces an approximate linear increase in the fibre loss [1], An in-situ loss measurement system as illustrated in Fig. 30, was created by splicing the processed iXBlue hollow-core ARF (HC-ARF) 206 to singlemode fibre (SMF) 204 on both sides. This was done using standard settings for SMF-SMF splicing and adapting the parameters to include fibre overlap. The SMF -HC-ARF- SMF system is connected between a 1550 nm laser 200 and a power meter 210. A controller 202 is used to control the laser 200.

[0189] The coupling loss was recorded to be on average 5.25 dB per splice 208 with a 1 dB polarization dependence across the spliced fibre. The splice loss is considered to mainly originate from numerical aperture (NA) mismatch between fibre types and the destruction of the capillary microstructure during fusion splicing. The error in the loss measurement comes from the fabrication introducing polarization dependence.

[0190] The loss incurred from fabricating a 5 gm wide microchannel of increasing length was recorded. A single circular 5x5 gm micro channel was found to give a loss increase of around 0.1 dB. Comparatively in the case of the longer channels, the scale of the loss increases, most likely due to the larger area for any remaining debris to entire the fibre core and cause scattering loss. The loss is confirmed to increase linearly with the microchannel length for a constant 5 pm width, as shown in Fig. 31.

[0191] As a final experiment, a 50 pm section of the entire width of the fibre cladding was ablated in layers towards the centre of the fibre. When the core was breached and the microchannel structure partly damaged, the loss incurred was 1.8 dB. Further destruction of the fibre and capillaries, towards the fibre centre, dramatically increased the loss by another 8.3 dB. This provides assurance that the capillary structure should be preserved. This increase in loss when the internal microstucture of the fibre is destroyed will limit the length of the fibre which can be used as well as the number and size of the microchannels before the loss is too great for highly sensitive detection.

[0192] Comparison with the simulation results show the effect of debris entering the core, where the simulation does not take into account scattering loss. Fig. 32 is an SEM image showing debris present in the core of the fibre provided by the University of Bath. The debris is predicted to be due to remaining debris from the cladding ablation entering the core upon microchannel fabrication. The loss can therefore be further improved by introducing an air flow system to remove any remaining debris after cladding ablation or the use of etching.

[0193] It may be desirable in some situations to retain the protective polyacrylate coating on the fibre to maintain structural integrity. Fig. 33 and Fig. 34 illustrate an experiment to fabricate microchannels in a fibre without manual stripping of the coating. Instead, only a small section of the coating was laser ablated in the region of the microchannel.

[0194] In order to fabricate microchannels into the core of coated ARF, a 70 pm long section of the outer polyacrylate coating was first removed by femtosecond laser ablation to provide easy placement for the microchannel through the remaining cladding. This was removed across the width of the coating to prevent debris entering the core upon microchannel fabrication, as illustrated schematically in Fig.33. Once the coating had been locally removed, the microchannels were fabricated within the exposed cladding region using the same method as described above. Using further debris removal via the transverse airflow, the loss incurred by fabrication of a microchannel through the coating was found to be 0.068 ± 0.009 dB, only a fractional increase from the earlier result with no coating present. With this technique, the structural integrity is far greater than removing a larger section using mechanical strippers with a negligible effect on fibre loss. Microscope images, via reflection illumination, of the top of the machined fibre are shown in Fig. 34 with increasing focus towards the fibre core.

[0195] The methods presented above can be used for fabrication of remote gas sensors using optical fibres and having improved sensitivity and response time provided by the long interaction length. The microchannels allow the entire fibre to be used as the active medium without a compromise on response time or the need for a pressure differential. Such fibre-based sensors can be used in extreme environments such as high temperature and pressure, for example using sapphire optical fibres. The devices may also be used for mid-IR wavelength experiments.

[0196] Fig. 35 shows an apparatus 250 in which a multicore optical fibre 110 fabricated as described above may be used as a 3D shape sensor. The multicore optical fibre 110 has a plurality of cores, into each of which a Bragg grating is inscribed.

[0197] A beam from a tuneable laser 200, controlled by a controller 202, is passed through a polarisation scrambler 212 and an optical circulator 214. A break out box 218 is provided that has separate optical connections to each of the cores. An optical switch 216 is used to control which of the cores of the fibre 110 the laser light is directed into. Light reflected from the Bragg gratings is directed via the optical circulator 214 into a photodetector 210.

