Optical fibre, fibre connector and optical detection system

Abstract

According to the present disclosure, there is provided an endoscopic fibre having a proximal end and a distal end, comprising a plurality of cores wherein one or more of the cores comprise excitation core(s) and one or more of the cores comprise collection core(s); wherein the excitation core or cores each comprises a solid core; wherein the collection core or cores each comprises a respective solid core with a refractive index that is different to a refractive index of the excitation core or cores; wherein the excitation core or cores are configured to transmit an optical excitation signal from the proximal end to a target region at or near the distal end; and wherein the collection core or cores are configured to transmit an optical detection signal from the target region at or near the distal end to the proximal end; and the difference in refractive index between excitation core(s) and the collection core(s) is such that cross-talk between mode(s) in the excitation core(s) and mode(s) in the collection core(s) is reduced or eliminated. There is also provided a fibre connector configured to connect to the fibre, a detection system comprising a laser source or other excitation light source connected to the excitation core(s) of the fibre, and a method of performing Raman spectroscopy comprising transmitting an excitation signal to a target region via the excitation core(s) of a fibre.

Description

[0001]OPTICAL FIBRE, FIBRE CONNECTOR AND OPTICAL DETECTION SYSTEM Field The present invention relates to an optical fibre apparatus, for example a multicore optical fibre apparatus for Raman sensing. Background Optical fibre apparatus comprising multiple cores and used for imaging are known. The cores of the fibre may be placed together to reduce the diameter of the optical fibre apparatus for endoscopy purposes. However, there may be a limit to how close to each other the cores may be placed in such known apparatus. When cores are too close together, light in one core may couple out of that core and into another core. Such coupling of light between cores may degrade a transmitted signal. Measuring Raman spectra through an optical fibre is usually complicated by the high intrinsic Raman scatter of the fibre material. When the material of the fibre is silica or doped silica, the unwanted Raman light generation is referred to as "silica background”. This background can be large enough to overwhelm any signal collected from a biological sample of interest at the distal end of the system. For example, many useful carbon-based Raman bands overlap spectrally with this background. For this reason, such systems use different fibres for excitation and collection in order to spatially separate the background from the collected signal. This can be problematic because the two fibres do not physically point at the same position and distal optics are required to correct for this, as well as to filter out the fibre-generated Raman signal. This increases the size of the probe. Common solutions such as the use of multiple fibres and distal optics are complex and bulky. One method of mitigating the coupling effect is to increase the index contrast between cores and the surrounding cladding, commonly measured using the numerical aperture (NA) of the fibre. Known fibres may obtain NA greater than 0.3 by using expensive glass doping and tempering techniques. It should also be noted that since different modes of propagation in the fibre experience a different set of interactions at the core / cladding interface, in more exact propagation models it is not possible to assign a single refractive index to the core or the cladding (and therefore to the NA) as different modes propagating in the fibre will experience different effective refractive indices. So although it is convenient to refer to a single refractive index for each material and a single NA for the fibre, the effective refractive index and effective NA experienced by an individual mode may be either higher or lower than the nominal value. The effective refractive indices of the modes are in between the refractive index of the core and cladding. They are never larger than the core’s refractive index. Yerolatsitis et al. (Ultra-low background Raman sensing using a negative-curvature fibre and no distal optics, Journal of Biophotonics (2019)) demonstrated that a single hollow-core fibre consisting of an excitation hollow-core and multiple Ge-doped collection cores can be used to minimise the excitation of this Raman silica background. In a hollow core there is only a weak interaction of the light with silica, minimising the generation of the silica background. Nevertheless, this type of fibre needs to be sealed to avoid any capillary action from liquid going through the hollow core of the fibre. Yerolatsitis et al. (Sub millimetre flexible fibre probe for background and fluorescence free Raman spectroscopy, Journal of Biophotonics (2021)) demonstrated an excitation hollow-core fibre spliced with a small piece of solid core fibres. To maintain the required collection efficiency, the solid core fibre needs to be of the order of few hundred micrometres. This complicates the manufacturing process, particularly for single-use probes, especially when the splicing procedure needs to be followed for both the distal and proximal end of the fibre. In addition, the underlining guiding mechanism between the multicore fibre and the hollow-core fibre is different. The light is guided through total internal reflection in the multicore fibre. Summary According to a first aspect, there is provided an endoscopic fibre having a proximal end and a distal end, comprising a plurality of cores wherein one or more of the cores comprise excitation core(s) and one or more of the cores comprise collection core(s); wherein the excitation core or cores each comprises a solid core; wherein the collection core or cores each comprises a respective solid core with a refractive index that is different to a refractive index of the excitation core or cores; wherein the excitation core or cores are configured to transmit an optical excitation signal from the proximal end to a target region at or near the distal end; and wherein the collection core or cores are configured to transmit an optical detection signal from the target region at or near the distal end to the proximal end; and the difference in refractive index between excitation core(s) and the collection core(s) is such that cross-talk between mode(s) in the excitation core(s) and mode(s) in the collection core(s) is reduced or eliminated. The reduction or elimination in cross-talk may be such that a magnitude of signal present in the collection core(s) caused by cross-talk from the excitation core(s) at operating wavelength(s) is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding signal in the excitation core(s). For example, the reduction or elimination in cross-talk may be such a magnitude of Raman signal present in the collection core(s) caused by cross-talk from the excitation core(s) of a Raman signal generated in the excitation core(s) by the excitation signal is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding generated Raman signal in the excitation core(s). The effective refractive index experienced by an electromagnetic mode propagating in the excitation core(s) at an operating wavelength, and the effective refractive index experienced by a corresponding electromagnetic mode propagating in the collection core(s) may be different. The difference in refractive index between the excitation core or cores and collection core or cores may be such as to provide substantially no cross-talk and / or mode coupling between the optical excitation signal and the optical collection signal in a wavelength range 200 nm to 1200 nm, optionally in the range 532 nm to 1064 nm. The refractive index of the excitation core or cores may be in the range 1.41 to 1.48. The refractive index of the collection core or cores may be in the range 1.41 to 1.48. The absolute difference in refractive index between the excitation core or cores and the collection core or cores may be in the range 0.0003 to 0.07. The excitation core or cores are each separated from the nearest collection core or cores by a distance of at least 10 μm and / or in a range 10 μm to 125 μm. The excitation core or cores may each have a diameter in the range 3 μm to 120 μm, and / or the collection core or cores may each have a diameter in the range 3 μm to 120 μm, and / or the excitation core or cores may be located substantially collinear and / or parallel to a longitudinal axis of the fibre, and / or the collection core or cores may be located substantially collinear and / or parallel to the axis of the fibre. The fibre may comprise a plurality of collection cores arranged concentrically around the one or more excitation cores. The fibre may comprise