Apparatus and methods for analysing optical fibre

Abstract

A method of analysing a hollow core antiresonant optical fibre having an inner cladding comprising at least one capillary defined by a wall with a wall thickness comprises: directing light onto the fibre for interaction with at least one surface of the capillary wall; detecting a portion of the light which has interacted with the at least one surface of the capillary wall to determine a power level of the detected portion; and using the power level to deduce information regarding the wall thickness.

Description

APPARATUS AND METHODS FOR ANALYSING OPTICAL FIBREBACKGROUND

[0001] Optical fibre can be manufactured by heating a glass preform assembly to soften it, and pulling or drawing the softened material down to a reduced diameter to produce the fibre. A preform for a conventional all-solid glass optical fibre has an internal structure comprising continuous glass with a varying transverse refractive index profile, and the intended structure for the fibre is a scaled version of this profile that is achieved inherently by careful drawing. Accordingly, it is only necessary' to monitor the external diameter of the fibre in order to produce fibre of the desired specification.

[0002] More recently, a variety of optical fibres have been developed which have a complex internal structure comprising glass and air, in which one or more longitudinal holes, lumen or capillaries extend internally along the fibre. Hollow core fibres are an example of this class of fibre. The proportions of the various structural features may be controlled by applying pressure inside one or more of the lumen during the drawing, and it is important to obtain the correct dimensions since these define the operating wavelength or wavelength range of the fibre. Accordingly, while it can be beneficial to measure and control the exterior width of the drawn fibre using conventional equipment, this is not sufficient to ensure that the fibre meets all its design requirements since these include the internal structure which is invisible to the known techniques. Any structural defect or imperfection introduced during fibre fabrication, such as a variation of the lumen wall thickness, can have a major impact on the performance and quality' of the finished fibre.

[0003] The embodiments described below are not limited to implementations which solve any or all of the disadvantages of environments such as those described above. Rather, this is background only.SUMMARY

[0004] The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not intended to identify key features or essential features of the claimed subject matter nor is it intended to be used to limit the scope of the claimed subject matter. Its sole purpose is to present a selection of concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0005] Examples disclosed herein operate to improve the analysis of the internal structure of optical fibres. By following the disclosed principles, substantial benefits, advantages and efficiencies can be achieved, thereby improving the performance of optical fibres and methods of their manufacture.

[0006] In some examples, there is provided a method of analysing a hollow core antiresonant optical fibre having an inner cladding comprising at least one capillary defined by a wall with a wall thickness, the method comprising: directing light onto the fibre for interaction with at least one surface of the capillary wall; detecting a portion of the light which has interacted with the at least one surface of the capillary w all to determine a power level of the detected portion; and using the power level to deduce information regarding the wall thickness.

[0007] In some examples, the method is carried out on the fibre as the fibre is being drawn in a draw process, and the method further comprises determining that the wall thickness differs from a specified value, and in response generating one or more control signals for adjusting the draw process to move the wall thickness closer to the specified value.

[0008] According to a second aspect of certain examples described herein, there is provided apparatus for analysing a hollow core antiresonant optical fibre having an inner cladding comprising at least one capillary defined by a wall with a wall thickness, the apparatus comprising: a light source operable to generate an output beam of light; a first optical system configured to derive a measurement beam from the output beam and direct the measurement beam onto the fibre for interaction w ith at least one surface of the capillary wall; a second optical system configured to collect a portion of the light which has interacted with the at least one surface of the capillary wall and direct the portion as a detector beam; a detector configured to detect the detector beam and output a signal representing the power level of the detected portion; and a processor configured to use the power level to deduce information regarding the wall thickness.

[0009] In some examples, the apparatus is suitable for mounting within an optical fibre draw tower in order to deduce information regarding the wall thickness of fibre as it is being drawn in a draw process, and the processor is further configured to determine if the wall thickness differs from a specified value, and in response generate one or more control signals for the draw tower for the purpose of adjusting the draw process to move the wall thickness closer to the specified value.

[0010] Many of the attendant features will be more readily appreciated as the same becomes better understood by reference to the following detailed description considered in connection with the accompanying drawings.DESCRIPTION OF THE DRAWINGS

[0011] The present description will be better understood from the following detailed description read in light of the accompanying drawings, wherein:FIG. 1 illustrates a schematic transverse cross-sectional view through an example hollow core antiresonant optical fibre;FIG. 2 illustrates a highly schematic representation of an apparatus for analysing optical fibre according to the present technology, in which light is transmitted through the fibre for detection;FIG. 3 illustrates a highly schematic representation of a second apparatus for analysing optical fibre according to the present technology, in which light is reflected from the fibre for detection;FIG. 4 illustrates a highly schematic representation of a third apparatus for analysing optical fibre according to the present technology, in which light is reflected from the fibre for detection;FIG. 5 illustrates a highly schematic representation of a combined optical system that can be included in the apparatus of FIG. 4;FIG. 6 illustrates two computer-generated broadband spectra of detected light transmitted through an optical fibre such as the FIG. 1 example, for two values of capillary w all thickness in the optical fibre.FIG. 7 shows two computer-generated broadband spectra of detected light reflected from an optical fibre such as the FIG. 1 example, for two values of capillary wall thickness in the optical fibre;FIG. 8 shows a graph of the variation of detected reflected light with capillary w all thickness in the optical fibre for two values of wavelength of the reflected light;FIG. 9 shows a graph of the variation of detected reflected light with capillary wall thickness in the optical fibre for three values of w avelength of the reflected light; andFIG. 10 show s a flow' chart of steps in an example method of optical fibre analysis according to an example of the disclosure.Like reference numerals are used to designate like parts in the accompanying drawings.DETAILED DESCRIPTION

[0012] Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed / described in detail in the interests of brevity. It will thus be appreciated that aspects and features of apparatus and methods discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.

[0013] The present disclosure presents methods for achieving non-destructive measurement, monitoring, assessment, interrogation or analysis of internal structural parameters of a class of microstructured optical fibre known as hollow core antiresonant fibre (ARF) (or antiresonant hollow core fibre), together with apparatus for carrying out the methods. The analysis can be carried out simply, quickly and conveniently during the process of drawing optical fibre from a preform or a cane, directly on the fibre as it is drawn, and the results immediately used to feed back to the drawing apparatus to effect real-time adjustments if the fibre is determined as not meeting its specification. It uses external non-contact measurements that nevertheless allow interrogation of the fibre interior, and is based on observ ation of a physical phenomenon upon which the optical performance of the fibre relies, thereby directly reflecting fibre characteristics of interest.

[0014] FIG. 1 shows a transverse cross-sectional view of an example hollow core antiresonant optical fibre of a type to which presently disclosed methods are applicable. The fibre 10 comprises an outer tubular cladding element 12, which has an inner surface 14. Within this is a structured (microstructured) inner cladding that comprises a plurality of tubular primary cladding capillaries 16, each of which is a tube comprising a wall 18 surrounding a hollow void, hole or lumen 20. In this example, there are six primary capillaries which are each of the same cross-sectional size and shape, and arranged inside the outer cladding element 12 in a ring, so that the longitudinal axes of each primary cladding capillary 16 and of the outer cladding element 12 are substantially parallel. The primary capillaries 16 are evenly spaced apart around the inner surface 14 of the outer cladding element 12 so that there is no contact between neighbouring capillaries 16, and each is in contact with and bonded to the inner surface 14 at an attachment zone 22. In alternative designs of ARF, the cladding capillaries may be positioned in contact with each other (in other words, not spaced apart as in FIG. 1). but spacing to eliminate this contact can improve the fibre’s optical performance. The spacing removes nodes thatarise at the contact points between adjacent capillaries and which tend to cause undesirable resonances that result in high losses. Accordingly, fibres with spaced-apart cladding capillaries may be referred to as nodeless antiresonant hollow core fibres.

