Patent Description:
Multimode optical fibres (MMF) have a core diameter that is much larger that the wavelength of light that it the fibre is configured to carry, and can thereby support more than one mode of light propagation. MMF have a wide range of biomedical and industrial applications. Their small total diameter (typically <NUM>-<NUM> microns) provide access to hard-to-access locations and permits their insertion in biological tissue with limited disruption of physiological structures and functions. For example, minimally invasive micro-endoscopes based on MMF have been developed for in vivo brain imaging in the context of neuroscience studies. Such systems might be adapted for surgical guidance in other clinical applications (e.g. ear nose and throat applications).

A range of MMF applications, including the micro-endoscopes mentioned above, require spatially resolving optical signals for imaging or sensing. Contrast mechanisms for the optical signals in these applications include scattering and fluorescence, both in linear and nonlinear regimes, but could also be based on polarisation. Imaging with MMF can be implemented with widefield and point-scanning illumination and has been demonstrated with many modalities, for example: one-photon and multiphoton point-scanning fluorescence, confocal reflectance, coherent anti-Stokes Raman scattering, light-sheet, and SOFI (Super Resolution Optical Fluctuation Imaging. MMF are also used for microfabrication.

The location of the light source for illumination defines the proximal location and the position of the proximal end of the fibre. The proximal location refers to the space where the light source is located, and the proximal end comprises the MMF facet closest to the light source into which the illumination light is coupled. The distal location refers to the space at the other end of the fibre where the optical signal for imaging or sensing is generated, and the distal facet is the MMF facet at the distal end of the optical fibre that collects the optical signal. Between the proximal and distal end is the MMF, which serves as a waveguide to carry the illumination from the proximal end to the distal end and the optical signal from the distal end to the proximal end, which is optically coupled to a detector. The distal space (adjacent to the distal end) may be required to be free of optical elements (e.g. in a case where minimal invasiveness is required).

Optical effects occurring inside the MMF upon propagation cause the optical field to be transformed in an unpredictable, yet deterministic manner. This complex transformation T must be known for homomorphic mapping of information between the distal and proximal space with high spatial resolution as well as for spatially defined sensing in the distal space. Using a vectorial notation, T, known as the transmission matrix (TM), expresses the relationship between an input field at the proximal facet x and the resulting output field at the distal facet y' and, reciprocally through time-reversal, an input field y at a distal facet and the resulting output field at the proximal facet x': <MAT> <MAT> where the superscript T indicates the transpose operation and the wavelength dependence of T has been made explicit. A definition for T may be valid for a narrow spectral bandwidth. This reciprocity of the transmission matrix is illustrated schematically in <FIG>.

The TM of an MMF is not only dependent on the illumination wavelength but also on the temperature and mechanical state of the fibre. This dependence on the fibre state S can be made explicit: <MAT> <MAT>.

In several dynamic systems, the MMF may be expected to be continually deformed, in ways that cannot necessarily be predicted. In such cases, the problem of determining the TM for each state is not tractable.

In most practical scenarios, a TM can be evaluated with the fibre in an initial state S<NUM> before its positioning (e.g. insertion in biological tissue). This approach can substantially simplify the evaluation of the TM upon continuous state changes (e.g. continuous deformation) using only proximal optical components, as the correction required to update the TM should also be continuous and small if evaluated in sufficiently close subsequent states. A correction matrix Cλ,Si-<NUM>→Si may be introduced to express the relationship between the TM of the current state Si to the previous state Si-<NUM>: <MAT> where <MAT>.

Several methods have been proposed for evaluating the TM. Methods that require access to the distal end include:.

<NUM> Čižmár, Tomáš, and Kishan Dholakia. "Shaping the light transmission through a multimode optical fibre: complex transformation analysis and applications in biophotonics. " Optics Express <NUM> (<NUM>): <NUM>-<NUM>. <NUM> <NPL>. <NUM> <NPL>.