a collection core of annular cross-section disposed substantially concentrically around the axis of the fibre and / or with a central axis co-linear with a central longitudinal axis of the fibre. The collection core may have an inner diameter in the range 10 μm to 150 μm and / or greater than 10 μm. The collection core may have an outer diameter in the range 13 μm to 1000 μm and / or less than 1000 μm. The excitation core or cores may be configured to transmit light of wavelength in the range 200 nm to 1200 nm, optionally in the range 532 nm to 1064 nm. The collection core or cores may be configured to transmit light in the wavelength range 200 nm to 1200 nm, optionally in the range 532 nm to 1500 nm. The fibre may have an outer diameter in the range 125 to 1000 μm and / or less than 1000 μm. The excitation core or cores may comprise at least one of silica or doped silica. There may be a difference between the absolute refractive index of the excitation core or cores and the absolute refractive index of cladding of the excitation core or cores of between 0.001 and 0.02. The collection core or cores may comprise doped silica, for example germanium-doped silica. There may be a difference between the absolute refractive index of the collection core or cores and the absolute refractive index of cladding of the collection core or cores of between 0.003 and 0.03. The excitation core or cores and the collection core or cores may be disposed in and / or substantially encased in a longitudinal direction by a first cladding material. The first cladding material may comprise fluorine-doped silica. A second cladding material may surround each of the excitation core or cores and / or may be disposed between the excitation core or cores and the first cladding material. The second cladding material may comprise fluorine-doped silica. A third cladding material may surround each of the collection core or cores and / or may be disposed between the collection core or cores and the first cladding material. The third cladding material may comprise silica or doped silica, for example fluorine- doped silica. The distal end of the fibre may be tapered in the longitudinal direction such that diameter of the fibre reduces with distance along the longitudinal direction at the distal end. A transverse extent of the tapering may be in the range 10% to 40% of the maximum diameter of the fibre, optionally in the range 5% to 70% of the maximum diameter of the fibre. The longitudinal extent of the tapering may extend over a range of length of at least 5 mm from the distal end of the fibre. The fibre may be configured for the collection of detection signals, optionally Raman detection signals, from the target region at or near the distal end. The difference in refractive index between the excitation core(s) and the collection core(s) may be such that there is substantially no cross-talk to the collection core(s) of Raman signals generated in the excitation core(s) by the excitation signal . In a further aspect, there is provided a fibre connector configured to connect to a fibre as claimed or described herein, comprising at least one collection fibre arranged such that when a face of the fibre connector is engaged with the proximal end of the fibre, the collection core(s) are aligned with the collection fibre(s). The numerical aperture of the or each collection fibre may be substantially the same as the numerical aperture of a corresponding one or more of the collection core(s). The fibre connector may further comprise at least one excitation fibre arranged such that when a face of the excitation connector is engaged with the proximal end of the fibre, the excitation core(s) are aligned with the excitation fibre(s). The fibre connector may be arranged such that the at least one excitation core of the fibre extends through the fibre connector when the fibre connector is connected to the fibre. The collection fibre(s) and the excitation fibre(s) or excitation core(s) may be bundled or otherwise attached together at a distal end of the connector and are separated and / or moveable relative to each other at a proximal end of the connector, such that the collection fibre(s) and the excitation fibre(s) or excitation core(s) are connectable to different connectors and / or devices. In a further aspect, there is provided a detection system comprising a laser source or other excitation light source connected to the excitation core(s) of a fibre as claimed or described herein, and a detection device connected to the collection core(s) of the fibre. The system may be a Raman spectroscopy system and the detection device may comprise a Raman spectrometer. The laser source or other excitation light source may be connected to the excitation core(s) of the fibre via a fibre connector as claimed or described herein, and / or the detection device may be connected to the collection core(s) of the fibre via a fibre connector as claimed or described herein. In a further aspect there is provided a method of performing Raman spectroscopy comprising transmitting an excitation signal to a target region via the excitation core(s) of a fibre as claimed or described herein, receiving, via the collection core(s) of the fibre a detection signal that comprises signal(s) produced in response to the excitation signal, and determining a Raman spectrum from the detection signal. In a further aspect, there is provided a multicore optical fibre with lower index core(s) for excitation and higher index core(s) for collection, prohibiting coupling between the two regions, enabling background-free Raman signal collection from a distal sample. A single all-solid fibre may be used for excitation. The excitation core typically carries higher optical power than the collection core(s) and generates a higher silica background. It is hence beneficial to reduce the coupling of guided modes in the excitation fibre to the guided modes in the collection fibre that can otherwise obscure the received signal of interest due to their strength. For example, in an embodiment, the effective refractive index of the mode of the excitation core is lower than the silica refractive index whereas the effective refractive indices of the modes of the collection cores are higher than the silica refractive index, prohibiting coupling between these two regions. The multicore optical fibre may consist of a single all-solid low index excitation core comprising silica core and Fluorine-doped silica cladding and several high index collection cores comprising germanium-doped core and silica cladding. In some embodiments, the excitation core may comprise Fluorine-doped silica. The fibre may be used to excite and collect Raman light from a distal sample. The refractive index difference between the two regions (excitation and collection) prohibits any unwanted coupling between the excitation core and the collection cores, allowing the background- free (silica background) collection of the Raman signal from the distal sample. Cores may be made from silica glass, that are either doped or not doped. In some embodiments, the core rods are made from at least one of silica, Ge-doped silica, fluorine doped silica, boron doped silica, aluminium doped silica, silicate glass or silica doped with any suitable dopant. Any suitable combinations of materials may be used to obtain desired difference in refractive index between excitation core(s) and collection core(s). Any suitable doping level may be used. For example, in some embodiments with Ge as the dopant, the doping level may be for example between 1 to 20mol%, for instance around 4 mol%. in some embodiments with F as the dopant, the doping level may be for example between 0.05 to 2.5mol%. The cladding rod or rods may be hollow silica capillaries. In some embodiments, the cladding rods are made from at least one of silica, Ge-doped silica, fluorine doped silica, boron doped silica, aluminium doped silica, silicate glass or silica doped with any suitable dopant. Any suitable materials for cladding may be used to obtain desired optical characteristics. In some embodiments, the core and cladding may comprise silica doped with the same dopant in different concentrations resulting in a difference in refractive index between the core and cladding. The use of an all-solid excitation fibre may make splicing unnecessary. The multicore optical fibre can simplify the requirements of developing endoscopic fibre probes for in- vivo Raman sensing, enabling easier mass-production and quicker turnaround. A hollow- core fibre is fabrication intensive to manufacture whereas the solid multicore fibre is much simpler to manufacture. The multicore region serving as collection fibre may be replaced by a ring shaped core. The apparatus may be configured to transmit at least one of visible light, ultraviolet light, infrared light. An optical coupler may be coupled to the optical fibre apparatus and to a light source and / or light detector. According to further aspects, which may be provided independently, the effective refractive index experienced by light traveling through excitation core(s) is significantly different from, and decoupled from, the effective refractive index of light passing through collection core(s). The excitation core(s) may have an absolute refractive index that is higher, lower, or the same as undoped silica, so long as the effective refractive index experienced by a mode traveling in the excitation core(s) does not couple to a mode of the collection core(s). Undoped silica may be used as the material either for a core or for a cladding, but any other suitable materials may be used, subject to a suitable difference in effective the refractive index of the excitation and collection cores is achieved and, for example, such that each core is arranged to have an appropriate cladding that has a lower refractive index than that of the corresponding core. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. For example, apparatus features may be applied as method features and vice versa. Brief description of the drawings Embodiments of the invention are now described, by way of non-limiting example, and are illustrated in the following figures, in which: - Figure 1 is a cross-sectional microscope image of a multicore optical fibre of an embodiment, showing one excitation fibre and six collection fibres; Figure 2 is a schematic illustration of the contrast in refractive indices of excitation fibres and collection fibres according to an embodiment; Figures 3a and 3b are schematic illustrations of refractive indices of excitation fibres and collection fibres in related examples; Figure 4 is a graph showing an experimentally obtained spectrum of generated silica background when light is coupled to excitation cores of two different lengths; Figure 5 is a schematic illustration of a detection system according to an embodiment; Figure 6 is a schematic illustration of the effect of tapering the distal end of the multicore optical fibre; Figure 7 is a graph showing an experimentally obtained spectrum of generated silica background in ethanol for a tapered and non-tapered distal end of the multicore optical fibre; Figure 8 is a near-field image of the cross-section of the tapered distal tip of the multicore fibre excited by light at its proximal end; Figure 9 is a schematic illustration of a fibre drawing apparatus; Figure 10 is a schematic of the cross-section of the tip of the multicore optical fibre; Figure 11 is a schematic illustration of the contrast in refractive indices of excitation fibres and collection fibres according to a further embodiment; Figures 12a to 12c are schematic illustrations of further embodiments; and Figure 13 is a schematic illustration a tip of an embodiment of a multicore optical fibre. Detailed Description Figure 1 is a cross-sectional microscope image of a multicore optical fibre 10 according to an embodiment. The fibre 10 has one excitation core 14 and six collection cores 12. In this embodiment, the excitation core 14 is located on a substantially central longitudinal axis of the multicore optical fibre 10. The excitation core 14 has a significantly smaller radius than the radius of the collection cores 12 in the embodiment of Figure 1. In other embodiments, the excitation core or cores may be of smaller, larger or equal diameter in comparison to the collection core or cores 12. Depending on the radius and the NA, the excitation core may be single-mode, few-mode or multimode. Similarly, the collection cores may be single-mode, few-mode or multimode. The collection cores 12 may be concentrically disposed around the excitation core 14. The excitation fibre 14 and collection fibres 14 are bundled together to form the multicore optical fibre 10. The excitation fibre 14 shown in Figure 1 comprises a single all-solid low index excitation core comprising a silica core and fluorine-doped silica cladding and six high index collection cores 12 comprising germanium-doped silica core and silica cladding. The ratio of core size to cladding size (for example, a ratio of core diameter to cladding diameter) may be referred to as a core to cladding ratio. The cladding size may also be referred to as an outer size. Figure 2 is a schematic illustration of the contrast in refractive indices of excitation core and collection fibre cores of an embodiment. The vertical axis shows refractive index of core material while the horizontal axis extends along the diameter of the multicore optical fibre and represents its width. In the configuration shown in Figure 2, the refractive indices of one excitation core and two collection cores are shown relative to undoped silica. The shaded elements in the figure represent the collection core and two excitation cores, as well as the surrounding cladding materials, and the vertical extents of the shaded elements represent the absolute refractive index. It can be seen that the cladding material surrounding the collection cores is silica (it has absolute refractive index equal to that of pure silica) and has a lower absolute refractive index than the absolute refractive index of the collection cores. It can also be seen that the cladding material surrounding the excitation core has a lower absolute refractive index than the absolute refractive index of the excitation core. The dashed lines represent the effective refractive indices of relevant operating modes in the collection cores and the excitation core. It can be seen that the absolute refractive index of the material of the collection cores is different to the absolute refractive index of the material of the excitation core. It can also be seen that the effective refractive indices (dashed lines) of modes of interest in the collection cores are significantly different to the effective refractive index (dashed line) of the mode of interest in the excitation core. In Figure 2, the fibre 20a comprises a single all-solid low index excitation fibre core 24a comprising silica, and at least two collection fibre cores 22a, comprising germanium- doped silica. In other embodiments, the cores can comprise any suitable other materials. The refractive index of the excitation fibre cores 24a may be lower than the refractive index of the corresponding collection fibre cores 22a. In the embodiment shown in Figure 2 that comprises the fibre 20a, all modes in the excitation fibres are separated from the modes of the collection fibres. Figures 3a and 3b shows related examples, not forming part of the invention, which have similar structure to the embodiment of Figure 2 but in which fibre 20b and 20c while similar in structure to fibre 20a do not successfully separate all modes in the excitation fibres (24b and 24c, corresponding to 24a in Figure 2) from the modes of the collection fibres (22b and 22c respectively, corresponding to 22a in Figure 2) in this example. In other embodiments, the cores can comprise any suitable other materials. A further embodiment is shown in Figure 11. In other embodiments, the multicore optical fibre can have any suitable number of collection fibre cores, for example one collection fibre core 22, for example six collection fibre cores 22, for example twenty collection fibre cores 22, between 1 and 20 collection cores, or any other suitable number of collection fibre cores 22. One excitation core 24 has been shown in Figure 2. In other embodiments, the apparatus can have more than one excitation core 24. The excitation core 14 in Figure 1 has been shown disposed substantially in the centre of and surrounded by collection fibre cores 12 in a direction transverse to the axis of the multicore optical fibre 0. In other embodiments, the excitation core may be offset from the central axis of the multicore optical fibre 10, such as at the outer region of the multicore optical fibre 10. In other embodiments, the excitation core 14 may not be surrounded by collection fibre cores 12 or only partially surrounded by collection fibre cores 12. The dashed lines in Figure 2 indicate the effective refractive indices of different orders of the guided modes in the cores and higher order modes have been labelled. The height of the solid blue bars in Figure 2 indicates the absolute refractive indices of the materials that comprise the multicore optical fibre 20. Figure 2 shows an optical fibre 20a for which the excitation core 24a has a lower refractive index than the collection cores 22a which is the first embodiment of the invention. The difference in refractive index eliminates or reduces coupling between guided modes in the excitation and collection cores. The excitation mode, and the collection modes, including the higher order modes, have significantly different effective refractive indices, prohibiting any mode mixing and therefore background interference. This contrast of refractive index eliminates or reduces the silica background generated