[0198] Deformation of the fibre 110 affects the local refractive index around the Bragg gratings. The 3D shape of the fibre can therefore be determined by measuring the differences in the Bragg wavelengths of Bragg gratings at different positions along the length and through the cross-section of the optical fibre 110.

[0199] It is desirable to retain the coating on multicore optical fibre. As the coated fibre is pulled through the system, it is imaged from the side. A comparison is made between the side-image of the coated fibre and a library of reference side-images of coated fibre at different rotations, to determine the rotational orientation of the fibre.

[0200] Using this rotational information, the objective lens is used to focus the beam onto a core of the coated fibre. The adaptive optical element is adapted to compensate for the aberration focussing in the core of the coated fibre. A Bragg grating is written into the core of the coated fibre. Optionally, the adaptive optical element is configured to have multiple beams to enable multiple cores to be written simultaneously in the coated fibre. In this way coated multicore fibre may be inscribed with Bragg gratings rapidly, without having to rotate the fibre as it is pulled though the system.

[0201] References

[0202] 1 P. Koziol, P. Jaworski, K. Krzempek, V. Hoppe, G. Dudzik, F. Yu, D. Wu, M.

[0203] Liao, J. Knight, and K. Abramski, “Fabrication of microchannels in a nodeless antiresonant hollow-core fiber using femtosecond laser pulses,” Sensors, Vol. 21, No. 22: 7591 (2021).

[0204] 2 X. Xiao, B. Xu, X. Xu, B. Du, Z. Chen, C. Fu, C. Liao, J. He, and Y. Wang, “Femtosecond laser auto-positioning direct writing of a multicore fiber bragg grating array for shape sensing,” Opt. Lett. Vol. 47, No. 4 pp. 758-761 (2022).

[0205] 3 L. Budd, A. Taranta, E. R. N. Fokoua, and F. Polled, “Non-destructive structural characterisation of double nested antiresonant nodeless fiber,” in Proc. Optica Advanced Photonics Congress, (Maastricht, Limburg, 2022).

[0206] 4 A. Ashkin, J. M. Dziedzic, and H. Stolen R, “Outer diameter measurement of low birefringence optical fibers by a new resonant backscatter technique,” Appl. Opt. Vol. 20, No. 13 pp. 2299-2303 (1981).

[0207] M. A. Bukshtab, H. C. Mulvad, R. Slavik, F. Poletti, and D. J. Richardson, “On the possibility of structural characterisation of hollow core fibres using whispering gallery modes excited by laser and broadband light,” in Proc.

[0208] Conference on Lasers and Electro-Optics Europe European Quantum Electronics Conference (CLEOZEurope-EQEC), (Munich, Germany, 2019).

[0209] A. W. Poon, R. K. Chang, and D. Q. Chowdhury, “Measurement of fibercladding diameter uniformity by use of whispering-gallery modes: nanometer resolution in diameter variations along millimeter to centimeter lengths,” Opt. Lett. Vol. 26, No. 23 pp. 1867-1869 (2001).

[0210] L. Shen, L. Gan, Z. Dong, B. Li, D. Liu, S. Fu, W. Tong, and M. Tang, “Endview image processing based angle alignment techniques for specialty optical fibers,” IEEE Photonics Journal, Vol. 9, No. 2: 7201508 (2017).

Claims

CLAIMS1. A method of determining a rotational alignment of an optical fibre, wherein a structure of the optical fibre varies around a longitudinal axis of the optical fibre, the method comprising:capturing one or more test measurements of the optical fibre; and determining the rotational alignment of the optical fibre based on the one or more test measurements and stored data,wherein the stored data is obtained based on a reference fibre having a known rotational alignment, a structure of the reference fibre corresponding to the structure of the optical fibre.

2. The method of claim 1, wherein determining the rotational alignment comprises:calculating a comparative metric between the one or more test measurements and the stored data; anddetermining the rotational alignment of the optical fibre based on the comparative metric,optionally wherein the comparative metric is a cross-correlation.

3. The method of claim 2, wherein:the one or more test measurements comprise one test measurement;the stored data comprise a plurality of reference measurements;calculating the comparative metric comprises calculating a comparative metric between the test measurement and each of the plurality of reference measurements; and determining the rotational alignment of the optical fibre based on the comparative metric comprises:identifying, based on the comparative metric, a first reference measurement of the plurality of reference measurements having a greatest similarity to the test measurement; anddetermining the rotational alignment of the optical fibre corresponding to the test measurement using the known rotational alignment of the reference fibre corresponding to the first reference measurement.