Methods have also been proposed that do not require a camera or light source at the distal end of the MMF, including:.

<NUM> Plöschner, M. & Ćižmár, T. Seeing through chaos in multimode fibres. Nature Photonics <NUM>, <NUM>-<NUM> (<NUM>). <NUM> <NPL>). <NUM> <NPL>). <NUM> <NPL>. <NUM> <NPL>). <NUM> <NPL>).

With the exception of Plöschner, all the other strategies require optical modulation beyond the distal facet. Plöschner requires that the TM, deformation and physical state of the MMF should be known, and allows a correction Cλ,Si-<NUM>→Si to be determined. In Ploschner, a theoretical approach is used to determine a TM for a bent fibre, based on prior knowledge of the shape of the bent fibre. Plöschner therefore is of limited practicality in contexts where the deformation of the fibre is not known or predictable. Furthermore, while this approach works well for a short fibre (<<NUM>) (which is fine for rodent applications) it is limited in how well it can deal with longer fibres with more complex conformation (including compound curvature). A <NUM> fibre length is not sufficient for many clinical and industrial applications. The calculations are also complex.

In Farahi a reference light is delivered to the distal end of the MMF using a single mode fibre that runs in parallel with the multimode fibre (either concentrically, in a double clad fibre, or via a coupling prism in the case of a laterally offset single mode fibre). The reference light is diffracted by a holographic pattern recorded in a polymer layer at the distal end of the multimode fibre and reflected back into the multimode fibre. The diffracted reference light reflected back into the multimode fibre approximates an object point source, or virtual beacon. The proximal intensity distribution of this virtual beacon (after transmission through the multimode fibre) is measured, and correlated to a data bank of patterns corresponding to different fibre conformations. The data bank also includes the TM associated with each fibre conformation. This enables the TM for the fibre conformation to be determined from the detected virtual beacon at the proximal end of the fibre.

In Gu a thin reflective surface with a checkboard pattern is placed at the distal end of the MMF. Each square of the checkerboard pattern has a different reflection coefficient, with the result that spatial modulation of the amplitude of the reflectance signal is achieved. Light reflected from the checkerboard pattern is imaged at the proximal end of the fibre by a camera, and the resulting pattern used to determine an updated TM from an initial TM (that may be predetermined with access to both ends of the fibre, using and if the methods i) ii) or iii) mentioned above).

In Chen many single mode fibres are placed at the distal end, and the approach is somewhat equivalent to Gu. This is not practical in biomedical applications and other industrial applications, where it is not possible to place a lot of optical equipment at the distal end of the MMF.

In Gordon an extension of the method proposed by Gu is described, in which thin reflectors are stacked on the distal end of the MMF, and further encoding is done in each layer based on polarisation and wavelength. This is a more general approach, as it extends to non-unitary MMF. The approach is essentially the same as Gu, but instead of requiring N references, corresponding to the number of elements of the TM diagonal, NxN references are needed, which enables determination of the full TM.

Farahi, Gu, and Gordon all require assembly of micro-optics - which involves multiple complex components at the distal end (e.g. coupled to the distal facet of the MMF). This is complex and not-straightforward.

Li discloses the use of a guide star, which could be a fluorescent bead attached to the tip of the fibre. The TM may be found by an optimisation algorithm that determines an approximate transmission matrix by optimising constructive interference onto the guide star. The TM determined using this method is valid over an isoplanatic patch, the size of which is determined by the number of propagation invariant modes that have power at the position of the guide star. In practice, a guide star at the interface between the core and cladding was found to be optimal.

Each of the existing methods for compensating for the transfer function of a MMF is complex in practical implementation. Methods and devices that make it easier to determine a MMF TM are desirable, particularly if they facilitate determining corrections for movement/deformation of the MMF.