in the excitation core 24 due to the relatively higher optical power transmitted through it from coupling to the collection fibre cores 22 and travelling to the proximal end to obscure the received optical signal. In Figure 2, light can be seen emitting from each fibre core, represented as vertical arrows, without coupling with each other. The reduction or elimination in cross-talk may be such that a magnitude of signal present in the collection core(s) caused by cross-talk from the excitation core(s) at operating wavelength(s) is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding signal in the excitation core(s). For example, the reduction or elimination in cross-talk may be such a magnitude of Raman signal present in the collection core(s) caused by cross-talk from the excitation core(s) of a Raman signal generated in the excitation core(s) by the excitation signal is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding generated Raman signal in the excitation core(s). Figures 3(a) and 3(b) show further optical fibres 20b, 20c. In the related example in Fig. 3(a), the cores have the same refractive indices and the consequent coupling of guided modes between the excitation fibre core 24 and collection fibre cores 22 is represented as curved arrows between the cores. In the related example in Fig 3(b), the cores have modes with similar effective refractive indices (indicated by the dashed lines) which leads to the coupling of guided modes between the excitation fibre core 24 and collection fibre cores 22, again represented as curved arrows between the cores. In each of Figures 3(a) and 3(b), the silica background generated in the excitation fibre core 24 may at least partially obscure the signal received at the proximal end of the collection fibre cores 22b or 22c. Figure 4 is a graph showing an experimentally obtained spectrum of generated silica background when light is coupled to excitation cores of two different lengths of a fibre according to the embodiment of Figure 1, in this case lengths of 2m and 1.8m. It can be seen that the silica background for the longer length is stronger than that for the shorter length due to the longer distance that light interacts with the material of the core. It is a feature of embodiments that by providing a difference in effective refractive index between the excitation core or cores and collection core or cores substantially no cross- talk and / or mode coupling at an operating frequency, or at least a reduction cross-talk and / or mode coupling at an operating frequency, can be provided between an optical excitation signal transmitted via the excitation core(s) and an optical collection signal passing through the collection core(s) for signals having a wavelength or wavelengths in an operating range, for example in a wavelength range 200 nm to 1200nm. In some embodiments the excitation signals have a wavelength selected from at least one of 248, 266, 355, 405, 457, 473, 491, 515, 532, 561, 640, 660, 785, 830, 980, or 1064 nm. The reduction or elimination in cross-talk may be such that a magnitude of signal present in the collection core(s) caused by cross-talk from the excitation core(s) at operating wavelength(s) is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding signal in the excitation core(s). For example, the reduction or elimination in cross-talk may be such a magnitude of Raman signal present in the collection core(s) caused by cross-talk from the excitation core(s) of a Raman signal generated in the excitation core(s) by the excitation signal is less than 10%, for example less than 1%, for example, less than 0.1%, optionally less than 0.01% of the corresponding generated Raman signal in the excitation core(s). In some embodiments the collection signals have wavelengths in a range above or below the excitation wavelength, Raman shifted by between 50cm-1 and 4500cm-1 or -50cm- 1 and -4500cm-1 , e.g. in the case of excitation at 532 nm collection in the ranges 535 to 700nm or 430 to 530 nm. In other embodiments the collection signals have wavelengths in the range 245 nm to 275nm, alternatively in the range 260 nm to 300 nm, alternatively in the range, 480 nm to 600 nm, alternately in the range 550 nm to 730 nm, alternately in the range 670 nm to 950 nm, alternately in the range 700 nm to 1020 nm and alternately in the range 860 nm to 1390 nm or any other suitable range of wavelength. The reduction or elimination of cross-talk and / or mode coupling can be particularly important in the context of endoscopic measurements, for example where both excitation and collection signals may need to travel over significant distances, for example from / to a source / detector outside a patient or other subject to / from a target region within the patient or other subject, for example in the lung or stomach or other anatomical region. For example, the length of the fibre may be greater than 30 cm, optionally greater than 50 cm or 1m, optionally in a range 30 cm to 3 m, or 50 cm to 4 m, or 50 cm to 2 m. As shown in Figure 4, a significant silica background signal from silica cores may be generated over such distances and, given frequencies often used for Raman measurements, that may have a significant effect on collected signals and thus on Raman spectra or other measurements. By providing excitation and collection cores with suitable refractive index differences such effects can be reduced Figure 5 is a schematic illustration of detection system 40, in this embodiment a Raman spectroscopy system, that includes a multicore optical fibre 10 of Figure 1. The multicore optical fibre 10 is in the form of an endoscopic fibre probe for insertion into a patient or other subject. The system of Figure 5 includes a fibre connector 42 that is configured to connect at a distal end to a proximal end of the multicore optical fibre 10, and to connect at a proximal end to a detection device. In the system of Figure 5, the fibre connector 42 is in the form of an all-fibre reformatter. In the system of Figure 5, the detection device comprises a spectrometer 44. The fibre connector 42 comprises a single-mode (low-index) excitation fibre 52 bundled together with six multi-mode (high-index) collection fibres 50. An image (a) of a distal end-face 46 of the fibre connector 42, and an image (b) of a proximal end-face 48 of the fibre connector 42, are shown in Figure 5. The design of the fibre connector 42 matches the design of the multicore fibre 10. In the system of Figure 5, at the distal end of the fibre connector 42 the excitation low-index core 14 is at the centre of the multi-fibre structure of the connector 42 surrounded by the six collection fibres 50 with the same numerical aperture (NA) as the collection cores 12 in the multicore fibre 10. A cross-sectional image (c) of a proximal end-face 54 of the multi-core fibre 10 is also shown in Figure 5. The fibre connector 42 is configured such that when the distal end- face 46 of the fibre connector 42 is engaged with the proximal end face 54 of the multicore fibre 10, the collection cores 12 of the multicore fibre 10 are aligned with the collection fibres of the fibre connector 42, thus enabling detection signals received via the collection cores 12 to pass into the collection fibres 50. It is a feature of the system of Figure 5 that the excitation core 14 does not terminate at the proximal end face 54 of the multicore fibre 10. Instead, the excitation core 14 extends through the distal end face 46 of the fibre connector 42 and exits the main part of the fibre connector 42 at a splitting point 56. The excitation core then extends from the splitting point 56 to an excitation light source in the form of laser source 58 that is included in the system 40. The laser source 58 is operable to transmit excitation signals in the form of laser light of an operating frequency, or range of operating frequencies, via the excitation core to the distal end of the multicore fibre 10, thereby to excite a target region as desired. In the system of Figure 5, additional cladding and / or outer layer(s) are provided around the excitation core 14 between the splitting point 14 and a proximal end of the excitation core 14 to protect the excitation core. In variants of the system, the excitation core 14 terminates at the proximal end face 54 of the multicore fibre 10 and is aligned with a further excitation fibre or core that is included in the connector 42 in such variants. The further excitation fibre or core is then connectable to the laser source 58 or other excitation light source. In some embodiments, this laser source may be single-frequency source operating at a wavelength of 532 nm, or 633 nm or 785 nm or 830 nm or 1064 nm. In some embodiments, the laser source 58 may be a 785 nm VHG-stabilised laser diode (part number : FPV785S) sold by Thor Labs. In other embodiments, the laser source 58 may be a 1060-1083 nm DBR laser diode (part number: DBR1064S) sold by Thor