4. The method of claim 2, wherein:the stored data comprise one reference measurement;the one or more test measurements comprise a plurality of test measurements captured at different rotational alignments of the optical fibre; andcalculating the comparative metric comprises calculating a comparative metric between the reference measurement and each of the plurality of test measurements;determining the rotational alignment of the optical fibre based on the comparative metric comprises:identifying, based on the comparative metric, a first test measurement of the plurality of test measurements having a greatest similarity to the reference measurement; anddetermining the rotational alignment of the optical fibre corresponding to the first test measurement using the known rotational alignment of the reference fibre corresponding to the reference measurement.

5. The method of claim 1 or 2, wherein:the stored data comprise a plurality of reference measurements;determining the rotational alignment comprises using a machine learning algorithm; andthe machine learning algorithm is trained using the plurality of reference measurements and the known rotational alignments of the reference fibre corresponding to the plurality of reference measurements,optionally wherein one or both of:a) one or more of the plurality of reference measurements corresponds to one or more different illumination conditions; andb) the machine learning algorithm comprises a neural network.

6. The method of claim 1, 2, 4, or 5, wherein:capturing the one or more test measurements comprises capturing a plurality of test measurements; andthe optical fibre is rotated about the longitudinal axis of the optical fibre between the capturing of each test measurement.

7. The method of claim 6, wherein a relative change in the rotational alignment of the optical fibre between the capturing of each test measurement is at most 30 degrees, optionally at most 20 degrees, optionally at most 10 degrees.

8. The method of claim 6 or 7, wherein:the structure of the optical fibre has rotational symmetry of at least order two about the longitudinal axis of the optical fibre; anda relative change in the rotational alignment of the optical fibre between the capturing of each test measurement is at most 50% of an angle between rotationally symmetric positions of the optical fibre about the longitudinal axis of the optical fibre, optionally at most 25%, optionally at most 10%.

9. The method of any preceding claim, wherein the stored data comprise one or more reference images that are captured image data or generated image data, optionally wherein the generated image data is based on modification of data relating to one or more images of the reference fibre.

10. The method of claim 9, wherein the one or more reference images correspond to a view of the reference fibre based on a spatial distribution of one or more data points in one or more dimensions.

11. The method of claim 9 or 10, wherein the one or more reference images correspond to a view from a side of the reference fibre.

12. The method of claim 10 or 11, wherein:the one or more test measurements comprise one or more test images of the optical fibre, optionally wherein the one or more test images correspond to a view from a side of the optical fibre;the spatial distribution of one or more data points in one or more dimensions comprises an intensity profile along a line, the line at an angle to a length of the reference fibre; anddetermining the rotational alignment comprises using an intensity profile along a corresponding line in the one or more test images, optionally wherein the line isperpendicular to the length of the optical fibre and reference fibre.

13. A method of obtaining stored data for use in a method of determining a rotational alignment of an optical fibre, the method of obtaining stored data comprising:capturing, generating and / or modifying a reference image of a reference fibre, a structure of the reference fibre corresponding to a structure of the optical fibre;capturing, generating and / or modifying an alignment image corresponding to a view from a longitudinal end face of the reference fibre, optionally wherein the longitudinal end face is cleaved; anddetermining a rotational alignment of the reference fibre when capturing, generating and / or modifying the reference image based on the alignment image.

14. The method of claim 13, wherein:capturing, generating and / or modifying the reference image comprises capturing a plurality of reference images;the reference fibre is rotated about the longitudinal axis of the reference fibre between the capturing, generating and / or modifying of each reference image;capturing, generating and / or modifying the alignment image comprises capturing, generating and / or modifying a plurality of alignment images corresponding to the plurality of reference images; anddetermining the rotational alignment comprises determining a rotational alignment of the reference fibre during the capturing, generating and / or modifying of each reference image based on the corresponding alignment image.

15. The method of claim 14, wherein a relative change in the rotational alignment of the reference fibre between the capturing of each reference image is at most 30 degrees, optionally at most 20 degrees, optionally at most 10 degrees.