[0015] The arrangement of the primary capillaries 16 in a ring around the inside of the outer cladding 12 creates a central space, cavity or void within the fibre 10, also with its longitudinal axis parallel to those of the outer cladding 12 and the primary capillaries 16. This is the hollow core 24 of the fibre 10, indicated by the dotted outline in FIG. 1. The fibre 10 also comprises six secondary cladding capillaries 26, one nested inside each of the primary' capillaries 16 (and therefore of a smaller diameter), and bonded to the inner surface of the respective primary capillary 16 at the attachment zone 22. so that the primary and secondary capillaries 16, 26 of each pair are at the same azimuthal position around the outer cladding 12. The primary (outer) capillaries 16 have a wall thickness to and the secondary (inner) capillaries 26 have a wall thickness ti. The smaller diameter of the secondary capillaries 26 compared to the primary capillaries 16 gives a separation z along the radial direction between the core-facing part of the second capillary walls and the core-facing part of the primary capillary' walls. A fibre with a microstructure of this type comprising nested capillaries may be referred to as a nested antiresonant nodeless fibre, or NANF. The secondary capillaries 26 act to reduce optical loss. Some fibre designs include additional still smaller tertiary capillaries nested inside the secondary capillaries 26. In other NANF designs, one or more additional capillaries may be nested at azimuthal locations around the primary capillary different from the attachment zone. A pair of additional capillaries may be equally or unequally displaced from the attachment zone, for example. Many other capillary configurations for the structured cladding of an ARF are possible, and the disclosure is not limited to the examples described above. For example, the capillaries may or may not be of circular cross-section, and / or may or may not be all of the same size and / or shape. The number of capillaries surrounding the core may be for example, four, five, six, seven, eight, nine, ten or more.

[0016] ARFs guide light by an antiresonant optical guidance effect provided by the structured inner cladding. The structure provides antiresonance for propagating wavelengths which are not resonant with a wall thickness of the cladding capillaries, in other words, for wavelengths in an antiresonance window which is defined by the cladding capillary wall thickness. This allows the hollow core of the fibre to support antiresonantly-guided optical modes. The structured cladding can also support claddingmodes able to propagate primarily inside the capillaries, in the glass of the capillary walls or in the spaces or interstices between the cladding capillaries and the fibre’s outer cladding. The loss of these additional non-core guided modes is generally very much higher than that of the core guided modes. The fundamental core guided mode typically has by far the lowest loss amongst the core guided modes. The antiresonance provided by a capillary wall thickness which is antiresonant with the wavelength of the propagating light acts to inhibit coupling between the fundamental core mode and any cladding modes, so that light is confined to the core and can propagate at very low loss. The core is bounded by the inwardly facing parts of the outer surfaces of the primary' cladding capillaries; this is the core boundary, and the material (glass or polymer, for example) of the capillary walls that make up this boundary provides the required antiresonance optical guidance effect or mechanism, and the capillary wall thickness defines the wavelength for which antiresonant optical guiding occurs in the ARF.

[0017] This optical guidance in a hollow core ARF can be understood in simple terms by considering that light is confined in the core by destructive interference (antiresonance) of Fresnel reflections from the pairs of glass-air boundaries provided by each capillary wall facing towards the core. In the FIG. 1 example, each secondary' capillary wall of thickness ti and each primary' capillary wall of thickness to provides a pair of glass-air boundaries. This destructive interference is an example of the well- known phenomenon of thin film interference which gives rise to colours in thin transparent materials such as bubbles and oil films. In these familiar examples, the reflected colour, or wavelength of light, depends on the thickness of the film, while in the ARF, the wavelength which is reflected and therefore guided along the core depends on the thickness of the capillary wall. Accordingly, it can be appreciated that the capillary wall thickness should be carefully specified in order that the fibre guides light at a desired wavelength. For maximum confinement of the guided mode in a nested cladding design, the antiresonant condition should be met at the desired wavelength for both the primary capillaries and the secondary’ capillaries (plus any additional capillaries). In other words, the wall thickness for all capillaries in the fibre should be substantially^ equal. In the FIG. 1 example, therefore, to and ti are preferably equal. However, they are depicted as unequal in order to illustrate a common fabrication error [9] that reduces the quality of drawn ARF.

[0018] The fabrication of hollow core ARF. such as the FIG. I example, by drawing the fibre down from a preform assembly, or from a cane previously drawn fromthe preform assembly, frequently includes the regulation of pressure inside the various tubes and capillaries that define the inner cladding and core in order to control the dimensions and proportions of the microstructure, including tube / capillary width and tube / capillary wall thickness

[0010] , For example, increasing the pressure in the primary capillaries with respect to that in the core will increase the diameter of the primary capillaries and reduce their wall thickness to. In this way. the final fibre structure can be in accordance with the required specification for the fibre performance, in particular providing capillary wall thicknesses that provide antiresonant guiding at the desired wavelength(s). However, as noted above, this can be difficult to achieve in practice, in part owing to the challenge of monitoring the fibre structure during the drawing to ensure the fabrication is proceeding correctly. The abi li ty to monitor without taking fibre samples for test, which is both wasteful and interferes with the draw process, and which allows the relevant features of the microstructure to be assessed, is of significant interest and importance.

[0019] The present disclosure proposes to utilise the same thin film interference effect that provides the antiresonant optical waveguiding in the core of an ARF to analyse the fibre microstructure in a manner than can be performed during drawing. It has been noted that such fibres, made from transparent silica, can show colours observable from the exterior of the fibre when appropriately illuminated. This arises from Fresnel reflections of the incident illuminating light at the pairs of air-glass boundaries of the thin capillary walls within the fibre, with the observed colour or wavelength being defined by the capillary' wall thickness. Since this is the same mechanism that confines the fundamental guided mode in the core of the fibre, the capillary wall thicknesses (“ti” and “to” in the FIG. 1 example) produce a thicknessdependent effect which is directly observable from the exterior of the fibre. The capillary wall thicknesses can therefore be measured by non-contact, non-destructive observation from outside the fibre. This information may be compared with the intended wall thickness, and used to control and adjust the draw parameters such as pressure and speed if the measured values differ from the intended values.

[0020] This may7be simply achieved by7directing light onto the fibre which will undergo Fresnel reflection from the capillary' walls, detecting either or both of a reflected component of the light or a transmitted component of the light, and comparing the detected light with that which would be expected from capillary walls of the desiredthickness. Any discrepancy indicates a deviation of the capillary wall thickness from the desired value, and the fibre drawing can be adjusted accordingly.