<CIT> discloses a system comprising: a light source; a spatial light modulator that phase shifts light from the light source into a first plurality of spatially independent modes; a beam splitter that splits light from the spatial light modulator into a first light beam and a second light beam; a first imager that images the first light beam; a multimode fiber coupler that couples a multimode fiber with the system; a second imager that images light from a distal end of the multimode fiber when coupled with the multimode fiber coupler for each of a second plurality of spatially independent modes; and a processor coupled with the spatial light modulator, the first imager and the second imager that creates a transmission matrix from the light received at the first imager and the second imager.

<CIT> discloses a method of characterizing an optical system, wherein the optical system comprises an optical fibre having a proximal end and a distal end, wherein the optical fibre comprises a non-single- mode optical fibre, and comprising a reflector assembly comprising a stack of reflectors disposed at the distal end of the optical fibre, wherein the stack of reflectors is arranged to provide different reflector matrices in dependence on illumination wavelength. The method comprises projecting calibration patterns at a plurality of characterization wavelengths onto the proximal end, then obtaining, at the proximal end, data relating to reflected calibration patterns. An instantaneous transmission matrix of the optical fibre is then determined using the data relating to the reflected calibration patterns and the reflector matrices.

<CIT> discloses a multimode waveguide illuminator and imager relying on a wave front shaping system that acts to compensate for modal scrambling and light dispersion by the multimode waveguide. A first step consists of calibrating the multimode waveguide and a second step consists in projecting a specific pattern on the waveguide proximal end in order to produce the desired light pattern at its distal end. The illumination pattern can be scanned or changed dynamically only by changing the phase pattern projected at the proximal end of the waveguide. The third and last step consists in collecting the optical information, generated by the sample, through the same waveguide in order to form an image.

According to the invention in claim <NUM>, there is provided a method of determining an optical transform imparted by a multimode optical fibre, wherein the multimode fibre comprises a proximal end, a distal end, and at least one modified region between the proximal end and distal end, the modified region configured to transmit light toward the proximal end in response to light propagating through the multimode optical fibre from the proximal end to the distal end, the method comprising:.

Determining the optical transform may comprise:.

The transmission matrix comprises a matrix of complex coefficients relating the input field amplitude and phase of each of a plurality of input modes to the output field amplitude and phase of each of a plurality of output modes. The correction matrix may comprise a set of corrections for correcting a transmission matrix (e.g. as defined in equation (<NUM>)).

Providing a modified region in the MMF is a more elegant solution than prior art methods that involve optics appended to the distal facet of the MMF, or that rely on optical components arranged about the distal end of the MMF.

The at least one modified region may comprise at least one fibre Bragg grating. A fibre Bragg grating can be written in a MMF, for example using laser micromachining. The at least one modified region may comprise at least one fluorescent colour centre. A fluorescent colour centre will emit light at a different wavelength to the illuminating light, which may make it possible to perform sensing using the MMF at the same time as determining the optical transform of the MMF (and using the optical transform to optimise/correct the sensing).

The at least one modified region may comprise one or more modified regions disposed in the fibre core and/or fibre cladding.

There may be a plurality of modified regions, with at least some of the modified regions at different lateral positions and/or at different longitudinal positions between the proximal and distal end.

The at least one modified region may comprise a plurality of fibre Bragg gratings, and at least some of the fibre Bragg gratings may have different: period, reflectivity and/or orientation/polarisation.

The at least one modified region may comprise one or more chirped fibre Bragg gratings.

The forward propagating light may be a first forward propagating light field and the backward propagating light may be a first backward propagating light field, and the method may further comprise:.

Correcting/optimising the image may comprise controlling an active optical element to modify the second forward propagating light field.

The active optical element may comprise a spatial light modulator configured to modify the spatial distribution of phase of the second forward propagating light field.

The method may comprise performing point scanning microscopy using a plurality of second forward propagating light fields.

Correcting/optimising the image may comprise computationally reconstructing an image from the results of detecting the second backward propagating light field.