Labs. In other embodiments, an interface between the excitation core(s) and corresponding further excitation fibre(s) or core(s) occurs at any desired point between the proximal end face 54 of the multicore fibre 10 and the connection to the excitation light source. A cross-sectional image (b) of a proximal end-face 46 of the fibre connector 42 is also shown in Figure 5, which shows an additional dummy fibre 52 between the collection fibres 50, and in a corresponding position to that of the excitation core 14 at the distal end of the fibre connector 42. The dummy fibre 52 terminates before reaching the distal end fibre connector 42 in some variants. In other embodiments, the proximal end-face 46 of the fibre connector 42 might be configured differently than in Figure 5, such as collection fibres 50 disposed in circular symmetry around the dummy fibre 52 or disposed of linearly on the proximal end-face 46 or any other suitable configuration. The dummy fibre is used to achieve a close pack arrangement when bundling the collection cores together for connecting them to a detection device. The proximal end of the fibre connector 42 is connected to a detection device, in the form of spectrometer 44 in the system of Figure 5. In some embodiments, the spectrometer 44 may be a WP 785 ER Raman Spectrometer (part number: WP-785-ER-IC) sold by Wasatch Photonics. In other embodiments, the spectrometer 44 may be a QE Pro Raman Series Spectrometer (part number: QEPRO-RAMAN-638 Spectrometer) sold by Ocean Insight. The reformatter in the form of fibre connector 42 is arranged such that light from the source 58 can be coupled to the excitation core 14 and a resulting Raman signal from a distal sample or other target region can be received through the collection cores 12 of the fibre probe 10. The splitting of the excitation core from the collection fibres at the splitting point means that the excitation light and collection light can be separated and provided from / to different components, for example the source 58 and spectrometer 44 in the system of Figure 5. The fibre connector 42 can be tapered at its distal end if required by the design of the multicore fibre, to ensure that the collector cores 14 align with the collection fibres 50. The multicore fibre 10 in the system of Figure 5 may also be tapered at its distal end. A cross-sectional image (d) of the distal end-face 62 of the multi-core fibre 10 is also shown in Figure 5. In other variants or embodiments, the distal tip of the multicore fibre 10 is untapered, or may have any other suitable shape. Figure 6 is a schematic illustration of the effect of tapering the distal end of the multicore optical fibre. The field of view (FoV) of an optical fibre core may be understood to be the three-dimensional boundary, which may for example be cone-shaped for a circular core end-face, delimiting the spatial extent of the light emitted by the core or, similarly, for instance the three-dimensional boundary delimiting the region from which it can receive a detection signal through coupling of an optical signal into a guided mode. Dashed lines in Figure 6 delimit the FoVs illustrated within and also represent the acceptance / exiting angles of light from the core. The exiting and acceptance angle depends on the Numerical Aperture of the core. Reducing the diameter of the core does not change the Numerical Aperture of the core. A region of overlap of the FoVs of an excitation fibre and collection fibre(s) may represent a target region that can be effectively interrogated by the multicore optical fibre. Such a region of overlap 70 is illustrated in Figure 6 for multicore fibre 10 as the shaded region where the FoVs of the excitation fibre core 24 and collection fibre core(s) 22 overlap in space. Similarly, a region of overlap 72 is also shown for a multicore optical fibre 74 that has identical characteristics to those of the multicore optical fibre 10, except that it has a tapered region 76 near the distal end of fibre 74. In the tapered region the excitation core 12 and the collection cores 14 gradually become closer to each other. The cores 12, 14 are not shown in the tapered region 76 in Figure 6 for clarity. The tapered region provides a reduction in the diameter of the distal end of the multicore optical fibre compared to the diameter at the proximal end. As illustrated schematically in from Figure 6, the tapering results in the region of overlap 72 moving closer to the distal end of the tapered multicore optical fibre 74, in comparison to the region of overlap for the non- tapered multicore optical fibre 10. Thus, using such a tapered fibre 73, a system such as that of Figure 5 is capable of collecting a Raman or other signal from closer to the distal end than is possible with a corresponding untapered multicore optical fibre 10. This can enable increased collection efficiency, especially when trying to interrogate samples through scattering or relatively opaque media such as can be the case with biological samples. Light at wavelengths of interest in biological samples may often only travel a few mm or so. Therefore, being able to effectively collect such light can significantly increase the collection efficiency of the system. Figure 7 is a graph showing an experimentally obtained spectrum of generated silica background in ethanol for a tapered and non-tapered distal end of the multicore optical fibre 10. As shown in the zoomed-in portion of the graph, the tapered multicore optical fibre 74 outperforms the optical multicore fibre 10 at the spectral peak for ethanol at 845nm. An improvement of between a factor of two and three was achieved in comparison with the non-tapered distal end. In this embodiment, the distal tip of the fibre was tapered to 40% of its original diameter. In other embodiments the tapering can be larger or smaller than 40%. In further embodiments, additional post-processing of the collected signal, lensing of end-faces / apertures of the fibre cores and / or angle-polishing the end-faces / apertures of the fibre cores can be provided, which can lead to further performance improvement for at least some measurements. Figure 8 is a near-field image of the cross-section of the distal tip of the tapered multicore fibre 74 with the excitation fibre core 14 excited by light at its proximal end. The configuration of the tapered multicore optical fibre 74 is similar to that shown in Figure 1 with one low-index excitation fibre 14 and six concentrically arranged collection fibres 12. Upon tapering down the structure, the light spreads out from the excitation fibre core 14 to the fluorine-doped cladding, therefore it can enable a better illumination of the target area. In addition, the separation between the collection fibre cores 22 and the excitation fibre cores 24 is reduced locally. By reducing this separation, an enhanced collection can be achieved, as discussed in relation to Figure 6. As shown also in Figure 8, although the light is spread to the fluorine-doped cladding region, the surrounding structure is still significantly darker confirming that the difference in the refractive index prohibits any unwanted coupling to the surrounding collection fibre cores 12. Any suitable method may be used to form the multicore fibre 10, for example using known fibre-drawing or other techniques. By way of example, Figure 9 is a schematic illustration of a fibre drawing apparatus 80 comprising heating elements 82 and a fibre pulling mechanism 84. The fibre drawing apparatus 80 can be used to form a multicore fibre using a stack and draw technique. Other components of the fibre drawing apparatus 80 have been omitted for clarity. Figure 9 is not illustrated to scale. In the present embodiment, the fibre drawing apparatus 80 is an item of telecommunications equipment and is configured to control diameter of a drawn fibre to within microns. In other embodiments, any suitable fibre drawing apparatus may be used. The fibre drawing apparatus 80 is configured to draw a fibre preform 86 comprising a core portion 88, comprising a plurality of suitably arranged core rods, and a cladding portion 34 comprising a cladding rod. To draw the fibre preform 86, the fibre preform is pulled by the pulling mechanism in a direction indicated by arrow 40 (which in Figure 1 is downwards). The fibre preform 86 is heated by heating elements 82 to a temperature of 2000 degrees centigrade so that it becomes soft and may be drawn. In other embodiments, the fibre preform 86 may be heated to any other suitable temperature. The fibre preform 86 is pulled by the pulling mechanism 22 so that it increases in length and decreases in cross- section. The output of the fibre pulling apparatus 10 is a core rod 102 having substantially the same ratio of core size to cladding size as the original fibre preform 86, but a much smaller cross-section. In some embodiments, the final diameter of the core rod 102 may be of the order of a few