16. The method of claim 14 or 15, wherein:the structure of the reference fibre has rotational symmetry of at least order two about the longitudinal axis of the reference fibre; anda relative change in the rotational alignment of the reference fibre between the capturing, generating and / or modifying of each reference image is at most 50% of an anglebetween rotationally symmetric positions of the reference fibre about the longitudinal axis of the reference fibre, optionally at most 25%, optionally at most 10%.

17. A method of preparing an optical fibre for machining by a machining apparatus comprising:determining a rotational alignment of the optical fibre using the method of any of claims 1 to 12; androtating the optical fibre to a target rotational alignment relative to the machining apparatus based on the determined rotational alignment.

18. A method of preparing an optical fibre for machining by a machining apparatus comprising:determining a rotational alignment of the optical fibre using the method of any of claims 1 to 12; andadjusting a configuration of the machining apparatus based on the determined rotational alignment.

19. The method of claim 18, wherein adjusting the configuration of the machining apparatus comprises adjusting the configuration such that the machining apparatus is configured to machine the optical fibre at a target position within a cross-section of the optical fibre,optionally wherein adjusting the configuration comprises one or more of:a) modifying a wavefront of an optical beam, optionally wherein modifying the wavefront comprises using an adaptive optical element, optionally wherein the adaptive optical element is a spatial light modulator;b) adjusting an aberration compensation of the machining apparatus; and c) adjusting a coma of the machining apparatus.

20. The method of any of claims 17 to 19, wherein:the machining apparatus is configured to machine the optical fibre using a laser focussed on the optical fibre through an objective lens; andthe one or more test images are captured by imaging through the objective lens.

21. A method of machining an optical fibre using a machining apparatus comprising: preparing the optical fibre for machining using the method of any of claims 17 to 20; andmachining the optical fibre using the machining apparatus,optionally wherein:the structure of the optical fibre and the reference fibre is a hollow core structure; andthe machining of the optical fibre comprises machining a channel fluidly connecting the hollow core with an exterior of the optical fibre.

22. The method of claim 21, wherein the method further comprises repeating the steps of preparing the optical fibre and machining the optical fibre for each of a plurality of target rotational alignments or target positions around the circumference of the optical fibre.

23. The method of claim 21 or 22, wherein the method further comprises:adjusting a position of the optical fibre and / or a configuration of the machining apparatus such that the machining apparatus is configured to machine the optical fibre at a different longitudinal position along a length of the optical fibre; andrepeating the steps of preparing the optical fibre and machining the optical fibre at the new longitudinal position.

24. The method of any preceding claim, wherein the optical fibre and the reference fibre are hollow core fibres, photonic crystal fibres, anti-resonant fibres, polarisation maintaining fibres, or side-hole fibres.

25. The method of any preceding claim, wherein the optical fibre and the reference fibre comprise one or more hollow capillaries surrounding the longitudinal axis of the optical fibre and the reference fibre.

26. The method of any preceding claim, wherein each of the optical fibre and the reference fibre comprises a plurality of cores.

27. The method of claim 26, wherein one or both of:a) the cores are twisted around the longitudinal axis of the optical fibre, and optionally of the reference fibre, along a length of the optical fibre and the reference fibre; andb) a core of the plurality of cores that is located away from the longitudinal axis of the optical fibre and the reference fibre is laser modified using its known rotational position, optionally where the laser modification comprises the inscribing of a Bragg grating.

28. An apparatus for determining a rotational alignment of an optical fibre, wherein a structure of the optical fibre varies around a longitudinal axis of the optical fibre, the system comprising:a sensor configured to capture one or more test measurements of the optical fibre; anda processing unit configured to determine the rotational alignment of the optical fibre based on the one or more test measurements and stored data,wherein the stored data is obtained based on a reference fibre having a known rotational alignment, a structure of the reference fibre corresponding to the structure of the optical fibre.

29. An apparatus for obtaining stored data for use in determining a rotational alignment of an optical fibre, the apparatus comprising:a first sensor configured to capture a reference image of a reference fibre, a structure of the reference fibre corresponding to a structure of the optical fibre;a second sensor configured to capture an alignment image corresponding to a view from a longitudinal end face of the reference fibre; anda processing unit configured to determine a rotational alignment of the reference fibre when capture the reference image based on the alignment image.

30. A system for machining an optical fibre, comprising:the apparatus for determining a rotational alignment of the optical fibre of claim 28; andan apparatus configured to machine the optical fibre based on the rotationalalignment of the optical fibre determined by the apparatus for determining the rotational alignment of the optical fibre.

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