The first forward propagating light field and the second forward propagating light field and/or the first backward propagating and the second backward propagating light field are multiplexed, so that the optical transform can be updated without interrupting imaging of the scene.

The multiplexing may comprise wavelength multiplexing and/or temporal multiplexing.

The laser micromachining may be is performed using adaptive optics, which modify wavefront properties of a laser system to counteract the effects of aberration on laser focus.

Determining on optical transform may comprise determining a correction matrix to take account of fibre deformation/conformation, wherein determining the optical transform comprises multiplying the correction matrix with an uncorrected transmission matrix.

The uncorrected transmission matrix may be determined by detecting, at the distal end of the fibre, forward propagating light coupled into the optical fibre at the proximal end of the fibre.

The method may further comprise correcting the transmission matrix for temperature determined from a modified region that comprises a fibre Bragg grating.

Also according to the invention in claim <NUM>, there is provided an apparatus for obtaining information for correcting an optical transform imparted by a multimode optical fibre, comprising:.

The apparatus may comprise a processor configured to determine the optical transform from the detected backward propagating light.

The apparatus according to the invention may be configured to perform the method according to the invention, including any of the optional features thereof.

The forward propagating light may be a first forward propagating light field and the backward propagating light is a first backward propagating light field; and.

The apparatus may be configured to use an optical transform determined from the detected first backward propagating light field to correct the image formed using the second backward propagating light field.

There is also provided an endoscope comprising the apparatus of the invention, including any optional features thereof.

Example embodiments will be described, by way of example only, with reference to the drawings, in which:.

Referring to <FIG>, a method of determining an optical transform imparted by a multimode optical fibre is shown. The multimode fibre comprises a proximal end, a distal end and at least one modified region between the proximal end and distal end. The modified region is configured to transmit backward propagating light toward the proximal end of the fibre in response to forward propagating light transmitted through the optical fibre from the proximal end towards the distal end. The modified region may, for example, comprise a reflective element (such as a fibre Bragg grating) or a fluorescent element. The modified region may be formed by laser machining of the optical fibre, and may be disposed in the fibre core or cladding.

At step <NUM>, forward propagating light is coupled into the proximal end of the MMF. The forward propagating light will reach the modified region in the MMF, and the modified region will cause a backward propagating light signal in response to the forward propagating light (e.g. by fluorescing in response to the forward propagating light, or by reflecting a portion of the forward propagating light).

At step <NUM>, the backward propagating light arising from the interaction of the forward propagating light with the modified region is detected, after it exists from the proximal end of the fibre. A beam splitter may be used to both transmit the forward propagating light into the fibre and to detect the backward propagating light from the modified region. The backward propagating light from the modified region may be imaged, to produce an intensity distribution of the backward propagating light at the proximal end of the fibre.

At step <NUM>, an optical transform TM is determined from the results of detecting the backward propagating light exiting the optical fibre at the proximal end. For example, the intensity distribution of the backward propagating light from the modified region may be used to determine a TM that includes conformation effects, or to determine a correction matrix for correcting a predetermined TM for changes in conformation of the optical fibre.

The detected backward propagating light from the modified region of the fibre can be used to characterise the optical transform imparted by the fibre without access to the distal end. The backward propagating light originates from one or more known positions in the optical fibre (corresponding with one or more modified regions disposed in the optical fibre). The techniques described in the background section can readily be used to characterise the TM in a way that takes into account conformation changes in the fibre to enable imaging using the MMF.

The TM can be used in several ways, which can be grouped into two categories.