millimetres. The pulling mechanism comprises a pulling belt (not shown). Tapering can also be provided, for example by selecting heating and / or further drawing of selection regions of the fibre, or using any other known techniques. Figure 10 is a schematic of the cross-section of the distal or proximal tip of the multicore optical fibre 10. The configuration of the multicore optical fibre 10 is similar to that shown in Figure 1 with one low-index excitation fibre core 14 and six concentrically arranged collection fibres cores 12. The excitation fibre core 14 is disposed in and / or substantially encased in a longitudinal direction by an excitation core cladding 64, which is referred to as the second cladding material. Each of the collection fibre cores 12 are disposed in and / or substantially encased in a longitudinal direction by a collection core cladding 66, which is referred to as the third cladding material. Each of the collection and excitation cores and claddings are disposed in and / or substantially encased in a longitudinal direction by a main cladding 68, which is referred to as the first cladding material. In some embodiments, the main cladding 68 or the first cladding material comprises Fluorine-doped silica. Any suitable materials may be used for the first cladding material to obtain desired optical characteristics. Figure 11 is a schematic illustration of the contrast in refractive indices of excitation fibre core and collection fibre cores for a further embodiment of the invention. The vertical axis shows refractive index while the horizontal axis extends along the diameter of the multicore optical fibre 20d and represents its width. The refractive indices of one excitation core 24d and two collection cores 22d are shown relative to undoped silica, and for clarity, the relative cladding indices are omitted. In this embodiment, the excitation core 24d has a higher refractive index than the collection cores 22d. This contrast of refractive index eliminates or reduces the silica background generated in the excitation core 24d due to the relatively higher optical power transmitted through it from coupling to the collection fibre cores 22d and travelling to the proximal end to obscure the received optical signal. In Figure 11, light can be seen emitting from each fibre core, represented as vertical arrows, without coupling with each other. Effective refractive indices of operating modes (horizontal dashed lines) are significantly different in the excitation core and the collection cores leading to little or no cross-talk Figure 12a to 12c illustrate schematically further embodiments. In each embodiment, effective refractive indices of operating modes (horizontal dashed lines) are significantly different in the excitation core and the collection cores leading to little or no cross-talk. Figure 12b shows an excitation core made with only using f-doped silica (slightly doped core and heavily doped cladding) whereas Figure 12c shows a fibre made using graded- index fibres. We can define the graded index using the standard formula for graded index fibres: where a is the core radius, and g is a parameter that defines the shape of the profile. In various embodiments g may be between 2 and 3. n1is the refractive index of the core and n2is the refractive index of the cladding An operating mode for the excitation core may be a mode corresponding to an operating wavelength of excitation signal input to the fibre to generate a Raman response from a target. The operating mode for the collection cores may be a mode corresponding to a wavelength or wavelengths of a Raman signal generated by the target in response to the excitation signal. An absolute difference between the refractive indices of the collection and excitation cores in turn results in an refractive index difference between the effective refractive indices of the guided modes. For example, if the excitation core has a core refractive index of 1.45 and cladding refractive of 1.44 and the collection cores have a core refractive index of 1.46 and cladding refractive index of 1.45 (e.g. similar to embodiment of Figure 12a) then due to this absolute difference, the modes of the cores are completely decoupled. In addition, if this was the other way around and the excitation core has higher refractive index than the collection cores (e,g, as in Figure 11) it would still work as long as there is a refractive index difference between the cores. Figure 13 is a schematic of the cross-section of the distal or proximal tip of an embodiment of the multicore optical fibre 10. The configuration of the multicore optical fibre 10 includes one low-index excitation fibre core 120 composed of undoped silica and one annular collection core 124 composed of germanium-doped silica. The annular collection core 124 has an annular cross-section. In various embodiments, the inner radius of the annular collection core may be between 10 μm and 125 μm, and the outer radius of the annular collection core may be between 20 μm and 600 μm. The excitation fibre core 120 is disposed in and / or substantially encased in a longitudinal direction by an excitation core cladding 122 with an annular cross-section, which is referred to as the second cladding material and is formed of Fluorine-doped silica. The annular collection core 124 is disposed in and / or substantially encased in a longitudinal direction by a collection core cladding 126, which is referred to as the third cladding material and is formed of undoped silica. Any suitable materials may be used for the second and third cladding and for the excitation and collection cores to obtain desired optical characteristics. In the above, the terms core rod and cladding rods are used. A core rod may refer to a rod that comprises a central portion comprising at least one material, the central portion being characterized by a first refractive index profile, and a cladding comprising a second material characterised by a second refractive index profile, wherein the cladding has a lower effective refractive index than the effective refractive index of the first portion, thereby to confine light to the central portion. The cladding rods may comprise hollow silica tubes. In some embodiments, a cladding rod may refer to a rod that comprising a core material characterised by a first effective refractive index profile and a cladding comprising a cladding material characterised by a second effective refractive index, wherein the second effective refractive index is higher than the first effective refractive index. In further embodiments, any desired number of stacking and drawing operations may be used. For example, the rods may be stacked or otherwise arranged and / or drawn down twice, three times, four times, or five times. Cores may be made from silica glass, that are either doped or not doped. In some embodiments, the core rods are made from at least one of silica, Ge-doped silica, fluorine doped silica, boron doped silica, aluminium doped silica, silicate glass. Any suitable combinations of materials may be used to obtain desired difference in refractive index between excitation core(s) and collection core(s). The cladding rod or rods may be hollow silica capillaries. In some embodiments, the cladding rods are made from at least one of silica, Ge-doped silica, fluorine doped silica, boron doped silica, aluminium doped silica, silicate glass. Any suitable materials for cladding may be used to obtain desired optical characteristics. Embodiments can provide a multicore optical fibre that has a plurality of core regions within a cladding region. The cladding region of the fibre may be formed from hollow air- filled tubes or capillaries. The formed multicore optical fibre has a longitudinal length with a distal end for receiving light and proximal end for transmitting light. In use, light can be introduced to the proximal end of the fibre and guided to the distal end of the fibre. Light is transmitted through the fibre via the core regions. Light travelling through the fibre is substantially confined to travel through the core regions by the cladding regions. The core regions may also be referred to as light confining regions. In some embodiments, the multicore fibre may be packaged with one or more sensing fibres and / or a capillary tube to form a multi-functional fibre apparatus. The imaging fibre, and / or sensing fibres and / or capillary tube may be placed within a further glass or polymer tube, which may be referred to as a package. The further glass or polymer tube may be shorter than the imaging fibre, and / or sensing fibres and / or capillary tube. The package contains the imaging fibre, sensing fibres and capillary tube, which are fixed in place using epoxy. The package may form an endoscope. Any suitable further outer protective layer may be provided if desired. Each feature disclosed in the description and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