[0021] FIG. 2 shows a simplified schematic representation of a first example apparatus configured to perform fibre structure monitoring in this way. The apparatus 30 is located in or near a fibre drawing installation, commonly called a draw tower, in which a heat source is applied to a preform or cane to soften the glass material and allow fibre to be drawn from it. This is generally represented by the element 32 in FIG. 2, from which a continuous length of hollow core ARF 10 is delivered along a draw direction D parallel to the longitudinal axis of the fibre 10. The apparatus 30 is installed so as to be able to interact with the fibre 10 as it is delivered, so the fibre 10 moves continuously past the apparatus 30.

[0022] The apparatus 30 comprises a light source 34 that generates and outputs a beam of light designated as a source beam 36. The source beam 36 is passed through a source optical system 38 which acts on the source beam 36 to adapt and modify the source beam into a desired beam format for a measurement beam 40 which is delivered from the source optical system 38 and directed onto the fibre 10. In this example the measurement beam 40 has a propagation direction substantially orthogonal to the draw direction D. The measurement beam 40 is incident on the fibre 10 and propagates through the fibre interior owing to the transparent nature of the material from which the fibre 10 is made. During this propagation, the measurement beam 40 undergoes Fresnel reflection at the walls of the capillaries inside the fibre 10. A portion of the light (not shown) in the measurement beam 40 is therefore reflected aw ay from the forward beam propagation direction, and a remaining portion of the light 40T is transmitted through the fibre 10 to be emitted out the far side of the fibre 10 (ignoring other loss effects). The transmitted light 40T is collected at a detector optical system 42 which acts to adapt the transmitted light 40T into a detection beam 44, which is delivered to and detected by an optical detector 46. The optical detector 46 outputs a detection signal 48 representing the power level, magnitude or amplitude, optionally as a function of wavelength, of the detection beam 44; this is provided to a processor 50. The processor 50, which comprises a microprocessor or other computing device, and a memory storing instructions for execution by the microprocessor (and optionally data for use in comparison with the detection signal or information derived therefrom), is configured to compare the detection signal 48 with one or more predetermined signal values known to correspond to the desired capillary wall thickness specified for the fibre. Alternatively, the capillarywall thickness in the fibre 10 may be extracted from or otherwise determined from the detection signal, and compared to the specified capillary wall thickness. If the comparison shows a good match between measurement and specification (such that any difference between the two is essentially zero, or below a predetermined acceptable threshold), the draw can be allowed to continue. If a difference is identified between measurement and specification that is unacceptable (such that the difference is non-zero, or is at or above a predetermined acceptable threshold), the processor 50 can generate one or more feedback control signals 52 which are supplied to the draw tower apparatus 32 in order to adjust the draw parameters in such a way that will bring the identified difference within the acceptable limit. The size of the threshold can be set to reflect acceptable manufacturing tolerances for the optical fibre. This monitoring process can operate continuously throughout drawing of the fibre 10 in order to provide real-time non-destructive assessment of the fibre’s interior structure and an ability to correct any identified defects in the structure.

[0023] The light source 34 may be a broadband light source or optical source (in that it outputs light over a broad range or spectrum of wavelengths). This may be a halogen lamp, a xenon lamp or a supercontinuum source, for example, which are operable to emit the broad range of wavelengths simultaneously. Alternatively, the light source 34 may be a tunable narrowband light source such as a tunable laser, which is operable to emit a single very narrow range of wavelengths at any one time, but which can be tuned to vary the wavelength. In this way, a broad spectrum of wavelengths can be provided in a non-simultaneous manner. In another alternative, the light source 34 may be a narrow band light source such as a laser that is not tunable, so that only a narrow range of wavelengths (e.g. a linewidth of less than 0.1pm, effectively a single wavelength) is obtainable. The source beam 36 may therefore be broadband or narrowband; alternatives for determining the assessment of the capillary wall thickness for these options are explained further below.

[0024] The source optical system 38 may comprise a variety of lenses, mirrors and other bulk optical components that operate in transmission or reflection. These can be combined and configured in order to shape the source beam 36 into the measurement beam 40 in such a way that substantially all of the measurement beam 40 is incident on the fibre 10. Accordingly, the measurement beam 40 is configured to have a transverse beam intensity profile with dimensions that are less than, the same as, or not greatly in excess of, the dimensions of the fibre 10 as seen from the side. A useful arrangement is asource optical system 38 which is anamorphic, in other words, which outputs a beam profile which is elongated, having a long dimension (major axis) and an orthogonal much shorter dimension (minor axis). If the minor axis does not greatly exceed the diameter of the fibre 10, and the measurement beam 40 is directed onto the fibre 10 with the major axis aligned along the longitudinal axis of the fibre 10 (also the draw direction D). most or all of the light in the measurement beam 40 is incident onto the fibre 10 in order to undergo Fresnel reflection, while enabling illumination of a larger amount of the fibre 10 at any one time. Alternatively, the measurement beam 40 may be shaped into a collimated beam, a focussed beam with a circular profile, or some other shape.

[0025] The detector optical system 42, which also comprises a variety of lenses, mirrors and other bulk optical components that operate in transmission or reflection, can be matched to the source optical system 38 but arranged to provide the reverse beam shaping. Hence, if the source optical system 38 is anamorphic, the detector optical system 42 may also be anamorphic. This returns the collected transmitted beam 40T to the original beam profile of the source beam 36.

[0026] If the light source provides a source beam 36 and hence a measurement beam 40 which has a broadband optical spectrum, the detector optical system can include an optically dispersive element such as a prism, a grating, a grism or a beam splitter, which spatially separates (disperses) the wavelength components of the detector beam 44 before the detector beam 44 is delivered to the detector 46. In such an arrangement, the detector 46 can comprise a one-dimensional or two-dimensional array of photodetector elements over which the wavelength components are dispersed so that the power level in each wavelength component can be separately detected (via tuning of the light source if the light source itself is not broadband). This allows the processor to determine a spectrum of the measurement beam (comprising a variation of power level (optical intensity) with wavelength across the broadband range). Alternatively, a single detector element can be used with a tunable light source to obtain a spectrum if the detection is synchronised with the tuning. If the measurement beam comprises a single narrowband wavelength component only (because the light source is an untunable narrow band source), the dispersive element may be omitted and the detector 46 may comprise a single photodetector element. In this case the processor determines a single power or intensity level rather than a spectrum. If the measurement beam comprises a broadband optical spectrum which substantially overlaps with the visible portion of the opticalspectrum than the detector 46 may conveniently be a conventional charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) detector.

[0027] FIG. 3 shows a simplified schematic representation of a second example apparatus for performing fibre structure monitoring by the currently disclosed method of detecting light which has been modified by interaction with the interior structural features of a fibre. This apparatus comprises the same components as that of the FIG. 2 example (although the draw tower apparatus 32 and the feedback control signals 52 are omitted for clarity), but they are differently arranged so that light reflected from the fibre interior, rather than light transmitted through the fibre interior, is detected. In some examples, light undergoes multiple reflections within the fibre interior prior to detection. Hence, the apparatus 60 comprises a light source 34 that outputs a source beam 36 for shaping in a source optical system 38 into a measurement beam 40 as before. The measurement beam 40 is incident on the fibre 10 at an angle of incidence which is non- orthogonal to fibre’s longitudinal axis (and hence also non-orthogonal to the surfaces of the capillaries in the fibre). This arrangement means that the portion 40R of the measurement beam 40 which is reflected from the fibre’s interior does not back- propagate along the incident beam direction, and can be conveniently collected by a detector optical system 44 which adapts the beam for delivery' to a detector 46 as before. The detector 46 outputs a detection signal 48 to a processor 50. also as before. The portion 40T of the measurement beam which is transmitted through the fibre 10 is uncollected in this example.