The first category encompasses methods in which a distal plane is illuminated with a sequence of non-uniform wide-field illuminations. A distal image can be reconstructed computationally for each image recorded with a proximal detector using the TM. In such an approach, the detector may be used to both determine the TM from the backward propagating light from the modified region, and to obtain the image data from which the distal image is computationally determined using the TM. In some embodiments the determination of the TM may be temporally multiplexed with the acquisition of imaging data (from which the image is determined), for example between each frame, or periodically, depending on how much movement of the MMF is anticipated during imaging (e.g. every <NUM> imaging frames, or every <NUM> imaging frames etc). The acquisition of multiple images enables averaging out the nonuniformities in the illuminations due to MMF optical effects. In some embodiments, different light sources and/or different detectors may be used to determine the TM and perform imaging at the same time, using wavelength multiplexing of: the forward propagating light used to interrogate the at least one modified region and the forward propagating light used to image a scene at the distal plane; and/or of the backward propagating light from the one or more modified region and the backward propagating light from the distal plane.

The second category uses the TM, not after data acquisition, but instead to modify the illumination. A wavefront shaping device (such as a spatial light modulator) may be used to modulate the input light field that is used for imaging, so as to shape the light at the distal plane. One implementation of this is to generate a diffraction-limited focal point in the distal plane. This point enables sensing or optical manipulation (e.g. nonlinear photo-polymerisation) in a spatially defined and limited location. The point can also be translated by updating the input field modulation in a fashion equivalent to raster-scanning for realising point-scanning microscopy. Suitable wavefront shaping devices include liquid crystal spatial light modulators and digital micromirror devices.

The methods of characterising the TM described herein are applicable to both categories of using the TM.

<FIG> illustrates, in simplified form, an apparatus <NUM> according to an embodiment, comprising a multimode optical fibre <NUM>, light source <NUM>, detector <NUM> and processor <NUM>.

The multimode optical fibre <NUM> comprises a modified region <NUM> near to the distal end thereof (e.g. within <NUM>, or <NUM> or <NUM> or closer), which may comprise a fibre Bragg grating. In some embodiments the modified region may be closer than <NUM> from the distal end (e.g. within <NUM> or <NUM>). The modified region <NUM> may be written using laser machining, which may employ active optics to modify the wavefront properties of the laser to correct for aberration of the laser focus (for example, as described in <CIT>). In some embodiments there may be a plurality of modified regions, examples of which will be described in more detail below.

The light source <NUM> is optically coupled to the proximal end of the fibre <NUM>, and transmits forward propagating light into the optical fibre <NUM>. The forward propagating light interacts with the modified region <NUM>, which results in backward propagating light.

The detector <NUM> is configured to detect the backward propagating light from the interaction of the forward propagating light with the modified region <NUM>, for example by imaging the resulting pattern of light at the proximal end of the fibre <NUM>.

The results from the detector <NUM> are provided to processor <NUM> (e.g. in the form of an image of the spatial distribution of light intensity), which determines the TM of the MMF <NUM> therefrom. Depending on the specific arrangement and nature of the modified region (or regions), any of techniques a) to f) described in the background section may be employed.

In embodiments employing the technique of Plöschner there may be one or many Bragg gratings <NUM>, which may be used to characterise conformal changes in the MMF <NUM>. One way to do this is to use the Bragg gratings as strain gauges. For example a distribution of Bragg gratings around the permieter of the fibre can be used to detect bending of the fibre in different directions. As the Bragg grating is strained, the Bragg wavelength defined by the grating spacing changes, which can be detected. Gratings with different Bragg wavelengths can be used, enabling wavelength multiplexed interrogating of the strain gauges. The conformal changes may be used to determine a correction matrix Cλ,Si-<NUM>→Si for updating a predetermined TM (as described in Plöschner).

In some embodiments, the modified region may comprise a single Bragg grating, and the reflected pattern imaged proximally. The intensity distribution may be correlated with a plurality of predetermined intensity distributions in a databank to determine a corresponding TM, following the technique described in Farahi (with the Bragg grating serving as a virtual beacon). A single grating may be sufficient to characterise a correction matrix (e.g. for relatively small changes in conformation of the fibre). Multiple gratings may be used to characterise a correction matrix for more significant conformation.