CLAIMS 1. An endoscopic fibre having a proximal end and a distal end, comprising a plurality of cores wherein one or more of the cores comprise excitation core(s) and one or more of the cores comprise collection core(s); wherein the excitation core or cores each comprises a solid core; wherein the collection core or cores each comprises a respective solid core with a refractive index that is different to a refractive index of the excitation core or cores; wherein the excitation core or cores are configured to transmit an optical excitation signal from the proximal end to a target region at or near the distal end; and wherein the collection core or cores are configured to transmit an optical detection signal from the target region at or near the distal end to the proximal end; and the difference in refractive index between excitation core(s) and the collection core(s) is such that there cross-talk between mode(s) in the excitation core(s) and mode(s) in the collection core(s) is reduced or eliminated.

2. A fibre according to claim 1, wherein the effective refractive index experienced by an electromagnetic mode propagating in the excitation core(s) at an operating wavelength, and the effective refractive index experienced by a corresponding electromagnetic mode propagating in the collection core(s) are different.

3. A fibre according to claim 1 or 2, wherein the difference in refractive index between the excitation core or cores and collection core or cores is such as to provide substantially no cross-talk and / or mode coupling between the optical excitation signal and the optical collection signal in a wavelength range 200 nm to 1200 nm, optionally in the range 532 nm to 1064 nm.