[0028] Features discussed above for the light source, the source optical system, the detector optical system, the detector and the processor are applicable also to this example. The depicted configuration of the apparatus 60 shows the fibre 10 (defined by its longitudinal axis) lying in the same plane as both the incident measurement beam 40 and the reflected beam portion 40R. This is not essential, however, and the fibre 10 may alternatively be arranged normal to the plane defined by the incident measurement beam 40 and the reflected portion 40R, or at some intermediate angle. In any arrangement, if the measurement beam is shaped into an elongate beam profile with the source optical system 38, it should be oriented such that the major axis is aligned with the longitudinal axis of the fibre

[0029] FIG. 4 shows a simplified schematic representation of a third example apparatus. In common with the FIG. 3 example, the apparatus 70 is configured to collect reflected light rather than transmitted light. In this case, however, the measurement beam40 is incident on the fibre 10 substantially orthogonally to the longitudinal axis of the fibre and the surfaces of the capillaries inside the fibre 10. This means that the reflected light 40R returns along substantially the same optical propagation path as the incident measurement beam 40. The back-propagating reflected light 40R needs to be separated from the forward-propagating measurement beam 40 in order for it to be detected; this is enabled by use of a single combined optical system 72 that combines the functions of the source optical system and the detector optical system in the previous examples.

[0030] Accordingly, a light source 34 provides a source beam 36 as before, and the source beam 36 is delivered to the combined optical system 72. This comprises optical components that provide the desired profile shaping and / or adaptation of the source beam 36 to produce the measurement beam 40, again as before, which is then directed onto the fibre 10. A portion 40T of the incident measurement beam 40 is transmitted through the fibre 10 and not detected, and a portion 40R of the incident measurement beam 40 undergoes Fresnel reflection at the capillary walls inside the fibre 10 and is directed back to the combined optical system 72 by virtue of the normal angle of incidence. The reflected light 40R passes through the optical components in the combined optical system 72 in the back propagation direction and therefore undergoes a reversal of the adaptation provided to produce the measurement beam 40. The combined optical system 72 additionally includes one or more optical elements that then direct at least a part of the adapted reflected light away from the backpropagation direction and out of the combined optical system along a different path in order to produce the detection beam 44. This is detected by a detector 46 that passes a representative electrical detection signal 48 to a processor 50, as before. The apparatus may additionally include a baffle 71 placed on the opposite of the fibre from the combined optical system 72, in order to inhibit any stray light from entering the combined optical system 72.

[0031] FIG. 5 shows a simplified schematic representation of an example of the combined optical system 72 of FIG. 4. The light source 34 generates the source beam 36 as previously described, which is delivered to the combined optical system 72. The source beam 36 is incident on a beam splitter 74 in the combined optical system, which is configured to transmit a part of the source beam 36 to the remaining beam shaping components of the combined optical system 72. In this example, the combined optical system 72 is configured to provide anamorphic beam shaping, and comprises a negative cylindrical lens 76 followed by a positive cylindrical lens 78. which together form an anamorphic telescope operable to shape the source beam 36 into an elongated beamprofile. The anamorphic telescope is followed by a positive spherical lens 80 that brings the elongated beam profile to an elongated focus 40F, thereby providing the required measurement beam 40. In operation, the fibre to be analysed (not shown) is placed substantially at the focus 40F, with its longitudinal axis normal to the optical propagation direction, to bring the measurement beam 40 into incidence on the fibre.

[0032] The normal incidence means that the light 40R reflected from the fibre is back-propagated along the original propagation direction, for collection by the spherical lens 80, and reverse shaping by the anamorphic telescope lenses 78, 76. It reaches beam splitter 74, where a part of it is reflected away from the backpropagation direction that would otherwise lead to the light source 34. This provides the detector beam 44. In the case where the light source 34 provides a broadband output (directly or via tuning of a narrowband output), the detector beam 44 is directed on a dispersive element 82 that provides the detector beam 44 as a spatially dispersed spectrum 84 which is directed to the detector 46 which produces the detection signal 48 as before.

[0033] In alternative arrangements, the light source 34 may provide a narrowband source beam 36 of fixed wavelength, in which case the dispersive element 82 may be omitted. Also, the anamorphic beam shaping is optional, and different optical components may be included in the combined optical system to provide a differently- shaped focus 40F, or a collimated measurement beam 40.

[0034] Apparatus that includes a combined optical source 72 may be preferred in that onty one set of optical components is required for handling both the measurement beam and the detector beam, so overall the apparatus can be made more compact and / or less costly, and access to only one side of the fibre 10 is required. However, optical losses at the beam splitter will reduce the amount of light available for detection from a given source beam, leading to an increased signal-to-noise ratio. These factors may need to be balanced when implementing the apparatus.

[0035] This may be addressed using polarisation, with the source beam being implemented as a beam of polarised light. This may be an intrinsic result of the type of light source, for example if the light source 34 is a polarised laser. Alternatively, a linear polarising element may be included to convert the source beam 36 to polarised light.FIG. 5 shows an example polariser 86 in phantom. This allow s the beam splitter 74 to be a polarising beam splitter arranged such that substantially all the source beam 36 passes through the beam splitter 74 to the beam shaping components. A quarter wave plate 88 (also in phantom) is placed after the beam splitter 74 to convert the transmitted linearlypolarised light to circularly polarised light. The measurement beam 40 is therefore circularly polarised, and when it is reflected from the capillary walls in the fibre as the reflected light 40R, the polarisation state is reversed (that is, from right circular to left circular, or vice versa). After passage of this light back through the quarter wave plate 88, the polarising beam splitter 74 is then able to reflect substantially all of the light to provide the detector beam 44. This configuration addresses the issue of optical losses at the beam splitter noted above.

[0036] Other alternatives are also possible for a combined optical source. For example, the beam splitter 74 may reflect the source beam 36 to the beam shaping components, instead of transmitting it, so that the returning detector beam 44 is transmitted to the detector 46 instead of being reflected by the beam splitter 74.

[0037] In order to implement an apparatus that operates at normal incidence to the fibre, but does not include a combined optical system, a beam splitter may be placed between beam shaping components of a source optical system and the fibre to transmit the measurement beam 40, and then intercept the returning reflected light 40R to reflect it to a different optical path that includes a detector optical system. This enables the analysis of reflected light at normal incidence but allows the separation of beam handling for the incident and reflected beams so that they may be handled differently if desired.

[0038] In another example, the light may be handled using a confocal optical system as descnbed in US 3,013,467 [11J. For example, a confocal lens system may be used in the combined optical system 72 in place of the anamorphic telescope shown in FIG. 5. The confocal arrangement allows “the provision of a microscopic optical system capable of rejecting all scattered light except that emanating from the central focal point, i.e. the illuminated point of the specimen. Such high selectivity of light reduces blurring, increases effective resolution, and permits unusually clear examination of thick and scattered specimens”. In the presently proposed methods and apparatus, the “illuminated point of the specimen” can be chosen to be a wall of a primary capillary or a secondary' capillary within the fibre, and the “scattered light” which is able to be rejected (and therefore removed from the detector beam collected by the detector) may be light reflected from the internal surface of the outer cladding (14 in FIG. 1) or another surface w hich does not belong to the capillary w all of interest.