In some embodiments, multiple Bragg gratings can be written at different locations in the fibre. A checkboard reflector following the approach of Gu can be generated by employing a similar spatial distribution of Bragg gratings near to the distal tip of the MMF <NUM>, with spatial modulation of the reflectance signal achieved by using Bragg gratings with different lengths (i.e. a different number of periodic variations in refractive index) with longer Bragg gratings providing higher reflectance at the Bragg wavelength.

The multiple Bragg gratings can be probed (by the forward propagating light from the light source <NUM>) at different times, or the light may be wavelength multiplexed. The light incident on the Bragg gratings may be shaped by using wavefront control at the light source <NUM>.

The approach set out in Gordon can be followed by employing Bragg gratings with different grating periods, and hence having different Bragg wavelengths. In some embodiments, two-dimensional arrays of Bragg gratings may be stacked (longitudinally near the distal end of the fibre), with each layer in the stack having a different Bragg wavelength. Different layers gratings may be configured with different diattenuation by changing the orientation of the gratings used to define them.

The approach set out in Li can be followed by disposing a modified region (Bragg grating or fluorescent centre) to serve as s guide star, from which the TM can be found that optimises constructive interference onto the guide star. The modified region may be disposed at the interface between the core and the cladding, to maximise the size of the isoplanatic patch. Multiple modified regions may be used to tile multiple isoplanatic patches into a single image. Each guide star may partial information of the transmission matrix. The transmission matrix basis can be expressed to make the information obtained from each guide star independent. This is also a good strategy for minimising the number of required guidestars for determining the transmission matrix.

Machine learning approaches can also be used. In one example, an image of the back-propagating light from the one or more modified regions is provided to a convolutional neural network (CNN). The CNN may be trained using controlled fibre deformations and corresponding TMs that have been determined with access to both ends of the fibre (for example using methods i) to iii) disclosed in the background section). The CNN may consequently be able to determine a TM directly from a detected image (spatial distribution of intensity) of the back propagating light at the proximal end of the fibre. In some embodiments, the CNN may be provided with image data corresponding with different wavelengths (e.g. reflected from gratings with different period) and with different polarisation states (e.g. reflected from gratings with differently oriented periodic variations in refractive index). The CNN may be configured to determine a correction matrix for correcting a predetermined TM corresponding with a specific conformation of the MMF (e.g. a straight fibre, or a fibre in a neutral starting position of a particular instrument).

<FIG> shows a further example embodiment of an apparatus <NUM> according to an embodiment, comprising: light source <NUM>, wavefront shaping device <NUM>, relay lenses <NUM>, aperture <NUM>, beam splitter <NUM>, imaging lens <NUM>, detector <NUM>, coupling lens <NUM> and multimode optical fibre (MMF) <NUM>.

The MMF <NUM> comprises a proximal end <NUM>, distal end <NUM>, core, <NUM>, cladding <NUM> and modified region <NUM> near to the distal end <NUM>.

The light source <NUM> is optically coupled to the proximal end <NUM> of the MMF <NUM> via the wavefront shaping device <NUM>, relay lenses <NUM>, aperture <NUM>, beam splitter <NUM> and coupling lens <NUM>. A light beam from the light source <NUM> is modified by the wavefront shaping device <NUM>, and then coupled into the proximal facet (at the proximal end) of the MMF <NUM> via the relay lenses <NUM>, aperture <NUM> and coupling lens <NUM>. The wavefront shaping device <NUM> may comprise a spatial light modulator, for example employing liquid crystal modulators that are configured to adjust the spatial distribution of phase of the light. The wavefront shaping device <NUM> may be configured to shape light incident on the modified region, or to perform imaging by shaping light incident on a distal plane (as described above).