4. A fibre according to any preceding claim, wherein the refractive index of the excitation core or cores is in the range 1.41 to 1.48; wherein the refractive index of the collection core or cores is in the range 1.41 to 1.48; wherein the absolute difference in refractive index between the excitation core or cores and the collection core or cores is in the range 0.0003 to 0.07.

5. A fibre according to any preceding claim, wherein the excitation core or cores are each separated from the nearest collection core or cores by a distance of at least 10 μm and / or in a range 10 μm to 125 μm.

6. A fibre according to any preceding claim, wherein the excitation core or cores each have a diameter in the range 3 μm to 120 μm; and / or wherein the collection core or cores each have a diameter in the range 3 μm to 120 μm; wherein the excitation core or cores are located substantially collinear and / or parallel to a longitudinal axis of the fibre; and wherein the collection core or cores are located substantially collinear and / or parallel to the axis of the fibre.

7. A fibre according to claim 6 wherein the fibre comprises a plurality of collection cores arranged concentrically around the one or more excitation cores.

8. A fibre according to any of claims 1 to 6, wherein the fibre comprises a collection core of annular cross-section disposed substantially concentrically around the axis of the fibre and / or with a central axis co-linear with a central longitudinal axis of the fibre.

9. A fibre according to claim 8 wherein the collection core has an inner diameter in the range 10 μm to 150 μm and / or greater than 10 μm, and an outer diameter in the range 13 μm to 1000 μm and / or less than 1000 μm.

10. A fibre according to any preceding claim wherein the excitation core or cores are configured to transmit light of wavelength in the range 200 nm to 1200 nm, optionally in the range 532 nm to 1064 nm; wherein the collection core or cores are configured to transmit light in the wavelength range 200 nm to 1200 nm, optionally in the range 532 nm to 1500 nm.

11. A fibre according to any preceding claim wherein the fibre has an outer diameter in the range 125 to 1000 μm and / or less than 1000 μm.

12. A fibre according to any preceding claim wherein the excitation core or cores comprise at least one of silica or doped silica, and there is a difference between theabsolute refractive index of the excitation core or cores and the absolute refractive index of cladding of the excitation core or cores of between 0.001 and 0.

02.

13. A fibre according to any preceding claim wherein the collection core or cores comprise doped silica, for example germanium-doped silica, and there is a difference between the absolute refractive index of the collection core or cores and the absolute refractive index of cladding of the collection core or cores of between 0.003 and 0.

03.

14. A fibre according to any preceding claim wherein the excitation core or cores and the collection core or cores are disposed in and / or substantially encased in a longitudinal direction by a first cladding material.

15. A fibre according to claim 14 wherein the first cladding material comprises fluorine-doped silica.

16. A fibre according to claim 14 or 15, wherein a second cladding material surrounds each of the excitation core or cores and is disposed between the excitation core or cores and the first cladding material.

17. A fibre according to claim 16 wherein the second cladding material comprises fluorine-doped silica.

18. A fibre according to any of claims 14 to 17 wherein a third cladding material surrounds each of the collection core or cores is disposed between the collection core or cores and the first cladding material.

19. A fibre according to claim 18 wherein the third cladding material comprises silica or doped silica, for example fluorine-doped silica.

20. A fibre according to any preceding claim wherein the distal end of the fibre is tapered in the longitudinal direction such that diameter of the fibre reduces with distance along the longitudinal direction at the distal end.

21. A fibre according to claim 20, wherein the transverse extent of the tapering is in the range 10% to 40% of the maximum diameter of the fibre, optionally in the range 5% to 70% of the maximum diameter of the fibre;wherein the longitudinal extent of the tapering extends over a range of length of at least 5 mm from the distal end of the fibre.

22. A fibre according to any preceding claim wherein the fibre is configured for the collection of detection signals, optionally Raman detection signals, from the target region at or near the distal end, and / or wherein the difference in refractive index between the excitation core(s) and the collection core(s) is such that there is substantially no cross- talk to the collection core(s) of Raman signals generated in the excitation core(s) by the excitation signal .

23. A fibre connector configured to connect to the fibre of any of claims 1 to 22, comprising at least one collection fibre arranged such that when a face of the fibre connector is engaged with the proximal end of the fibre, the collection core(s) are aligned with the collection fibre(s).

24. A fibre connector according to claim 23, wherein the numerical aperture of the or each collection fibre is substantially the same as the numerical aperture of a corresponding one or more of the collection core(s).

25. A fibre connector according to claim 23 or 24, further comprising at least one excitation fibre arranged such that when a face of the excitation connector is engaged with the proximal end of the fibre of any of claims 1 to 21, the excitation core(s) are aligned with the excitation fibre(s).

26. A fibre connector according to any of claims 23 to 25, arranged such that the at least one excitation core of the fibre according to any of claims 1 to 22 extends through the fibre connector when the fibre connector is connected to the fibre.

27. A fibre connector according to any of claims 23 to 26, wherein the collection fibre(s) and the excitation fibre(s) or excitation core(s) are bundled or otherwise attached together at a distal end of the connector and are separated and / or moveable relative to each other at a proximal end of the connector, such that the collection fibre(s) and the excitation fibre(s) or excitation core(s) are connectable to different connectors and / or devices.

28. A detection system comprising a laser source or other excitation light source connected to the excitation core(s) of the fibre of any of claims 1 to 22, and a detection device connected to the collection core(s) of the fibre of any of claims 1 to 22.

29. A detection system according to claim 28, wherein the system is a Raman spectroscopy system and the detection device comprises a Raman spectrometer.

30. A detection system according to claim 28 or 29, wherein the laser source or other excitation light source is connected to the excitation core(s) of the fibre via the fibre connector according to any of claims 23 to 27, and / or wherein the detection device is connected to the collection core(s) of the fibre via the fibre connector according to any of claims 23 to 27.

31. A method of performing Raman spectroscopy comprising transmitting an excitation signal to a target region via the excitation core(s) of a fibre according to any of claims 1 to 22, receiving, via the collection core(s) of the fibre according to any of claims 1 to 22, a detection signal that comprises signal(s) produced in response to the excitation signal, and determining a Raman spectrum from the detection signal.

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