[0039] Other examples of apparatus may include more than one measurement beam. In other words, two or more measurement beams are generated and used to illuminate the fibre. For example a first measurement beam may be directed tointerrogate the wall of a primary capillary (by suitable placement of the focus if a noncollimated beam shape is used) and a second measurement beam may be directed to interrogate the wall of a secondary' capillary’. More generally, the beams and fibre may be arranged such that a different internal structural thickness feature of interest is in the field of view of each of two or more measurement beams. If narrow band illumination is used, each measurement beam may comprise a different wavelength (from multiple narrow band light sources, or by filtering to extract different wavelengths from the output of a broadband source, for example) for ease of identifying detected light associated with each fibre feature. In other words, measurements from different fibre features are separated by wavelength.

[0040] As an alternative, a single measurement beam may be delivered in a reconfigurable way (by movement of the optical components under control of the processor, for example) so that it can be redirected periodically in order to place each of a plurality of internal fibre thickness features in the field of view of the beam in turn. In this way. multiple fibre features can be analysed over time. In other words. measurements from different fibre features are separated by time. For example, the measurement beam may be rotated or oscillated with respect to the fibre.

[0041] In any of the examples of apparatus, optical fibres may be included to carry any or all of the source beam, the measurement beam and the detector beam over at least a part of their propagation paths. Optical components including lenses, diffraction or dispersion elements and reflective elements may be directly integrated onto the ends of the optical fibres. This can enhance miniaturisation of the apparatus, and the use of fibres can aid in maintaining optical alignment of the various components, increasing stability of the apparatus and enabling more flexibility in the arrangement of parts.

[0042] In general, the source optical system, the detector optical system and the combined optical system may comprise one or more optical components able to direct and if necessary shape the light beam before and after its interaction with the capillary walls in the fibre in order to achieve the formatting of the beam desired by the user and to collect a usable amount of light for detection after the interaction.

[0043] FIG. 6 shows an example of a detection signal that can be obtained by detecting some or all of the transmitted portion of the measurement beam, such as by use of the example apparatus in FIG. 2. The detection signal, output by the detector that collects the detection beam, represents the power amplitude or intensity of the light in the detection beam. In this example, which is obtained by computer modelling, and whichdoes not include spectral features derived from scattering or WGM resonances, the power amplitude is the percentage of the incident light in the measurement beam which is transmitted through the fibre (shown on the vertical axis), and this is measured by the detector over a broadband wavelength range, in this case 0.4 pm to 1.8 pm (shown on the horizontal axis), so the detection signal can be considered to be a spectrum. The broadband range is obtainable by use of a broadband light source or a tunable narrowband light source, as explained above.

[0044] FIG. 6 includes two example spectra, corresponding to two different capillary wall thicknesses. The spectrum shown as a solid line is for a wall thickness of 0.49 pm. while the dashed line is for a wall thickness of 0.51 pm. The spectra show a generally periodic variation with interrogating wavelength, as the Fresnel reflection cycles through constructive and destructive interference as the mismatch between wavelength and wall thickness varies. From this it can be appreciated that different thicknesses of wall produce different spectra. The variation of the transmitted power level is shifted to longer wavelengths as the wall thickness increases.

[0045] Hence, the detected spectrum can yield information about the thickness of the capillary w alls in the fibre being assessed. This may be utilised in a number of ways.

[0046] If a fibre is being manufactured to a specification with a predetermined capillary wall thickness, a detected spectrum such as that of FIG. 6 can be compared with the spectrum which is known to be obtained from a fibre having a capillary wall thickness of that specification. The known spectrum may be obtained by computer modelling or from measurements on a fibre known to have the correct specification, for example. If the detected spectrum and the known spectrum match, either exactly or to within a given tolerance (difference is below a predetermined threshold), the conclusion is that the manufacturing is correct and can be continued with the same parameters. If the detected spectrum and the known spectrum differ, either at all or outside a given tolerance (difference is above a predetermined threshold), this indicates that the wall thickness is incorrect and outside the specification; this information can be used to initiate a modification of the manufacturing parameters. If the detected spectrum differs by being shifted to longer w avelengths than the predetermined spectrum, this indicates that the wall thickness is too large. The manufacturing parameters can be adjusted to produce a thinner capillary wall, for example by increasing the fibre draw speed or increasing the pressure in the capillaries. If the detected spectrum is shifted to shorter wavelengths than the predetermined spectrum, this indicates that the wall thickness is toosmall. The manufacturing parameters can be adjusted to produce a thicker capillary' wall, for example by decreasing the fibre draw speed or decreasing the pressure in the capillaries. The comparison of the spectra is carried out by the processor, and the adjustment can be effected by generation of feedback control signals at the processor which are sent to the fibre draw apparatus, as discussed with regards to FIG. 2.

[0047] Alternatively or additionally, the detected spectrum can be used to obtain an actual measurement of the capillary wall thickness, since the shape of the spectrum is a known function of wall thickness. For example, the processor can be configured to perform mathematical curve fitting of the detected spectrum using the knoyvn function in order to extract a value for the capillary wall thickness. Alternatively, the processor may have access to a database of stored spectra for known values of wall thickness, and the fitting may comprise pattern matching or comparison of the detected spectrum to the stored spectra in order to determine yvhich is the closest fit. The yvall thickness value corresponding to the stored spectrum with the closest fit is then identified as being the value of the wall thickness in the fibre under assessment. If the fibre being assessed is currently being drawn (rather than measurements being made post-production) this measurement of the yvall thickness can be used to inform any adjustments that may be required in the manufacturing if the measured value differs from a specified value. The processor can generate appropriate control signals for feedback to the draw apparatus as mentioned above.

[0048] As noted earlier, for fibres having primary capillaries and secondary capillaries (and optionally tertiary' or higher level nested capillaries), optimum confinement of the guided optical mode is obtained w hen the capillaries all have the same wall thickness. Therefore, in situations where measurements are detected for more than one capillary or capillary type in a fibre, such as by the use of different measurement beams or by directing the same measurement beam to different parts of the fibre interior in turn, the tyvo or more detected spectra can be compared to one another in order to ascertain if the capillary wall thicknesses are the same, or similar to within a predetermined tolerance or threshold. If the spectra differ by an amount in excess of the threshold, steps can be taken to adjust the drawing parameters to reduce the difference and therefore bring the w all thicknesses into better coincidence. This can be additional or alternative to any control to push any or each of the wall thicknesses towards a specified thickness for the fibre.

[0049] Entirely equivalent approaches can be used in configurations where the apparatus is arranged to detect the reflected portion of the measurement beam, such as the examples of FIG. 3 and FIG. 4. Ignoring other losses, the reflected power level is simply the incident power level minus the transmitted power level, so the reflected power level is complimentary to the transmitted power level and hence is similarly related to the capillary wall thickness.