The beam splitter <NUM> is configured to couple forward propagating light from the light sources <NUM> into the proximal end <NUM> of the MMF <NUM>, and to coupled backward propagating light emerging from the proximal end <NUM> of the MMF to the sensor <NUM>, via the imaging lens <NUM>. The detector <NUM> may comprise a focal plane array, at which an image of the backward propagating light is formed. The detector <NUM> may be used to detect the backward propagating light from the modified region <NUM> for characterisation of the TF of the MMF <NUM>, and may also be used to form an image of a scene adjacent to the distal end <NUM> of the MMF <NUM> (as described above) using the TF to correct the image. Using the TF to correct/optimise the image may comprise generating a point scanning illuminating light field at the distal scene using the wavefront shaping device <NUM>, and/or may comprise computationally reconstructing an image of the distal scene employing the TF (from one or more images taken from the detector <NUM>).

<FIG> show an end view and side view respectively of an example of a modified region according to an embodiment comprising a Bragg grating <NUM> written near the distal end of a MMF. In this example embodiment, the Bragg grating <NUM> is centrally located, on the axis of the optical fibre core <NUM>.

<FIG> show an end view and side view respectively of an example of a modified region according to an embodiment comprising a point structure <NUM>, such as a fluorescent centre written near the distal end of a MMF. In this example embodiment, the point structure <NUM> is centrally located, on the axis of the optical fibre core <NUM>.

<FIG> shows an example embodiment in which a plurality of modified regions in the form of Bragg gratings <NUM> are disposed in the core <NUM> near the distal end of the MMF. The Bragg gratings <NUM> are arranged in a one dimensional array along a radius of the fibre core, with different gratings <NUM> arranged at different radial positions.

<FIG> shows an example embodiment in which a plurality of modified regions in the form of Bragg gratings <NUM> are disposed at different circumferential locations at the perimeter of the fibre core <NUM>, near a distal end of the MMF. In the example the gratings <NUM> are equally spaced around the circumference of the fibre core <NUM>, but this is not essential.

<FIG> shows a side view of multiple modified regions comprising Bragg gratings <NUM>, disposed on the longitudinal axis of the MMF at different longitudinal positions near to the distal end of the MMF. Although a single Bragg grating at each longitudinal position is shown, there may be more than one Bragg grating at each longitudinal position, and the modified regions may be dispersed both laterally (e.g. in different radial and circumferential positions at a particular longitudinal direction) and longitudinally.

<FIG> illustrates a side view of multiple modified regions comprising Bragg gratings <NUM> written near to the edge of the core <NUM>. The Bragg gratings <NUM> may be evenly distributed along the length of the MMF, with each longitudinal location having a evenly spaced circumferential array of Bragg gratings <NUM> disposed near to the edge of the core <NUM> (or in the cladding <NUM>). In such an arrangement each grating is sensitive to bending (with bending at each longitudinal location imparting different strain at different circumferential locations, depending on the direction of bending, which can be detected by a changed in the Bragg wavelength of each grating. Such an arrangement allows conformal changes in the MMF to be sensed. The conformal changes can be used in a calculation as disclosed in Plöschner to determine a correction matrix for updating a TM (which may have been predetermined by techniques i) to iii), for example with access to both proximal and distal ends of the MMF.

<FIG> shows an MMF <NUM> according to an embodiment in which multiple modified regions <NUM> can be provided in a distal end region <NUM> of the MMF <NUM>. The multiple modified regions may comprise Bragg gratings, and may be provided at a series of longitudinal positions, with each longitudinal position comprising a plurality of gratings (e.g. as shown in any of <FIG>, <FIG>). The distal end region <NUM> of the MMF may be kept substantially free from conformation changes, either by a support structure (such as a rigid support tube) or by the specimen being imaged (e.g. by tissue surrounding the MMF, in the case of invasive endoscopy). The modified regions <NUM> in the distal end region <NUM> of the MMF may be used to determine a TM (or a correction matrix) that takes account of conformation changes in the rest of the MMF <NUM>.