[0050] FIG. 7 shows examples of detected signals based on reflected light, computer generated via the same modelling as the FIG. 6 graphs, so that each detected signal is shown as a spectrum of the reflected power level (vertical axis) as it varies across a broadband wavelength range of the measurement beam, from 0.4 pm to 1.8 pm. Again, spectra are included for two capillary wall thickness values. 0.49 pm (solid line) and 0.51 pm (dashed line). The peaks and troughs of the generally periodic spectra are the inverse of those in the transmitted light spectra of FIG. 6, as expected, but otherwise the behaviour of the curves is the same. In particular, the spectrum shifts to longer wavelengths as the wall thickness increases, as before.

[0051] As can be further appreciated from FIG. 6 and FIG. 7, at any given incident wavelength, the magnitude of the detected power level varies with capillary wall thickness. This relationship can be used to obtain information about the wall thickness from a measurement beam which is narrow band (effectively single wavelength) only, in other words, in an apparatus that is not configured to obtain broadband spectral measurements as in FIG. 6 and FIG. 7. If the relationship between wall thickness and power level is known and stored in the processor for the measurement wavelength which is used, a simple power level measurement can be made and used to calculate the capillary wall thickness. The processor can generate appropriate feedback control signals to adjust the fibre drawing if the measured thickness differs from a specified thickness, as previously described.

[0052] FIG. 8 shows a graph of tw o example relationships betw een the power level (as a proportion of the incident power) and capillary wall thickness when the reflected portion of light is detected. In order to maximise the variation in power level with w all thickness and therefore improve the resolution of the measurement, the measurement w avelength can be chosen appropriately. In the examples of FIG. 6 and FIG. 7. it can be seen that for wall thicknesses around 0.5 pm, wavelengths at which the transmittance / reflectance changes most rapidly with capillary wall thickness are about 0.45 pm, 0.52 pm, 0.66 pm and 1.16 pm. Accordingly, FIG. 8 shows a power / thicknessrelationship for a 0.8 pm interrogating wavelength (solid line) and a 1.2 pm interrogating wavelength (dashed line); it can be seen that the slope of the relationship is steepest for wall thicknesses around of 0.5 pm. For other wall thicknesses of interest, the operating wavelength can be adjusted accordingly. In order to provide an apparatus suitable for analysing a variety of optical fibres of differing specification, the light source could be implemented as a tunable narrowband laser, the output of which is tuned in accordance with the capillary wall thickness which is to be measured. Note that the relationship is periodic and therefore not single-valued with regard to wall thickness - a detected power level could correspond to two or more wall thicknesses. Therefore, some a priori knowledge of the wall thickness is useful in order to exclude incorrect thickness values that give the same power measurement.

[0053] Note also that the most appropriate individual wavelengths for assessing capillary' wall thickness according to the theory' relating to FIG. 8 are not those at which the fibre is designed to transmit. In fact, at and near the intended transmission wavelength, the transmitted / reflected power level is largely insensitive to wall thickness (corresponding to the peaks and troughs in FIG. 6 and FIG. 7).

[0054] This leads to a further analysis technique. As noted above in the context of assessing different fibre features, two or more measurement beams of different wavelength can be used, produced from multiple narrow band light sources or by extracting different wavelengths from a broadband output. It is possible to select one measurement beam wavelength to correspond to high sensitivity for the capillary wall thickness of interest, as described above. Also, an additional measurement beam is provided at a wavelength corresponding to low sensitivity. For example, for a wall thickness around 0.5 pm, FIG. 6 and FIG. 7 indicate that at measurement wavelengths around 0.42 pm, 0.49 pm, 0.59 pm, 0.73 pm, 0.97 pm and 1.45 pm, the detected power level is substantially insensitive to changing wall thickness; these are wavelengths at which the transmittance and reflectance spectra have maxima or minima, and therefore a zero or small rate of change.

[0055] The detector beam at any wavelength will include amplitude variations arising from factors other than the wall thickness, such as reflections from the inner and outer surfaces of the outer cladding of the fibre, and variations caused by relative movement between the fibre and the apparatus (recall that the fibre is continuously moving during the draw process). Note that the modelled data of FIG. 6. FIG. 7 and FIG. 8 does not include such contributions. The detector beam obtained from the additionalmeasurement beam, at the low sensitivity wavelength, in which variations will be caused largely by these extraneous factors only and not by the wall thickness, can be used to exclude or discount these contributions, which are effectively noise, from the detector beam of interest, obtained from the measurement beam at the high sensitivity wavelength. Appropriate processing for the two detection signals will be apparent to the skilled person, in order to identify the noise variations from the “insensitive’' signal and subtract or otherwise remove them from the “sensitive” signal. In this way, the desired output measurement is improved by being made more sensitive to changes in wall thickness. FIG. 9 shows a graph of three example relationships between the power level of the reflected light portion and the capillary wall thickness. The solid line and the dashed line correspond to interrogating wavelengths of 0.8 pm and 1.2 pm as in FIG. 8, selected for high sensitivity at wall thicknesses around 0.5 pm. Also shown is the power / thickness relationship for a 1 pm interrogating wavelength (dot-dash line), which is insensitive to wall thicknesses around 0.5 pm, as can be appreciated from the presence of a minimum in the curve at this point.

[0056] It is evident from FIG. 6 to FIG. 9 that the detected light level is dependent on the wavelength of the light. To properly interpret the detection signal, the processor therefore requires knowledge of the measurement beam wavelength(s). For simple measurements acquired from a single narrowband interrogating beam, this is trivial, and the processor merely requires the relevant power / thickness relationship at that wavelength, such as any of the curves in FIG. 8 and FIG. 9. More generally, the processor may either be provided with the w avelength of the light source, such as the bandwidth covered by a broadband source, or it may determine the wavelength on detection, in other words by wavelength-dependent detection. The first arrangement is appropriate for a tunable light source, and requires no dispersive element before the detector, and a zero-dimensional detector (point detector, single element detector). The processor can control the tuning of the light source, and therefore is aware of the wavelength at any given time, and can designate the detected light level at that time to the appropriate wavelength to build up a spectrum. The second arrangement is suitable for a broadband light source, and requires a dispersive element to spatially disperse the detector beam over a one- or two-dimensional detector array, as mentioned previously.

[0057] For those methods employing two or more measurement beams at different wavelengths, either arrangement may be used. A tunable light source may be switched between the wavelengths of interest (effectively a simple form of tuning) andthe light detected using a single element detector. Alternatively, the wavelengths can be emitted simultaneously (from separate sources or by appropriate filtering to extract the wavelengths from a broadband output), and a dispersive element used to direct the different wavelengths in the detected beam onto separate single element detectors. For two wavelengths only, a beam splitter is an adequate dispersive element, for example.

[0058] It will be appreciated from the foregoing description that it is possible to configure the processor to output suitable feedback control signals to control manufacturing parameters of a fibre drawing process (such as draw speed and lumen pressure), or to output an actual value of the thickness of the capillary' wall which has been analysed, or both.

[0059] From the preceding description, it will be appreciated that the proposed analysis apparatus and methods can be usefully employed for the purpose of real-time non-destructive assessment of optical fibre during manufacture which allows feedback control of the manufacturing in order to provide ARF fibre with a desired specification. However, the approach can also be used to make the same measurements on existing fibres, after fabrication. Applications might include post-fabrication quality control, testing of purchased fibres, and characterisation of unknow n fibres or new fibre designs. Accordingly, the methods can be applied both “on the fly” to a continuously advancing length of fibre as it is drawn from a preform or a cane, or as a one-off measurement on a static fibre, post-fabrication. For the former situation, apparatus according to the examples herein may be installed in a fibre draw tower.

[0060] FIG. 10 shows a flow chart that summarises steps in an example method according to the present disclosure. In a first step SI a hollow core optical fibre with a microstructured inner cladding comprising longitudinal capillaries to configure the fibre for antiresonant optical waveguiding is provided for analysis. The fibre may be in a postfabrication state, or more usefully, may be in the process of being drawn from a preform or a cane in a fibre drawing tower. In a second step S2, a beam of light is directed onto the fibre in such a way that it interacts with a wall of at least one of the capillaries, in particular that a portion of the light is transmitted and a portion is reflected, after undergoing Fresnel reflections at the interfaces between the capillary- yvall surfaces and the surrounding air. In a third step S3, the transmitted light or the reflected light (or optionally both) is detected, and in a fourth step S4. a poyver level of the detected light is determined. The light may be single wavelength, so that the power level is a singlevalued measurement, or the light may comprise a broadband spectrum of wavelengths sothat the power level is a power spectrum. In a fifth step S5, the power level obtained in step S4 is used to deduce information about the thickness of the wall of the capillary with which the light has interacted. In various examples, the information includes one or more of a value for the thickness of the wall, or a determination that the wall thickness does or does match a specified thickness value for the fibre, or a determination that two or more wall thicknesses in the fibre do or do not have the same thickness.

[0061] Finally, if the method is being performed on fibre that is actively being drawn, the method can include an optional step S6, in which the information about the thickness of the capillary wall is used to prompt the generation of feedback control signals for operation of the fibre drawing tower. In particular, if the information indicates that there is a difference between the actual wall thickness of the fibre and a specified value of the wall thickness which it is intended that the fibre should have in order to efficiently guide a particular wavelength of light, and the difference is in excess of a threshold considered to be within acceptable manufacturing tolerances, feedback control signals are produced in order to modify the draw process so as to correct the wall thickness by moving it towards the specified value.

[0062] The optical fibre assessment methods proposed herein are applicable generally to hollow' core antiresonant fibres. The fibre may be made from materials known for the fabrication of existing designs of lumen-containing optical fibres, in particular glass materials such as silica. The various tubes and capillaries from which the structured fibre is composes may be made from the same material or from different materials. Types of glass include "silicate glasses" or “silica-based glasses”, based on the chemical compound silica (silicon dioxide, or quartz), of which there are many examples. Other glasses suitable for optical applications and from which the filament may usefully be made include, but are not limited to, chalcogenide, tellurite glasses, fluoride glasses, and doped silica glasses. The glass materials may include one or more dopants for the purpose of tailoring the optical properties, such as modifying absorption / transmission or enabling optical pumping.

[0063] The following paragraphs set out several examples of the present disclosure. Examples include any combination of the following:

[0064] Clause A. A method of analy sing a hollow core antiresonant optical fibre having an inner cladding comprising at least one capillary’ defined by a wall with a wall thickness, the method comprising: directing light onto the fibre for interaction with at least one surface of the capillary wall; detecting a portion of the light which hasinteracted with the at least one surface of the capillary wall to determine a power level of the detected portion; and using the power level to deduce information regarding the wall thickness.

[0065] Clause B. The method of clause A in which the detected portion is light which has been transmitted through the capillary' wall and reflected from the at least one surface of the capillary’ wall.

[0066] Clause C. The method of clause A or clause B in which the light comprises a broadband range of wavelengths, and the power level is determined across the range of wavelengths in order to obtain a power level spectrum.

[0067] Clause D. The method of clause C, comprising deducing information regarding the wall thickness by comparing the power level spectrum with a predetermined power level spectrum corresponding to a specified value of wall thickness in order to identify a difference between the power level spectrum and the predetermined power level spectrum, and determining that the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

[0068] Clause E. The method of clause C, comprising deducing information regarding the wall thickness by performing mathematical curve fitting of the power level spectrum to extract a value for the wall thickness known to correspond to the power level spectrum.

[0069] Clause F. The method of clause C, comprising deducing information regarding the wall thickness by comparing the power level spectrum to a set of predetermined power level spectra with corresponding known wall thicknesses to determine a predetermined power level spectrum to which the power level spectrum best matches, and identifying the known wall thickness corresponding to the determined predetermined powder level spectrum as the value of the wall thickness.

[0070] Clause G. The method of clause A or clause B, wherein the light comprises a narrow range of wavelengths, further comprising deducing information regarding the wall thickness using a predetermined relationship between power level of the detected portion and w all thickness at the narrow range of wavelengths to extract a value for the wall thickness.

[0071] Clause H. The method of any one of clauses E, F or G, further comprising comparing the value for the wall thickness with a specified value of wall thickness in order to identify a difference betw een the value and the specified value, and determiningthat the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

[0072] Clause I. The method of clause D or clause H, in which the method is carried out on the fibre as the fibre is being drawn in a draw process, the method further comprising, when it is determined that the wall thickness differs from the specified value, generating one or more control signals for adjusting the draw process to move the wall thickness closer to the specified value.

[0073] Clause J. An apparatus for analysing a hollow core antiresonant optical fibre having an inner cladding comprising at least one capillary defined by a wall with a wall thickness, the apparatus comprising a light source operable to generate an output beam of light; a first optical system configured to derive a measurement beam from the output beam and direct the measurement beam onto the fibre for interaction with at least one surface of the capillary' wall; a second optical system configured to collect a portion of the light which has interacted with the at least one surface of the capillary wall and direct the portion as a detector beam; a detector configured to detect the detector beam and output a signal representing the power level of the detected portion; and a processor configured to use the power level to deduce information regarding the wall thickness.

[0074] Clause K. An apparatus according to clause J, in which the collected portion is light which has been transmitted through the capillary wall and reflected from the at least one surface of the capillary' wall.

[0075] Clause L. An apparatus according to clause J or clause K, in which the processor is configured to deduce information regarding the wall thickness by determining a value of the wall thickness.

[0076] Clause M. An apparatus according to clause J or clause K, in which the light source is operable to generate light comprising a broadband range of wavelengths, and the detector is configured to output the signal as the power level across the range of wavelengths in order to provide a power level spectrum to the processor.

[0077] Clause N. An apparatus according to clause M, in which the processor is configured to deduce information regarding the wall thickness by comparing the power level spectrum with a predetermined power level spectrum corresponding to a specified value of wall thickness in order to identify a difference between the power level spectrum and the predetermined power level spectrum, and determining that the wall thickness matches the specified value if the difference is below a threshold ordetermining that the wall thickness differs from the specified value if the difference is above the threshold.

[0078] Clause O. An apparatus according to clause M, in which the processor is configured to deduce information regarding the wall thickness by performing mathematical curve fitting of the power level spectrum to extract a value for the wall thickness known to correspond to the power level spectrum.