<FIG> shows a plurality of Bragg gratings 261a, 261b, 261c disposed near a distal end <NUM> of a MMF in the core <NUM>. Each of the Bragg gratings 261a, 261b, 261c has a different length (number of regions of different refractive index), with the result that each Bragg grating 261a, 261b, 261c has a different reflectance. This enables a checkerboard reflector to be implemented within the fibre that imposes a spatial encoding on the intensity of the reflected light from the modified regions (analogous to the approach of Gu).

<FIG> shows a plurality of Bragg gratings 261a, 261b, 261c disposed near a distal end <NUM> of a MMF in the core <NUM>. Each of the Bragg gratings 261a, 261b, 261c has a different spatial period, with the result that each Bragg grating 261a, 261b, 261c has a different Bragg wavelength (at which it will have maximum reflectance). Different wavelength Bragg gratings may be used to simplify interrogation of multiple modified regions at the same time (by wavelength multiplexing), and also facilitates an approach to determining the TM that is similar to that disclosed in Gordon.

<FIG> shows a plurality of Bragg gratings 261a, 261b disposed near a distal end <NUM> of a MMF, in the core <NUM>. Each of the Bragg gratings 261a, 261b has a different orientation of the regions of different refractive index, with the result that each Bragg grating 261a, 261b will have peak reflectance for a different polarisation of light. Polarisation specific Bragg gratings may be used to simplify interrogation of multiple modified regions at the same time (by polarisation multiplexing), and also facilitates an approach to determining the TM that is similar to that disclosed in Gordon.

<FIG> shows a plurality of modified regions L1-L16, disposed in a two dimensional array in the MMF fibre core <NUM> at a longitudinal location near the distal end of the MMF. The modified regions L1-L16 may comprise Bragg gratings, which may be configured to impose a spatial intensity modulation on the light reflected from the modified regions (e.g. by varying the length of the grating at each location).

<FIG> shows a plurality of modified regions 261a, 261b, 261c at each of a plurality of longitudinal positions. The modified regions comprise Bragg gratings, and each longitudinal position comprises Bragg gratings with a particular grating period, so that each layer of Bragg gratings is configured to reflect a specific wavelength of light. Each longitudinal position may be provided with an array of Bragg gratings, as shown in <FIG>. This embodiment may be applicable for determining the TM as disclosed in Gordon, with the stacked arrays of Bragg gratings analogous to the stacked metasurfaces in Gordon.

<FIG> shows an embodiment in which a single Bragg grating <NUM> is provided in the core <NUM> at the edge of the core <NUM>, near the distal end of the fibre. In this embodiment the Bragg grating may serve as a guide star, to determine a TM that enables an isoplanatic patch, as described by Li.

Although embodiments with Bragg gratings have been described, some embodiments may employ fluorescent light from the modified regions, rather than reflected light. Since the fluoresced light will have a different wavelength, the sensing of the fluoresced light from the modified regions may be detected at the same time as sensing or imaging using the optical fibre (corrected by the TM determined from the fluoresced light from the modified regions). A secondary focus can be generated on the fluorescent modified region using holography, and temporal multiplexing between multiple fluorescent structures is also possible.

Claim 1:
A method of determining an optical transform imparted by a multimode optical fibre (<NUM>), wherein the multimode optical fibre (<NUM>) comprises a proximal end (<NUM>), a distal end (<NUM>), and at least one modified region (<NUM>) between the proximal end (<NUM>) and distal end (<NUM>), the modified region (<NUM>) configured to transmit light toward the proximal end (<NUM>) in response to light propagating through the multimode optical fibre (<NUM>) from the proximal end (<NUM>) to the distal end (<NUM>), the method comprising:
coupling forward propagating light into the proximal end (<NUM>) of the multimode optical fibre;
detecting, at the proximal end (<NUM>), backward propagating light transmitted from the at least one modified region (<NUM>) in response to the forward propagating light; and
determining an optical transform from the detected backward propagating light.