[0079] Clause P. An apparatus according to clause M, in which the processor is configured to deduce information regarding the wall thickness by comparing the power level spectrum to a set of predetermined power level spectra with corresponding known wall thicknesses to determine a predetermined power level spectrum to which the power level spectrum best matches, and identifying the known wall thickness corresponding to the determined predetermined power level spectrum as the value of the wall thickness.

[0080] Clause Q. An apparatus according to clause J or clause K, in which the light source is operable to generate light comprising a narrow range of wavelengths, further configured to deduce information regarding the wall thickness using a predetermined relationship between power level of the detected portion and wall thickness at the narrow range of wavelengths to extract a value for the wall thickness.

[0081] Clause R. An apparatus according to clause L, clause O, clause P or clause Q, in which the processor is further configured to compare the value for the wall thickness with a specified value of wall thickness in order to identify a difference between the value and the specified value, and determine that the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

[0082] Clause S. An apparatus according to clause N or clause R suitable for mounting within an optical fibre draw tower in order to deduce information regarding the wall thickness of fibre as it is being draw n in a draw process, the processor further configured to, when it determines that the w all thickness differs from the specified value, generate one or more control signals for the draw tower for the purpose of adjusting the draw process to move the wall thickness closer to the specified value.

[0083] Clause T. An apparatus according to any one of clauses J to S, in w hich the first optical system is configured to derive the measurement beam such that it has an elongated transverse beam profile, and direct the measurement beam onto the fibre with the major axis of the beam profile aligned parallel to a longitudinal axis of the fibre.

[0084] Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person.

[0085] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

[0086] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items.

[0087] The operations of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks maybe deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

[0088] The term 'comprising' is used herein to mean including the method blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements.

[0089] It will be understood that the above description is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments. Although various embodiments have been described above with a certain degree of particularity7, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this specification.

Claims

CLAIMS1 . A method of analysing a hollow core antiresonant optical fibre (10) having an inner cladding comprising at least one capillary (16) defined by a wall (18) with a wall thickness, the method comprising: directing light onto the fibre for interaction with at least one surface of the capillary wall (18); detecting a portion of the light which has interacted wi th the at least one surface of the capillary wall (18) to determine a power level of the detected portion; and using the power level to deduce information regarding the wall thickness.

2. A method according to claim 1. in which the detected portion is light which has been transmitted through the capillary wall and reflected from the at least one surface of the capillary wall.

3. A method according to claim 1 or 2, in which the light comprises a broadband range of wavelengths, and the power level is determined across the range of wavelengths in order to obtain a power level spectrum.

4. A method according to claim 3, comprising deducing information regarding the wall thickness by comparing the power level spectrum with a predetermined power level spectrum corresponding to a specified value of wall thickness in order to identify a difference between the power level spectrum and the predetermined power level spectrum, and determining that the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

5. A method according to claim 3, comprising deducing information regarding the wall thickness by performing mathematical curve fitting of the power level spectrum to extract a value for the wall thickness know n to correspond to the power level spectrum.

6. A method according to claim 3, comprising deducing information regarding the wall thickness by comparing the power level spectrum to a set of predetermined power level spectra with corresponding known wall thicknesses to determine a predetermined power level spectrum to which the power level spectrum best matches, and identifying a known wall thickness corresponding to the determined predetermined power level spectrum as the value of the wall thickness.

7. A method according to any one of claims 1 or 2, wherein the light comprises a narrow range of wavelengths, further comprising deducing information regarding the wall thickness using a predetermined relationship between a power level of the detected portion and a wall thickness at the narrow range of wavelengths to extract a value for the wall thickness.

8. A method according to any one of claims 5 to 7. further comprising comparing the value for the wall thickness with a specified value of wall thickness in order to identify a difference between the value and the specified value, and determining that the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

9. A method according to claim 4 or claim 8, in which the method is carried out on the fibre as the fibre is being drawn in a draw process, the method further comprising, when it is determined that the w all thickness differs from the specified value, generating one or more control signals for adjusting the draw process to move the wall thickness closer to the specified value.

10. Apparatus (30) for analysing a hollow? core antiresonant optical fibre (10) having an inner cladding comprising at least one capillary' (16) defined by a wall (18) with a w all thickness, the apparatus comprising: a light source (34) operable to generate an output beam of light (36); a first optical system (38) configured to derive a measurement beam (40) from the output beam (36) and direct the measurement beam (40) onto the fibre (10) for interaction with at least one surface of the capillary wall (18); a second optical system (42) configured to collect a portion of the light which has interacted with the at least one surface of the capillary wall (18) and direct the portion as a detector beam (44); a detector (46) configured to detect the detector beam (44) and output a signal (48) representing a power level of the detected portion; and a processor (50) configured to use the power level to deduce information regarding the w all thickness.

11. Apparatus according to claim 10, in which the collected portion is light which has been transmitted through the capillary’ yvall and reflected from the at least one surface of the capillary wall.

12. Apparatus according to claims 10 or 11, in which the processor is configured to deduce information regarding the wall thickness by determining a value of the wall thickness.

13. Apparatus according to claims 10 or 11, in which the light source is operable to generate light comprising a broadband range of wavelengths, and the detector is configured to output the signal as the power level across the range of wavelengths in order to provide a power level spectrum to the processor.

14. Apparatus according to claim 13, in which the processor is configured to deduce information regarding the wall thickness by comparing the power level spectrum with a predetermined power level spectrum corresponding to a specified value of wall thickness in order to identify a difference between the power level spectrum and the predetermined power level spectrum, and determining that the wall thickness matches the specified value if the difference is below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

15. Apparatus according claim 13, in which the processor is configured to deduce information regarding the wall thickness by performing mathematical curve fitting of the power level spectrum to extract a value for the wall thickness known to correspond to the power level spectrum.

16. Apparatus according to claim 13, in which the processor is configured to deduce information regarding the wall thickness by companng the power level spectrum to a set of predetermined power level spectra with corresponding known wall thicknesses to determine a predetermined power level spectrum to which the power level spectrum best matches, and identifying the know wall thickness corresponding to the determined predetermined power level spectrum as the value of the wall thickness.

17. Apparatus according to any claims 10 or 1 1, in wfiich the light source is operable to generate light comprising a narrow^ range of wavelengths, further configured to deduce information regarding the wall thickness using a predetermined relationship between a power level of the detected portion and a wall thickness at the narrow range of wavelengths to extract a value for the w all thickness.

18. Apparatus according to claim 12, claim 15, claim 16 or claim 17, in which the processor is further configured to compare the value for the wall thickness with a specified value of wall thickness in order to identify a difference between the value and the specified value, and determine that the wall thickness matches the specified value if the differenceis below a threshold or determining that the wall thickness differs from the specified value if the difference is above the threshold.

19. Apparatus according to claim 14 or claim 18, suitable for mounting within an optical fibre draw tower in order to deduce information regarding the wall thickness of fibre as it is being drawn in a draw process, the processor further configured to, when it determines that the wall thickness differs from the specified value, generate one or more control signals for the draw tower for the purpose of adjusting the draw process to move the wall thickness closer to the specified value.

20. Apparatus according to any one of claims 10 to 19, in which the first optical system is configured to derive the measurement beam such that it has an elongated transverse beam profile, and direct the measurement beam onto the fibre with the major axis of the beam profile aligned parallel to a longitudinal axis of the fibre.

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